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AMER. ZOOL., 21:233-242 (1981) Logical Levels of Steroid Hormone Action in the Control of Vertebrate Behavior1 ARTHUR P. ARNOLD Department of Psychology and Brain Research Institute, University of California, Los Angeles, California 90024 SYNOPSIS. Three steroid-sensitive neural systems are reviewed to suggest where hormones act to modify neuroendocrine or behavioral functions: the system controlling the ovulatory surge of luteinizing hormone in laboratory rats, the system controlling male copulatory behavior in the rat, and the system controlling passerine bird vocalizations. In each, steroids act at several levels, including the final common path. This generalization is discussed in light of some earlier conceptualizations of the levels of hormone action in behavioral systems. INTRODUCTION herent picture of how internal and external stimuli affect instinctive behavior. One class of internal events which he considered was hormonal influences. Tinbergen summarized briefly the famous ethological hierarchical model of centers which control instinctive behavior. The highest centers represented major instincts (e.g., reproduction), and such centers controlled lower centers representing specific instincts such as fighting, courtship, locomotion, which in turn controlled lower centers which coordinate specific movements involved in each behavior. In discussing where hormones might act in this hierarchy, Tinbergen considered the early work of Beach and others, and concluded that "it is probable that hormones act exclusively on the higher centers" (p. 124). He reached this conclusion because of the specificity with which hormones activate only certain behaviors. Since the animal uses the same muscles for different instinctive behaviors (Tinbergen, 1951, p. 74) an action of a hormone at a low level in the hierarchy would seem to be incompatible with the hormone's selective effect on the behavior. In explaining the hierarchical model, Tinbergen used the example of the three-spined stickleback (Gasterosteus aculeatus). The lowest level of the hierarchy of centers controlling reproductive behaviors is comprised of the muscles involved in the behaviors, e.g., swimming muscles. The same muscles are used for both the zig-zag 1 From the Symposium on Social Signals—Compar- courtship swimming of the male stickleative and Endocrine Aspects presented at the Annual back, and for swimming during appetitive Meeting of the American Society of Zoologists, 27- behaviors involved in food search. In this Sex steroids have profound yet specific effects on the behavior of vertebrates. For example, in zebra finches (Poephila guttata) castration reduces (and testosterone therapy increases) the frequency of courtship, copulatory, and aggressive behaviors, whereas other behaviors are relatively unaffected (Arnold, 1975). At what logical levels do steroids act to bring about such changes in behavior? For example, do they act on sensory organs or afferents to the nervous system, on motor pattern generators, on general arousal mechanisms, or at other functional levels? The word logical is used here to imply that we wish to determine the function of steroid target tissues not only in terms of their physiology (e.g., primary sensory neurons) but also in terms of their logical function within the network of tissues responsible for the behavior. For example, a steroid hormone acting on a primary sensory neuron could modify at least two logical functions: it could alter the elicitation of the behavior by sensory stimuli, or it could alter the feedback of sensory information to a central pattern generator which is instrumental in assessment of the output of the generator. In his classical treatise on mechanisms and evolution of animal behavior, Tinbergen (1951) attempted to synthesize a co- 30 December 1979, at Tampa, Florida. 233 234 ARTHUR P. ARNOLD instance, Tinbergen most likely would have concluded that hormones affecting courtship swimming induce a selective enhancement of certain patterns of muscle activity rather than all contractions of the swimming muscles, and these patterns must be controlled at higher levels in the nervous system. Thirty years after Tinbergen published this model in summary form, we still think in terms of hierarchies of neural systems involved in controlling behavior. This is partly because the notion of hierarchy has been common in the best described neural systems involved in invertebrate behavior patterns. But it is also because any diagram of neural connections in a system underlying vertebrate behavior involves neurons synapsing on others in a more or less linear fashion (although loops, branches, or parallel pathways are possible) leading eventually to the motor effectors for the behavior. We have now reached the point that, although we do not yet know the entire logical or functional circuitry for any single hormone-sensitive vertebrate behavior pattern (which would be a prerequisite for finding all of the sites where hormones act to modify the behavior), we are familiar enough with several neural systems to make at least two conclusions about logical levels of steroid hormone action. The first conclusion is that sex steroids probably act at many logical levels to modify neural functioning, which leads to changes in behavior. Secondly, in some species, sex steroids act not only at logically high levels (integrative levels rather high in the hierarchy, far removed from the motor effectors), but also at very low logical levels, in the final common neural path and in the muscles themselves. To draw these conclusions, I wish to examine three hormonesensitive neural systems. pattern, yet there are functional aspects which are similar to the two behavioral systems discussed below. In rats, there is an increase in plasma estradiol levels on the morning of proestrus which precedes release of luteinizing hormone releasing h o ^ mone (LHRH) from the median eminence into the portal blood supply to the pituitary (Sarkar et al., 1976). Subsequently, there is a large increase in LH secretion by the pituitary which induces ovulation. LHRH appears to have complex effects on the pituitary during the LH surge. It not only causes the release of LH, but also has a self-priming effect. Exposure of the pituitary to LHRH increases its sensitivity to subsequent LHRH, so that constant amounts of LHRH are able to induce progressively greater releases of LH (Fink, 1979). Estradiol is intimately implicated in triggering the ovulatory surge of LH. When the proestrus increase in estradiol levels is counteracted with antiestrogens, the surge of LH does not occur, and simulation of the proestrus rise in estradiol with exogenous estradiol injections will induce an LH surge and ovulation (Brown-Grant, 1977). There is evidence that estradiol exerts this influence by acting at a number of sites and by a variety of mechanisms. First of all, it is clear that estradiol acts on the pituitary to increase the secretion of LH induced by a given amount of LHRH (Beck et al., 1978; Fink, 1979). Estradiol is also important for the LHRH self-priming effect on the pituitary, since the magnitude of this LHRH-induced increase in pituitary sensitivity to LHRH is determined by estradiol levels (Fink, 1979; Blake, 1978). A second site of action of estradiol in inducing the LH surge is in the medial preoptic area (MPOA). This region has been implicated consistently in the control of ovulation. When the medial basal hyESTRADIOL CONTROL OF THE pothalamus and pituitary are isolated from OVULATORY LH SURGE rostral afferents with knife cuts, ovulation The first system is the neural network persists only if the rostral knife cut is ancontrolling the ovulatory surge of lutein- terior to the MPOA (Schwartz, 1972). Furizing hormone (LH) secretion by the an- thermore, electrical stimulation of the terior pituitary of rats. This system is not MPOA is very effective in eliciting LHRH a behavioral one since the endpoint is en- and LH secretion (Chiappa et al., 1977; docrine secretion rather than a behavior Cramer and Barraclough, 1971). Since LEVELS OF STEROID ACTION ON BEHAVIOR MPOA neurons contain LHRH, it is possible that they are the secretory neurons responsible for the rise in LHRH. However, the stimulation in MPOA may activate nerve fibers or cell bodies which syn#pse on others in a multisynaptic pathway, which eventually fires LHRH-containing neurons elsewhere, causing them to discharge LHRH. Significantly, estradiol increases the amount of LHRH and LH released by MPOA stimulation. A priori, it could do this by acting on (a) afferents to the MPOA (e.g., especially from the hippocampus, amygdala, or septum, which contain estradiol receptors, Pfaff and Keiner, 1973); (b) on cells fired by stimulation of the MPOA; (c) on neural elements which receive direct or indirect inputs from fibers or cells activated by MPOA stimulation and which are involved in LHRH release; and/or (d) on the pituitary itself. Goodman (1978) has provided strong evidence that the MPOA itself may be a site of action, since implants of estradiol into the MPOA (but not into the medial basal hypothalamus) cause an LH surge, even though both types of implants raise pituitary estradiol levels by comparable amounts. This result fits nicely with autoradiographic localization of estradiol concentrating cells, since the MPOA contains many such cells (e.g., Pfaff and Keiner, 1973; Stumpf, 1968). There is also evidence compatible with estradiol's action on limbic regions afferent to the MPOA, since if these afferents are sectioned, electrical stimulation of the MPOA is significantly less effective in triggering LH release (Chiappa et al., 1977). However, this last finding does not necessarily imply an action of estradiol outside of the MPOA and hypothalamus, since the sensitivity of MPOA neurons to estradiol could be dependent on their proper innervation. Since stimulation of the hippocampus and amygdala have been reported to affect MPOA-stimulation induced LH release and spontaneous ovulation (Kawakami et al., 1973; Velasco and Taleisnik, 1969), and because these regions contain cells that accumulate estradiol (Pfaff and Keiner, 1973), one can also construct a plausible but speculative argument that estra- 235 diol acts in these regions to modulate the release of LHRH which is controlled by the MPOA. In summary, it is likely that estradiol triggers and modulates the ovulatory surge of LH secretion by acting at three (and possibly more) levels in the CNS-pituitary circuit: in the pituitary itself, in the MPOA, and possibly in other limbic regions. The actions of estradiol in the MPOA and pituitary may be partially redundant, since each has the effect of increasing LH secretion. These results also suggest action of estradiol at the final common path of this circuit controlling LH secretion, i.e., in the pituitary itself. In turning to studies of modulation of behavior by steroids, we face some problems. The literature on neurohumoral control of LH secretion is an order of magnitude larger than the literature on mechanisms of steroid control of any one vertebrate behavior pattern, so we must rely on much less information. Secondly, LH secretion varies in magnitude but not in quality. This is not true for behavior. Each behavior pattern is a complex and variable sequence of muscular contractions so that the steroid may change the behavior both qualitatively and quantitatively. Therefore, precise measurement of the steroid's effect has often been incomplete. ANDROGEN EFFECTS ON MALE R A T COPULATORY BEHAVIOR The first behavioral system to be discussed here is masculine copulatory behavior of the laboratory rat. A large number of CNS regions have been implicated in the control or modulation of male copulatory behavior, including the MPOA, medial forebrain bundle, olfactory tubercle, mammillary bodies, amygdala, hippocampus, neocortex, diencephalic-mesencephalic junction, and spinal cord (reviewed by Malsbury and Pfaff, 1974; Hart, 1974; Sachs, 1978; Larsson, 1979). The roles of the amygdala, hippocampus, and neocortex appear to be minor, or modulatory, since sexual behavior persists after lesioning these areas, although such lesions cause deficits in this behavior. Yet the olfactory tubercle, amygdala, and hippocam- 236 ARTHUR P. ARNOLD pus should be potential sites of action of steroids, since these contain cells which accumulate estrogens and/or androgens (e.g., Pfaff and Keiner, 1973; McEwen, 1976; Stumpf and Sar, 1978). However, steroid effects on these regions which influence male sex behavior have apparently not been investigated extensively. Good evidence exists, however, for steroid actions in the MPOA to facilitate mounting, intromission, and ejaculation, since lesions of this region eliminate these male sexual behaviors in rats, electrical stimulation augments these behaviors, and androgen implants in this region in castrates reinstate the behaviors. Significantly, cell bodies of neurons in the MPOA concentrate radioactivity heavily after injections of tritiated estradiol, testosterone, or dihydrotestosterone in autoradiographic studies (e.g., Pfaff, 1968; Stumpf, 1968; Stumpf and Sar, 1978). The evidence for androgen action in the lower spinal cord to modulate sexual reflexes is nearly as strong as the evidence for its action in the MPOA. Hart (1978a, b) has demonstrated that male rats possess penile flip reflexes which are elicited by holding back the preputial sheath of the penis. These flips are decreased by castration and enhanced by administration of testosterone propionate, and they occur more frequently following mid-thoracic transection of the spinal cord. Implants of testosterone propionate in the spinal cord increase the flips in castrated males (Hart and Haugen, 1969), suggesting that testosterone propionate or its metabolites act in the spinal cord to modulate these reflexes. It is unclear to what extent these reflexes, elicited by artificial stimulation, are similar to reflexes during normal copulatory behavior. However, even if the reflexes are labeled "artificial," it is quite likely that they are controlled by neural circuits which are normally involved in erection, intromission, and/or ejaculation. Sachs and Garinello (1978) and Kurtz and Santos (1979) have demonstrated that males exposed to a female and allowed to initiate a series of intromissions will show much shorter latencies for penile reflexes than controls not exposed to females, and that sexual exhaustion depresses all such reflexes. These studies support the notion that the penile reflexes are a measure of the function of neural circuits involved in copulation, and give credence to the assertion that androgens act on the spinal conj to modulate copulatory behavior. To search for cellular accumulation of androgen which would correlate with a spinal action of androgen on this system, Breedlove and Arnold (1980) located the motor neurons which innervate the striated muscles of the penis, which are almost certainly those responsible for the penile flip reflexes. When horseradish peroxidase is injected into the bulbocavernosus or levator ani muscles, it is retrogradely transported to a discrete motor nucleus in the ventromedial lumbar spinal cord. In autoradiograms, these motor neurons accumulate hormone after injection of tritiated testosterone, dihydrotestosterone, but not after estradiol (Breedlove and Arnold, 1980, which extends earlier work of Sar and Stumpf, 1977). This pattern of accumulation agrees perfectly with the pattern of steroid activation of the penile reflexes, since of these three hormones, only estradiol is ineffective in reinstating the reflexes in castrated males (Hart, 1978a). These results raise the likelihood that androgens act on these motor neurons to influence the function of neural circuits involved in some aspect of the genital reflexes demonstrated by Hart, and hence in copulatory behavior. The autoradiographic studies located androgen-labeled cells in other spinal nuclei, including lamina X, intermediolateral nucleus, and the ventrolateral motor neurons, among which are found the motor neurons of the ischiocavernosus. Thus there are several potential sites of action of androgens even within the spinal cord. At present we do not know precisely how the penile striated muscles (bulbocavernosus, ischiocavernosus, and levator ani) participate in male copulatory behavior in rats. However, in dogs the ischiocavernosus and bulbospongiosus (the homolog of the bulbocavernosus) are responsible for vascular pressure increases in the penis which mediate erection and copulatory LEVELS OF STEROID ACTION ON BEHAVIOR lock in that species (Purohit and Beckett, 1976). In the stallion, these muscles contract during intromission and ejaculation (Beckett et al., 1975). Thus it is likely that these muscles alter erectile mechanisms in 0ats, too. However, in the human male the homologs of these muscles are also involved in control of micturation (Warwick and Williams, 1973), so that we cannot be certain that the motor neurons that accumulate androgen are activated exclusively during copulation in rats. In addition to effects of androgen on the CNS, Beach and his co-workers have suggested a peripheral action on the penis which may be important in copulatory behavior (Beach and Levinson, 1950; Beach, 1971). This is based primarily on a strong correlation between testosterone's influence on the amount of copulatory behavior and its influence on the size and morphology of the cornified papillae on the glans penis, which are thought to be important for its tactile sensitivity. It is known that gross alterations in tactile sensitivity of the penis produced by topical anesthetics will lead to decrements in ability to intromit (reviewed by Malsbury and Pfaff, 1974), but there seems to be no direct evidence that the sensitivity of the glans penis is altered by androgens. Indeed, Cooper and Aronson (1974) could detect no sensory effect of androgens on the penis of cats. It is possible to administer non-aromatizable androgens at moderate doses2 (fluoxymesterone or dihydrotestosterone) to castrates, which affect the morphology of the penile papillae but have little effect on copulatory behavior or penile reflexes (Beach and Westbrook, 1968; Feder, 1971; Hart, 1978a). While these results indicate that androgenic effects on the papillae are not sufficient to reinstate copulatory behavior or penile reflexes, they have little bearing on the question of whether androgens modulate the sensitivity of the penis. Although most authors interested in an2 Dihydrotestosterone does have effects on penile reflexes when given in higher doses (Hart, 1978a, 1979) and on copulatory behavior when given in combination with estradiol (Davis and Barfield, 1979). 237 drogen-induced copulatory behavior have considered changes in the sensitivity of the penis in detail, few have considered direct androgenic effects on the penile striated muscles as possibly important for copulatory behavior. These muscles are among the most androgen-sensitive muscles in the body of the rat. Castration reduces the sarcoplasmic mass of the levator ani (Venable, 1966), and castration and administration of androgen cause a host of biochemical and physiological changes in this muscle which alter its size and functional capacity (Buresova and Gutmann, 1971; Tucek et al., 1976; Vyskocil and Gutmann, 1977; Hanzlikova and Gutmann, 1978). Although some of these steroid effects could be exerted via an action on the androgensensitive motor neurons of these muscles (Buresova et al., 1972), at least some of the metabolic and functional effects have been demonstrated in cultured muscles, suggesting direct actions on the muscle (Buresova and Gutmann, 1971). The levator ani and bulbocavernosus appear to be virtually identical in their biochemical affinity for androgens (Krieg et al., 1974). Each possesses high affinity receptors for testosterone and dihydrotestosterone, and the numbers of these receptors is 5-7 times greater than in other striated muscles (Tremblay et al., 1977; Krieg and Voight, 1977). Both dihydrotestosterone and testosterone are effective in stimulating growth of the levator ani (Liao and Fang, 1969). Thus, these muscles could be one site of action of androgens in their influences on male copulatory behavior. If one were to list the sites of action of androgen related to copulatory behavior in the order of our current certainty regarding their involvement, the list might appear as follows: (1) MPOA, (2) spinal cord, possibly at the level of the motor neurons controlling penile striated muscles, (3) the penile striated muscles themselves, and (4) certain steroid-concentrating limbic brain regions. The evidence is clearest for the MPOA. It is weaker for the penile motor neurons and their target muscles because we have little idea so far how implants of steroids in these places would affect copulatory behavior. Yet 238 ARTHUR P. ARNOLD N SYRIN FIG. 1. Some of the brain regions thought to be involved in vocal control in zebra finches are shown in highly schematic form in this parasagittal view, together with anatomical projections discovered in the canary by Nottebohm et al. (1976) and Nottebohm and Kelley (1978). Black dots indicate the presence of cells labeled by androgens (Arnold et al., 1976; Arnold, 1979, 19806). there can be little doubt that these structures play an important role in copulation. The evidence for copulation-related effects of androgen on other CNS areas is rather speculative at present. ANDROGEN EFFECTS ON BIRD SONG The second behavioral system to be discussed is the neural system involved in the control of song and other vocalizations in the zebra finch. Zebra finch males sing a quiet courtship song which is oriented toward the female, and together with specific patterns of movement, body orientation, and feather erection, serves as an attractive stimulus for females. Song is reduced in its occurrence after castration, and androgens reinstate it (Prove, 1974; Arnold, 1975). This behavior is controlled by an interconnected system of discrete neural regions (Fig. 1) which were originally described in the canary (Nottebohm et al., 1976; Nottebohm and Kelley, 1978; Kelley and Nottebohm, 1979). Lesions of the hyperstriatum ventrale pars caudale (HVc) and nucleus robustus archistriatalis (RA) disrupt song in the canary. HVc neurons project to RA, and RA in turn projects to nucleus intercollicularis (ICo), which has long been implicated in vocal control in many bird species (e.g., Phillips and Peek, 1975), and to the tracheosyringeal motor neurons of the hypoglossus (nXIIts), which innervate the muscular vocal organ, or syrinx. Two other brain regions, the magnocellular nucleus of the anterior neostriatum (MAN) and Area X of the 1^ bus parolfactorius are also thought to be involved in song control since they have heavy monosynaptic connections with either HVc or RA (Fig. 1). The connections of the song system described in the canary by Nottebohm and his colleagues have been confirmed in the zebra finch (M. E. Gurney, personal communication; Lewis et al., 1979; Arnold, 1980a). In suggesting where androgens might act to influence singing behavior in zebra finches, we must at present rely almost exclusively on autoradiographic data which establish which brain regions involved in song contain cells which specifically accumulate sex steroids. Of the brain regions shown in Figure 1, HVc, MAN, RA, and nXIIts contain neurons which accumulate hormone after injection of tritiated testosterone or dihydrotestosterone, but not after estradiol (Arnold et al., 1976; Arnold, 1979, 19806), and ICo contains labeled cells after injection of any of these. Furthermore, the syringeal muscles themselves are quite sensitive to androgens. Castration reduces the size of these muscles, and testosterone propionate administration reverses this effect (Arnold, 1974; Luine et al., 1980). The syringeal tissues contain specific high affinity androgen binding proteins (Lieberburg and Nottebohm, 1979). The activities of acetylcholinesterase and choline acetyltransferase in the syrinx and tracheosyringeal nerve are regulated by androgen titers in the blood (Bleisch et al., 1979). Although it is not clear at present precisely how much of the androgen effects on these enzyme activities in the syringeal muscles can be attributed to an effect on the motor neurons of nXIIts which innervate the muscles, it is quite likely that there is a direct action on the muscles themselves, in view of the high levels of androgen receptors in the syrinx itself. When considering this evidence in the LEVELS OF STEROID ACTION ON BEHAVIOR aggregate it is tempting to suggest that as in the neural systems controlling the ovulatory surge of LH and male rat copulatory behaviors, (a) sex steroids may act at many levels in the song system to influence this ^jehavior, and (b) one of these levels is the ™ final common pathway of the system, i.e., the motor neurons controlling song and the syringeal muscles themselves. However, we must hesitate before accepting this conclusion fully at present, since for all of the neural regions which accumulate androgens except nXIIts, we have no direct evidence that androgens affect the function of these neurons, and in no case is it apparent that such an action actually is important for vocal behavior. Nevertheless, the parallels between these three neural systems are sufficiently great that this lends weight to the notion that multilevel actions of steroids may not be uncommon in the control of neuroendocrine and behavioral functions. In fact, Pfaff and Modianos (1980) have suggested that estrogen exerts its effects on lordosis behavior of the female rat via actions on neurons in various interconnected regions of the brain, each of which is influential in control of lordosis. Similarly, Kelley (1980) has presented autoradiographic evidence of multi-level sites of action of androgens in circuits controlling reproductive vocalizations of the frog Xenopus laevis. In these two neural systems, there is evidence that sex steroids act on structures which carry relevant sensory information. This is clearest in the rat, where estradiol increases the size of receptive fields of sensory neurons innervating the perineal region of the female (Komisaruk et al, 1972; Kow and Pfaff, 1973). Since this influence of estradiol is detected even when the pudendal nerve carrying the sensory fibers is severed from the CNS, this effect does not depend on centrifugal influences of the CNS on the periphery. In Xenopus, a mesencephalic auditory nucleus contains cells that accumulate sex steroids, and this accumulation could mediate possible androgen influences on auditory processes relevant to Xenopus mating calls ) (Kelley, 1980). Significantly, other brain 239 regions implicated in the control of calling accumulate steroids, including the laryngeal motor neurons (Kelley et al., 1975; Kelley, 1980). If sex steroids do act at many levels in circuits controlling behavior this might seem to imply that there is considerable redundancy in hormonal activation of a particular behavior. To support this view, it is possible to point to studies which demonstrate that implants of testosterone into the MPOA of male rats activate male sex behavior. If an implant at one locus in the neural circuit is sufficient to activate the behavior in a castrate, then it would seem that the other sites could only serve a redundant function. This view may be incorrect. Because there may be leakage of the hormone to sites distant from the implants, even in studies which look for and fail to find such leakage, the activation of the behavior may depend on marginal steroid effects on such distant sites. Furthermore, because copulatory behavior evoked by such implants is rarely normal in its topography (Hart, 1974), other sites of action can be invoked. It is quite conceivable that at each level, the steroid exerts a specific effect which is not functionally equivalent to that exerted at any other level. This should not be confused with redundancy. To turn more specifically to the question posed at the beginning of this paper, what can be said about the logical levels at which steroids exert their action? At present we can have only the barest glimpse of an answer to this intriguing question: Since the anatomical sites of steroid action are likely to be multiple and diverse, so are the logical functions performed by these sites. The only sites of action which have a clear logical function in the three systems discussed here are the motor neurons and muscles in the behavioral systems, and the pituitary in the LH control system. Each of these is part of the final common path. Yet the motor neuronal and muscular action of steroids were what Tinbergen (1951) specifically thought would be unlikely. As discussed above, there seem to be two main considerations which led Tin- 240 ARTHUR P. ARNOLD bergen, quite logically, to the speculation that steroids must modify the function only of centers which are high in the hierarchy. (1) The action of hormones on motor neurons and muscles seems to be incompatible with a specific action on a particular behavior, since any given muscle is often involved in more than one behavior. (2) It seems unlikely that the "switching" function of hormones (i.e., increasing the frequency of certain behaviors at the expense of others) would be exerted at the motor neuronal or muscular level. For example, if this were the only site of action in the zebra finch song system, and androgens modified the frequency of the behavior at the motor neuronal level, one is faced with the improbable notion of a castrate male "singing to himself." That is, the neuronal pattern for song would be generated in the brain but is not expressed in muscular contractions of the syrinx because of a lack of androgen effects on the motor neurons and muscle fibers. It is possible to reconcile these two points with our present knowledge of sites of steroid action. With regard to the first point, it is true that the syringeal muscles of zebra finches are active during both song control and breathing, and in rats the penile muscles could be involved in the control of micturation, in addition to their sexual function. Yet it would still be possible for androgens to act on the syringeal motor neurons and muscles to influence singing without also markedly changing the function of the muscles during breathing. For example, if androgens increase the activity of choline acetyltransferase in the hypoglossal motor neurons (Bleisch et al, 1979), this might allow them to synthesize enough acetylcholine to keep pace with increased use of these neurons which requires more discharge of acetylcholine. Yet the amount of acetylcholine release per impulse in the motor neurons might stay constant so that there would be no effect on the ability of these neurons to function properly when driving the muscles during breathing. Although this is highly speculative, it serves to demonstrate that certain effects of androgens at this level need not be non-specific. It also helps to resolve the second consideration, since one can imagine androgen effects on motor neurons which would not play a role in altering the frequency of a behavior (i.e., a switching role), but would merely alloiQ the motor neurons and muscles to handle the increase in usage demanded if there is a switch in the behavioral repertoire. Thus, Tinbergen was probably right, that hormones are most likely to switch behaviors on or off by acting centrally, at logically high levels in the neural circuitry. Yet there may be other, non-switching functions played by steroids, which can occur at any level. There is currently great need for detailed studies of the changes exerted by steroids at each level, so that a better picture may emerge of the logical functions which are modulated by steroids. ACKNOWLEDGMENTS The author's research has been supported by NSF grant BNS 7705973 and USPHS grant 5-SO7 RR07009-14. REFERENCES Arnold, A. P. 1974. Behavioral effects of androgen in zebra finches (Poephila gutlata) and a search for its sites of action. Ph.D. Diss., The Rockefeller University. Arnold, A. P. 1975. The effect of castration and androgen replacement on song, courtship, and aggression in zebra finches (Poephila guttala). J. Exp. Zool. 191:309-326. Arnold, A. P. 1979. Hormone accumulation in the brain of the zebra finch after injection of various steroids and steroid competitors. Soc. Neurosci. Abstr. 5:434. Arnold, A. P. 1980a. Sexual differences in the brain. Amer. Sci. 68:165-172. Arnold, A. P. 19804. Quantitative analysis of sex differences in hormone accumulation in the zebra finch brain: Methodological and theoretical issues. J. Comp. Neurol. 189:421-436. Beach, F. A. 1971. Hormonal factors controlling the differentiation, development, and display of copulatory behavior in the ramstergig and related species. In E. Toback, L. R. Aronson, and E. Shaw (eds.), Biopsychology of development, pp. 2 4 9 - 296. Academic Press, New York. Beach, F. A. and G. Levinson. 1950. Effects of androgen on the glans penis and mating behavior of castrated male rats. J. Exp. Zool. 114:159-171. Beach, F. A. and W. H. Westbrook. 1968. Dissociation of androgenic effects on sexual morphology and behavior in male rats. Endocrinology 83:395-398. LEVELS OF STEROID ACTION ON BEHAVIOR Beck, L. V., M. Bay, A. F. Smith, D. King, and R. Long. 1978. Steroid priming of the luteinizing hormone response to luteinizing hormone releasing hormone. J. Endocrinol. 77:293-299. Beckett, S. D., D. F. Walker, R. S. Hudson, T. M. Reynolds, and R. C. Purohit. 1975. Corpus A spongiosum penis pressure and penile muscle activity in the stallion during coitus. Am. J. Vet. Res. 36:431-433. Blake, C. A. 1978. Neurohumoral control of cyclic pituitary LH and FSH release. Clinics in Obstetrics and Gynaecology 5:305-327. Bleisch, W., V. Luine, F. Nottebohm, and B. McEwen. 1979. Testosterone effects on choline acetylase and acetylcholinesterase in the Xllth cranial nerve and syringeal muscle of the zebra finch. Soc. Neurosci. Abstr. 5:476. Breedlove, S. M. and A. P. Arnold. 1980. Hormone accumulation in a sexually dimorphic motor nucleus of the rat spinal cord. Science 210:564566. Brown-Grant, K. 1977. Physiological aspects of the steroid hormone-gonadotropin interrelationship. In R. O. Greep (ed.), Reproductive physiology II, Int. Rev. Physiol. 13:57-83. University Park Press, Baltimore. Buresova, M. and E. Gutmann. 1971. Effect of testosterone on protein synthesis and contractility of the levator ani muscle of the rat. J. Endocrinol. 50:643-651. Buresova, N., E. Gutmann, and V. Hanzlikova. 1972. Differential effects of castration and denervation on protein synthesis in the levator ani muscle of the rat. J. Endocrinol. 54:3-14. Chiappa, S. A., G. Fink, and N. M. Sherwood. 1977. Immunoreactive luteinizing hormone releasing factor (LRF) in pituitary stalk plasma from female rats: Effects of stimulating diencephalon, hippocampus, and amygdala. J. Physiol. 267:625640. Cooper, K. K. and L. R. Aronson. 1974. Effects of castration on neural afferent responses from the penis of the domestic cat. Physiol. and Behav. 12:93-107. Cramer, O. N. and C. A. Barraclough. 1971. Effect of electrical stimulation of the preoptic area on plasma LH concentrations in proestrus rats. Endocrinology 88:1175-1183. Davis, P. G. and R. J. Barfield. 1979. Activation of masculine sexual behavior by intracranial estradiol benzoate implants in male rats. Neuroendocrinology 28:217-227. Feder, H. A. 1971. The comparative actions of testosterone propionate and 5 alpha-androstan-17 beta-ol-3 one propionate on the reproductive behavior, physiology and morphology of male rats. J. Endocrinol. 51:241-252. Fink, G. 1979. Feedback actions of target hormones on hypothalamus and pituitary with special reference to gonadal steroids. Ann. Rev. Physiol. 41:571-585. Goodman, R. L. 1978. The site of the positive feedback action of estradiol in the rat. Endocrinology 102:151-159. 241 Hanzlikova, V. and E. Gutmann. 1978. Effect of castration and testosterone administration on the neuromuscular junction in the levator ani muscle of the rat. Cell Tiss. Res. 189:155-166. Hart, B. L. 1974. Gonadal androgen and sociosexual behavior of male mammals: A comparative analysis. Psych. Bull. 81:383-400. Hart, B. L. 1978a. Reflexive mechanisms in copulatory behavior. In T. E. McGill, D. A. Dewsbury, and B. D. Sachs (eds.), Sex and behavior, pp. 205242, Plenum, New York. Hart, B. L. 1978*. Hormones, spinal reflexes, and sexual behavior. In J. B. Hutchison (ed.), Biological determinants of sexual behavior, pp. 319-348. Wiley, New York. Hart, B. L. 1979. Activation of sexual reflexes of male rats by dihydrotestosterone but not estrogen. Physiol. and Behav. 23:107-109. Hart, B. L. and C. M. Haugen. 1968. Activation of sexual reflexes in male rats by spinal implantation of testosterone. Physiol. and Behav. 3:735738. Kawakami, M., E. Terasawa, F. Kimura, and K. Wakabayashi. 1973. Modulatory effect of limbic structures on gonadotropin release. Neuroendocrinol. 12:1-16. Kelley, D. B. 1980. Auditory and vocal nuclei of frog brain concentrate sex hormone. Science 207:553555. Kelley, D. B., J. I. Morrell, and D. W. Pfaff. 1975. Autoradiographic localization of hormone-concentrating cells in the brain of an amphibian Xenopus laevis. I. Testosterone. J. Comp. Neurol. 164:47-61. Kelley, D. B. and F. Nottebohm. 1979. Projections of a telencephalic auditory nucleus—Field L—in the canary. J. Comp. Neurol. 183:455-470. Komisaruk, B. R., N. T. Adler, and J. B. Hutchison. 1972. Genital sensory field: Enlargement by estrogen treatment in female rats. Science 178:12951298. Kow, L.-M. and D. W. Pfaff. 1973. Effects of estrogen treatment on the size of receptive field and response threshold of pudendal nerve in the female rat. Neuroendocrinology 13:299-313. Krieg, M., R. Szalay, and K. D. Voigt. 1974. Binding and metabolism of testosterone and of 5 alphadihydrotestosterone in bulbocavernosus/levator ani (BCLA) of male rats: In vivo and in vitro studies. J. Steroid Biochem. 5:453^159. Krieg, M. and K. D. Voigt. 1977. Biochemical substrate of androgenic actions at a cellular level in prostate, bulbocavernosus/levator ani and in skeletal muscle. Acta Endocrinol. Suppl. 85:4389. Kurtz, R. G. and R. Santos. 1979. Supraspinal influences on the penile reflexes of the male rat: a comparison of the effects of copulation, spinal transection, and cortical spreading depression. Horm. Behav. 12:73-94. Larsson, K. 1979. Features of the neuroendocrine regulation of masculine sexual behavior. In C. Beyer (ed.), Endocrine control of sexual behavior, pp. 77-163. Raven Press, N.Y. 242 ARTHUR P. ARNOLD Lewis, J. W., S. M. Ryan, L. L. Butcher, and A. P. Arnold. 1979. Identification of catecholaminergic projections to Area X in the zebra finch. Soc. Neurosci. Abstr. 5:143. Liao, S. and S. Fang. 1969. Receptor-protein for androgens and the mode of action of androgens on gene transcription in ventral prostate. Vitam. Horm. 27:17-90. Lieberburg, I. and F. Nottebohm. 1979. High-affinity androgen binding proteins in syringeal tissues of song birds. Gen. Comp. Endocrinol. 37:286293. Luine, V., F. Nottebohm, C. Harding, and B. S. McEwen. 1980. Androgen affects cholinergic enzymes in syringeal motor neurons and muscle. Brain Res. 192:89-107. Malsbury, C. W. and D. W. Pfaff. 1974. Neural and hormonal determinants of mating behavior in adult male rats. A review. In L. D. DiCara (ed.), Purohit, R. C. and S. D. Beckett. 1976. Penile pressures and muscle activity associated with erection and ejaculation in the dog. Am. J. Physiol. 231:1343-1348. Sachs, B. D. 1978. Conceptual and neural mechanisms of masculine copulatory behavior. In T. E. McGill, D. A. Dewsbury, and B. D. Sachs (eds.)^ Sex and behavior, pp. 267—296. Plenum Press, N.Y. Sachs, B. D. and L. D. Garinello. 1978. Interaction between penile reflexes and copulation in male rats. J. Comp. Physiol. 92:759-767. Sar, M. and W. E. Stumpf. 1977. Androgen concentration in motor neurons of cranial nerves and spinal cord. Science 197:77—79. Sarkar, D. K., S. A. Chiappa, and G. Fink. 1976. Gonadotropin-releasing hormone surge in proestrus rats. Nature 264:461-462. Schwartz, N. B. 1972. Reproduction: Gonadal funcLimbic and autonomic nervous system research, pp. tion and its regulation. Ann. Rev. Physiol. 85-136. Plenum Press, N.Y. 34:425-472. McF.wen, B. S. 1976. Gonadal steroid receptors in Stumpf, W. E. 1968. Estradiol concentrating neuneuroendocrine tissues. In F. Naftolin et al. rons: Topography in the hypothalamus by drymount autoradiography. Science 162:1001-1003. (eds.), Subcellular mechanisms in reproductive neuroendocrinology, pp. 277-304. Elsevier, Amster- Stumpf, W. E. and M. Sar. 1978. Anatomical distridam. bution of estrogen, androgen, progestin, corticosteroid, and thyroid hormone target sites in Nottebohm, F. and D. B. Kelley. 1978. Projections the brain of mammals: Phylogeny and ontogeny. to efferent vocal control nuclei of the canary telAmer. Zool. 18:435-445. encephalon. Soc. Neurosci. Abstr. 4:101. Nottebohm, F., T. M. Stokes, and C. M. Leonard. Tinbergen, N. 1951. The study of instinct. Oxford 1976. Central control of song in the canary, SeUniv. Press, N.Y. rinus canarius. J. Comp. Neurol. 165:457-486. Tremblay, R. R., J. Y. Dube, M. A. Ho-Kim, and R. Lesage. 1977. Determination of rat muscle anPfaff, D. W. 1968. Autoradiographic localization of drogen-receptor complexes with methyltrienoradioactivity in rat brain after injection of tritiated sex hormones. Science 161:1355-1356. lone. Steroids 29:185-195. Pfaff, D. W. and M. Keiner. 1973. Atlas of estradiol Tucek, S., D. Kostirova, and E. Gutmann. 1976. Tesconcentrating cells in the central nervous system tosterone induced changes of choline acetylof the female iat.j. Comp. Neurol. 151:121-158. transferase and cholinesterase activities in rat levator ani. J. Neurol. Sci. 27:353-362. Pfaff, D. W. and D. Modianos. 1980. Neural mechanisms of female reproductive behavior. In R. Velasco, M. E. and S. Taleisnik. 1969. Effect of hipGoy and D. Pfaff (eds.), Neurobiology of reproducpocampal stimulation on the release of gonadotion. Plenum Press, N.Y. (In press) tropin. Endocrinology 85:1154-1159. Phillips, R. E. and F. W. Peek. 1975. Brain organi- Venable,J. H. 1966. Morphology of the cells of norzation and neuromuscular control of vocalization mal, testosterone deprived and testosterone stimin birds. In P. Wright, P. G. Caryl, and D. M. ulated levator ani muscles. Am. J. Anat. Vowles (eds.), Neural and endocrine aspects of be119:271-302. havior in birds, pp. 243-274. Elsevier, Amster- Vyskocil, F. and E. Gutmann. 1977. Electrophysiodam. logical and contractile properties of the levator Prove, E. 1974. Der Einfluss von Kastration und ani muscle after castration and testosterone Testosteronsubstitution auf das Sexualverhalten administration. Pfluegers Arch. 368:105-109. mannlicher Zebrafinken (Taeniopygia guttata cas- Warwick, R. and P. L. Williams. 1973. Gray's anatomy. tanotis Gould). J. fuer Ornithologie 115:338-347. 35th British edition, p. 531. W. B. Saunders, Philadelphia.