Download Serotonin synaptic receptors in the mammalian central

Serotonin synaptic receptors in the
mammalian
central nervous system,
JAMES P. BENNETT,
JR., AND SOLOMON
H. SNYDER
Departments _ Ph.armacology and l"xperime_tal Therapeutics and Psychiatry aml Behavioral Sciet_ces,
Johns Hopkins University School oJ Medici_w, Baltimore, Maryland 21205
Physiological
serotonin
(5-HT) receptors, as is the case for other biogenicamine
receptors, can bedemonstrated by microionu)phoresis
1o be
present on both the neurons that synthesize the amine transmitter
Cautoreceptors") (3, 7) and neurons that
receive synapticinput
of the transmitter (postsynaptic
receptors) (14). Virtually all of the neurons
containing
5-HT as demonstrated
by histofluorescence are found in the raphenuclei
groups located in the upper brainstem and lower midbrain
(8). The
tonic firing rates of raphe cells are decreased
potently and reversibly
by
iontophoretic
application of 5-HT or
d-LSD but are not altered appreciably
by 2-bromo-LSD,
a psychedelically inactive LSD analogue (2, 14). Forebrain neurons that received anatomically and physiologically
defined 5HT input from the raphe nuclei are
also inhibited by exogenously
applied
5-HT; d-LSD is weaker as an inhibitory agent on these forebrain neurons
compared to its potency on raphe
neurons (14). In some cases 2-bromoLSD is about equal to d-LSD as an
inhibitory agent at 5-HT postsynaptic
sites (l).
Tritium-labeledd-LSD
(d-[aH]LSD)
binding sites in mammalian
brain
have been characterized
using both
equilibrium dialysis (12, 13) and rapid
filtration techniques
(4, 5, 15). In all
cases, d-[3H]LSD binds saturab[y, reversibly, stereospecificaUy
(I-LSD is
more than 1,000 times weaker than
d-LSD as a displacing agent), and with
apparent equilibrium dissociation constants in the nanomolar
range. AIthough d-[aH]LSD binding is highest
in some areas receiving a prominent
5-HT input (i.e., striatum), very little
if any binding occurs to presynaptic
RECEPTORS FOR NEUROTRANSM|TTERS
5-HT elements. Electrolytic lesions of
the raphe nuclei that result in degenerationof5-HTaxonsand
nerveendings do not alter forebrain d-[aH]LSD
binding (4, 5). Ofall neurotransmitter
substances
examined,
5-HT
is the
most effective displacing agent with
ha|f-maximal
displacement
(ms0) of
d-[aHlLSD binding occurring at 2002,000nM 5-HT (4, 5, 15). LSD analogues, including
methysergide
and
2-bromo-LSD,
are potent displacing
agents (4, 5, 15).d-[3H]LSD therefore
binds to rat forebrain
membranes
at
sites that are not components
of the
presynaptic
5-HT system bu[ appear
to be associated with synaptic 5-HT
function.
Under appropriate
conditions[all]
5-HT binds saturab]y, reversibly and
with high affinity (K, = 8 nM) to forebrain particulate preparations.
In rat
(6) and human (10) brain [3H]5-HT
binding shows a regional distribution
very similar to d-[aH]LSD
binding,
Simultaneously
performed
saturation
studies in rat brain show that the regional variations in both ligands' binding can be accounted
for by changes
in the apparent maximum number of
binding sites as opposed to changes
in the apparent binding affinity constants. In several forebrain
regions
the ratio of d-[aH]LSD to [3H]5-HT
binding site density is about 2 (6).
Bound [_H]5-HT
is displaced potently by LSD analogues with d-LSD
having an ms0 of about 10 nM, very
similar to the affinity of d-LSD on
d-[aH]LSD binding (6). Displacement
of [3H]5-HT binding is more sensitive
than d-[aH]LSD binding to structural
modifications
in the tryptamine
nucleus. Compounds
wilh 5-hydroxy
substituents are about two orders of
magnitude more potent than homo-
logues without the 5-hydroxy
strucrare (6). Classical peripheral
5-HT
amagonis_s
(methiothepin,
cyproheptadine,
methysergide,
mianserin)
are 4-100
times more potent
at
displacingd-[aH]LSD
than [aH]5-HT
binding (6).
Since raphe nuclei lesions do not
lower forebrain
[3H]5-HT
binding
(6), [3H]5-HT does not bind significantly to presynaptic elements of 5HT neurons.
Neonatal interruption
of the development
of the 5-HT norvoussystem by theintracerebralinjection of 40/_g of 5,7-dihydroxytryptamine does not affect the postnatal development of binding sites for d-[3H]
LSD or [3H ]5-HT (6). The subcellular
distributions
of both d-[aH]LSD and
[aH]5-HT binding are nearly identical, with enrichment
in synaptosoma|
and synaptic membrane subfractions
(6).
d-[aH]LSD and [aH]5-HT
binding
are both reduced
about 50-70%
in
basal ganglia (caudate, putamen, globus pallidus) but not cerebral cortex
of brains from patients dying from
Huntington's
disease, a hereditary
neurodegenerative
disorder (9, 11).
In rat striata unilaterally depleted of
neurons by stereotaxic microinjection
of kainic acid, both ligands' binding
is reduced
50-60%
(16). Taken together these findings suggest that the
majority ofd-[aH]LSD
and [SH]5-HT
binding occurs to neurons as opposed
to glia or other
nonneuronal
elements.
In spite of the potent neurophysio-
' From Session I, Receptorsfor Neurotransmitters,ofthe
FASEBConference
on Recept0rs
presented at the
61st Annual Meeting
of the
Federationof American Societies for Experimental Biology, Chicago, IL, April 4, 1977.
137
logical actions of bo_h d-l.Sl) and 5HT on presynaplic elemems td 5-lIT
neurons,
neither
substance
can be
dentonstraled
to bind significanlly It)
raphe neurons, axons, or nerve endtugs. 'l'he binding sties ira bolh ligantis appear lo be associatt'd cxchlsivclv
with the serolonevgic as Ol)pt_setl l_)
ollter neurolransmiuer
syslems. Bod_
ligands appear to bind primarily
to
neurons, and the ontogeny of binding
sites in rat brain is independem
of
functional input fi'om 5-HT neurons,
Ahhtmgh
d-l.Sl) ,tntl 5-1tT binding
sites show many similarities, il is uulikely that both ligantls bind to ideutical macromo]ecular
slructures. 5-HT
a,nagonisls
;ll'e II|Ol'e
pO|ell| OII d['qt]l.Sl)compared
to [:q-tl5-H'l'binding sites, and tryptamines
with 5-hydroxy suhstiluenls are more potent on
[:_H]5-HT compared
1o d-[_H]LSD
binding sites. Such discrepancies
are
besl ttnderstood with a two-state model
forrecei)torfuncfioningwherein
separate antagonistand agonist-prefer-
ring ctmli)rmations
exist (17), In this
model, d-l.Sl), which functions neurtq)hysi,_h_gically as a 5-HT mixed
agonisl-antagonist,
binds to both con['Ol'ltta|iolls
of the receptor with approximately
etlual aftinity and thus
can be ttisplaced
potently by 5-HT
antagonists. 5-HT, on the other hand,
binds primarily to the agonist conformation, shows much greater selectivity for tryptamines
with 5-HT-like
structures,
and is displaced
more
weakly by 5-HT antagonists.
[]
7. Carlsson, A., W. Kehr, M. Lindquist, T.
Magnusson
and C. V. Atack. Regulation
of monoamine
metabolism in the central
nerw_us system. Pharmacol. Rev. 24: 371,
1972.
8. Dahlstrom,
A., and K. Fuxe. Evidence for
the existence of monoamine-containing
neurons
in the central nerw_us system.
1. Demonstration
of monoanfines
in the
cell bodies of brainstem
neurons.
Acta
Physiol. Scand. 62, Suppl. 232: 1, 1965.
9. Enna, S.J.,J. P. Bennett, Jr., D. B. Bylund,
S.H. Snyder, E. D. Bird and L. L. lversen. Alterations
of hrain neurotransmitter receptor
binding
in Huntington's
chorea. Brain Res. 116: 531, 1976.
10. Enna, S.J.,J. p. Bennett, Jr., D. B. Bylund,
I. Creese, D. R. Burr, M. E. Charness,
H. I. Yamamura, R. Simantov and S.H.
Snyder. Neurotransmitter
receptor binding: regional
distribution
in human
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D.B. Bylund, H. I. Yamamura, L.L.
Iversen and S. H. Snyder. Huntington's
chorea--changes
in neurotransmitter
receptors in the brain. N. Engl. J. Med.
294: 1305, 1976.
12. Farrow, J. T., and H. Van Vunakis. Bind-
ing of d-lysergic
acid diethylamide
to
subcellular
fractions from rat brain. Nature (London)237:
164, 1972.
Farrow, J. T., and H. Van Vunakls. Characteristicsofd-lysergicaciddiethylamide
binding to subcellular
fractions derived
from rat brain. Biochem. Pharmacol. 22:
1103, 1973.
Haigler, H.J., and G. K. Aghajanian.
Lysergic acid diethylamide
and serotonin:
a comparison
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neurons and neurons
receiving a serotonergic input. J. Pharmacol. Exp. Ther.
188: 688, 1974.
Loveil, R.A.,andD.
X. Freedman. Stereospecific receptor sites ['or d-lysergic acid
diethylanfide
in rat brain: effects of neurotransmitters,
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other psychotropic
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Schwarez, R., J. p. Bennett, Jr., and J. T.
Coyle, Jr. Loss of striatal serotonin synaptic receptor binding induced by kainic
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P. Bennett, Jr. Neurotransmitter
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FEDERATION PROCEEDINGS VOL. 37, NO 2 " FEBRUARY1978