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C 2007 the Authors
C 2007 American Headache Society
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ISSN 0017-8748
doi: 10.1111/j.1526-4610.2007.00714.x
Published by Blackwell Publishing
Research Submission
Sensitization of the Trigeminal Sensory System During
Different Stages of the Rat Estrous Cycle: Implications
for Menstrual Migraine
Vincent T. Martin, MD; James Lee; Michael M. Behbehani, PhD
Objectives.—To determine if the sensitization of the trigeminal system changes after dural activation of the
trigeminal nerve during different stages of the rat estrous cycle.
Background.—The specific mechanisms through which ovarian hormones trigger menstrual migraine are currently unknown. Past animal studies have suggested that the response properties of the trigeminal nucleus caudalis
(TNC) may change during the different phases of the rat estrous cycle, but none have been performed in an experimental paradigm for migraine headache.
Methods.—Sixty-one cycling female Sprague–Dawley rats were used for these experiments. The stage of the
estrous cycle of each animal was identified by examination of the cellular morphology of vaginal lavage. The animals
were anesthetized and a 7 mm portion of the skull was removed that was centered over the sagittal sinus. A tungsten
electrode was used to record from neurons in the TNC or CI-CIII regions. Only neurons that had both dural
and cutaneous receptive fields were used for these experiments. Facial receptive field sizes (RFS) were mapped and
neurophysiologic response properties of the TNC/CI-CIII neurons to cutaneous and dural stimuli was ascertained
before and after application of capsaicin to the dura. One-way and repeated measure analysis of variance were used
to compare changes in RFS and response properties of TNC/CI-CIII neurons from animals during different stages
of the rat estrous cycle.
Results.—When data were analyzed individually for each stage, there was greater enlargement of cutaneous
receptive fields and enhanced sensitivity of the trigeminal system to cutaneous stimuli during proestrus as compared
to metestrus and diestrus after dural activation with capsaicin (P values <.05). When data were pooled from
stages with similar hormonal milieus, the percent change in the response magnitude of TNC neurons to electrical
stimulation of the dura was greater and receptive field enlargement was larger from the proestrous/estrous group
compared to those from the metestrous/diestrous group after administration of capsaicin (P values <.05).
Conclusions.—There is enhanced sensitization of the trigeminal system during the later halves of proestrus
and estrus, which represent stages of the rat estrous cycle during and immediately following an abrupt decline in
ovarian hormones. If similar changes occur during the human menstrual cycle these results could have important
implications for menstrual migraine.
Key words: migraine headache, menstrual migraine, estrogen, ovarian hormones, trigeminal nucleus caudalis, sensitization
Abbreviations: TNC trigeminal nucleus caudalis, RFS receptive field size, PSTH peristimulus time histogram,
ANOVA analysis of variance, TMJ temporomandibular joint
(Headache 2007;47:552-563)
From the Department of Internal Medicine, University of Cincinnati College of Medicine Cincinnati, OH (Dr. Martin); and Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, OH (Dr. Behbehani and
Mr. Lee).
Address all correspondence to Dr. Vincent T. Martin, Division of General Internal Medicine, University of Cincinnati College of
Medicine, 231 Albert Sabin Way, ML 6603, Cincinnati, OH 45267-0535.
Accepted for publication November 14, 2006.
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Estrogen withdrawal during the perimenstrual
time period plays a critical role in the triggering of menstrual migraine in susceptible women.
Somerville1,2 administered an intramuscular injection
of estradiol valerate shortly before menstruation and
found that the onset of menstrual migraine could be
delayed by artificially raising serum estradiol levels
during the perimenstrual time period. Intramuscular
injection of progesterone prior to menstruation did
not delay the onset of menstrual migraine.3 He later
administered a short-acting estrogen preparation to
menstrual migraineurs during the mid-follicular phase
of the menstrual cycle and found that it did not trigger a migraine.4 The above data suggest that “estrogen
withdrawal” during the perimenstrual time is the primary mechanism through which menstrual migraine
is triggered and that several days of estrogen priming
prior to “estrogen withdrawal” are necessary to induce
menstrual migraine.
The specific mechanisms through which “estrogen withdrawal” modulates menstrual migraine are
currently unknown. It is known however that nociceptive responses within the trigeminal system may
be hormonally dependent in female rats. Okamoto
et al5 reported that the sensitivity of neurons from the
trigeminal nucleus caudalis (TNC) is enhanced during the proestrous stage of the rat estrous cycle after
chemical stimulation of the temporomandibular joint.
Bereiter et al6 demonstrated a greater enlargement of
cutaneous receptive fields of trigeminal neurons during the estrous stage than the diestrous stage in female
rats. Therefore, the sensitivity of TNC neurons appears
to be enhanced during proestrus and estrus of the rat
estrous cycle.
Past studies however have not determined if different “hormonal milieus” of ovarian hormones can
alter the neurophysiologic properties of the trigeminal system in an experimental model for migraine
headache. Dural activation of the trigeminal system
in rodents has been used for the study of migraine
headache and migraine specific medications.7,8 Testing during the later parts of the proestrous and estrous
stages allowed us to determine the effects of “estrogen withdrawal” on the trigeminal system since these
time periods occur during and following an abrupt decline in serum estradiol levels. The specific objective
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of this study was to determine if the sensitization of
TNC neurons changes during different stages of the
rat estrous cycle before and after dural activation of
the trigeminal nerve with capsaicin.
METHODS
The protocols for these experiments were approved by the Institutional Animal Care and Use
Committee and conformed to the guidelines set by the
National Institutes of Health guide for the care and
use of laboratory animals. Female Sprague–Dawley
rats weighing between 220 and 270 g purchased from
Harlan (Indianapolis, IN) were used in all studies. The
animals were maintained in 12 hours dark/light cycle
with the light cycle starting at 06:00.
The animals were first anesthetized with chloral
hydrate (400 mg/kg) or urethane (1.2 g/kg) and their
jugular veins were cannulated. Since there were no
differences between the results obtained by the use
of chloral hydrate or urethane, the data were pooled.
The stage of the estrous cycle of each animal was identified between 09:30 and 10:00 by examination of the
cellular morphology of vaginal lavage. Following these
procedures the animals was placed in a stereotaxic instrument. During the experiment the animal’s body
temperature was maintained at 38◦ C using a circulating warm water heating pad. The skull was removed
and the dura covering the sagittal sinus, confluence of
sinuses and 5 mm areas surrounding them was exposed
and covered with saline. The neck muscle overlying the
cervical cord and the skull over the caudal brain stem
was removed. A laminectomy was performed exposing the cervical cord and then the dura above the caudal brainstem and the cervical cord was removed. The
exposed muscle, caudal brainstem and cervical cord
were covered with warm agar dissolved in saline. A
bipolar ball stimulating electrode was placed on the
dura centered on the sagittal sinus at the junction between the sagittal and the confluence of sinuses and
then the exposed dura was covered with mineral oil.
A tungsten electrode with resistance between 5 and
7 megaohms was used to record from neurons in the
TNC or CI-CIII regions. Conventional electrophysiological instrumentation was used for measurement of
single unit activity. An online computer was used for
data acquisition and analysis. For recording of baseline
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activity and responses to cutaneous stimulation a rate
histogram with bin width of 1 second was constructed
in real time.
Brushing of the facial area surrounding the eye
was used as a search stimulus while the electrode penetrated into the TNC or spinal cord. Once a neuron
that responded to periocular stimulation was isolated
its response properties was ascertained by using air
puff, brushing the skin and pressure applied with a
wooden applicator or Von Frey filament. For each cell
the receptive field was mapped and drawn on a facsimile of a rat face. Following this mapping procedure,
the response to stimulation of the dura was tested by
application of constant current pulses (40 µs at 1Hz)
to the ball electrodes. The intensity of the current was
adjusted to a level that produced maximum number of
spikes and then the intensity was reduced to approximately 80% of the maximum intensity. A peristimulus
time histogram (PSTH) with bin width of 1 ms was constructed using a prepulse duration of 40 ms, postpulse
duration of 200 ms and 300 sweeps. Following these
measurements, the mineral oil was washed out and
then 20 µL of capsaicin containing 6 µg of capsaicin
(3 mg capsaicin dissolved in saline, ethanol and Tween
80 at 8:1:1 ratio by volume and then diluted 10-fold)
was applied to the dura. After 5 minutes, the receptive
fields were mapped as described above. After this procedure a second PSTH was constructed. At the end of
this procedure the dura was washed with repeated application of saline using a push–pull system. Between
60 and 90 minutes after washing the dura the receptive fields and responses properties of the cells were
measured and a third PSTH was constructed for each
cell. Only the data from animals that recovered from
application of capsaicin as measured by return of the
dura response to baseline were considered in the final
analysis. For marking the recording site, 10 µA of current was passed through the electrode for 1 minute. For
histological examination of the recording site, the animals were perfused intracardially with saline followed
by 5% formalin and the brain and cervical spinal cord
were removed and cut into 40 µm sections and the site
of the recording was noted. Because of the consistency
in the correlation between the recording site obtained
by histological measurements and the stereotaxic co-
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ordinate, histological examination was not performed
in all animals.
STATISTICS
One-way analysis of variance (ANOVA) with a
Tukey post hoc test or ANOVA with repeated measures were used to compare outcome measures during
different stages of the rat estrous cycle both before and
after application of capsaicin to the dura. The Tukey
post hoc test corrected for multiple comparisons. The
outcome measures for the study were the following:
1. Percent change in the area of cutaneous receptive fields,
2. Neuronal firing rates after cutaneous stimulation,
3. Response magnitude (number of spikes multiplied by the duration of response) after electric
stimulation of the dura, and
4. Peak latency (time from onset of electrical stimulation to the peak amplitude of neuronal firing).
For measurement of the area of receptive fields,
the receptive field was mapped on a grid containing
1 mm squares. The number of squares was counted
for each measured receptive field to estimate the
receptive field size (RFS). The percent increase in
RFS after application of capsaicin was calculated by
the following formula: [RFS areaAfter Capsaicin – RFS
areaBefore Capsaicin ]/RFS areaBefore Capsaicin × 100. A P
value <.05 was considered significant.
In some of the analyses the data from the proestrous and estrous stages were combined and compared to data from the metestrous and diestrous stages.
The justification for combining these data was that
the late proestrous and estrous stages occur during or
shortly after an abrupt withdrawal of serum estradiol
simulating the hormonal milieu of the perimenstrual
time period in humans. Estradiol levels are relatively
constant during the metestrous stage and gradually rise
during the diestrous stage.
RESULTS
Sixty-one animals were used in this study. Only
one neuron was tested per animal in order to have
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Fig 1.—The location of neurons that had periocular receptive
fields and responses to electrical stimulation of the dura. As
indicated, the majority of neurons were located in the neck of
the dorsal horn that included lamina V. Because of the overlap
of the recording sites, one symbol represents more than one
cell.
consistent time related data. This allowed measurements that were within 3 hours (between 13:00 and
16:00) of the day for all animals. Only neurons that
maintained their baseline characteristics throughout
the procedure were considered in the final analysis. Because some neurons were lost before the final PSTH
was obtained, the number of animals where the receptive field was mapped before and after capsaicin is
larger that the number of animals where PSTH be-
A
*#
65
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fore, during and after recovery was measured. The
recording sites of the individual neurons are shown in
Figure 1.
Cutaneous Receptive Field Size.—All neurons that
responded to stimulation of the dura had a cutaneous
receptive field (measured based on the response to
pressure) in the periocular region and in some neurons
the receptive field extended near the mandibular joint.
Before application of capsaicin to the dura the RFS of
animals during proestrus was slightly larger than during the other stages but because of the significant variability the statistical tests showed no difference in the
RFS between the stages of the estrous cycle (P > .05).
Application of capsaicin to the dura caused an enlargement of the RFS that ranged between 5 and 60%. As
shown in Figure 2, the percent increase in RFS after
application of capsaicin was greater during proestrus
(n = 17) than during the metestrus (n = 15) and disestrus (n = 15, all P values <.05). The percent increase
in RFS during estrus (n = 15) did not significantly differ from the other stages. Using the combined data
from the various stages, the percent increase in RFS
from the proestrous/estrous groups was significantly
larger than that from the metestrous/diestrous groups
(P < .05).
Neuronal Firing Rates to Cutaneous Stimulation.—
The baseline rate of neurons tested during stimulation
of the cutaneous receptive field ranged between 0.1
B
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% increase in RFS
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Fig 2.—Percent increase in cutaneous receptive field size (RFS) after application of capsaicin. Panel A shows the percent increase
in RFS during the individual stages (E = estrus; M = metestrus; D = diestrus; P = proestrus). Panel B is the percent increase in
RFS for the combined stages (M + D = metestrus/diestrus, P + E = proestrus/estrus). The symbols (∗) and (#) indicate P values
<.05 between 2 groups. Standard error bars are shown for each outcome measure.
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Fig 3.—A to D show the mean firing rate (MFR) of trigeminal nucleus caudalis/CI-CIII neurons following stimulation of the
cutaneous receptive field at each stage of the estrous cycle before and after application of capsaicin to the dura. Panel A shows the
MRF for animals in estrus, panel B is the MFR for animals in metestrus, panel C is the MFR for animals in diestrus, and panel D
is the MFR for animals in proestrus. B/B and B/A are the MFR at baseline before and after application of capsaicin to the dura,
P/B and P/A are the same metric for air puff, T/B and T/A are the same metric for touch, Pr/B and Pr/A are the same metric for
pressure, and Pi/B and Pi/A are the same metric for pinch. Asterisks indicate a P value <.05 between the MFR before and after
application of capsaicin to the dura. Standard error bars are shown for each outcome measure.
and 15 spikes per second. Ninety percent of all neurons were classified as wide dynamic range neurons.
All neurons responded to air puff, brush, pressure and
pinch. The sensitivity of TNC/CI-CIII dorsal horn neurons to cutaneous stimulation changed following application of capsaicin to the dura during certain stages
of the estrous cycle (Fig. 3). The firing rates of neurons to air puff, touch and pressure recorded during
the proestrus were significantly larger after application of capsaicin to the dura than the response to the
same stimuli before capsaicin application (all P values <.05). Firing rates to pinch however did not differ
before and after capsaicin during proestrus. The same
measurements made from animals in other stages of
the cycle did not show any significant differences between the firing rates for any of the stimuli before and
after application of capsaicin.
Response to Electrical Stimulation of the Dura.—
The majority of neurons recorded in the peristimulus
interval were silent. For neurons that had baseline activity the firing range was between 0.1 and 15 spikes
per second. Following application of capsaicin there
was an increase in the firing during the peristimulus interval, but this increase was not statistically significant
from baseline in any of the four stages. Stimulation of
the dura produced responses with onset latencies between 9 and 16 ms. Assuming that the distance between the recording and stimulation sites were approximately 10 mm, this latency corresponds to a conduction
velocity between 1.0 m/s and 1.6 m/s, which is within
the range of conduction velocity through c-fibers. The
peak response occurred between 16 and 55 ms.
Following application of capsaicin to the dura
there was a difference in the response magnitude
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B
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response duration x Numer of spikes
Response duration x Number of spikes
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M+D
Fig 4.—The response magnitude of trigeminal nucleus caudalis/CI-CIII neurons to electrical stimulation of the dura before and after
administration of capsaicin (CAP). The response magnitude was defined as the number of spikes × the duration of the response from
the peristimulus time histogram. Panel A is the response magnitude for each stage (E = estrus, M = metestrus, D = diestrus, and
P = proestrus). Although not statistically different note that the response magnitude was numerically higher within the individual
estrous and proestrous stages after administration of capsaicin when compared to the metestrous and diestrous stages. Panel B is
the response magnitude during the combined stages (P + E= proestrous/estrous, M + D = metestrous/diestrous). Note that the
response magnitude was significantly greater after stimulation of the dura with capsaicin for the combined proestrous/estrous stages
when compared to prior to administration. Asterisks indicate significance at P < .05. Standard error bars are shown for each outcome
measure.
(number of spikes multiplied by the duration of the
response) between the stages of the estrous cycle
(Fig. 4). When using combined data from the stages,
the response magnitude was significantly larger during the proestrous/estrous stages as compared to the
metestrous/diestrous stages (P value <.05). The response magnitude however did not differ statistically between the individual stages despite greater increases after application of capsaicin during proestrus
and estrus when compared to metestrus and diestrus.
Figure 5 shows peristimulus time histograms demonstrating the responses of individual neurons to electrical stimulation of the dura during different stages of
the rat estrus cycle before and after capsaicin.
As shown in Figure 6, the peak latency measured
before and after capsaicin application (second PSTH)
to the dura was longer during proestrus than at any
other stages, but this difference did not reach statistical
significance (P > .05).
COMMENTS
The results of this study indicate that chemical
stimulation of dura produces sensitization of dural and
cutaneous sensory processing that is dependent upon
the stage of the estrus cycle. When data were analyzed individually for each stage, there was greater
enlargement of cutaneous receptive fields and enhanced sensitivity of the trigeminal system to cuta-
neous stimuli during proestrus as compared to the
metetrus and diestrus after application of capsaicin
to the dura. There was also a trend toward larger
receptive fields and greater percent increases in response magnitudes during estrus than during metetrus
and diestrus after dural activation with capsaicin.
When the data were pooled from stages with similar hormonal milieus, the percent increase in the response magnitude of TNC neurons to dural stimulation was greater and receptive field enlargement
was larger from the proestrous/estrous stages compared to those from the metestrous/diestrous stages.
The increased sensitization of the trigeminal system
during proestrus and estrus is likely secondary to the
varying “hormonal milieus” of ovarian hormones encountered during different stages of the rat estrous
cycle.
The rat estrous cycle is divided into 4 oneday stages: metestrus, diestrus, proestrus, and estrus.9
Serum estradiol levels are low and relatively nonfluctuating during metestrus and early diestrus, but
begin to rise during the later part of diestrus. During proestrus, serum estradiol levels abruptly increase
during the early part (00:00 to 12:00 hours), then decline rapidly during the later part (12:00 to 24:00 hours)
of the stage. Since our measurements were taken between 13:00 and 16:00 hours on the day of the proestrous stage we postulate that serum estradiol levels
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April 2007
Fig 5.—The peristimulus time histograms showing the responses of individual neurons to electrical stimulation of the dura. Panels
A and B are from a cell recorded in a rat in estrus before and after application of capsaicin (CAP), respectively. Panels C and D are
the same metric recorded in a rat in metestrus. Panels E and F are the same metric for an animal in diestrus and panels G and H
are the same metric for a rat in proestrus. Arrows denote the onset of electrical stimulation. Note the enhanced response of neurons
during estrus and proestrus after the administration of capsaicin (panels B and H).
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Before Cap
After Cap
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Headache
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Fig 6.—The peak latency measured from peristimulus histograms recorded from animals at each stage of the estrous cycle before and after application of capsaicin (CAP) to the dura.
The peak latency refers to the time from the onset of electrical
stimulation to the peak firing rate of neurons within the TNC
and CI-CIII regions. E = estrus, M = metestrus, D = diestrus,
and P = proestrus; B and A denote responses before and after
capsaicin application to the dura. There were no statistically significant differences between the stages. Standard error bars are
shown for each value.
were declining at the time of testing. The estrous stage
follows the proestrous stage and has serum estradiol
levels that remain low and relatively non-fluctuating.
(Fig. 7)
The later part of the proestrous stage and the entire estrous stage share a similar “hormonal milieu” to
the perimenstrual time period of the human menstrual
cycle. The later half of the proestrous stage resembles the late luteal phase of the human menstrual cycle
since estradiol levels decline abruptly during this time
period while the estrous stage resembles the early follicular phase as this stage follows an abrupt withdrawal
of serum estradiol and progesterone. Sensitization of
the trigeminal system was first observed during the
late proestrous stage, but carried over into the estrous
stage. If similar sensitization of the trigeminal system
occurs during the peri-menstrual time period of female
migraineurs this could have important implications for
menstrual migraine (see below).
Our results are similar to those obtained in a
human experimental model for trigeminal sensitization.10 In this study capsaicin was injected intradermally into the forehead of 14 women during the luteal
Fig 7.—Hormonal changes during the rat estrous cycle. This
graph depicts the changes in plasma estradiol (–$–) and progesterone (–!–) levels that occur during the rat estrous cycle.
Each stage lasts for 24 hours and starts at midnight (M). Serum
estradiol levels are low and relatively non-fluctuating during the
metestrous and early diestrous stages, but begin to rise during
the later part of the diestrous stage. During the proestrous stage,
serum estradiol levels abruptly increase and peak at noon (N)
and then decline rapidly during the later part (12:00 to 24:00
hours) of the stage. The estrous stage follows the proestrous
stage and has serum estradiol levels that remain low and relatively non-fluctuating. Vertical lines denote the start and end
of each stage of the estrous cycle. Black rectangles (!) denote
the time of neurophysiologic recordings in our study. Figure
adapted with permission from Butcher R. Endocrinology. 1974;
94(6):1704-1708.
and menstrual phases of the menstrual cycle. They reported greater peak pain intensities and larger areas of
brush-induced allodynia during the menstrual phase
than during the luteal phase after administration of
capsaicin. These results support the results of our study
suggesting an enhanced sensitization of the trigeminal
system after a decline in serum estradiol and/or progesterone levels as occurs during the menstrual phase
of the female menstrual cycle.
Animal models also suggest that trigeminal sensitization varies with the phase of the rat estrous cycle. Okamoto et al5 found that the response magnitude and duration of response were greater during
proestrus than during diestrus within neurons in the
superficial laminae at the spinomedullary junction after activation of the temporomandibular joint (TMJ)
with bradykinin. In addition, the receptive field area of
TMJ units was 30% greater from proestrous animals
than diestrous animals. Beireter et al11 demonstrated
that the receptive field size of trigeminal neurons was
560
increased in female rats during estrus when compared
to those in diestrus. They also found that receptive
field size was increased in ovariectomized animals after administration of estradiol benzoate, but 2 days of
estrogen priming was required to observe a significant
enlargement.
Cutaneous pain sensitivity varies with the phase of
the menstrual cycle in human studies of cephalic and
non-cephalic pain.12 Studies of non-cephalic pain have
reported increased thresholds to pressure pain,13,14
cold pressor tests,14-16 ischemic pain,17,18 and thermal
heat17 during the follicular phase compared to all other
phases, but electrical stimulation18 elicited the highest
thresholds during the luteal phase. Studies of cephalic
pain have reported the opposite with lower thresholds to pressure pain during the menstrual phase (eg,
early follicular phase) and higher thresholds during
the luteal phase.10 In addition, other forms of cephalic
pain such as temporomandibular joint disorder tend to
worsen during menstrual time periods.19 These largely
discrepant results might suggest cephalic and noncephalic types of pain are modulated differently by
ovarian hormones.
Capsaicin was used in this study to activate dural
afferents of the trigeminal nerve. Capsaicin is an agonist of calcium permeable ion channels called TRPV1
receptors, which are a subtype of vanalloid receptors
and are located on sensory neurons.20 Agonism of
TRPV1 receptors leads to an inward current, which
induces an action potential in afferent nerves. Schepelmann et al21 demonstrated that capsaicin was more
effective than inflammatory mediators in activating
dural nociceptors. Capsaicin can induce the release of
calcitonin gene related peptide from trigeminal afferents in animal models and has been used to model the
effects of migraine specific medications in experimental paradigms for migraine headache.22,23 Therefore,
activation of trigeminal afferents with capsaicin during the late proestrous and estrous stages of the rat
estrous cycle (eg, a time period during which there is
estrogen withdrawal) was thought to represent a potential experimental model for the study of menstrual
migraine.
The differences in sensitization observed in our
study only occurred after activation of trigeminal afferents with capsaicin. It is possible that the neuro-
April 2007
physiologic changes induced by the varying hormonal
milieus of the proestrous and estrous stages do not depolarize neurons to a significant enough degree to lead
to spontaneous firing. It may be necessary to activate
dural afferents with potent activators such as capsaicin
to detect differences in the neurophysiologic properties of the trigeminal system during different stages of
the rat estrous cycle.
We recorded from neurons within the TNC as well
as from those within the dorsal horns of CI-CIII. Unfortunately the number of neurons obtained from each
site was not large enough to analyze each location separately. Therefore, we pooled data obtained from all
4 sites for our analysis. We believe this was justified
since neurons within these sites have been thought to
be functionally equivalent.
Application of capsaicin to the dura significantly
increased the firing rates of neurons to cutaneous stimulation during proestrus for all stimulus intensities except for those that were the most intense (eg, pinch or
the largest forced produced by Von Frey filaments).
One explanation for the absence of sensitization with
intense stimuli could be depolarization block. This occurs when an intense depolarization of the membrane
leads to a paradoxical inactivation of sodium channels
causing a decreased rather than an increased response.
Intracellular recording will be required to establish if
this mechanism is responsible for the results we obtained.
We estimated a conduction velocity of 1 to
1.6 m/s after electrical stimulation of dural trigeminal afferents, which is within the range encountered
with c-fibers. There also was a trend toward higher
conduction latencies during proestrus both before and
after administration of capsaicin, but because of the
variability in this measure no statistical difference
was noted. One might speculate that “estrogen withdrawal” during proestrus preferentially affects conduction within the slower conducting c-fibers since
higher conduction latencies have lower conduction velocities. Obviously future studies with larger numbers
of animals will need to be performed to confirm or
refute this hypothesis.
The increase in cutaneous receptive field size
noted in proestrus may have resulted from changes
in the neurophysiologic properties of the synapse
Headache
between first and second order trigeminal neurons.
The hormonal changes encountered during this stage
might increase the number or affinity of postsynaptic
glutmatergic receptives or could enhance postsynaptic
release of nitric oxide. These changes could enhance
neurotransmission within previously silent afferents
within the TNC and thus lead to an expanded cutaneous receptive field.
Potential Mechanisms for the Results Obtained.—
Our results demonstrate enhanced sensitization of the
trigeminal system occurring during the late proestrous
and estrous stages of the rat estrous cycle, but we cannot be entirely certain which hormonal changes may
have led to the sensitization. We suspect however that
“estrogen withdrawal” is the most likely cause for the
sensitization since that was the predominant hormonal
change during these time periods. We cannot exclude
the possibility that a period of “estrogen priming” as
occurs during the early to mid proestrous stage is necessary prior to estrogen withdrawal to sensitize the system. The presence and/or withdrawal of progesterone
might have played a role in the sensitization as well.
We think this is less likely since progesterone levels
have an abrupt rise during the time period from 12:00
to 18:00 hours during the proestrus stage, which represented the time period during which our recordings
were obtained, and then fall abruptly prior to the start
of estrus. To explain our results one would have to postulate that both a rise in progesterone levels during late
proestrus and withdrawal of serum progesterone levels during the estrus stage enhance sensitization of the
trigeminal system. Future neurophysiological studies
will need to be conducted on ovariectomized animals
both before and after withdrawal of estrogen and progestereone to determine which hormonal changes may
be the most relevant to this enhanced sensitization.
The specific mechanisms through which ovarian
hormones sensitize the trigeminal system are unknown. Estrogen and progesterone however have
important effects on excitatory and inhibitory neurotransmitter systems thought to be relevant to neurotransmission within the trigeminal system. These
effects have been extensively reviewed in a recent
publication.24
Implications for Menstrual Migraine.—Sensitization of the trigeminal system during or after a decline
561
in ovarian hormones could have important implications for menstrual migraine. For example, sensitization of the trigeminal system could play a role in the
triggering of a migraine attack by increasing the resting membrane potential of the TNC and/or trigeminal
nerve reducing their threshold for activation from a
number of stimuli. Another possibility would be that
sensitization could change the response of the trigeminal system after its activation. A more vigorous activation of trigeminal system during the perimenstrual
time period might explain why some studies have reported that menstrual migraines are more severe, disabling and have a longer duration than non-menstrual
migraines.25,26
Limitations.—There are several limitations to this
study. First, serum estradiol levels were not measured
during the late proestrous stage and therefore we can
theorize that our recordings occurred during a decline
in estrogen levels, but we cannot definitively confirm
this. Second, there are hormonal differences between
the rat estrous cycle and the human menstrual cycle
and therefore a direct extrapolation of this data to
humans should be made with caution. For example,
serum estradiol levels decline from 85 to 20 pg/mL
during late proestrus while they decline from 250–300
to 25–50 pg/mL during the late luteal phase of the human menstrual cycle. Patterns of progesterone secretion are different in rats and humans. There are 2 progesterone peaks during the rat estrous cycle occurring
during diestrus and proestrus while only one occurs
during the mid-luteal phase of the human menstrual
cycle. Third, our sampling technique of neurons only
identified those that had both dural and cutaneous receptive fields. It is unknown whether our results would
have been similar in TNC and CI-CIII neurons with
only a dural receptive field. Those with both dural
and cutaneous receptive fields however probably have
more relevance to migraine as cutaneous allodynia is
experienced by 79% of migraineurs during a migraine
attack.27 Cutaneous allodynia of the ipsilateral face
is explained by TNC neurons that receive convergent
input from both cutaneous and dural sources.
CONCLUSIONS
Our results suggest that sensitization of the
trigeminal system is dependent on the varying
562
hormonal milieus encountered during the rat estrous
cycle. The proestrous and estrous phases demonstrated enhanced sensitivity after administration of
capsaicin to the dura while the metestrous and diestrous stages did not demonstrate any enhanced sensitivity. Since our recordings were performed during the
later part of the proestrous and estrous stages we postulate that “estrogen withdrawal” accounted for sensitization noted during these times. If similar changes occur within the trigeminal system of female migraineurs
this could explain both the triggering and maintenance of menstrual migraine. Clearly future studies
are needed to delineate the exact mechanisms through
which ovarian hormones sensitize the trigeminal system.
Conflict of Interest: None
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