Putative neurotransmitters involved in discharging gonadotropin-releasing neurohormones and the action of LH-releasing hormone on the CNS

Pergamon PresB

Life Sciences Vol . 16, pp . 833-852 Printed in the U.S .A .

MINIREVIEW PUTATIVE NEUROTRANSMITTERS INVOLVED IN DISCHARGING GONADOTROPIN-RELEASING NEUROHORMONES AND THE ACTION OF LH-RELEASING HORMONE ON THE CNS S .M . McCann and R .L . Moss Department of Physiology, The University of Texas Health Science Center at Dallas, Southwestern Medical School, Dallas, Texas 75235

Recent advances have made it abundantly clear that the release of adenohypophysial hormones is controlled by a family of peptide neurohormones which are secreted into the hypophysial portal vessels to stim ulate or inhibit the release of particular pituitary hormones

(1-4) .

At

least three of these neurohormones have been isolated, their structure determined, and synthesis of the molecules has been accomplished .

Two

factors have been postulated to regulate the secretion of FSH and LH by the gland, FSH-releasing factor (FRF) (LRH)

(6) .

(5) and an LH-releasing hormone

On the other hand, prolactin, which is predominantly under

inhibitory hypothalamic control, inhibiting factor

(PIF)

appears to be inhibited by a prolactin-

(7), but there is suggestive evidence for the

existence of a prolactin-releasing factor

(PRF) as well (8,9) .

The structure of the LH-releasing factor was elucidated by Matsuo et al .

(10,11) who then synthesized the molecule .

LH-releasing factor,

or hormone, as it is now frequently called, is active in all vertebrate species so far examined, including man.

it is a decapeptide and anti-

bodies raised against it can block ovulation and inhibit gonadotropin release (12) .

Furthermore, radioimmunoassays developed to measure the

decapeptide have localized it to the same portions of the preoptichypothalamic region which contain biologically assayable LH-releasing factor (13) .

Increases in titers of the decapeptide have been found in plasma 833

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in situations associated with increased gonadotropin release (14)

so that

it appears that the decapeptide is indeed the natural product. The capacity of the decapeptide to release FSH has led some workers to postulate that there is no distinct FRF and that hypothalamic control over FSH release is mediated via the decapeptide (15) .

It is known that

FSH and LH release do not always proceed in parallel, and those who espouse the unitary hypothesis of hypothalamic gonadotropin control postulate that interactions on the gonadotropha between the decapeptide and the steroid milieu can account for differential release of FSH and LH .

We believe

that a separate FRF will ultimately be isolated since several groups have reported partial purification of this factor (16-18), and since it is possible to dissociate FSH and LH release in experiments employing either hypothalamic stimulation (19) or destructive lesions in this part of the brain (20) . Resolution of this point will require the isolation of a distinct FRF . With this introduction to serve as a background, we will review the evidence for the role of putative synaptic transmitters in altering the release of FRF, LRH and PIF and the recent evidence which suggests that LRH may play a role in the induction of mating behavior . Localization of LRH and other releasi~ fac tors with in the preoptichypothalamic region . As determined by both bioaasay and radioi~unoassay of extracts from frozen hypothalamic sections, LRH is localized to a medial basal region extending caudally and ventrally from the suprachiasmatic region to the region of the arcuate nucleus and median eminence

(13) .

Since lesions

in the suprachiasmatic region led to a decrease in stored LRH in the median eminence on measurement sometime later (21), it was postulated that LRH is formed in neurons, and that some of these have cell bodies located as far rostrally as the suprachiasmatic region with long axons projecting caudally to the median eminence, there to release the hormone into hypophysial

Vol . 16, No . 6 portal vessels .

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83 5

Since considerable LRH was still present in the basal

hypothalamus sometime after suprachiasmatic lesions,

it was also postulated

that another population of neurons has its cell bodies located more caudally, probably in the arcuate nucleus and relatively short axons which project to the median eminence . Recently, it has been possible to study the localization of LRH by means of ~+~~~ohistochemical methods .

Unfortunately, results from the

various groups are not in complete agreement.

All groups agree that axons

of LRH neurone course in a rostral-caudal direction through the lateral aspects of the median eminence and terminate in juxtaposition to the hypophysial portal capillaries .

Harry and his collaborators have traced these

axons back to nerve cell bodies in the preoptic, anterior hypothalamic, and arcuate nuclear regions (22), which would be in complete agreement with the work in which the hormone was localized by assay of frozen sections cut through the hypothalamus .

On the other hand, some groups

(23,24) can

only trace the axons back to the caudal limits of the optic chiaem and toward the arcuate nucleus, but see no cell bodies which are ~+~+~noreactive . Still another group finds activity in cell bodies within the arcuate nucleus (25) .

Lastly, LRH has been reported to be in the tanycyte ependymal cells

lining the floor of the third ventricle and also in the organum vasculosum of the lamina terminalis which is a circumventricular organ lying just over the optic chiasm

(25) .

The discrepancies among the various groups

may be attributable to the use of different species, antibodies, and to possible non-specific staining . The preponderance of evidence is consistent with the origin of LRH in neurosecretory neurons which synthesize the hormone, transport it by axoplasmic flow and store it in axon terminals in the external layer of the median eminence .

In further support of the neuronal origin of LRH

are the demonstrations of LRH-containing granules in axon terminals in the external layer (26)

and its isolation from synaptosomes in experiments

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employing ultracentrifugation (27) . Since the hormone is also found in ependymal elements and in the organum vasculosum, the possibility exists that it may be released from the organum vasculosum into the ventricular system and be transported to the median eminence for uptake by ependymal cells .

These could transport the neurohormone

to the portal vessels since their processes terminate in juxtaposition to portal capillaries .

in this connection, the intraventricular injection

of LRH does result in appearance of LRH in portal vessels and stimulation of LFI release, although only about 5$ of the injected hormone reaches the portal vessels (28) . The localization of LRH differs from that of other releasing factors . For example, TRF is found in the region of the nucleus interstitialis stria terminalia and in a medial zone extending caudally to the dorsomedial nucleus and ventrally to the median eminence where a large amount of the factor is stored

(29) .

In general, it appears that each releasing and inhibiting

factor has a distinct localization within the hypothalamus, but that the majority of the activity is usually stored in the median eminence .

Thus,

the median eminence subserves a role as a storage site for releasing factors analagous to that of the neural lobe as a storage site for neurohypophysial principals . The localization of LRH just described reveals that these neurons a

would be in possible synaptic contact with a host of putative synaptic transniitters .

For example, the rostral LRH neurons would be in the same

general region in which numerous noradrenergic terminals are located (30) . Serotoninerqic terminals are found in abundance in the region of the suprachiasmatic nucleus (30,31) .

Terminals of possible histaniinergic neurons

are localized largely to the median eminence region (32), and the tuberoinfundibular dopaminergic pathway (30,33) lies within the caudal region which contains the largest amount of LRH .

Because of the wide distribution

of cholinergic terminals within the hypothalamus

(34), acetylcholine cannot

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be ruled out as a possible transmitter modulating the release of these factors .

The axons of the tuberoinfundibular dopaminergic tract terminate

in the external layer of the median eminence in juxtaposition to hypophysial portal vessels which raises the possibility that dopamine might be released into portal capillaries and have a direct action on the anterior lobe (30) . Evidence for catecholaminergic control of gonadotro~n- release . On the basis of a variety of experiments, there is little doubt that release of gonadotropins from the adenohypophysis is under adrenergic control .

In early studies it was shown that the adrenergic blocking drug,

dibenamine, could block ovulation (35) ; however, this was presumed to be via a direct action on the anterior lobe since epinephrine infusions into the gland elicited ovulation (36) .

Subsequent studies revealed that this

was probably an artifact caused by the acidity of the solutions (37) . in vitro studies . When various amines were incubated with pituitaries in vitro, little effect was observed on the release of gonadotropins, but when a coincubation system was used in which ventral hypothalami were incubated together with anterior lobes, it was observed that the addition of dopamine to the incubation medium increased the release of both FSH (38)

and LH

(39) .

Since

the catecholamine did not alter the action of added gonadotropin-releasing factors,

it was concluded that it evoked a release of these factors from

the ventral hypothalamic fragments .

This release was prevented by the

alpha receptor blocker, phentolamine, or by the dopamine receptor blocker, haloperidol, but was uninfluenced by a beta receptor blocker.

Dopamine

produced a dose-related release and there was little effect of norepinephrine .

It was puzzling that alpha receptor blockers could prevent this

action of dopamine and equally puzzling was the fact that addition of reserpine to the medium also blocked the action of dopamine

(40) .

This latter finding

suggested that dopamine was active only after uptake and re-release from

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It is conceivable that it may be converted to norepinephrine,

which is the active agent .

The ineffectiveness of norepinephrine could

be explained by postulating ite more rapid inactivation when added to the medium . Interestingly enough, the releasing action of dopamine was blocked by estradiol in this in vitro system (41), and this blockade could be prevented by the addition of either puromycin or cyclohexamide, inhibitors of protein synthesis.

This suggested to us that at least part of the negative

feedback action of estrogen to inhibit gonadotropins might be mediated by an action on the releasing factor neurone which resulted in the production of an inhibitory peptide or protein which then blocked the action of dopamine on the releasing factor neuron .

This would be analagoua to the apparent

ability of thyroxin to block the action of thyrotropin-releasing factor on the pituitary after synthesis of an intermediary peptide or protein (42) . in vivo experiments . It was important to determine if dopamine was also active to release gonadotropins in vivo .

In order to circumvent the blood brain barrier,

the catecholamine was injected directly into the third ventricle in ani orals bearing permanent third ventricular cannulae . tion, increases in plasma FSH, LH,

T"RH ,

Following its injec-

and gonadotropin-releasing activity

in portal vessels have been reported (43-46) .

In hypophysectomized female

rata, the prior injection of estradiol into the third ventricular cannula blocked the increase in

T" RH

following intraventricular dopamine, a result

which agrees with the prior in vitro studies (44) . In the female, the hormonal background appeared to modify the response to dopamine

(43)

in that the catecholamine was ineffective in ovariectomized

animals and in animals in estrus or dieatrua day 1 of the estrous cycle . Dopamine was only effective on diestrus day 2 and proestrus and was most

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effective in ovariectomized animals which had been primed with estrogen and progesterone .

This latter preparation is maximally sensitive to LRH .

Thus, the steroid background appeared to determine the response to the catecholamine .

Recently, it has been shown that the preovulatory discharge

of LH can be advanced by L-DOPA in the rat which is consistent with this supposition (47) . In the in vivo test situation, norepinephrine was less active than dopamine and epinephrine was the least effective catecholamine (45,46)t however, in proeatrous rats anesthetized with Nembutal, Rubinstein and Sawyer (48)

reported that norepinephrine was the effective agent to induce ovulation

after its intraventricular injection, whereas dopamine was ineffective . in a more recent study in rabbits, Sawyer et al . (49) have reported that intraventricular dopamine will block the increase in plasma LH in response to subsequent intraventricular injection of norepinephrine . In later work it has been difficult to reproduce the effect of dopamine in male rats

(50), and it should be noted that the response of dopa-

mine in the male was quite small in earlier experiments (43) . The response to intraventricular dopamine could be blocked by phentolamine or haloperidol but was not influenced by the beta receptor blocker, propanolal

(431 in agreement with the in vitro studies.

The blockade by the

alpha receptor blocker, phentolamine, is puzzling if dopamine is acting as such and is consistent with the possibility already mentioned that dopamine may be converted to norepinephrine which is then the active agent . Receptor blockers have also been used in various physiological states to determine their effect on gonadotropin titers .

We have already cited

the old observation that the alpha blocker, dibenamine, would inhibit ovulation (35) .

In subsequent studies it has been shown that phentolamine, another

alpha Mocker, can inhibit the post-castration rise in gonadotropins (51) and can also block the pulsatile release of LH which occurs in the ovariectomized monkey (52) .

Pimozide, a dopamine receptor Mocker, on the other

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hand, produced a slight but nonsignificant reduction in the post-castration rise in LH and a small but significant reduction in the post-castration rise in FSH (51) . Another approach has been to use drugs which either interfere with or augment catecholamine synthesis .

Alpha methyl tyrosine, which inhibits

tyrosine hydroxylase, leading to a reduction in synthesis of catecholamines, can block the post-castration rise in gonadotropins .

The block can be

partially rever®ed by the administration of either L-DOPA or dihydroxyphenylserine

(DOPS)

to reinitiate the synthesis of both dopamine and norepinephrine,

or of only norepinephrine, respectively

(51) .

In addition to blocking

the response to removal of steroid negative feedback, alpha methyl tyrosine can also block the stimulatory effects on gonadotropin release of either estrogen or progesterone in estrogen-primed rats

(Sla, 52a) .

Here again,

partial or complete reversal of the blockade can be achieved by reinitiating norepinephrine synthesis.

Similarly, alpha methyl tyrosine can block the

preovulatory discharge of gonadotropins, which is thought to be brought about by rising titers of estrogen and possibly progesterone on proestrus (53) . When drugs, such as diethyldithiocarbamate or U14624, which block dopamine beta hydroxylase and lead to a selective impairment in norepinephrine synthesis were used, similar results were obtained, namely, interference with gonadotropin release in the castrate, in the estrogen or estrogen, progesterone-primed rat, and in the case of the normal preovulatory release (S1,S1a,52a,53) .

Again, partial or complete restoration of gonadotro-

pin release could be obtained by reinitiating norepinephrine synthesis with DOPS to bypass the block .

The dosages of the various drugs which

were used were shown to produce the expected alterations in hypothalamic stores of norepinephrine (54)

lending credence to the idea that the effects

were indeed related to altered transmission across noradrenergic synapses . In order to localize the noradrenergic synapse involved in gonadotropin

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84 1

release, the preoptic area was stimulated in rats using parameters of stimulation known to release LH (55) .

Drugs which interfered with norepineph-

rine synthesis significantly impaired the response to stimulation and it could be restored using other drugs, such as L-DOPA or DOPS in the case of alpha methyl tyrosine-imposed block, which would restore the synthesis of norepinephrine .

On the other hand, stimulation in the arcuate-median

eminence region also released LH and this release was not altered by inhibitors of catecholamine synthesis .

Presumably in this situation the LRH neurons

or their axons were directly stimulated . It would appear that a noradrenergic synapse may lie in the preoptic or anterior hypothalamic area caudal to the site of preoptic stimulation, such that interference with norepinephrine synthesis blocks transmission of the stimulus to the median eminence .

We postulate that noradrenergic

terminals in this region synapse with LRH neurons whose cell bodies lie here and that increased impulse traffic across this synapse may mediate the increased release of LRH and LH which occurs in response to estrogen or progesterone and during the preowlatory discharge of gonadotropins . Since the enzyme which converts norepinephrine into epinephrine has now been demonstrated in various hypothalamic regions (SSa),

it is possible

that the actions we have attributed to norepinephrine could be mediated by epinephrine instead . Studies of amine turnover . Another approach to the problem is to determine the turnover of hypothalamic catecholamines in situations associated with altered release of gonadotropins.

This approach has given further evidence for a role of norepinephrine

in the control of gonadotropin release since the turnover of hypothalamic norepinephrine is increased on proestrus (56), at the time of the preovulatory release of gonadotropins . in castrated animals,

Brain norepinephrine turnover is also increased

another situation in which gonadotropin release is

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84 2

enhanced (57) .

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On the other hand, the turnover of dopamine in the tubero-

infundibular dopaminergic pathway appears to be reduced during proestrus, at a time when gonadotropin release is enhanced, which led Fuxe and coworkers to postulate that dopamine has an inhibitory role in gonadotropin release (30,58,59) .

Opposed to this concept is their own finding that the DA receptor

blocker, Pimozide, will block pregnant mare's serum (PMS)-induced ovulationr however, they report that some ergot derivatives which appear to be dopamine agonists, as well as apomorphine, a known dopamine agonist, will also block PMS-induced ovulation (59) .

Since gonadotropins were not measured in these

studies, it is not possible to determine if the blocking action is via inhibition of gonadotropin release. A complex series of events takes place on the afternoon of proestrus and the release of prolactin as well as gonadotropins is enhanced .

An

alternative hypothesis would be that a reduced DA turnover on the after noon of proestrus would induce the rise in prolactin which occurs on proestrus .

Dopamine clearly functions to inhibit prolactin release (see below) .

This coupled with the increased transmission across a noradrenergic synapse in the preoptic or anterior hypothalamic area leading to increased LRH release could account for the preovulatory discharge of FSH, LH and prolactin. Other putative synaptic transmitters involved in gonadotropin release. Large doses of subcutaneously administered atropine were shown to block ovulation in the pioneering experiments of Sawyer and Everett (60) . More recently it has been possible to block gonadotropin and prolactin release by either subcutaneous or intraventricular injection of atropine sulfate (61) .

The dose required is rather large, being one-fourth the

LD50 for intraventricular injections .

This raises some question as to

whether or not atropine is acting specifically .

Recently, Martini's group

has reported that acetylcholine in a large dose can increase FSH and LH release from a coincubation system of ventral hypothalami and pituitaries

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84 3

in vitro and can increase LH release following injection into the ventricular system (62,63) .

In our own experiments we have had little success in stimulat-

ing gonadotropin release by intraventricular injections of carbachol (Libertun and McCann, unpub . data), and, furthermore, systemic administration of pilocarpine or eserine produced immediate inhibition of gonadotropin and prolactin release followed by a delayed release in ovariectomized, estrogenprimed rats

(64) .

Thus, it is still too early to conclude that cholinergic

synapses play a role in mediating hypothalamic control over gonadotropin and prolactin release . Both serotonin and the pineal indole, melatonin, have been shown to be capable of inhibiting gonadotropin and augmenting prolactin release following their injection into the third ventricle (43,65,66) ; however, since the inhibitor of serotonin biosynthesis, parachlorophenylalanine, had little effect on gonadotropin and prolactin release in male rats

(54),

it is still too early to conclude that serotonin has a physiological role . in recent experiments it has been shown that methysergide, a serotonin receptor blocker, can block the stress-induced release of prolactin (67) and parachlorophenylalanine was reported to block suckling-induced prolactin release which suggests a role for the serotonin system in prolactin control (68) . Histamine has long been known to be concentrated in the basal tuberal region

(69)

and appears to be found in synaptosomes there (70), suggesting

that histaminergic endings are closely related to releasing factor neurons in the median eminence .

It has recently been shown that intraventricular

histamine, at a relatively high dose, will release both gonadotropins and prolactin in ovariectomized, estrogen-primed animals and that a relatively low dose of methylhistadine to block histamine decarboxylase can lower prolactin (71) .

Furthermore, diphenhydramine, an antihistiminic drug,

was capable of blocking stress-induced prolactin release (71) .

Therefore,

there is suggestive evidence for a possible role for histamine in the regula-

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tion of prolactin and possibly gonadotropin release . T9e have been unsuccessful in modifying gonadotropin and prolactin release with relatively large doses of intraventricular amino acids or gamma amino butyric acid (Libertun and McCann, unpub . data) ; however, Ondo (72) has recently reported that very large doses of intraventricularly injected gamma amino butyric acid can release gonadotropins in the male rat . Comparison of the adrenergic control of gonadotropin and prolactin release . As ind ü:ated before, prolactin appears to be predominantly under inhibitory hypothalamic: control .

Experiments similar to those already described

for gonadotropins point to dopamine as an inhibitory transmitter w}rich can release PIF, and dopamine, itself, may have a direct inhibitory effect on pituitary prolactin release after its secretion into hypophysial portal vessels .

For example, alpha methyl tyrosine leads to a rapid elevation

in scrum prolactin (59,73) which can be brought back to below initial values by the injection of L-DOPA to reinitiate catecholamine synthesis (54) . On tree ctaer hair3, if DOFS is injected to reinitiate only norepinephrine synthesis, no decrease, and even a small rise, supervenes .

A variety of

pharmacological tests of this sort (54,73) have led to the almost inescapable conclusion that it is dopamine rather than norepinephrine which is the inhibitory transmitter .

Furthermore, the intraventricular injection of

dopamine cr the dopamine agonist, apomorphine, leads to a rapid decline in prolactin, wüeroas norepinephrine produces a slight increase (74,75) . The dopamine receptor bloc}:er, Pimozide, when implanted into the median eminence leads to a rapid rise in prolactin, w}rereas similar implants in the pituitary produce only a small response (76) .

This has led to the

view that dopamine acts primarily by releasing PIF which then inhibits prolactin release .

The possibility that the catecholamine may act directly

on the gland cannot be ruled out since dopamine inhibits prolactin release by pituitaries incubated in vitro (77-KO), and has recently been shown

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Neurotransmitters-Peptide Hormone Release

to also be capable of inhibiting its release when injected into cannulated portal vessels (81) .

Furthermore, when L-DOPA was injected into rats with

hypothalamic lesions to eliminate neural control over the gland, a reduction in the elevated plasma levels of prolactin resulted, suggesting that L-DOPA is taken up by the gland and converted to dopamine which then inhibits the release of prolactin (82) .

That dopamine is not the only inhibitor

and that there really is a PIF is indicated by the fact that incubation of dopamine with ventral hypothalamic fragments releases an inhibiting factor whose action on the pituitary unlike that of dopamine was not blocked by Ilaloperidol

(79) and that there is too little dopamine in hypothalamic

extracts to account for its prolactin-inhibiting activity in vivo

(83) .

Thus, we currently believe that dopamine acts to release PIF from peptidergic neurons in the median eminence and that it may also be secreted into portal blood and inhibit the gland directly .

Proof of the physiologic

significance of the direct action of dopamine on the pituitary will require its demonstration in portal blood .

So far this has not been accomplished

(84), but this may be attributed to the relative insensitivity of the asçay method used . The action- of LRF to induce matin~be havior . The fact that the preovulatory release of gonadotropins, presumably induced by release of LRH, precedes the onset of mating behavior, coupled with the fact that LRH is localized in the brain to the same regions which are known to be involved in inducing mating behavior

(85), namely, the

preoptic anterior hypothalamic region, led us to evaluate its possible role in induction of mating behavior

(86,87) .

It has long been known that progesterone can induce mating in the ovariectomized, estrogen-primed rat .

It occurred to us that LRII might

be active to induce mating in the estrogen-primed female .

The most reliable

index of sexual receptivity in the female is lordosis behavior which follows

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mounting by the male .

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Animals primed with the dose of estrone used in

this study rarely evinced this type of behavior ; however, following the subcutaneous injection of LRH, nearly all of the animals exhibited lordosis in response to mounting .

As expected, a very high incidence of lordosis

was also seen in estrogen-primed animals treated with progesterone .

The

sexual behavior elicited by LRH was quite similar to that observed in the progesterone-treated animals except for the fact that the animals seldom exhibited ear wiggling, and sometimes showed some aggressive behavior as evidenced by hind kicking.

Estrogen-priming was essential since LRH failed

to induce mating behavior when it was injected alone. The possibility that LRH was acting via release of gonadotropins was ruled out by injecting large doses of either FSH or LH .

This possibility

was also ruled out in the experiments of Pfaff (88) who induced mating behavior with LRH in hypophysectomized, estrogen-primed females .

A pos-

sible contribution of adrenal progesterone was also eliminated by showing that LRH was equally effective in the ovariectomized, adrenalectomized animal as in the ovariectomized animal

(87) .

Thyrotropin-releasing factor

was chosen as another releasing factor to evaluate, and it exhibited no action whatsoever . The minimal effective dose of LRH to produce this effect is approximately 150 ng given subcutaneously, which is a reasonably high dose in terms of that required to produce gonadotropin release .

The response is

almost all-or-none in that there was a very narrow dose-response relationship, nearly all animals responding by the time a dose of 500 nq was reached . It would seem reasonable to believe that LRH is acting centrally to induce mating behavior .

To test this hypothesis, the releasing hormone

was microinjected into either the preoptic area or into the median eminencearcuate region

(87) .

In the former locus, significant mating behavior

was induced with doses which were ineffective when given systemically . There was a definite latency between the injection of LRH and the

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Neurotransmitters-Peptide Hormone Release

induction of mating behavior .

84 7

Good behavior usually was not elicited prior

to two hours and continued for 6 to 8 hours after injection (87) .

This

suggests that LRH is not acting in this situation as a synaptic transmitter her_ se .

Perhaps it does initiate activity in post-synaptic neurons which

would lead to the synthesis of some intermediary substance which then evokes mating behavior .

Further studies are necessary to determine the mechanism

of action of LRH to induce mating . It has been more difficult to demonstrate an effect of LRH on mating behavior in the male rat; however, it now appears that it does shorten the latency to intromission and ejaculation in normal males and in castrate males treated with suboptimal doses of testosterone propionate (87) . Further evidence for an action of LRH on the nervous system has recently been obtained using the technique of microiontophoresis onto neurons. Kawakami et al .

(89)

, Moss et al .

(90)

and Dyer et al .

(91)

have observed

alterations in firing rate of neurons following the iontophoretic application of LRH .

Further studies of this type may clearly delineate the action

of the neurohormone on the brain . Although LRH can induce mating behavior in spayed, estrogen-primed females, it does not appear to be capable of altering the time of onset or extending the duration of mating behavior in the normal female rat (87) . If LRH plays an essential role in the induction of mating behavior in the normal female, its rate of release must already be optimal for the induction of mating behavior on proestrus . It will of course be of extreme interest to determine if LRH can alter sex behavior in the human.

Studies to answer this question are underway

in a number of laboratories, but definitive results are not yet forthcoming . Do other releasing factors have an action on the brain as well as on the pituitary?

Thyrotropin-releasing factor, in contrast to LRH, appears

to be widely distributed throughout much of the brain (92-94) and it has been reported to have behavioral actions (95-97) .

Similarly, melanocyte-

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stimulating hormone-inhibiting factor has been reported to have actions on the central nervous system . elsewhere (98,99) .

The details of these effects can be found

Although much work remains to be done, it now appears

possible that the actions of the releasing factors on the CNS may be as important as their actions on the pituitary. REFERENCES 1.

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Vale, W ., G . Grant and R. Guillemin, Frontiers _in Neuroendocrinology ([+i .F . Ganong and L . Martini, eds .), p . 375, Oxford University Press, New York (1973) .

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McCann, S.M ., C.P . Fawcett and L . Krulich, Endocrine Physiology , vol . 5, (S .M . McCann, ed .), pp . 31-65, [dTP Press, Lancaster (1974) .

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Nicoll, C .S ., R.P . Fiorindo, C .T . McKennee and J.A . Parsons, Hypophysiotropic Hormones _of _the Hypothalamus : Assay and Chemistry (J . Meites, ed .), p 115, William and Wilkins, Baltimore (1970) .

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