Charles W. Malsbury

Neuropeptides and male sexual behavior

Evidence is rapidly accumulating that a number of neuropeptides are involved in the central control of male sexual behavior. This is consistent with their neuroanatomical distribution, i.e., in CNS loci previously implicated in the control of this behavior such as the medial preoptic area, and with recent findings that the peptide content of some of these regions is regulated by testosterone or its metabolites. Most of the work has been done using rats, but relevant human studies have been included whenever such material has been available. At this point there are relatively few studies which directly demonstrate the involvement of peptides in this behavior. Inhibitory and facilitatory actions, however, have been demonstrated following injections of peptides, peptide antisera, or antagonists into the CNS of male rats. Significant new developments include demonstrations that injections of substance P and A-MSH directly into the medial preoptic area can facilitate this behavior, while ventricular injection of an oxytocin antagonist can produce a powerful inhibition. The emerging picture is that GnRH, oxytocin, A-MSH and substance P stimulate, while CRF, beta-endorphin, prolactin, and neuropeptide Y are inhibitory. The inhibitory peptides CRF, beta-endorphin and prolactin are related, as they are released in response to stress. This may be relevant to the low level of sexual motivation in some depressed men. Questions concerning sites of action and mechanisms of action which mediate the behavioral effects which have been demonstrated remain largely unanswered.

Facilitation of lordosis by injection of substance P into the midbrain central gray.

Behavioral experiments tested the idea that the substance P (SP) innervation of the midbrain central gray (MCG) may be involved in the hormonal induction of sexual receptivity in female rats. SP, a SP antiserum or a reported SP antagonist were injected bilaterally into the MCG in ovariectomized, estrogen-treated females, and the lordosis response was recorded at repeated intervals. In the first experiment, three doses of SP (50,500 and 1,000 ng/cannula), a single dose of LHRH (50 ng/cannula) or vehicle were given to separate groups of females. All three doses of SP produced a rapid and long-lasting (3 h) increase in lordosis scores in moderately receptive females in tests with either manual stimulation or male rats. This facilitation was similar in latency, magnitude and duration to that produced by LHRH. In the second experiment, the basic findings of experiment 1 were replicated using blind testing. As no dose-response relation was established in experiment 1, a lower dose of SP (10 ng/cannula) was used in addition to doses of 50 and 500 ng/cannula also used in experiment 1. All three doses produced similar long-lasting increases in lordosis scores as in experiment 1. MCG injections of SP also increased lordosis scores in a second series of tests using manual stimulation alone. This demonstrates that the SP-induced facilitation does not depend on an interaction between the injections and stimuli delivered only by the male rat, eg., vaginal stimuli or ultrasonic calls. The question of the importance of endogenous SP for receptivity was examined in experiment 2 using MCG injections of a SP antiserum or the SP analogue, (D-Pro2, D-Trp7,9)-SP, which has been reported to block the excitation of locus coeruleus neurons by SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitation of sexual receptivity by hypothalamic and midbrain implants of progesterone in female hamsters

In the first experiment, ovariectomized female hamsters were stereotaxically implanted with bilateral guide cannulae aimed at the medial preoptic area (POA), ventromedial hypothalamus (VMH), or ventral tegmentum (VTA). The following week these females were injected SC with 10 micrograms estradiol benzoate (EB) and then had 27-gauge cannulae containing crystalline progesterone inserted through the guide tubes. Sexual receptivity was observed in 3 of 11 animals with VMH implants of progesterone, in 2 of 10 with VTA progesterone, but in none with POA implants. In the second experiment, the amount of intracranial progesterone was increased by mechanically expelling a 1.5 micrograms progesterone pellet from the tip of each cannula insert. This treatment facilitated receptivity in 10 of 20 hamsters with VTA implants and in 9 of 32 VMH-implanted animals. This induction of receptivity required approximately 2 hr. Progesterone pellets in the POA, mammillary region, and lateral mesencephalon were generally ineffective. In hamsters, progesterone into either the VMH or the VTA is sufficient to facilitate receptivity, although neither site is highly sensitive to progesterone. These results differ from those in recent studies in rats and this difference may reflect important species differences in the control of lordosis.

Peptidergic control of male rat sexual behavior: the effects of intracerebral injections of substance P and cholecystokinin.

Behavioral experiments examined the roles of substance P (SP) and cholecystokinin (CCK) in male rat copulatory behavior. Male copulatory behavior was recorded subsequent to injections of different doses of CCK and SP into the medial preoptic-anterior-hypothalamic area (MPOA-AH), caudate/putamen (CP), or the lateral ventricles (LV) in sexually experienced male rats. In the first experiment, three different doses of SP (10, 100, and 200 ng/cannula) injected bilaterally into the MPOA-AH produced marked changes in several components of male copulatory behavior. Latencies were most affected. All three doses significantly shortened the interval to initiate copulation, and the 10 and 100 ng, but not 200 ng dose also significantly reduced ejaculation latencies. Injections of 10 ng of SP into the CP did not affect sexual behavior, while injections into the LV produced changes different from those of MPOA-AH injections. These data argue for some degree of site specificity of the effects of the MPOA-AH injections. Bilateral injections of 10 ng of SP into the MPOA-AH, were incapable of inducing copulatory behavior in castrated rats deprived of testosterone. Injections of an undiluted SP antiserum (2 microliters/cannula) into the MPOA-AH produced a dramatic impairment of male copulatory behavior. These injections significantly lengthened amount, intromission, and ejaculation latencies, while having no effect on the number of mounts or intromissions prior to ejaculation. In contrast, bilateral injections of CCK-8 (10, 100, and 200 ng/cannula) into the MPOA-AH failed to affect any parameter of male copulatory behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

A sex difference in the pattern of substance P-like immunoreactivity in the bed nucleus of the stria terminalis

The caudal part of the dorsomedial bed nucleus of the stria terminalis (BNST) contained a discrete region of dense substance P (SP) staining in both male and female rats. However, the pattern of this staining differed markedly between the sexes. Males exhibited a densely stained capsule surrounding a less dense core. In females, this core was barely detectable (small and present on only 1 or 2 sections) or absent. The perimeter of this area of dense staining was traced and the area inside calculated. The mean area was found to be 2.4 times larger in males. The component of the BNST which receives this SP innervation was identified on Cresyl violet-stained sections. The area occupied by this distinct cell group was 32% larger in males. The areas of the suprachiasmatic nuclei and whole coronal sections through the BNST were measured in the same way and found not to differ between the sexes.

Sexual receptivity: Brain sites of estrogen action in female hamsters

In order to investigate the brain sites of estrogen action, ovariectomized hamsters were stereotaxically implanted with unilateral 27 gauge cannulae containing estradiol. Groups of females received implants into either the lateral septum, bed nucleus of the stria terminalis, preoptic area, anterior hypothalamus, ventromedial hypothalamus, arcuate nucleus, corticomedial amygdala or mesencephalic central gray. Another set of animals received implants containing cholesterol. One week later the animals were injected with progesterone and 4-5 hours later tested for sexual receptivity. The most receptivity and the most consistent response was seen in females with estradiol implants in the ventromedial hypothalamus. Only a few scattered animals in the other anatomical groups showed any receptivity. Only in animals with implants in the anterior hypothalamus was there any evidence of leakage of estrogen into peripheral circulation as measured by uterine weight. There was no response in females with cholesterol implants. Our results suggest that the ventromedial hypothalamus is the most sensitive brain area for the estrogenic induction of female sexual receptivity in hamsters.

Two organizational effects of pubertal testosterone in male rats: transient social memory and a shift away from long-term potentiation following a tetanus in hippocampal CA1

The organizational role of pubertal androgen receptor (AR) activation in synaptic plasticity in hippocampal CA1 and in social memory was assessed. Earlier data suggest pubertal testosterone reduces adult hippocampal synaptic plasticity. Four groups were created following gonadectomy at the onset of puberty: rats given testosterone; rats given testosterone but with the AR antagonist flutamide, present during puberty; rats given testosterone at the end of puberty; and rats given cholesterol at the end of puberty. A tetanus normally inducing long-term potentiation (LTP) was used to stimulate CA1 in the urethane-anesthetized adults during the dark phase of their cycle. Social memory was assessed prior to electrophysiology. Social memory for a juvenile rat at 120 min was seen only in rats not exposed to AR activation during puberty. Pubertal AR activation may induce the reduced social memory of male rats. Early CA1 LTP occurred following tetanus in rats with no pubertal testosterone. Short-term potentiation occurred in rats exposed to pubertal testosterone. Unexpectedly, rats with pubertal AR activation developed long-term depression (LTD). The same pattern was seen in normal male rats. Lack of LTP during the dark phase is consistent with other data on circadian modulation of CA1 LTP. No correlations were seen among social memory scores and CA1 plasticity measures. These data argue for two organizational effects of pubertal testosterone: (1) CA1 synaptic plasticity shifts away from potentiation toward depression; (2) social memory is reduced. Enduring effects of pubertal androgen on limbic circuits may contribute to reorganized behaviors in the postpubertal period.

Sex difference in the substance P-immunoreactive innervation of the medial nucleus of the amygdala

Discrete fields of substance P-immunoreactive fibers are present within the posterior dorsal division of the medial nucleus of the amygdala and the posterior medial bed nucleus of the stria terminalis in adult male and female rats. We previously reported a sex difference in the extent of this innervation of the bed nucleus. In the present study, we have replicated our earlier finding and found an equally dramatic sex difference in the amygdala. Morphometry revealed that the areas of dense staining in both the medial amygdala and the medial bed nucleus were more than twice as large in male brains.

Neurotrophic Effects of Testosterone on the Medial Nucleus of the Amygdala in Adult Male Rats

Our previous reports of major sex differences in the substance P‐immunoreactive (SPir) innervation of the medial posterior divisions of the bed nucleus of the stria terminalis (BST) and medial nucleus of the amygdala in rats raised the question of the hormonal regulation of this innervation. We now report the results of two experiments which examined the effects of castration of adult males on the SPir innervation of these regions. In experiment 2 we asked whether castration might also alter the cytoarchitecture of these regions.

Proteins Regulated by Gonadal Steroids in the Medial Preoptic and Ventromedial Hypothalamic Nuclei of Male and Female Rats

Protein profiles of brain areas mediating effects of steroid hormones on copulation were compared between animals in gonadal steroid states predictive of either the presence or absence of copulatory activity. A broad range of proteins present in micropunches of tissue from the medial preoptic area (MPO) and from the ventromedial hypothalamus (VMH) were compared between male and female rats with gonadal steroids present or absent. Half of the animals of each gender were gonadectomized 1 month prior to sacrifice. The remaining males were left intact, while the remaining females were gonadectomized, implanted with estrogen capsules, and injected with progesterone prior to sacrifice. These females were screened for sexual receptivity immediately prior to sacrifice. Proteins from the MPO and VMH of each animal were separated by two-dimensional gel electrophoresis, silver stained, and quantified by computerized optical densitometry. Several proteins differed in density between gels of high-steroid males and and females and between high-steroid and absent-steroid animals of one or both genders. Two previously reported sex differences were replicated and found to depend on activational effects of gonadal steroids. Several interesting reversal patterns were noted between MPO and VMH, including three proteins that were affected by gonadectomy in the MPO of males, but not females, and in the VMH of females, but not males, thus correlating with sexual function. These included serum albumin (a possible index of local area blood flow) and neuron-specific enolase, a glycolytic enzyme of anaerobic metabolism. A probable genetic polymorphism was discovered at a locus whose expression appears to be regulated by gonadal steroids.(ABSTRACT TRUNCATED AT 250 WORDS)