Projections from ventrolateral hypothalamic neurons containing progestin receptor- and substance P-immunoreactivity to specific forebrain and midbrain areas in female guinea pigs

Substance P in the medial amygdala regulates aggressive behaviors in male mice

Behavioral and clinical studies have revealed a critical role of substance P (SP) in aggression; however, the neural circuit mechanisms underlying SP and aggression remain elusive. Here, we show that tachykinin-expressing neurons in the medial amygdala (MeATac1 neurons) are activated during aggressive behaviors in male mice. We identified MeATac1 neurons as a key mediator of aggression and found that MeATac1→ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl) projections are critical to the regulation of aggression. Moreover, SP/neurokinin-1 receptor (NK-1R) signaling in the VMHvl modulates aggressive behaviors in male mice. SP/NK-1R signaling regulates aggression by influencing glutamate transmission in neurons in the VMHvl. In summary, these findings place SP as a key node in aggression circuits.

Expanding the Role of Tachykinins in the Neuroendocrine Control of Reproduction

Reproductive function is driven by the hormonal interplay between the gonads and brain-pituitary axis. Gonadotropin-releasing hormone (GnRH) is released in a pulsatile manner, which is critical for the attainment and maintenance of fertility, however, GnRH neurons lack the ability to directly respond to most regulatory factors, and a hierarchical upstream neuronal network governs its secretion. We and others proposed a model in which Kiss1 neurons in the arcuate nucleus (ARC), so called KNDy neurons, release kisspeptin (a potent GnRH secretagogue) in a pulsatile manner to drive GnRH pulses under the coordinated autosynaptic action of its cotransmitters, the tachykinin neurokinin B (NKB, stimulatory) and dynorphin (inhibitory). Numerous genetic and pharmacological studies support this model; however, additional regulatory mechanisms (upstream of KNDy neurons) and alternative pathways of GnRH secretion (kisspeptin-independent) exist, but remain ill defined. In this aspect, attention to other members of the tachykinin family, namely substance P (SP) and neurokinin A (NKA), has recently been rekindled. Even though there are still major gaps in our knowledge about the functional significance of these systems, substantial evidence, as discussed below, is placing tachykinin signaling as an important pathway for the awakening of the reproductive axis and the onset of puberty to physiological GnRH secretion and maintenance of fertility in adulthood.

Sexually dimorphic neurons in the ventromedial hypothalamus govern mating in both sexes and aggression in males

Sexual dimorphisms in the brain underlie behavioral sex differences, but the function of individual sexually dimorphic neuronal populations is poorly understood. Neuronal sexual dimorphisms typically represent quantitative differences in cell number, gene expression, or other features, and it is unknown whether these dimorphisms control sex-typical behavior exclusively in one sex or in both sexes. The progesterone receptor (PR) controls female sexual behavior, and we find many sex differences in number, distribution, or projections of PR-expressing neurons in the adult mouse brain. Using a genetic strategy we developed, we have ablated one such dimorphic PR-expressing neuronal population located in the ventromedial hypothalamus (VMH). Ablation of these neurons in females greatly diminishes sexual receptivity. Strikingly, the corresponding ablation in males reduces mating and aggression. Our findings reveal the functions of a molecularly defined, sexually dimorphic neuronal population in the brain. Moreover, we show that sexually dimorphic neurons can control distinct sex-typical behaviors in both sexes.

Sex differences in the neural circuit that mediates female sexual receptivity

Female sexual behavior in rodents, typified by the lordosis posture, is hormone-dependent and sex-specific. Ovarian hormones control this behavior via receptors in the hypothalamic ventromedial nucleus (VMH). This review considers the sex differences in the morphology, neurochemistry and neural circuitry of the VMH to gain insights into the mechanisms that control lordosis. The VMH is larger in males compared with females, due to more synaptic connections. Another sex difference is the responsiveness to estradiol, with males exhibiting muted, and in some cases reverse, effects compared with females. The lack of lordosis in males may be explained by differences in synaptic organization or estrogen responsiveness, or both, in the VMH. However, given that damage to other brain regions unmasks lordosis behavior in males, a male-typical VMH is unlikely the main factor that prevents lordosis. In females, key questions remain regarding the mechanisms whereby ovarian hormones modulate VMH function to promote lordosis.

The involvement of substance P in the induction of aggressive behavior

Aggression is a complex social behavior that involves a similarly complex neurochemical background. The involvement of substance P (SP) and its potent tachykinin receptor (NK1) in the induction of both defensive rage and predatory attack appears to be a consistent finding. However, an overall understanding of the nature of the SP involvement in the induction of aggressive behavior has not yet been fully achieved. The aim of this review is to summarize and present the current knowledge with regards to the role of SP in the induction of aggressive behavior and to synopsize: (a) its biochemical profile, and (b) the exact anatomical circuits through which it mediates all types of aggressive behavior. Future studies should seriously consider the potential use of this knowledge in their quest for the treatment of mood and anxiety disorders.

Co-localization of midbrain projections, progestin receptors, and mating-induced fos in the hypothalamic ventromedial nucleus of the female rat

In female rats, sexual behavior requires the convergence of ovarian hormone signals, namely estradiol and progesterone, and sensory cues from the male on a motor output pathway. Estrogen and progestin receptors (ER and PR) are found in neurons in the hypothalamic ventromedial nucleus (VMH), a brain region necessary for lordosis, the stereotypic female copulatory posture. A subset of VMH neurons sends axonal projections to the periaqueductal gray (PAG) to initiate a motor output relay, and some of these projection neurons express PR. Previous studies showed that VMH neurons are activated during mating, based on the expression of the immediate early gene Fos. Many of the activated neurons expressed ER; however, it is not known if such activated neurons co-express PR. Fluorogold, a retrograde tracer, was injected into the PAG of ovariectomized rats to label neurons projecting from the VMH. Hormone-treated animals then were mated, and their brains were immunohistochemically stained for PR and Fos. Of the Fos-positive neurons, 33% were double-labeled for PR, 19% were double-labeled with Fluorogold, and 5% were triple-labeled for Fos, PR, and the retrograde tracer. The majority of triple-labeled neurons were found in the rostral, rather than caudal, portion of the VMH. These results show that PR-containing neurons are engaged during sexual behavior, which suggests that these neurons are the loci of hormonal-sensory convergence and hormonal-motor integration.

Neurokinin-1 receptor-expressing neurons in the amygdala modulate morphine reward and anxiety behaviors in the mouse

Mice lacking the neurokinin-1 (NK1) receptor, the preferred receptor for the neuropeptide substance P (SP), do not show many of the behaviors associated with morphine reward. To identify the areas of the brain that might contribute to this effect, we assessed the behavioral effects of ablation of neurons expressing the NK1 receptor in specific regions of the mouse brain using the neurotoxin substance P-saporin. In a preliminary investigation, bilateral ablation of these neurons from the amygdala, but not the nucleus accumbens and dorsomedial caudate putamen, brought about reductions in morphine reward behavior. Subsequently, the effect of ablation of these neurons in the amygdala on anxiety behavior was assessed using the elevated plus maze (EPM), before conditioned place preference (CPP), and locomotor responses to morphine were measured. Loss of NK1 receptor-expressing neurons in the amygdala caused an increase in anxiety-like behavior on the EPM. It also brought about a reduction in morphine CPP scores and the stimulant effect of acute morphine administration relative to saline controls, without affecting CPP to cocaine. NK1 receptor-expressing neurons in the mouse amygdala therefore modulate morphine reward behaviors. These observations mirror those observed in NK1 receptor knock-out (NK1-/-) mice and suggest that the amygdala is an important area for the effects of SP and the NK1 receptor in the motivational properties of opiates, as well as the control of behaviors related to anxiety.

Substance P

Substance P is considered to be an important neuropeptide in nociceptive processes. Although substance P was described more than 60 years ago, there is still controversy about its exact role in nociception. This article reviews the current knowledge about the function of substance P in pain. Special emphasis is put on how to use this knowledge in the development of new ways to treat pain.

Estrogen modulation of neuropeptides: somatostatin, neurotensin and substance P, in the ventrolateral and arcuate nuclei of the female guinea pig

In the guinea pig, steroid target cells reside in the ventrolateral hypothalamic nucleus (VLH), an important site in the mediation of female receptive behavior, and in the arcuate nucleus (AR), a structure essential for stimulation effects of ovarian hormones on gonadotropin secretion. However, the mechanisms by which these steroid-dependent reproductive neuroendocrine processes occur are only partially understood. Estrogen is known to affect the hypothalamus content of certain neuropeptides. In the present study, we investigated the effects of estradiol benzoate (EB) on immunoreactivity of neurons containing one of three following neuropeptides: somatostatin (SOM), neurotensin (NT) and substance P (SP) in VLH and AR. The number of immunoreactive (IR)-neurons was quantified in anatomically matched sections through VLH and AR of ovariectomized (OVX), OVX + EB and OVX + oil-treated guinea pigs. Analysis of variance revealed that the number of SOM-IR and SP-IR neurons significantly increased in all regions of VLH of OVX + EB-treated guinea pigs as compared to OVX or OVX + oil-treated animals (P < 0.01) but showed no EB effect on the number of NT-IR neurons. Although the number of SOM-IR and NT-IR neurons slightly increased following treatment with EB in AR, analysis of variance revealed no significant change. The present results provide additional information relevant to possible involvement of these neuropeptides in facilitation of female typical sexual behavior.

Multiple peptides infrequently coexist in progesterone receptor-containing neurons in the ventrolateral hypothalamic nucleus of the guinea-pig: an immunocytochemical triple-label analysis of somatostatin, neurotensin and substance P

Progesterone plays an important role in regulating reproductive behaviour in guinea-pigs through actions exerted at the ventrolateral nucleus (VL), an area of the brain that contains progesterone receptors (PR) and neuroactive peptides, somatostatin (SOM), neurotensin (NT) and substance P (SP). Previous double-label analyses provided evidence that a substantial proportion of these neuropeptidergic cells contain PR. By means of triple-label immunofluorescence histochemistry, we examined whether PR are colocalized with two neuropeptides (SOM + NT or SP + SOM or SP + NT) within the same neurons in the VL. Ovariectomized guinea-pigs were primed with estradiol to induce PR immunoreactivity, and treated with colchicine to visualize immunoreactive (IR) neuropeptidergic cells. Both monoclonal mouse PR and polyclonal rabbit neuropeptide antibodies were used in double staining and in elution-restaining experiments. In the whole VL, the proportion of each coexisting peptide with PR obtained after double immunofluorescence appeared in decreasing order as: SOM (34%)>NT (25%)>SP (20%). Occasional colocalization was seen between PR and two neuropeptides throughout the rostrocaudal extent of the VL. Combining our various quantitative observations, we found that, of the total population of PR-IR neurons containing any combination of SOM, NT and SP, only about 1.5% contained SOM and NT, 2% contained SP and SOM and 1.6% contained SP and NT. These results indicate that while many PR-IR neurons also contain SOM or NT or SP in the guinea-pig VL, there may be very few PR-IR neurons that express more than one of these three peptides.

Behavioral responses to Depo-Provera, Fadrozole, and estradiol in castrated, testosterone-treated cynomolgus monkeys (Macaca fascicularis): the involvement of progestin receptors

Sexual motivation and behavior decreased in male cynomolgus monkeys given either Depo-Provera (medroxyprogesterone acetate, MPA), which reduces androgen uptake by brain, or the nonsteroidal aromatase inhibitor, Fadrozole, which virtually eliminates the conversion of testosterone (T) to estradiol (E2) in brain. This suggested that both unchanged T and E2 are important for the control of male primate sexual behavior, but combined treatment with MPA and Fadrozole did not have the anticipated summatory effects in intact males: the behavioral decrements when MPA-treated males were given Fadrozole were about half those observed when Fadrozole was given alone. The present study tested the hypothesis that Fadrozole suppressed the behavioral effects of MPA by preventing the induction by E2 of progestin receptors in the brain to which MPA binds. Eight castrated, T-treated males were each tested with an estrogenized female i) during baseline, ii) during MPA treatment, iii) during treatment with MPA and Fadrozole together, and iv) with E2 treatment added to condition (iii) (256 1-h behavior tests). All dosages were those used in previous studies. Sexual motivation, as reflected in mounting attempts and mounting attempt latencies, was further diminished by E2 treatment in males receiving both MPA and Fadrozole, but ejaculatory activity was not changed. Immunocytochemistry demonstrated that the distributions of progestin and androgen receptors were little affected by MPA treatment, and that progestin receptor immunoreactivity was almost completely abolished in the brains of males receiving both MPA and Fadrozole but present in those receiving additional E2 treatment, findings that supported the hypothesis.

Hypothalamic estrogen receptor-immunoreactivity in prepubertal vs adult female guinea pigs

Juvenile guinea pigs (18-20 days old) rarely display lordosis in response to estradiol and progesterone treatments that elicit sexual behavior in adult females. Nor do immature animals release a preovulatory-like surge of luteinizing hormone in response to estradiol. In vitro radioligand binding assays have revealed similar concentrations of estrogen receptors in the hypothalamus and preoptic area of prepubertal and adult guinea pigs. The aim of the present study was to compare estrogen receptor-immunoreactivity in a variety of forebrain regions of immature and adult guinea pigs, to determine whether age differences in estrogen receptor levels in more discrete portions of the hypothalamus and preoptic area exist. Forebrain tissue from juvenile (17 days) and adult females (> 6 weeks), ovariectomized 6 days previously, was processed for estrogen receptor-immunoreactivity, using Abbott Laboratories' H222 anti-human estrogen receptor antibody. Juveniles had estrogen receptor-immunoreactive cells in all of the same regions as adults: medial preoptic area, medial preoptic nucleus, bed nucleus of the stria terminalis, periventricular, paraventricular, dorsomedial and arcuate nuclei, ventrolateral and anterior hypothalamic regions, and amygdala. Among the areas in which estrogen receptor-immunoreactivity was quantified (medial preoptic area, medial preoptic nucleus, anterior periventricular nucleus, arcuate nucleus and ventrolateral hypothalamus), the only region in which an age difference in estrogen receptor-immunostaining was observed was the rostral portion of the ventrolateral hypothalamus. Juvenile females had, on average, 30% fewer estrogen receptor-immunoreactive cells in a sample of this region than adults (440 +/- 25 vs. 626 +/- 25, P = 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothalamic ovarian steroid hormone-sensitive neurons involved in female sexual behavior

Estradiol and progesterone regulate sexual behaviors in guinea pigs and rats, at least in part, through interaction with intracellular steroid hormone receptors. In the present review of work from the laboratory of the authors, we summarize recent work which has focused on one of the sites of hormone action in female guinea pigs--the ventrolateral hypothalamus. We summarize results of earlier experiments in which the regulation of steroid hormone receptors in this area was assessed after hormonal treatments with predictable effects on female sexual behavior. We then review the results of recent tract-tracing experiments in which we have examined the projections from the steroid receptor-immunoreactive neurons in this region, as well as the afferent projections from other neuroanatomical areas, including neurons which themselves contain estrogen receptors. We also present studies on the afferent input into these neurons by noradrenergic neurons and discuss the possibility that noradrenergic input influences steroid hormone sensitivity in these neurons. Finally, we discuss the results of experiments in which Fos-immunocytochemistry was used in rats to identify the neurons responding to a particular tactile stimulus associated with female reproduction, i.e., vaginal-cervical stimulation. These experiments further define a complex neuroanatomical system, in which many of the elements are estradiol or progesterone-responsive, that is involved in the hormonal regulation of sexual behavior in guinea pigs and rats.