Gonadal steroid-dependent GAL-IR cells within the medial preoptic nucleus (MPN) and the stimulatory effects of GAL within the MPN on sexual behaviors

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Sim1 Neurons Are Sufficient for MC4R-Mediated Sexual Function in Male Mice

Sexual dysfunction is a poorly understood condition that affects up to one-third of men around the world. Existing treatments that target the periphery do not work for all men. Previous studies have shown that central melanocortins, which are released by pro-opiomelanocortin neurons in the arcuate nucleus of the hypothalamus, can lead to male erection and increased libido. Several studies specifically implicate the melanocortin 4 receptor (MC4R) in the central control of sexual function, but the specific neural circuitry involved is unknown. We hypothesized that single-minded homolog 1 (Sim1) neurons play an important role in the melanocortin-mediated regulation of male sexual behavior. To test this hypothesis, we examined the sexual behavior of mice expressing MC4R only on Sim1-positive neurons (tbMC4Rsim1 mice) in comparison with tbMC4R null mice and wild-type controls. In tbMC4Rsim1 mice, MC4R reexpression was found in the medial amygdala and paraventricular nucleus of the hypothalamus. These mice were paired with sexually experienced females, and their sexual function and behavior was scored based on mounting, intromission, and ejaculation. tbMC4R null mice showed a longer latency to mount, a reduced intromission efficiency, and an inability to reach ejaculation. Expression of MC4R only on Sim1 neurons reversed the sexual deficits seen in tbMC4R null mice. This study implicates melanocortin signaling via the MC4R on Sim1 neurons in the central control of male sexual behavior.

Maternally involved galanin neurons in the preoptic area of the rat

Recent selective stimulation and ablation of galanin neurons in the preoptic area of the hypothalamus established their critical role in control of maternal behaviors. Here, we identified a group of galanin neurons in the anterior commissural nucleus (ACN), and a distinct group in the medial preoptic area (MPA). Galanin neurons in ACN but not the MPA co-expressed oxytocin. We used immunodetection of phosphorylated STAT5 (pSTAT5), involved in prolactin receptor signal transduction, to evaluate the effects of suckling-induced prolactin release and found that 76 % of galanin cells in ACN, but only 12 % in MPA were prolactin responsive. Nerve terminals containing tuberoinfundibular peptide 39 (TIP39), a neuropeptide that mediates effects of suckling on maternal motivation, were abundant around galanin neurons in both preoptic regions. In the ACN and MPA, 89 and 82 % of galanin neurons received close somatic appositions, with an average of 2.9 and 2.6 per cell, respectively. We observed perisomatic innervation of galanin neurons using correlated light and electron microscopy. The connection was excitatory based on the glutamate content of TIP39 terminals demonstrated by post-embedding immunogold electron microscopy. Injection of the anterograde tracer biotinylated dextran amine into the TIP39-expressing posterior intralaminar complex of the thalamus (PIL) demonstrated that preoptic TIP39 fibers originate in the PIL, which is activated by suckling. Thus, galanin neurons in the preoptic area of mother rats are innervated by an excitatory neuronal pathway that conveys suckling-related information. In turn, they can be topographically and neurochemically divided into two distinct cell groups, of which only one is affected by prolactin.

Galanin and Cognition

Since the neuropeptide galanin’s discovery in 1983, information has accumulated that implicates it in a wide range of functions, including pain sensation, stress responses, appetite regulation, and learning and memory. This article reviews the evidence for specific functions of galanin in cognitive processes. Consistencies as well as gaps in the literature are organized around basic questions of methodology and theory. This review shows that although regularities are evident in the observed behavioral effects of galanin across several methods for measuring learning and memory, generalization from these findings is tempered with concerns about confounds and a restricted range of testing conditions. Furthermore, it is revealed that many noncognitive behavioral constructs that are relevant for assessing potential roles for galanin in cognition have not been thoroughly examined. The review concludes by laying out how future theory and experimental work can overcome these concerns and confidently define the nature of the association of galanin with particular cognitive constructs.

Puberty onset in female rats: relationship with fat intake, ovarian steroids and the peptides, galanin and enkephalin, in the paraventricular and medial preoptic nuclei

Puberty is a time of rapid change, including a marked increase in fat consumption and body fat accrual, particularly in females. The mechanisms underlying these changes are unknown. Building on the results obtained in adult rats, the present study in pubertal rats focused on the orexigenic peptides, galanin (GAL) and enkephalin (ENK), in the paraventricular nucleus (PVN) and medial preoptic nucleus (MPN), which are known to be responsive to female steroids and have a role in both energy balance and reproductive function. The present study examined female rats maintained on pure macronutrient diets from before weaning (day 15) to day 70. After an initial burst in protein intake (days 21-35), rats showed an increase, specifically in preference for fat, from 15% to 30%. In rats examined at different ages before (day 30) and after (days 45 and 60) puberty, this rise in fat intake was associated with a marked increase, from days 30-45, in levels of oestradiol and progesterone and in GAL and ENK mRNA or peptide levels, specifically in the PVN and MPN, but not other hypothalamic areas examined. This positive relationship with increased fat intake, steroids and peptides across ages was also observed when comparing pubertal rats that naturally preferred fat (> 25% of total diet) with those consuming little fat (< 15%) or rats that reached puberty at an early age (days 30-34) with those that were late (days 37-40). These rats with early puberty onset exhibited a strong fat preference 3-4 days before vaginal opening, which was positively related to steroid levels, GAL, fat intake and body fat accrual after puberty. These findings suggest that, in addition to providing a signal for puberty onset, early fat ingestion acting through mechanisms involving the steroids and orexigenic peptides may be related to long-term patterns of eating and body weight regulation.

Increased caloric intake on a fat-rich diet: role of ovarian steroids and galanin in the medial preoptic and paraventricular nuclei and anterior pituitary of female rats

Previous studies in male rats have demonstrated that the orexigenic peptide galanin (GAL), in neurones of the anterior parvocellular region of the paraventricular nucleus (aPVN) projecting to the median eminence (ME), is stimulated by consumption of a high-fat diet and may have a role in the hyperphagia induced by fat. In addition to confirming this relationship in female rats and distinguishing the aPVN-ME from other hypothalamic areas, the present study identified two additional extra-hypothalamic sites where GAL is stimulated by dietary fat in females but not males. These sites were the medial preoptic nucleus (MPN), located immediately rostral to the aPVN, and the anterior pituitary (AP). The involvement of ovarian steroids, oestradiol (E(2)) and progesterone (PROG), in this phenomenon was suggested by an observed increase in circulating levels of these hormones and GAL in MPN and AP with fat consumption and an attenuation of this effect on GAL in ovariectomised (OVX) rats. Furthermore, in the same four areas affected by dietary fat, levels of GAL mRNA and peptide immunoreactivity were stimulated by E(2) and further by PROG replacement in E(2)-primed OVX rats and were higher in females compared to males. Because both GAL and PROG stimulate feeding, their increase on a fat-rich diet may have functional consequences in females, possibly contributing to the increased caloric intake induced by dietary fat. This is supported by the findings that PROG administration in E(2)-primed OVX rats reverses the inhibitory effect of E(2) on total caloric intake while increasing voluntary fat ingestion, and that female rats with higher GAL exhibit increased preference for fat compared to males. Thus, ovarian steroids may function together with GAL in a neurocircuit, involving the MPN, aPVN, ME and AP, which coordinate feeding behaviour with reproductive function to promote consumption of a fat-rich diet at times of increased energy demand.

Androgen receptor expressing neurons that project to the paraventricular nucleus of the hypothalamus in the male rat

Androgen receptors are distributed throughout the central nervous system and are contained by a variety of nuclei that are known to project to or regulate the paraventricular nucleus (PVN) of the hypothalamus, the final common pathway by which the brain regulates the hypothalamic-pituitary-adrenal (HPA) response to homeostatic threat. Here we characterized androgen receptor staining within cells identified as projecting to the PVN in male rats bearing iontophoretic or crystalline injections of the retrograde tracer FluoroGold aimed at the caudal two-thirds of the nucleus, where corticotropin-releasing hormone-expressing neurons are amassed. Androgen receptor (AR) and FluoroGold (FG) double labeling was revealed throughout the limbic forebrain, including scattered numbers of cells within the anterior and posterior subdivisions of the bed nuclei of the stria terminalis; the medial zone of the hypothalamus, including large numbers of AR-FG-positive cells within the anteroventral periventricular and medial preoptic cell groups. Strong and consistent colabeling was also revealed throughout the hindbrain, predominantly within the periaqueductal gray and the lateral parabrachial nucleus, and within various medullary cell groups identified as catecholaminergic, predominantly C1 and A1 neurons of the ventral medulla. These connectional data predict that androgens can act on a large assortment of multimodal inputs to the PVN, including those involved with the processing of various types of sensory and limbic information, and provide an anatomical framework for understanding how gonadal status could contribute to individual differences in HPA function.

Distribution of galanin and galanin transcript in the brain of a galanin-overexpressing transgenic mouse

The distribution of galanin mRNA-expressing cells and galanin-immunoreactive (IR) cell bodies and processes was studied in the brain of mice overexpressing galanin under the PDGF-B promoter (GalOE mice) and of wild type (WT) mice, both in colchicine-treated and non-treated animals. In this abstract, we only describe the results in GalOE mouse. A widespread ectopic expression of galanin (both mRNA and peptide) was found, that is a situation when neither transcript nor peptide could be seen in WT mice, not even after colchicine treatment. However, in some regions, such as claustrum, basolateral amygdala, thalamus, CA1 pyramidal cells, and Purkinje cells only galanin mRNA could be detected. In the forebrain galanin was seen in the mitral cells of the olfactory bulb, throughout the cortex, in the basolateral amygdaloid nucleus, claustrum, granular and pyramidal cell layers of the hippocampus, subiculum and presubiculum. In the thalamus, the anterodorsal, mediodorsal, intermediodorsal and mediodorsal lateral nuclei, the reuniens and reticular nuclei showed ectopic expression of galanin. Within the hypothalamus, neurons of the suprachiasmatic nucleus contained galanin. In the mesencephalon, the geniculate nucleus, nucleus ruber, the mesencephalic trigeminal and reticulotegmental nuclei ectopically expressed galanin. In the cerebellum, galanin was observed in the Purkinje cells and in the lateral and interposed cerebellar nuclei. In the pons, sensory and motor nuclei of the trigeminal nerve, the laterodorsal and dorsal tegmental nuclei, the pontine, reticulotegmental and gigantocellular reticular nuclei expressed galanin. Within the medulla oblongata, labeled cells were detected in the facial, ambiguus, prepositus, lateral paragigantocellular and lateral reticular nuclei, and spinal trigeminal nucleus. High densities of galanin-IR fibers were found in the axonal terminals of the lateral olfactory tract, the hippocampal and presumably the cerebellar mossy fibers system, in several thalamic and hypothalamic regions and the lower brain stem. Possible functional consequences of galanin overexpression are discussed.

Synthesis of galmic: a nonpeptide galanin receptor agonist

Galanin is a neuropeptide with a wide variety of biological functions. Few nonpeptide ligands, capable of activating galanin receptors, are available today. Based on known pharmacophores of galanin and the tripeptidomimetic galnon, a combinatorial library was formulated, synthesized, and screened against the galanin receptor. An active compound, galmic, was identified and tested in vitro and in vivo for its affinity and efficacy at galanin receptors. The present work describes the total synthesis of galmic, the synthesis of its oxazole precursors, the coupling of the building blocks into a linear trimer, and the macrolactamization reaction.

Galanin in pituitary adenomas

Tumor galanin content was measured in extracts from human pituitary adenomas using a specific RIA method for monitoring human galanin. Twenty-two out of twenty-four tumors contained galanin with notably high levels in corticotroph adenomas, varying levels in clinically inactive tumors, and low levels in GH secreting adenomas. Tumor galanin and ACTH contents were closely correlated in all tumors. In four young patients with microadenomas and highly active Mb Cushing tumor galanin was inversely related to tumor volume. The molecular form of tumor galanin, studied with reverse-phase HPLC, was homogeneous with the majority of tumor galanin coeluting with standard human galanin. In the tumors analysed with in situ hybridization there was a good correlation between galanin peptide levels and galanin mRNA expression. In some tumors galanin mRNA and POMC levels coexisted, in others they were essentially in different cell populations. Levels of plasma galanin-LI were not related to tumor galanin concentration, and galanin levels were in the same range in sinus petrosus close to the pituitary venous drainage as in peripheral blood. Corticotrophin releasing hormone injections in two patients caused ACTH, but no detectable galanin release into sinus petrosus. Our results demonstrate that corticotroph, but not GH adenomas, express high levels of galanin, in addition to ACTH, and that in some tumors both polypeptides are synthesised in the same cell population. However, galanin levels in plasma were not influenced by the tumor galanin content.

Development of galanin-like immunoreactivity in the brain of the brown trout (Salmo trutta fario), with some observations on sexual dimorphism

The development of galanin-like immunoreactive (GAL-ir) cells and fibers was investigated in the brain of brown trout embryos, alevins, juveniles, and adults (some spontaneously releasing their gametes). The earliest GAL-ir neurons appeared in the preoptic region and the primordial hypothalamic lobe of 12-mm embryos. After hatching, new GAL-ir neurons appeared in the lateral, anterior, and posterior tuberal nuclei, and in late alevins, GAL-ir neurons appeared in the area postrema. In juveniles, further GAL-ir populations appeared in the nucleus subglomerulosus and magnocellular preoptic nucleus. The GAL-ir neuronal groups present in juveniles were also observed in sexually mature adults, although the area postrema of males lacked immunoreactive neurons. Moreover, spawning males exhibited GAL-ir somata in the olfactory bulb and habenula, which were never observed in adult females or in developing stages. In adults, numerous GAL-ir fibers were observed in the ventral telencephalon, preoptic area, hypothalamus, neurohypophysis, mesencephalic tegmentum, ventral rhombencephalon, and area postrema. Moderate to low GAL-ir innervation was seen in the olfactory bulbs, dorsomedial telencephalon, epithalamus, medial thalamus, optic tectum, cerebellum, and rhombencephalic alar plate. There were large differences among regions in the GAL-ir innervation establishment time. In embryos, GAL-ir fibers appeared in the preoptic area and hypothalamus, indicating early expression of galanin in hypophysiotrophic centers. The presence of galanin immunoreactivity in the olfactory, reproductive, visual, and sensory-motor centers of the brain suggest that galanin is involved in many other brain functions. Furthermore, the distribution of GAL-ir elements observed throughout trout development indicates that galaninergic system maturation continues until sexual maturity.

Restricted distribution of galanin receptor 3 (GalR3) mRNA in the adult rat central nervous system

Recent molecular cloning studies have established the existence of a third rat galanin receptor subtype, GalR3, however its precise distribution in the mammalian central nervous system (CNS) is not well established. In the present study, we examined the regional and cellular distribution of GalR3 mRNA in the CNS of the rat by in situ hybridization. Our findings indicate that GALR3 mRNA expression in the rat brain is discrete and highly restricted, concentrated mainly in the preoptic/hypothalamic area. Within the hypothalamus, GalR3 expression was confined to the paraventricular, ventromedial and dorsomedial hypothalamic nuclei. In addition to these hypothalamic nuclei, GalR3 mRNA-expressing cells were observed in the medial septum/diagonal band of Broca complex, the bed nucleus of the stria terminalis, the medial amygdaloid nucleus, the periaqueductal gray, the lateral parabrachial nucleus, the dorsal raphe nucleus, the locus coeruleus, the medial medullary reticular formation and in one of the circumventricular organs, the subfornical organ. In the spinal cord, a faint but specific ISH signal was observed over the laminae I-II with a few moderately labeled cells distributed in laminae V and X. The neuroanatomical distribution of GalR3 suggests it might be involved in mediating documented effects of galanin on food intake, fluid homeostasis, cardiovascular function and nociception.

Anticonvulsant activity of a nonpeptide galanin receptor agonist

Galanin is a neuropeptide with a wide variety of biological functions, including that of a strong endogenous anticonvulsant. No nonpeptide ligands, capable of activating galanin receptors, are available today. Based on known pharmacophores of galanin, a combinatorial library was designed, synthesized, and screened at the rat hippocampal galanin receptor. A low molecular weight galanin receptor agonist, 7-((9-fluorenylmethoxycarbonyl)cyclohexylalanyllysyl)amino-4-methylcoumarin (galnon) was found to displace (125)I-galanin with micromolar affinity at Bowes cellular and rat hippocampal membranes. Autoradiographic binding assay on rat spinal cord sections confirmed the ability of galnon to displace (125)I-galanin from its binding sites. Galnon inhibited adenylate cyclase activity, suggesting an agonist action at galanin receptors. When injected i.p. galnon reduced the severity and increased the latency of pentylenetetrazole-induced seizures in mice and reversed the proconvulsant effects of the galanin receptor antagonist M35, injected into a lateral ventricle. Intrahippocampal injection of galnon also shortened the duration of self-sustaining status epilepticus in rats, confirming its agonist properties in vivo. Pretreatment of rats with antisense peptide nucleic acid targeted to galanin receptor type 1 mRNA abolished the effect of galnon, suggesting mediation of its anticonvulsant properties through this receptor subtype. These findings introduce a systemically active nonpeptide galanin agonist anticonvulsant.

Inhibition of lordosis behavior in male and female rats by androgens and progesterone

Several studies suggest that when manipulated experimentally in adulthood, the lordosis response to estrogen can be increased dramatically in male rats. Because adult-gonadectomized (Gx) animals were used in these studies, the lack of testicular hormones in adulthood may have been a factor. To examine this possibility, adult-Gx rats were implanted with blank (Bk)-, testosterone (T)-, 5alpha-dihydrotestosterone (DHT)-, or progesterone (P)-filled capsules, alone or in combination. We report a new finding, that a combined treatment of T plus P (T+P) at physiological doses for the male, but not T or P alone, reduced lordosis significantly in males, with and without estrogen priming. T+P did not inhibit lordosis in females, nor did this specific treatment affect open field, aggressive, and male copulatory behaviors. In confirming studies done with much higher doses, DHT reduced lordosis in both sexes. DHT and T+P also reduced lordosis in adrenalectomized/Gx males. Mechanisms responsible for the T+P inhibition of lordosis in males are not known, but they may include an upregulation of androgen receptors by P, and this possibility is discussed.

Strain and sex differences in the morphology of the medial preoptic nucleus of mice

The medial preoptic nucleus (MPO), which is involved in sexual and maternal behaviors, contains neuronal clusters that have been described as being sexually dimorphic in size and neuropeptide content in a variety of species. A subnucleus in DBA/2J (D2) inbred mice, called the pars compacta of the MPO (MPOpc), is absent in C57BL/6J (B6) inbred mice (Robinson et al. [1985] J. Neurogenet. 2:381-388). We report here on experiments that further characterize strain and sex differences in medial preoptic morphology in D2 and B6 inbred mice. A prominent MPOpc, located within the caudal part of the MPO and dorsal to the suprachiasmatic nucleus, was present in both male and female D2 animals but was absent from B6 animals. MPOpc neurons were darkly stained for Nissl substance and larger than neurons in the surrounding MPO. In D2 brains, galanin-immunoreactive (-ir), oxytocin-ir, vasopressin-ir, and NADPH diaphorase-positive neurons were concentrated within the MPOpc. Fewer similar neurons in the comparable region of the MPO of B6 mice suggests that the absence of the MPOpc is due to absence of these neurons rather than a less compact organization. In D2 animals, the density of galanin-ir neurons in the MPOpc was sexually dimorphic, with higher numbers of galanin-ir neurons in females. Strain differences in galanin-ir, oxytocin-ir, vasopressin-ir, and NADPH diaphorase staining appeared to be limited to the MPOpc. Cholecystokinin-immunoreactive neurons, which have been reported to be numerous in the sexually dimorphic central subdivision of the MPO of rats, were sparse in the MPO of D2 and B6 mice. Confirmation of the MPOpc as an accessory magnocellular neurosecretory nucleus was obtained by finding labeling of MPOpc neurons after injection of DiI into the posterior pituitary.