Distribution of galanin-immunoreactive cells within sexually dimorphic components of the medial preoptic area of the male and female rat

Neuronal activation of the sexually dimorphic nucleus of the preoptic area in female and male rats during copulation and its sex differences

The medial preoptic area, which plays an essential role in the control of sexual behavior in rats, contains a sexually dimorphic nucleus that consists of neurons expressing calbindin-D28 K (Calb) that is referred to as the CALB-SDN. The CALB-SDN is larger and contains more Calb neurons in males than in females. The physiological functions of the CALB-SDN are not fully understood; however, CALB-SDN neurons are activated during sexual behavior in males, suggesting that the male CALB-SDN is involved in regulation of sexual behavior. However, no information exists about the physiological functions of the female CALB-SDN. In the present study, we performed an immunohistochemical analysis of c-Fos, a neuronal activity marker, in the CALB-SDN of female and male rats that had copulated with conspecifics of the opposite sex to determine whether neurons of the female CALB-SDN are activated during copulation and whether the neuronal activity of the CALB-SDN differs between sexes. The numbers of c-Fos-immunoreactive cells with or without Calb-immunoreactivity (c-Fos⁺/Calb⁺ and c-Fos⁺/Calb^- cells) were greater in the CALB-SDN of rats that had copulated than in rats that had not copulated in each sex. Although the number of Calb⁺ cells in the CALB-SDN was smaller in females than in males, the increase in the number of c-Fos⁺/Calb⁺ cells in the female CALB-SDN with copulation was comparable to that in the male CALB-SDN with copulation. The increase in the number of c-Fos⁺/Calb^- cells in the CALB-SDN with copulation was more prominent in males than in females. These results suggest that CALB-SDN neurons are activated during copulation in both sexes. The patterns of neuronal activation in the CALB-SDN during copulation may differ between sexes.

Male-predominant galanin mediates androgen-dependent aggressive chases in medaka

Recent studies in mice demonstrate that a subset of neurons in the medial preoptic area (MPOA) that express galanin play crucial roles in regulating parental behavior in both sexes. However, little information is available on the function of galanin in social behaviors in other species. Here, we report that, in medaka, a subset of MPOA galanin neurons occurred nearly exclusively in males, resulting from testicular androgen stimulation. Galanin-deficient medaka showed a greatly reduced incidence of male-male aggressive chases. Furthermore, while treatment of female medaka with androgen induced male-typical aggressive acts, galanin deficiency in these females attenuated the effect of androgen on chases. Given their male-biased and androgen-dependent nature, the subset of MPOA galanin neurons most likely mediate androgen-dependent male-male chases. Histological studies further suggested that variability in the projection targets of the MPOA galanin neurons may account for the species-dependent functional differences in these evolutionarily conserved neural substrates.

Gender differences in neurodevelopment and epigenetics

The concept that the brain differs in make-up between males and females is not new. For example, it is well established that anatomists in the nineteenth century found sex differences in human brain weight. The importance of sex differences in the organization of the brain cannot be overstated as they may directly affect cognitive functions, such as verbal skills and visuospatial tasks in a sex-dependent fashion. Moreover, the incidence of neurological and psychiatric diseases is also highly dependent on sex. These clinical observations reiterate the importance that gender must be taken into account as a relevant possible contributing factor in order to understand the pathogenesis of neurological and psychiatric disorders. Gender-dependent differentiation of the brain has been detected at every level of organization--morphological, neurochemical, and functional--and has been shown to be primarily controlled by sex differences in gonadal steroid hormone levels during perinatal development. In this review, we discuss howthe gonadal steroid hormone testosterone and its metabolites affect downstream signaling cascades, including gonadal steroid receptor activation, and epigenetic events in order to differentiate the brain in a gender-dependent fashion.

Ventromedial and medial preoptic hypothalamic ibotenic acid lesions potentiate systemic morphine analgesia in female, but not male rats

Sex differences in systemic morphine analgesia occur with male rodents displaying significantly greater analgesic magnitudes and potencies than females. Neonatal androgenization, and to a lesser degree, adult ovariectomy enhance systemic morphine analgesia in female rats, implicating both organizational and activational effects of gonadal hormones. The neuroanatomical circuits sensitive to sex-related hormones by which females display a smaller opiate analgesic effect is not clear, but the ventromedial (VMH) and medial preoptic (MPOA) hypothalamic nuclei are critical in the monitoring of estradiol and other sex hormone levels. To assess the contribution of these nuclei to sex and adult gonadectomy differences in systemic morphine analgesia, intact male, intact female and adult ovariectomized (OVEX) female rats received bilateral saline (SAL) or ibotenic acid (IBO) microinjections into either the VMH or MPOA. Following surgeries, baseline tail-flick latencies over 120 minutes (min) were assessed over 4 days in all nine groups with intact females tested in the estrus phase of their cycle. All animals then received an ascending series of morphine (1.0, 2.5, 5.0, 7.5, 10.0mg/kg) injections 30min prior to the tail-flick test time course with 8-12 day inter-injection intervals between doses. Baseline latencies failed to differ between SAL-treated intact males and females, but were significantly higher in SAL-treated OVEX females. Both VMH IBO and MPOA IBO lesions increased baseline latencies in intact male and female rats, but not in OVEX females. SAL-treated intact males (ED(50)=4.0mg/kg) and SAL-treated OVEX females (ED(50)=3.5mg/kg) displayed significantly greater potencies of systemic morphine analgesia than SAL-treated intact females (ED(50)=6.3mg/kg), confirming previous gender and gonadectomy differences. Neither VMH IBO (ED(50)=3.7 mg/kg) nor MPOA IBO (ED(50)=4.1mg/kg) males differed from SAL-treated males in the potency of systemic morphine analgesia. In contrast, VMH IBO (ED(50)=4.1mg/kg) and MPOA IBO (ED(50)=3.5mg/kg) intact females displayed significantly greater potencies in systemic morphine analgesia than SAL-treated intact females. However, VMH IBO OVEX (ED(50)=3.5mg/kg) and MPOA IBO OVEX (ED(50)=3.9 mg/kg) failed to differ from SAL-treated OVEX females in the potency of systemic morphine analgesia. The magnitudes of systemic morphine analgesia as measured by Maximum Percentage Effect values displayed similar patterns, but lesser degrees, of effects. These data suggest that VMH and MPOA nuclei act to tonically inhibit endogenous pain-inhibitory circuits in the intact female, but not intact male brain, and that removal of circulating gonadal hormones by OVEX and/or excitotoxic destruction of these estrogen receptor accumulating nuclei disinhibit the female analgesic response to systemic morphine.

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.

Neuronal nitric oxide synthase and calbindin delineate sex differences in the developing hypothalamus and preoptic area

Throughout the hypothalamus there are several regions known to contain sex differences in specific cellular, neurochemical, or cell grouping characteristics. The current study examined the potential origin of sex differences in calbindin expression in the preoptic area and hypothalamus as related to sources of nitric oxide. Specific cell populations were defined by immunoreactive (ir) calbindin and neuronal nitric oxide synthase (nNOS) in the preoptic area/anterior hypothalamus (POA/AH), anteroventral periventricular nucleus (AVPv), and ventromedial nucleus of the hypothalamus (VMN). The POA/AH of adult mice was characterized by a striking sex difference in the distribution of cells with ir-calbindin. Examination of the POA/AH of androgen receptor deficient Tfm mice suggests that this pattern was in part androgen receptor dependent, since Tfm males had reduced ir-calbindin compared with wild-type males and more similar to wild-type females. At P0 ir-calbindin was more prevalent than in adulthood, with males having significantly more ir-calbindin and nNOS than have females. Cells that contained either ir-calbindin or ir-nNOS in the POA/AH were in adjacent cell groups, suggesting that NO derived from the enzymatic activity of nNOS may influence the development of ir-calbindin cells. In the region of AVPv, at P0, there was a sex difference with males having more ir-nNOS fibers than have females while ir-calbindin was not detected. In the VMN, at P0, ir-nNOS was greater in females than in males, with no significant difference in ir-calbindin. We suggest that NO as an effector molecule and calbindin as a molecular biomarker illuminate key aspects of sexual differentiation in the developing mouse brain.

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.

Sexually dimorphic immunoreactivity of galanin and colocalization with arginine vasotocin in the chicken brain (Gallus gallus domesticus)

The bed nucleus of the stria terminalis medialis (BSTM) of adult chickens (Gallus gallus domesticus) was previously shown to synthesize arginine vasotocin (AVT) in males only and coincides spatially and temporally with steroid activity regulating male reproductive behavior. Galanin has been shown to be a potent modulator of the behavioral and neuroendocrine responses in the mammalian BSTM and in other sexually dimorphic brain regions. In the present study of adult chickens the morphological relationship of AVT and galanin was examined by immunohistochemical analysis of two limbic structures, the BSTM and the lateral septum (SL). The analysis also included the hypothalamic nuclei supraopticus (SON) and paraventricularis (PVN). In males galanin and AVT were both synthesized in the BSTM, while in females neither galanin nor AVT was present. Furthermore, in the males galanin and AVT were colocalized in the majority of neurons within BSTM and in fibers of the SL. In both sexes galanin neurons in the PVN were scattered between the distinct clusters of AVT neurons and there was no colocalization of galanin and AVT in single PVN neurons. Furthermore, AVT immunoreactivity was significantly higher in the SON than in the PVN in both sexes. In the SON, galanin was colocalized with AVT in significantly more neurons in hens than in males (P </= 0.05%). These results demonstrate that the distributions of galanin and AVT are sexually dimorphic not only in the limbic BSTM but also in the hypothalamic SON. It is tempting to speculate that galanin in the SON is involved in regulation of oviposition as an AVT-dependent female-specific function.

Protection from neuronal damage evoked by a motivational excitation is a driving force of intentional actions

Motivation may be understood as an organism's subjective attitude to its current physiological state, which somehow modulates generation of actions until the organism attains an optimal state. How does this subjective attitude arise and how does it modulate generation of actions? Diverse lines of evidence suggest that elemental motivational states (hunger, thirst, fear, drug-dependence, etc.) arise as the result of metabolic disturbances and are related to transient injury, while rewards (food, water, avoidance, drugs, etc.) are associated with the recovery of specific neurons. Just as motivation and the very life of an organism depend on homeostasis, i.e., maintenance of optimum performance, so a neuron's behavior depends on neuronal (i.e., ion) homeostasis. During motivational excitation, the conventional properties of a neuron, such as maintenance of membrane potential and spike generation, are disturbed. Instrumental actions may originate as a consequence of the compensational recovery of neuronal excitability after the excitotoxic damage induced by a motivation. When the extent of neuronal actions is proportional to a metabolic disturbance, the neuron theoretically may choose a beneficial behavior even, if at each instant, it acts by chance. Homeostasis supposedly may be directed to anticipating compensation of the factors that lead to a disturbance of the homeostasis and, as a result, participates in the plasticity of motivational behavior. Following this line of thought, I suggest that voluntary actions arise from the interaction between endogenous compensational mechanisms and excitotoxic damage of specific neurons, and thus anticipate the exogenous compensation evoked by a reward.

Sex differences in the location of immunochemically defined cell populations in the mouse preoptic area/anterior hypothalamus

The preoptic area/anterior hypothalamus (POA/AH) is sexually dimorphic in many vertebrates. We have defined specific cell populations within the POA/AH using immunocytochemical markers for estrogen receptor beta (ERbeta) and the R1 subunit of the GABA(B) receptor (GABA(B)R1). Our previous finding of sex differences in cell migration in this region in embryonic day 15 mice led us to examine sex differences in the location or size of chemically identified cell groups. At embryonic day 17 (E17), cells containing immunoreactive (ir) ERbeta in females were located more dorsal and lateral than those in males. In contrast to this positional sex difference seen at E17, ERbeta expression at P0 and adulthood showed a sex difference in cell number and area of immunoreactivity with a higher expression of ERbeta in males than females. Furthermore, in animals that were genetically deprived of gonadal and adrenal hormones by virtue of a disrupted gene coding for steroidogenic factor 1, cells containing ir ERbeta followed a female phenotype for location at E17 and a female phenotype for number of ir cells at P0 regardless of genetic sex, suggesting that circulating hormones may be influencing cell position in the POA/AH. A second phenotypically identified cell group containing ir GABA(B)R1 also had a sex difference in cell positions at E17. Females expressed GABA(B)R1 in cells with a more dorsal position than in males. These results provide support for the suggestion that sex differences in cellular organization in the developing hypothalamus arise from sex differences in cell migration.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Immunoreactive galanin expression in ovine gonadotropin-releasing hormone neurones: no effects of gender or reproductive status

The neuropeptide, galanin, has been implicated to play a significant role in numerous physiological functions, including reproduction. Studies on several species have shown that galanin enhances gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone secretion. In rodents, a subset of GnRH neurones expresses galanin in a sexually dimorphic manner and it has been suggested that this may underpin the differences in GnRH secretion observed between the sexes. However, there are few data available for other species. Previous studies in sheep have shown that the distribution of GnRH neurones overlaps with galanin cells. The primary objectives of our study were to determine whether GnRH and galanin coexist in the sheep brain and, importantly, if a sex difference is apparent in the colocalization of these two peptides. Using immunocytochemistry coupled to high temperature antigen retrieval, we found that all GnRH neurones in the ovine brain colocalize with galanin. There is also a distinct population of galanin neurones that do not secrete GnRH. In addition, the distribution of galanin-immunoreactive cells was similar to that previously reported for colchicine treated ewes and, in agreement with earlier studies, the number of GnRH neurones did not differ between rams and ewes or between ewes killed at different stages of the oestrous cycle. These results suggest that, in sheep, GnRH and galanin may be cosecreted but the functional significance of this coexpression and possible cosecretion remains to be elucidated.

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.

Sexual differentiation of the human hypothalamus

Functional sex differences in reproduction, gender and sexual orientation and in the incidence of neurological and psychiatric diseases are presumed to be based on structural and functional differences in the hypothalamus and other limbic structures. Factors influencing gender, i.e., the feeling to be male or female, are prenatal hormones and compounds that change the levels of these hormones, such as anticonvulsants, while the influence of postnatal social factors is controversial. Genetic factors and prenatal hormone levels are factors in the determination of sexual orientation, i.e. heterosexuality, bisexuality or homosexuality. There is no convincing evidence for postnatal social factors involved in the determination of sexual orientation. The period of overt sexual differentiation of the human hypothalamus occurs between approximately four years of age and adulthood, thus much later than is generally presumed, although the late sexual differentiation may of course be based upon processes that have already been programmed in mid-pregnancy or during the neonatal period. The recently reported differences in a number of structures in the human hypothalamus and adjacent structures depend strongly on age. Replication of these data is certainly necessary. Since the size of brain structures may be influenced by premortem factors (e.g. agonal state) and postmortem factors (e.g. fixation time), one should not only perform volume measurements, but also estimate a parameter that is not dependent on such factors as, i.e., total cell number of the brain structure in question. In addition, functional differences that depend on the levels of circulating hormones in adulthood have been observed in several hypothalamic and other brain structures. The mechanisms causing sexual differentiation of hypothalamic nuclei, the pre- and postnatal factors influencing this process, and the exact functional consequences of the morphological and functional hypothalamic differences await further elucidation.

Projections of the sexually dimorphic calcitonin gene-related peptide neurons of the preoptic area determined by retrograde tracing in the female rat

The medial preoptic area of the rat exhibits morphologic sex differences and is implicated in the control of sexually dimorphic behavior and function. Neurons expressing calcitonin gene-related peptide (CGRP) within the anteroventral periventricular (AVPV) and medial preoptic nucleus (MPN) of the medial preoptic area exhibit female-dominant sex differences in number through organizational and activational effects of gonadal steroids. The present study used retrograde tracing experiments to establish the projections of the AVPV and MPN CGRP neurons in the female rat. After the intraperitoneal administration of Fluoro-Gold to female rats (n = 5), we were unable to detect retrograde tracer in any CGRP-immunoreactive cells of the hypothalamus. Intracerebral injections of 50- to 100-nl volumes of Fluoro-Gold into the mediobasal hypothalamus resulted in up to 70% of CGRP neurons in the AVPV and MPN containing retrograde tracer. Similar large volume tracer depositions in the lateral septum, periaqueductal gray, two likely CGRP projection sites, resulted in no labeling of preoptic CGRP neurons. Experiments using small volume (30-nl) injections of Fluoro-Gold and green fluorescent microspheres at multiple sites in the mediobasal hypothalamus (n = 18) revealed that approximately 60% of AVPV and 30% of MPN neurons expressing CGRP were projecting to the region of the tuberal and ventral premammillary nuclei, with a minor projection to the dorsomedial nucleus. These findings demonstrate a major projection of the preoptic CGRP neurons to the posterior hypothalamus in the female rat and support further a functional role for these neurons in the sexually dimorphic regulation of reproductive functioning.

Sexually dimorphic activation of galanin neurones in the ferret's dorsomedial preoptic area/anterior hypothalamus after mating

Male ferrets in breeding condition possess three times as many galanin-immunoreactive (IR) neurones as oestrous females in the sexually dimorphic dorsomedial preoptic area/anterior hypothalamus (dmPOA/AH). Using Fos-IR as a marker of activation, we investigated whether mating with intromission differentially activates this sexually dimorphic group of galanin-IR neurones in male and female ferrets. Male ferrets that intromitted had a significantly greater percentage of galanin-IR neurones in the dmPOA/AH that were colabelled with nuclear Fos-IR than oestrous females that received an intromission. Intromissive stimulation augmented Fos-IR in an equal percentage of galanin-IR neurones in both sexes in the medial amygdala (MA) and bed nucleus of the stria terminalis (BNST). Peripheral anosmia induced by bilateral occlusion of males' nares did not reduce the mating-induced activation of galanin-IR neurones in the dmPOA/AH, and there was a significant correlation among individual males between intromission duration and the percentage of dmPOA/AH galanin-IR neurones colabelled with Fos-IR. Exposure of castrated, testosterone propionate-treated male ferrets to either soiled bedding or to volatile odours from oestrous females failed to induce nuclear Fos-IR in galanin-IR neurones located in the dmPOA/AH, BNST or MA, suggesting that the mating-induced activation of galanin-IR forebrain neurones in male ferrets depends more on genital-somatosensory than on olfactory inputs. The observed sex dimorphism in the mating-induced activation of galanin-IR neurones in the dmPOA/AH raises the possibility that these neurones perform a mating-dependent function that occurs only in males.

Sexually dimorphic effects of testosterone on preoptic area calcitonin gene-related peptide mRNA expression depend upon neuron location and differential estrogen and androgen receptor activation

Experiments examined activational roles of gonadal steroids on the sexually dimorphic, calcitonin gene-related peptide-expressing neurons of the rat preoptic area. Gonadectomy of male rats followed by treatment with testosterone, dihydrotestosterone, or estrogen demonstrated that the tonic suppressive influence of testosterone on cellular levels of calcitonin gene-related peptide mRNA expression in the medial preoptic nucleus and anteroventral periventricular nucleus occurred through either ER- or AR-mediated mechanisms (P < 0.05). The gonadectomy of adult female rats demonstrated little tonic influence of ovarian steroids upon calcitonin gene-related peptide mRNA levels. However, the administration of male levels of testosterone to ovariectomized rats resulted in reduced calcitonin gene-related peptide mRNA expression within the medial preoptic nucleus (P < 0.05) and, strikingly, a 3-fold induction in calcitonin gene-related peptide mRNA expression in the anteroventral periventricular nucleus (P < 0.01). Testosterone's effects in the medial preoptic nucleus and anteroventral periventricular nucleus of the female required both ER and AR activation. Dual labeling immunocytochemical studies revealed that less than 10% of calcitonin gene-related peptide neurons in the male expressed ARs compared with approximately 50% in the female. These investigations reveal that sexually differentiated region- and steroid receptor-specific mechanisms function in association with the sex differences in circulating gonadal steroids to maintain the sexually dimorphic nature of calcitonin gene-related peptide expression in the preoptic area of the adult rat.

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.

Galanin immunoreactivity in mouse basal forebrain: sex differences and discrete projections of galanin-containing cells beyond the blood-brain barrier

The distribution of galanin-immunoreactive (GAL-IR) cell bodies in the basal forebrain of mice was investigated. The overall pattern of staining for GAL in the area of brain analyzed was similar to that reported in other species with noticeable variations. Distinctive groups of GAL-IR cells were present in the bed nucleus of stria terminalis (BNST), supraoptic nucleus, retrochiasmatic supraoptic nucleus (SOR), magnocellular paraventricular nucleus, arcuate nucleus (ARC) and the nucleus circularis which is one of the cell groups belonging to the accessory magnocellular system. Comparison of the number of GAL-IR cells between the sexes indicated sexual dimorphism in the BNST, SOR and the ARC. As compared with female mice, the mean number of GAL-IR cells/section in the BNST and the SOR was higher and that in the ARC was lower in the males. Unlike in rats, the preoptic area contained mostly scattered GAL-IR cell bodies. Intraperitoneal injection of the retrograde tracer fluoro-gold in male mice resulted in uptake of fluoro-gold by selective GAL-IR cell groups in the basal forebrain suggesting that only some of these cell groups may project outside the blood-brain barrier whereas others may be involved in intracerebral neural transmission.

The preoptic area/anterior hypothalamus of different strains of mice: sex differences and development

While sex differences in neural morphology in the preoptic area/anterior hypothalamus (POA/AH) have been demonstrated in many species, their existence in mice have been controversial. Given the increased use of transgenic and gene-disrupted mice, we characterized sex differences using Nissl stains, and the immunocytochemical location of estrogen receptor-alpha (ER-alpha) and galanin in the POA/AH of two widely used strains, C57BL/6 and 129SvEv, and a mixed strain (C57BL/6x129Sv); the wild-type littermates of steroidogenic factor-1 (SF-1) gene-disrupted mice. Cell grouping was not a reliable marker of sex. In adults, cells located beneath the anterior commissure (AC) were reliably larger in females than males in 129SvEv, but not in the other strains. Caudally, cells in a group medial to the medial extension of the bed nucleus of the stria terminalis (BST) were significantly larger in males than females in C57BL/6J and SF-1 gene-disrupted wild-types. Cell groups discernible by embryonic day (E) 18 were not sexually dimorphic for cell size in C57BL/6J mice at E18 or postnatal day (P) 4. The pattern of distribution of cells containing ER-alpha was similar among the strains, reduced in the group medial to the BST; a pattern established by P0. Galanin-containing cells and fibers were seen from E15 to adulthood ventral to the AC. Caudally, a smaller group ventromedial to the BST was found only in 129SvEv adults. Sex differences in neural morphology which develop within the POA/AH depend upon multiple factors, particularly including genetic background.

Evidence for oestrogen receptor alpha-immunoreactivity in gonadotrophin-releasing hormone-expressing neurones

Having used the cingulate cortex to demonstrate the validity of our methods for detecting hitherto unrecognized oestrogen receptor alpha (ERalpha)-immunoreactive neurones, we have now employed immunoprecipitation and double-label immunohistochemistry to investigate whether the ERalpha protein is present in gonadotrophin-releasing hormone (GnRH)-containing cells. The immortalized GnRH cell line GT1-7 and GnRH neurones within the rat preoptic area were found to possess ERalpha-immunoreactivity (ERalpha-IR). These observations indicate that oestrogen may regulate the synthesis and release of GnRH by direct actions on GnRH neurones.

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

More GAL-I cells exist within sexually dimorphic cell groups of the medial preoptic nucleus (MPN) in male rate than females, a large percentage of estrogen-concentrating cells within MPN cell groups are also GAL-immunoreactive (GAL-IR), and significantly more GAL-IR cells are visible with estrogen or its precursor, testosterone. Gonadal steroids also increase the size (diameter) of MPN GAL-IR cells and the number of GAL-IR cell processes within a portion of the MPN called the "GAL-IR MPOA plexus," which exists in males only. GAL microinjected into the MPN stimulated male-typical sexual behaviors, with more testosterone required in females than males. Immunoneutralization with anti-GAL serum inhibited male-typical sexual behavior, indicating a role for endogenous GAL within the MPN. Microinjection of GAL into the MPN also stimulated female-typical sexual behaviors in estrogen-treated females and males, and GAL within the MPN dramatically overrode an inhibition of lordosis by dihydrotestosterone in rats of both sexes.

Differential functions of hypothalamic galanin cell grows in the regulation of eating and body weight

Evidence suggests that hypothalamic galanin (GAL) has a variety of functions related to energy and nutrient balance, reproduction, water balance, and neuroendocrine regulation. The focus of this chapter is the role of GAL in eating and body weight regulation. Findings described herein demonstrate that GAL, in a cell group of the anterior region of the paraventricular nucleus (aPVN) that projects to the median eminence, has a role in the control of fat intake, fat metabolism, and body fat. This function of aPVN GAL neurons is carried out in close relation to circulating insulin and glucose. Galanin-expressing perikarya in the medial preoptic area (MPOA) have a similar function, although GAL here operates in association with the female steroids estrogen and progesterone. These GAL cell groups of the aPVN and MPOA contrast with those in the arcuate nucleus as well as the magnocellular vasopressin-containing neurons of the PVN and supraoptic nucleus, which show no relation to fat balance. This evidence reveals differential functions for the distinct GAL neuronal cell groups of the hypothalamus.

Effects of estrous cyclicity on the expression of the galanin receptor Gal-R1 in the rat preoptic area: a comparison with the male

Variations in the number of galanin receptor (Gal-R1)-expressing cells and levels of Gal-R1 messenger RNA (mRNA) were determined in the preoptic area in intact female rats throughout the phases of the estrous cycle and compared with those in the male. Female and male Wistar rats were fixed by perfusion with 4% paraformaldehyde. Cryostat sections were hybridized with a 35S-labeled antisense Gal-R1 riboprobe. The number of Gal-R1 mRNA-expressing cells was lower in the rostral preoptic area than in the medial preoptic area. During the estrous cycle, the highest number of Gal-R1 mRNA-expressing cells in the rostral preoptic region was detected at 0800 h on proestrus, whereas in the medial preoptic area, the maximum number was observed at 1800 h on estrus. Gal-R1 mRNA levels in individual cells were low during diestrus and increased at estrus in both areas. In the male, the number of mRNA-expressing cells and the hybridization signal were significantly lower than those in females during estrus. The results demonstrate that Gal-R1 gene expression in the preoptic area varies during the estrous cycle and is low in males. Short term treatment of ovariectomized rats with estradiol plus progesterone caused significantly decreased preoptic Gal-R1 mRNA levels compared with those after treatment with estrogen only. These observations suggest that in the preoptic area, expression of Gal-R1 is influenced by progesterone. The variation in Gal-R1 expression is likely to influence the extent to which galanin can influence the preoptic cells implicated in the control of neighboring GnRH cells.

The sexually dimorphic nucleus of the hypothalamus contains GABA neurons in rat and man

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is the most striking structure displaying a morphological sex difference in the rat brain. A potentially homologous nucleus has been identified in the human hypothalamus. The objective of the present study was to pursue the putative homology of the rat and human SDN-POA by determining whether they express the same transmitter phenotype. We employed in situ hybridization histochemistry for GAD mRNA to show whether the neurons of the SDN-POA produce GABA. In both the rat and human, high levels of GAD65 and GAD67 mRNA are present in most, if not all, SDN-POA neurons. No sex difference is evident in the level of expression in either the rat or human. The data indicate that neurons of the SDN-POA in both the rat and human are GABA-producing and argue for the homology of these nuclei in the rat and human hypothalamus.

Galanin microinjected into the medial preoptic nucleus facilitates female- and male-typical sexual behaviors in the female rat

Galanin (GAL) microinjected within the sexually dimorphic medial preoptic nucleus (MPN) facilitates male-typical sexual behaviors in the male rat, a response that requires the presence of testosterone. As in the male, GAL-immunoreactive cells located within the MPN of the female also concentrate gonadal steroids and become less immunoreactive after gonadectomy. Thus, to investigate sexual behaviors in the female and to determine whether effects are comparable to those obtained in the male, GAL was microinjected unilaterally within the MPN of female rats. We report that GAL stimulated female-typical lordosis behavior after estrogen priming, and that the effect was not due to general arousal as measured by nonspecific locomotor activities. In a separate experiment, GAL microinjected within the MPN dose-responsively increased mount frequencies and decreased mount latencies in testosterone-primed females. A higher dose of testosterone was required in females for this stimulation of male-typical sexual behavior than required in a previous experiment in males.

Brain peptides and obesity: pharmacologic treatment

Obesity results from an imbalance between nutrient ingestion and metabolism, with more calories being ingested than utilized. The brain plays an important role in coordinating these complex behavioral and physiological functions, operating through multiple neurochemical systems with distinct properties. This review focuses on two hypothalamic peptide systems, neuropeptide Y (NPY) and galanin (GAL), that illustrate how the brain operates through different mechanisms to control the body's nutrient stores, in different states or conditions. These peptides have different behavioral and physiological effects and are, themselves, differentially responsive to feedback signals from circulating steroids, peptides, and nutrients. They can be distinguished by their relation to natural feeding patterns and endogenous hormones and by their specificity of action in relation to natural biological rhythms. The neuroanatomical substrates involved in these actions of NPY and GAL are also distinct. The neurocircuit mediating NPY's actions originates in the arcuate nucleus and terminates in the medial portion of the paraventricular nucleus; the GAL-containing neurons, in contrast, are concentrated in the lateral portion of the paraventricular nucleus, in addition to the medial preoptic area, which contribute to local GAL innervation as well as projections to the median eminence. Regarding their distinct functions, the evidence suggests that the NPY system is more closely related to patterns of carbohydrate ingestion and carbohydrate utilization, channeling nutrients towards the synthesis of fat. It is most strongly activated at the start of the active feeding cycle or after weaning, in close association with the adrenal steroid, corticosterone. The GAL system, in contrast, is more closely associated with patterns of fat consumption and signals related to fat oxidation. This peptide system is most active during the middle of the feeding cycle or immediately after puberty, in close association with the gonadal steroids. The gene expression and synthesis of these peptides in their respective neuronal cell groups is inhibited by circulating insulin and altered by dietary nutrients. Disturbances in sensitivity to insulin and steroid feedback regulation in the brain are believed to be involved in producing abnormal patterns of peptide function that result in overeating and body weight gain.

Specificity of hypothalamic peptides in the control of behavioral and physiological processes

This review summarizes two model systems for understanding how brain neurochemicals, in conjunction with peripheral endocrine and metabolic processes, may be active in controlling very different functions in relation to energy and nutrient balance. As proposed, these systems are unquestionably oversimplified; however, they generate testable hypotheses for future investigations that will help to advance and revise these working models, as well as those of other peptide systems in the brain. Under normal conditions, these peptide systems are behaviorally and endocrinologically specific, and they are activated at very different periods of the daily cycle and at different stages of development. However, under pathologic conditions, their specificity and rhythmicity may be greatly disturbed. This occurs in states involving hypercortisolemia along with hyperinsulinemia or insulin deficiency, when these peptide systems become chronically activated. To determine whether this increased activity actually contributes to conditions of hyperphagia and obesity, and, thus, whether a reversal of this neurochemical activity may help in the treatment of these conditions, critical studies with various pharmacological manipulations are required. Of equal importance are investigations examining the development of these pathologic conditions, from birth to maturity, and their associated disturbances in neurochemical and endocrine processes. A thorough understanding of gene expression in localized brain areas and the contribution of various transcription factors to this process should allow the identification and development of methods that are useful in the treatment, as well as prevention, of disturbed patterns of nutrient intake, fat deposition, and body weight gain.