Preoptic-anterior Hypothalamic Area Research Articles

Distribution of estrogen target sites in the 2-day-old mouse forebrain and pituitary gland during the ‘critical period’ of sexual differentiation

The present study provides a detailed anatomical description of estrogen target cells in the mouse forebrain and pituitary gland during the sexual imprinting stage of the brain. Six 2-day-old mice (3 males and 3 females) were s.c. injected with 16α-[ 125I]iodo-11β-methoxy-17β-estradiol ([ 125I]MIE 2) and two additional mice (one male and one female) were s.c. injected with 1000x unlabeled 17β-estradiol 1 h before [ 125I]MIE 2 to check the specificity of estradiol binding. Two hours after injection the mice were decapitated, the brains dissected, frozen, sectioned, and processed for thaw mount autoradiography. The highest intensity of nuclear labeling was observed in the preoptic-anterior hypothalamic area, amygdala and cortex entorhinalis. Strong labeling was present in the cerebral cortex and moderate to strong labeling in the lateral septum, bed nucleus of stria terminalis and pituitary gland. Weak to moderate labeling was observed in the bulbus olfactorius, circumventricular organs, basal ganglia, ventral striatum, thalamus, hippocampus and pineal gland. No sex differences were observed in the intensity of labeling and distribution of the estrogen target sites. The topographic distribution of estrogen-concentrating cells in the hypothalamus of the 2-day-old mouse forebrain was similar to the adult pattern but differed prominently in the cerebral cortex, entorhinal cortex and thalamus: the cerebral cortex showed an extensive and intensive labeling, the intensity of labeling in the entorhinal cortex greatly exceeded that observed in the adult and the nucleus anterior medialis thalami was distinctly labeled.

Hypothalamic indomethacin fails to block fever induced in rats by central macrophage inflammatory protein-1 (MIP-1)

This investigation examined the extent to which the activity of a prostaglandin (PG) in the anterior hypothalamic, preoptic area (AH/POA) of the rat plays a role in the intense feverr induced by macrophage inflammatory protein-1 (MIP-1) applied directly to this anatomical region. For the microinjection of both a PG synthesis inhibitor, indomethacin, and MIP-1 into sites within the AH/POA, guide cannulae were implanted chronically just above this pyrogen-reactive region. Postoperatively, the body temperature (T b) of each rat was monitored in the inrestrained condition by means of a colonic thermistor probe. MIP-1 microinjected into the AH/POA in a 0.5-μl volume evoked a biphasic fever when given in a dose of 5.6 picograms (pg) and a monophasic fever in a dose of 28 pg. The latency of the febrile response was ordinarily 15 min with an asymptote of 1.5°C reached ordinarily within 2.0–2.5 h. When the cytokine-reactive site in the AH/POA was pretreated with indomethacin microinjected in an efficacious dose of 0.5 μg, the MIP-1 fever evoked by 5.6 pg was not inhibited. Further, pretreatment of AH/POA sites with indomethacin prior to the higher 28-pg dose of MIP-1 delayed the febrile response but did not block it. As a systemic control, indomethacin also was administered intraperitoneally in a dose of 5.0 mg/kg, again 15 min prior to the microinjection of MIP-1 into the AH/POA. In this case, indomethacin only partially attenuated but did not block the fever evoked by either dose of MIP-1. These results suggest that, although the synthesis of a PG in the periphery could contribute to the central pyrexic action of MIP-1, this cytokine exerts its febrile action within thermosensitive and pyrogen-reactive neurons independently of the local synthesis and/or release of a PG of the E type. Therefore, it is evident that a PGE-independent mechanism within the AH/POA participates in the sequence of cellular events underlying the pathogenesis of a centrally mediated febrile response.

Glucocorticoid-induced hypertension in rats: role of the central muscarinic cholinergic system

Betamethasone was administered on alternate days to rats, and the role of the central cholinergic system in the development of hypertension assessed. After 15 days of treatment the systolic blood pressure of treated rats was significantly higher than that of control rats. Peripheral administration of atropine but not of methyl atropine reduced systolic pressure in glucocorticoid-treated rats and had no effect in controls. Therefore, [3H]quinuclidinyl benzylate binding, sodium-dependent high-affinity choline uptake and choline acetyltransferase studies were performed in the septal area, anteroventrolateral medulla (AVLM), anterior hypothalamic preoptic area (AH/PO) and hypothalamus. The density of muscarinic receptors was increased in the hypothalamus and AVLM of treated rats without significant changes in affinity. Choline acetyltransferase activity significantly decreased in the AVLM and increased in the AH/PO. In addition, a decrease in the hypothalamus and an increase in the AH/PO of sodium-dependent high-affinity choline uptake was observed in glucocorticoid-treated rats. These results suggest the presence of an enhanced muscarinic cholinergic activity in several brain nuclei in rats with glucocorticoid-induced hypertension. This activation could be due to pre- and post-synaptic hypersensitivity.

Intrahypothalamic Mu‐, not Delta‐ or Kappa‐Opioid Receptor Activation Causes Growth Hormone Secretion

Abstract The possible effects of opioid receptor agonists on growth hormone (GH)-releasing factor or somatostatin neurons were examined by measuring the effects of localized intracerebral injections of mu-, delta- and kappa-selective agonists on GH secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of opioid agonists in the preoptic anterior hypothalamic area and medial basal hypothalamus. In the medial basal hypothalamus, injections of the mu-agonist DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) caused dose-responsive increases in GH, the maximally effective dose being 0.001 nmoles. Injection of 10,000-fold higher doses of the delta-agonist DPDPE ([D-Pen, D-Pen]enkephalin) and the kappa-agonist U50,488H were also effective in stimulating GH secretion. In the preoptic anterior hypothalamic area, DAGO caused dose-responsive increases in GH, the maximally effective dose being 0.01 nmoles. U50.488H was ineffective at 1,000-fold higher doses while DPDPE was effective at 100- to 1,000-fold higher doses. We conclude that hypothalamic mu-opioid receptor activation on or near somatostatin or GH-releasing factor neurons causes GH secretion. Opioids capable of acting on other opioid receptors may also stimulate GH secretion, though only at doses that seem likely to affect mu-receptors.

The participation of histaminergic receptors of the rostral hypothalamus on the tonic release of luteinizing hormone (LH) in adult spayed rats under estrogen and progesterone treatment

The influence of histaminergic sites in the preoptic-anterior hypothalamic area (POA-AHA) on the basal release of luteinizing hormone (LH) under a continuous regimen of estradiol, progesterone, or both was studied in ovariectomized rats. Different groups of animals were subjected to the following experimental schedule: at day 1, rats received a s.c. silastic implant filled with oil, estradiol, progesterone, or estradiol plus progesterone. Seven days later (day 7), animals were implanted into the POA-AHA with microinjection cannulae. At day 8 and 9, the different groups of rats were microinjected with 1 microliter of saline solution containing 35 nMol of pyrilamine or metiamide, or 20 nMol of alpha-fluoro-methyl-histidine. At day 10, blood samples were taken through a permanent jugular cannulae implanted in situ the day before. LH concentrations were determined in plasma by RIA. Results showed that the increase of LH plasma levels induced by the ovariectomy was inhibited by the estrogen implant, as expected. Treatment of metiamide or alpha-fluoro-methyl-histidine did not affect the pattern of LH secretion. Nevertheless, treatment of metiamide induced a transient increase in the gonadotropin concentrations that extended for two hours (16:00 and 17:00 H). No change in LH plasma levels was observed in rats bearing the progesterone implant. Treatments (pyrilamine, metiamide, or alpha-fluoro-methyl-histidine into the POA-AHA) had no effect. The transient increase in the hormone levels observed in rats treated with pyrilamine in the estrogen-implanted rats was absent in rats bearing the estrogen-progesterone implant.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivity in the sites of fever production by prostaglandin E1 within the hypothalamus of the rat.

1. The febrile sensitivity of male Sprague-Dawley rats to microinjections of prostaglandin E1 (PGE) was investigated at three different locations in the rostromedial hypothalamic region. These were the preoptic anterior hypothalamic area (PO-AH), the organum vasculosum laminae terminalis (OVLT) and the rostral third ventricle (3V). 2. Stainless-steel cannula guide tubes were implanted in the OVLT region of one group of animals, within the PO-AH area of a second group and into the third ventricle of a third group of rats. After their recovery, the febrile response of each group was tested to a variety of doses of PGE, each administered in a volume of 1 microliter sterile 0.9% saline, via a sterile cannula inserted into the implanted guide tubes. Metabolic, vasomotor and rectal temperature changes were monitored continuously for the duration of the fevers. 3. Surprisingly, not only did the introduction of PGE into the OVLT region produce fevers, but the sensitivity of this region to PGE in the production of fever greatly exceeded that of the PO-AH area and the third ventricle. Fevers produced by microinjection of PGE into the PO-AH and 3V were identical. 4. Doses of PGE as low as 0.5 ng injected into the OVLT produced fevers of 0.5 degrees C. The fever dose threshold for the OVLT region was one-fifth those of the PO-AH area and the 3V, and the slope of the OVLT dose-response curve was twice those of the PO-AH and the 3V dose-response curves. 5. This study demonstrates that there is an anatomically distinct, regional sensitivity in the febrile responsiveness to PGE within the hypothalamus. These results are interpreted as evidence that the site of action of PGE in the production of fever is located within or immediately adjacent to the OVLT region, rather than within the medial PO-AH neuropil as has been believed previously.

Developmental sex differences in brain aromatase activity are related to androgen level

Sex differences in the metabolism of testosterone (T) in the developing brain of quail were examined using an in vitro microassay. During each developmental stage (day before hatching, hatching and 2 days after hatching) aromatase activity was higher in hypothalamic areas than in a control neostriatal area. There was no sex difference in oestradiol-17β (E 2) formation in the late embryonic brain or at hatching. But aromatase activity in the male preoptic-anterior hypothalamic area was 50% higher than in females by day 2. No regional differences in brain 5β-reductase activity were detected at any of the developmental stages sampled. There was a sex difference in production of catabolic 5β-reduced metabolites. Male 5β-reductase activity declined continuously from high embryonic levels in all areas, whereas female enzyme activity showed an increase at hatching. In contrast to plasma progesterone, levels of T were higher in the male than in the female by day 1 after hatching. We suggest that elevated circulating T in the male after hatching may account for the sexual dimorphism in brain aromatase activity.

Injection of prostaglandin E 2 into the anterior hypothalamic preoptic area activates brown adipose tissue thermogenesis in the rat

E series prostaglandins (PGE) are known to elicit potent hyperthermia when injected into the anterior hypothalamic preoptic area (POAH) in rats, but the effector mechanisms mediating the rise in temperature are not well defined. In the present study, microinjection of PGE 2 into the POAH dose-dependently increased non-shivering thermogenesis in brown adipose tissue (BAT) in urethananesthetized rats, bringing about a marked and sustained rise in interscapular BAT (IBAT) and core temperatures. The effect of intra-POAH PGE 2 injection on IBAT and core temperatures could be blocked by systemic pretreatment with the sympathetic ganglionic blocker chlorisondamine chloride or the β-adrenergic receptor blocker propranolol, thus implicating the involvement of the sympathetic system. Furthermore, the increase in IBAT and core temperatures induced by intra-POAH PGE 2 could be blocked by prior injection of the local anesthetic procaine or the GABA receptor agonist muscimol into the ipsilateral ventromedial hypothalamic nucleus (VMH). Taken together, the results suggest that PGE 2 increases body temperature by acting in the POAH to stimulate heat production in BAT via a sympathetic efferent mechanism located in the VMH.

Reduced aminergic synthesis in the hypothalamus of the infertile, genetically diabetic (C57BL/KsJ-db/db) male mouse

Diabetes mellitus is commonly associated with reproductive neuroendocrinopathy in both humans and animal models for the disease. Diabetes-associated reproductive failure in the male is a result of multilevel dysfunction within the hypothalamo-pituitary-testicular axis. In view of the known effects of diabetes on hypothalamic gonadotropin-releasing hormone (GnRH) and gonadotropins in chemically-induced animal models for diabetes, we examined hypothalamic aminergic activities (important to the regulation of GnRH release), circulating gonadotropin levels and testicular morphology in the infertile, genetically diabetic (C57BL/KsJ-db/db) male mouse. Groups of 2-5 month old (average age: 3.4 months) and 6-11 month old (average age: 8.8 months) diabetic mice were compared with age-matched non-diabetic (C57BL/KsL(-)+/?) male mice. Diabetic mice in both age groups were markedly obese and hyperglycemic. Hypothalamic serotonin synthesis was inhibited in the preoptic area-anterior hypothalamus (POA-AH) in both 2-5 month old and 6-11 month old diabetic mice as well as in the mediobasal hypothalamus-median eminence (MBH-ME) of 6-11 month old diabetic mice. Catecholamine synthesis (norepinephrine and dopamine) was reduced in the POA-AH of 2-5 month old diabetic mice and in the MBH-ME of 6-11 month old mice. These aminergic changes were associated in 2-5 month old diabetic mice with reduced circulating levels of LH and in 6-11 month old diabetic mice, of both LH and FSH. In 6-11 month old diabetic mice, testes were characterized by a thickened tunica albuginea, numerous Sertoli cells and the near absence of any spermatogenic cells. The epididymis from these diabetic mice was devoid of spermatozoa.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroidal Control of Behavioural, Neuroendocrine and Brain Sexual Differentiation: Studies in a Carnivore, the Ferret

More than 30 years have passed since the publication ( I ) showing that transplacental administration of testosterone to female guinea-pigs caused permanent changes in their ability to exhibit reproductive behaviours in response to ovarian steroids in adulthood. Over the intervening years articles have appeared which have extended this type of observation to numerous other mammalian species, including rat, hamster, gerbil, mouse, dog, ferret, pig, sheep, rhesus monkey, and marmoset (2, 3). In many instances inferences have been made about the probable existence of steroid-induced changes in the structure and function of the nervous system which underlie these changes in behaviour. These so-called ‘organizational’ effects of perinatal steroid exposure on the developing nervous system are often distinguished from the later ‘activational’ effects of these same steroids on the expression of sociosexual behaviours in adulthood. The number of different mammalian species used for mechanistic studies on the organizational effects of steroids, such as determining the precise perinatal timing of hormone action, the identity of the steroid(s) responsible for behavioural sexual differentiation, and the neural sites of steroid action, has been relatively small, and generally restricted to rodents. In the late 1970’s sex steroids acting during development were first implicated in the differentiation of sexually dimorphic telencephalic structures in passerine song birds which control singing associated with courtship (4, 5) . Since this early work, numerous studies have exploited the canary and zebra finch to explore mechanisms of steroid action in controlling the ontogeny of song in the male and the underlying changes in the nervous system. Other studies using the rat have explored in detail the role of androgen and oestrogen in controlling the differentiation in males of patterns of reproductive behaviour (2) as well as aspects of neural sex dimorphisms in the preoptic area/anterior hypothalamus (POA/AH) (6) and in the lumbar spinal cord (7). The concentration of mechanistic (behavioural, biochemical, neuroanatomical) studies of steroidal effects on sexually dimorphic behavioural potential in a few avian and rodent species is understandable, in light of their relative availability, low expense, and rapid time-course of neural development. Despite the substantial progress already made using these lower vertebrate models in promoting our understanding of how sex steroids influence neural and behavioural development, extrapolation of the results obtained in these studies to higher mammals including man cannot occur in the absence of some degree of empirical verification. Obviously, experimental manipulations of steroidal exposure during the development of human subjects would be unethical, thus we are left with experiments of nature (e.g. congenital adreno-genital syndrome,androgen insensitivity syndrome, and Sa-reductase deficient pseudohermaphroditism; details in later sections) with which to ask questions about the role of sex steroids in controlling psychosexual development in man. This work using clinical populations has been complemented by studies (8, 9) on two populations of rhesus female pseudohermaphrodites, created by transplacental administration of either testosterone or dihydrotestosterone (DHT) and by a study of female pseudohermaphroditic marmosets created by neonatal treatment with testosterone (10). While highly informative as far as they go, this research has not led to mechanistic studies using non-human primates to explore such questions as the critical perinatal periods of steroid action, the relative role of oestrogens versus androgens in controlling behavioural sexual differentiation in the male, or the relationship between steroidal control of sex dimorphisms in brain structure and behaviour. We have used the European ferret (Mustelufuro) in a n effort to develop a model system in which such questions can be addressed in a higher mammal. This carnivore is ideally suited for such experiments in that it is commercially available at a reasonable cost, is easily kept and handled in the laboratory, and produces relatively large litters (6 to 12 kits) which are born in an altricial state after a relatively short gestation (41 days) and which reach adulthood within 5 months. Normally, ferrets breed seasonally in response to lengthening photoperiod. A commercial breeder (Marshall Farms, North Rose, NY, USA) provides pregnant ferrets for our experiments throughout the year by the judicious use of artificial photoperiods in their breeding facility. Upon arrival in the laboratory, all ferrets used in our research are continuously

Effect of early exposure to δ-9-tetrahydrocannabinol on the levels of opioid peptides, gonadotropin-releasing hormone and substance P in the adult male rat brain

The effects of neonatal exposure to δ-9-tetrahydrocannabinol (THC) on the adult animal brain neurochemistry and pain perception were evaluated. Newborn rat pups were culled to a litter size of 8 (males and females) and treated either with THC (2 mg/kg) or oil (control) daily, during days 1–4 after birth. After weaning, the THC-treated males were housed 4 per cage. During the juvenile period (day 50), the THC-treated animals exhibited significantly lower baseline tail-flick values (a measure of pain perception) than the control. However, as adults, the THC-treated animals exhibited significantly higher sensitivity to pain following 5 mg/kg morphine challenge. Furthermore, the THC-treated animals had significantly elevated β-endorphin and methionine-enkephalin levels in almost all the brain areas sampled for the study. In addition, the neonatally THC-treated rats exhibited significantly higher levels of substance P (SP) and significantly lower levels of gonadotropin releasing hormone (GnRH) in the anterior hypothalamus-preoptic area. The SP and GnRH levels did not differ among the THC-treated and control animals in the medial basal hypothalamus. The results of this study indicate that even a very low dose of THC administered during the neonatal period has a long-lasting effect on the brain neurochemistry. In particular, neonatal administration of THC appears to alter functioning of the endogenous opioid system.

Medial preoptic-anterior hypothalamic area involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the ovariectomized rat

The objective of this study was to examine whether specific activation of mu-opioid receptors at the level of the medial preoptic-anterior hypothalamic area (MPOA-AHA) could suppress pulsatile LH release. The experiments were done using rats that had been ovariectomized (OVX) 24 hr before on diestrus 2, animals in which we have previously demonstrated an active endogenous opioid peptide suppression of pulsatile LH release (2). DAGO, DPDPE, or U50488H, specific agonists of mu-, delta- and kappa-opioid receptors, respectively, were continuously applied directly to the MPOA-AHA by means of push-pull perfusion. Perfusion of the MPOA-AHA with 0.5 μg DAGO/hr suppressed LH pulse amplitude. This effect of DAGO was not due to spread to the third ventricle and subsequent diffusion via the CSF to another CNS site, since push-pull perfusion with this dose of DAGO in the region just dorsal to or in the posterior hypothalamus was ineffective in altering LH pulse amplitude. The response to DAGO was dose-dependent since a higher dose (4.8 μg/hr) markedly suppressed both LH pulse amplitude and frequency. The same doses of DPDPE and U50488H (0.5 and 4.8 μg/hr) had no effect on pulsatile LH secretion, providing support for mu receptor involvement in the DAGO-induced suppressive action. These data demonstrate MPOA-AHA involvement in the suppression of pulsatile LH release by a mu-opioid agonist in the OVX rat.

Is androgen‐dependent aromatase activity sexually differentiated in the rat and dove preoptic area?

Aromatase activity is higher in the male than in the female anterior hypothalamic-preoptic area (POA) in both the avian and the rodent adult brain. This sex difference is abolished after castration of the male and restored by androgen treatment. Gonadectomy has no effect on POA aromatase in the female. The aim of this study was to find out whether sex dimorphism in adult POA aromatase is only due to a sex difference in circulating gonadal hormones or dependent upon sexual differentiation of the brain. Aromatase activity was measured in vitro in microdissected POA samples using a sensitive radiometric assay. We examined the effects of gonadectomy and testosterone treatment on enzyme activity in adult rats and doves of both sexes. We also studied the effects of neonatal gonadectomy and hormone substitution in male and female rats. The results suggest that levels of POA aromatase in the adult depend primarily on gonadal activity, but that mechanisms involved in the regulation of aromatase activity and enzyme induction may be sex-specific and could result from sexual differentiation of the brain in early life. Further work will be required to determine the developmental stage when this occurs and the exact mechanism(s) responsible for increased sensitivity of the adult male POA to the inductive effect of testosterone.

Macrophage inflammatory protein-1: Unique action on the hypothalamus to evoke fever

Macrophage inflammatory protein (MIP-1) administered systemically causes a fever not blocked by a prostaglandin (PGE) synthesis inhibitor. The purpose of this study was to examine the central mechanism of pyrexic action of this cytokine in the unrestrained rat. After guide cannulae for microinjection were implanted stereotaxically just above the anterior hypothalamic preoptic area (AH/POA), the body temperature of each rat was monitored by a colonic thermistor probe. Saline control vehicle or MIP-1 was microinjected into the AH/POA in one of eight concentrations ranging from 0.0028–9.0 ng per 0.5 μ1 volume. MIP-1 induced a biphasic or monophasic fever of short latency characterized by an inverse dose-response curve. The potency of MIP-1 was in the femtomolar (10 −15) range with the lowest dose of 0.028 ng producing a fever of over 2.0°C with a latency of 15 min or less. To determine whether a PGE mediates MIP-1 fever, indomethacin was administered either intraperitoneally in a dose of 5.0 mg/kg or directly into the MIP-1 injection site in a dose of 0.5 μg/0.5 μl, both injected 15 min before MIP-1. Pretreatment of the injection site in the AH/POA with indomethacin failed to prevent the febrile response evoked by MIP-1 injected at the same locus. Further, the dose of systemic indomethacin, which blocks PGE-induced fever in the rat, attenuated only partially the MIP-1 fever. The results demonstrate that MIP-1 is the most potent endopyrogen discovered thus far, and that its action is directly in the region of the hypothalamus which contains both thermosensitive and pyrogen-sensitive neurons. The local action of MIP-1 on cells of the AH/POA in evoking fever is unaffected by the PGE inhibitor which indicates, therefore, that a cellular mechanism operates in the hypothalamus to evoke fever independently of the central synthesis of a PGE.

Central action of endogenous sugar acid (2-buten-4-olide): Comparison with local anesthesia in hypothalamus

Rats were trained to discriminate cue tone stimuli (CTS) predicting reward (CTS +) [juice or intracranial self-stimulation (ICSS)], or aversion (CTS −) (mild electric shock or tail pinch). Unit activity in the lateral hypothalamus (LHA) and lateral preoptic-anterior hypothalamic area (1POA-AHA) of the rat was recorded during CTS learning. The effects of local anesthesia of the amygdala (AM), ventral tegmental area (VTA) or LHA by procaine hydrochloride, and the effects of intraperitoneal or intravenous 2-buten-4-olide (2-B4O) on LHA neural activity and licking behavior were compared. LHA neurons differentiated between rewarding and aversive stimuli, and acquired corresponding discrimination of CTS + and CTS −. In the 1POA-AHA, neurons responded similarly to CTS +, rewarding stimuli, CTS − and aversive stimuli. Procainization of the AM suppressed LHA neural responses to CTS1 + predicting juice, and stopped licking for juice. Procainization of the VTA suppressed LHA neural responses to CTS2 + predicting ICSS, and stopped licking for ICSS. LHA procainization suppressed both licking for juice and ICSS. Both intraperitoneal and intravenous 2-B4O stopped licking for juice and ICSS, but did not influence LHA responses to CTS1 + or CTS2 +. The results suggest that dynamic interaction of AM-LHA-VTA are important for CTS + learning, and 2-B4O acts directly on LHA neurons while maintaining afferent sensory inputs to the LHA.

Sexually Dimorphic Areas in the Brain of Whiptail Lizards

The whiptail lizard species Cnemidophorus inornatus exhibits sexually dimorphic mating behaviors. We report that complementary sexual dimorphisms exist in two hypothalamic regions in male and female C. inornatus. The anterior hypothalamus-preoptic area (AH-POA), which is involved in male-typical mounting and intromission behaviors, is larger in males. The ventromedial hypothalamus (VMH), which controls female-typical receptivity, is larger in females. The all-female whiptail lizard Cnemidophorus uniparens, a direct descendant of C. inornatus, regularly and reliably displays both male-like mounting and female-like receptive (pseudosexual) behaviors. Rather than having a male-like AH-POA, however, the AH-POA as well as the VMH of C. uniparens resemble that of female C. inornatus. The biological basis of pseudosexual behavior in the parthenogen appears to be due to an unusual sensitivity to ovarian progesterone rather than to a difference in brain morphology.

Estradiol stimulates preoptic area-anterior hypothalamic proGnRH-GAP gene expression in ovariectomized rats

The decapeptide gonadotropin-releasing hormone (GnRH) and the 56-amino acid GnRH-associated peptide (GAP) are derived from a common precursor translated from the proGnRH-GAP mRNA. Studies using solid-phase hybridization techniques (i.e., Northern blot analysis, dot blot analysis, or in situ hybridization autoradiography) have yielded a controversy as to whether estradiol stimulates, inhibits, or has any effect on proGnRH-GAP gene expression in the preoptic area-anterior hypothalamus (POA-AH) of the ovariectomized (OVX) rat. Using a sensitive and quantitative solution hybridization-nuclease protection assay, which ensures complete hybridization of target RNA to probe RNA, we examined the effects of OVX and estradiol replacement on the amount of proGnRH-GAP mRNA in individual POA-AH dissections. Rats sacrificed at different intervals after OVX showed a significant time-dependent decrease (34–60%) in the levels of POA-AH proGnRH-GAP mRNA relative to sham-operated animals; OVX rats treated with estradiol, however, had proGnRH-GAP mRNA levels comparable to those of sham-OVX animals. To verify these observations, levels of the proGnRH-GAP peptide, measured by radioimmunoassay with antibodies directed against the cleavage and amidation site between the GnRH and the GAP portions of the precursor molecule, were also found to decrease (37%) after OVX and increase (63–85%) following estradiol replacement, relative to intact rats. These data support the view that estradiol stimulates the levels of both proGnRH-GAP mRNA and its primary translation product in the POA-AH region of the OVX rat.

Enhancement of the febrile responses of rats to endogenous pyrogen occurs within the OVLT region

The febrile responses of male Sprague-Dawley rats to a semi-purified endogenous pyrogen (EP) derived from human monocytes are markedly enhanced 3 days after the animals are intravenously injected with a variety of immunoadjuvants. The present study was designed to investigate the site within the body at which these substances act to produce this febrile-enhancing phenomenon. Stainless steel microinjection cannula guide tubes were implanted within the region of the organum vasculosum lamina terminalis (OVLT) of the rats and control febrile dose-response curves to EP were established. Minute quantities of the immunoadjuvants zymosan, lipopolysaccharide endotoxin, and the synthetic adjuvant peptide, muramyl dipeptide, were microinjected into the OVLT region and 3 days later, the febrile responses of the animals were retested. In each case the febrile response elicited by a standard dose of EP was more than doubled, the slope of the fever dose-response curve was tripled, and the dose threshold was lowered by a factor of four to five. These responses are identical with those produced when much larger amounts of these immunoadjuvants are injected intravenously, and, thus, we conclude that the site of action of these substances in enhancing fever in response to EP resides in or near the OVLT region. It is proposed that EP stimulates a type of reticuloendothelial cell residing within the OVLT to release prostaglandin E, which in turn crosses the blood-brain barrier to effect the changes in the thermoregulatory neurons of the preoptic anterior hypothalamic area that result in fever.

Modulation of a viscerosomatic reflex by electrical and chemical stimulation of hypothalamic structures in the rat

Ventromedial forebrain structures were stimulated electrically with short (10-ms) trains of pulses to test for effects on a viscerosomatic reflex. Stimulation at many hypothalamic sites led to an attenuation or even a complete inhibition of reflex activity. The most sensitive sites, however (i.e. those requiring currents of 50 μA or less to inhibit the reflex), were located in the anterior hypothalamus/preoptic area (AH/POA) and rostrally in the diagonal band of Broca (DBB). At certain sites the effects of electrical stimulation were compared with those of microinjection of an excitatory amino acid (DL-homocysteic acid) which is known to excite neuronal cell bodies and not axons. The results of this part of the study indicated that activation of cell bodies located in the ventromedial AH/POA (from the level of the optic chiasma caudally to the level of DBB rostrally) mediate, at least in part, the inhibitory effects on visceral afferent processing. These data are discussed in relation to a possible role of AH/POA in the spinal processing of nociceptive information of visceral origin.

The probable role of histamine in the rostral hypothalamus on the prolactin and luteinizing hormone release induced by estrogen in conscious spayed rats.

The participation of histamine (HA) sensitive sites in the preoptic-anterior hypothalamic area (POA-AHA) on prolactin (PRL) and luteinizing hormone (LH) surge induced by estrogen was studied in ovariectomized rats. Different groups of animals were subjected to the following experimental schedule: On day "0" rats were stereotaxically implanted into the POA-AHA with microinjection cannulae. On day "1", rats were injected s.c. with estrogen. On days "2" and "3", animals were microinjected into the POA-AHA with different drugs, according to the type of experiment, and at day "4", through a silastic cannula implanted previously in the jugular vein, blood samples were taken each hour between 15:00-21:00 h. In the plasma, PRL and LH concentrations were measured by RIA. Four experiments were performed. In Experiment 1, animals at 12:00 h were injected into the POA-AHA with pyrilamine maleate (an H 1-histamine antagonist), metiamide (an H2-histamine antagonist) or saline as control. In Experiment 2, rats at 12:00 h were injected into the POA-AHA with alpha-fluormethyl-histidine (an inhibitor of histamine synthesis) or the combined administration of pyrilamine and metiamide. In Experiment 3, rats previously microinjected with the histamine synthesis inhibitor were microinjected with 4-methyl-histamine (an H 2-histamine agonists) or 2-pyridilethyl-amine (an H 1-histamine agonist) and in Experiment 4, rats were microinjected at 09:00 h with metiamide, pyrilamine, fluor-methyl-histidine or saline as control. Results showed that in animals treated with pyrilamine or metiamide at noon the prolactin surge induced by estrogen was affected (inhibited by metiamide and shortened by pyrilamine, Experiment 1) and LH surge slightly affected. Rats that received FMH or the combined administration of the histamine antagonists the prolactin and LH surge were abolished (Experiment 2). Only the treatment of the H 2-histamine agonist was able to reproduce the prolactin increase in rats treated with FMH. Nor the H 1 or H 2-histamine agonists were effective in reproducing the LH surge in these animals (Experiment 3). Animals that received saline at 09:00 h into the POA-AHA, the prolactin and LH surges were abolished. Results confirm that histamine in the POA-AHA is important for the expression of prolactin and LH surge induced by estrogen and suggest that H 1- and H 2-histamine receptors are involved in the complex timing mechanisms of the rostral hypothalamus that control both hormone release in rats.