Oestrogenic activity of an environmentally persistent alkylphenol in the reproductive tract but not the brain of rodents

Compounds with oestrogenic actions present in the environment as a result of human activity may represent a threat to health and reproductive efficiency in human and wildlife populations. We show here that parenteral administration of octylphenol, a recently described environmental oestrogen derived from one group of non-ionic surfactants, is active in stimulating oestrogen-dependent uterine growth in prepubertal rats, but has no influence on perinatal sexual differentiation of the rat brain. These results extend previous in vitro findings to show that alkylphenols exert weak oestrogenic activity in vivo in mammals.

Increased fos expression in preoptic calcitonin gene-related peptide (CGRP) neurones following mating but not the luteinizing hormone surge in female rats

The functional relationship between sexually dimorphic neural populations and sex differences in reproductive functioning is unclear. The present study has investigated the function of the sexually dimorphic, estrogen-receptive, calcitonin gene-related peptide (CGRP) neurones in the female preoptic area by examining patterns of Fos immunoreactivity within these cells in relation to the luteinizing hormone surge and lordosis behaviour. In the first experiment, ovariectomized rats were treated with estradiol alone or estradiol plus progesterone to induce the luteinizing hormone surge. The percentage of CGRP neurones with Fos-positive nuclei was not different in estradiol alone (18 +/- 4%) and estradiol/progesterone-treated (24 +/- 3%) rats although the number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 2-fold (P < 0.01) in estrogen/progesterone-treated rats and 40 +/- 5% of luteinizing hormone-releasing hormone neurones were found to express Fos in this group. In the second experiment, ovariectomized rats were treated with estradiol and progesterone and either, mated with a single male or placed in an empty cage, for 30 min. The number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 4-fold in mated rats (P < 0.01) and the percentage of CGRP neurones with Fos-positive nuclei increased from 24 +/- 3% to 38 +/- 2% (P < 0.01) in mated animals. No differences were detected in the number of luteinizing hormone-releasing hormone neurones with Fos-positive nuclei in mated and non-mated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen regulation of GABAA receptor subunit mRNA expression in preoptic area and bed nucleus of the stria terminalis of female rat brain

This study has examined whether circulating estrogens are involved in regulating gamma-aminobutyric acid (GABA)A receptor mRNA expression in regions of the female rat brain known to contain estrogen receptors (ERs). In situ hybridization experiments using 35S-labeled oligonucleotides specific for alpha 2, beta 3, and gamma 1 subunit mRNAs of the GABAA receptor demonstrated that all three mRNAs were abundant in only the medial preoptic nucleus (MPN), where they were expressed by the vast majority of cells, and specific regions of the bed nucleus of the stria terminalis including the principle encapsulated nucleus (PrN-BNST) and bed nucleus of the anterior commissure (BNAC). Estrogen treatment of ovariectomized rats for 7 d resulted in significant 30-60% increases in alpha 2 and gamma 1, but not beta 3, subunit mRNA expression in the MPN and PrN-BNST. Estrogen treatment for 24 hr resulted in levels of mRNA expression intermediate between those of controls and animals treated with estrogen for 7 d. No changes in subunit mRNA expression were detected for any subunit in the BNAC or cingulate cortex. Double-labeling immunocytochemistry experiments using antibodies directed against the alpha 2 subunit of the GABAA receptor and the ER, revealed that 67 +/- 3% of alpha 2 subunit-immunoreactive cells in the MPN also contained ER immunoreactivity. Cells expressing alpha 2 subunits in the PrN-BNST were also found to possess ERs while those in the BNAC and cingulate cortex did not. These findings suggest the possibility that ER-containing cells in the MPN and PrN-BNST express an alpha 2 beta 3 gamma 1 isoform of the GABAA receptor that has its alpha 2 and gamma 1 subunits regulated by circulating estrogen concentrations. Together, our observations indicate that estrogen may regulate GABAA receptor mRNA expression at a transcriptional level and that this is only likely to occur within regions of the rat brain possessing ERs.

Changing patterns of Fos expression in brainstem catecholaminergic neurons during the rat oestrous cycle

Brainstem catecholaminergic neurons are believed to play an important role in the activation of luteinising hormone-releasing hormone (LHRH) neurons on the afternoon of proestrus which results in the luteinising hormone (LH) surge. To examine the respective roles of brainstem A1 and A2 neurons and the adjoining C1 and C2 adrenergic cells at this time, we have examined the patterns of Fos-immunoreactivity within tyrosine hydroxylase (TH) and phenylethanolamine-N-methyl transferase (PNMT) neurons during diestrus and proestrus. Initial studies demonstrated that the LH surge commenced at approximately 15:00 h in proestrous animals and that peak plasma levels of LH were observed between 16:00 and 17:00 h. Groups of cycling female rats (n = 6) were then perfused between 09:00 and 11:00 (diestrus early) and 18:00 to 19:30 h (diestrus late) on diestrus and at the same times on proestrus (proestrus early and proestrus late). Double-labelling immunocytochemistry revealed little Fos expression by adrenergic neurons of the C1 or C2 cell groups and this did not change significantly between any of the experimental groups. Analysis of the A2 region was divided into rostral, middle and caudal divisions and all regions showed a significant (P < 0.01) increase in the number of Fos-expressing TH neurons (up to 35% of TH cells) in proestrus early animals compared with diestrus and proestrus late rats. In the A1 region, a significant increase in the number of TH neurons expressing Fos (approximately 33%) was detected in both proestrus early (P < 0.05) and diestrus early (P < 0.01) rats compared with animals perfused in the late afternoon (approximately 12%).(ABSTRACT TRUNCATED AT 250 WORDS)

Central inhibitory effects of muscimol and bicuculline on the milk ejection reflex in the anaesthetized rat

1. In order to determine whether GABAergic mechanisms are involved in the control of the milk ejection reflex in the rat, we examined the effects of central administration of a GABAA receptor agonist (muscimol) and antagonist (bicuculline) on the milk ejection reflex in the urethane-anaesthetized rat. 2. Intracerebroventricular (i.c.v.) injection of both muscimol (n = 17), at doses of 5, 10 and 20 ng, and bicuculline (n = 15), at doses of 0.01, 0.1 and 0.3 microgram, inhibited the milk ejection reflex in a dose-dependent manner. The bicuculline-induced inhibition was accompanied by desynchronization of the electroencephalogram and, at the highest dose, by alteration in the sensitivity of the mammary gland to oxytocin. No significant effect on the milk ejection reflex was seen with i.c.v. isotonic saline (n = 5). 3. Injection of 20 (n = 5) or 40 ng (n = 2) muscimol or 0.1 microgram bicuculline (n = 5) i.c.v. did not significantly alter the rise in intramammary pressure evoked by electrical stimulation of the neurohypophysis. 4. Bilateral 400 nl microinfusions directly into the supraoptic nuclei of either muscimol (20-100 ng microliter(-1); n = 10) or bicuculline (0.15 micrograms microliter(-1); n = 5) [corrected] resulted in an inhibition of the milk ejection reflex, which was not accompanied by desynchronization of the electroencephalogram. 5. The effects of i.c.v. injections of muscimol (15 and 20 ng) and bicuculline (0.01, 0.12 and 0.3 microgram) on the electrical activity of twenty-seven antidromically identified supraoptic magnocellular neurones were examined. Both compounds resulted in an inhibition of the background firing of oxytocinergic and vasopressinergic cells, and delayed the occurrence of high frequency bursts in oxytocin neurones. In five supraoptic neurones, bicuculline induced a transient activation before inhibition. 6. The powerful inhibitory action on the milk ejection reflex of both muscimol and bicuculline provides evidence for the importance of GABA neurones in maintaining the functional integrity of the mechanisms which allow the intermittent and pulsatile release of oxytocin during suckling.

Cellular localization and differential distribution of GABAA receptor subunit proteins and messenger RNAs within hypothalamic magnocellular neurons

The inhibitory neurotransmitter GABA plays an important role in regulating the activity of magnocellular oxytocin and vasopressin neurons located in the supraoptic and paraventricular nuclei through occupancy of GABAA receptors. However, the GABAA receptor is a hetero-oligomeric protein comprised of different subunits and the subunit types expressed in a given receptor complex appear critical for its sensitivity to GABA, benzodiazepines and/or steroids. Thus, in order to understand fully the GABAergic control of oxytocin and vasopressin secretion, definition of the GABAA receptors synthesized by magnocellular neurons in the supraoptic and paraventricular nuclei is required. In the supraoptic nucleus, antibodies directed against the alpha 1, alpha 2 and beta 2/3 subunits of the GABAA receptor revealed similar strong antigen distribution on all magnocellular neurons. Using sequential double-immunoperoxidase staining, immunoreactivity for all three subunits was observed on both oxytocin and vasopressin neurons of the supraoptic nucleus. In contrast, only alpha 2 subunit immunoreactivity was detected on the cell bodies of oxytocin and vasopressin neurons in the paraventricular nucleus. No sex differences were detected. In situ hybridization experiments using 35S-labelled oligonucleotides showed that all supraoptic neurons expressed alpha 1, alpha 2 and beta 2 subunit messenger RNA transcripts while magnocellular neurons in the paraventricular nucleus were only enriched in alpha 2 subunit messenger RNA. Quantitative analysis showed that the expression of alpha 1 and beta 2 subunit messenger RNAs in the paraventricular nucleus was half that observed in the supraoptic nucleus while expression of beta 3 subunit messenger RNA was very low in both nuclei. These results show that all oxytocin and vasopressin neurons located in the supraoptic nucleus synthesize and express alpha 1, alpha 2 and beta 2 subunits of the GABAA receptor while those in the paraventricular nucleus are only immunoreactive for the alpha 2 subunit. These observations suggest, therefore, that at least two pharmacologically distinct GABAA receptor isoforms exist on supraoptic neurons and that these are different to those expressed by paraventricular magnocellular cells. Thus, in addition to providing a definition of the subunits likely to form specific GABAA receptor isoforms on magnocellular neurons, this study gives direct evidence for GABAA receptor heterogeneity between supraoptic and paraventricular neurons, but not between oxytocin and vasopressin cells.

Distribution of estrogen receptor-immunoreactive cells in monkey hypothalamus: relationship to neurones containing luteinizing hormone-releasing hormone and tyrosine hydroxylase

The precise sites and mechanisms by which gonadal steroids influence the activity of neuroendocrine cells controlling pituitary hormone secretion are poorly understood. The present study has determined the distribution of estrogen receptor (ER)-immunoreactive cells in the monkey hypothalamus and examined whether ERs are expressed by luteinising hormone-releasing hormone (LHRH)-and/or dopamine-containing neurones. The distribution of ER-immunoreactive cells was determined in ovariectomised (n = 2) and estrogen plus progesterone-treated (n = 2) cynomolgus macaques and in a single ovariectomised African green monkey. Large numbers of cells immunoreactive for the ER were detected in the preoptic area, bed nucleus of the stria terminalis, periventricular area and ventromedial and arcuate nuclei of all monkeys irrespective of the steroid status. Smaller numbers of ER-immunoreactive cells were found in the paraventricular, but not supraoptic nucleus. Double-labeling experiments in sections from all 5 monkeys revealed that none of the 432 LHRH neurons examined possessed detectable ER immunoreactivity. Neurones stained for tyrosine hydroxylase (TH) were identified in the A11, A12, and A14 cell groups and, although A11 and A12 neurones were intermingled amongst and found adjacent to ER-immunoreactive cells, none of the 1,652 TH-immunoreactive cells examined contained ER immunoreactivity. These results show that ER-immunoreactive cells in the monkey hypothalamus are distributed in a manner similar to that observed in other mammalian species although not all brain regions reported to contain progesterone receptors (PRs) in these species of monkey were found to express ERs. The double-labelling experiments provide further evidence that LHRH neurones do not possess ERs and indicate that, as in other species, estrogen influences on primate LHRH neurones are indirect and/or non-genomic in nature. Unlike the rat and sheep, no evidence was found for ER immunoreactivity in hypothalamic dopaminergic neurones of the monkey. The discrepancy in ER and reported PR receptor localisation within specific hypothalamic nuclei as well as in dopaminergic neurones raises the possibility that not all PR-containing cells may express ERs in the primate hypothalamus.

Localization of GAT-1 GABA transporter mRNA in rat striatum: cellular coexpression with GAD67 mRNA, GAD67 immunoreactivity, and parvalbumin mRNA

The cellular localization and neurochemical phenotype of cells expressing the GAT-1 GABA transporter was investigated in the adult rat dorsal striatum using single and dual in situ hybridization and immunocytochemical techniques. Cellular sites of GAT-1, GAD67, and parvalbumin mRNAs were visualized using a combination of radioactive and alkaline phosphatase-labeled oligonucleotides and emulsion autoradiography; GAD67 immunoreactivity was detected using a polyclonal antibody (K2) and 3'3"-diaminobenzidine. Two types of GAT-1-positive striatal cells were detected: (1) those expressing an abundance of GAT-1 mRNA, and (2) those expressing low/undetectable amounts of message. This study focused on the striatal cells expressing an abundance of GAT-1 mRNA; these cells accounted for approximately 3-5% of all striatal neurons and were detected scattered sparsely throughout the striatal complex. Dual in situ hybridization and immunocytochemical studies established that all cells enriched in GAT-1 mRNA also expressed high levels of GAD67 mRNA and were strongly GAD67 immunopositive; the converse was also found to be the case, the two hybridization signals having identical distribution patterns. Further dual in situ hybridization studies established that approximately 60% of these high GAD67/GAT-1 cells expressed parvalbumin mRNA, a marker of one population of striatal interneurons, and had an average cross-sectional area of 152.40 microns 2. The chemical phenotype of the remaining 40% of high GAD67/GAT-1 cells was not determined, although the average cross-sectional area of these cells (102.48 microns 2) was significantly smaller than GAT-1/GAD67/parvalbumin cells; these cells were detected in all striatal regions and are likely to correspond to another population of striatal GABAergic interneuron.

Modelling the luteinizing hormone-releasing hormone pulse generator

Pituitary hormones are released in pulses as a result of episodic patterns of electrical activity in neuroendocrine neurons. The mechanisms underlying such pulsatility have, however, been difficult to elucidate. For example, the luteinizing hormone-releasing hormone neurons regulating reproductive functioning have a sparse and scattered distribution within the hypothalamus which has made definitive electrophysiological investigation impracticable. Little is known not only of their electrical characteristics, but also of the critical neural components with which they interact to form the so-called "luteinizing hormone-releasing hormone pulse generator". We have used here a neural modelling approach, based on the FitzHugh-Nagumo model of a single neuron, to provide a simple dynamical network model of this neuroendocrine pulse generator. We have found that the minimal components required to generate pulsatile luteinizing hormone secretion arise from combining luteinizing hormone-releasing hormone neurons with reciprocally connected inhibitory interneurons and an external stimulatory input. Local GABA neurons and ascending noradrenergic and/or adrenergic inputs have been used as the biological basis for these respective components. The network displays a wide repertoire of behaviours comparable with experimental observations, including some thought previously to be paradoxical. The capacity of this model network to display complex behavioural features interpretable against experimental evidence suggests that this type of modelling may become a necessary adjunct to empirical studies of pulsatile neuroendocrine systems.

Extracellular GABA concentrations in rat supraoptic nucleus during lactation and following haemodynamic changes: an in vivo microdialysis study

Morphological and pharmacological evidence suggest that the dense GABAergic innervation of the supraoptic nucleus is important for regulating the electrical activity of vasopressin and oxytocin neurons. We have employed the technique of intracranial microdialysis to examine extracellular GABA concentrations in the supraoptic nucleus of the anaesthetized rat and questioned whether differences exist in the dynamics of GABA release between virgin and lactating rats, and if events during lactation or following blood pressure manipulation alter endogenous GABA levels in this nucleus. No significant differences were detected between virgin and lactating animals in either basal or 100 mM potassium ion-evoked GABA release. The inclusion of the GABA uptake blocker nipecotic acid (0.5 mM) into the dialysate resulted in a six- to eight-fold increase (P < 0.01) in GABA outflow in both groups of animals. In lactating rats, GABA outflow measured at 4 min intervals was not altered during a 60 min period of suckling by a full litter of pups and no significant change in GABA outflow was detected in relation to individual milk ejections. In virgin rats, removal of 1.5-2 ml of blood resulted in a 30-60 mmHg fall in blood pressure and a non-significant decline in GABA outflow. Replacement of blood resulted in an abrupt 50 mmHg increase in blood pressure and a significant 22% increase in GABA outflow (P < 0.01), but no change in aspartate or methionine concentrations. Repeated intravenous injections of the alpha-adrenoceptor agonist, metaraminol, similarly evoked approximately 50 mmHg increments in blood pressure and a 26% increase in GABA outflow (P < 0.05). Electrical stimulation of the diagonal band of Broca for 10 min produced a two-fold increase in GABA outflow from the supraoptic nucleus (P < 0.05). These results show that the overall profile of basal and potassium-stimulated GABA concentrations in the supraoptic nucleus is not substantially different between lactating and virgin rats. In lactating animals we have found that GABA levels are not altered in response to suckling or at the time of high-frequency firing by oxytocin neurons to induce milk ejection. In contrast, our data further support the hypothesis that GABA inputs to supraoptic neurons are part of a baroreceptor reflex, relaying through the diagonal band of Broca, to signal periods of acute hypertension and inhibit the firing of vasopressin neurons. Such observations suggest the physiological importance of GABA inputs to the supraoptic nuclei and indicate that GABA may be used in a stimulus-specific manner to influence the activity of magnocellular neurons.

Somatostatin-immunoreactive neurones in the hypothalamic ventromedial nucleus possess oestrogen receptors in the male and female rat

Neurones containing oestrogen receptors (ERs) in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) are believed to play an important role in mediating oestrogen's regulatory influence on reproductive behaviour. As somatostatin (SOM)-immunoreactive neurones are found exclusively within the ventrolateral division of VMN, this study has used double-labelling immunocytochemistry techniques to evaluate whether ER-immunoreactive cells in the VMNvl synthesize SOM in both the male and female rat. Rats gonadectomized 1 week earlier were perfused and brain sections through the mediobasal hypothalamus processed for ER and SOM immunoreactivity using the H222 monoclonal rat ER antibody and a polyclonal rabbit SOM antiserum. Within the VMN, cells immunoreactive for SOM were found predominantly in the rostral portion of the VMNvl while ER-immunoreactive cells were distributed throughout the VMNvl. Sequential double-labelling studies revealed that many ER-containing cells in the rostral VMNvl were also immunoreactive for SOM. A semi-quantitative analysis of double-labelled cells in the rostral VMNvl of male and female rats, respectively, estimated that 52 +/- 2% and 55 +/- 8% of SOM-IR cells possess ERs while 35 +/- 1% and 28 +/- 3% of ER-IR cells synthesise SOM in the rostral VMNvl. No sex differences were detected at this level. These results show that approximately half of the SOM-synthesizing neurones in the rostral VMN possess ERs and indicate that SOM should now be considered alongside enkephalin and Substance P as a putative mediator of oestrogen's regulatory influence on reproduction through the VMN of the rat.

Expression of GABAA receptor alpha 2 sub-unit mRNA by periventricular somatostatin neurones in the rat hypothalamus

Pharmacological evidence suggests that GABA may play an important role in regulating the secretory and synthetic activity of the hypothalamic periventricular somatostatin (SOM) neurones controlling growth hormone secretion. In this study, we have utilized a dual labelling in situ hybridization technique to examine whether the alpha 2 sub-unit of the GABAA receptor, which is abundant in this region, is expressed by periventricular SOM neurones. Neurones expressing SOM were detected using an alkaline phosphatase-labelled antisense oligonucleotide and the alpha 2 sub-unit with an 35S-labelled antisense oligonucleotide. Hybridization experiments with the alpha 2 sub-unit probe alone confirmed the high level of mRNA expression for this sub-unit in the rat periventricular region and simultaneous hybridization experiments with both probes revealed that > 90% (93 +/- 2%) of periventricular SOM neurones express the alpha 2 sub-unit of the GABAA receptor. These results provide the first direct evidence that periventricular SOM cells possess GABAA receptors and suggest that the great majority of these neurones synthesize a GABAA receptor containing the alpha 2 sub-unit.

Analysis of brainstem A1 and A2 noradrenergic inputs to the preoptic area using microdialysis in the rat

Noradrenergic inputs to the preoptic area (POA) are involved in regulating a variety of homeostatic functions. However, the accurate measurement of endogenous noradrenaline (NA) release in the POA has been difficult to achieve and consequently little has been done to characterise the different noradrenergic pathways. By combining the technique of intracranial microdialysis with tissue pre-loading of [3H]NA we have developed a sensitive index of NA release in the POA [8]. Using this method we have now examined and compared the effects of electrical stimulation of the brainstem A1 and A2 cell groups on NA release in the POA. Anaesthetised proestrus rats were implanted with microdialysis probes either unilaterally or bilaterally in the POA and stimulating electrodes positioned in either the A1 or A2 regions. Electrical stimulation (10 Hz, 10s on/off for 20 min) of the A1 region resulted in repeatable, calcium-dependent increases in radioactivity outflow from the ipsilateral POA (P < 0.01). A1-evoked release was twice as large as that observed after equivalent 10 Hz electrical stimulation of the A2 region (P < 0.05). In experiments using bilateral POA microdialysis and A1 stimulation, a significant increase in release from the contralateral POA, amounting to approximately 80% of that observed in the ipsilateral POA, was observed. Experiments involving the blockade of A1-stimulated release in the ipsilateral POA by perfusion with a calcium-free medium demonstrated that increases in radioactivity measured in the contralateral POA were not originating from the ipsilateral POA.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical evidence for oestrogen receptors within GABA neurones located in the perinuclear zone of the supraoptic nucleus and GABAA receptor beta 2/beta 3 subunits on supraoptic oxytocin neurones

The mechanisms by which oestrogen modulates the biosynthetic and secretory activity of magnocellular oxytocin neurones are poorly understood. Using an antibody directed against the oestrogen receptor (ER), the distribution of ER-immunoreactive (-IR) cells in relation to the supraoptic nucleus (SON) was examined. Although no ER-IR cells were detected within the SON, a small population of immunoreactive cells separate from those in the preoptic area was identified in the perinuclear zone of the SON. Double-labelling experiments with an antibody specific for glutamic acid decarboxylase (GAD), the neuronal enzyme producing gamma aminobutyric acid (GABA), revealed that approximately 60% of perinuclear zone ER-IR cells contained GAD. A further set of immunocytochemistry experiments using an antibody raised against the beta 2 and beta 3 sub-units of the GABAA receptor revealed immunoreactivity in the SON. Double-labelling experiments demonstrated that both oxytocin-IR and non-oxytocin-IR neurones in the SON were immunoreactive for beta 2 and/or beta 3 sub-units of the GABAA receptor. These studies have identified ERs within a GABAergic neural population in the perinuclear zone of the SON and shown that magnocellular oxytocin neurones in the SON possess GABAA receptors comprised of beta 2 and/or beta 3 sub-units. In conjunction with previous evidence that the perinuclear zone GABA neurones are an important source of GABA terminals in the SON, these results provide a morphological basis for the hypothesis that perinuclear zone GABA neurones may be part of a steroid-sensitive neural circuitry transmitting oestrogen input to oxytocin neurones in the SON.

Perinatal and adult factors responsible for the sexually dimorphic calcitonin gene-related peptide-containing cell population in the rat preoptic area

Neurons containing calcitonin gene-related peptide in the medial preoptic nucleus exhibit the largest neurochemically defined sex difference in the rat preoptic area with a 20-fold difference in cell numbers. The gonadal steroid hormones responsible for this sexual dimorphism have been investigated by examining calcitonin gene-related peptide immunoreactivity in the preoptic area of adult rats receiving a variety of perinatal and adult gonadal steroid manipulations. Cells immunoreactive for calcitonin gene-related peptide were examined in two populations within the preoptic area, one in its ventrolateral aspect and the other located in the lateral division of the medial preoptic nucleus. Cell profile counts estimate numbers of calcitonin gene-related peptide-containing cells in the medial preoptic nucleus of the female to be 22.2 +/- 3.0 cells/section compared with 1.0 +/- 0.2 in the male (P < 0.01). No sex differences existed in the preoptic ventrolateral population of calcitonin gene-related peptide cells (males 4.3 +/- 0.2, females 4.4 +/- 0.6 cells/section). Gonadectomy of male rats on postnatal day 2 resulted in the appearance of a calcitonin gene-related peptide-containing cell population in the medial preoptic nucleus which was indistinguishable from intact female rats (19.3 +/- 2.2 cells/section). Gonadectomy of adult male rats resulted in a modest increase in calcitonin gene-related peptide cell numbers within the medial preoptic nucleus (8.8 +/- 0.4 cells/section) and this was fully reversed by replacement of testosterone (0.7 +/- 0.2 cells/section).(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitonin gene-related peptide (CGRP): immunocytochemical identification of a neuropeptide synthesised by ventral paraventricular magnocellular neurones in the sheep

The distribution of calcitonin gene-related peptide (CGRP)-immunoreactive neurones was examined in the hypothalamus and pituitary gland of the short-term ovariectomised ewe. A large number of magnocellular CGRP-immunoreactive neurones were identified in the ventral paraventricular nuclei (PVN); few were found in the dorsal PVN and supraoptic nuclei. Parvicellular CGRP-immunoreactive neurones were identified in low density scattered throughout the preoptic region, anterior and basal hypothalamus and region of the stria terminalis. A dense CGRP innervation of the median eminence and neural lobe of the pituitary was observed. These observations reveal substantial species differences in CGRP immunoreactivity compared with the rat and show that magnocellular CGRP-synthesising neurones in the sheep are essentially restricted to, and define, the ventral PVN. This suggests a functionally distinct role for this previously neglected division of the PVN within the ovine hypothalamo-neurohypophyseal system.

Characterization of tritiated noradrenaline release from the rat preoptic area with microdialysis in vivo

Present techniques are unable to provide a sensitive and accurate index of noradrenergic activity in the rat preoptic area. In this study, we have examined the brainstem A1 noradrenergic input to the preoptic area using a new technique whereby [3H]noradrenaline is preloaded into the preoptic area and release of radioactivity from this region is measured subsequently using microdialysis in vivo. Electrical stimulation of the ipsilateral A1 area for 20 min at 5, 10, and 15 Hz evoked significant increases in dialysate radioactivity that were repeatable and frequency-dependent. After removal of calcium from the perfusion medium, basal release of radioactivity was markedly reduced and the effect of A1 stimulation abolished. Changing to a 100 mM K+ medium evoked an increase in the release of radioactivity that was sixfold greater than that seen after A1 stimulation. Separation of the dialysate with HPLC showed that 33% of the increase in measured radioactivity after A1 stimulation was directly attributable to [3H]noradrenaline and the remainder to the metabolites vanillylmandelic acid, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglycol. In contrast, the increase in radioactivity after K+ depolarization was due almost completely to [3H]noradrenaline. Addition of 10 microM clonidine to the perfusion medium markedly reduced basal release of radioactivity, but had no effect on evoked release following A1 stimulation. Conversely, perfusion with 10 microM yohimbine had no effect on basal release, but significantly increased evoked release after A1 stimulation. These results now provide a characterization of noradrenergic activity in the preoptic area and indicate the importance of the A1 noradrenergic input to this region. The technique of measuring radioactivity with microdialysis after preloading with [3H]noradrenaline provides a relatively simple, sensitive index of noradrenergic activity in vivo with good temporal resolution.

Distribution of estrogen receptor-immunoreactive cells in the preoptic area of the ewe: co-localization with glutamic acid decarboxylase but not luteinizing hormone-releasing hormone

Using immunocytochemical techniques we have examined the distribution of cells containing estrogen receptors (ERs) in the preoptic and anterior hypothalamic regions of short-term (1 week) ovariectomized ewes. Subsequent double-labelling experiments examined the co-localization patterns of ER and luteinizing hormone-releasing hormone (LHRH) or glutamic acid decarboxylase (GAD) immunoreactivities. ER-immunoreactive (-IR) cells were identified throughout the central and medial aspects of the preoptic area in a continuum which begins at the level of the organum vasculosum of the lamina terminalis and terminates in the caudal anterior hypothalamic area. A conspicuous sub-population of densely clustered ER-IR cells was identified within this distribution extending from the central region of the preoptic area into the bed nucleus of the stria terminalis. ER-IR cells were also identified in the ventrolateral septum and supraoptic nuclei. Double-labelling experiments showed that although rostral LHRH neurons were surrounded by ER-IR cells, they did not themselves possess ER immunoreactivity. In marked contrast, we estimate that approximately 40% of GAD-IR cells in the central aspect of the medial preoptic area are immunoreactive for the ER and that these cells account for nearly 30% of all ER-IR cells in this region. These results indicate that, in common with other species, LHRH neurons in the ewe do not possess ERs and suggest therefore, that these neurons are unlikely to be modulated directly by circulating estrogens. However, large numbers of adjacent GABA neurons possess ERs and may comprise a major neuronal population mediating gonadal steroid input to LHRH neurons.

Absence of estrogen receptor immunoreactivity in somatostatin (SRIF) neurons of the periventricular nucleus but sexually dimorphic colocalization of estrogen receptor and SRIF immunoreactivities in neurons of the bed nucleus of the stria terminalis

The mechanisms by which somatostatin (SRIF) neurons in the periventricular nucleus and bed nucleus of the stria terminalis (BNST) are differentially regulated by gonadal steroids are unknown. Studies have shown an overlap in the distribution of estrogen receptor and SRIF immunoreactivities in these areas, giving rise to the possibility that SRIF neurons are regulated directly by gonadal steroids. In this study we have used double labeling immunocytochemistry to address the question of whether SRIF neurons in the periventricular region and BNST possess estrogen receptors in male and female rats. Short-term (2- to 4-day) gonadectomized rats with or without colchicine pretreatment for 16-18 h were processed for immunocytochemical staining using monoclonal estrogen receptor (H222) and polyclonal SRIF antisera. Single staining for SRIF and the estrogen receptor in rostral hypothalamic areas showed that the only sites of significant overlap between the two immunoreactivities were in the dorsal half of the periventricular region and posterior BNST. No sex differences were detected in the numbers of SRIF- or estrogen receptor-immunoreactive cells in the dorsal periventricular region. Sequential double-staining techniques demonstrated that periventricular SRIF neurons do not possess estrogen receptor immunoreactivity in either the male or female rat. However, double-labeled cells were readily detected in the posterior division of the BNST in male rats treated with colchicine. No colocalization was detected in the BNST of the female. These results show that the periventricular SRIF neurons controlling GH secretion do not possess estrogen receptor immunoreactivity in either sex and suggest that effects of testosterone and estradiol on SRIF messenger RNA expression in these cells through the estrogen receptor are indirect. In contrast, we provide evidence of a substantial sex difference in the localization of estrogen receptors within SRIF neurons of the BNST.

Immunocytochemical identification of oestrogen receptors in the ovine pars tuberalis: localization within gonadotrophs

The pars tuberalis (PT) of the pituitary gland shows dense binding of melatonin and consequently this region may be involved in modulating seasonal reproduction. Oestrogen is well established as a critical gonadal steroid in controlling seasonally via its ability to alter luteinizing hormone (LH) release. Using immunocytochemistry techniques with antibodies specific for the oestrogen receptor (ER) and the ovine βLH (oβLH) subunit, we have identified large populations of ER-immunoreactive (-IR) and LH-IR cells in the anteroventral region of the ovine PT. In contrast, few ER- or LH-IR cells were identified in the anterodorsal or posterior regions of the PT. Double-labelling experiments revealed that all ER-IR cells in the PT are also immunoreactive for LH. These results show that cells immunoreactive for the ER are concentrated in the anteroventral aspect of the PT and that these receptors are located in the nuclei of the PT gonadotrophs. These results suggest that the anteroventral PT, a region which also expresses high melatonin binding, may be a site of integrated oestrogen and melatonin action on LH secretion from the PT.

Immunocytochemical identification of oestrogen receptors in preoptic neurones containing calcitonin gene-related peptide in the male and female rat

Using single- and double-labelling immunocytochemistry with antibodies specific for the oestrogen receptor and calcitonin gene-related peptide (CGRP), we have demonstrated oestrogen receptor immunoreactivity in the sexually dimorphic CGRP-immunoreactive (IR) population of the medial preoptic area (MPOA). In the short-term gonadectomised female approximately 80% of preoptic CGRP-IR neurones were immunoreactive for the oestrogen receptor. In short-term gonadectomised males, a small population of CGRP-IR cells was visualised in the MPOA only after colchicine treatment. Approximately 30% of CGRP-IR cells in the male were oestrogen receptor-IR, accounting for 2% of the total population of cells containing oestrogen receptors in this area. In the colchicine-treated female, it is estimated that 10-15% of preoptic oestrogen receptor-IR cells contain CGRP. These results indicate that CGRP is synthesised by preoptic neurones with oestrogen receptors. Furthermore, the identification of oestrogen receptors in the sexually dimorphic CGRP population suggests that these neurones may be directly linked with gonadal steroid-dependent, sex-specific functioning of the MPOA.

Identification of a sexually dimorphic neural population immunoreactive for calcitonin gene-related peptide (CGRP) in the rat medial preoptic area

The medial preoptic area (MPOA) of the rat exhibits morphological sex differences and is implicated in sex-specific functioning and behaviour. Using immunocytochemistry, the distribution and numbers of cells containing calcitonin gene-related peptide (CGRP) were examined in the MPOA of adult male and female rats. In the intact female rat, CGRP-immunoreactive (-IR) cells were found in a continuum within the MPOA extending from the caudal aspects of the organum vasculosum of the lamina terminalis through the anteroventral periventricular nucleus (AVPv) to the region of the medial preoptic nucleus (MPN). An additional small group of CGRP-IR cells was noted at the level of the caudal MPNin the ventrolateral (VL) region. Compared with males, the AVPv and MPN regions of the female contained over 25-fold more CGRP-IR cells (P < 0.01). The VL region contained similar numbers of CGRP-IR cells in both sexes. Ovariectomy 1 month earlier, with or without subsequent 17-beta estradiol treatment, had no effect on the numbers or distribution of CGRP-IR cells in the MPOA. Gonadectomy of male rats resulted in a significant increase (P < 0.01) in the numbers of CGRP-IR cells in the AVPv and MPN regions. Subsequent administration of testosterone propionate for 1 week reduced (P < 0.05) numbers of CGRP-IR cells to levels observed in the intact male. Neurones containing CGRP in the VL group were not altered by gonadal steroid manipulation. This study shows that CGRP neurones in the AVPv/MPN region are sexually dimorphic.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of oestrogen receptors in preoptic neurons containing neurotensin but not tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone in the male and female rat

The neurochemical identity of preoptic neurons containing oestrogen receptors was investigated in the male and female rat using a sequential double-staining immunocytochemistry procedure. Single-immunostaining revealed large populations of cells with nuclear immunoreactivity to the oestrogen receptor in the medial preoptic area of the male and female rat. Optimal double-staining of sections for the oestrogen receptor and one of several neuropeptides or tyrosine hydroxylase, was achieved with short-term (two- to four-day) gonadectomized rats treated with colchicine where necessary. Neurotensin-immunoreactive cells were distributed in a sexually dimorphic manner in the region of the anteroventral preoptic nucleus and exhibited oestrogen receptor immunoreactivity in both sexes. Double-labelled cells in this area of the female rat comprised 50% and 11% of the total neurotensin- and oestrogen receptor-containing cell populations, respectively, compared with 25% and 4% in the male (P less than 0.01). The numbers of neurotensin-immunoreactive cells in the region of the medial preoptic nucleus were similar in male and female rats with double-labelled cells making up 20-38% and 3-5% of the total numbers of cells containing neurotensin and oestrogen receptors, respectively, in both sexes. Neurons immunoreactive for tyrosine hydroxylase were distributed in a gender-specific manner within the anterior periventricular area but were not immunoreactive for the oestrogen receptor in either sex. Following colchicine treatment, cholecystokinin-immunoreactive cells were identified predominantly within periventricular regions of the preoptic area and similarly, did not possess immunoreactivity to the oestrogen receptor in either the male or the female rat. Neurons containing luteinizing hormone-releasing hormone were found immediately lateral to the cell populations containing oestrogen receptors and immunoreactivity to the oestrogen receptor was not identified within any neurons containing luteinizing hormone-releasing hormone in either the male or female rat. The absence of oestrogen receptor immunoreactivity in neurons containing tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone suggests that gonadal steroids acting through this receptor do not influence these cells directly in either sex. In particular, it appears that gender-specific patterns of luteinizing hormone secretion cannot be attributed to sex differences in oestrogen receptor localization within luteinizing hormone-releasing hormone neurons. These experiments also show that the sexually dimorphic neurotensin neurons in the preoptic area possess oestrogen receptors and that female rats have larger number of neurons co-localizing neurotensin and oestrogen receptors. As such, these neurons may be involved in mediating sex-specific actions of the gonadal steroids in the preoptic area.

Expression of glutamic acid decarboxylase messenger RNA in rat medial preoptic area neurones during the oestrous cycle and after ovariectomy

Evidence suggests that medial preoptic area (MPOA) neurones containing gamma-aminobutyric acid (GABA) are modulated directly by oestrogen. We have used an alkaline phosphatase-labelled antisense oligonucleotide probe to examine glutamic acid decarboxylase67 (GAD) mRNA expression within individual cells of the MPOA, diagonal band of Broca (DBB) and parietal cortex in rats killed at noon on each day of the oestrous cycle and after ovariectomy (n = 4-5). As a fall in extracellular GABA concentrations occurs in the MPOA on the afternoon of proestrus, the GAD67 mRNA content of cells was also examined in proestrous rats at 15:00h immediately prior to the preovulatory luteinising hormone (LH) surge. The MPOA was found to have an intermediate number of GAD67 mRNA-containing cells compared with the DBB and cortex (P less than 0.01) but expressed the lowest mean hybridisation signal (P less than 0.01). The parietal cortex had significantly fewer (P less than 0.01) GAD mRNA-containing cells than either the MPOA or DBB but these contained higher mean density of signal (P less than 0.01). The hybridisation signal for GAD mRNA was abolished by either ribonuclease pre-treatment or the use of excess non-labelled probe. No significant (P greater than 0.05) differences in GAD67 mRNA were detected in animals killed at noon throughout the oestrous cycle or after ovariectomy. On the afternoon of proestrus (15:00h) there was a significant 40% reduction in mean GAD67 mRNA content within cells of only the MPOA compared with noon (P less than 0.05). The numbers of cells in the MPOA expressing GAD67 mRNA were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotensin-lmmunoreactive Neurons in the Rat Medial Preoptic Area are Oestrogen-Receptive

The identity of neurons influenced by oestrogen is critical for the understanding of ovarian steroid actions in the brain. The medial preoptic area (MPOA) contains one of the largest oestrogen-receptive cell populations in the rat brain and participates in several oestrogen-dependent functions, including the regulation of luteinizing hormone (LH) secretion and sexual behaviour. Using double immunostaining procedures with antibodies specific for the oestrogen receptor and neurotensin, a neuropeptide implicated in the regulation of LH secretion within this area, we found that approximately half of the neurotensin-immunoreactive neurons in the MPOA also displayed immunoreactivity for the oestrogen receptor. We estimate that oestrogen-receptive neurotensin neurons represent 5% to 12% of all oestrogen receptor-positive cells in the MPOA. Our results provide morphological evidence that neurotensin mediates oestrogen-dependent mechanisms within the brain and suggest that oestrogen may act through preoptic neurotensin neurons to aid in the generation of the LH surge.