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

Insulin-induced hypoglycaemia suppresses pulsatile luteinising hormone secretion and arcuate Kiss1 cell activation in female mice

Stress suppresses pulsatile luteinising hormone (LH) secretion in a variety of species, although the mechanism underlying this inhibition of reproductive function remains unclear. Metabolic stress, particularly hypoglycaemia, is a clinically-relevant stress type that is modelled with bolus insulin injection (insulin-induced hypoglycaemia). The present study utilised ovariectomised C57BL/6 mice to test the hypothesis that acute hypoglycaemia suppresses pulsatile LH secretion via central mechanisms. Pulsatile LH secretion was measured in 90-minute sampling periods immediately prior to and following i.p. injection of saline or insulin. The secretion of LH was not altered over time in fed animals or acutely fasted (5 hours) animals following an i.p. saline injection. By contrast, insulin elicited a robust suppression of pulsatile LH secretion in fasted animals, preventing LH pulses in five of six mice. To identify the neuroendocrine site of impairment, a kisspeptin challenge was performed in saline or insulin pre-treated animals in a cross-over design. LH secretion in response to exogenous kisspeptin was not different between animals pre-treated with saline or insulin, indicating normal gonadotrophin-releasing hormone cell and pituitary responses during acute hypoglycaemia. Based on this finding, the effect of insulin-induced hypoglycaemia on arcuate kisspeptin (Kiss1) cell function was determined using c-Fos as a marker of neuronal activation. Insulin caused a significant suppression in the percentage of Kiss1 cells in the arcuate nucleus that contained c-Fos compared to saline-injected controls. Taken together, these data support the hypothesis that insulin-induced hypoglycaemia suppresses pulsatile LH secretion in the female mouse via predominantly central mechanisms, which culminates in the suppression of the arcuate Kiss1 population.

Absence of Female-Typical Pheromone-Induced Hypothalamic Neural Responses and Kisspeptin Neuronal Activity in α-Fetoprotein Knockout Female Mice

Pheromones induce sexually dimorphic neuroendocrine responses, such as LH secretion. However, the neuronal network by which pheromones are converted into signals that will initiate and modulate endocrine changes remains unclear. We asked whether 2 sexually dimorphic populations in the anteroventral periventricular and periventricular nuclei that express kisspeptin and tyrosine hydroxylase (TH) are potential candidates that will transduce the olfactory signal to the neuroendocrine system. Furthermore, we assessed whether this transduction is sensitive to perinatal actions of estradiol by using female mice deficient in α-fetoprotein (AfpKO), which lack the protective actions of Afp against maternal estradiol. Wild-type (WT) and AfpKO male and female mice were exposed to same- versus opposite-sex odors and the expression of Fos (the protein product of the immediate early gene c-Fos) was analyzed along the olfactory projection pathways as well as whether kisspeptin, TH, and GnRH neurons are responsive to opposite-sex odors. Male odors induced a female-typical Fos expression in target forebrain sites of olfactory inputs involved in reproduction in WT, but not in AfpKO females, whereas female odors induced a male-typical Fos expression in males of both genotypes. In WT females, opposite-sex odors induced Fos in kisspeptin and TH neurons, whereas in AfpKO females and WT males, only a lower, but still significant, Fos expression was observed in TH but not in kisspeptin neurons. Finally, opposite-sex odors did not induce any significant Fos expression in GnRH neurons of both sexes or genotypes. Our results strongly suggest a role for fetal estrogen in the sexual differentiation of neural responses to sex-related olfactory cues.

C1 neurons: the body's EMTs

The C1 neurons reside in the rostral and intermediate portions of the ventrolateral medulla (RVLM, IVLM). They use glutamate as a fast transmitter and synthesize catecholamines plus various neuropeptides. These neurons regulate the hypothalamic pituitary axis via direct projections to the paraventricular nucleus and regulate the autonomic nervous system via projections to sympathetic and parasympathetic preganglionic neurons. The presympathetic C1 cells, located in the RVLM, are probably organized in a roughly viscerotopic manner and most of them regulate the circulation. C1 cells are variously activated by hypoglycemia, infection or inflammation, hypoxia, nociception, and hypotension and contribute to most glucoprivic responses. C1 cells also stimulate breathing and activate brain stem noradrenergic neurons including the locus coeruleus. Based on the various effects attributed to the C1 cells, their axonal projections and what is currently known of their synaptic inputs, subsets of C1 cells appear to be differentially recruited by pain, hypoxia, infection/inflammation, hemorrhage, and hypoglycemia to produce a repertoire of stereotyped autonomic, metabolic, and neuroendocrine responses that help the organism survive physical injury and its associated cohort of acute infection, hypoxia, hypotension, and blood loss. C1 cells may also contribute to glucose and cardiovascular homeostasis in the absence of such physical stresses, and C1 cell hyperactivity may contribute to the increase in sympathetic nerve activity associated with diseases such as hypertension.

Differential effects of systemic interleukin-1β on gene expression in brainstem noradrenergic nuclei

AIMS

The cytokine, interleukin-1β (IL-1β), is known to produce specific effects on the neuroendocrine system such as suppression of the reproductive axis and stimulation of the stress axis. The mechanism by which IL-1β produces these differential effects is not clear. Since norepinephrine (NE) is involved in these effects, we hypothesized that IL-1β acts on brainstem noradrenergic nuclei to affect gene transcription of NE synthesizing enzymes, cytokines and associated transcription factors.

MAIN METHODS

Adult female Sprague Dawley rats in proestrus were divided into two groups. Control animals received PBS-BSA and the treatment group received 5 μg of rat recombinant IL-1β i.p. at noon. They were sacrificed in groups at 1, 3 and 5 pm (n=6/group) for measurement of tyrosine hydroxylase (TH) mRNA by qPCR or at 3 pm for mRNA analysis by qPCR array.

KEY FINDINGS

TH mRNA levels decreased gradually with time in both control and IL-1β-treated rats in the ventrolateral medulla. In the nucleus of solitary tract, TH mRNA levels were significantly reduced by IL-1β treatment at 5 pm. In the locus coeruleus, TH mRNA levels increased significantly at 5 pm with IL-1β treatment compared to controls. In the second set of animals analyzed by qPCR array, there were several fold increases in the expression of certain cytokines, chemokines, and transcription factors in specific noradrenergic nuclei.

SIGNIFICANCE

Systemic administration of IL-1β causes significant changes in the expression of tyrosine hydroxylase and several chemokines in brain stem noradrenergic nuclei, thereby mediating its neuroendocrine effects.

Estradiol-17beta-responsive A1 and A2 noradrenergic cells of the brain stem project to the bed nucleus of the stria terminalis in the ewe brain: a possible route for regulation of gonadotropin releasing hormone cells

We have studied brain stem cells in the ewe brain that project to the bed nucleus of the stria terminalis (BNST) and determined if these cells are activated by estradiol-17beta. This would predicate an indirect role in the estradiol-17beta regulation of gonadotropin releasing hormone (GnRH) cells, since these receive input from the BNST. Ovariectomized ewes received 50 mug estradiol-17beta benzoate (i.m.) 1 h prior to brain collection, so that activated cells could be identified by Fos immunohistochemistry. Retrograde tracer (FluoroGold; FG), was injected into the three divisions of the BNST and labeled cells were mapped to the A1 and A2 regions and the parabrachial nucleus (PBN) of the brain stem. With FG injection into the dorsal and lateral BNST, all FG-containing cells in the caudal A1 and 45% of those in A2 stained for dopamine-beta-hydroxylase (DBH), indicating noradrenergic type. No FG-labelled cells in the PBN were DBH-positive. In A1 and A2 respectively, 42% and 46% of FG-labelled cells were Fos-positive, with no double-labeling in cells of the PBN. In ewes receiving FG injections into the ventral BNST, estrogen receptor (ER)alpha-immunoreactive nuclei were found in 82% of A1-FG labeled and 38% of A2-FG labeled cells. No FG-labelled cells of the PBN were ERalpha-positive. Anterograde tracing from A1 with microruby injection identified projections to the PBN, BNST and preoptic area (POA). Thus, A1 and A2 noradrenergic neurons project to the BNST in the ewe brain, express ERalpha and are activated by estradiol-17beta. These noradrenergic, estrogen-responsive cells may provide indirect input to GnRH cells, via the BNST.

Ovarian-steroid modulation of locus coeruleus activity in female rats: involvement in luteinising hormone regulation

The noradrenergic nucleus locus coeruleus (LC) has been reported to regulate luteinising hormone (LH) secretion in female rats. Both oestrogen and progestin receptors have been demonstrated in LC neurones, suggesting that these cells are possibly responsive to variations in circulating levels of ovarian steroids. We therefore evaluated changes in the activity of LC neurones during the oestrous cycle and after ovarian-steroid treatment in ovariectomised (OVX) rats, as determined by immunoreactivity to Fos-related antigens (FRA), which comprises all of the known members of the Fos family. Effects of ovarian steroids on the firing rate of LC neurones were also determined in a slice preparation. The number of FRA/tyrosine hydroxylase (TH)-immunoreactive (ir) neurones in the LC increased from 14.00-16.00 h on pro-oestrus, coinciding with the onset of the LH surge and rise in plasma progesterone. FRA immunoreactivity was unaltered during dioestrus. Oestradiol-treated OVX rats (OVX+E) displayed marked reduction in FRA/TH-ir neurones in LC compared to oil-treated OVX rats. Accordingly, oestradiol superfusion significantly reduced the spontaneous firing rate of LC neurones in slices from OVX rats. Compared to OVX+E, oestradiol-treated rats injected with progesterone at 08.00 h (OVX+EP) exhibited higher number of FRA/TH-ir neurones in the LC at 10.00 h and 16.00 h, and great amplification of the LH surge. Bath application of progesterone significantly increased the spontaneous firing rate of OVX+E LC neurones. Our data suggest that ovarian steroids may physiologically modulate the activity of LC neurones in females, with possible implications for LH secretion. Moreover, oestradiol and progesterone appear to exert opposite and complementary effects (i.e. whereas oestradiol inhibits, progesterone, after oestradiol priming, stimulates LC activity).

The role of noradrenaline in the generation of the preovulatory LH surge in the ewe

Increasing plasma estrogen (E) levels during the follicular phase of the estrous cycle trigger the pre-ovulatory surge of gonadotropin-releasing hormone (GnRH)/LH. Noradrenaline (NA)-producing cells of the brain stem are involved in regulating GnRH cells and project to the preoptic area (POA) and bed nucleus of stria terminalis (BnST). Input to GnRH cells may be direct or indirect, via relay neurons in the POA/BnST. To investigate this, we ascertained whether an alpha(1)-adrenergic antagonist would block/delay the LH surge in ovariectomised (OVX), E-treated ewes. E benzoate (EB) (50microg) was injected (i.m.) and Doxazosin (100nmol/h) or vehicle was infused into the third ventricle 2-26h after EB injection. Doxazosin reduced the magnitude of the LH surge, but did not affect timing. To determine if NA is released in the POA/BnST of cyclic ewes, we immunostained dopamine-beta-hydroxylase (DBH) in terminal fields. Reduced numbers of varicosities staining for DBH indicates release of NA. The number of varicosities immunostained for DBH was reduced in the dorsal and lateral BnST during the follicular phase and during the preovulatory LH surge compared to the luteal phase. These data suggest that noradrenergic mechanisms are involved in generation of the GnRH/LH surge via projections to the BnST and relay to GnRH cells. Since Doxasozin reduced the magnitude of the LH surge in the E-treated OVX ewe, and release of NA in cyclic ewes occurred during the follicular phase of the estrous cycle, we speculate that NA is a permissive factor in surge generation. Thus, increased noradrenergic activity is not a trigger mechanism for initiation of the surge.

Estrogen alters c-Fos response to immobilization stress in the brain of ovariectomized rats

Estrogen receptors are widely expressed in the brain, where estrogen modulates central nervous function. In this study, we investigated the effect of estrogen on the emotional stress response in the brain by comparing the CNS patterns of c-Fos expression in response to immobilization stress (IMO) in ovariectomized rats with placebo treatment (OVX + Pla) vs. ovariectomized rats supplemented with 17beta-estradiol (OVX + E2). Increased c-Fos immunoreactive neurons in response to IMO were observed in cerebral cortex, septum, thalamus, hypothalamus, midbrain, pons and medulla oblongata in accordance with previous findings. When OVX + E2/Stress were compared with OVX + Pla/Stress, the numbers of c-Fos immunoreactive cells were significantly lower in the lateral septum, paraventricular hypothalamic nucleus, dorsomedial hypothalamic nucleus, medial amygdaloid nucleus, lateral periaqueductal gray, laterodorsal tegmental nucleus and locus coeruleus, while they were significantly higher in paraventricular thalamic nucleus and nucleus of the solitary tract. These data suggest that neuronal activities in these areas are influenced bidirectionally by systemic estrogen level.

Fast Fos: rapid protocols for single- and double-labeling c-Fos immunohistochemistry in fresh frozen brain sections

Immunohistochemical localization of c-Fos immunoreactivity has been used successfully for over a decade to visualize patterns of neuronal activity in the brain and spinal cord. These experiments are extremely useful in identifying physiological or pharmacological activation of specific populations of neurons. Unfortunately, conventional c-Fos immunohistochemical protocols are very time and resource intensive. We have adapted and optimized established c-Fos immunohistochemistry (IHC) methodologies for use with fresh frozen brain tissue mounted directly onto slides. The resultant rapid protocols, which we refer to as "Fast Fos", include applications for single- and double-labeling, utilizing either enzyme-substrate or fluorescent detection systems. These protocols provide increased assay throughput and reproducibility, which can be further enhanced by use of an automated slide stainer. Taken as a whole, the c-Fos IHC protocols described in this report provide a flexible system for the identification of neuronal activation that substantially reduces time and resource expenditure while increasing quality and reproducibility of data.

Interrelationships between Hormones, Behavior, and Affect during Adolescence: Understanding Hormonal, Physical, and Brain Changes Occurring in Association with Pubertal Activation of the Reproductive Axis. Introduction to Part III

This paper summarizes the goals of this section and considers current knowledge about the association between hormonal changes that occur over pubertal development and the changes in behavior and brain function over the adolescent period. It reviews the cascade of neural and hormonal changes that occur with puberty; discusses mechanisms by which these changes can affect higher-order brain processes; reviews the current limited state of knowledge about links between puberty and changes in affect regulation in the adolescent period; identifies hurdles that have made progress in our understanding of these relationships difficult; and suggests areas for future investigation that will allow us to obtain a much more comprehensive understanding of these interrelationships. This overview of the physiological processes occurring at puberty indicates that puberty (1) encompasses changes in a number of neural systems; (2) results in altered secretion of a number of hormones; (3) involves hormones that are secreted in a pulsatile manner so that collection of a single blood sample does not clearly delineate hormone profiles; and (4) shows considerable individual variation in the rate of progression and in hormone secretion during progression. The important role that gonadal steroid hormones play throughout development and adulthood in regulating plastic changes in neuronal structure and function is noted, highlighting the need for further studies to determine the extent to which the dramatic increases in circulating steroid hormones at puberty modulate brain circuits that underlie changes in social behaviors, risk-taking behaviors, and cognitive function at adolescence.

Oestradiol-dependent and -independent modulation of tyrosine hydroxylase mRNA levels in subpopulations of A1 and A2 neurones with oestrogen receptor (ER)alpha and ER beta gene expression

Oestradiol (E2) induces luteinizing hormone-releasing hormone (LHRH) hypersecretion, thereby triggering LH surge release in ovariectomized (OVX) rats. Neural signals responsible for the surge are marked by a morning increase in LHRH gene expression and an afternoon increase in LHRH release. Evidence suggests that subpopulations of noradrenergic neurones may be responsible for one or both of these signals. To further investigate this issue, we examined effects of E2 on the activity of A1 and A2 noradrenergic neurones, as reflected in changes in tyrosine hydroxylase (TH) mRNA expression, on the day of LH surge release. We then used dual-label in situ hybridization to determine whether E2-induced changes occurred primarily in A1 and A2 subdivisions wherein most noradrenergic neurones expressed oestrogen receptor (ER)alpha and/or ER beta mRNA. We found that in all subdivisions, levels of TH mRNA were higher in E2- than oil-treated rats at 12.00 h. These differences resulted from a decline in TH mRNA expression in oil-treated rats, as well as a rise in levels in E2-treated rats between 10.00 h and 12.00 h. During the afternoon, TH mRNA expression in most A1 and A2 subdivisions peaked at 14.00 h when LH surge release began. However, in all but the middle and caudal A2 subdivisons, levels were similar in E2-treated and control rats at this time. This was attributable to a widespread increase in TH mRNA expression between 12.00 h and 14.00 h in OVX rats. There was no evidence that E2 induced changes in TH mRNA expression preferentially in regions wherein most neurones contained ER alpha or ER beta mRNA. Our findings suggest that E2 activation of middle and caudal A2 neurones, in conjunction with the widespread E2-independent activation of noradrenergic neurones in other subdivisions, may play a role in the induction of LH surge release.

17beta-Estradiol increases, aging decreases, c-Fos expression in the rat accessory olfactory bulb

In the present paper we investigated the c-Fos immunoreactivity in the accessory olfactory bulb (AOB) of juvenile, adult and old rats of both sexes, as well as the effect of 17beta-estradiol on the expression of this immediate early gene. Basal c-Fos expression in the olfactory bulb decreased with age, and estrogen treatment caused an increase in the number of neurons expressing c-Fos in the AOB. The results indicate that both aging and estrogen have roles in the ability of neurons to co-ordinate genetic activity. Our observations may explain the decrease in age-related changes of brain plasticity, and provide data for the understanding of hormonally regulated neuronal plasticity.

Immunotoxic destruction of distinct catecholamine subgroups produces selective impairment of glucoregulatory responses and neuronal activation

The toxin-antibody complex anti-d(beta)h-saporin (DSAP) selectively destroys d(beta)h-containing catecholamine neurons. To test the role of specific catecholamine neurons in glucoregulatory feeding and adrenal medullary secretion, we injected DSAP, unconjugated saporin (SAP), or saline bilaterally into the paraventricular nucleus of the hypothalamus (PVH) or spinal cord (T2-T4) and subsequently tested rats for 2-deoxy-D-glucose (2DG)-induced feeding and blood glucose responses. Injections of DSAP into the PVH abolished 2DG-induced feeding, but not hyperglycemia. 2DG-induced Fos expression was profoundly reduced or abolished in the PVH, but not in the adrenal medulla. The PVH DSAP injections caused a nearly complete loss of tyrosine hydroxylase immunoreactive (TH-ir) neurons in the area of A1/C1 overlap and severe reduction of A2, C2, C3 (primarily the periventricular portion), and A6 cell groups. Spinal cord DSAP blocked 2DG-induced hyperglycemia but not feeding. 2DG-induced Fos-ir was abolished in the adrenal medulla but not in the PVH. Spinal cord DSAP caused a nearly complete loss of TH-ir in cell groups A5, A7, subcoeruleus, and retrofacial C1 and a partial destruction of C3 (primarily the ventral portion) and A6. Saline and SAP control injections did not cause deficits in 2DG-induced feeding, hyperglycemia, or Fos expression and did not damage catecholamine neurons. DSAP eliminated d(beta)h immunoreactivity but did not cause significant nonspecific damage at injection sites. The results demonstrate that hindbrain catecholamine neurons are essential components of the circuitry for glucoprivic control of feeding and adrenal medullary secretion and indicate that these responses are mediated by different subpopulations of catecholamine neurons.

Mating-activated brainstem catecholaminergic neurons in the female rat

Central catecholaminergic systems play an important role in the control of reproductive activities including sexual behavior, luteinizing hormone (LH) and prolactin secretion. It has been reported that catecholaminergic neurons in the locus coeruleus (A6) are activated by mating in rabbits and ferrets, animals known as reflex ovulators. This study used Fos as a marker of neuronal activity to examine whether brainstem catecholaminergic neurons are activated by mating in the spontaneous ovulator, the female rat. Proestrous rats receiving intromissions (mated group) from males or mounts-without-intromission (mounted group) were sacrificed along with rats taken directly from their home cage (control group) 90 min after the beginning of mating or mounting. Double-label immunocytochemistry was used to examine the expression of c-Fos in catecholaminergic neurons labeled by tyrosine hydroxylase (TH) antibody, or adrenergic neurons labeled by phenylethanolamine-N-methyl transferase (PNMT) antibody. Double label immunofluorescent immunohistochemistry was used to determine the number of neurons containing the estrogen receptor (ERalpha) that were activated by mating in these brain areas. The results showed that mating-with-intromissions induced a significant increase in the percentage of TH/Fos colabeled neurons in both A1 and A2 cells compared to mounting-without-intromission or control. In both these areas, over 50% ERalpha-ir neurons were activated after mating while mounting-without-intromission did not affect the percentage of colabeled Fos/ERalpha neurons. In A6 region, neither the expression of Fos nor the percentage of TH/Fos colabeled cells was influenced by either mating or mounting compared to controls. The percentage of PNMT-containing neurons colabeled with Fos was not different in C1 and C2 among the three experimental groups. The results indicate that catecholaminergic neurons were activated by mating in A1 and A2 but not in adjoining adrenergic C1 and C2 cells. In contrast to the findings that catecholaminergic neurons in A6 are activated by mating in induced ovulators, mating did not affect neuronal activity in A6 neurons in the female rat. In A1 and A2 areas, a high percentage of neurons containing ERalpha were activated by mating suggesting both tactile and hormonal information may converge on these populations of neurons. The activated catecholaminergic neurons in A1 and A2 may be an important pathway by which sensory information generated during sexual interaction modulates both behavior and pituitary function.

Expression of estrogen receptor-alpha and c-Fos in adrenergic neurons of the female rat during the steroid-induced LH surge

Epinephrine is an important neurotransmitter that is synthesized in relatively few neurons of the medullary regions C1-C3. Epinephrine is involved, among others in the control of most neuroendocrine systems, such as corticotropin releasing hormone-, gonadotropin releasing hormone- and oxytocin/vasopressin-containing neurons as part of complex feedback loop systems that often include interactions with the gonadal or adrenal steroid hormones. In order to determine if the interactions between gonadal steroid hormones with the adrenergic neurons are direct or involve steroid-receptive interneurons that in turn innervate the adrenergic neurons, dual immunohistochemistry was applied to identify if estrogen receptor-alpha (ERalpha) protein was expressed by adrenergic, phenylethanolamine-N-methyl transferase (PNMT)-positive neurons and if estradiol can activate these neurons as determined by the transient expression of the transcription factor c-Fos. The results show that an average of 22% of all PNMT neurons in the C1 region, 38% in C2 and 42% in the C3 region express estrogen receptor-alpha protein with the highest numbers of dual labeled neurons in the central levels of the C1-C3 regions. Overall, the percentages of dual labeled PNMT/ERalpha neurons did not change during the steroid-induced LH surge. In contrast, the percentage of c-Fos expressing PNMT neurons changed significantly during the LH surge. Thus, c-Fos immunoreactivity was highest in all three regions at 1200 h with 69% of the PNMT neurons in C1, 60% in C2 and 79% in C3 co-expressing c-Fos. C-Fos expression was lowest before and after the surge with 39% of the PNMT neurons in the C2 region containing c-Fos at 0800 h, 52% c-Fos-positive PNMT neurons in C1 and 54% in area C3. The results show that many adrenergic neurons are direct targets for estradiol and that most PNMT neurons in the brainstem are activated during the initiation of the steroid-induced LH surge which suggests that epinephrine is one of the triggers that stimulates GnRH release during the surge.

Molecular activation of noradrenergic neurons in the rabbit brainstem after coitus

Our previous studies indicate that coitus in female rabbits induces a gonadotropin-releasing hormone (GnRH) surge that is preceded by an increase in hypothalamic norepinephrine (NE) release. The additional findings of an enhanced tyrosine hydroxylase (TH) mRNA expression in the female brainstem after coitus, in addition to the appropriate topographic distribution of TH and dopamine-beta-hydroxylase (DBH), lead us to hypothesize that coital signals are relayed to hypothalamic GnRH-secreting neurons via brainstem NE-containing perikarya. Here we analyzed coitally activated areas in the brainstem by in situ hybridization of the oncogene c-fos, as well as the expression of TH mRNA at 0, 30 and 60 min postcoitus using specific 35S-labeled probes for c-fos and TH. To establish the identity of activated brainstem neurons, we immunocytochemically double-labeled cells with specific antibodies against Fos protein and DBH at 90 min postcoitus. Both c-fos and TH mRNAs were present at 0 min (control) in the A1, A2 and A6 brainstem-noradrenergic areas. At 30 min after coitus the expression of both genes significantly increased (P<0.01) in the A1 and A2 areas. By 60 min postcoitus the expression of c-fos mRNA decreased to control levels, while that of TH mRNA remained stimulated. Double-labeling of Fos and DBH indicated that the number of dual-labeled neurons increased (P<0.05) over control levels only in the A1 and A2 areas (not in A6) at 90 min postcoitus. These findings support the hypothesis that coitus activates transcriptional/translational events within brainstem NE neurons that culminate in the release of hypothalamic NE and hence a GnRH surge.

Modulation of the sleep electroencephalogram by estrogen replacement in postmenopausal women

OBJECTIVE

We performed an examination of the effects of estrogen replacement on the sleep electroencephalogram in postmenopausal women.

STUDY DESIGN

A sleep electroencephalogram was recorded in 11 postmenopausal women with and without estrogen administered by skin patch (50 microg of estradiol per day).

RESULTS

Estrogen enhanced rapid-eye-movement sleep (50 +/- 4 vs 39 +/- 5 minutes, P <.05) and reduced time awake (12 +/- 5 vs 20 +/- 6 minutes, P <.05) during the first 2 sleep cycles. The normal decrease in slow-wave sleep and delta activity from the first to the second cycle (in percentage from the first cycle) was restored by estrogen (-56% +/- 9% vs -5% +/- 14% and -20% +/- 6% vs -2% +/- 5%; P <.05, respectively). Sigma electroencephalographic activity was increased by estrogen from the first to the second half of the night but decreased during baseline.

CONCLUSION

Estrogen treatment after menopause can help to restore the normal sleep electroencephalogram pattern, which in turn might contribute to improved cognitive functioning.

Oestrogen receptors in the brainstem of the female sheep: relationship to noradrenergic cells and cells projecting to the medial preoptic area

Oestrogen regulates the secretion of gonadotropin releasing hormone (GnRH) and this could be mediated by noradrenergic systems originating in the brainstem. Whilst it is known that noradrenergic cells possess oestrogen receptors (ER), it is not known whether ER-immunoreactive (-ir) cells in the brainstem project to the regions of the hypothalamus in which GnRH neurons are found. We have used dual-label immunocytochemistry to determine the extent to which ER-alpha is found in noradrenergic cells in the brainstem of the ovariectomized (OVX) ewe. Noradrenergic/adrenergic cells were identified by immunostaining for dopamine beta-hydroxylase (DBH). Cells that stained for both DBH and ER were found in both the A1 and A2 cell groups, with the highest levels found in the most caudal regions. In the A1 group, at the most caudal extent, 73% of ER-ir cells were DBH-positive and 19% of DBH-ir cells were ER-positive. The degree of co-localization decreased in a linear manner towards the rostral brainstem. In the caudal half of A2, 9-14% of ER-ir cells were DBH-positive and 20-25% of DBH cells were ER-positive. Less than 2% of DBH-ir cells in the A5 group were dual-labelled and none of the cells in the A6 and A7 groups were ER-positive. The retrograde tracer FluoroGold was injected into the preoptic area of nine OVX ewes and labelled cells were examined in the brainstem to determine the extent of co-localization of ER. Only injections in the rostroventral part of the medial preoptic area near to the organum vasculosum of the lamina terminalis resulted in the labelling of cells in the brainstem. One ewe with very strong labelling of the brainstem was selected for detailed mapping. In the ventrolateral medulla, half the ER-ir cells in the most caudal regions were retrogradely labelled. Almost all the ER-ir cells in the mid-region of the ventrolateral medulla were retrogradely labelled but no co-localization of retrograde tracer and ER was observed rostral to obex. There were many ER-ir cells and retrogradely-labelled cells in the nucleus of the solitary tract but only a few double-labelled cells. Similarly, numerous ER-ir cells and retrogradely labelled cells were observed around the lateral edges of the caudal fourth ventricle and across to the lateral parabrachial nucleus but there were few double-labelled cells. These results suggest differential regulation of noradrenergic cells by oestrogen, with a direct action of the hormone confined to the cells in the most caudal region of the A1 and A2 cell groups. The cells of the caudal ventrolateral medulla which contain ER-ir cells that project to the preoptic area may be important in the mediation by noradrenaline of the actions of oestrogen on GnRH secretion in the ewe.

Identification and characterization of estrogen receptor alpha-containing neurons projecting to the vicinity of the gonadotropin-releasing hormone perikarya in the rostral preoptic area of the rat

Gonadal steroids exert a powerful regulatory influence upon the functioning of gonadotropin-releasing hormone (GnRH) neurons despite the apparent absence of gonadal steroid receptors in these cells. By using retrograde-tracing techniques combined with dual-labeling immunocytochemistry, we show here that distinct populations of estrogen receptor alpha (ERalpha)-containing neurons located in the hypothalamus and caudal brainstem project to the vicinity of the GnRH perikarya located in the rostral preoptic area (rPOA). The strongest estrogen-receptive afferent projection to this area originated from neurons located in the anteroventral periventricular and medial preoptic nuclei of the preoptic area. Approximately 50% of arcuate nucleus neurons projecting to the rPOA were demonstrated to synthesize either neuropeptide Y or beta-endorphin, but little evidence was found for ERalpha immunoreactivity in either of these specific subpopulations. Over 80% of all tyrosine hydroxylase-expressing neurons in the arcuate nucleus expressed ERalpha, but none projected to the rPOA. In the caudal brainstem, the A1 and A2 norepinephrine neurons comprised nearly all of the retrogradely labeled neurons. However, only the A2 afferents expressed ERalpha immunoreactivity, whereas the A1 afferents coexpressed neuropeptide Y. These observations, combined with the anterograde labeling data of others, provide neuroanatomical evidence for the existence of specific estrogen-receptive neuronal cell populations that project to the rPOA and may be involved in the estrogen-dependent transsynaptic regulation of GnRH neurons in the rat.

Fluctuating estrogen and progesterone receptor expression in brainstem norepinephrine neurons through the rat estrous cycle

Norepinephrine (NE) neurons within the nucleus tractus solitarii (NTS; A2 neurons) and ventrolateral medulla (A1 neurons) represent gonadal steroid-dependent components of several neural networks regulating reproduction. Previous studies have shown that both A1 and A2 neurons express estrogen receptors (ERs). Using double labeling immunocytochemistry we report here that substantial numbers of NE neurons located within the NTS express progesterone receptor (PR) immunoreactivity, whereas few PRs are found in ventrolateral medulla. The evaluation of ERa and PR immunoreactivity in NE neurons through the estrous cycle revealed a fluctuating pattern of expression for both receptors within the NTS. The percentage of A2 neurons expressing PR immunoreactivity was low on metestrus and diestrus (3-7%), but increased significantly to approximately 24% on proestrous morning and remained at intermediate levels until estrus. The pattern of ERalpha immunoreactivity in A2 neurons was more variable, but a similar increment from 11% to 40% of NE neurons expressing ERa was found from diestrus to proestrus. Experiments in ovariectomized, estrogen-treated and estrogen-plus progesterone-treated rats revealed that PR immunoreactivity in A2 neurons was induced strongly by estrogen treatment, whereas progesterone had no significant effect. The numbers of ERalpha-positive NE neurons were not influenced by steroid treatment. These observations provide direct evidence for PRs in NE neurons of the brainstem and show that cyclical patterns of gonadal steroid receptor expression exist in A2, but not A1, neurons through the rat estrous cycle. The expression of PR in A2 neurons appears to be driven principally by circulating estrogen concentrations. The fluctuating levels of ERalpha and PR expression in these brainstem NE neurons may help generate cyclical patterns of biosynthetic and electrical activity within reproductive neural networks.

Endotoxin disrupts the estradiol-induced luteinizing hormone surge: interference with estradiol signal reading, not surge release

Three experiments were conducted to investigate whether the immune/inflammatory stimulus endotoxin disrupts the estradiol-induced LH surge of the ewe. Ovariectomized sheep were set up in an artificial follicular phase model in which luteolysis is simulated by progesterone withdrawal and the follicular phase estradiol rise is reproduced experimentally. In the first experiment, we tested the hypothesis that endotoxin interferes with the estradiol-induced LH surge. Ewes were either infused with endotoxin (300 ng/kg/h, i.v.) for 30 h beginning at onset of a 48-h estradiol stimulus or sham infused as a control. Endotoxin significantly delayed the time to the LH surge (P < 0.01), but did not alter surge amplitude, duration, or incidence. The second experiment tested the hypothesis that the delaying effects of endotoxin on the LH surge depend on when endotoxin is introduced relative to the onset of the estradiol signal. Previous work in the ewe has shown that a 14-h estradiol signal is adequate to generate GnRH and LH surges, which begin 6-8 h later. Thus, we again infused endotoxin for 30 h, but began it 14 h after the onset of the estradiol signal. In contrast to the first experiment, endotoxin given later had no effect on any parameter of the LH surge. In the third experiment, we tested the hypothesis that endotoxin acts during the first 14 h to disrupt the initial activating effects of estradiol. Estradiol was delivered for just 14 h, and endotoxin was infused only during this time. Under these conditions, endotoxin blocked the LH surge in five of eight ewes. In a similar follow-up study, endotoxin again blocked the LH surge in six of seven ewes. We conclude that endotoxin can disrupt the estradiol-induced LH surge by interfering with the early activating effects of the estradiol signal during the first 14 h (reading of the signal). In contrast, endotoxin does not disrupt later stages of signal processing (i.e. events during the interval between estradiol signal delivery and surge onset), nor does it prevent actual hormonal surge output. Thus, endotoxin appears to disrupt estrogen action per se rather than the release of GnRH or LH at the time of the surge.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Identification of alpha1B adrenergic receptor protein in gonadotropin releasing hormone neurones of the female rat

Noradrenaline is an important neurotransmitter which regulates GnRH release from the median eminence in the female rat during both basal GnRH secretion and the preovulatory or steroid hormone-induced GnRH-mediated LH surge. However, it is not clear at which sites in the brain this predominantly stimulatory influence is exerted nor is it known which adrenergic receptor subtype(s) mediate(s) the effects of noradrenaline. In order to determine if the GnRH neurones in the septum-diagonal band-preoptic area and/or their axon terminals in the median eminence are direct targets for noradrenaline, immunohistochemical triple-labelling studies were conducted to localize simultaneously GnRH peptide, dopamine-beta-hydroxylase and alpha1B adrenergic receptor protein. The results show that about 80% of all GnRH neurones examined contained patches of immunoreactive alpha1B adrenergic receptor protein at or near the plasma membrane and that some of these alpha1B adrenergic receptors were adjacent to dopamine-beta-hydroxylase containing axons. The GnRH neurones which did not contain alpha1B adrenergic receptors were preferentially located in the rostral portion of the septum and diagonal band while all GnRH neurones in the caudal septum, diagonal band and in the preoptic area expressed alpha1B adrenergic receptors. In the median eminence, a few alpha1B adrenergic receptor patches were seen in the external layer and these receptors were only rarely observed to be associated with GnRH containing axon terminals. The results suggest that the effects of noradrenaline on GnRH release are, at least in part, mediated by the activation of alpha1B adrenergic receptors which are located on most GnRH perikarya while the median eminence is not a likely site at which GnRH release is regulated by alpha1B adrenergic receptors.

Daily cycle of fos expression within hypothalamic POMC neurons of the male rat

The activity of hypothalamic pro-opiomelanocortin (POMC) neurons is known to display a circadian cycle. We hypothesized that the existence of a c-Fos responsive element (AP-1 site) within the POMC gene sequence might reflect the ability of POMC neurons to express c-fos proto-oncogene during circadian increase of their neuronal activity. To this aim, adult male rats previously kept under a controlled 12 h light/12 h dark schedule were sacrificed every 4 h throughout the 24 h cycle and their brains processed for Fos and/or POMC immunocytochemistry. Here we show that, specifically during the dark period of the cycle, the mediobasal hypothalamic area spontaneously exhibits a strong Fos immunoreactivity, whereas very low Fos labelling was detected during the light period. As postulated, the simultaneous visualisation of both Fos and POMC antigens allowed us to show that this nocturnal induction of Fos occurs almost exclusively at the nuclear level of POMC-producing neurons. These results not only highlight the mechanisms underlying the physiological functioning of the hypothalamic POMC system, but also demonstrate the feasibility of using c-fos expression as a useful tool to assess the pharmacological effect of drugs on the activity of POMC neurons as is the case for many other neuronal systems. Such drugs might be relevant in the treatment of psychosis since an alteration of POMC-related peptide transmission has been reported in the brains of both schizophrenic and depressive patients.

Effect of the estrous cycle on olfactory bulb response to vaginocervical stimulation in the rat: results from electrophysiology and Fos immunocytochemistry experiments

To determine whether the stage of the estrous cycle modified the response of olfactory bulb neurons to vaginocervical stimulation, (1) vaginocervical stimulation was applied to animals in proestrus-estrus and metestrus-diestrus and the extracellular electrophysiological response of units in the mitral cell layer of the main olfactory bulb was compared, and (2) the effect of vaginocervical or sham stimulation and the effect of the estrous cycle on the number of neurons stained immunocytochemically for Fos in the main and accessory olfactory bulb was examined. Animals in proestrus-estrus had basal firing rates of 21.8 +/- 1.8 spikes per 5 s and vaginocervical stimulation produced an increase in firing rate. In contrast, animals in metestrus-diestrus had a slower basal firing rate (14.3 +/- 2.3 spikes per 5 s) and vaginocervical stimulation produced a decrease in the firing rate. For animals in proestrus-estrus, vaginocervical stimulation increased the number of Fos-stained cells in the granular cell layer of the accessory olfactory bulb, and in the glomerular and in external plexiform layers of the main olfactory bulb. In contrast, the number of Fos-stained cells decreased in the granular cell layer of the main olfactory bulb after stimulation was applied to animals in proestrus-estrus. The number of Fos-stained cells in the granular layer of the accessory olfactory bulb and the granular and glomerular cell layers of the main olfactory bulb was modulated by the estrous cycle. Therefore, olfactory bulb activity, measured both electrophysiologically and by Fos staining, was affected by the estrous cycle and vaginocervical stimulation, and the two variables interacted. It is likely that integration of interoceptive and environmental stimulation is important for the normal expression of sexual behavior in the female rat.

Differential expression of estrogen receptor and neuropeptide Y by brainstem A1 and A2 noradrenaline neurons

The release of noradrenaline and neuropeptide Y appears to be regulated by estrogen in a co-ordinated fashion within specific brain regions. The present study has used double and triple-labelling immunocytochemical procedures to determine the patterns of nuclear estrogen receptor and neuropeptide Y expression by brainstem A1 and A2 noradrenergic neurons in the female rat. Estrogen receptor-immunoreactive cells were detected within the ventrolateral medulla, nucleus tractus solitarius, area postrema and, in the very caudal medulla, the reticular nuclei and spinal nucleus of the trigeminal nerve. Cells double labelled for the estrogen receptor and dopamine-beta-hydroxylase were identified in largest numbers (up to seven double-labelled cells per 30-microm-thick coronal section) in the caudal-most medulla, where approximately 30% of A1 and 60% of A2 neurons were immunoreactive for the estrogen receptor. These percentages reduced in a linear fashion in more rostral sections and at the level of the area postrema, no co-expression was evident in the ventrolateral medulla and only 10% of A2 neurons displayed estrogen receptor immunoreactivity. Fluorescence double-labelling studies undertaken in colchicine-treated rats revealed that 50% and 90-100% of tyrosine hydroxylase-immunoreactive cells were positive for neuropeptide Y in the rostral ventrolateral medulla and nucleus tractus solitarius (up to 15 double-labelled cells per section), respectively. This pattern of co-expression also showed a rostrocaudal bias, but in the opposite direction, such that none of the caudal-most A1 and only 10% of caudal A2 neurons were immunoreactive for neuropeptide Y. Triple-labelling experiments revealed the presence of a total of only three triple-labelled cells in the ventrolateral medulla and none in the nucleus tractus solitarius of four rats. Double-labelling studies examining estrogen receptor and neuropeptide Y co-expression similarly found only three double-labelled cells in the ventrolateral medulla. These findings provide immunocytochemical evidence for a clear rostrocaudal topography in nuclear estrogen receptor synthesis by A1 and A2 neurons and show a reverse rostrocaudal bias in neuropeptide Y expression by these cells. The absence of any substantial neuropeptide Y and estrogen receptor co-expression in A1 and A2 neurons indicates that these two proteins are very likely to be differentially expressed by brainstem noradrenergic neurons. Such observations provide further evidence for the biosynthetic and functional heterogeneity of brainstem noradrenergic cells and suggest that A1 and A2 neurons transmitting information on estrogen status within the brain are unlikely to utilize neuropeptide Y as a co-transmitter.

Neuronal expression of Fos-like protein along the afferent pathway of the milk-ejection reflex in the sheep

This study was designed to reveal the relay stations of the afferent branch of the milk-ejection reflex in the sheep, by examining the effect of nipple stimulation on the expression of Fos protein along the spino-hypothalamic axis. Immunocytochemical detection of Fos protein after manual nipple stimulation in nonlactating ewes revealed immunolabeled neurons located exclusively ipsilaterally in the 3rd and 4th lumbar spinal ganglia, the medial part of laminae I-III of the 3rd and 4th lumbar spinal segments, the lateral cervical nucleus, the dorsal column nuclei, and bilaterally in the hypothalamic paraventricular nucleus. These findings selectively demonstrate for the first time those cell groups mediating the neuroendocrine effects of nipple stimulation.

Estrogen uncouples noradrenergic activation of Fos expression in the female rat preoptic area

The preoptic area of the rat brain is a site at which gonadal steroids act to regulate sexual behaviour and gonadotrophin secretion. The expression of the immediate-early gene product, Fos, in the preoptic area was investigated in conscious ovariectomised, vehicle and estrogen-treated animals which had received an intracerebroventricular (i.c.v.) infusion of noradrenaline, and also in anaesthetised proestrous and ovariectomised rats following electrical stimulation of the brainstem A1 or A2 noradrenergic cell groups. In ovariectomised oil-treated rats, a third ventricular infusion of noradrenaline (45 micrograms) resulted in a significant (P < 0.05) increase in the numbers of Fos-immunoreactive cell nuclei throughout the preoptic area, compared to vehicle controls. In contrast, Fos expression in animals which had received estrogen replacement showed no change in response to i.c.v. noradrenaline compared with saline-treated controls. In anaesthetised, ovariectomised animals electrical stimulation of the A1 cell group resulted in a significant increase (P < 0.05) in Fos-like immunoreactivity compared with sham controls, specifically within the ventral preoptic area whilst stimulation of the A2 cell group had no significant effect. In anaesthetised, proestrous rats receiving electrical stimulation no significant changes in Fos-like immunoreactivity were detected within the preoptic area after either A1 or A2 stimulation compared with paired controls. These results show that noradrenaline-induced Fos expression in the preoptic area is dependent on estrogen status and suggest that the estrogenic regulation of reproductive functions may thus involve altered responses to noradrenaline in sub-populations of preoptic neurones.

Relationship of neuronal nitric oxide synthase immunoreactivity to GnRH neurons in the ovariectomized and intact female rat

The present study has used a rat neuronal nitric oxide synthase (nNOS) antibody to examine the relationship of nNOS immunoreactivity to GnRH neurons in the ovariectomized and intact diestrous and proestrous rat. A striking band of nNOS-immunoreactive cells was identified in the rostral preoptic area which began in the median preoptic nucleus and organum vasculosum of the lamina terminalis and formed an inverted Y-type distribution above the rostral third ventricle at the level of the anteroventral periventricular nucleus. Another band of nNOS-immunoreactivity was found extending through the internal zone of the median eminence into the arcuate nucleus. Although nNOS immunoreactivity was not detected within GnRH neuronal cell bodies in any of the experimental groups, GnRH perikarya located in the rostral preoptic area, but not elsewhere, were found to be surrounded by nNOS-containing cells. In the median eminence, nNOS and GnRH immunoreactivities were distributed separately in the internal and external zones, respectively. These results provide evidence that, regardless of their pattern of activity, GnRH neurons in the female rat do not express nNOS. Instead, a close anatomical relationship between nNOS-immunoreactive cells and GnRH perikarya and fibers has been identified within specific sub-regions of the rostral preoptic area and in the median eminence. Such findings are compatible with a role for NO at both sites in regulating the release of GnRH throughout the estrous cycle.