Arousal-related reticular neurons during reduced oxygen tension: resilience and recovery of electrical activity

Whole-cell patch clamp recordings of the electrical activity of large medullary reticular formation neurons, in nucleus gigantocellularis, were performed under control conditions and under conditions of hypoxia or anoxia. Neurons were discovered whose activity was remarkably resilient during and after the reduction or loss of oxygen. Such cells may relate to the ability of the newborn brain to survive hypoxia/anoxia, and also may demonstrate the preservation of neurons involved in generalized CNS arousal, as would be appropriate for activating behavioral responses to the reduction or loss of oxygen.

Thyroid hormone action: nongenomic modulation of neuronal excitability in the hippocampus

Years of effort have failed to establish a generally-accepted mechanism of thyroid hormone (TH) action in the mature brain. Recently, both morphological and pharmacological evidence have supported a direct neuroactive role for the hormone and its triiodinated metabolites. However, no direct physiological validation has been available. We now describe electrophysiological studies in vivo in which we observed that local thyroxine (T4) administration promptly inhibited field excitatory postsynaptic potentials recorded in the dentate gyrus (DG) with stimulation of the medial perforant pathway, a result that was found to be especially pronounced in hypothyroid rats. In separate in vitro experiments, we observed more subtle but statistically significant responses of hippocampal slices to treatment with the hormone. The results demonstrate that baseline firing rates of CA1 pyramidal cells were modestly reduced by pulse-perfusion with T4. By contrast, administration of triiodothyronine (T3) was often noted to have modest enhancing effects on CA1 cell firing rates in hippocampal slices from euthyroid animals. Moreover, and more reliably, robust firing rate increases induced by norepinephrine were amplified when preceded by treatment with T3, whereas they were diminished by pretreatment with T4. These studies provide the first direct evidence for functional, nongenomic actions of TH leading to rapid changes in neuronal excitability in adult rat DG studied in vivo and highlight the opposing effects of T4 and T3 on norepinephrine-induced responses of CA1 cells studied in vitro.

Reverse engineering the lordosis behavior circuit

Reverse engineering takes the facts we know about a device or a process and reasons backwards to infer the principles underlying the structure-function relations. The goal of this review is to apply this approach to a well-studied hormone-controlled behavior, namely the reproductive stance of female rodents, lordosis. We first provide a brief overview on the considerable amount of progress in the analysis of female reproductive behavior. Then, we propose an analysis of the mechanisms of this behavior from a reverse-engineering perspective with the goal of generating novel hypotheses about the properties of the circuitry elements. In particular, the previously proposed neuronal circuit modules, feedback signals, and genomic mechanisms are considered to make predictions in this manner. The lordosis behavior itself appears to proceed ballistically once initiated, but negative and positive hormonal feedback relations are evident in its endocrine controls. Both rapid membrane-initiated and slow genomic hormone effects contribute to the behavior's control. We propose that the value of the reverse-engineering approach is based on its ability to provide testable, mechanistic hypotheses that do not emerge from either traditional evolutionary or simple reductionistic perspectives, and several are proposed in this review. These novel hypotheses may generalize to brain functions beyond female reproductive behavior. In this way, the reverse-engineering perspective can further develop our conceptual frameworks for behavioral and systems neuroscience.

Voltage-dependent calcium channels in ventromedial hypothalamic neurones of postnatal rats: modulation by oestradiol and phenylephrine

Oestradiol actions in the hypothalamus play an important role in reproductive behaviour. Oestradiol treatment in vivo induces alpha(1b)-adrenoceptor mRNA and increases the density of alpha(1B)-adrenoceptor binding in the hypothalamus. Oestradiol is also known to modulate neuronal excitability, in some cases by modulating calcium channels. We assessed the effects of phenylephrine, an alpha(1)-adrenergic agonist, on low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium channels in ventromedial hypothalamic (VMN) neurones from vehicle- and oestradiol-treated female rats. Whole-cell and gramicidin perforated-patch recordings were obtained, with barium as the charge carrier. In the absence of phenylephrine, oestradiol treatment increased the magnitude of LVA currents compared to controls, but had no effect on HVA currents. Phenylephrine enhanced HVA currents in a significantly greater proportion of neurones from oestradiol-treated rats (76%) than from vehicle-treated (41%) rats. The L-channel blocker nifedipine abolished this oestradiol effect on phenylephrine-enhanced HVA currents. Preincubating slices with the N-type channel blocker omega-conotoxin GVIA completely blocked the phenylephrine response, suggesting that the N-type channel is essential. Phenylephrine also stimulated LVA currents in approximately two-thirds of neurones in slices from both vehicle- and oestradiol-treated rats. Our data show that oestradiol increases LVA currents in the VMN. Oestradiol also amplifies alpha(1)-adrenergic signalling by increasing the proportion of neurones showing phenylephrine-stimulated HVA currents mediated by N- and L-type calcium channels. In this way, oestradiol may increase excitatory responses to arousing adrenergic inputs to VMN neurones governing oestradiol-dependent reproductive behaviour.

Hormonal induction of lordosis and ear wiggling in rat pups: gender and age differences

To assess how early can estrogens induce female mating behaviors, rat pups 8-29 days old (D8-D29, respectively) were injected twice daily with estradiol benzoate (E) or oil (O) followed by progesterone (P) or oil, and then observed for the estrogen-dependent ear wiggling (EW) and lordosis in response to natural stimulation from male rats. In female pups treated with E + E + P, the incidence of EW appeared as early as D13 and increased gradually to reach maximum at D18, when all pups tested showed EW. EW also occurred in E + E + O females, but never in O + O + P females or in any E + E + P male. Lordosis in E + E + P, as well as E + E + O, female pups occurred later, starting at D15. O + O + P females or E + E + P males never display lordosis. To explore the possibilities that the age and gender differences are due to distribution and/or function of estrogen receptor-alpha (ERalpha) or progesterone receptor (PR), separate pups were used for immunocytochemical (ICC) staining of these receptors in the hypothalamic ventromedial nucleus (VMN). There was no age difference in female pups in the density of ERalpha or the induction of PR between D11/D12, when no sexual behavior was observed, and D19/D20, when almost all pups tested performed the behaviors. There were gender differences: male pups had less ERalpha than females at D19/D20, though not at D11/D12, and did not respond to E in the induction of PR in the VMN. These results show that ERs and their signaling systems in the VMN of rat pups are functional at least after D11 but only in females, and that the gender differences appeared to be due to differences in the molecular biology of ERalpha.

Acute estradiol application increases inward and decreases outward whole-cell currents of neurons in rat hypothalamic ventromedial nucleus

Acute estradiol (E2) can potentiate the excitatory responses of hypothalamic ventromedial nucleus (VMN) neurons to neurotransmitters. To investigate the mechanism(s) underlying the potentiation, the whole-cell patch voltage clamp technique was used to study VMN neurons in hypothalamic slices prepared from female juvenile (3-5 weeks) rats. A voltage step and/or ramp was applied every 5 min to evoke whole-cell currents before, during and after a treatment with E2 (10 nM), corticosterone (10 nM) or vehicle for up to 20 min. Acute E2 increased inward currents in 38% of neurons tested. Their average peak inward current amplitudes started to increase within 5 min and reached the maximum of 163% of pretreatment level (Pre) at 20 min of treatment before recovering toward Pre. These increases are significantly greater than the Pre and corresponding vehicle controls and non-responsive neurons. Outward currents were decreased significantly by E2 in 27% of E2-treated cells, down to 60% of Pre levels. E2 also appeared to affect the kinetics of the inward and outward currents of estrogen-responsive neurons. Whenever observed, the effects of acute E2 were reversible after a 5- to 10-min washing. Probability analysis indicates that E2 affected the inward and the outward currents independently. The E2 effects are specific in that they were not produced by similar treatment with vehicle or corticosterone. Pharmacological characterizations using ion replacement and channel blockers showed that the inward currents were mediated practically all by Na(+) and the outward currents mainly by K(+). Thus, acute E2 can enhance inward Na(+) and attenuate outward K(+) currents. Since both effects will lead to an increase in neuronal excitability, they may explain our previous observation that E2 potentiates the excitation of VMN neurons.

Sex and estrogenic effects on coexpression of mRNAs in single ventromedial hypothalamic neurons

Regulated gene expression in single neurons can be linked to biophysical events and behavior in the case of estrogen-regulated gene expression in neurons in the ventrolateral portion of the ventromedial nucleus (VMN) of the hypothalamus. These cells are essential for lordosis behavior. What genes are coexpressed in neurons that have high levels of mRNAs for estrogen receptors (ERs)? We have been able to isolate and measure certain mRNAs from individual VMN neurons collected from rat hypothalamus. Large numbers of neurons express mRNA for ERalpha, but these neurons are not identical with the population of VMN neurons expressing the likely gene duplication product, ERbeta. An extremely high proportion of neurons expressing either ER also coexpress mRNA for the oxytocin receptor (OTR). This fact matches the known participation of oxytocin binding and signaling in sexual and affiliative behaviors. In view of data that ER and OTR can signal through PKCs, we looked at coexpression of selected PKCs in the same individual neurons. The most discriminating analysis was for triple coexpression of ERs, OTR, and each selected PKC isoform. These patterns of triple coexpression were significantly different for male vs. female VMN neurons. Further, individual neurons expressing ERalpha could distribute their signaling across the various PKC isoforms differently in different cells, whereas the reverse was not true. These findings and this methodology establish the basis for systematic linkage of the brain's hormone-sensitive signaling pathways to biophysical and behavioral mechanisms in a well studied mammalian system.

Female oxytocin gene-knockout mice, in a semi-natural environment, display exaggerated aggressive behavior

Compared to results from a generation of neuropharmacological work, the phenotype of mice lacking the oxytocin (OT) peptide gene was remarkably normal. An important component of the current experiments was to assay OT-knockout (OTKO) and wild-type (WT) littermate control mice living under controlled stressful conditions designed to mimic more closely the environment for which the mouse genome evolved. Furthermore, our experimental group was comprised of an all-female population, in contrast to previous studies which have focused on all-male populations. Our data indicated that aggressive behaviors initiated by OTKO during a food deprivation feeding challenge were considerably more intense and diverse than aggressive behaviors initiated by WT. From the measures of continuous social interaction in the intruder paradigm, it emerged that OTKO mice were more offensively aggressive (attacking rumps and tails) than WT. In a test of parental behaviors, OTKO mice were 100% infanticidal while WT were 16% infanticidal and 50% maternal. Finally, 'alpha females' (always OTKO) were identified in each experiment. They were the most aggressive, the first to feed and the most dominant at nesting behaviors. Semi-natural environments are excellent testing environments for elucidating behavioral differences between transgenic mice and their WT littermates which may not be ordinarily discernible. Future studies of mouse group behavior should include examining female groupings in addition to the more usual all-male groups.

Acute estrogen potentiates excitatory responses of neurons in rat hypothalamic ventromedial nucleus

In a previous behavioral study, brief application of a membrane-limited estrogen to neurons in rat hypothalamic ventromedial nucleus (VMN) facilitated lordosis behavior-inducing genomic actions of estrogen. Here, electrophysiological recordings from single neurons were employed to characterize these membrane-initiated actions. From rat hypothalamic slices, electrical activity was recorded from neurons in the ventrolateral VMN, the cell group crucial for estrogen induction of lordosis. In addition to the resting activity, neuronal responses to histamine (HA) and N-methyl-d-aspartate (NMDA) were also recorded before, during, and after a brief (10-15 min) application of estradiol (E, 10 nM). These two transmitters were chosen because their actions are mediated by different mechanisms: HA through G protein-coupled receptors and NMDA by ligand-activated ion channels. Vehicle applications did not affect either resting activity or neuronal responses. In contrast, acute E exposure modulated neuronal responses to transmitters, with no significant effect on the resting activity. It potentiated excitatory responses to HAs (20 out of 48 cells tested) and to NMDA (10 out of 19 cells), but attenuated inhibitory responses to HA (3 out of 6 units). Both of these hormonal actions would increase VMN neuronal excitation. In separate experiments, neuronal excitation was found to be suppressed by anesthetics, which would block E's induction of lordosis when administered at the time of estrogen application. These data are consistent with the notion that increasing electrical excitation of VMN neurons can be a mechanism by which acute E exposure facilitates the lordosis-inducing genomic actions of estrogens.

Potentiation of the excitatory action of NMDA in ventrolateral periaqueductal gray by the mu-opioid receptor agonist, DAMGO

Several lines of evidence have suggested that mu-opioids, generally regarded as inhibitory, also have effects that stimulate neural activity. To look for possible excitatory opioid action in the rat periaqueductal gray (PAG), we first re-examined data from a previous study and found that met-enkephalin could evoke a delayed, sluggish excitation, suggestive of modulation by the opioid on the action of certain excitants. This observation, coupled with other studies that show mu-opioids can modulate NMDA receptor activation, prompted us to perform extracellular recording of the responses of single ventrolateral PAG (vlPAG) neurons in brain slices to DAMGO, a mu-opioid, and to NMDA. When applied alone, DAMGO at nM concentrations, like met-enkephalin, often evoked the delayed excitation and occasionally an inhibition. When applied after a brief exposure to NMDA, DAMGO at doses as low as 0.1 nM potentiated the excitation produced by a subsequent pulse of NMDA. This occurred, depending on cell type, in 23-100% of vlPAG neurons. The potentiating action of DAMGO was blocked by naloxone, suggesting it was mediated by mu-opioid receptors. Characterization of these mu-opioid actions revealed that the potentiation and the delayed excitation, unlike the inhibition, was not blocked by another opioid antagonist, nalmefene, nor by an inhibitor of the G protein of the G(i) class, N-ethylmaleimide. Moreover, the potentiating action was distinct from the inhibition in that it was: (a) enhanced by repeated opioid applications, (b) exhibited low effective doses, (c) had a long time course (minutes to develop and last tens of minutes) and (d) was present in distinct though overlapping cell populations. These data reveal an unconventional action of opioids in PAG neurons, that is, a potentiation of excitation produced by NMDA. This effect appeared mechanistically distinct from opioid inhibition or disinhibition and may be related to established examples of direct opioid excitation. These observations may help understanding behaviorally important mechanisms linked to acute and chronic opioid functions in the vlPAG.

Early membrane estrogenic effects required for full expression of slower genomic actions in a nerve cell line

Interpretations of steroid hormone actions as slow, nuclear, transcriptional events have frequently been seen as competing against inferences of rapid membrane actions. We have discovered conditions where membrane-limited effects potentiate later transcriptional actions in a nerve cell line. Making use of a two-pulse hormonal schedule in a transfection system, early and brief administration of conjugated, membrane-limited estradiol was necessary but not sufficient for full transcriptional potency of the second estrogen pulse. Efficacy of the first pulse depended on intact signal transduction pathways. Surprisingly, the actions of both pulses were blocked by a classical estrogen receptor (ER) antagonist. Thus, two different modes of steroid hormone action can synergize.

Brain glucose-sensing mechanisms: ubiquitous silencing by aglycemia vs. hypothalamic neuroendocrine responses

Interest in brain glucose-sensing mechanisms is motivated by two distinct neuronal responses to changes in glucose concentrations. One mechanism is global and ubiquitous in response to profound hypoglycemia, whereas the other mechanism is largely confined to specific hypothalamic neurons that respond to changes in glucose concentrations in the physiological range. Although both mechanisms use intracellular metabolism as an indicator of extracellular glucose concentration, the two mechanisms differ in key respects. Global hyperpolarization (inhibition) in response to 0 mM glucose can be reversed by pyruvate, implying that the reduction in ATP levels acting through ATP-dependent potassium (K-ATP) channels is the key metabolic signal for the global silencing in response to 0 mM glucose. In contrast, neuroendocrine hypothalamic responses in glucoresponsive and glucose-sensitive neurons (either excitation or inhibition, respectively) to physiological changes in glucose concentration appear to depend on glucokinase; neuroendocrine responses also depend on K-ATP channels, although the role of ATP itself is less clear. Lactate can substitute for glucose to produce these neuroendocrine effects, but pyruvate cannot, implying that NADH (possibly leading to anaplerotic production of malonyl-CoA) is a key metabolic signal for effects of glucose on glucoresponsive and glucose-sensitive hypothalamic neurons.

Presynaptic and postsynaptic relations of mu-opioid receptors to gamma-aminobutyric acid-immunoreactive and medullary-projecting periaqueductal gray neurons

The ventrolateral portion of the periaqueductal gray (PAG) is one brain region in which ligands of the mu-opioid receptor (MOR) produce analgesia. In the PAG, MOR ligands are thought to act primarily on inhibitory [e.g., gamma-aminobutyric acidergic (GABAergic)] neurons to disinhibit PAG output rather than directly on medullary-projecting PAG neurons. In this study, the ultrastructural localization of MOR immunolabeling was examined with respect to either GABAergic PAG neurons or PAG projection neurons that were labeled retrogradely from the rostral ventromedial medulla. Immunoreactivity for MOR and GABA often coexisted within dendrites. Dual-labeled profiles accounted for subpopulations of dendrites containing immunoreactivity for either MOR (65 of 145 dendrites; 45%) or GABA (65 of 183 dendrites; 35%). In addition, nearly half of PAG neuronal profiles (148 of 344 profiles) that were labeled retrogradely from the ventromedial medulla contained MOR immunoreactivity. MOR was distributed equally among retrogradely labeled neuronal profiles in the lateral and ventrolateral columns of the caudal PAG. With respect to the presynaptic distribution of MOR, approximately half of MOR-immunolabeled axon terminals (35 of 69 terminals) also contained GABA. Some MOR and GABA dual-immunolabeled axon terminals contacted unlabeled dendrites (11 of 35 terminals), whereas others contacted GABA-immunoreactive dendrites (15 of 35 terminals). Furthermore, axon terminals synapsing on medullary-projecting PAG neurons sometimes contained immunoreactivity for MOR. These data support the model that MOR ligands can act by inhibiting GABAergic neurons, but they also provide evidence that MOR ligands may act directly on PAG output neurons. In addition, MOR at presynaptic sites could affect both GABAergic neurons and output neurons. Thus, the disinhibitory model represents only partially the potential mechanisms by which MOR ligands can modulate output of the PAG.

Hypothalamic glucose sensor: similarities to and differences from pancreatic beta-cell mechanisms

Glucose-responsive neurons in the ventromedial hypothalamus (VMH) are stimulated when glucose increases from 5 to 20 mmol/l and are thought to play an essential role in regulating metabolism. The present studies examined the role of glucose metabolism in the mechanism by which glucose-responsive neurons sense glucose. The pancreatic, but not hepatic, form of glucokinase was expressed in the VMH, but not cerebral cortex, of adult rats. In brain slice preparations, the transition from 5 to 20 mmol/l glucose stimulated approximately 17% of the neurons (as determined by single-cell extracellular recording) from VMH but none in cortex. In contrast, most cells in both VMH and cortex were silent below 1 mmol/l and active at 5 mmol/l glucose. Glucosamine, 2-deoxyglucose, phloridzin, and iodoacetic acid blocked the activation of glucose-responsive neurons by the transition from 5 to 20 mmol/l glucose. Adding 15 mmol/l mannose, galactose, glyceraldehyde, glycerol, and lactate to 5 mmol/l glucose stimulated glucose-responsive neurons. In contrast, adding 15 mmol/l pyruvate to 5 mmol/l glucose failed to activate glucose-responsive neurons, although pyruvate added to 0 mmol/l glucose permitted neurons to maintain activity. Tolbutamide activated glucose-responsive neurons; however, diazoxide only blocked the effect of glucose in a minority of neurons. These data suggest that glucose-responsive neurons sense glucose through glycolysis using a mechanism similar to the mechanism of pancreatic beta-cells, except that glucose-responsive neurons are stimulated by glycerol and lactate, and diazoxide does not generally block the effect of glucose.

Neural oxytocinergic systems as genomic targets for hormones and as modulators of hormone-dependent behaviors

At the molecular level, estradiol turns on the gene for oxytocin in a subset of paraventricular hypothalamic neurons and turns on the gene for the oxytocin receptor in other limbic and hypothalamic cell groups. As a result, oxytocin deposition, whose signal is transduced both through G alpha (q/11) and Gi to stimulate phosphatidylinositol turnover, facilitates electrical activity in certain hypothalamic neurons. Consequently, affiliative behaviors including those closely associated with reproduction--mating behaviors and parental behaviors--are promoted. One important aspect of this effect is the preservation of instinctive behaviors associated with reproduction, in the face of disturbances due to mild stress.

In the ventromedial nucleus of the rat hypothalamus, GABA-immunolabeled neurons are abundant and are innervated by both enkephalin- and GABA-immunolabeled axon terminals

Immunohistochemical-labeling for the neurochemicals gamma-aminobutyric acid (GABA) and enkephalin are abundant in the ventromedial nucleus of the hypothalamus (VMN). In VMN, both GABA and enkephalin may function to regulate feeding behavior, as well as other hormone-controlled behaviors. Importantly, in several brain areas, enkephalin is often thought to modulate GABAergic neurotransmission. Therefore, we used dual-labeling immunohistochemistry with electron microscopic analysis to study the circuitry of neurons containing GABA- and/or enkephalin-labeling within the VMN. Somato-dendritic profiles containing GABA-labeling were three fold more abundant than GABA-labeled axon terminals (117 soma or dendrites vs. 34 axons). In addition, axon terminals containing GABA-labeling sometimes synapsed onto GABA-labeled somata or dendrites (25% or 9/34). In contrast, under these conditions labeling for enkephalin was primarily restricted to axon terminals, which were very abundant throughout VMN. Enkephalin-containing terminals accounted for a large fraction (25% 23/92) of the axons in contact with GABA-labeled dendrites, although they also contacted unlabeled dendrites. These observations suggest that a population of VMN neurons are GABAergic. These may be either local circuit 'interneurons' or projection neurons. In addition, GABA-labeled VMN neurons may be regulated by either enkephalin or GABA. These morphologic observations provide the basis for disinhibitory mechanisms to function within the VMN.

Mapping of neural and signal transduction pathways for lordosis in the search for estrogen actions on the central nervous system

Estrogen can act on the brain to regulate various biological functions and behavior. In attempts to elucidate the estrogen action, the rodent female reproductive behavior, lordosis, was used as a model. Lordosis is an estrogen-dependent reflexive behavior and, hence, is mediated by discrete neural pathways that are modulated by estrogen. Therefore, a strategy of mapping the pathways, both neural and biochemical, and examining them for estrogen effect was used to localize and subsequently analyze the central action of estrogen. Using various experimental approaches, an 'inverted Y-shaped' neural pathway both sufficient and essential for mediating lordosis was defined. The top portion is a descending pathway conveying the permissive estrogen influence which originated from hypothalamic ventromedial nucleus relayed via midbrain periaqueductal grey down to medullary reticular formation, the top of the spino-bulbo-spinal reflex arc at the bottom. This estrogen influence alters the input-output relationship, shifting the output toward more excitation. With this shift in output, estrogen can enable the otherwise ineffective lordosis-triggering sensory stimuli to elicit lordosis. In the ventromedial nucleus, the origin of the estrogen influence, a multidisciplinary approach was used to map intracellular signaling pathways. A phosphoinositide pathway involving a specific G protein and the activation of protein kinase C was found to be involved in the mediation of lordosis as well as a probable target of the permissive estrogen action. The action of estrogen on this signal transduction pathway, a potentiation, is consistent with and, hence, may be an underlying mechanism for the estrogen influenced shift toward excitation. Thus, further investigation on this specific signal transduction pathway should be helpful in elucidating the action of estrogen on the brain.

Sexual reinforcement in the female rat

Sexual reinforcement in the female rat was studied in a preparation that allowed continuous operant responding for access to a male rat leading to intromission. Experiment 1 used a high operant level nose-poke response to test the possible reinforcing effects of some components of access to a male. A simple tone stimulus used as a conditioned reinforcer and two odor stimuli, target male bedding and emulsified preputial gland, were tested. None of these contingent events altered responding above or below operant level. Access to the male, which was always accompanied by intromission, immediately increased response rate when it was made contingent upon the nose-poke response. Performance on fixed-ratio schedules was erratic, and response rate was low in comparison to typical food-reinforced responding. An interresponse-time analysis indicated, however, that some effect of the ratio contingency may have been present. In Experiment 2, several modifications of the procedure were tested with the objective of creating a more tractable preparation for behavior analysis. Response type and the hormone delivery method were changed, and 2 target males were used instead of 1. The latter tripled the average number of reinforcers earned in a single session. Differences between sexual and other reinforcers are discussed in terms of procedural, quantitative, and motivational aspects of the sexual reinforcement procedure.

Estradiol regulation of nitric oxide synthase mRNAs in rat hypothalamus

Expression and estrogen regulation of the genes for nitric-oxide (NO)-synthesizing enzymes (NO synthase, NOS) were investigated by in situ hybridization. This study focused on regions of the hypothalamus that contain estrogen receptors and regulate specific neuroendocrine functions related to female sexual behavior and food intake, among others. Ovariectomized (OVX) rats were treated with vehicle or 3 micrograms/100 g estradiol benzoate (EB) for 7 days. Brains were sectioned and hybridized with antisense riboprobes for neuronal NOS, macrophage NOS and endothelial NOS. In the hypothalamus, mRNA was clearly detectable only for the neuronal NOS with the probes used. A strong hybridization signal was observed in the supraoptic paraventricular and ventromedial nuclei (SON, PVN and VMN, respectively). Quantitative analysis showed an increase in neuronal NOS mRNA in the VMN of the OVX rats treated with EB. The increase was mainly in the ventrolateral aspect of the VMN. No significant changes were observed in the hypothalamic SON and PVN. The data suggest that the expression of neuronal NOS mRNA in VMN can be regulated by estrogen.

Functional analysis of opioid receptor subtypes in the ventromedial hypothalamic nucleus of the rat

Effects of [Met5]enkephalin and agonists selective for mu-, delta- and kappa-opioid receptors were tested in vitro on neurons of the hypothalamic ventromedial nucleus of ovariectomized, estrogen-primed rats. Brain slices were perfused with artificial cerebrospinal fluid and opioid drugs were applied by bolus injection into the perfusion line. Single unit activity was recorded extracellularly. The majority of ventromedial hypothalamic nucleus neurons tested exhibited marked inhibitory responses to [Met5]enkephalin. The inhibition was blocked by naloxone, by the selective delta-opioid receptor antagonist naltrindole and, to a lesser extent, by the mu-opioid receptor antagonist beta-funaltrexamine. The kappa-opioid receptor antagonist nor-binalmorphimine had virtually no effect on [Met5]enkephalin inhibition. Agonists selective for delta-([D-Pen2,D-Pen5]enkephalin, DPDPE) and for mu-([D-Ala2,MePhe4,Gly-ol5]enkephalin, DAGO) opioid receptors also potently inhibited the ventromedial hypothalamic nucleus neurons while the kappa-opioid receptors agonist U50,488 only produced a small inhibition in a smaller number of units. These results provide functional evidence that [Met5]enkephalin, a potential opioid transmitter in the ventromedial hypothalamic nucleus, can exert an inhibitory effect by acting on delta-and mu-opioid receptors.

Hypothalamic cellular and molecular mechanisms helping to satisfy axiomatic requirements for reproduction

In the absence of universal equations expressing neurobiological findings, the safest theoretical approach for the neuroendocrinologist is to start from axiomatic requirements for biologically adaptive neural mechanisms, in our case for reproduction. From this emerge two themes: the likely importance of interactions between internal (hormonal) and external signals in controlling gene expression relevant to reproductive functions; and, second, the vision of molecular interactions on DNA subserving environmental impacts on reproduction. The first theoretical notion has so far yielded data showing a role for synaptic inputs during the onset of estradiol actions for the hormone's induction of enkephalin mRNA, a finding which parallels earlier behavioral results. As well, noxious somatosensory inputs interact with estrogens and progesterone in their influence on enkephalin gene expression. The second theme led to novel investigations of thyroid influences on reproductive molecular biology and behavior, including the ability of exogenous or endogenous thyroid hormones to reduce female mating responses. Since elevated thyroid hormone levels could signal environmental cold, our experiments offer the possibility of explaining ethological facts at a molecular level. More generally, nuclear hormone receptor interactions on the surface of DNA may offer a new level of neural integration revealed first by hormone effects in neuroendocrine cells.

Thyrotropin-releasing hormone (TRH) has independent excitatory and modulatory actions on lamina IX neurons of lumbosacral spinal cord slices from adult rats

Coronal and horizontal slices of the lumbar and sacral spinal cord, respectively, of ovariectomized adult rats, either treated with estrogen (OVX+E) or untreated (OVX), were used to test the neuronal actions of TRH and its metabolite, cyclo(His-Pro) (or cHP). Both coronal slices, which possess only short stumps of ventral roots (VRs), and horizontal slices, in which long sections of VRs were preserved, were used for extracellular recording of single motor and other types of neurons. Methodological comparisons between these two types of slices showed that the length of VRs preserved had no significant effect on the characteristics of motoneurons (MNs). In coronal slices, MNs in medial and lateral lamina IX (MNM and MNL, respectively) were identified by antidromic activation. Of these lumbar MNs, estrogen treatment lowered the antidromic activation threshold for MNM but not MNL. Because MNM innervate the back muscles crucial for the execution of the estrogen-dependent lordosis, the observed estrogen effect may contribute to the hormone's induction of the sexual behavior. The recorded MNs and other types of neurons were subjected to bath applications of TRH, cHP, and neurotransmitters. TRH was found to be capable of evoking an early, shorter-lasting neuronal excitation and/or a late, longer-lasting modulation of neuronal responses to transmitters. Each neuronal action could occur with or without the other, and the occurrence of the excitation did not affect the probability of whether a modulation would occur later. The modulatory, but not the excitatory, action appeared to be shared by cHP, because cHP could also modulate neuronal responses in similar, if not identical, ways as TRH did, but could neither stimulate neurons nor mimic TRH in desensitizing TRH-evoked excitation. The modulatory actions of the two peptides were not affected by estrogen. Although the excitatory action was desensitized by repeated TRH applications, the modulatory action did not appear to be attenuated but instead was often enhanced by repeated administrations of TRH and/or cHP. These results, together with the essentially identical findings from our previous study on hypothalamic neurons, indicate that the excitatory and the modulatory actions of TRH are independent of each other and, hence, are mediated by different subcellular mechanisms.

In vitro electro-pharmacological and autoradiographic analyses of muscarinic receptor subtypes in rat hypothalamic ventromedial nucleus: implications for cholinergic regulation of lordosis

Muscarinic agonists can act through the hypothalamic ventromedial nucleus (VMN) to facilitate lordosis. To elucidate the neuronal mechanism(s) underlying this muscarinic facilitation, effects of muscarinic agents on the single-unit activity of VMN neurons recorded in brain tissue slices of estrogen-primed female rats were analyzed. All the agonists tested, including acetylcholine (ACh), oxotremorine-M (OM), carbachol (CCh) and McN-A-343 (McN), evoked primarily excitation (80-100%), some inhibition (0-20%) and occasional biphasic responses (0-8%). By comparing the response magnitude and the effectiveness in evoking a response, the rank order for evoking excitation, the primary response, was found to be: OM > CCh > ACh approximately McN, which is consistent with that (OM > CCh > McN) for facilitating lordosis reported by others. This consistency and the frequency of its occurrence suggest that the excitatory electric action of the muscarinic agonists is related to their facilitatory behavioral effect. Experiments with antagonists selective for M1 (pirenzepine), M2 (AF-DX 116) and M3 (4-DAMP and p-F-HHSiD) indicate that muscarinic excitations are mediated by M1 and/or M3, but not M2. Since M1 receptors have been shown to be neither sufficient nor necessary to mediate the muscarinic facilitation, M3 receptor may be crucially involved in this behavioral effect. Autoradiographic assays of binding to [3H]4-DAMP with or without pirenzepine and AF-DX 116, also indicate the presence of M3 receptors in the VMN. Quantitative analyses show that the M3 binding was not affected by the in vivo estrogen priming required to permit muscarinic agonists to facilitate lordosis. Thus, while the excitation mediated by M3 is likely to be involved in muscarinic facilitation of lordosis, the regulation of M3 receptor density does not seem to be involved in the permissive

Functional analyses of alpha 1-adrenoceptor subtypes in rat hypothalamic ventromedial nucleus neurons

Activation of alpha 1-adrenoceptors in rat hypothalamic ventromedial nucleus can excite neurons and facilitate female sexual behavior. To identify the alpha 1-adrenoceptor subtype(s) involved, the alpha 1B-adrenoceptor-specific antagonist chloroethylclonidine (100 microM) and/or the alpha 1A-adrenoceptor-selective antagonist 5-methyl urapidil (1 or 2.5 microM) or WB-4101 (0.1-10 microM) were applied to a recording chamber bathing the hypothalamic slice containing the ventromedial nucleus. In all the neurons tested, both types of antagonists blocked, often completely, excitatory responses to nonselective alpha 1-adrenoceptor agonists. Since the doses used were unlikely to make these antagonists nonselective, the results suggest that activation of both alpha 1A- and alpha 1B-adrenoceptor subtypes was necessary for alpha 1-adrenoceptor agonists to evoke an excitation, or that with the present application method--injection into the continuously perfused chamber--chloroethylclonidine did not act specifically. In preincubation (at 37 degrees C for 90 min) where it was reported to act by specific alkylation, chloroethylclonidine (100 microM) but not the vehicle abolished the excitation evoked by an alpha 1-adrenoceptor agonist, but not that by carbachol or other excitants. Also, either in bath application or incubation, chloroethylclonidine worked equally efficiently on slices from ovariectomized rats, that reportedly contain few alpha 1B-adrenoceptors, and from those treated with estrogen which induces alpha 1B-adrenoceptors selectively, suggesting that alpha 1B-adrenoceptor was necessary even when in low abundance.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro electrophysiological characterization of midbrain periaqueductal gray neurons in female rats: responses to GABA- and Met-enkephalin-related agents

Met-Enkephalin, which can be induced by estrogen in the ventromedial nucleus of hypothalamus (VMH), has been proposed to help mediate estrogenic action on lordosis behavior by acting on midbrain periaqueductal gray (PAG) neurons. Also, in the PAG, GABA may locally regulate the levels of lordosis behavior through GABAA receptors. Therefore, we examined the effects of both Met-enkephalin and GABA-related agents on neuronal activity of PAG neurons in slices. Overall, 72.6% of the PAG neurons were inhibited by GABA and 60.9% of GABA-responsive neurons were also excited by the GABAA receptor antagonist, bicuculline methiodide (BMI), suggesting that many of GABA-responsive PAG neurons are tonically inhibited by GABAergic neurons through GABAA receptors. Dorsal PAG neurons were more responsive to BMI than ventral PAG neurons. Moreover, in the middle part of the dorsal PAG, where prominent inhibitory behavioral effects of BMI have been reported, BMI excited 94% of GABA-responsive PAG neurons from estrogen-treated animals, significantly more than observed in ovariectomized control (50%). The most frequent action of Met-enkephalin on PAG neurons was inhibitory (38 out of 149 recorded neurons) although it excited 12 neurons. A dose-dependent increase of inhibitory action of enkephalin was found in the estrogen-primed group but not in the ovariectomized control group while higher doses of enkephalin failed to excite any more neurons in both groups. Most frequently (90%), enkephalin inhibited the same neurons as those on which GABA had the inhibitory effects. Conversely, these neurons composed about 50% of the entire GABA-responsive PAG neurons. Moreover, 76% of neurons inhibited by enkephalin were found to be tonically inhibited by endogenous GABA through GABAA receptors. It is argued, therefore, that increased enkephalinergic influences from the VMH to the PAG in estrogen-treated females could participate in the PAG neuronal control of lordosis by acting on the same neurons as are innervated by intrinsic GABAergic neurons. Since GABAA agonists actually facilitate lordosis in the PAG, these PAG neurons inhibited by both GABA and enkephalin may themselves facilitate behaviors which are antagonistic to lordosis, such as defensive behaviors.

Activation of protein kinase C in the hypothalamic ventromedial nucleus or the midbrain central gray facilitates lordosis

Many neurotransmitters and neuropeptides can act through the hypothalamic ventromedial nucleus (VMN) or midbrain central gray (MCG) to facilitate lordosis. Since these lordosis-facilitating agents can also stimulate the phosphoinositide (PI) second-messenger pathway, it was hypothesized that direct activation of this pathway can also potentiate the behavior. To evaluate this possibility, a phorbol ester, TPA (12-O-tetradecanoyl phorbol 13-acetate), was used to activate a key enzyme, protein kinase C (PKC), of the PI pathway in ovariectomized (OVX) rats either primed or not primed with estrogen. These female rats were paired with males for mating tests before and after an intracerebral infusion of TPA, and both the lordosis quotient (LQ) and the lordosis strength (LS) were measured. Bilateral infusion of TPA (5 micrograms/0.5 microliter or 0.2 microgram/0.2 microliter, but not 0.1 microgram/0.2 microliter/side) into the VMN or MCG of estrogen-primed subjects facilitated both LQ and LS in 30 min, peaked at 60-90 min, and the facilitation lasted for more than 180 min. This facilitatory effect of TPA was: (1) not observed in OVX rats not primed with estrogen; (2) not observed if the infused TPA did not reach both sides of the VMN or MCG; (3) not mimicked by 4 alpha-phorbol 12,13-didecanoate, which does not activate PKC; (4) blocked by PKC inhibitors (H7 10 mM or staurosporine 1 microM, 0.2 microliter/side), which by themselves did not facilitate lordosis; and (5) was not affected by pretreatment of the progestin antagonist RU486.(ABSTRACT TRUNCATED AT 250 WORDS)

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Injection of cobalt protoporphyrin into the medial nuclei of the hypothalamus elicits weight loss

Intracerebroventricular administration of small amounts (0.1 mumol/kg body wt) of cobalt protoporphyrin (CoPP), a synthetic analogue of heme, results in transient hypophagia and prolonged reduction in body weight of rats. Statistically significant hypophagia is detectable within 3 h of CoPP infusion. These changes are accompanied by prompt and sustained reductions in running wheel revolutions, a measure of spontaneous locomotor activity. Bilateral intrahypothalamic injections of CoPP at far lower doses (4 nmol/rat) resulted in similar findings following infusion into the paraventricular, dorsomedial, and ventromedial nuclei, but the compound had no such effect when injected into the thalamus or the lateral hypothalamic area. These effects were also observed following microinjection of the natural metalloporphyrin, heme, into the medial hypothalamic nuclei. Inorganic cobalt, iron, protoporphyrin, and magnesium protoporphyrin injected similarly were without such effect. These findings provide further evidence that the site of the previously described actions of CoPP in reducing food intake and body weight in rats resides, at least in part, in the medial hypothalamus. Furthermore, this study expands the spectrum of metalloporphyrins that act in the central nervous system to elicit these changes from synthetic compounds such as CoPP to heme, the natural, physiological metalloporphyrin.

Effects of lordosis-relevant neuropeptides on midbrain periaqueductal gray neuronal activity in vitro

Certain neuropeptides can facilitate lordosis by acting on midbrain periaqueductal gray (PAG) in estrogen-primed female rats. Here, we investigated responses of individual PAG neurons in vitro, to five neuropeptides: substance P (SP), luteinizing hormone-releasing hormone (LHRH), prolactin (PRL), oxytocin (OT), and thyrotropin-releasing hormone (TRH). Substance P, OT, and TRH excited spontaneous activity of PAG neurons through neurotransmitter-like actions in a dose-dependent manner, whereas LHRH and PRL virtually never affected PAG neurons this way. Oxytocin acted through oxytocin receptors located on the recorded PAG neurons, since excitatory actions of OT were 1) not abolished by synaptic blockade, 2) mimicked by the OT-specific agonist [Thr4, Gly7]OT but not by arginine vasopressin, and 3) blocked by the OT-specific antagonist [d(CH2)5,Tyr(Me)2,Orn8]vasotocin. Although LHRH had no neurotransmitter-like action on spontaneous activity of PAG neurons, it, as well as SP, could modulate responses of some dorsal PAG neurons to GABAA and GABAB agonists or norepinephrine. Neuromodulatory actions of LHRH and SP could help facilitate lordosis through PAG neurons.

α1-Adrenergic agonists act on the ventromedial hypothalamus to cause neuronal excitation and lordosis facilitation: electrophysiological and behavioral evidence

To see if activation of central alpha 1-adrenergic receptors can cause facilitation of lordosis in rats, the behavioral effects of centrally administered alpha 1-agonists, methoxamine (MA) and phenylephrine (PhE), and related agents were studied. In ovariectomized rats treated with estrogen, infusion of MA, PhE, or a beta-agonist isoproterenol, into the lateral ventricle, or bilateral infusions of MA or PhE into the ventromedial hypothalamus (VMH) facilitated lordosis. Conversely, intra-VMH infusion of the alpha 1-antagonist prazosin (PZ) inhibited lordosis. Intra-VMH infusion of isoproterenol or an alpha 2-agonist clonidine, had no effect. Neither was the intra-VMH infusion of MA effective if: (i) the rats were not primed with estrogen; (ii) the tips of the cannulae were outside the VMH; or (iii) it was preceded by an intra-VMH infusion of the alpha 1b-antagonist, chloroethylclonidine (CEC). These results not only verify implications from recent studies that alpha 1-receptors in the hypothalamus are important for lordosis facilitation, but further show that the adrenergic facilitatory effect are: (i) mediated specifically by alpha 1b-subtype of the alpha 1-receptor, (ii) estrogen-dependent, and (iii) site-specific to VMH. To investigate neural mechanisms potentially underlying the lordosis-facilitating effect of alpha 1-activation, the actions of MA and PhE on the electrical activity of single neurons of the ventromedial nucleus of the hypothalamus (VMN) in vitro were studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological actions of oxytocin on hypothalamic neurons in vitro: neuropharmacological characterization and effects of ovarian steroids

Oxytocin (OT) neurotransmission in the brain has a facilitatory effect on sexual receptivity in rats. This effect of OT is dependent on priming by ovarian steroids, estrogen and progesterone. These steroids modulate OT binding in specific brain nuclei, including the ventrolateral portion of the ventromedial hypothalamic nucleus (vlVMN). In the present study, single-unit activity was recorded from the vlVMN in hypothalamic slices to characterize the electrophysiological actions of OT. To examine the effects of ovarian steroids on OT actions, we used brain slices prepared from ovariectomized rats either treated with estrogen or not, and some slices were treated with progesterone in vitro. OT had little modulatory action on neuronal responses to other agents, but affected the activity of large numbers of vlVMN units. Of those neurons affected, 94% responded with excitation. This predominant stimulatory action of OT is consistent with its lordosis-facilitating effect, because increases in the activity of VMN neurons are generally associated with the facilitation of lordosis. Pharmacological analyses with selective OT agonists and antagonists as well as structurally related peptides showed that the excitatory action of OT is mediated by OT receptors. Estradiol modulated several aspects of OT transmission. First, it increased neuronal responsiveness to OT, especially at the lowest concentration used (0.2 nM). In addition, it caused neuronal responses to OT to correlate significantly with responses to acetylcholine and norepinephrine, which also can act on the ventromedial hypothalamus to facilitate lordosis. Finally, estradiol enhanced the excitability of laterally projecting neurons, which have been implicated in lordosis. In estrogen-pretreated slices, addition of progesterone in vitro caused little further effect on responses of individual neurons to exogenous OT. Altogether, the present electrophysiological findings are consistent with the hypothesis that estrogen potentiates OT action by increasing functional OT receptors preferentially in lordosis-relevant neurons, thereby enabling OT to efficiently facilitate female reproductive behavior.

PLC-alpha: a common mediator of the action of estrogen and other hormones?

The phosphatidyl inositol (PI) second messenger pathway may mediate diverse effects of estrogen, including its potentiation of the effects of other hormones. Both estradiol (E2) and luteinizing hormone-releasing hormone (LHRH) induce a putative isoform of PI-specific phospholipase C-alpha (PLC-alpha). PLC-alpha catalyzes PI hydrolysis, which in turn can increase protein kinase C (PKC) activation, Ca2+ mobilization, and arachidonic acid metabolism. Estrogen activates the PI pathway, and components of the PI pathway can mimic or enhance some effects of estrogen. Furthermore, estrogen potentiates effects of several hormones (e.g., LHRH, prolactin, and insulin) which can also act through the PI system. PLC-alpha may therefore provide a common second messenger pathway mediating the potentiation by E2 of the effects of other hormones; in addition it may also mediate some or all of the many actions of E2, since components of the PI pathway can have secretory, trophic, toxic, and neuromodulatory effects.

Effects of GABA and related agents on the electrical activity of hypothalamic ventromedial nucleus neurons in vitro

Functional and neurochemical evidence suggests significant GABA participation in the basomedial hypothalamus. We have investigated electrophysiological effects of GABA using in vitro recording from hypothalamic tissue slices. Exogenous GABA inhibited 94 out of 121 ventromedial hypothalamic (VMN) neurons tested. In sixty-one percent of these GABA-responsive neurons, the inhibitory action of GABA was blocked by GABAA antagonists, bicuculline methiodide (BMI) and picrotoxin (PTX). Nevertheless, many (27/69) GABA-responsive neurons were not sensitive to GABAA blockers: BMI and PTX failed to antagonize inhibitory action of GABA. Most, if not all, of these inhibitions can be accounted for by GABAB effects, since baclofen powerfully inhibited 42 of 44 neurons tested. In addition to blocking the inhibitory action of exogenous GABA, BMI (55%) and PTX (36%) also caused changes of neuronal activity indicating blockade of intrinsic GABAergic action. Altogether, our results showed that, in the VMN, GABA acts through not only GABAA but also GABAB receptors to inhibit neuronal activity, and that there is tonic inhibition by intrinsic GABA neurons. These GABA actions may participate in behaviorally-relevant VMN hypothalamic mechanisms.

The effects of the TRH metabolite cyclo(His-Pro) and its analogs on feeding

Cyclo(His-Pro), or cHP, is a putative metabolite of thyrotropin-releasing hormone (TRH), and, like TRH, can inhibit food intake but requires higher doses. In attempts to improve the anorectic effects of cHP through modification of its structure, a number of its analogs were synthesized. These analogs or cHP itself were administered to rats either by intracerebroventricular (ICV) infusion or systemic injection, and their effects on food intake were measured. None of the synthetic analogs was more potent than cHP, although several analogs demonstrated comparable potencies to the parent compound. Interestingly, one cHP analog reversed the suppressive effect and stimulated feeding. This reversal, as well as the preservation of the anorectic effect by some but not all the analogs, suggests that the cHP effect on feeding does require specific structural features.

Cyclo(His-Pro) potentiates the reduction of food intake induced by amphetamine, fenfluramine, or serotonin

Electrophysiological and pharmacological evidence suggests that cyclo(His-Pro) (cHP) could reduce food intake by modulating the actions of relevant neurotransmitters. We tested this hypothesis by giving rats a combination of cHP or its analogs centrally and an anorectic, amphetamine or fenfluramine, systemically. Compared to saline control, cHP at doses too low to affect food intake by itself significantly potentiated the reduction of food intake by amphetamine. This potentiation is thought to be due to cHP modulation of norepinephrine (NE) action, because at the low dose used amphetamine acts mainly through NE to inhibit food intake. The modulation has specific requirements for cHP structure, since it was mimicked by one but not two other analogs tested. The anorectic effect of fenfluramine was also potentiated and prolonged by cHP at a dose not effective by itself. Since fenfluramine is known to act by increasing brain serotonin (5-HT), the potentiation was apparently a result of an interaction between cHP and 5-HT effects. To examine this interaction more directly, we administered both cHP and 5-HT centrally. Again, cHP potentiated the reduction of food intake caused by 5-HT. Thus the neuromodulation of feeding-relevant neurotransmitter effects, following NE and 5-HT, is probably a mechanism by which cHP reduces food intake.

Histamine excites arcuate neurons in vitro through H1 receptors

The electrophysiological effects of histamine on neurons of the hypothalamic arcuate nucleus of the female rat were tested with extracellular single unit recordings in an in vitro slice preparation. Histamine increased the spontaneous neuronal firing rate in 63% of the arcuate cells tested. An inhibitory response to histamine was seen in only one of the 117 neurons tested. The excitatory response to histamine showed dose dependency and was stable during synaptic blockade (by high magnesium and low calcium concentrations) and across a temperature range of 29-37 degrees C. Administration of histaminergic type 1 (pyrilamine and chlorpheniramine) and type 2 (cimetidine) receptor blockers revealed that the excitatory responses to histamine were mediated by type 1 receptors. The same neurons were also tested for responses to norepinephrine, serotonin, acetylcholine and substance P. A significant correlation was found between responses to histamine and substance P: all units excited by substance P were also excited by histamine. This subclass of histamine-responsive arcuate neurons may play a role in the regulation of the anterior pituitary, since histamine and substance P have similar effects on LH and prolactin secretion.

Responses of hypothalamic paraventricular neurons in vitro to norepinephrine and other feeding-relevant agents

To investigate paraventricular hypothalamic neuronal actions responsible for the effects of neurotransmitters on feeding, and to test the notion that a single population of cells there could account for feeding effects, hypothalamic slices containing the paraventricular nucleus (PVN) were prepared from rats. Electrophysiological responses of individual PVN neurons to feeding-inducing agents norepinephrine (NE) and gamma-aminobutyric acid (GABA), and to anorexic agents serotonin (5-HT) and histamine (Hist) were examined. NE inhibited neuronal activity through alpha 2-adrenergic receptors, and excited through alpha 1-receptors. alpha 2-receptors are known to mediate the behavioral effect of NE. NE inhibited most clearly those neurons that otherwise fired continuously in this type of in vitro preparation. GABA affected the activity of 37% of the neurons tested, primarily by inhibition. The inhibitory action of GABA can be related to its feeding-inducing effect. GABA in PVN can also attenuate excitatory responses and enhance inhibitory responses to NE or 5-HT. 5-HT caused excitatory and inhibitory responses with the former action outnumbering the latter by approximately 3 to 1. Since this would result in a net excitation, it appears that 5-HT in PVN inhibits feeding mainly by exciting neuronal activity. Hist excited 72% and inhibited only 2% of PVN neurons. The excitation was blocked by H1-antagonists, which have been shown to mediate Hist effect on feeding. Comparing across neurons, the inhibitory response to NE was correlated with that to GABA, but not with any responses to 5-HT or Hist. The excitatory responses to Hist correlated with 5-HT responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulatory actions of luteinizing hormone-releasing hormone on electrical activity of preoptic neurons in brain slices

Single unit activity was recorded from 378 neurons, in two preoptic nuclei rich in luteinizing hormone-releasing hormone neurons, using in vitro brain tissue slices which were prepared form either ovariectomized or ovariectomized plus estradiol-treated rats. To test possible transmitter-like actions, agents were injected into the perfusion medium. Luteinizing hormone-releasing hormone excited 46%, inhibited 7%, and evoked biphasic responses in 2% of the 250 units tested. By comparison, two other peptides, thyrotropin-releasing hormone and cholecystokinin-octapeptide sulfated were exclusively excitatory, acting on 55 and 67% of the neurons, respectively. The response to thyrotropin-releasing hormone, cholecystokinin-octapeptide sulfated, and neurotransmitters were prompt, large, and consistent from trial to trial. In contrast, responses to luteinizing hormone-releasing hormone were usually delayed, small, and variable. Responses to the agents tested were not affected by in vivo estradiol treatment. Possible modulatory actions of luteinizing hormone-releasing hormone were tested by comparing the responses of single neurons to norepinephrine and serotonin before and after an application of luteinizing hormone-releasing hormone. In 39 and 20% of the 119 neurons tested, the norepinephrine responses were potentiated and attenuated, respectively, by luteinizing hormone-releasing hormone. In 46 serotonin-responsive neurons, 28% were potentiated and 22% attenuated. These neuromodulatory actions of luteinizing hormone-releasing hormone were specific in affecting only certain responses of certain neurons, and they were not duplicated on the same neurons by thyrotropin-releasing hormone. It appears that luteinizing hormone-releasing hormone may be a neuromodulator in the preoptic area.

Transmitter and peptide actions on hypothalamic neurons in vitro: implications for lordosis

This article summarizes a series of studies using brain tissue slices of rats to record single-unit activity from the hypothalamic ventromedial nucleus (VMN) and preoptic area (POA), both of which are crucial for the regulation of the estrogen-dependent feminine mating behavior, lordosis. In these studies the actions of acetylcholine (ACh), serotonin (5HT), norepinephrine (NE), luteinizing hormone-releasing hormone (LHRH), arginine-vasopressin (AVP), and oxytocin (OXY) on neuronal activity were investigated. The results show that these agents could evoke either direct responses or neuromodulatory changes from VMN or POA cells in vitro. Comparison of the net neuronal actions of each of these agents with their effects on lordosis behavior revealed interesting correlations. All the excitatory agents, i.e., ACh, AVP, OXY, and LHRH, have been indicated by intracerebral application studies to be facilitatory on lordosis. The inhibitory agent, 5HT, could inhibit lordosis, when applied to the VMN and its vicinity. Such correlations indicate that these transmitters and peptides can facilitate or inhibit lordosis by increasing or decreasing, respectively, the frequency of action potentials in the types of hypothalamic neurons recorded here.

Responses of ventromedial hypothalamic neurons in vitro to norepinephrine: dependence on dose and receptor type

Application of norepinephrine (NE) at 12.5 microM in the bath surrounding hypothalamic slices from ovariectomized rats could evoke excitation, inhibition, or biphasic inhibition-excitation from single neurons in the ventromedial nucleus. Whether the rats were treated with estrogen or not did not alter the distribution of the type of neuronal responses to NE in vitro. Altering the composition of the bathing solution to achieve synaptic blockade did not abolish or alter the type of responses, indicating that all these types of NE responses, including both phases of the biphasic response, were mediated by postsynaptic receptors. Experiments with varying doses of NE showed that the inhibitory response could be evoked at doses lower than those required to evoke the excitatory response. The effective dose for 50% of the responsive neurons (ED50) was lower than 1.25 microM for inhibitions and higher than 5 microM for excitations. Using specific adrenergic receptor agonists and antagonists, it was found that the excitation and the inhibition were mediated, primarily, by alpha 1- and alpha 2-receptors, respectively. beta-Receptors played only a minor role, but might be related to both excitation and inhibition. Study with adrenergic agents further revealed that different types of adrenergic receptors co-localized not only in neurons showing the biphasic response, but also in a major portion of neurons showing monophasic excitation or inhibition. Because of the co-localization and the differential sensitivities to NE, alteration of the dose of NE or the ratio of excitatory/inhibitor receptors co-localized on a neuron should be able to reverse the type of a neuronal response to NE.

Neuropeptides TRH and cyclo(His-Pro) share neuromodulatory, but not stimulatory, action on hypothalamic neurons in vitro: implication for the regulation of feeding

Electrical activity of neurons in rat hypothalamic ventromedial nucleus was recorded in tissue slices, to investigate central neural mechanisms underlying reduction of food intake caused by TRH and its metabolite, cyclo(His-Pro) [cHP]. Application of TRH had two actions: stimulation of neuronal activity, which was desensitized on closely repeated applications; and modulation of neuronal responses to neurotransmitters, even in the absence of the stimulatory action. The neuromodulatory but not the direct stimulatory action could also be achieved by cHP. The neuromodulatory action is more likely to be a neural mechanism underlying the inhibition of feeding, while other biological functions, unique to TRH, may depend on direct stimulation. In this way, TRH could achieve different biological results through different modes of action on hypothalamic neurons.

Electrophysiological test of an amphiphilic beta-structure in LHRH action

Micropipettes were used to record electrical activity from single neurons in hypothalamic tissue slices, in the hypothalamic arcuate nucleus (ARC), and in the periventricular and suprachiasmatic preoptic nuclei (POA). Responses were measured following in vitro application of luteinizing hormone releasing hormone (LHRH), and two analogues: LHRH models 1 and 3. Model 1 (pyroGlu-His-Trp-Ser-Phe-Thr-Ile-Lys-Ile-ThrNH2) had amino acid substitutions in residues 5-10 designed to form an amphiphilic beta-strand structure. Model 3 (pyroGlu-His-Trp-Ser-Phe-Gly-Ile-Lys-Pro-SerNH2) was also designed to possess amphiphilic characteristics, but also more closely to resemble the native peptide. Electrical recording results showed that LHRH was able both to excite or inhibit different hypothalamic neurons, and that it was more effective in the preoptic area than in the arcuate nucleus. Responses to LHRH model 3 were strongly correlated with responses to LHRH, in their occurrence and their direction, for both of the brain regions studied. Moreover, LHRH models 1 and 3 also retained the neuromodulatory effects of LHRH on cellular responses to norepinephrine and serotonin. Thus, LHRH analogues, designed to possess an amphiphilic beta-structure, preserved some of the properties of LHRH when tested electrophysiologically in the central nervous system.

CCK-8 stimulation of ventromedial hypothalamic neurons in vitro: a feeding-relevant event?

Bath application of sulfated or non-sulfated cholecystokinin octapeptide (CCK-8s or CCK-8ns, respectively) at concentrations of 25 to 250 nM stimulated the firing activity of 40 to 80% of neurons recorded from the ventromedial nucleus (VMN) in hypothalamic slices maintained in vitro. On the basis of molarity or the percentages of neurons affected, CCK-8s was about 2 to 10 times more potent than CCK-8ns. However, qualitatively, the two forms of CCK-8 were virtually identical: both had a stimulatory action on VMN neurons; both affected VMN neurons in a dose-dependent fashion; both could desensitize their own stimulatory action; and both could cross-desensitize the stimulatory action of the other. These results indicate that not only CCK-8s but also CCK-8ns, which is biologically inactive peripherally, can serve as excitatory neurotransmitters in the VMN, and that both peptides stimulated neurons through the same or a similar neuronal mechanism. It was also found that in the VMN, the stimulatory action of CCK-8 correlated with the actions of norepinephrine, and affected all of the VMN neurons responsive to glucose. Since the actions of glucose and norepinephrine on the activity of VMN neurons are feeding-relevant, our data support the notion that, in addition to acting as a peripheral satiety agent, CCK-8 can also act as a neurotransmitter centrally to mediate satiety.

Vasopressin excites ventromedial hypothalamic glucose-responsive neurons in vitro

Effects of vasopressin (AVP), oxytocin (OXY), norepinephrine (NE), and glucose on the single-unit activity of hypothalamic ventromedial nucleus (VMN) in tissue slices were studied. While AVP was exclusively excitatory on 58% of the neurons, OXY could be excitatory or inhibitory and affected only 42% of the neurons. There was no correlation between the responses to these two peptides. Each of these two peptides could desensitize neuronal response to itself, but did not cross-desensitize responses to each other. These results indicate that AVP and OXY do not act on the same population of VMN neurons through the same cellular mechanism. Furthermore, only the responses to AVP were correlated to responses to glucose and NE, two agents relevant to central regulation of feeding. This correlation with responses to feeding-relevant agents and the exclusively excitatory action on the VMN, which is involved in the regulation of feeding, suggest that AVP can play a role in the regulation of feeding, particularly the feeding induced by the injection of NE into the paraventricular nucleus, that is known to alter AVP release.

Single-unit activity of hypothalamic arcuate neurons in brain tissue slices. Effects of anterior pituitary hormones, cholecystokinin-octapeptide, and neurotransmitters

Extracellular single-unit activity was recorded from hypothalamic arcuate nucleus (ARC) in brain tissue slices. Adult female Sprague-Dawley rats, ovariectomized or ovariectomized plus estrogen treated for at least 1 week, were used. Resting activity and responses of ARC neurons to six anterior pituitary hormones, or (as a positive control) cholecystokinin-octapeptide sulfate (CCK-8S), and a battery of four neurotransmitters including norepinephrine, serotonin, dopamine, and glutamate were recorded. A total of 263 neurons were recorded. Estrogen treatment did not cause any significant changes in the firing patterns nor in responses to most agents tested except for CCK-8S. A large percentage of the ARC neurons were either silent (40%) or slow-firing units (43% fired less than twice/s). Only a small percentage of ARC neurons (20-30%) responded to the anterior pituitary hormones, and these responses were small, delayed increases in firing despite the fact that CCK-8S stimulated more than half of the neurons with large responses. Glutamate was also excitatory, but not quite as effective as CCK-8S. Norepinephrine and serotonin were equally effective in eliciting a neuronal response (over 70% of units responded with an excitation or inhibition). Dopamine acted like norepinephrine, but was less potent. Since anterior pituitary hormones only weakly affected ARC neurons, these electrophysiological data give scant support to the notion that short-loop feedback is accompanied by electrical changes. In the ARC, however, CCK-8S may play some functional roles that are influenced by estrogen.