Parvocellular Neuroendocrine Cells Research Articles

Sex differences in α-adrenergic receptor function contribute to impaired hypothalamic metaplasticity following chronic intermittent ethanol exposure.

Individuals with alcohol use disorder (AUD) exhibit maladaptive responses of the hypothalamic-pituitary-adrenal (HPA) axis to stress, which has been linked to high rates of relapse to drinking among abstinent individuals. Corticotropin-releasing factor (CRF) parvocellular neuroendocrine cells (PNCs) within the paraventricular nucleus of the hypothalamus (PVN) are critical to stress-induced HPA axis activation. Here, we investigate sex differences in synaptic transmission and plasticity in PNCs following the application of the stress-associated neurotransmitter norepinephrine (NE) in a rat model of AUD. Adult Sprague-Dawley rats were exposed to 40 days of chronic intermittent ethanol (CIE) vapor and 30 to 108 days of protracted withdrawal. We measured changes in holding current, evoked synaptic currents, and short-term glutamatergic plasticity (STP) in putative PNCs following the application of NE (10μM) with and without the selective α1 adrenergic receptor (AR) antagonist prazosin (10μM) or the α2AR antagonist atipamezole (10μM). The experiments were performed using whole-cell patch clamp recordings in slices from CIE rats and air-exposed controls. NE application caused two distinct effects: a depolarizing, inward, postsynaptic current and a reduction in amplitude of an evoked glutamatergic excitatory postsynaptic current (eEPSC). Both effects were sex- and CIE-specific. Prazosin blocked the postsynaptic inward current, while atipamezole blocked the NE-mediated suppression of eEPSCs. Additionally, STP formation was facilitated following NE application only in stress-naïve males and this response was lost in stressed animals exposed to a 30-min restraint stress following CIE exposure. Furthermore, NE + prazosin restored STP formation in stressed CIE males. NE exerts excitatory and inhibitory effects on CRF PVN PNCs, and both effects are influenced by sex and CIE. Behavioral and hormonal responses to stress are influenced by STP formation within the PVN, which is lost following CIE and restored with the preapplication of prazosin. The selective blockade of α1AR may, therefore, ameliorate CIE-induced deficits in HPA responses to stress in a sex-specific manner.

Prostaglandin E2 depresses GABA release onto parvocellular neuroendocrine neurones in the paraventricular nucleus of the hypothalamus via presynaptic receptors.

Inflammation-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis and the ensuing release of anti-inflammatory glucocorticoids are critical for the fine-tuning of the inflammatory response. This immune-induced neuroendocrine response is in large part mediated by prostaglandin E2 (PGE2 ), the central actions of which ultimately translate into the excitation of parvocellular neuroendocrine cells (PNCs) in the hypothalamic paraventricular nucleus. However, the neuronal mechanisms by which PGE2 excites PNCs remain incompletely understood. In the present study, we report that PGE2 potently depresses GABAergic inhibitory synaptic transmission onto PNCs. Using whole-cell patch clamp recordings obtained from PNCs in ex vivo hypothalamic slices from rats, we found that bath application of PGE2 (0.01-100μmolL-1 ) concentration-dependently decreased the amplitude of evoked inhibitory postsynaptic currents (eIPSCs) with maximum effects at 10μmolL-1 . The PGE2 -mediated depression of eIPSCs had a rapid onset and was long-lasting, and also was accompanied by an increase in paired pulse ratio. In addition, PGE2 decreased the frequency but not the amplitude of both spontaneous IPSCs and miniature IPSCs. These results collectively indicate that PGE2 acts at a presynaptic locus to decrease the probability of GABA release. Using pharmacological approaches, we also demonstrated that the EP3 subtype of the PGE2 receptor mediated the actions of PGE2 on GABA synapses. Taken together, our results show that PGE2 , via actions of presynaptic EP3 receptors, potently depresses GABA release onto PNCs, providing a plausible mechanism for the disinhibition of HPA axis output during inflammation.

Nitric Oxide Synthase Inhibitor Attenuates the Effects of Repeated Restraint Stress on Synaptic Transmission in the Paraventricular Nucleus of the Rat Hypothalamus.

Corticotropin-releasing hormone (CRH)-synthesizing parvocellular neuroendocrine cells (PNCs) of the hypothalamic paraventricular nucleus (PVN) play a key role in the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Several studies have demonstrated that synaptic inputs to these cells may undergo stress-related enhancement but, on the other hand, it has been reported that exposition to the same stressor for prolonged time periods may induce a progressive reduction in the response of the HPA axis to homotypic stressors. In the present study rats were subjected to 10 min restraint sessions, repeated twice daily for 3 or 7 days. Miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were then recorded from PNCs in ex vivo hypothalamic slice preparations obtained 24 h after the last restraint. Restraint stress repeated over 3 days resulted in increased mean frequency and decreased rise time and decay time constant of mEPSCs, accompanied by a decrease in the excitability of PNCs, however, no such changes were evident in slices obtained from rats subjected to restraint over 7 days. There were no changes in mIPSCs after repeated restraint. Administration of the unspecific nitric oxide synthase (NOS) blocker Nω-Nitro-L-arginine (L-NNA) before each restraint, repeated over 3 days, prevented the occurrence of an increase in mEPSC frequency. However, animals receiving L-NNA and subjected to repeated restraint had similar changes in PNCs membrane excitability and mEPSC kinetics as stressed rats not receiving L-NNA. Comparison of the effects of a single 10 min restraint session followed by either an immediate or delayed (24 h) decapitation revealed an increase in the mean mEPSC frequency and a decrease in the mean mIPSC frequency in slices prepared immediately after restraint, with no apparent effects when slice preparation was delayed by 24 h. These results demonstrate that restraint, lasting 10 min and repeated twice daily for 3 days, induces a selective and long-lasting enhancement of excitatory synaptic input onto PNCs, partially by a NOS-dependent mechanism, and reduces PNC excitability, whereas prolongation of repeated stress for up to 7 days results in an adaptation.

Sensitization of the Hypothalamic-Pituitary-Adrenal Axis in a Male Rat Chronic Stress Model.

Stress activation of the hypothalamic-pituitary-adrenal (HPA) axis is regulated by rapid glucocorticoid negative feedback. Chronic unpredictable stress animal models recapitulate certain aspects of major depression in humans, which have been attributed to impaired glucocorticoid negative feedback. We tested for an attenuated HPA sensitivity to fast glucocorticoid feedback inhibition in male rats exposed to a chronic variable stress (CVS) paradigm. In vitro, parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus recorded in slices from CVS rats showed an increase in basal excitatory synaptic inputs and a decrease in basal inhibitory synaptic inputs compared with neurons from control rats. There was no difference between control and CVS-treated rats in the rapid glucocorticoid suppression of excitatory synaptic inputs, a fast feedback mechanism. In vivo, CVS-treated rats showed an increase in ACTH secretion at baseline and after both iv CRH and acute stress and no impairment of the corticosterone suppression of the ACTH response, compared with controls. In an in vitro pituitary preparation, an increase in basal ACTH release, a small increase in CRH-induced ACTH release, and no decrement in the glucocorticoid suppression of ACTH release were seen in pituitaries from CVS rats. Thus, CVS does not suppress rapid glucocorticoid negative feedback at the hypothalamus or pituitary, but increases the synaptic excitability of paraventricular nucleus CRH neurons and the CRH sensitivity of the pituitary. Therefore, increased HPA activity in chronically stressed male rats is due to sensitization of the HPA axis, rather than to desensitization to rapid glucocorticoid feedback.

Editorial: Plasticity of GABAergic synapses.

Editorial: Plasticity of GABAergic synapses.

Embedded synaptic feedback in the neuroendocrine stress axis.

Neural regulation of blood glucocorticoid levels is critical for defence of homeostasis during physiological or psychoemotional challenges. In mammals, this function is carried out by the neuroendocrine stress axis, coordinated by parvocellular neuroendocrine cells (PNCs) of the paraventricular hypothalamic nucleus. Feedback regulation of PNCs by glucocorticoids provides complex experience-dependent shaping of neuroendocrine responses. We review recent evidence for metaplastic actions of glucocorticoids as 'circuit breakers' at synapses directly regulating PNC excitability and explore how such mechanisms may serve as substrates for stress adaptation.

Stress-induced plasticity of GABAergic inhibition.

GABAergic neurotransmission is highly plastic, undergoing dynamic alterations in response to changes in the environment, such as following both acute and chronic stress. Stress-induced plasticity of GABAergic inhibition is thought to contribute to changes in neuronal excitability associated with stress, which is particularly relevant for stress-related disorders and seizure susceptibility. Here we review the literature demonstrating several mechanisms altering GABAergic inhibition associated with stress, including brain region-specific alterations in GABAA receptor (GABAAR) subunit expression, changes in chloride homeostasis, and plasticity at GABAergic synapses. Alterations in the expression of specific GABAAR subunits have been documented in multiple brain regions associated with acute or chronic stress. In addition, recent work demonstrates stress-induced alterations in GABAergic inhibition resulting from plasticity in intracellular chloride levels. Acute and chronic stress-induced dephosphorylation and downregulation of the K+/Cl− co-transporter, KCC2, has been implicated in compromising GABAergic control of corticotropin-releasing hormone (CRH) neurons necessary for mounting the physiological response to stress. Acute stress also unmasks the capacity for both long-term potentiation and long-term depression, in distinct temporal windows, at GABAergic synapses on parvocellular neuroendocrine cells (PNCs) in the paraventricular nucleus (PVN) of the hypothalamus. This review highlights the complexity in the plasticity of GABAergic neurotransmission associated with stress and the relationship to neuronal excitability, including alterations in GABAAR expression, synaptic plasticity at GABAergic synapses, and changes in chloride homeostasis.

Beyond inhibition: GABA synapses tune the neuroendocrine stress axis

We recently described a novel form of stress-associated bidirectional plasticity at GABA synapses onto hypothalamic parvocellular neuroendocrine cells (PNCs), the apex of the hypothalamus-pituitary-adrenal axis. This plasticity may contribute to neuroendocrine adaptation. However, this GABA synapse plasticity likely does not translate into a simple more and less of inhibition because the ionic driving force for Cl(-) , the primary charge carrier for GABAA receptors, is dynamic. Specifically, stress impairs a Cl(-) extrusion mechanism in PNCs. This not only renders the steady-state GABA response less hyperpolarizing but also makes PNCs susceptible to the activity-dependent accumulation of Cl(-) . Accordingly, GABA synapse plasticity impacts both the robustness of GABA voltage response and dynamic Cl(-) loading, imposing nonlinear influences on PNC excitability during circuit activities. This theoretical consideration predicts roles for GABA transmission far more versatile than canonical inhibition. We propose potential impacts of GABA synapse plasticity on the experience-dependent fine-tuning of neuroendocrine stress responses.

Short‐term plasticity impacts information transfer at glutamate synapses onto parvocellular neuroendocrine cells in the paraventricular nucleus of the hypothalamus

Glutamatergic synaptic inputs onto parvocellular neurosecretory cells (PNCs) in the paraventricular nucleus of the hypothalamus (PVN) regulate the hypothalamic-pituitary-adrenal (HPA) axis responses to stress and undergo stress-dependent changes in their capacity to transmit information. In spite of their pivotal role in regulating PNCs, relatively little is known about the fundamental rules that govern transmission at these synapses. Furthermore, since salient information in the nervous system is often transmitted in bursts, it is also important to understand the short-term dynamics of glutamate transmission under basal conditions. To characterize these properties, we obtained whole-cell patch clamp recordings from PNCs in brain slices from postnatal day 21-35 male Sprague-Dawley rats and examined EPSCs. EPSCs were elicited by electrically stimulating glutamatergic afferents along the periventricular aspect. In response to a paired-pulse stimulation protocol, EPSCs generally displayed a robust short-term depression that recovered within 5 s. Similarly, trains of synaptic stimuli (5-50 Hz) resulted in a frequency-dependent depression until a near steady state was achieved. Application of inhibitors of AMPA receptor (AMPAR) desensitization or the low-affinity, competitive AMPAR antagonist failed to affect the depression due to paired-pulse and trains of synaptic stimulation indicating that this use-dependent short-term synaptic depression has a presynaptic locus of expression. We used cumulative amplitude profiles during trains of stimulation and variance-mean analysis to estimate synaptic parameters. Finally, we report that these properties contribute to hamper the efficiency with which high frequency synaptic inputs generate spikes in PNCs, indicating that these synapses operate as effective low-pass filters in basal conditions.

A synaptocentric view of the neuroendocrine response to stress

The essential role of parvocellular neuroendocrine cells (PNCs) in the paraventricular nucleus of the hypothalamus (PVN) is to translate real or perceived challenges into a comprehensive glucocorticoid (GC) hormone response. Synaptic inputs encoding physical and psychological stress engage the hypothalamic-pituitary-adrenal axis (HPA) by increasing PNC activity, and corticotropin-releasing hormone production and release. Following robust recruitment in response to stress, GCs feedback to dampen PNC responses. Here we review the contributions of glutamate and GABA synapses in PVN to the initiation and termination of the stress response. The reliability of HPA responses to a given stress can vary as a function of prior experience. Within this context, we examine possible synaptic correlates that allow this neuroendocrine system to learn and adapt following stress challenges.

Repeated Stress Impairs Endocannabinoid Signaling in the Paraventricular Nucleus of the Hypothalamus

Endocannabinoids (eCBs) are ubiquitous retrograde signaling molecules in the nervous system that are recruited in response to robust neuronal activity or the activation of postsynaptic G-protein-coupled receptors. Physiologically, eCBs have been implicated as important mediators of the stress axis and they may contribute to the rapid feedback inhibition of the hypothalamic-pituitary-adrenal axis (HPA) by circulating corticosteroids (CORTs). Understanding the relationship between stress and eCBs, however, is complicated by observations that eCB signaling is itself sensitive to stress. The mechanisms that link stress to changes in synaptic eCB signaling and the impact of these changes on CORT-mediated negative feedback have not been resolved. Here, we show that repetitive immobilization stress, in juvenile male rats, causes a functional downregulation of CB(1) receptors in the paraventricular nucleus of the hypothalamus (PVN). This loss of CB(1) receptor signaling, which requires the activation of genomic glucocorticoid receptors, impairs both activity and receptor-dependent eCB signaling at GABA and glutamate synapses on parvocellular neuroendocrine cells in PVN. Our results provide a plausible mechanism for how stress can lead to alterations in CORT-mediated negative feedback and may contribute to the development of plasticity of HPA responses.

Brain-Derived Neurotrophic Factor Silences GABA Synapses Onto Hypothalamic Neuroendocrine Cells Through a Postsynaptic Dynamin-Mediated Mechanism

In the paraventricular nucleus of the hypothalamus (PVN), experimental stress paradigms that suppress gamma-aminobutyric acid (GABA) inputs to parvocellular neuroendocrine cells (PNCs) also increase the expression of brain-derived neurotrophic factor (BDNF). In the adult CNS, BDNF regulates the efficacy of GABAergic transmission, but its contributions to functional changes at inhibitory synapses in the PVN have not been investigated. Analysis of quantal transmission revealed a decrease in the frequency of miniature inhibitory postsynaptic currents (mIPSCs) in response to BDNF with no accompanying changes in their amplitude. These effects were completely blocked by prior inclusion of the TrKB receptor antagonist K252a in the patch pipette. Inclusion of a dynamin inhibitory peptide in the patch pipette also blocked the effects of BDNF, consistent with an all-or-none removal of clusters of postsynaptic GABAA receptors. Finally, to confirm a decrease in the availability of postsynaptic GABAA receptors, we tested the effects of BDNF on focal application of the GABAA agonist muscimol. Postsynaptic responses to muscimol were reduced after BDNF. Collectively, these data indicate that BDNF remodels functional synaptic contacts putatively by reducing the surface expression of postsynaptic GABAA receptors.

Activity‐dependent release and actions of endocannabinoids in the rat hypothalamic supraoptic nucleus

Exogenous cannabinoids have been shown to significantly alter neuroendocrine output, presaging the emergence of endogenous cannabinoids as important signalling molecules in the neuroendocrine control of homeostatic and reproductive functions, including the stress response, energy metabolism and gonadal regulation. We showed recently that magnocellular and parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus and supraoptic nucleus (SON) respond to glucocorticoids by releasing endocannabinoids as retrograde messengers to modulate the synaptic release of glutamate. Here we show directly for the first time that both of the main endocannabinoids, anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), are released in an activity-dependent fashion from the soma/dendrites of SON magnocellular neurones and suppress synaptic glutamate release and postsynaptic spiking. Cannabinoid reuptake blockade increases activity-dependent endocannabinoid levels in the region of the SON, and results in the inhibition of synaptically driven spiking activity in magnocellular neurones. Together, these findings demonstrate an activity-dependent release of AEA and 2-AG that leads to the suppression of glutamate release and that is capable of shaping spiking activity in magnocellular neurones. This activity-dependent regulation of excitatory synaptic input by endocannabinoids may play a role in determining spiking patterns characteristic of magnocellular neurones under stimulated conditions.

Rapid Central Corticosteroid Effects: Evidence for Membrane Glucocorticoid Receptors in the Brain

Glucocorticoid secretion occurs in a circadian pattern and in response to stress. Among the broad array of glucocorticoid actions are multiple effects in the brain, including negative feedback regulation of hypothalamic hormone secretion. The negative feedback of glucocorticoids occurs on both rapid and delayed time scales, reflecting different regulatory mechanisms. While the slow glucocorticoid effects are widely held to involve regulation of gene transcription, the rapid effects are too fast to invoke genomic mechanisms. We provide a brief overview of multiple lines of evidence for membrane-associated glucocorticoid receptors in the brain, with an emphasis on our recent findings of a rapid, G protein-dependent glucocorticoid action in the rat hypothalamus. We have observed a novel mechanism of rapid glucocorticoid inhibition of parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus (PVN) mediated by the retrograde release of endocannabinoids and suppression of synaptic glutamate release. This acute glucocorticoid action may underlie the rapid inhibitory effect of glucocorticoids on hypothalamic neuroendocrine function, and provides a potential model for the rapid glucocorticoid effects that occur in several areas of the brain.