A neurobehavioral model for rapid actions of corticosterone on sensorimotor integration.

Stress-induced corticosterone (CORT) secretion that causes a rapid blockade of courtship clasping by male roughskin newts (Taricha granulosa) is mediated by a specific neuronal membrane receptor for CORT. Amplectic clasping, which can be triggered by pressure on the ventral body surface and cloaca, is controlled by the influence of medullary neurons on the spinal cord. Using clasping as a simple neurobehavioral model, we have focused our analysis of CORT effects on clasping by examining the steroid's effects on neurophysiological properties of medullary neurons, especially medullary reticulospinal neurons, the principal output cells from the brain to the spinal cord. Systemic CORT caused, within 3 min of injection, diverse reductions in reticulospinal neuron excitability. Another rapid CORT effect on medullary neurons was to depress responsiveness to pressure on the cloaca. Experiments with chronically implanted, freely moving newts revealed that the rapid CORT effects are quite specific to neural processes related to clasping. CORT injections rapidly blocked clasping in response to cloacal stimuli and concurrently depressed neuronal responses to cloacal pressure and firing associated with clasping. Activity of reticulospinal neurons was often associated with nonclasping movements and this activity was rarely altered by CORT. Thus, CORT mainly affected aspects of neuronal function related to clasping. In other neurophysiological experiments, we found that the neuropeptides vasotocin and corticotropin-releasing hormone modified the neural effects of CORT. Prior exposure of medullary neurons to either of these neuropeptides caused systemic CORT administration to rapidly potentiate neuronal responses to cloacal stimuli, indicating that the direction and potency of CORT effects depend critically on the prevailing neuroendocrine state of the brain.     

Rapid corticosterone-induced impairment of amplectic clasping occurs in the spinal cord of roughskin newts (taricha granulosa)

Courtship clasping, a reproductive behavior in male roughskin newts (Taricha granulosa), is rapidly blocked by an action of corticosterone (CORT) at a specific neuronal membrane receptor. The CORT-induced impairment of clasping in behaving newts appears to be mediated partly by an elimination of clasping-related activity in medullary reticulospinal neurons. Previous studies of rapid CORT actions in Taricha have focused on the brain, so existence of CORT action in the spinal cord or peripheral nervous system has not been assessed. The present study used newts with a high cervical spinal transection to examine potential spinal or peripheral CORT effects on clasping by the hindlimbs in response to pressure on the cloaca. Spinal transection causes clasps elicited by cloacal stimulation to be very sustained beyond the termination of the eliciting stimulus. In spinally transected newts, CORT caused a dose-dependent depression in the duration as well as quality of the clasp that appeared within 10 min of injection. CORT selectively impaired the usual sustained maintenance of a clasp after termination of cloacal stimulation, but not clasp elicitation during stimulation. These effects were not produced by dexamethasone, a synthetic glucocorticoid that binds poorly to the CORT membrane receptor. The CORT effect on clasp maintenance but not clasp elicitation implies selective action on an intraspinal generator for clasping but not on sensory or efferent neuromuscular aspects of the response. These results indicate the presence in the newt spinal cord of the CORT membrane receptor that exerts functional effects distinctly different from those on the brainstem.     

Rapid, Corticosterone-Induced Disruption of Medullary Sensorimotor Integration Related to Suppression of Amplectic Clasping in Behaving Roughskin Newts (Taricha granulosa)

Endogenously secreted or injected corticosterone (CORT) rapidly suppresses courtship clasping in male roughskin newts (Taricha granulosa) by an action on a specific neuronal membrane receptor. Previous studies, using immobilized newts, showed that CORT administration rapidly depresses excitability of reticulospinal neurons and attenuates medullary neuronal responsiveness to clasp-triggering sensory stimuli. The present study used freely moving newts to examine clasping responses and concurrently record sensorimotor properties of 67 antidromically identified reticulospinal and other medullary reticular neurons before and after CORT injection. Before CORT, reticulospinal neurons fired in close association with onset and offset of clasps elicited by cloacal pressure. Reticulospinal neurons also showed firing correlates of nonclasping motor events, especially locomotion. Neuronal activity was typically reduced during clasping and elevated during locomotion. Medullary neurons that were not antidromically invaded (unidentified neurons) usually showed sensorimotor properties that resembled those of reticulospinal neurons. Intraperitoneal CORT (but not vehicle) reduced the probability and quality of hindlimb clasping in response to cloacal pressure, especially within 5–25 min of injection. Simultaneously, responses of reticulospinal and unidentified neurons to cloacal pressure and occurrence of clasping-related activity were attenuated or eliminated. CORT effects were relatively selective, altering clasping-related neuronal activity more strongly than activity associated with nonclasping motor events. The properties of CORT effects indicate that the hormone impairs clasping by depressing processing of clasp-triggering afferent activity and by disrupting the medullary control of clasping normally mediated by reticulospinal neurons. The rapid onset of these CORT effects implicates a neuronal membrane receptor rather than genomic action of the steroid.     

Corticosterone suppresses vasotocin-enhanced clasping behavior in male rough-skinned newts by novel mechanisms interfering with V1a receptor availability and receptor-mediated endocytosis

In rough-skinned newts, Taricha granulosa, exposure to an acute stressor results in the rapid release of corticosterone (CORT), which suppresses the ability of vasotocin (VT) to enhance clasping behavior. CORT also suppresses VT-induced spontaneous activity and sensory responsiveness of clasp-controlling neurons in the rostromedial reticular formation (Rf). The cellular mechanisms underlying this interaction remain unclear. We hypothesized that CORT blocks VT-enhanced clasping by interfering with V1a receptor availability and/or VT-induced endocytosis. We administered a physiologically active fluorescent VT conjugated to Oregon Green (VT–OG) to the fourth ventricle 9 min after an intraperitoneal injection of CORT (0, 10, 40 μg/0.1 mL amphibian Ringers). The brains were collected 30 min post-VT–OG, fixed, and imaged with confocal microscopy. CORT diminished the number of endocytosed vesicles, percent area containing VT–OG, sum intensity of VT–OG, and the amount of VT-V1a within each vesicle; indicating that CORT was interfering with V1a receptor availability and VT-V1a receptor-mediated endocytosis. CORT actions were brain location-specific and season-dependent in a manner that is consistent with the natural and context-dependent expression of clasping behavior. Furthermore, the sensitivity of the Rf to CORT was much higher in animals during the breeding season, arguing for ethologically appropriate seasonal variation in CORT's ability to prevent VT-induced endocytosis. Our data are consistent with the time course and interaction effects of CORT and VT on clasping behavior and neurophysiology. CORT interference with VT-induced endocytosis may be a common mechanism employed by hormones across taxa for mediating rapid context- and season-specific behavioral responses.     

Electrophysiological actions of benzodiazepines

Electrophysiological investigations have revealed that benzodiazepines, applied either locally or systemically, reduce central nervous system excitability. The studies summarized here indicate that this depression of excitability by benzodiazepines is a result of an increase in gamma-aminobutyric acid (GABA) mediated inhibition. This increase in inhibition may result from benzodiazepines increasing the activity of some GABAergic neurons and also from a modulatory action of benzodiazepines on GABA actions at some postsynaptic receptor sites. The modulatory action is observed with doses of benzodiazepines that do not cause any direct effects on neuronal excitability or membrane polarization. Specificity tests indicate that benzodiazepines do not enhance inhibition mediated by glycine or monoamines such as norepinephrine or serotonin. Results of experiments with a convulsant benzodiazepine compound, which causes a specific reduction in GABA-mediated inhibition, are also presented, The data are discussed in terms of a model in which the benzodiazepine receptor, the GABA receptor, and the chloride ionophore are functionally linked. Furthermore, it is proposed that some postsynaptic actions of GABA may be continually regulated by the occupancy of a benzodiazepine receptor, and that occupancy of the benzodiazepine receptor may be permissive for the GABA-elicited increase in chloride ion permeability.     

Intracranial administration of corticotropin-like peptides increases incidence of amphibian reproductive behavior

Male rough-skinned newts (Taricha granulosa) exhibit an increase in sexual behavior (amplectic clasping) following intracerebroventricular (ICV) injection of adrenocorticotropin (ACTH 1–39), ACTH 4–10, or melanocyte-stimulating hormone (αMSH). In contrast, intraperitoneal (IP) administration of ACTH 1–39 or corticosterone significantly decreases the incidence of sexual behavior in male newts. These results suggest that a corticotropin-like peptide acts centrally to enhance sexual behavior and that systemic ACTH acts on the interrenal tissue to inhibit sexual behavior by stimulating the release of corticosterone.     

Adrenal steroids as modulators of nerve cell function

Adrenal steroids modulate the function of nerve cells. Some, but not all actions of these steroids take place after binding to intracellular receptor systems and translocation of the steroid-receptor complex into the cell nucleus. Studies on the rat brain revealed heterogeneity of receptors. One population of receptor sites is present in abundance in extrahypothalamic limbic brain regions, e.g. neurons of the hippocampus, septum and amygdala. This neuronal receptor system displays a stringent binding specificity towards corticosterone, which is the naturally occurring glucocorticoid of the rat. Focussing the studies on the corticosterone receptor system in hippocampal neurons has provided further insight in the understanding of some of the actions of the steroid. Certain hippocampus-associated behaviors and indices of neurotransmission (serotonin) were disturbed after removal of the adrenals, but selectively restored after replacement with a low dose of corticosterone. The specificity, localization and dose-dependency of the corticosterone action on behavior and neurotransmission corresponds to the properties of its receptor system. The responsiveness to corticosterone is altered after changes in number of receptor sites. Chronic stress or high doses of exogenous corticosterone cause a long-term reduction. Other factors involved in regulation of receptor number are the neurotransmitter serotonin and neuropeptides related to ACTH and vasopressin. These substances restore changes in number of hippocampal corticosterone receptor sites due to aging, endocrine or neural deficiencies. Our results show that the number of corticosterone receptors is a sensitive index for brain functioning. Thus, the receptor system mediates some of the modulatory actions of corticosterone on nerve cell function and it may adjust its capacity under the influence of neural and endocrine factors.     

Localization of mineralocorticoid receptors at mammalian synapses

In the brain, membrane associated nongenomic steroid receptors can induce fast-acting responses to ion conductance and second messenger systems of neurons. Emerging data suggest that membrane associated glucocorticoid and mineralocorticoid receptors may directly regulate synaptic excitability during times of stress when adrenal hormones are elevated. As the key neuron signaling interface, the synapse is involved in learning and memory, including traumatic memories during times of stress. The lateral amygdala is a key site for synaptic plasticity underlying conditioned fear, which can both trigger and be coincident with the stress response. A large body of electrophysiological data shows rapid regulation of neuronal excitability by steroid hormone receptors. Despite the importance of these receptors, to date, only the glucocorticoid receptor has been anatomically localized to the membrane. We investigated the subcellular sites of mineralocorticoid receptors in the lateral amygdala of the Sprague-Dawley rat. Immunoblot analysis revealed the presence of mineralocorticoid receptors in the amygdala. Using electron microscopy, we found mineralocorticoid receptors expressed at both nuclear including: glutamatergic and GABAergic neurons and extra nuclear sites including: presynaptic terminals, neuronal dendrites, and dendritic spines. Importantly we also observed mineralocorticoid receptors at postsynaptic membrane densities of excitatory synapses. These data provide direct anatomical evidence supporting the concept that, at some synapses, synaptic transmission is regulated by mineralocorticoid receptors. Thus part of the stress signaling response in the brain is a direct modulation of the synapse itself by adrenal steroids.     

Neuroimmune interactions in guinea pig stomach and small intestine

Enteric neuroimmune interactions in gastrointestinal hypersensitivity responses involve antigen detection by mast cells, mast cell degranulation, release of chemical mediators, and modulatory actions of the mediators on the enteric nervous system (ENS). Electrophysiological methods were used to investigate electrical and synaptic behavior of neurons in the stomach and small intestine during exposure to beta-lactoglobulin in guinea pigs sensitized to cow's milk. Application of beta-lactoglobulin to sensitized preparations depolarized the membrane potential and increased neuronal excitability in small intestinal neurons but not in gastric neurons. Effects on membrane potential and excitability in the small intestine were suppressed by the mast cell stabilizing drug ketotifen, the histamine H(2) receptor antagonist cimetidine, the cyclooxygenase inhibitor piroxicam, and the 5-lipoxygenase inhibitor caffeic acid. Unlike small intestinal ganglion cells, gastric myenteric neurons did not respond to histamine applied exogenously. Antigenic exposure suppressed noradrenergic inhibitory neurotransmission in the small intestinal submucosal plexus. The histamine H(3) receptor antagonist thioperamide and piroxicam, but not caffeic acid, prevented the allergic suppression of noradrenergic inhibitory neurotransmission. Antigenic stimulation of neuronal excitability and suppression of synaptic transmission occurred only in milk-sensitized animals. Results suggest that signaling between mast cells and the ENS underlies intestinal, but not gastric, anaphylactic responses associated with food allergies. Histamine, prostaglandins, and leukotrienes are paracrine signals in the communication pathway from mast cells to the small intestinal ENS.     

糖皮质激素非基因组效应的研究

在过去的几十年间,人们认为糖皮质激素（glucocorticoid,GC）仅仅是通过改变基因的表达来发挥其生理作用,这个过程需要几个小时来完成。然而,近年来越来越多的证据表明GC对激素分泌、神经元兴奋性、机体行为及细胞形态、糖类代谢等具备快速效应,这些过程往往在数秒钟或者分种内完成,这种作用机制被称为GC的非基因组作用机制。GC的非基因组作用主要可能通过两种不同的机制起作用：（1）通过细胞膜上或者细胞质内结构未知的糖皮质激素受体（glucocorticoid Recptor,GR）来发挥非基因组作用,即为特异性非基因组效应,（2）GC主要通过改变细胞膜理化作用来发挥效应。也称为非特异性非基因组效应（non-specific nongenomic effects,NSNE）。本文通过阐述近年来GC的非基因组的作用的最新研究进展并且讨论了这些非基因组作用临床治疗过程中的联系。对糖皮质激素基因组和非基因组作用机制的深入了解有助于指导我们在临床合理用药并减少其副作用。     

A neurotrophic hypothesis of depression: role of synaptogenesis in the actions of NMDA receptor antagonists

Molecular and cellular studies have demonstrated opposing actions of stress and antidepressant treatment on the expression of neurotrophic factors, particularly brain-derived neurotrophic factor, in limbic structures of the brain. These changes in neurotrophic factor expression and function result in structural alterations, including regulation of neurogenesis, dendrite length and spine density in hippocampus and prefrontal cortex (PFC). The deleterious effects of stress could contribute to the reduced volume of these brain regions in depressed patients. Conversely, the actions of antidepressant treatment could be mediated in part by blocking or reversing the atrophy caused by stress and depression. Recent studies have identified a novel, rapid-acting antidepressant, ketamine, in treatment-resistant depressed patients that addresses the limitations of currently available agents (i.e. delayed onset of action and low response rates). We have found that ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, causes a rapid induction of synaptogenesis and spine formation in the PFC via stimulation of the mammalian target of the rapamycin signalling pathway and increased synthesis of synaptic proteins. These effects of ketamine rapidly reverse the atrophy of PFC neurons caused by chronic stress and correspond to rapid behavioural actions of ketamine in models of depression. Characterization of a novel signalling pathway also identifies new cellular targets that could result in rapid and efficacious antidepressant actions without the side effects of ketamine.     

Brain mineralocorticoid receptor diversity: Functional implications

Mineralocorticoid receptors (MRs) in neurons of the anterior hypothalamus and the periventricular brain regions mediate aldosterone-selective actions on sodium hemeostasis, salt appetite and cardiovascular regulation. Corticosterone is not effective in these neurons, possibly because it is enzymatically inactivated. However, MRs in limbic brain regions, notably in the hippocampal neurons, do already respond to very low concentrations of both corticosterone and aldosterone. The MR-mediated effects stabilize neuronal transmission and appear critical for neuronal integrity of a sub-region of the hippocampus: the dentate gyrus. Higher concentrations of corticosterone induced by stress and the circadian rise progressively activate the lower affinity glucocorticoid receptors (GRs), which in coordination with MR-mediated actions then facilitate adaptive processes required for recovery of homeostasis. It is postulated that this balanced MR- and GR-mediated action of corticosterone is of critical importance for regulation of the stress response and behavioural adaptation.     

The effects of sex steroids and vasotocin on behavioral responses to visual and olfactory sexual stimuli in ovariectomized female roughskin newts

Previous studies have found that vasotocin (AVT) administration to male roughskin newts (Taricha granulosa) enhances courtship clasping as well as appetitive responses to specific sexual stimuli and that treating female newts with androgens plus AVT induces the expression of male-typical courtship clasping (the selective clasping of females). However, the unique and/or interactive effects of sex steroids and AVT on appetitive responses to specific sexual stimuli have not yet been determined. To first identify male-typical, sexually dimorphic appetitive responses to female sexual stimuli, we tested intact newts during the breeding season and found that males, but not females, are attracted to female visual and pheromonal sexual stimuli. We then used ovariectomized (ovx) females implanted with empty silastic capsules (Blk) or with capsules containing testosterone (T), dihydrotestosterone (DHT), or estradiol (E2) and then injected with either saline or AVT to determine the effects of steroids and AVT, alone or in combination with each other, on male-typical behavioral responses to those stimuli. E2 treatment depressed responses toward female visual stimuli independently of AVT. On the other hand, only T-implanted, AVT-injected females displayed male-typical behavioral responses toward female olfactory stimuli, preferring to spend more time in proximity to female-scented than unscented newt models and selectively clasping the female-scented models. Together, these results support the conclusion that sex steroids and AVT influence behavioral responses to sexual stimuli via sensory-specific mechanisms. Furthermore, they suggest that T and AVT interact within the brain to influence sensorimotor processing in the pathways that integrate olfactory sexual stimuli into male-typical courtship behaviors.     

Prolactin receptor in regulation of neuronal excitability and channels

Prolactin (PRL) activates PRL receptor isoforms to exert regulation of specific neuronal circuitries, and to control numerous physiological and clinically-relevant functions including; maternal behavior, energy balance and food intake, stress and trauma responses, anxiety, neurogenesis, migraine and pain. PRL controls these critical functions by regulating receptor potential thresholds, neuronal excitability and/or neurotransmission efficiency. PRL also influences neuronal functions via activation of certain neurons, resulting in Ca²⁺ influx and/or electrical firing with subsequent release of neurotransmitters. Although PRL was identified almost a century ago, very little specific information is known about how PRL regulates neuronal functions. Nevertheless, important initial steps have recently been made including the identification of PRL-induced transient signaling pathways in neurons and the modulation of neuronal transient receptor potential (TRP) and Ca²⁺-dependent K⁺ channels by PRL. In this review, we summarize current knowledge and recent progress in understanding the regulation of neuronal excitability and channels by PRL.     

Prolactin receptor in regulation of neuronal excitability and channels

Prolactin (PRL) activates PRL receptor isoforms to exert regulation of specific neuronal circuitries, and to control numerous physiological and clinically-relevant functions including; maternal behavior, energy balance and food intake, stress and trauma responses, anxiety, neurogenesis, migraine and pain. PRL controls these critical functions by regulating receptor potential thresholds, neuronal excitability and/or neurotransmission efficiency. PRL also influences neuronal functions via activation of certain neurons, resulting in Ca²⁺ influx and/or electrical firing with subsequent release of neurotransmitters. Although PRL was identified almost a century ago, very little specific information is known about how PRL regulates neuronal functions. Nevertheless, important initial steps have recently been made including the identification of PRL-induced transient signaling pathways in neurons and the modulation of neuronal transient receptor potential (TRP) and Ca²⁺-dependent K⁺ channels by PRL. In this review, we summarize current knowledge and recent progress in understanding the regulation of neuronal excitability and channels by PRL.     

Benzodiazepine pharmacology of cultured mammalian CNS neurons

Many neurons cultured from the embryonic mammalian central nervous system (CNS) express benzodiazepine receptors while some neurons differentiate specific transmitter phenotypes like glutamic acid decarboxylase (GAD), the synthetic enzyme for gamma-aminobutyric acid (GABA). The benzodiazepine receptors in these cultured neurons are often, if not always coupled to a practically ubiquitous GABA-mediated function, activation of Cl- ion conductance. The transmitter signal serves to inhibit neuronal excitability and is facilitated by clinically important benzodiazepines. Here we review some details regarding the pharmacological actions of benzodiazepines on membrane excitability.     

Antagonists of NMDA glutamate receptors selectively affect synaptic mechanisms of the nociceptive sensitization in the snail

The effects of N-methyl-D-aspartate (NMDA) glutamate receptor antagonists on the mechanisms of nociceptive sensitization were studied in LPl1 and RPl1 neurons of the semiintact preparation of a Helix lucorum snail. Application of sensitizing stimuli on the head part of the control preparation led to a depolarization of the membrane and increase in its excitability. A depression of responses of neurons evoked by tactile or chemical sensory stimulation during the short-term period and significant facilitation of responses during the long-term period of sensitization were observed. Sensitization performed under conditions of application of NMDA antagonists (AP5 or MK801) produced similar changes in membrane potential, membrane excitability, and neuronal responses evoked by tactile stimulation of the head or foot. However, the chemical stimulation of the head under these conditions evoked a significant depression of responses during the short- and long-term sensitization periods. The results suggest that the NMDA glutamate receptor antagonists selectively affect the plasticity induction mechanisms of the command neuron synaptic inputs, which mediate the chemical sensory stimulation from the snail's head.