Neurohypophysial responses to emotional stress after deafferentation of sino-aortic baroreceptors in rats

Influence of the ventral hippocampal formation on plasma vasopressin, hypothalamic-pituitary-adrenal axis, and behavioral responses to novel acoustic stress

The ventral hippocampal formation (vHF) seems to constrain diverse responses to psychological stimuli, and disruption of this function may underlie severe neuropsychiatric diseases. In particular, the ventral subiculum inhibits hypothalamic-pituitary-adrenal axis (HPA) activity following psychological, but not systemic, stressors. Despite the difficulty in interpreting such HPA responses, they have been relied upon to further characterize vHF function, because increased HPA axis activity is implicated in neuropsychiatric disturbances, and reliance on behavioral and cognitive data is even more problematic. Plasma arginine vasopressin (pAVP), which is inhibited by psychological stimuli and is also implicated in diverse neuropsychiatric diseases, provides a less ambiguous measure of CNS function. To test if its inhibition by psychological stress is also mediated by the vHF, we conducted two studies. In the first, pAVP and behavioral responses to novel acoustic stress were assessed in rats with bilateral excitotoxic lesions of the ventral subiculum and the ventral hippocampus. The subiculum lesions blocked the fall in pAVP and enhanced escape behaviors, whereas the hippocampal lesions produced responses intermediate to those in the subiculum-lesioned and control rats. In the second study, the pAVP response was similarly blocked by small lesions restricted to those vHF subfields which project to the neuroendocrine hypothalamus, compared to the response in animals with lesions in other vHF subfields. These results indicate that discrete projections from the vHF inhibit the pAVP response to psychological stimuli, and suggest that pAVP may provide a reliable probe of vHF activity.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Role of adrenoceptors in vasopressin, oxytocin and prolactin responses to conditioned fear stimuli in the rat

Conditioned fear or novel environmental stimuli suppress vasopressin (VP) and augment oxytocin (OT) and prolactin (PRL) release in rats. We examined the effects of intracerebroventricular (i.c.v.) injections of adrenoceptor antagonists on these neuroendocrine responses to conditioned fear or novel environmental stimuli in male rats. A beta1 antagonist, metoprolol, blocked the VP but not the OT or PRL response to conditioned fear stimuli, but did not abolish neuroendocrine responses to novel environmental stimuli. A beta2 antagonist, ICI118551, impaired the PRL but not the VP or OT response to fear or novel environmental stimuli. In rats injected with a alpha1 adrenoceptor antagonist, benoxathian, conditioned fear stimuli did not significantly induce the VP, OT or PRL responses. The effects of benoxathian were not due to a general reduction of arousal, since benoxathian did not prevent the VP, OT or PRL response to novel environmental stimuli. These data suggest that beta1 adrenoceptors play a selective role in the VP response to conditioned fear stimuli, as do beta2 adrenoceptors in the prolactin response to conditioned fear and novel environmental stimuli. We conclude that alpha1 adrenoceptors play a facilitative role in VP, OT, PRL responses to conditioned fear stimuli.

Role of noradrenergic projections to the bed nucleus of the stria terminalis in neuroendocrine and behavioral responses to fear-related stimuli in rats

The bed nucleus of the stria terminalis (BNST) receives dense noradrenergic projections from the brainstem and has been claimed to play a role in expression of a variety of stress responses. Fear-related stimuli suppress vasopressin and facilitate oxytocin release from the neurohypophysis and induce behavioral suppression. Here we investigated in male rats whether conditioned fear stimuli increase noradrenergic activity in the BNST and whether depletion of epinephrine content in the BNST prevents neuroendocrine and behavioral responses to fear stimuli. Environmental stimuli previously paired with electric footshocks increased the ratio of 3-methoxy-4-hydroxyphenylglycol to norepinephrine contents in the BNST, suggesting that the stimuli activated noradrenergic projections to the BNST. 5-Amino-2, 4-dihydroxy-alpha-methylphenylethylamine, a neurotoxin relatively selective for noradrenergic fibers, when injected into the BNST 7 days before measurement, decreased the content of norepinephrine by 95% and that of dopamine or serotonin by about 50%. In the rats that received the neurotoxin, the suppressive vasopressin but not the augmentative oxytocin response to intermittent footshocks was abolished. In the experiments with conditioned fear stimuli, the neurotoxin given before training partially but significantly impaired the suppressive vasopressin and behavioral responses to testing stimuli. The neurotoxin given after training, however, did not prevent the vasopressin, oxytocin or behavioral responses. The results suggest that noradrenergic fibers in the BNST mediate the suppressive vasopressin but not the augmentative oxytocin response to nonassociatively applied fear stimuli and that they modulate, in a facilitative fashion, acquisition but not retention or recall of the emotional memory associated with the vasopressin and behavioral responses to conditioned fear stimuli.

Norepinephrine depletion impairs neuroendocrine responses to fear but not novel environmental stimuli in the rat

Emotional stimuli suppress vasopressin secretion and potentiate oxytocin and prolactin secretion by the pituitary in the rat. We studied effects of central norepinephrine depletion on these hormonal responses to novel environmental or fear stimuli. Male Wistar rats were injected intracerebroventricularly with 5-amino-2,4-dihydroxy-alpha- methylphenylethylamine, a selective neurotoxin to noradrenergic fibers. The neurotoxin treatment reduced the hypothalamic content of norepinephrine by 71% but did not significantly affect the dopamine content. Novel environmental stimuli suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected rats. The neurotoxin did not block the neuroendocrine responses. Intermittently applied electric footshocks also induced the similar neuroendocrine responses in the vehicle-injected rats. The neurotoxin significantly reduced the neuroendocrine responses. The drug, however, did not significantly alter vasopressin release after continuously applied footshocks. Environmental stimuli previously paired with footshocks (conditioned fear stimuli) suppressed vasopressin secretion and augmented secretion of oxytocin and prolactin in the vehicle-injected animals. Motor activity was suppressed during the conditioned fear stimuli. The neurotoxin impaired the neuroendocrine and behavioral responses whether the drug was injected before or after the conditioning. These data demonstrate the distinction between the neural mechanisms underlying the neuroendocrine responses to fear and to novel stimuli, suggesting that noradrenergic neurons are selectively involved in the hypothalamo-hypophysial responses to fear stimuli.

A benzodiazepine, chlordiazepoxide, blocks vasopressin and oxytocin release after footshocks but not osmotic stimulus in the rat

Noxious as well as hypertonic stimuli potentiate vasopressin and oxytocin secretion in rats. Neurohypophysial vasopressin- and oxytocin-secreting neurons receive inhibitory synaptic inputs mediated by gamma-aminobutyric acid (GABA). Benzodiazepines modulate GABA-A receptor activity in a facilitatory fashion. It is thus possible that benzodiazepines suppress vasopressin and oxytocin release after noxious stimuli. To test this hypothesis, we investigated whether chlordiazepoxide impairs the enhanced release of vasopressin and oxytocin after noxious or hypertonic stimuli in male rats. Chlordiazepoxide (5-20 mg/kg, i.p.) blocked dose-dependently the vasopressin and oxytocin responses to footshocks. Chlordiazepoxide, however, did not impair the hormonal responses to hypertonic stimulus. The results demonstrate that chlordiazepoxide selectively prevents vasopressin and oxytocin release after noxious stimuli and therefore suggest that the sites of chlordiazepoxide actions are not on the vasopressin or oxytocin neurons in rats.

Effects of a histamine H2-receptor antagonist, ranitidine on the vasopressin and oxytocin responses to novelty stress in the rat

Effects of an intraperitoneally (i.p.) administered histamine H2-receptor antagonist, ranitidine, on plasma levels of vasopressin and oxytocin were studied in male rats under unstressed or stressed conditions. In the rats injected i.p. with the vehicle (saline) solution, plasma vasopressin level was significantly lower and plasma oxytocin level was significantly higher after weak electric foot shocks (10 ms pulses of 0.8 mA, 50 Hz and 1 s duration, repeated at 30 s intervals for a period of 5 min) than those levels in the unshocked control rats. Ranitidine injected i.p. at a dose of 100 mg per kg body weight blocked the suppressive vasopressin but not the facilitatory oxytocin response to the shocks. Novel environmental stimuli were applied to rats in such a way that the animals were transferred to an experimental room, placed in a white-painted plastic pail and administered an intermittent 2 kHz and 70 dB pure tone of 2 s duration that was repeated at 10 s intervals for 2 min. In the rats injected i.p. with the vehicle solution, plasma vasopressin level was lower and oxytocin level was higher after the novel stimuli than in the unstimulated control rats. Ranitidine injected i.p. at a dose of 100 mg per kg body weight blocked the suppressive vasopressin but not the facilitatory oxytocin response to the novel stimuli. Ranitidine administered i.p. at doses of 10, 20, 50 and 100 mg per kg body weight was tested for the suppressive vasopressin response to the novel stimuli given for periods of 2 or 5 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of novelty stress on vasopressin and oxytocin secretion by the pituitary in the rat

Effects of novel environmental stimuli on vasopressin and oxytocin secretion by the pituitary were studied in dehydrated male rats. As the novel environmental stimuli, rats were transferred to an experimental room, placed in a box painted black and given a pure tone auditory stimulus of 2 kHz. Exposure of rats to the novel environmental stimuli for a period of 2 min decreased plasma concentrations of vasopressin and increased plasma levels of adrenocorticotrophic hormone (ACTH) and prolactin, but did not significantly change the plasma level of oxytocin. The stimuli, however, became ineffective for producing the suppressive vasopressin response as the period of exposure was prolonged to more than 5 and up to 30 min, although the prolonged stimuli were still effective for inducing facilitatory ACTH and prolactin responses. After repeated exposures of rats to the environmental stimuli once a day for 5 or 10 days, the stimuli became disabled from producing the suppressive vasopressin response. However, the rats were still capable of responding to the novel stimuli of another kind. All these data suggest that novelty stress suppresses vasopressin secretion but does not change oxytocin secretion. In order to test the possibility that glucocorticoids expectedly secreted by the adrenals in response to the stress might have suppressed vasopressin secretion, a large amount of dexamethasone was administered to the rat before testing. Dexamethasone pretreatment depressed plasma levels of ACTH and vasopressin as reported previously and blocked the facilitatory ACTH response to the novelty stress. However, dexamethasone treatment did not affect the suppressive vasopressin response to the novelty stress.(ABSTRACT TRUNCATED AT 250 WORDS)