Medial posterior hypothalamic input is involved in adrenocortical activation following forebrain limbic stimulation

Role of serotonin in the amygdala in hypothalamo-pituitary-adrenocortical responses

In view of the fact that the amygdala (AMG) and hypothalamic serotonin (5-HT) have an excitatory effect on the hypothalamo-pituitary-adrenal (HPA) axis, we have investigated the role of 5-HT in the AMG on this response. In intact freely moving rats, a mildly stressing short photic stimulation caused depletion of median eminence CRH-41, due to its release into the portal circulation and a rise in serum ACTH and corticosterone levels. This effect was significantly inhibited in rats in which 5-HT was depleted in the AMG following local 5,7-dihydroxytryptamine administration, which did not affect hypothalamic 5-HT content. Also, local pretreatment with ketanserin (a 5-HT2 receptor antagonist) in the AMG had the same inhibitory effect on the HPA axis response. These results indicate that AMG 5-HT has an important role in the activation of the HPA axis following neural stimulation and that 5-HT2 receptors are involved.

Hypothalamic mechanisms mediating glutamate effects on the hypothalamo-pituitary-adrenocortical axis

The effect of local administration of glutamate into the hypothalamic paraventricular nucleus (PVN) on the hypothalamo-pituitary adrenocortical (HPA) axis was studied in male rats. Glutamate caused CRH-41 depletion from the median eminence (ME) and a consequent rise in ACTH and corticosterone (CS) serum levels. In rats pretreated with systemic dexamethasone (dex) these effects were completely inhibited. The administration of the glucocorticoid receptor antagonist RU-38486 abolished the inhibitory effect of dex on the adrenocortical discharge. In addition, the depletion of hypothalamic norepinephrine (NE) and serotonin (5-HT) by specific neurotoxins administered into the ventral noradrenergic blundle or into the raphe nuclei respectively, inhibited the response of serum ACTH and CS following PVN glutamate administration. These data indicate that glutamate stimulated the HPA axis via the release of ME CRH-41 into the portal circulation. This response is steroid sensitive involving type II glucocorticoid receptors. Hypothalamic NE and 5-HT participate in the glutamate induced HPA axis activation.

Posterior hypothalamic deafferentation or 5,7-dihydroxytryptamine inhibit corticotropin-releasing hormone, ACTH and corticosterone responses following photic stimulation

The effect of posterior hypothalamic deafferentation (PHD) or hypothalamic serotonin (5-HT) depletion by 5,7-dihydroxytryptamine on corticotropin-releasing hormone (CRH), ACTH and corticosterone (CS) responses, following photic stimulation, was investigated in male rats. In intact animals, photic stimulation caused median eminence (ME) CRH depletion and a rise in serum ACTH and CS levels. In rats with PHD, in which previous studies have demonstrated 5-HT depletion, or with neurotoxin induced hypothalamic 5-HT reduction, these responses were markedly inhibited. This indicates that the blockage in the release of ME CRH into the portal circulation in rats with hypothalamic 5-HT depletion is responsible for the inhibition of the pituitary-adrenocortical responses following photic stimulation.

Limbic pathways and hypothalamic neurotransmitters mediating adrenocortical responses to neural stimuli

One of the major phenomena related to the stress response is the activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. This axis consists of corticotropin releasing factor-41 in the paraventricular nucleus of the hypothalamus (PVN), which in response to a variety of stimuli is released into the portal circulation and stimulates pituitary ACTH secretion and subsequently adrenocortical discharge. The mechanisms involved in the activation are not uniform and the responses to various stimuli are mediated by different neural pathways. Since extrahypothalamic limbic structures play a significant role in the HPA function, it is the purpose of this review to describe the neural pathways between the hippocampus, septum and amygdala and the hypothalamus in relation to adrenocortical activity and the differential role of the medial forebrain bundle as well as the effects of various hypothalamic deafferentation on the transmission of the neural impulses to the hypothalamus. Also, the importance of norepinephrine and serotonin in the activation of the HPA axis will be delineated.