The afferent input and functional organization of the hypothalamus in reactions regulating pituitary-adreno-cortical activity

Rapid Nongenomic Glucocorticoid Actions in Male Mouse Hypothalamic Neuroendocrine Cells Are Dependent on the Nuclear Glucocorticoid Receptor

Corticosteroids act classically via cognate nuclear receptors to regulate gene transcription; however, increasing evidence supports rapid, nontranscriptional corticosteroid actions via activation of membrane receptors. Using whole-cell patch clamp recordings in hypothalamic slices from male mouse genetic models, we tested for nongenomic glucocorticoid actions at glutamate and gamma aminobutyric acid (GABA) synapses in hypothalamic neuroendocrine cells, and for their dependence on the nuclear glucocorticoid receptor (GR). In enhanced green fluorescent protein-expressing CRH neurons of the paraventricular nucleus (PVN) and in magnocellular neurons of the PVN and supraoptic nucleus (SON), dexamethasone activated postsynaptic membrane-associated receptors and G protein signaling to elicit a rapid suppression of excitatory postsynaptic inputs, which was blocked by genetic deletion of type I cannabinoid receptors and a type I cannabinoid receptor antagonist. In magnocellular neurons, dexamethasone also elicited a rapid nitric oxide-dependent increase in inhibitory postsynaptic inputs. These data indicate a rapid, synapse-specific glucocorticoid-induced retrograde endocannabinoid signaling at glutamate synapses and nitric oxide signaling at GABA synapses. Unexpectedly, the rapid glucocorticoid effects on both excitatory and inhibitory synaptic transmission were lost with conditional deletion of GR in the PVN and SON in slices from a single minded-1-cre-directed conditional GR knockout mouse. Thus, the nongenomic glucocorticoid actions at glutamate and GABA synapses on PVN and SON neuroendocrine cells are dependent on the nuclear GR. The nuclear GR, therefore, is responsible for transducing the rapid steroid response at the membrane, or is either a critical component in the signaling cascade or regulates a critical component of the signaling cascade of a distinct membrane GR.

Rapid neuromodulation by cortisol in the rat paraventricular nucleus: an in vitro study

1. We have used a range of in vitro electrophysiological techniques to investigate the mechanism of rapid cortisol neuromodulation of parvocellular neurones in the rat paraventricular nucleus. 2. In our study, we found that cortisol (10 microM) increased spontaneous action-current firing frequency to 193%. This effect was insensitive to the glucocorticoid intracellular-receptor antagonist mifepristone. 3. Cortisol (0.1-10 microM) had no detectable effects on whole-cell GABA current amplitudes, or GABA(A) single-channel kinetics. 4. Cortisol (10 microM) inhibited whole-cell potassium currents in parvocellular neurones by shifting the steady-state activation curve by 14 mV to the right. 5. Additionally, in a cell line expressing both the glucocorticoid intracellular receptor and recombinant, fast inactivating potassium channels (hKv1.3), cortisol (1 and 10 microM) inhibited potassium currents by shifting their steady-state activation curves to the right by 12 mV (10 microM cortisol). This effect was also insensitive to the cortisol antagonist, mifepristone. 6. These data suggest that inhibition of voltage-gated potassium channels may contribute to the rapid neuromodulatory effects of cortisol, possibly by direct interaction with the ion channel itself.

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.

Modifications in single cell activity in the rat midbrain during the iontophoretic application of cortisol

In view of the modulatory role of the midbrain in the regulation of adrenocorticotrophic functions, the effects of iontophoretic application of cortisol to central gray and midbrain reticular formation units was studied. Two-thirds of the cells responded by a change in the rate of firing, mainly by inhibition, which in some cells lasted more than 1 min. These modifications in the excitability of midbrain neurons, produced by changing concentrations of glucocorticoids, may affect indirectly hypothalamic neuroendocrine mechanisms controlling ACTH secretion.

Animal models and human depressive disorders

Clinical depressive disorders are complex in presentation, dissimilar in origins and course, and often pleomorphic in character. An adequate understanding of their origins, biological substrates, and amenability to established and novel forms of therapy demands biological and social interventions which cannot always readily or ethically be carried out in a clinical setting. One useful complementary approach to clinical research utilizes preclinical models for laboratory investigations in parallel. The present paper reviews current approaches to modelling depression using animals, with particular emphasis upon phylogenetic constraints, systematic validity and reliability, and nosological limitations. Preclinical models are useful and necessary adjuncts for adequately understanding depression in humans. However, their utility remains a direct function of a continuing dialogue between clinical and laboratory research, and demands scrupulous observation and methodological rigor on the part of both clinicians and experimental researchers.

Participation of the hypothalamus in the feedback regulation of the pituitary-adrenocortical system

The activity of the pituitary-adrenocortical system, which was judged according to the level of corticosteroids in the blood, and the electrical response of the anterior, medial, and lateral hypothalamus to intravenous injection of hydrocortisone (1 mg/kg) were investigated in chronic experiments on rabbits with implanted semimicroelectrodes. It was shown that hydrocortisone induces an inhibition of the pituitary-adrenocortical function and changes the multineuronal activity of the hypothalamus. In the anterior hypothalamus, pools of neurons predominated, the discharge frequency of which was increased, while in the medial and lateral hypothalamus it was decreased. It is concluded that the corticosteroid feedback mechanisms are correlated with excitation of the anterior hypothalamus and inhibition of the medial hypothalamus. The key words are: hypothalamus, multineuronal activity corticorsteroids, and feedback.

Electrical activity of hypothalamus during activation of pituitary-adrenocortical system

Multiunit activity in anterior (AHA), medial (MHA) and lateral (LHA) areas of hypothalamus during activation of the pituitary-adrenocortical system was measured in chronic experiments on rabbits. Immobilization causing blood-corticosteroid levels to rise was followed by the excitation of most pools of neurons in MHA, and by inhibition in AHA and LHA. When immobilization failed to induce stimulation of MHA (as a result of pretreatment with large doses of cortisol), it did not cause blood-corticosteroid levels to rise, either. The switching off activity in AHA and MHA is considered to be a manifestation of functional changes which are responsible for stress-induced activation of the adrenocortical system.