The effects of iontophoretic cortisol on evoked activity was studied in rat tuberal hypothalamic units. Evoked activity from visual, auditory, sciatic nerve and hippocampal stimulation was examined in same neurons both before and during local cortisol administration. Hippocampal stimuli were studied in isolation and as conditioning stimuli for one of the sensory modalities. A constant background glutamate current was frequently used in order to shorten observation periods and to enhance inhibition of firing. Even though the predominant effect of cortisol on spontaneous firing was to reduce the firing rate, the effect of the hormone on evoked responses of all kinds was to reduce the size of the response, independent of sign, and even to reverse the nature of the response, from control facilitation to inhibition or from control inhibition to facilitation. Evoked responses in many units whose spontaneous activity was not affected by cortisol were altered by hormone. Of particular interest are a group of cells which are normally silent and respond to afferent stimulation with inhibition. The responses in these cells are reduced or changed to excitation in the presence of cortisol and they may represent local inhibitory interneurons. The findings are related to the negative feedback of adrenocorticosteroids on CRF and ACTH release. © 1981.
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