LESIONS OF THE MEDIAL GENICULATE NUCLEI SPECIFICALLY BLOCK CORTICOSTERONE RELEASE AND INDUCTION OF C-FOS MESSENGER-RNA IN THE FOREBRAIN ASSOCIATED WITH AUDIOGENIC STRESS IN RATS

Audiogenic stress is known to activate the hypothalamo-pituitary-adren ocortical (HPA) axis in rats. The goal of the present study was to det ermine whether the medial geniculate nuclei (including all auditory nu clei of the thalamus), which are obligatory relays in the transmission of auditory information to the forebrain, are critically involved in HPA activation by audiogenic stress. To this end, corticosterone level s and regional brain activity indexed by c-fos mRNA induction, elicite d by 30 min of 105 dB white noise, were measured. Compared with unoper ated and sham-operated rats, complete medial geniculate nuclei lesions blocked corticosterone release normally induced by loud noise. The ef fects of the lesions were specific to loud noise insofar as corticoste rone release in response to restraint or ether stress was not reduced in lesioned rats. We have determined previously that audiogenic stress is associated with a specific regional pattern of c-fos mRNA inductio n. Rats sustaining complete medial geniculate lesions demonstrated a b lockade of c-fos mRNA induction in several audiogenic stress responsiv e regions, also known to directly innervate medial parvocellular neuro ns of the paraventricular hypothalamic nucleus. Thus, in addition to b lockade in the paraventricular hypothalamic nucleus, c-fos mRNA induct ion in the lesioned animals was abolished in the bed nucleus of the st ria terminalis, especially its anterior medial and ventral aspects, th e septohypothalamic nucleus, and the anteroventral preoptic area, comp ared with unoperated and sham-operated rats. Several additional region s in the lesioned rats failed to show reliable c-fos mRNA induction co mpared with naive rat controls. Nearly all other regions that showed r eliable c-fos mRNA induction in the unoperated and sham-operated rats displayed either similar or slightly reduced levels in complete medial geniculate-lesioned rats, suggesting that these regions are not part of a critical HPA activational circuit in response to audiogenic stres s. On the basis of these results, putative circuits from the medial ge niculate nuclei to the paraventricular nucleus of the hypothalamus inv olved in activation of the HPA axis by audiogenic stress are discussed .