The role of the medial forebrain bundle in the mediation of the hypothalamic-hypophyseal-adrenal responses to acute neurogenic stress.

Adult male rats, intact (N) or bearing bilateral lesions in the medial forebrain bundle (MFB), were acutely exposed to visual, audiogenic, or thermal stress, during the following which serum ACTH and corticosterone (CS) levels were determined. The marked elevations in serum concentrations of both ACTH and CS which occurred in intact animals following photic stimulation were absent in MFB rats. The normal ACTH and CS responses to audiogenic stress were slightly delayed and attenuated, respectively, in the operated group, whereas the responses of both hormones to exposure to elevated environmental temperature were partially blocked in lesioned, as compared to intact, animals. These results demonstrate that the MFB is involved in mediating the ACTH, as well as the CS secretory responses to acute stress exposure. More specifically, the role of the MFB, vís-à-vis the hypothalamo-hypophyseal-adrenal axis, appears to be in the transmission of sensory input from rostral brain areas posteriorly.