Opposite medial thalamic unit responses to rewarding and aversive brain stimulation.

Medial thalamus receives fibers from both medial forebrain bundle (MFB) and hindbrain and midbrain reticular formation (RET). The MFB and RET stimulations are rewarding and aversive respectively. In 32 unanesthetized cerveaz~ isolt! rats, 158 units were recorded. The MFB and RET effects converge on two thirds of the units recorded in the dorsal medial and paracentral nuclei of thalamus and are opposite in the following ways: post-stimulus pattern of unit discharge during 7 Hz stimulation; slow-wave recruiting with MFB, but not RET, 7 Hz stimulation; and at a “desynchronization” stimulus frequency (20 Hz), MFB elicits decreased unit discharge and RET elicits increased unit discharge, compared to the 7 Hz rates. In the intralaminar and parafasicular nuclei, post-train (60 Hz, 0.2 set) decreases and increases in unit firing lasting seconds are often elicited with MFB and RET trains respectively. Stimulation of hypothalamic sites outside MFB did not elicit these MFB effects; parafasicular stimulation did not elicit the RET effects. The MFB effects were not seen in habenula or ventral basal thalamus. Hippocampal, habenular, and ventral medial thalamic units did not show opposite MFB and RET effects. Threshold currents for the opposite 1\lFB and RET effects are similar to those eliciting self-stimulation and escape respectively in several operated rats tested both behaviorally and neurophysiologically.