Reward-associated excitation and pain-associated inhibition lasting seconds in rat medial pallidal units

Prolonged changes during 10 sec poststimulus periods in unit firing rate following stimuli (0.2 sec, 100 Hz, 0.5 msec cathodal pulses) at reward sites in medial forebrain bundle and aversive midbrain reticular sites have been observed in unanesthetized postcollicular cerveau isole rats. However, longlasting medial forebrain bundle-elicited inhibition and reticular-elicited excitation converged on single cells only in intralaminar thalamus. Since these opposite effects had a comparatively long duration, the present study postulated that a related brain region might also show opposite medial forebrain bundle and reticular responses, but the inverse of those seen in intralaminar thalamus. Using similar methods, convergence on single units of prolonged excitation by medial forebrain bundle and by reticular stimuli was typical in internal capsule and to a lesser extent amygdala. Ventral anterior-ventral lateral nuclei of thalamus were characterized by converging inhibition. Contrasting actions of the stimuli which might involve presynaptic interactions were observed in lateral pallidum. However, direct opposite effects, medial forebrain bundle-elicited excitation and reticular-elicited inhibition, were demonstrated and confined anatomically to a medial pallidal region. The opposite responses summated or cancelled when the medial forebrain bundle and reticular stimuli were given simultaneously. A medial pallidal-intralaminar thalamus system may be central to the neural coding of certain affective states, integrating these with motor control and sensory information.