Electrophysiological evidence that the mediodorsal nucleus of the thalamus is a relay between the ventral pallidum and the medial prefrontal cortex in the rat

The neural connections from the ventral pallidum (VP) through the mediodorsal nucleus of the thalamus (MD) to the medial prefrontal cortex (MPC) were investigated. Extracellular recordings were made from 219 neurons in the medial and lateral portions of the MD and the VP and the MPC were stimulated. The most frequent response to VP stimulation was inhibition and inhibition preceded by excitation. Also, the most frequent response of MD units to MPC stimulation was inhibition and inhibition preceded by excitation. Nineteen of 26 MD units, activated antidromically by MPC stimulation, responded orthodromically to VP stimulation. The most frequent orthodromic response of these MD output neurons was inhibition and inhibition preceded by excitation. GABA iontophorized onto MD neurons reduced their rate of discharge. GABA and picrotoxin iontophorized onto MD neurons did not influence the inhibitory or excitatory responses to VP stimulation. These electrophysiological results support previous anatomical findings of connections between the VP and the MPC by way of the MD. MD output neurons to the MPC receive mostly inhibitory inputs from VP afferents. A high proportion of MD neurons respond orthodromically to both VP and MPC stimulation, suggesting the convergence of synaptic inputs from these structures to the same MD units.