Relationships between the prefrontal cortex and the basal ganglia in the rat: physiology of the corticosubthalamic circuits.

The prelimbic-medial orbital areas (PL/MO) of the prefrontal cortex are connected to the medial part of the subthalamic nucleus (STN) through a direct projection and an indirect circuit that involves the core of the nucleus accumbens (NAcc) and the ventral pallidum (VP). In the present study, the influence of the PL/MO on the discharge of STN cells has been characterized. The major pattern of the responses observed after stimulation of PL/MO consisted of two excitatory peaks often separated by a brief inhibitory period. The early excitation was most likely to be caused by the activation of direct cortical inputs because its latency matches the conduction time of the prefrontal STN projections. The late excitation resulted from the activation of the indirect PL/MO-STN pathway that operates through a disinhibitory process. Indeed, the late excitation was no longer observed after acute blockade of the glutamatergic corticostriatal transmission by CNQX application into the NAcc. A similar effect was obtained after the blockade of the GABAergic striatopallidal transmission by bicuculline application into the VP. Finally, the brief inhibition that followed the early excitation was likely to result from the activation of a feedback inhibitory loop through VP because this inhibition was no longer observed after the blockade of STN inputs by CNQX application into the VP. This study further indicates the implication of STN in prefrontal basal ganglia circuits and underlines that in addition to a direct excitatory input, medial STN receives an indirect excitatory influence from PL/MO through an NAcc-VP-STN disinhibitory circuit.