Inhibitory substantia nigra inputs to the pedunculopontine neurons.

Responses of 43 pedunculopontine area (PPN area) neurons to electrical stimulation of the substantia nigra (SN) were studied in anesthetized rats. An intracellular recording technique was used to demonstrate that SN stimulation evoked hyperpolarizing potentials, which were identified by intracellular injections as inhibitory postsynaptic potentials (IPSPs). These IPSPs were often followed by a rebound depolarization that originates several spike potentials. These IPSPs were characterized as monosynaptic, with latencies varying from 1.0 to 8.5 ms. Similar results were observed in some animals with chronic unilateral coronal lesion just rostral to subthalamic nucleus (STH), which severed the rostral afferents. PPN are neurons were also antidromically activated by SN stimulation. Two PPN area projection neurons were clearly identified. Mean latency of one group was 0.71 ms; mean latency of the second group was 5.16 ms. The morphological analysis of a neuron inhibited by SN stimulation and labeled with horseradish peroxidase (HRP) demonstrated that the soma was fusiform in shape, with the axon originating in the soma and collaterals and a large dendritic field extending in the ventrodorsalis direction. The results indicate that the PPN area is reciprocally connected with the SN, which elicits an inhibitory effect on PPN area neurons.