Intracellular analysis of excitatory subthalamic inputs to the pedunculopontine neurons

Response patterns of the pedunculopontine (PPN) area neurons to electrical stimulation of the subthalamic nucleus (STH) were investigated in anesthetized rats. Intracellular recordings demonstrated that STH stimulation evoked short duration (mean value: 11.6 ms) depolarizing potentials which were identified as excitatory postsynaptic potentials (EPSPs) by intracellular current injection. These potentials were considered monosynaptic because the latency was constant in spite of changes in stimulus intensities. The conduction velocities of STH-PPN area afferents ranged from 1.2 to 8.3 m/s (mean value: 1.6 m/s). Similar results were obtained in rats with chronic unilateral coronal lesion just rostral to STH which eliminated the rostral afferents. In some neurons, EPSPs were followed by IPSPs. PPN area neurons were also antidromically activated by STH stimulation. Some of these neurons could also be antidromically activated by substantia nigra stimulation. We could differentiate PPN area projection neurons into two groups based on their conduction velocity. One group had a conduction velocity of 6.0 m/s and the second one 1.7 m/s. The morphology of 5 HRP-labeled PPN area neurons was analyzed. The somata were fusiform, polygonal and oval in shape, and the size varied from 152 to 1310 micron 2. Two to nine dendrites emerged from the somata and extended either radially or more in one direction (e.g. rostrocaudal). Axons arose, in general, from a proximal dendritic trunk close to the soma except one and branched near the soma. These data indicate a reciprocal connection between STH and PPN area and that STH exerts excitatory influence on PPN area neurons.