An electrophysiological characterization of projections from the pedunculopontine area to entopeduncular nucleus and globus pallidus in the cat.

Stimulation of the pedunculopontine region (PPN), in which neurons are filled by horseradish peroxidase injected into the entopeduncular nucleus (ENTO) or globus pallidus (GP) of the cat, excites ENTO and GP neurons both orthodromically and antidromically. Stimulus threshold mapping experiments and intracellular records of EPSPs provide evidence that the orthodromic excitation may be produced monosynaptically by the axons of PPN neurons. Antidromic excitation of ENTO axons from stimulation in PPN or the thalamus may elicit IPSPs in ENTO neurons via the action of recurrent collaterals. An excitatory synaptic action of PPN neurons on pallidal cells could be a partial basis for the high discharge rate characteristic of these neurons in awake animals.