THE PALLIDOSTRIATAL PROJECTION IN THE RAT - A RECURRENT INHIBITORY LOOP

The pallidostriatal projection in the rat was investigated employing t he PHA-L tracing technique. Following iontophoretic injections into th e lateral aspect of the globus pallidus external segment, the ipsilate ral striatum showed patches of dense anterograde labeling separated by areas containing sparse anterograde labeling and isolated retrogradel y labeled neurons. The densely labeled patches did not correspond to a ny known compartments of the striatum. The retrogradely labeled neuron s consistently showed similar distribution and morphological features reminiscent of striatal type II projection neurons. As all projection neurons of the striatum and all pallidal neurons are GABAergic, the co mplementary pattern of anterogradely and retrogradely labeled profiles from the globus pallidus suggest a possible mechanism whereby a horiz ontal inhibition may be exerted on groups of striatal neurons via the striato-pallido-striatal pathway.