Neostriatal enkephalin-immunoreactive neurones project to the globus pallidus.

The origin of enkephalin-immunoreactive nerve terminals in the globus pallidus was investigated by combining immunocytochemistry with stereotaxic injection of neurotoxic agents (colchicine and kainic acid) and microknife deafferentations. The intracerebral administration of colchicine, irrespective of whether in the caudate putamen or in globus pallidus, induces the appearance of enkephalin-immunoreactive cell bodies and fibres in the caudate putamen. No immunoreactive cell bodies were depicted in the globus pallidus after this treatment. Kainic acid injections in the caudate putamen produced topographic depletions of enkephalin-immunoreactive terminals in the globus pallidus. The more anterior injections produced medial-anterior depletions, while posterior injections gave latero-posterior depletions. Injections in the globus pallidus produced only a non-specific loss of fluorescence restricted to the tip of the cannula. Coronal microknife cuts produced a combination of build-up and depletion of enkephalin immunofluorescence according to the position of the cut. The build-up of immunoreactive materials was always observed in the caudate-putamen side of the cut while depletions observed in the globus pallidus were related to the extent of deafferentation of this nucleus from the caudate putamen. All these observations confirmed the neostriatal origin (caudate putamen) of the enkephalinergic fibres present in the paleostriatum (globus pallidus).