Synaptic relationships between cortical and dopaminergic inputs and intrinsic GABAergic systems within intrastriatal...

Synaptic relationships between gamma-aminobutyric acid-ergic systems intrinsic to intrastriatal striatal grafts and inputs from host adult rat neocortex and substantia nigra were investigated using a variety of neuroanatomical techniques. The input from host frontal cortex was demonstrated using an anterograde degeneration technique, whilst a double immunocytochemical procedure, using the chromogens diaminobenzidine and benzidine dihydrochloride was utilized to visualize the tyrosine hydroxylase (TH)- and glutamate decarboxylase (GAD)-immunoreactive systems. Only areas receiving dense TH-immunoreactive innervation were examined for synaptic interactions since these areas were judged as being striatal in origin. Examples of synaptic interactions were observed between cortical and TH-immunoreactive inputs; between cortical input and GAD-immunoreactive neuronal elements within TH-immunoreactive inputs and a variety of GAD-immunoreactive neuronal elements within the striatal grafts. No interactions were seen between cortical input and GAD-immunoreactive neuronal perikarya or dendrites, possibly because of technical limitations since the GAD-immunoreactivity did not extend into the distal dendrites where cortical input is predominantly located, nor between all three systems. The results suggest that the formation of new synaptic connections in a pattern reminiscent of that seen in control neostriatum may be responsible, in part at least, for the behavioural recovery in motor skills seen in rats following intrastriatal striatal transplants. They also demonstrate that the host adult brain retains sufficient plasticity and may play an important role in the control of synaptic output from the transplant.