Electrophysiological characteristics of cells within mesencephalon suspension grafts

Both spontaneous and evoked extracellular electrophysiological activity of neurons within fetal mesencephalon suspension grafts to the dopamine-depleted striatum of rats were examined. In some cases, extracellular recording was combined with intracellular labeling to identify recorded neurons. Grafted rats displaying a complete cessation of ipsilateral rotations following amphetamine administration were examined at post-implantation time intervals of two, four, five, eight and nine months. Four separate classes of neurons were distinguished within the transplanted striatum based on electrophysiological properties. The first of these groups, the type I cells, appeared to be non-grafted striatal neurons. When spontaneously active, these striatal-like cells fired bursts of action potentials separated by periods of decreased activity. Evoked responses in these cells were characteristic of striatal cells. Type I cells which were intracellularly labeled were found outside the grafts and displayed the characteristic morphology of the medium spiny neuron of the neostriatum. The other three cell classes displayed electrophysiological properties similar to neurons recorded in situ within the reticular formation, substantia nigra pars compacta and substantia nigra pars reticulata. Neurons from these three groups which were labeled with an intracellular marker were found to lie within the suspension grafts. The spontaneous activity of the pars compacta dopaminergic-like neurons was predominantly irregular, with some cells also firing in a regular or pacemaker-like pattern. Infrequently, irregular firing dopaminergic-like neurons displayed episodes of doublet bursting. Many of the grafted neurons responded to electrical stimulation of prefrontal cortex and striatum, indicating that the graft was receiving functional inputs from host neurons. Comparison of the firing rate and pattern of grafted neurons to in situ mesencephalic neurons as a function of time following grafting suggested that the grafted neurons and/or the neuronal circuitry is slowly developing within the host environment. A prolonged time-course for the maturation of the graft may be reflected in the time required to achieve improvements in some behavioral deficits following transplantation. However, the relatively rapid recovery of drug-induced rotational asymmetry following grafting suggests that this form of recovery may not require mature functioning of the grafted neurons.