In the present study, we have re-examined the heterogeneous nature of intrastriatal striatal transplants derived from embryonic day 14–15 rat striatal primordia implanted into the previously excitotoxically lesioned striatum of adult rats, using in situ hybridization histochemistry to localize neurotransmitter-related messenger RNAs. These grafts are characterized by discrete patches of DARPP-32 messenger RNA expression, which cover approximately one-third of the cross-sectional graft area. The messenger RNAs encoding forpreproenkephalin (the enkephalin precursor), preprotachykinin (precursor to substance P), choline acetyltransferase, as well as the D 1 and D 2 dopamine receptors, which are abundant in the normal striatum, were all present in the striatal grafts and were expressed almost exclusively in the DARPP-32-positive graft regions. In these graft regions, the expression of the neurotransmitter-related messenger RNAs was generally similar to that seen in the intact striatum, although the level of expression of preproenkephalin and preprotachykinin messenger RNAs varied notably among the patches of expression. Cellular analysis performed on individual patches showed that the expression per cell of preproenkephalin and preprotachykinin messenger RNAs was inversely related, such that patches with higher than normal preproenkephalin messenger RNA levels displayed lower than normal preprotachykinin messenger RNA levels, and vice versa. Moreover, messenger RNA expression for the dopamine D 2 receptor was overall lower than that for the dopamine D 1 receptor, both with respect to the level per cell and the number of positive cells within the DARPP-32 patches. Glutamate decarboxylase messenger RNA was expressed throughout the grafts, in 98% of all neurons located in the DARPP-32-positive regions and in 75% of all neurons in the non-DARPP-32 regions of the graft. Interestingly, the cellular expression of glutamate decarboxylase messenger RNA was considerably higher in the non-DARPP-32 expressing regions than that in the DARPP-32 messenger RNA-rich areas, where it approximated that of the intact striatum. Furthermore, grafted neurons located outside the DARPP-32-expressing regions displayed similar levels of expression to those found in the overlying cortex and in the closely adjacent globus pallidus.To further characterize the DARPP and non-DARPP graft compartments, messenger RNAs encoding the α 1 and β 2 subunits of the GABA A receptor were studied. These receptor subunits, which exhibit a high expression in the host cortex and pallidum but little in the intact striatum, were found in discrete patches situated outside, but often closely associated with, the DARPP-32-rich areas of the graft. Many of the grafted neurons expressing the α 1 or β 2 subunit messenger RNAs of the GABA A receptor were similar in size and levels of expression to those in the host globus pallidus.These results demonstrate that intrastriatal striatal grafts consist of at least two distinct compartments; a striatal compartment clearly defined by DARPP-32, choline acetyltransferase, preproenkephalin, preprotachykinin, and dopamine D 1 and D 2 receptor messenger RNA expression, as well as by low-to-moderate glutamate decarboxylase gene expression, and a non-striatal compartment defined by neurons expressing high levels of glutamate decarboxylase and GABA A α 1 and β 2 subunit messenger RNAs, similar to other forebrain structures, such as the cerebral cortex and globus pallidus.
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