Ultrastructural observations of the cholinergic neuron in the rat striatum as identified by acetylcholinesterase pharmacohistochemistry.

In order to examine the ultrastructural features of the cholinergic neuron in the striatum (caudatoputamen) of the rat, cytochemistry for acetylcholinesterase was conducted 2-12 h after intramuscular injection of the irreversible acetylcholinesterase inhibitor diisopropylphosphorofluoridate. Light microscopic examination of Epon sections reacted from acetylcholinesterase showed that only large-sized cells in the striatum (25-35 microns in the long axis) were stained intensely. In the case of longer survival periods (10-12 h), some lightly stained cells (medium-sized) were seen dispersed amongst the large acetylcholinesterase-rich cells. Electron microscopic observations were made on ultrathin sections of selected large acetylcholinesterase-rich neurons that were first studied by light microscopy. The nucleus of these cells has an eccentric position and possesses several indentations of the nuclear envelope. The cytoplasm contains abundant organelles, many exhibiting features unique to this cell type. Many stacks of granular endoplasmic reticulum, arranged in a parallel manner and forming typical Nissl bodies, were observed in the periphery of the perikarya, and many distinct golgi complexes were seen in the perinuclear zone. At all post-diisopropylphosphorofluoridate survival times, heavy deposits of acetylcholinesterase reaction product were found within the perikarya of this cell type, for the most part within the cisternae of the granular endoplasmic reticulum. At the longer post-diisopropylphosphorofluoridate survival times, reaction product within the cytoplasm was very dense and appeared to have reached a maximum level. At these times reaction product also appeared in the secondary and tertiary dendritic branches of the large-sized neurons. Of the other cell types in the striatum, two types of medium-sized cells displayed a light deposit of reaction product in their perikarya, but this was observed only at longer recovery times (8-12 h). The majority of cells in the striatum lacked reaction particles. Throughout the early post-diisopropylphosphorofluoridate period, the recovery of enzyme activity in the neuropil was moderate compared to that seen within cell bodies. These findings indicate that the large-sized neuron is the only striatal structure that shows rapid regeneration of acetylcholinesterase activity during the early recovery phase after diisopropylphosphorofluoridate administration. Previous studies have indicated that this type of neuron represents the cholinergic interneuron of the striatum. (ABSTRACT TRUNCATED AT 400 WORDS)