Morphological characteristics of the acetylcholinesterase-containing neurons in the CNS of DFP-treated monkeys.

The distribution of acetylcholinesterase (AChE, EC3.1.1.7) within extrapyramidal and related structures was studied in 4 monkeys following the i.m. administration of bis-(1-methylethyl) phosphorofluoridate (di-isopropylfluorophosphate: DFP). In 1 animal, sacrificed 4 hr after the injection of 0.43 mg/kg C, AChE is virtually absent in all structures. In the other 3 animals, which received 0.20 mg/kg DFP 10, 12 and 18 hr prior to sacrifice, AChE activity is greatly reduced in the neuropil of those structures which normally show intense AChE activity in pharmacologically unmanipulated monkeys. As a consequence of the lower background AChE activity in the latter 3 DFP-treated monkeys, the perikarya and processes of several groups of neurons can be readily identified. The perikarya and processes of two types of neurons of the neostriatum, representing a small percentage of all neurons in this structure, are intensely stained. They apparently correspond to the chromatic giant aspiny neurons and the achromatic medium-size "spidery aspiny" neurons. Most perikarya and processes of the neurons of the medial and lateral divisions of the pallidum, as well as the morphologically similar neurons of the pars reticulata of the substantia nigra, display light to moderate AChE activity. The pars compacta of the substantia nigra contains a small number of intensely stained elongated and triangular neurons and numerous moderately stained ovoid neurons whose processes are very lightly reactive. The cell bodies of the neurons of the subthalamic nucleus and associated groups of neurons (nucleus ansae peduncularis) are moderately stained whereas the perikarya and processes of the subnucleus compactus of the pedunculopontine tegmental nucleus are intensely and moderately stained, respectively. Numerous large and multipolar neurons associated with the lenticular nucleus (intralamellar groups) or the pallidum (peripallidal groups) including the nucleus ansae lenticularis have somata and processes which show an intense AChE activity. Certain of the latter groups of neurons, partially interspersed with the neurons of the substantia innominata, correspond in part to the nucleus basalis of Meynert. The present data are compatible with the idea that the large AChE neurons of the neostriatum may be the source of the acetylcholinesterasic striatopallidal and strionigral fibers. The similarity between the neurons of the pallidum and those of the pars reticulata of the substantia nigra suggests that the latter structure may represent a caudal extension of pallidal tissue. Therefore, the nigrothalamic projection, which has been claimed to originate in the pars reticulata of the substantia nigra, would correspond to pallidothalamic projections.