Immunohistochemical demonstration of differential substance P‐, met‐ enkephalin‐, and glutamic‐acid‐decarboxylase‐containing cell body and axon distributions in the corpus striatum of the cat

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Abstract
The immunohistochemical localization of neuronal cell bodies and axons reactive for substance P (SP) and methionine‐enkephalin (ME) was investigated in the corpus striatum of the adult cat brain and compared with that of glutamate decarboxylase (GAD), synthetic enzyme for γ‐aminobutyric acid. Striatal cell bodies reactive for ME could be identified only in colchicine treated cats, are medium size, ovoid striatal cells, and are found in large numbers in a more or less even distribution throughout the caudate nucleus, putamen, and nucleus accumbens. The striatial region most densely occupied by ME‐immunoreactive cells is the ventral and central part of the caudate head. Modest numbers of larger ME‐reactive neurons are dispersed throughout the entopeduncular nucleus and the pars reticulata of the sub‐stantia nigra. Striatal cells of medium size reactive for SP could be identified, with or without colchicine, in largest numbers in the medial half of the caudal three‐fourths of the putamen and in clusters of irregular size and shape in the head of the caudate nucleus. Cells reactive for SP are also common in layer II and the islands of Calleja of the olfactory tubercle. We could not reliably visualize GAD‐positive cell bodies in the striatum, even with colchicine treatment; however, they could be seen readily in all pallidal structures such as the globus pallidus, ventral pallidum, entopeduncular nucleus, and substantia nigra.Axons reactive for ME are found mainly in the globus pallidus where they form a dense and even network throughout the nucleus. The globus pallidus is almost devoid of SP reactivity except near its extreme caudal pole. Conversely, SP‐immunoreactive axons form dense meshworks in the entopeduncular nucleus and substantia nigra where ME immunoreactivity is minimal. Fewer, but still ample numbers, of SP‐reactive axons are present also in the ventral tegmental and retrorubral areas of the midbrain tegmentum and in the ventral pallidum of the basal forebrain, but only sparse ME‐reactive axons are present in these areas. This differential distribution of SP‐ and ME‐containing axons in the pallidal and nigral structures stands in contrast to the relatively homogeneous and dense distribution of GAD‐containing axons throughout the dorsal and ventral pallidum, entopeduncular nucleus, and substantia nigra. These data suggest that (1) the striatum can be dissected into component parts on the basis of the histochemical properties of its cells, (2) the output structures of the corpus striatum (entopeduncular nucleus, substantia nigra, ventral pallidum) are more influenced by SP than ME mechanisms, (3) the globus pallidus, which is involved primarily with an intrinsic corpus striatal circuit through the subthalamic nucleus, is more influenced by ME than SP mechanisms, and (4) the ventral pallidum is cornparable to the entopeduncular nucleus and nigra in that it receives manyy SP‐ and GAD but apparently only a few ME‐containing axons.