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We determined the distribution pattern of cholinergic neurons in laboratory shrews (Suncus murinus) based on comparative morphology by an immunohistochemical method using an anticholine acetyltransferase (ChAT) antibody. ChAT-positive neurons were observed in the striatum, magnocellular nucleus and basal nucleus, pontine tegmentum and cranial nerve motor nuclei (IV, V, VII, X and XII). Specific findings were as follows: many ChAT-positive neurons were observed in the locus ceruleus (LC) and raphe dorsalis (RD), and the colocalization of ChAT-positive neurons and tyrosine hydroxylase in the LC and that of ChAT-positive neurons and serotonin in the RD were clarified. On the other hand, in the rat brain, ChAT-positive neurons were present in the central amygdaloid nucleus; however, this was not observed in the brains of laboratory shrews. Laboratory shrews are among the most primitive mammalian species and have a different distribution pattern of aminergic neurons, other than cholinergic neurons, from that in rats. A unique distribution pattern of ChAT-positive neurons in the laboratory shrew brain was thus noted.

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Monoaminergic neuron systems were studied in the frontal sections of the house-shrew brain by using the peroxidase-antiperoxidase technique with antisera to tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC) and serotonin (5-HT). The distribution of monoaminergic neurons was found to be similar to that in the rat brain. However, in the house-shrew brain proper, TH-and AADC-positive cells were found in the diagonal band, stria terminalis, dorsal tegmental nucleus and area postrema, whereas some TH-positive but AADC-negative neurons were found in the diagonal band, lateral septal area, olfactory tubercles, caudoputamen, caudal level of the lateral habenular nucleus and around the dorsal vagal nucleus. Moreover, 5-HT neurons were observed in the motor cortical area, diagonal band, lateral septal area, caudoputamen, lamina intercalaris, periaqueductal gray matter, locus coeruleus, and superior olive. D neurons (AADC-positive but TH-or 5-HT-negative) were observed in the frontal cortex, medial vestibular nucleus, and area postrema. In the dorsal vagal nucleus and its dorsal portion, TH-positive cells started to appear at postnatal day 5 (P5), and the cell number increased between P10-P20, but decreased by adulthood, while D4 and D5 cells first appeared at P1 and gradually decreased until adulthood.Immunoelectron microscopy showed AADC-positive reaction products in the cytoplasmic matrix and granule vesicles in the suprachiasmatic nucleus. Catecholaminergic or serotoninergic nerve fibers and their terminals were widely distributed throughout the brain. Nevertheless, no distinctive distribution or projection of AADC-positive fibers was found.