Anatomical distribution of serotonin-containing neurons and axons in the central nervous system of the cat

By using a monoclonal antibody to serotonin (5-HT), an immunohistochemical study was undertaken to provide a comprehensive description of the 5-HT-con taining neurons and of the distribution of their axonal processes in the ca t brain and spinal cord. The localization of cell bodies was comparable to that previously reported in studies using formaldehyde-induced fluorescence and other 5-HT antibodies, with a large proportion of labeled neurons in t he raphe nuclei and a minor, yet not negligible number, in the ventral, lat eral, and dorsal reticular formation. The ascending efferent non-varicose a xons were best visualized in sagittal sections and mainly seen taking a ros troventral direction through the tegmentum. The varicose axons could be gro ssly classified into thin and large fibers, according to the size and shape of the immunoreactive varicosities, which were elongated (up to 2 mum in l ength and 1 mum in width) or round (2-4 mum in diameter). Varicose axonal a rborizations invaded almost every region of the gray matter and avoided lar ge myelinated bundles except in the spinal cord. Variations in the density of the plexuses of immunoreactive fibers generally followed the anatomical divisions and were also observed within nuclei, especially in laminated str uctures. Only the superior olivary complex could be regarded as devoid of 5 -HT-containing axons. A few areas contained extremely rich fiber plexuses. These were the olfactory tubercle, nucleus accumbens, ventral mesencephalon , periventricular gray from the hypothalamus to the pens, facial nucleus, s ubdivisions of the inferior olive, and the intermediolateral nucleus in the spinal cord. Varicose axons formed tight pericellular arrays in the neocor tex, mainly the ectosylvian gyrus, and in the lateral septum and medullar m agnocellular nucleus. These data, combined with those of the literature con cerning the synaptic versus non-synaptic mode of termination of the 5-HT-im munoreactive varicosities and the high number of distinct; receptors, are i ndicative of the multiple possible actions of serotonin in the central nerv ous system. J. Comp. Neurol. 433:157-182, 2001. (C) 2001 Wiley-Liss, Inc.