INTERDIGITATION OF NITRIC-OXIDE-SYNTHASE-CONTAINING, TYROSINE HYDROXYLASE-CONTAINING, AND SEROTONIN-CONTAINING NEURONS IN AND AROUND THE LATERODORSAL AND PEDUNCULOPONTINE TEGMENTAL NUCLEI OF THE GUINEA-PIG

The topography of neurons containing nitric oxide synthase (NOS) and m onoamines was investigated in the guinea pig mesopontine tegmentum. NO S-containing neurons were identified with NADPH-diaphorase (NADPH-d) h istochemistry, and monoamine-containing neurons were identified with t yrosine hydroxylase (TH) and serotonin (5-HT) immunocytochemistry. The distribution of NADPH-d positive cells was centered on the laterodors al tegmental (LDT) and pedunculopontine tegmental (PPT) nuclei. Diapho rase-containing cells had a mean soma diameter of 23.0 +/- 4.1 mu m (n = 160) and were distributed inhomogeneously, with numerous cells foun d within densely packed clusters. A nearest-neighbor analysis revealed that these cells were closely spaced, with up to 20% within one cell diameter and more than 50% within two cell diameters of a neighboring NADPH-d cell. Within the LDT and PPT, NADPH-d positive cells were mixe d with smaller, diaphorase-negative cells (diam: 12.8 +/- 3.3 mu m; n = 182; P << 0.01). TH-containing cells were not organized into a compa ct LC as in rat and their distribution more closely resembled that obs erved in cat. On average, TH-containing cells (diam: 21.2 +/- 4.8 mu m ; n = 160) were smaller than NADPH-d cells (P < 0.01). 5-HT-containing cells were mainly located in the raphe nuclei, as in other species. 5 -HT-containing cells (diam: 18.2 +/- 4.4 mu m; n = 161) were smaller o n average than both the NADPH-d (P < 0.01) and TH-containing cells (P < 0.01). An analysis of the overlap in soma distributions revealed tha t TH-containing cells were largely interdigitated with NADPH-d-contain ing cells. As much as 78% of the area occupied by the NADPH-d cells of LDT was contained within the area occupied by TH cells. Substantial n umbers of TH and 5-HT immunoreactive processes were seen in both LDT a nd PPT. Varicose 5-HT and TH-containing fibers, as well as thicker, po ssibly dendritic processes containing TH were often seen in close appo sition to NADPH-d containing somata and proximal dendrites. These resu lts support the hypothesis that NADPH-d cells of both the PPT and LDT receive input from TH and 5-HT cells. Moreover, the clustered substruc ture of LDT and PPT and the extensive overlap of NADPH-d and TH-contai ning somata raise the possibility that the membrane permeable messenge r nitric oxide plays a role in modulating TH-containing somata and the ir processes as well as 5-HT-containing processes in the LDT and PPT. (C) 1995 Wiley-Liss, Inc.