Interdigitation of nitric oxide synthase-, tyrosine hydroxylase-, 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 monoamines was investigated in the guinea pig mesopontine tegmentum. NOS-containing neurons were identified with NADPH-diaphorase (NADPH-d) histochemistry, and monoamine-containing neurons were identified with tyrosine hydroxylase (TH) and serotonin (5-HT) immunocytochemistry. The distribution of NADPH-d positive cells was centered on the laterodorsal tegmental (LDT) and pedunculopontine tegmental (PPT) nuclei. Diaphorase-containing cells had a mean soma diameter of 23.0 +/- 4.1 microns (n = 160) and were distributed inhomogeneously, with numerous cells found 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 mixed with smaller, diaphorase-negative cells (diam: 12.8 +/- 3.3 microns; n = 182; P << 0.01). TH-containing cells were not organized into a compact LC as in rat and their distribution more closely resembled that observed in cat. On average, TH-containing cells (diam: 21.2 +/- 4.8 microns; 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 microns; n = 161) were smaller on 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 that TH-containing cells were largely interdigitated with NADPH-d containing 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 numbers of TH and 5-HT immunoreactive processes were seen in both LDT and PPT. Varicose 5-HT and TH-containing fibers, as well as thicker, possibly dendritic processes containing TH were often seen in close apposition to NADPH-d containing somata and proximal dendrites. These results support the hypothesis that NADPH-d cells of both the PPT and LDT receive input from TH and 5-HT cells. Moreover, the clustered substructure of LDT and PPT and the extensive overlap of NADPH-d and TH-containing somata raise the possibility that the membrane permeable messenger nitric oxide plays a role in modulating TH-containing somata and their processes as well as 5-HT-containing processes in the LDT and PPT.