Coexpression of serotonin and nitric oxide in the raphe complex: cortical versus subcortical circuit.

Several lines of evidence have implicated a direct reciprocal interaction between serotonin and nitric oxide (NO). The goal of this investigation was, therefore, to examine the coexpression of tryptophan hydroxylase (TPH; the rate limiting enzyme for the synthesis of serotonin) and neuronal NO synthase (nNOS) in the ascending cortical projecting raphe nuclei (B6-B9 subgroups), when compared with the descending spinal cord projecting raphe nuclei (B1-B3 subgroups). Our data demonstrated that: (1) a significant number of raphe-cortical projecting neurons was identified not only in the midline subgroup of dorsal raphe (B6, 7) but also in the median raphe (B8), as well as in the supralemniscal nucleus (B9); (2) serotonergic cortical projecting neurons from these three raphe nuclei exhibited a high (>80%) percentage of coexpression with nNOS immunoreactivity; (3) similarly, serotonin transporter immunoreactive fibers in the medial prefrontal cortex were also double-labeled with nNOS immunoreactivity; (4) in contrast, the descending spinal cord projecting raphe nuclei revealed only TPH but not nNOS immunoreactivity. Our present findings suggest the existence of a direct interaction between serotonin and NO in the ascending cortical projecting raphe system. In contrast, a different strategy appears to operate the descending spinal cord projecting raphe system.