Expression of nitric oxide and CGMP in sympathetic neurons of the rat RVLM and spinal cord

Spinal preganglionic neurons (SPN) receive direct inputs from a number of supraspinal sites including the rostral ventrolateral medulla (RVLM) of the brainstem. Together, these regions provide the efferent output of central neural pathways controlling the circulation. Subpopulations of nitric oxide (NO) containing cells can be identified in both regions and are thought be involved in controlling vasomotor tone 1; 2. One of the most widely recognised actions of NO is an increase in cyclic guanosine monophosphate (cGMP). In this study, we wanted to determine the expression of cGMP in these central cardiovascular regions, in relation to the NO synthase cell groups. Animals were anaesthetised, perfused with saline or IBMX (0.5mM, which acts to inhibit phosphodiesterases and hence cGMP breakdown), and then 4% formalin. Double labelling with antibodies directed against cGMP and nNOS or PNMT was detected using confocal microscopy. Without the use of IBMX, no cGMP staining was detected in the RVLM or IML regions. In IBMX treated animals, we noted a predominant colocalisation of neuronal NOS and cGMP in SPN, while in the RVLM, very few cells containing cGMP could be detected except for the nucleus ambiguus. Experiments were also performed after rats had undergone baroreceptor unloading. In the IML, it is suggested that under basal conditions, NO synthase and cGMP are present in the same cell and that NO may act in an autocrine role, in contrast to the purported paracrine role suggested for NO. These results support a role for NO in the aintenance of cardiovascular tone, and in particular regulation at the level of the spinal cord.

1 Anderson CR. Neurosci Lett. 1992; 139: 280-284.
2 Iadecola C, Faris PL, Hartman BK, Xu X. Brain Res. 1993; 603: 173-179.