THE NO-CGMP PATHWAY IN THE RAT LOCUS-COERULEUS - ELECTROPHYSIOLOGICAL, IMMUNOHISTOCHEMICAL AND IN-SITU HYBRIDIZATION STUDIES

The effect of two nitric oxide (NO) donors, SIN-1 and DEA/NO, as well as of the inactive SIN-1 derivative molsidomin, was studied on locus c oeruleus (LC) neurons in a slice preparation using intracellular recor dings. In addition, the effect of the guanylate cyclase inhibitor ODQ was analysed. Furthermore, the effect of NO donors on cyclic guanosine monophosphate (GMP) levels in the LC was studied using the indirect i mmunofluorescence technique, and the expression of soluble guanylyl cy clase with in situ hybridization. In 36 of 66 LC neurons extracellular application of SIN-1 and DEA/NO caused a hyperpolarization and a decr ease in apparent input resistance. In almost 20% of neurons SIN-1 incr eased the firing rate. No effect could be recorded with the brain-inac tive SIN-1 derivative molsidomin. The membrane permeable cGMP analogue 8-bromo-cGMP imitated the action of SIN-1. The hyperpolarizing effect of SIN-1 and DEA/NO was attenuated by preincubation with the guanylyl cyclase inhibitor ODQ. The immunohistochemical analysis revealed lack of cGMP immunostaining in non-stimulated slices, whereas SIN-1 dramat ically increased this staining in about 40% of the LC neurons, and the se neurons were all tyrosine hydroxylase positive, that is noradrenerg ic. A large proportion of the LC neurons expressed soluble guanylyl cy clase mRNA. The present and previous results suggest that NO, released from a small number of non-noradrenergic neurons in the LC, mainly ha s an inhibitory influence on many noradrenergic neurons, by upregulati ng cGMP levels via stimulation of soluble guanylyl cyclase. As nitric oxide synthase is present only in a small number of non-noradrenergic neurons (Xu et al., 1994), a few neurons may influence a large populat ion of noradrenergic LC neurons, which in turn may control activity in many regions of the central nervous system.