Constitutive nitric oxide signal transduction pathways in association with preautonomic neuronal subpopulations in the hypothalamic PVN

We have recently shown that constitutive nitric oxide (NO) within the PVN inhibits the activity of preautonomic sympathetic neurons via activation of GABAergic inhibitory inputs. Recent work (Li DP et al 2004) suggests that NO inhibition of presympathetic PVN neurons is mediated via a cGMP dependent pathway. The goal of this work was to determine whether a cGMP also mediates constitutive NO actions within the PVN. We visualized PVN NO-receptive cells by detecting basal levels of immunoreactive cGMP, in conjunction with immunohistochemistry for nNOS , oxytocin (OT) or vasopressin (VP), and a fluorescent retrograde tracer injected into either the upper thoracic spinal cord, RVLM or NTS. nNOS was found predominantly in magnocellular neuroendocrine subdivisions of the PVN, and showed strong co-localisation with cGMP. On the other hand, few preautonomic cells were found to express detectable cGMP. Low or non- detectable cGMP levels were found associated with either neuronal somata or terminals of eGFP-labeled GABAergic PVN interneurons in a transgenic mouse expressing eGFP under the control of the GAD67 gene. Similar results were observed in GAD67 immunoreactive terminals associated with preautonomic PVN neurons. Thus, while pharmacologically enhanced NO levels potentiate inhibitory synaptic inputs through a cGMP-dependent pathway, our data indicate that a different signaling mechanism may underlie constitutive NO actions on preautonomic PVN neurons.