Proceedings of The Physiological Society

University of Leeds (2002) J Physiol 544P, S011

Communications

Functional role of nitric oxide in spinal sympathetic networks activated by PVN stimulation in anaesthetised rats

L. Smith, Z. Yang and J.H. Coote

Department of Physiology, University of Birmingham, Birmingham B15 2TT, UK


There are contrasting reports on the action of nitric oxide (NO) in spinal regulation of sympathetic neurone activity. In vivo experiments on rabbits showed NO donors given intrathecally facilitated sympathetic activity to the kidney (Hakim et al. 1995), whereas in vitro experiments on slices of rat spinal cord showed that NO potentiated glycine inhibitory currents in immature sympathetic preganglionic neurones (Wu & Dun, 1996). The latter study indicated that the NO effect followed synaptic excitation of the neurones. Therefore in rats anaesthetised with urethane, chloralose mixture (650 mg kg-1, 50 mg kg-1) I.V. we determined whether NO acted to enhance or depress synaptic excitation of spinal sympathetic neurones involved in the pathway from the paraventricular nucleus (PVN) of the hypothalamus to renal sympathetic nerve. Recordings were made of renal sympathetic nerve activity (RSNA) and arterial blood pressure. For activation of neurones in PVN a glass micropipette containing D,L-homocysteic acid (DLH) was placed stereotaxically, and sites marked with pontamine sky blue for later histological identification. Changes in the efficacy of spinal circuits were induced by intrathecal (I.T.) application of drugs via a catheter inserted into the subarachnoid space via the foramen magnum so that its tip lay at T10. Drugs were administered in a volume of 10 ml washed in with 20 ml of artificial CSF. Statistical analysis was performed using a Student's two-tailed, paired t test. Rats were killed by overdose of urethane anaesthetic at the end of experiments. I.T. application of the NO precursor L-arginine (100 nM) significantly reduced RSNA by 12 ± 3 % (P ?le? 0.05). An increase of RSNA of 69 ± 14 % produced by I.T. glutamate (0.2 M) was enhanced to 94 ± 15 % by preceding the same dose with I.T. L-NMMA (200 nM) (P ?le? 0.05). Similarly, an increase in RSNA of 83 ± 15 % produced by microinjection of DLH (100 nl, 0.2 M) into PVN was potentiated by 126 ± 21 % (P ?le? 0.05) when elicited following I.T. L-NMMA (200 nM). I.T. L-NMMA alone had no significant effect on baseline RSNA activity. I.T. application of the glycine receptor antagonist strychnine (3 mM) prior to PVN stimulation also enhanced the PVN-RSNA response. I.T. strychnine alone had no significant effect on baseline RSNA. The potentiation of PVN-RSNA responses by strychnine was not further increased by prior I.T. L-NMMA, indicating that the effects of block of glycine receptors and NO synthesis are not additive. These results suggest that NO acts as a retrograde inhibitory messenger following PVN synaptic activation of spinal sympathetic neurones and that glycine interneurones may be involved in this action.

This work was funded by The Wellcome Trust.

All procedures accord with current UK legislation.

Where applicable, experiments conform with Society ethical requirements