A population of sympathetic preganglionic neurones (SPNs) are electrotonically coupled (Logan et al. 1996). Oscillations in membrane potential, manifest as low-pass filtered remnants of presynaptic action potentials in coupled SPN, are a characteristic feature of these neurones. Nitric oxide (NO) induces membrane hyperpolarisation and inhibits oscillatory activity in coupled SPN (Spanswick et al. 1996). One explanation for these observations is that NO modulates electrical synapses in SPN. We therefore used whole-cell recording techniques in an isolated spinal cord preparation to investigate the effects of NO-dependent signalling on electrical synapses in SPNs.

Neonatal Wistar rats (6-14 days) were terminally anaesthetised with halothane (4 % in O₂), decapitated, the spinal cord removed and 400 mm transverse thoracolumbar spinal cord slices prepared as described previously (Logan et al. 1996).

Simultaneous whole-cell recordings were made from 12 pairs of SPNs. In 3 pairs, electrotonic coupling was directly demonstrated by injection of current pulses into one SPN, which produced membrane responses in both SPNs. The mean coupling coefficients (CC) and junctional conductances (G_j) were 0.22 ± 0.02 (mean ± S.E.M.) and 0.91 ± 0.06 nS, respectively.

Bath application of the NO donor, S-nitrosoglutathione (SNOG, 100-200 mM) induced membrane hyperpolarisation, cessation of spontaneous activity and a decrease in neuronal input resistance in all neurones tested (n = 13). In the presence of SNOG (n = 3), mean CC and G_j were 0.23 ± 0.03 and 0.96 ± 0.15 nS, respectively. Consequently, NO had no significant effect on either CC or G_j (P = 0.23 and 0.60, respectively, Student’s paired t test). A principle mode of action of NO is to raise intracellular cGMP levels via the activation of soluble guanylyl cyclase. Therefore the effects of 8-bromo-cGMP were compared with those of the NO donor. 8-Bromo-cGMP (50 mM) mimicked the effects of SNOG, inducing membrane hyperpolarisation, and a reduction in neuronal input resistance in all neurones tested (n = 2). CC and G_j ranged from 0.19 to 0.26 and 0.76 to 1.01 nS, respectively, in control conditions. Corresponding values in the presence of 8-Bromo-cGMP were 0.17 to 0.24 and 0.87 to 0.95 nS, respectively. Thus, 8-Bromo-cGMP was also without effect on CC and G_j.

These data suggest that NO, via cGMP-dependent and -independent pathways, acts to inhibit electrotonically coupled SPN, but is without direct effect on electrical synapses.

This work was supported by BHF and ERF.

All procedures accord with current UK legislation.