Mechanisms of orexin‐induced depolarizations in rat dorsal motor nucleus of vagus neuronesin vitro

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Abstract
1 Whole-cell patch-clamp recordings were made from neurones of the dorsal motor nucleus of the vagus (DMNV), including Fluoro-gold-labelled parasympathetic preganglionic neurones (PPNs), in slices of the rat medulla. In the latter case, rats had received an i.p. injection of Fluoro-gold solution (10 μg) 2-3 days earlier. 2 Superfusion of orexin A or B (10-300 nm) caused a slow depolarization in approximately 30 % of the DMNV neurones, including PPNs. Orexin-induced depolarizations, which persisted in TTX (0.5 μm)-containing Krebs solution, were reduced by 70 % in a low-Na+ (26 mm) Krebs solution, indicating the involvement of Na+ ions. A significant change in orexin-induced depolarizations was not obtained in either a high-K+ (7 mm) or Cd2+ (100 μm) Krebs solution. 3 Inclusion of the hydrolysis-resistant guanine nucleotide GDP-β-S in the patch solution significantly reduced the orexin A- or B-induced depolarizations. 4 Under whole-cell voltage-clamp conditions, the orexin-induced inward current declined with hyperpolarization, but did not reverse polarity in the potential range between -120 and 0 mV. In low-Na+ solution, the orexin-induced current was reduced, and the I–V curve reversed polarity at about -105 mV; the response was further reduced and the reversal potential shifted to -90 mV in a low-Na+, high-K+ Krebs solution. 5 It is concluded that the peptides orexin A and B, acting on orexin receptors, which are GTP-binding-protein coupled, are excitatory to DMNV neurones. In addition, more than one conductance, which may include a non-selective cation conductance and a K+ conductance, appears to be involved in the orexin-induced depolarization.