Pharmacology of Nicotinic Receptors in PreBötzinger Complex That Mediate Modulation of Respiratory Pattern

Abstract

Nicotine regulates respiratory pattern by modulating excitatory neurotransmission affecting inspiratory neurons within the preBötzinger Complex (preBötC). The nicotinic acetylcholine receptor (nAChR) subtypes mediating these effects are unknown. Using a medullary slice preparation from neonatal rat, we recorded spontaneous respiratory-related rhythm from the hypoglossal nerve (XIIn) and patch-clamped inspiratory neurons in the preBötC simultaneously. The α7 nAChR antagonists α-bungarotoxin or methyllycaconitine (MLA) had little effect on the actions of low concentrations of nicotine (0.5 μM), which included an increase in respiratory frequency; a decrease in amplitude of XIIn inspiratory bursts; a tonic inward current associated with an increase in membrane noise; an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), and; a decrease in the amplitude of inspiratory drive current in voltage-clamped preBötC inspiratory neurons. These nicotinic actions were completely reversed by dihydro-β-erythroidine (DH-β-E) or hexamethonium and reduced by d-tubocurarine. Comparable concentrations of RJR-2403 (0.5–1 μM), an agonist selective for α4β2 nAChRs, increased respiratory frequency to 186% and decreased the amplitude of XIIn inspiratory bursts to 83% of baseline. In voltage-clamped preBötC inspiratory (including pacemaker) neurons, RJR-2403 induced a tonic inward current of −15.2 pA associated with an increase in membrane noise, increased the frequency to 157% and amplitude to 106% of spontaneous EPSCs, and decreased the amplitude of inspiratory drive current to 80% of baseline. MLA had little effect on RJR-2403 actions, while DH-β-E completely reversed them. These results suggest that the predominant subtype of nAChRs in preBötC in neonatal rats that mediates the modulation of respiratory pattern by low concentrations of nicotine is an α4β2 combination and not an α7 subunit homomer. We do not exclude the possibility that co-assembly of α4β2 with other subunits or other nAChR subtypes are also expressed in preBötC neurons. The parallel changes in the cellular and systems level responses induced by different nicotinic agonists and antagonists support the idea that modulation of excitatory neurotransmission affecting preBötC inspiratory neurons is a mechanism underlying the cholinergic regulation of respiratory pattern ( Shao and Feldman 2001 ). This study provides a useful model system for evaluating potential therapeutic cholinergic agents for their respiratory effects and side effects.