Association of nicotinic acetylcholine receptors with central respiratory control in isolated brainstem-spinal cord preparation of neonatal rats

Abstract

Nicotine exposure is a risk factor in several breathing disorders Nicotinic acetylcholine receptors (nAChRs) exist in the ventrolateral medulla, an important site for respiratory control. We examined the effects of nicotinic acetylcholine neurotransmission on central respiratory control by addition of a nAChR agonist or one of various antagonists into superfusion medium in the isolated brainstem-spinal cord from neonatal rats. Ventral C4 neuronal activity was monitored as central respiratory output, and activities of respiratory neurons in the ventrolateral medulla were recorded in whole-cell configuration. RJR-2403 (0.1-10µM), α4β2 nAChR agonist induced dose-dependent increases in respiratory frequency. Non-selective nAChR antagonist mecamylamine (0.1-100µM), α4β2 antagonist dihydro-β-erythroidine (0.1-100µM), α7 antagonist methyllycaconitine (0.1-100µM), and α-bungarotoxin (0.01-10µM) all induced dose-dependent reductions in C4 respiratory rate. We next examined effects of 20µM dihydro-β-erythroidine and 20µM methyllycaconitine on respiratory neurons. Dihydro-β-erythroidine induces hyperpolarization and decreases intraburst firing frequency of inspiratory and preinspiratory neurons. In contrast, methyllycaconitine has no effect on the membrane potential of inspiratory neurons, but does decrease their intraburst firing frequency while inducing hyperpolarization and decreasing intraburst firing frequency in preinspiratory neurons. These findings indicate that α4β2 nAChR is involved in both inspiratory and preinspiratory neurons, whereas α7 nAChR functions only in preinspiratory neurons to modulate C4 respiratory rate.

Identification of a Cholinergic Modulatory and Rhythmogenic Mechanism within the Lamprey Respiratory Network

“…All the respiratory responses to ACh-related drugs are mediated at the level of the pTRG, thus confirming its importance in respiratory rhythm generation. The finding that ACh has an excitatory effect on respiration is consistent with previous results obtained in mammals (Gesell et al, 1943;Murakoshi et al, 1985;Monteau et al, 1990;Burton et al, 1994Burton et al, , 1995Bianchi et al, 1995;Shao and Feldman, 2000Hatori et al, 2006;Shao et al, 2008). Noticeably, the present findings show that ACh affects respiratory activity through a specific action on the pTRG.…”

“…Most probably, ACh is released from neurons located in close proximity with or even within the pTRG. Varied results have been reported on the modulatory role of endogenously released ACh in in vitro rodent preparations (Murakoshi et al, 1985;Monteau et al, 1990;Burton et al, 1994Burton et al, , 1995Shao and Feldman, 2000Hatori et al, 2006). In contrast with these previous results, we show that ACh affects respiration in the lamprey by acting solely on nAChRs.…”

Central cholinergic regulation of respiration: nicotinic receptors

“…These pharmacological characteristics suggest that the predominant subtype of preBötC nAChRs that mediates the modulation of respiratory pattern by low concentrations of nicotine is an α4β2* combination and not an α7 subunit homomer. Results of pharmacological studies [78] using the en bloc brainstem-spinal cord preparation from neonatal rats where respiratory motor activity can be recorded from C4 nerve roots are basically consistent with those of slice preparations. DHβE hyperpolarizes and decreases intraburst firing frequency in both inspiratory and preinspiratory (pre-I) neurons in the rostral ventrolateral medulla.…”

“…DHβE hyperpolarizes and decreases intraburst firing frequency in both inspiratory and preinspiratory (pre-I) neurons in the rostral ventrolateral medulla. MLA has no effects on the membrane potential of inspiratory neurons but hyperpolarizes and decreases intraburst firing frequency in pre-inspiratory neurons [78] . These authors conclude that α4β2 nAChRs mediate cholinergic modulation of both inspiratory and preinspiratory neurons whereas α7 nAChRs are only involved in cholinergic modulation of pre-inspiratory neurons.…”

Nicotine increases chemoreflex sensitivity to hypoxia in non-smokers

“…Some studies have also implicated the nicotinic receptors in the signal transduction arising from central chemosensitive cells [26][27][28], and therefore we used sympathetic and ventilatory responses to hyperoxic hypercapnia (7% carbon dioxide in 93% oxygen) to rule out a central chemoreflex effect of nicotine. Finally, an isometric handgrip was used as an internal control to exclude a non-specific facilitation to excitatory stimuli by nicotine.…”