ATP is released during hypoxia from the ventrolateral medulla (VLM) and activates purinergic P2 receptors (P2Rs) at unknown loci to offset the secondary hypoxic depression of breathing. In this study, we used rhythmically active medullary slices from neonatal rat to map, in relation to anatomical and molecular markers of the pre-Bötzinger complex (preBötC) (a proposed site of rhythm generation), the effects of ATP on respiratory rhythm and identify the P2R subtypes responsible for these actions. Unilateral microinjections of ATP in a three-dimensional grid within the VLM revealed a "hotspot" where ATP (

BackgroundHypoglossal (XII) motoneurons innervate tongue muscles and are vital for maintaining upper-airway patency during inspiration. Depression of XII nerve activity by opioid analgesics is a significant clinical problem, but underlying mechanisms are poorly understood. Currently there are no suitable pharmacological approaches to counter opiate-induced suppression of XII nerve activity while maintaining analgesia. Ampakines accentuate α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor responses. The AMPA family of glutamate receptors mediate excitatory transmission to XII motoneurons. Therefore the objectives were to determine whether the depressant actions of μ-opioid receptor activation on inspiratory activity includes a direct inhibitory action at the inspiratory premotoneuron to XII motoneuron synapse, and to identify underlying mechanism(s). We then examined whether ampakines counteract opioid-induced depression of XII motoneuron activity.Methodology/Principal FindingsA medullary slice preparation from neonatal rat that produces inspiratory-related output in vitro was used. Measurements of inspiratory burst amplitude and frequency were made from XII nerve roots. Whole-cell patch recordings from XII motoneurons were used to measure membrane currents and synaptic events. Application of the μ-opioid receptor agonist, DAMGO, to the XII nucleus depressed the output of inspiratory XII motoneurons via presynaptic inhibition of excitatory glutamatergic transmission. Ampakines (CX614 and CX717) alleviated DAMGO-induced depression of XII MN activity through postsynaptic actions on XII motoneurons.Conclusions/SignificanceThe inspiratory-depressant actions of opioid analgesics include presynaptic inhibition of XII motoneuron output. Ampakines counteract μ-opioid receptor-mediated depression of XII motoneuron inspiratory activity. These results suggest that ampakines may be beneficial in countering opiate-induced suppression of XII motoneuron activity and resultant impairment of airway patency.

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y 1 Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mM) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 μM) and DAMGO (50 μM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K + channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mM) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y 1 R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

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