Regulation of ventral surface CO 2/H +-sensitive neurons by purinergic signalling

Abstract

Central chemoreception is the mechanism by which the brain regulates breathing in response to changes in tissue CO 2/H +. A brainstem region called the retrotrapezoid nucleus (RTN) contains a population of CO 2/H +-sensitive neurons that appears to function as an important chemoreceptor. Evidence also indicates that CO 2-evoked ATP release from RTN astrocytes modulates activity of CO 2/H +-sensitive neurons; however, the extent to which purinergic signalling contributes to chemoreception by RTN neurons is not clear and the mechanism(s) underlying CO 2/H +-evoked ATP release is not fully elucidated. The goals of this study are to determine the extent to which ATP contributes to RTN chemoreception both in vivo and in vitro, and whether purinergic drive to chemoreceptors relies on extracellular Ca 2+ or gap junction hemichannels. We also examine the possible contribution of P2Y1 receptors expressed in the RTN to the purinergic drive to breathe. We show that purinergic signalling contributes, in part, to the CO 2/H + sensitivity of RTN neurons. In vivo, phrenic nerve recordings of respiratory activity in adult rats show that bilateral injections of pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonate (PPADS, a P2 receptor blocker) decreased the ventilatory response to CO 2 by 30%. In vitro, loose-patch recordings from RTN neurons show that P2 receptor blockers decreased responsiveness to both 10% and 15% CO 2 also by 30%. In the slice, the contribution of purinergic signalling to RTN chemoreception did not increase with temperature (22-35°C) and was retained in low extracellular Ca 2+ medium. Conversely, the gap junction blockers carbenoxolone and cobalt decreased neuronal CO 2/H + sensitivity by an amount similar to P2 receptor antagonists. Inhibition of the P2Y1 receptor in the RTN had no effect on CO 2 responsivness in vitro or in vivo; thus, the identity of P2 receptors underlying the purinergic component of RTN chemoreception remains unknown. These results support the possibility that CO 2/H +-evoked ATP release is mediated by a mechanism involving gap junction hemichannels. © 2012 The Authors. The Journal of Physiology © 2012 The Physiological Society.