Adenosinergic modulation of respiratory neurones in the neonatal rat brainstem in vitro

1. The mechanism underlying adenosinergic modulation of respiration was examined in vitro by applying the whole-cell patch-clamp technique to different types of respiration-related neurones located in the rostral ventrolateral medulla of neonatal rats (0-4 days old). 2. The adenosine A(1)-receptor agonist (R)-N-6-(2-phenylisopropyl)-adenosine (R-PIA, 10 mu M; n = 31) increased the burst distance of rhythmic C4 inspiratory discharges and decreased the duration of inspiratory discharges (control: 8.00 +/- 2.49 s and 918 +/- 273 ms; R-PIA: 12.10 +/- 5.60 s and 726 +/- 215 ms). 3. Expiratory neurones demonstrated a reversible decrease in input resistance (R-in), a depression of action potential discharges and a hyperpolarization of the membrane potential (V-m) during application of R-PIA (1-10 mu M). Similar responses of R-in and V-m to R-PIA were evident after synaptic activity had been blocked by 0.5 mu M tetrodotoxin (TTX). 4. Some of the biphasic expiratory (biphasic E) neurones, but none of the inspiratory neurones, demonstrated changes in R-in or V-m during R-PIA application. With TTX present, R-PIA did not alter V-m or R-in in biphasic expiratory or inspiratory neurones. 5. Furthermore, R-PIA decreased the spontaneous postsynaptic activities of all neurones examined. The effects of R-PIA on respiratory activity, R-in and V-m could be reversed by the A(1)-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 200 nM). 6. Our data suggest that the modulation of respiratory output induced by adenosinergic agents can be explained by (1) a general decrease in synaptic transmission between medullary respiration-related neurones mediated by presynaptic A(1)-receptors, and (2) an inactivation, via membrane hyperpolarization, of medullary expiratory neurones mediated by postsynaptic A(1)-receptors. Furthermore, our data demonstrate that inactivation of expiratory neurones does not abolish the respiratory rhythmic activity, but only modulates respiratory rhythm in vitro.