Sympathetic and parasympathetic component of bradycardia triggered by stimulation of NTS P2X receptors

We have previously shown that activation of P2X purinoceptors in the subpostremal nucleus tractus solitarius (NTS) produces a rapid bradycardia and hypotension. This bradycardia could occur via sympathetic withdrawal, parasympathetic activation, or a combination of both mechanisms. Thus we investigated the relative roles of parasympathetic activation and sympathetic withdrawal in mediating this bradycardia in chloralose-urethane anesthetized male Sprague-Dawley rats. Microinjections of the selective P2X purinoceptor agonist alpha,beta-methylene ATP (25 pmol/50 nl and 100 pmol/50 nl) were made into the subpostremal NTS in control animals, after atenolol (2 mg/kg i.v.), a beta1-selective antagonist, and after atropine methyl bromide (2 mg/kg i.v.), a muscarinic receptor antagonist. The bradycardia observed with activation of P2X receptors at the low dose of the agonist is mediated almost entirely by sympathetic withdrawal. After beta1-adrenergic blockade, the bradycardia was reduced to just -5.1 +/- 0.5 versus -28.8 +/- 5.1 beats/min in intact animals. Muscarinic blockade did not produce any significant change in the bradycardic response at the low dose. At the high dose, both beta1-adrenergic blockade and muscarinic blockade attenuated the bradycardia similarly, -37.4 +/- 6.4 and -40.6 +/- 3.7 beats/min, respectively, compared with -88.0 +/- 11 beats/min in control animals. Double blockade of both beta1-adrenergic and muscarinic receptors virtually abolished the response (-2.5 +/- 0.8 beats/min). We conclude that the relative contributions of parasympathetic activation and sympathetic withdrawal are dependent on the extent of P2X receptor activation.