Abstract
<p>The cholinergic innervation of the rat parasympathetic cardiac ganglion was verified by anatomical and histochemical techniques. A tissue culture method designed for electrophysiological studies on principal cells of the cardiac ganglia is described. The passive and active membrane properties of cultured cardiac neurons were studied under whole cell current and voltage clamp conditions. The cultured neurons have an input resistance of 1 G$\Omega$ and action potentials are generated in response to injected currents up to a stimulus frequency of 20 Hz. A detailed description of the pharmacological and ion selectivity properties of receptor-mediated ionic currents in cardiac neurons is presented. Whole-cell responses of cardiac neurons to the agonists, acetylcholine (ACh) and adenosine triphosphate (ATP) are similar in that both evoke excitatory responses of similar magnitude and time course but are pharmacologically distinct. The pharmacological profile of the purinergic (ATP) receptor-channel obtained from the relative potencies for activation was ATP = 2-methylthioATP $>$ $\beta\sb{\gamma}$methyleneATP $>$ $\beta\sb{\gamma}$methylene ATP and inhibition by the receptor antagonist, reactive blue 2, indicate that ATP-induced currents are mediated by the activation of P$\sb{\rm 2y}$ receptors. Single ATP-evoked channels studied in cell-attached and excised membrane patches had a conductance of 80 and 50 pS, respectively. ACh-induced currents were inhibited by ganglionic nicotinic receptor antagonists but not by atropine suggesting that they are mediated by nicotinic receptor activation. ACh-induced channel currents were obtained in excised membrane patches and had conductances of 25 and 66 pS. Ion substitution experiments demonstrated the cation selectivity of nicotinic and purinergic receptor operated channels, and in particular, that both agonist-activated channels have a high permeability to Ca$\sp{2+}$.</p>