Basic features of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP.

An extracellular recording method is described which permits in suitable model tissues (e.g. guinea-pig or mouse vas deferens) study of the nerve impulse in sympathetic terminals and the release of transmitter from sites inside or outside the recording electrode. Negative- or positive-going potentials were assumed to reflect the excitatory junction current (EJC) caused by transmitter released inside or outside the electrode, respectively, and hence termed 'EJCi' (i for inside) or 'EJCo' (o for outside). The EJCo were shown to be Ca2+-dependent, blocked by addition of tetrodotoxin or guanethidine, resistant to the alpha 1-adrenoceptor blocking agent prazosin but suppressed by desensitization of P2-purinoceptors by alpha,beta-methylene ATP, and hence, presumably, are caused by release of ATP as a sympathetic co-transmitter. The amplitude of the EJCo was voltage-dependent and increased with the length and frequency of stimulus trains within the range of 1-50 shocks at 0.1-2.5 Hz. In conclusion, combined use of EJCi and EJCo provides a useful tool for physiological and pharmacological analysis of pre- and post-junctional events associated with the secretion of a sympathetic co-transmitter, presumably ATP.