A NOVEL ELECTROPHYSIOLOGICAL APPROACH TO MONITOR PULSE BY PULSE THE CONCENTRATION OF RELEASED NORADRENALINE AT THE PRESYNAPTIC ALPHA(2) ADRENOCEPTORS OF SYMPATHETIC-NERVES IN RAT TAIL ARTERY

The excitatory junction current (EJC) evoked by electrical stimulation of postganglionic sympathetic nerves of rat tail artery with 100 puls es at 2 Hz, at 1.3 mmol/l external Ca2+ was used as a measure of the p er pulse release of ATP. In controls the EJCs were initially facilitat ed, then gradually depressed during the stimulus train. The first EJC was slightly depressed by the alpha(2)-adrenoceptor antagonist yohimbi ne, but starting from the 4th pulse the EJCs were enhanced. Yohimbine increased the early facilitation without markedly modifying the subseq uent depression. The yohimbine-induced enhancement of EJCs caused by p ulses 11-100 was, thus, constant. The noradrenaline reuptake blocker c ocaine depressed the EJCs, abolished the early facilitation and slight ly enhanced the depression. These effects of cocaine were reversed by further addition of yohimbine. The alpha(2)-adrenoceptor agonist xylaz ine (1 and 10 mu mol/l) dose dependently depressed the EJCs starting f rom the first pulse. The inhibitory effect of 1 mu mol/l xylazine, but not that of 10 mu mol/l xylazine, declined with train length. The inh ibition of individual EJCs caused by activation of presynaptic alpha(2 )-adrenoceptors was used to monitor the concentration of released nora drenaline at these receptors. The ratio of individual EJCs in the pres ence and absence of yohimbine was assumed to reflect, pulse by pulse, the relative concentration of released noradrenaline at the presynapti c alpha(2)-adrenoceptors, and hence termed [NA](alpha 2). For comparis on, the concentration of endogenous noradrenaline was monitored electr ochemically by differential pulse amperometry with a carbon fibre micr oelectrode; this signal is termed [NA](CF). [NA](alpha 2) and [NA](CF) grew during the first 7-10 or 14-16 pulses, respectively, and then re mained relatively constant throughout the stimulus train. Cocaine caus ed [NA](alpha 2) and [NA](CF) to continue to grow during the first 35 and 50 pulses, and enhanced their peak levels by 180% and 320%, respec tively. For comparison with the effects on the EJCs mediated via presy naptic alpha(2)-adrenoceptors, those caused by varying external Ca2+ l evel were examined. At 0.65 mmol/l Ca2+ the amplitude of the first EJC was smaller than that at 1.3 mmol/l Ca2+, but the facilitation of lat er EJCs was enhanced and the subsequent depression reduced. An increas e in external Ca2+ to 2.6 mmol/l had the opposite effects. All effects on EJCs caused by changes in external Ca2+ were maximal for the first EJC and then declined with train length. The results show (i) that ch anges in the amplitude of individual EJCs during a 2 Hz train, caused by activation of presynaptic alpha(2)-adrenoceptors, may by used to mo nitor pulse by pulse the concentration of neurally released noradrenal ine at these receptors ([NA](alpha 2)), (ii) that [NA](alpha 2) grows during the first ten pulses to a plateau which is maintained. until th e end of the stimulus train, (iii) that an exogenous alpha(2)-adrenoce ptor agonist or changes in the external Ca2+ concentration affect the release probability in all varicosities uniformly, (iv) that activatio n of presynaptic alpha(2)-adrenoceptors by released endogenous noradre naline preferentially inhibits release from 'weak' varicosities while 'strong' varicosities were immune, and (iv) that the degree of activat ion of these receptors controls the per pulse release, and thereby the level of the [NA](alpha 2) plateau at steady state.