INVOLVEMENT OF PERTUSSIS-TOXIN-SENSITIVE AND TOXIN-INSENSITIVE MECHANISMS IN ALPHA-ADRENOCEPTOR MODULATION OF NORADRENALINE RELEASE FROM RAT SYMPATHETIC NEURONS IN TISSUE-CULTURE

1 Sympathetic neurones derived from superior cervical ganglia of neona tal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Thre e week old cultures of neurones were loaded with [H-3]-noradrenaline t o label endogenous neurotransmitter stores. Release of noradrenaline w as evoked by depolarization with raised extracellular K+ in the presen ce of desipramine and corticosterone to prevent uptake of released cat echolamine. 2 Potassium (55 mmol l-1) depolarization for 30 s caused m ore than a four fold increase in H-3 overflow from basal levels but th is increase was reduced by up to 40% in the presence of exogenous nora drenaline (1 mumol l-1). The inhibition by noradrenaline of depolariza tion-evoked overflow was blocked by the alpha1/alpha2-adrenoceptor ant agonist, phentolamine. Phentolamine alone did not increase K+-evoked H -3 overflow. 3 The alpha2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K+-evoked overflow, while the alpha1-adrenoceptor antagonist, prazosi n, was without effect at concentrations up to 0.1 mumol l-1. 4 The bet a-adrenoceptor antagonist, propranolol, neither reduced K+-evoked over flow nor increased the degree of inhibition caused by the addition of 1 mumol l-1 noradrenaline. 5 The alpha2-adrenoceptor agonist, clonidin e (I mumol l-1) was less effective than noradrenaline at inhibiting K-evoked overflow, while the alpha1-adrenoceptor agonist, phenylephrine (1 mumol l-1) had no significant effect. 6 The L-channel calcium bloc ker, nicardipine (1 mumol l-1) significantly inhibited H-3 overflow ev oked by K+. In the presence of L-channel block, however, noradrenaline still inhibited residual evoked overflow. 7 In the presence or absenc e of nicardipine, pertussis toxin pretreatment (I nmol l-1) reduced, b ut did not prevent, the effect of noradrenaline (I mumol l-1). Pertuss is toxin alone caused a significant enhancement of K+-evoked H-3 overf low. 8 The data indicate that on postganglionic neurones of cultured r at sympathetic ganglia there are alpha2-adrenoceptors that modulate ne urotransmitter release, but no functional beta-adrenoceptors that medi ate an enhancement of transmitter release. The data suggest further th at in this preparation the mechanism of alpha2-adrenoceptor modulation may involve pertussis toxin sensitive and insensitive G-proteins and effects on calcium channels other than L-type.