Involvement of pertussis toxin‐sensitive and ‐insensitive mechanisms in α‐adrenoceptor modulation of noradrenaline release from rat sympathetic neurones in tissue culture

Abstract

1 Sympathetic neurones derived from superior cervical ganglia of neonatal rats and maintained in tissue culture were used to investigate the modulation of neurotransmitter release by presynaptic receptors. Three week old cultures of neurones were loaded with [3H]-noradrenaline to label endogenous neurotransmitter stores. Release of noradrenaline was evoked by depolarization with raised extracellular K+ in the presence of desipramine and corticosterone to prevent uptake of released catecholamine. 2 Potassium (55 mmol 1-) depolarization for 30 s caused more than a four fold increase in 3H overflow from basal levels but this increase was reduced by up to 40% in the presence of exogenous noradrenaline (1 timol 1`). The inhibition by noradrenaline of depolarization-evoked overflow was blocked by the aI/a2-adrenoceptor antagonist, phentolamine. Phentolamine alone did not increase K+-evoked 3H overflow. 3 The a2-adrenoceptor antagonist, yohimbine, produced a concentration-dependent block of the inhibition by noradrenaline of K+-evoked overflow, while the ax-adrenoceptor antagonist, prazosin, was without effect at concentrations up to 0.1 Lmol 1'. 4 The P-adrenoceptor antagonist, propranolol, neither reduced K+-evoked overflow nor increased the degree of inhibition caused by the addition of 1 1tmol 1' noradrenaline.5 The M2-adrenoceptor agonist, clonidine (I iLmol 11) was less effective than noradrenaline at inhibiting K+-evoked overflow, while the a,-adrenoceptor agonist, phenylephrine (1,umol 1I) had no significant effect.6 The L-channel calcium blocker, nicardipine (1 timol 1-') significantly inhibited 3H overflow evoked by K+. In the presence of L-channel block, however, noradrenaline still inhibited residual evoked overflow. 7 In the presence or absence of nicardipine, pertussis toxin pretreatment (1 nmol 1-') reduced, but did not prevent, the effect of noradrenaline (1 tLmol 1'). Pertussis toxin alone caused a significant enhancement of K+-evoked 3H overflow. 8 The data indicate that on postganglionic neurones of cultured rat sympathetic ganglia there are M2-adrenoceptors that modulate neurotransmitter release, but no functional P-adrenoceptors that mediate an enhancement of transmitter release. The data suggest further that in this preparation the mechanism of a2-adrenoceptor modulation may involve pertussis toxin sensitive and insensitive G-proteins and effects on calcium channels other than L-type.

α2‐Adrenoceptors couple to inhibition of R‐type calcium currents in myenteric neurons

“…α 2 ‐Adrenoceptors couple to the inhibitory G o /G i G‐proteins 27,33,40–42 . In the present study, the inhibitory effect of UK 14 304 on I Ca 2+ was blocked by PTX, a selective G o /G i G‐protein toxin.…”

Antioxidant treatment restores prejunctional regulation of purinergic transmission in mesenteric arteries of deoxycorticosterone acetate-salt hypertensive rats

“…The splanchnic vasculature is densely innervated by sympathetic nerves that release norepinephrine, adenosine 5′-triphosphate (ATP) and neuropeptide Y (NPY) which all constrict vascular smooth muscle (Donoso et al, 1997). Norepinephrine constricts arteries by activating α 1 -adrenergic receptors (α 1 ARs) on vascular smooth muscle cells and norepinephrine release from sympathetic nerves is regulated by prejunctional α 2 -adrenergic receptors (α 2 ARs)(Msghina et al, 1991; Hill et al, 1993; Stärnje and Stärnje, 1995). Purinergic P2X 1 type receptors are cation channels activated by ATP.…”

“…Norepinephrine can also inhibit ATP release via activation of prejunctional α 2 ARs on sympathetic nerves (Driessen et al, 1993; Stärnje and Stärnje, 1995). A 1 adenosine receptors and α 2 ARs link to pertussis-toxin sensitive G-proteins to cause inhibition of nerve terminal calcium channels and inhibition of transmitter release from sympathetic nerves (Hill et al, 1993; Zhu and Ikeda, 1993). …”

α 2 -Adrenoceptor-mediated inhibition of cultured sympathetic neurons: changes in α 2A/D -adrenoceptor-deficient mice

“…Results in peripheral sympathetically innervated preparations, however, have been inconclusive (see Starke et al 1989), and so have been studies on cultured sympathetic ganglion cells: the α 2 -adrenergic inhibition of noradrenaline release from chick neurons was abolished by pertussis toxin (Boehm et al 1992), but the inhibition in rat neurons was partly (Hill et al 1993;Koh and Hille 1997) or even totally (Schwartz 1997) pertussis toxin-insensitive. In WT neurons, no inhibition by UK 14,304 whatsoever remained after pertussis toxin, irrespective of whether the neurons were stimulated by trains of 8 pulses at 100 Hz (Fig.…”