Omega-conotoxin GVIA and nifedipine inhibit Ca2+ action potentials in rat locus coeruleus neurons

Effects of potassium channel openers and their antagonists on rat locus coeruleus neurones

1. Intracellular recordings were obtained from a pontine slice preparation of the rat brain containing the locus coeruleus (LC). Two openers of ATP-sensitive potassium (K(ATP)) channels, RO 31-6930 (10 microM) and cromakalim (100 microM) decreased the spontaneous discharge of action potentials without altering their amplitude or duration. Neither compound changed the resting membrane potential. 2. Of two K(ATP) channel blockers, tolbutamide (300 microM) increased the firing rate, while glibenclamide (3 microM) only tended to do so. In addition, both compounds antagonized the effect of RO 31-6930 (10 microM). Neither glibenclamide (3 microM) nor tolbutamide (300 microM) altered the resting membrane potential. 3. Tetrodotoxin (0.5 microM) depressed the firing, but did not influence the inhibitory action of RO 31-6930 (10 microM). The excitatory amino acid antagonist, kynurenic acid (500 microM), did not change the spontaneous discharge of action potentials. 4. Small shifts (2-4 mV) of the membrane potential by hyper- or depolarizing current injections markedly decreased and increased the firing rate, respectively. 5. Noradrenaline (100 microM) hyperpolarized the cells and decreased their input resistance. This effect was not antagonized by glibenclamide (3 microM) or tolbutamide (300 microM). Ba2+ (2 mM), a blocker of both ATP-sensitive and inwardly rectifying potassium channels, abolished the effects of RO 31-6930 (10 microM) and noradrenaline (100 microM). 6. These data suggest that K(ATP) channels are present on the noradrenergic LC neurones, but are not coupled to alpha 2-adrenoceptors.

Excitatory effects of adenosine 5'-triphosphate on rat locus coeruleus neurones

Pontine slices of the rat brain were used for extracellular recording of the frequency of spontaneous action potentials of locus coeruleus (LC) neurones. In the absence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP; 0.3-30 mumol/l) and 2-methylthio ATP (0.3-100 mumol/l), but not ATP (1-100 mumol/l) increased the firing rate. In the presence of DPCPX 0.1 mumol/l, all three purinoceptor agonists were active, the potency order being alpha,beta-meATP greater than 2-methylthio ATP = ATP. Preincubation of the slices with tetrodotoxin (TTX) 0.5 mumol/l decreased the spike discharge but did not alter the percent facilitatory effect of alpha,beta-meATP 30 mumol/l. There was no desensitization to alpha,beta-meATP 10 mumol/l on repeated or continuous application. Suramin 100 mumol/l selectively depressed the effect of alpha,beta-meATP 30 mumol/l without interfering with the effect of equiactive concentrations (10-100 mumol/l) of glutamic acid. The concentration-response curve of alpha,beta-meATP was shifted in a parallel manner to the right by suramin 10 mumol/l. While DPCPX 0.1 mumol/l facilitated firing, suramin 100 mumol/l did not change it. In conclusion, LC neurones may possess P2-purinoceptors of an unidentified type, which share some P2x characteristics.