Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100nM-10μM), noradrenaline (10μM-1 mM) and adrenaline (10μM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents-intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 μM-1 mM) and adrenaline (20μM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 μM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 μM), propranolol (20 μM) or prazosin (1 μM); it was reduced by a concentration of phentolamine about 100 times higher than its K_e for α₂-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an α₂-adrenoceptor.