Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles

Norepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles . In order to distinguish unambiguously between mitral cells and granule ce lls of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electro physiological criteria. At NE concentrations lower than 5 mum there was onl y one effect of NE upon voltage-gated conductances; NE blocked a high-volta ge-activated Ca2+-current in mitral cells of both the main and the accessor y olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca2+-currents was mimicked by the alpha (2)-recepto r agonists clonidine and alpha -methyl-NE. As a second effect, NE or clonid ine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the sponta neous synaptic activity in mitral cells of either olfactory bulb. The ampli tude of glutamate-induced currents in granule cells was modulated neither b y clonidine nor by alpha -methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha (2)-receptor-mediated block of Ca2+-curr ents in mitral cells appeared to be a wide-spread disinhibition of mitral c ells in the accessory olfactory bulb as well as in the main olfactory bulb.