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Physiologisches Institut, Universität Göttingen, Germany.

1. The intracellular calcium concentration ([Ca2+]i) in cultured olfactory bulb neurones of Xenopus laevis tadpoles was imaged using the calcium indicator dyes fluo-3 and Fura Red as well as a laser scanning microscope. 2. Upon extracellular application of brief pulses of a solution with high potassium concentration (high [K+]o), an increase in [Ca2+]i occurred in all neurones observed. During the first 2 days in culture this increase was highest. At later stages (more than 2 days in culture) the increase in [Ca2+]i was non-homogeneous and highest in the dendritic processes. 3. Nifedipine (10 microM) reduced the high [K+]o-induced increase in [Ca2+]i. The reduction was greatest in somata and proximal dendrites. 4. With nifedipine in the bath, the high [K+]o-induced increase of [Ca2+]i was further reduced by the application of omega-conotoxin GVIA (1 microM). The omega-conotoxin-sensitive Ca2+ influx occurred predominantly on dendritic processes. 5. Noradrenaline (NA), as well as the alpha 2-adrenergic receptor agonist clonidine, reduced the high [K+]o-induced increase of [Ca2+]i. This reduction occurred mainly on dendritic processes. 6. Our results suggest a highly non-homogeneous spatial distribution of voltage-gated Ca2+ channels in cultured olfactory bulb neurones. L-type channels were found mainly on somata and their density seemed to decrease on the dendrites with increasing distance from the soma. In contrast, nifedipine-insensitive N-type channels were mainly observed on dendrites and were blocked by omega-conotoxin. NA, as well as clonidine, markedly blocked Ca2+ influx through dendritic N-type Ca2+ channels.

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