ELECTROPHYSIOLOGICAL CHARACTERIZATION OF DOPAMINERGIC AND NONDOPAMINERGIC NEURONS IN ORGANOTYPIC SLICE CULTURES OF THE RAT VENTRAL MESENCEPHALON

The aim of the present study was to characterize electrophysiologicall y neurones in organotypic cultures of the rat ventral mesencephalon an d to compare these results with results published for the same neurone s in other types of preparation. Intracellular recordings were obtaine d in 3- to 8-week-old organotypic slice cultures of the ventral mesenc ephalon prepared from newborn rats. Dopaminergic neurones were disting uished from non-dopaminergic neurones by staining with the autofluores cent serotonin analogue 5,7-dihydroxytryptamine and briefly viewing th e preparation with short exposures to ultraviolet (UV) light (365 nm). Short exposures to UV light did not affect the electrophysiological p roperties. There were no significant differences between dopaminergic and non-dopaminergic neurones with regard to resting membrane potentia l or action potential threshold and amplitude, and in both types of ne urone spontaneous burst activity and glutamatergic excitatory postsyna ptic potentials were seen. There were differences in the following par ameters, which can be used to distinguish between the two types of neu rone. Dopa minergic neurones had broad action potentials (2-9 ms), hig h input resistance (mean 81 M Omega), were silent or fired spontaneous ly at a low frequency (0-9 Hz), and no spontaneous GABA(A)-ergic inhib itory postsynaptic potentials or inward rectification were present. In contrast, non-dopaminergic neurones had fast action potentials (0.6-3 .2 ms), low input resistance (mean 32 M Omega), were silent or fired s pontaneously at relatively high firing frequency (0-28 Hz), and someti mes inhibitory postsynaptic potentials and inward rectification were s een. In the presence of 1 mu M tetrodotoxin and 10 mM tetraethylammoni um, Ca2+ spikes could be evoked in both dopaminergic and non-dopaminer gic neurones. Dopaminergic neurones in 3- to 8-week-old organotypic sl ice cultures have a number of distinguishing electrophysiological char acteristics similar to those recorded in other types of acute or cultu red preparations. However, some intrinsic regulatory mechanisms, namel y the slow oscillatory potentials, inward rectification and the K+ cur rent, I-A, seem to be missing in the cultured neurones.