Brain Research, 593 (1992) 51-56 © 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05,00 51 BRES 18151 A voltage-clamp analysis of NMDA-induced responses on dopaminergic neurons of the rat substantia nigra zona compacta and ventral tegmental area Nicola B. Mercuri, Francesca Stratta, Paolo Calabresi and Giorgio Bernardi Clinica Neurologica, il Universtta' di Roma, Roma (Italy) (Accepted 12 May 1992) Key words: intracellular recording; Slice; Ventral mesencephalon; Excitatory amino acid receptor "£he effects of NMDA-receptor activation on dopaminergic neurons of the rat substantia nigra zona compacta and ventral tegmental area were studied by using in vitro intracellular electrophysiological recordings (current and voltage-clamp). NMDA depolarized the membrane and increased the firing activity. A voltage-dependent inward current and a reduction of the apparent input conductance were observed in voltage-clamp experiments. Interestingly, the peak amplitude of the inward current occurred at approximately -60 mV. The NMDA-induced responses were reduced by the application of DL-2-amino-5-phophonovaleric acid (APV). The NMDA-induced current was unaffected by potassium channel blockers, was present in low-sodium solutions or in solutions treated with Trx; but was reduced or blocked in low-calcium solutions containing cobalt, in addition, no reduction of the apparent input conductance was observed either in the solutions without magnesium or in those with low-sodium. Our data indicate that the activation of NMDA receptors produces a powerful excitatory stimulus on the dopaminergic neurons of the ventral mesencephalon and this may be primarily the result of a voltage-dependent ir.tlux of calcium ions. The degeneration of the dopaminergic cells after application of neurotoxins may be explained by their peculiar response to NMDA. INTRODUCTION Among the excitatory amino acid receptor sub- classes, the one activated by N-methyl-D-aspartate (NMDA) is involved in several aspects of neuronal function and pathology. In fact, NMDA receptors are involved in long-term potentiation s, in homeostasis of intracellular calcium 6'26'35, in kindling 9'14, in stroke or hypoxia-induced brain lesions t5'45, in neurodegenera- tire disorders, and in causing neuronal death as a consequence of their excessive stimulation 7'31'42. More- over, NMDA antagonists were recently proposed as therapeutic agents for the treatment of the symptoms of Parklnson's disease and as neuroprotective agents against dopaminergic cell damage ~6'44. NMDA recep- tors were shown to be located on different subpopula- tions of dopaminergic ceils 19 and they are depleted in the substantia nigra of parkinsonian patients 38. Recent experiments have also demonstrated that NMDA an- tagonists protected dopaminergic neurons when they were exposed to methamphetamine, and 1-methyl-4- phenyl-pyridinium ion (MPP + )42-44. In light of the involvement of the ,~opaminergic neuronal system in psychomotor functions, it is therefore important to understand the membrane mechanisms which underlie the effects of NMDA.receptors stimulation on the dopamine-containing cells. Extracellular electrophysio- logical recordings in vitro have already shown that NMDA increase the firing rate when applied to these cells 41. By using single-electrode voltage-clamp tech- niques in the present investigation, we examined the effects of NMDA on the membrane of rat mesen- cephalic dopaminergic cells in an in vitro slice prepara- tion. MATERIALS AND METHODS Albino Wistar rats (150-350 g) of both sexes were sacrificed by a blow on the chest under ether anesthesia. The brain was removed and slices of the mesencephalon (300-350 gm thick) containing the substantia nigra were cut by an Oxford vibratome. A single slice was then transferred to a recording chamber and continuously super- fused at a rate of 2.5 ml/min, with a solution maintained at 35°C and Correspondence: N.B. Mercuri, Clinica Neurologica, 11 Universita' di Roma, Via Orazio Raimondo, 00173 Rome, Italy. Fax: (39) (6) 7233063.