^*Department of Neurophysiology, Institute of Physiology, Medical Faculty, Ruhr-Universität Bochum, Universitätsstr, 150, D-4630 Bochum 1,F.R.G.

^bMRC Anatomical Neuropharmacology Unit, Department of Pharmacology, University of Oxford, Oxford, U.K.

^‡Department of Functional Neuroanatomy, Institute of Experimental Medicine, Hungarian Academy of Sciences, P.O. Box 67, H-1450 Budapest, Hungary