FUNCTIONAL-ORGANIZATION OF CHROMAFFIN CELLS AND CHOLINERGIC SYNAPTIC TRANSMISSION IN RAT ADRENAL-MEDULLA

Optical recordings of membrane depolarization and whole-cell patch-cla mp recordings of membrane potentials and currents were obtained from c hromaffin cells in slices of rat adrenal medulla. The stimulation of s planchnic nerve fibers caused a discontinuous spread of electrical act ivity across the slice. Cells in clusters with diameters of about 80 m u m were excited simultaneously, suggesting that the adrenal medulla i s organized into descrete cell complexes with common innervation. The electrical properties of chromaffin cells in situ were in agreement wi th previous reports on cultured cells. A fraction of the recorded cell s displayed excitatory postsynaptic currents (EPSCs) of 0.2-1 nA upon the stimulation of presynaptic nerve fibers. The EPSC was blocked by h examethonium, suggesting that nicotinic ACh receptors were involved. T he decay phase of the EPSC was well fit by the sum of two exponentials with time constants of 6.3 and 57.3 ms. The relative amplitude of the fast component was 84.1%. These two exponentials may reflect activati on of both fast and slow time-constant ACh receptor channels by presyn aptic release of ACh. There were multiple peaks in the EPSC amplitude histograms in low-[Ca2+] saline, the first peak was at 37 pA. To resol ve the quantal size, miniature EPSCs were recorded in a tetrodotoxin-c ontaining high-[K+] solution. The miniature EPSC amplitude histograms were also multimodal with the first peak at 25 pA, which probably repr esents the quantal size of the synapse. The second and third peaks wer e at the integer multiples of the first one.