CAFFEINE REDUCES THE EFFICACY OF ELECTRORECEPTOR CELL SYNAPSES - AN ELECTROPHYSIOLOGICAL SINGLE-UNIT IN-VIVO STUDY

Ampullary electroreceptor organs of catfish, Ictalurus melas, were exp osed apically to caffeine solutions at concentrations of 0, 5, 7.5, 10 , and 15 mM. Recording sinusoidally-modulated activity of single-unit afferents reveals a dose-dependent decrease in mean afferent activity and sensitivity. A rebound effect of average activity occurs after caf feine is washed out. After 25 min exposure to 15 mM caffeine the peak of the gain curve shifts from 8 Hz to 4 Hz. The corresponding phase ch aracteristic shows an increased phase lag with a maximum shift of 35 d egrees at 20 Hz. The latency between stimulus and response increases f rom 12 to 19 ms; the recovery time after onset of the pulse decreases with 60 ms. The most probable explanation for the recorded effects is that caffeine reduces the availability of intracellular Ca2+ by blocki ng of the inositol triphosphate receptors in the endoplasmic reticulum . This in turn would affect many intracellular properties and processe s. The unavailability of Ca2+ could reduce the synaptic efficacy and i ncrease latency by suppressing fusion of synaptic vesicles with the pr esynaptic membrane and by depressing vesicle transport. The change in frequency response corresponds in part to reduction of the apical memb rane surface area of the receptor cells, and in part to the increased latency. Accumulation of glutamate-containing vesicles could account f or the higher mean activity and modulation amplitude in the lower freq uency range after caffeine is washed out. Caffeine might act postsynap tically by inducing hyperpolarization of the terminals of the primary afferents. (C) 1997 IBRO. Published by Elsevier Science Ltd.