Neuropharmacological analysis of synaptic transmission in the Lorenzinian ampulla of the skate Raja clavata.

Dissected ampullae of Lorenzini of the skate (Raja clavata) were studied with the aim of determining the synaptic transmitter between electroreceptor cell and afferent fibre. Resting activity and stimulus-evoked activity in response to electrical pulses were recorded in single afferent units at constant perfusion with normal and test solutions containing different putative neurotransmitters. Presynaptic transmitter release was blocked by Mg2+ (up to 50 mM) to investigate the effects of the test substances upon the postsynaptic membrane. L-Glutamate (L-GLU) and L-aspartate (L-ASP), both at concentrations between 10(-7) and 10(-3) M, enlarged strongly resting and stimulus-evoked discharge frequency in the afferent fibre. If transmission was blocked by high Mg2+, resting discharge frequency could be restored by L-GLU or L-ASP. The glutamate agonists quisqualate (10(-8)-10(-5) M) and N-methyl-D-aspartate (10(-5)-10(-3) M) enlarged spontaneous activity in the afferent fiber. The same was found for kainic acid (10(-9)-10(-5) M). Taurine at concentrations between (10(-5) and 10(-3) M caused a concentration-dependent decrease in afferent activity. The same was found for gamma-aminobutyric acid (GABA; 10(-5)-10(-4) M), and for the catecholamines adrenaline and noradrenaline, both in concentrations between 10(-5) and 10(-3) M. Serotonine (10(-5)-10(-3) M) and dopamine (10(-5)-10(-3) M) had no effect on resting or evoked activity in the Lorenzinian ampulla afferents.(ABSTRACT TRUNCATED AT 250 WORDS)