Presynaptic effects of octopamine, serotonin, and cocktails of the two modulators on neuromuscular transmission in crustaceans

The effect of the biogenic amines octopamine and serotonin, and of both ami nes combined (cocktails) on transmitter release at neuromuscular junctions of two crustaceans was studied. octopamine (10(-8) mol l(-1) to 10(-6) mol l(-1)) either enhanced or decreased evoked transmitter release through pres ynaptic effects.. The results were identical for the slow and the fast exci ter in the closer muscle of the crab, and for the exciter in the opener mus cle of the crayfish. Application of serotonin always resulted in a strong i ncrease of release. However, this potentiating effect of serotonin was redu ced in strength by subsequent application of cocktails consisting of seroto nin and octopamine. In all experiments, a cocktail of serotonin and octopam ine was less effective than serotonin alone. The decrease in the mean quant al content m by octopamine was due to a reduction of the probability of rel ease p. Since both amines are synthesized in the central nervous system and are released from neurohaemal organs into the haemolymph bathing the neuro muscular junctions, the results suggest that the two amines, when present t ogether, modulate transmitter release in an antagonistic way, and that the level of the two determines synaptic efficacy.