Tetrodotoxin: effects on fish and frog melanophores.

Tetrodotoxin blocks the response of killifish melanophores to electrical stimulation of their melanin-aggregating nerves. The direct response of killifish or frog melanophores to hormones is not affected. Since there is no effect on melanin aggregation resulting from the release of adrenergic neurotransmitter in killifish melanophores, we conclude that tetrodotoxin acts by blocking nerve conduction.