Transmitterlike action of serotonin in phase shifting a rhythm for the Aplysia eye

The presence of serotonin (5-HT) and the characteristecs of the phase-response curve for 5-HT indicate that 5-HT acts as a transmitter of circadian information in the Aplysia eye. Additional evidence for such a neurotransmitter role of 5-HT in the eye is presented. The isolated eye has the capacity to synthesize 5-HT from exogenous tryptophan. The receptors mediating the phase-shifting effects of 5-HT have a high affinity for 5-HT (threshold for phase shifting is ≤ 10^-7 M). Also these receptors demonstrate a high degree of structural specificity for 5-HT. Some structurally similar indoles to 5-HT do not cause phase shifts (5-hydroxytryptophan, 5-hydroxyindoleacetic acid), whereas others (bufotenine, tryptamine, LSD) do cause phase shifts but are less effects than 5-HT. Furthermore phase shifts in the rhythm are not produced by other monoamine neurotransmitters (dopamine, octopamine) or acetylcholine. Changes in membrane conductance to Na⁺, Cl^-, or Ca²⁺ do not appear to be involved in phase shifting by 5-HT. Since large reductions in extracellular Ca²⁺ did not affect phase shifting by 5-HT, 5HT is acting either directly on the circadian pacemaker cell(s) or on cells electronically coupled to the pacemaker cell(s).