Transmitterlike action of serotonin in phase shifting a rhythm from the Aplysia eye.
The presence of serotonin (5-HT) and the characteristics 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 effect of 5-HT have a high affinity for 5-HT (threshold for phase shifting is less than or equal to 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 effective 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, 5-HT is acting either directly on the circadian pacemaker cell(s) or on cells electronically coupled to the pacemaker cell(s).
D000818:Animals; D001048:Aplysia / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D002940:Circadian Rhythm; D005123:Eye / Q000378:metabolism*; D066298:In Vitro Techniques; D018377:Neurotransmitter Agents / Q000096:biosynthesis*; D009799:Ocular Physiological Phenomena; D012701:Serotonin / Q000096:biosynthesis*; D012964:Sodium / Q000378:metabolism
D002712:Chlorides; D018377:Neurotransmitter Agents; D012701:Serotonin; D012964:Sodium; D002118:Calcium