AJP - Regulatory, Integrative and Comparative Physiology, Vol 242, Issue 3 333-R338, Copyright © 1982 by American Physiological Society
Transmitterlike action of serotonin in phase shifting a rhythm from the Aplysia eye
G. Corrent and A. Eskin
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).
