PHASE SHIFTING THE CIRCADIAN RHYTHM FROM THE APLYSIA EYE: INVOLVEMENT OF 5-HT AND CYCLIC AMP
CORRENT, GEORGE F.
Doctor of Philosophy
The circadian rhythm (CR) of compound optic nerve potentials (CAPs) from the isolated eye of Aplysia californica can be phase shifted by 5-HT (serotonin). Data are presented in this thesis which suggest that 5-HT acts as a transmitter of temporal information to the circadian pacemaker (CP) in the Aplysia eye. Six h. 5-HT treatments produce both advance and delay phase shifts, are effective at low concentrations (10('-7)M), and alter both the spontaneous activity of the eye and the response of the eye to light. In addition, evidence is presented that the eye contains 5-HT (50ng/mg protein), can synthesis 5-HT from tryptophan, and that the 5-HT stored in the eye can be depleted by Hi-K('+) (depolarizing) treatments. The effects of 5-HT on the eye also show a degree of stereospecificity since three structurally related indoleamines, LSD, Br-LSD, and bufotenine acted as agonists of 5-HT when added to the eye, while other amines (5-hydroxytryptophan, tryptamine, 7-methyltryptamine, and others) and two putative neurotransmitters (Dopamine, Acetylcholine) did not mimic the effects of 5-HT. These results suggest that 5-HT may act as a neurotransmitter in the Aplysia eye. 5-HT appears to be producing its effects on the CR by acting either directly on the CR cell(s), or on cells electrotonically coupled to the CR cell(s), since treatments which block transmitter release (HiMg('2+)-LoCa('2+)-EGTA) do not inhibit phase shifting by 5-HT. The phase shifting effects of 5-HT may be mediated by changes in cellular cAMP, since 8-benzylthio-cAMP (2 x 10('-3)M), a cAMP analog, mimics 5-HT by producing both advance and delay phase shifts when given during the same phases as 5-HT. Papaverine (2 x 10('-4)M), which is a phosphodiesterase inhibitor that has been shown to increase cAMP levels in Aplysia tissue, also mimicked the phase shifting effects of 5-HT. IBMX (5 x 10('-4)M), another phosphodiesterase inhibitor, did not produce significant phase shifts when given alone, but did potentiate the phase shifting effects of subthreshold doses of 5-HT (10('-8)M). The results presented in this thesis are significant because 5-HT is the first endogenous substance shown to produce phase shifts in Aplysia and one of the few natural substances known to produce effects on the CR from any system. The data suggest that 5-HT may be part of a pathway which transmits entrainment information to the CP cell(s) in the eye, and that the effects of 5-HT may be mediated by changing levels of cAMP. 5-HT may provide a natural marker for investigating entrainment pathways and mechanisms in the Aplysia eye.