Photoendocrine transduction in cultured chick pineal cells. III. Ouabain (or dark) pulses can block, overcome, or alter the phase response of the melatonin rhythm to light pulses.

A photoentrainment pathway, circadian pacemakers, and the apparatus for regulated melatonin production all reside within chick pineal cells. Pulses of white light (L), or darkness (D), or ouabain (Ob), in otherwise constant red light, induce phase-dependent phase shifts in the rhythm of melatonin output displayed by these cells in static culture. The phase response curves (PRCs) (which describe the relationship between the phase at which a pulse is given and the resulting phase shift) for Ob and for D are quite similar, and differ from the PRC for L pulses. Here, we describe the effects of pulses of Ob, L, and D, in combination, on subsequent phase of the melatonin rhythm. Ob pulses can block, overcome, or alter the phase response to light pulses, depending on the phases and concentrations used. Under appropriate conditions, D pulses can, like Ob, convert the phase response to a light pulse from phase delays to phase advances. Such alterations in the response to a second pulse (L) caused by a first pulse (Ob or D) implies a rapid resetting of the phase of the underlying pacemaker. The interactions of Ob, L, and D are consistent with, but do not require, the convergence of their entrainment pathways on the same oscillating component of the pacemaker.