‘Strong’ phase response curve for the circadian rhythm of locomotor activity in a cockroach (Nauphoeta cinerea)

Abstract

ANALYSIS of entrainment in circadian systems depends on a knowledge of the phase resetting effects of the light (or temperature) pulses perturbing the circadian oscillation. Thus when a light pulse of a particular intensity and/or duration falls at certain phases of the free-running oscillation it may generate phase-advances (+Δφ) in subsequent activity; when it falls at other phases it may generate phase-delays (−Δφ). A plot of such phase changes, both magnitude and sign, as a function of the phase of the oscillation so perturbed, is called a phase response curve (PRC) (refs 1, 2). In a systematic study of the resetting effects of light pulses on the rhythm of pupal eclosion in Drosophila pseudoobscura, Winfree³ demonstrated two topologically different types of PRC, depending on the strength of the signal. When light pulses were ‘weak’ (< 50 s blue light, 10 µW cm⁻²) phase shifts were small. When pulses were ‘strong’ (> 50 s) phase shifts became abruptly larger, up to 10–12 h. Because of the way Winfree's data were graphically presented, weak resetting curves were called type 1 and strong curves type 0. An abrupt change from type 1 to type 0 has been observed only in circadian systems which control physiological events such as pupal eclosion (refs 1–3 and D.S. in preparation) and oviposition⁵ in populations of insects. Rhythms of locomotor activity in single animals, on the other hand, have only shown low amplitude type 1 curves^6,7. Entrainment in ‘physiological’ rhythms is in most cases mediated by extraoptic photoreceptors and a hormonal output⁸. ‘Behavioural’ rhythms, on the other hand, most frequently involve entrainment by way of the organised photoreceptors^9,10 and an electrical (neural) output^10–12. Truman used these differences to distinguish two types of biological clock¹³. Here, however, we report PRCs which switch from type 1 to type 0 in the locomotor activity rhythm of a cockroach Nauphoeta cinerea, which might from earlier literature have been expected to show only type 1 resetting.