Circadian rhythms in the suprachiasmatic nucleus are temperature-compensated and phase-shifted by heat pulses in vitro

Abstract

Temperature compensation and the effects of heat pulses on rhythm phase were assessed in the suprachiasmatic nucleus (SCN). Circadian neuronal rhythms were recorded from the rat SCN at 37 and 31 degrees C in vitro. Rhythm period was 23.9 +/- 0.1 and 23.7 +/- 0.1 hr at 37 and 31 degrees C, respectively; the Q(10) for tau was 0.99. Heat pulses were administered at various circadian times (CTs) by increasing SCN temperature from 34 to 37 degrees C for 2 hr. Phase delays and advances were observed during early and late subjective night, respectively, and no phase shifts were obtained during midsubjective day. Maximum phase delays of 2.2 +/- 0.3 hr were obtained at CT 14, and maximum phase advances of 3.5 +/- 0.2 hr were obtained at CT 20. Phase delays were not blocked by a combination of NMDA [AP-5 (100 microM)] and non-NMDA [CNQX (10 microM)] receptor antagonists or by tetrodotoxin (TTX) at concentrations of 1 or 3 microM. The phase response curve for heat pulses is similar to ones obtained with light pulses for behavioral rhythms. These data demonstrate that circadian pacemaker period in the rat SCN is temperature-compensated over a physiological range of temperatures. Phase delays were not caused by activation of ionotropic glutamate receptors, release of other neurotransmitters, or temperature-dependent increases in metabolism associated with action potentials. Heat pulses may have phase-shifted rhythms by directly altering transcriptional or translational events in SCN pacemaker cells.

Fig. 1.

Representative recordings at 37°C (A) and 31°C (B); each of the six recordings is from a different animal. Each point is the mean (±SE) hourly firing rate of 10–12 SCN neurons. Circadian time 0 is the time of lights on in the donor colony; all tissue slices were prepared between CT 0 and 1 hr on day 1. Shaded areasrepresent projected dark phases of the LD cycle in the donor colony (LD 12/12). Vertical dashed lines represent the mean time of peak firing rates at each temperature on each day. TheQ₁₀ for tau was 0.99.

Fig. 2.

SCN temperature during a representative heat pulse. Solid vertical lines indicate the start and end times of the heat pulse. Dashed vertical lines indicate the start and end times of drug administration. Data are plotted in 5 min intervals.

Fig. 3.

Representative recording trials in which tissue slices were heat-pulsed at CT 20 (A), 14 (B), or 5 (C).Vertical black bars indicate time of heat pulse. Other symbols as in Figure 1.

Fig. 4.

Representative recording from one tissue slice obtained on day 3 after a heat pulse was administered at CT 20 on day 1. Time of peak firing rate was advanced by 3 hr. Vertical black bar indicates time of heat pulse. Other symbols as in Figure1.

Fig. 5.

Phase response curve to heat pulses in which tissue temperature was increased from 34 to 37°C for 2 hr; data are plotted at the midpoint of each pulse (A). Each symbol is the mean (±SE) of three to six trials, except at CTs 16 and 18 (n = 2 each). Maximal phase delays and advances were obtained with heat pulses applied in early and late, respectively, subjective night. Significant phase shifts were not obtained during most of subjective day (CT 0–12). Note that this curve closely resembles a photic phase response curve. Phase-shift magnitude increased linearly with pulse durations (B) of 2 (n = 6), 4 (n = 2), or 6 (n = 2) hr; solid line, first order regression (r = 0.95; p < 0.0001).

Fig. 6.

Mean (±SE) phase delays produced by each drug treatment. Control groups are TTX (1 μm), AP-5/CNQX, or the heat pulse applied alone. All other treatments were applied in the presence of the heat pulse. Numbers in parenthesesindicate number of recording trials for each group. There were no significant differences among the three controls compared to untreated slices (p > 0.05) or among treatments coadministered with the heat pulse (p > 0.05).

Fig. 7.

Representative recordings of neuronal rhythms after coapplication of a heat pulse at CT 14 with TTX (1 μm) (A) or AP-5/CNQX (B). Drugs were bath-applied to the tissue beginning 15 min before, and ending 45 min after, the heat pulse (Fig.2). Vertical white bars indicate the onset and termination of drug applications, and black barsindicate the time of the heat pulse. Other symbols as in Figure1.