Acute ethanol impairs photic and nonphotic circadian phase resetting in the Syrian hamster

Abstract

Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal, we used microdialysis to characterize the pharmacokinetics of intraperitoneal injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase resetting of the circadian locomotor activity rhythm. Peak EtOH levels (approximately 50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of approximately 3 h. EtOH treatment dose-dependently attenuated photic phase advances but had no effect on phase delays and, contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase advances. In a complementary experiment using reverse microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase advances were blocked, similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such actions could underlie the disruption of circadian rhythmicity associated with alcohol abuse.

Fig. 1.

Pharmacokinetics of ethanol (EtOH) in the suprachiasmatic nucleus (SCN) (n = 5 hamsters) following ip injection (Inj) of EtOH (2.0 g/kg). Peak levels (estimated at ∼50 mM) occurred within 20–40 min of injection, and the half-life (t_1/2) for clearance was 3 h. Bars represent means ± SE.

Fig. 2.

Coronal section through the hypothalamus showing the probe tract (T) of a microdialysis probe targeted to the lateral margin of the SCN. 3V, third ventricle; OC, optic chiasm.

Fig. 3.

Histologically verified probe tip locations relative to the SCN for each of the microdialysis experiments. ▵, Pharmacokinetics of EtOH after ip injection; •, reverse dialysis of EtOH; ○, reverse dialysis of vehicle.

Fig. 4.

EtOH dose dependently attenuates photic phase-advance shifts to a light pulse delivered late in the dark phase [Zeitgeber time (ZT) 18.5; left] but has no effect on phase-delay responses to a light pulse delivered at ZT 14 (right). The highest dose of EtOH (2.0 g/kg) had no phase-resetting effect of its own at ZT 18.5 in the absence of a light pulse (middle). For each time point, bars with different letters are significantly different (P < 0.05). Bars represent means ± SE.

Fig. 5.

Representative double-plotted actograms of general locomotor activity showing EtOH attenuation of photic phase-advance responses to light pulses delivered at ZT 18.5. A and B: vehicle-injection. C and D: EtOH (2.0 g/kg) injection. Arrows denote the time of injection.

Fig. 6.

Inhibition of photic phase-advance responses to a light pulse delivered late in the dark phase (ZT 18.5) by direct reverse-microdialysis perfusion of EtOH to the SCN. The EtOH perfusion had no phase-resetting effect at ZT 18.5 in the absence of a light pulse. Bars with different letters are significantly different (P < 0.05). Bars represent means ± SE.

Fig. 7.

Representative double-plotted actograms of general locomotor activity showing direct reverse-microdialysis perfusion of EtOH to the SCN attenuates photic phase-advance responses to light pulses delivered at ZT 18.5. A: microdialysis perfusion of vehicle [artificial cerebrospinal fluid (ACSF)]. B: reverse-microdialysis perfusion of EtOH (50 mM). Arrows denote the onset of the 1.5-h perfusion.

Fig. 8.

EtOH (2.0 g/kg) inhibition of nonphotic phase-advance responses to the serotonin 5-HT_1a,7 agonist (±)8-OH-DPAT (5.0 mg/kg) administered during the middle of the light phase (ZT 6; left). EtOH had no phase-resetting effect when administered before vehicle (dimethyl sulfoxide) injection at ZT 6 (right). Bars with different letters are significantly different (P < 0.05). Bars represent the means ± SE.