Endogenous regulation of serotonin release in the hamster suprachiasmatic nucleus

Abstract

Serotonin (5-HT) has been strongly implicated in the regulation of the mammalian circadian clock located in the suprachiasmatic nuclei (SCN). However, little is known of the pattern of neuronal 5-HT release in the SCN or of the factors involved in regulating its release. Using in vivo microdialysis, we demonstrated the existence of a daily rhythm in the output of 5-HT in the SCN of freely behaving hamsters. This rhythm was characterized by a sharp increase in release from a nadir during late midday to peak levels at the light/dark transition. Output declined to basal levels throughout the remainder of the night. A similar pattern also was evident under constant darkness, with increased 5-HT output occurring at the onset of subjective night. Locomotor activity induced by exposure to a novel running wheel had a pronounced phase-dependent effect on 5-HT release in the SCN, with stimulation during the light phase and suppression during the late dark phase. Systemic application of the somatodendritic 5-HT1A agonist BMY 7378 had a significantly greater suppressive effect on 5-HT release in the SCN during the late dark phase compared with mid light phase, indicating that a variation in raphe autoreceptor response may underlie the time-dependent effects of wheel running on 5-HT release. Collectively, these results show that the daily rhythm in output of 5-HT in the SCN is generated endogenously, and that behavioral state can strongly influence serotonergic activity in the circadian clock in a phase-dependent manner.

Fig. 1.

Representative coronal section through the midposterior aspect of the SCN showing a microdialysis cannula tract (C) at the lateral margin of the nucleus. Section is stained with cresyl violet. 3V, Third ventricle;OC, optic chiasm.

Fig. 2.

Pharmacologically induced changes in SCN microdialysate 5-HT concentrations. Top, Middle, Effects of localized perfusions with ACSF solutions containing high [K⁺] or no Ca²⁺, respectively, on 5-HT output. Solid bars denote the 2 hr duration of the treatments. Bottom, Effect of intraperitoneal injection of 8-OH-DPAT on 5-HT output. Arrow designates the time of injection. The sample interval was 20 min for all treatments; n = 4–5 for each experiment; *p < 0.05 compared with pretreatment levels.

Fig. 3.

Daily profile of 5-HT release in the SCN region in hamsters maintained under 14/10 hr light/dark cycle. Solid bar denotes the dark phase. Each pointrepresents the mean ± SEM for 12 animals. *p< 0.05 versus mean light phase level.

Fig. 4.

Daily profile of 5-HT release in the SCN region assessed over two consecutive 14/10 hr light/dark cycles. Solid bars denote the dark phase. Each pointrepresents the mean ± SEM for 5 animals. *p< 0.05 versus mean light-phase level.

Fig. 5.

Daily profile of 5-HT release from the SCN region in hamsters maintained under constant dark (DD) for a minimum of 2 weeks. CT 12 represents the onset of subjective night as assessed by actograms. Each point represents the mean ± SEM for 6 animals. *p < 0.05 versus mean subjective day level.

Fig. 6.

Averaged release of 5-HT in the SCN under LD (n = 12) and DD (n = 6). In both conditions, overall release of 5-HT is enhanced during subjective night. *p < 0.01 versus respective subjective day.

Fig. 7.

Profiles of novel wheel-running-induced alterations in 5-HT release from the SCN region during midday (ZT 4–7) and during the late dark phase (ZT 19–22). Solid bardenotes the 3 hr period of confinement in the wheel. *p < 0.05 relative to prerunning baseline. Data from all animals in each group are included irrespective of running intensity. Inset, Bars represent total number of revolutions over the 3 hr wheel-running period for both groups. *p < 0.05 between groups.

Fig. 8.

Time of day difference in the effect of novel wheel running on 5-HT release averaged over the entire 3 hr running period from the SCN region. The vertical position and length of thesolid bars denote the mean level of 5-HT release and period of wheel running, respectively. Data from all animals in each group are included irrespective of running intensity. Solid bar at the top denotes the dark phase. Groups with different letters are different from each other;p < 0.05.

Fig. 9.

Individual profiles of 5-HT release from the SCN collected before, during, and after 2 hr of novelty-induced wheel-running. Solid bar denotes the period of confinement in the wheel. A, 5-HT measured without reuptake blocker (citalopram) added to the ACSF. B, 5-HT measured with citalopram added to the ACSF. Note differences in vertical axis scales. Sample interval is 20 min.

Fig. 10.

Time of day difference in the inhibitory effect of intraperitoneal injection of BMY 7378 on 5-HT release from the SCN region. Arrow denotes time of injection. Sampling interval is 20 min. *p < 0.05 versus same time point at ZT 6; ^!p < 0.05 versus saline control.