5-HT 1A autoreceptor antagonist-induced 5-HTrelease in the hamster suprachiasmatic nuclei: effects on circadian clock resetting Michael C. Antle a , J. David Glass b , Ralph E. Mistlberger a, * a Department of Psychology, Simon Fraser University, Burnaby, B.C., Canada, V5A 1S6 b Department of Biological Sciences, Kent State University, Kent, Ohio, OH, 44242, USA Received 27 December 1999; received in revised form 27 January 2000; accepted 27 January 2000 Abstract Serotonin (5-HT) has been implicated in phase shifting of mammalian circadian rhythms by non-photic stimuli. This study tests whether pharmacological induction of endogenous 5-HT release can shift circadian phase in the Syrian hamster. Systemic injections of the 5-HT 1A antagonist WAY100635 during the mid-subjective day signi®cantly increased 5-HT in dialysate from the hamster suprachiasmatic nucleus (SCN) circadian pacemaker by ,50% for 40±60 min. However, this was not associated with phase shifts or with potentiation of phase shifts induced by a 3 h bout of running. These results indicate that enhanced 5-HT release in the SCN or possibly other regions is not suf®cient to induce phase shifts in the subjective day. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Circadian rhythms; Serotonin; WAY100635; Non-photic entrainment; Microdialysis; Wheel running Locomotor activity stimulated in the mid-subjective day (rest phase) of hamsters can induce signi®cant phase advance shifts of circadian rhythms [6]. The master pace- maker controlling rhythmicity in mammals is located in the suprachiasmatic nucleus (SCN). The SCN receive photic input via a glutamatergic pathway from the retina, and are thought to receive inputs relating to non-photic stimuli (de®ned here as locomotor activity) via a neuropeptide Y (NPY) pathway from the intergeniculate lea¯et (IGL) and possibly also a serotonergic (5-HT) pathway from the median raphe (MR) [20]. The body of evidence supporting a role for NPY in non-photic phase resetting is internally consistent. However, the case for 5-HT is mixed [19,20]. In Syrian hamsters, extracellular 5-HT levels in the SCN increase during spontaneous nocturnal activity and during bouts of running stimulated by con®nement to a novel wheel in the mid-subjective day [8]. At this circadian phase, small phase advances can be induced in this species by systemic injections of the 5-HT agonist 8-OH-DPAT [18] and by electrical stimulation of the MR [17]. In the rat SCN slice, 5-HT agonists induce large phase advances via the 5-HT 7 receptor, and similar shifts have also been reported follow- ing intraventricular injections in vivo [10,19]. Microdrop applications of 5-HT as brief as 5 min have proven effective [14]. In mice, neurotoxic lesions that deplete SCN 5-HT prevent entrainment of circadian rhythms to daily schedules of forced or voluntary activity [13]. However, activity- induced phase shifts in hamsters are not affected by 5-HT lesions or receptor antagonists [2,6,16]. Moreover, 5-HT agonists induce either very small [7] or no phase shifts when applied directly to the hamster SCN in vivo [11,18]. The present study was designed to further assess the role of 5-HT in the induction of non-photic phase shifts in the Syrian hamster. The potent and selective 5-HT 1A antagonist WAY100635 [3] was used to increase endogenous 5-HT release in the SCN of hamsters at rest or during 3 h of running in a novel wheel. Changes in SCN 5-HT were assessed in a separate group using in vivo microdialysis. If 5-HT release within the SCN mediates non-photic phase shifts, then WAY100635, at doses that raise endogenous SCN 5-HT, should mimic these phase shifts and might also potentiate activity-induced shifts. Young adult male hamsters (80±100; 100±120 g for cannulation experiments) were obtained from Charles River, (Montreal, Quebec) or raised from breeding pairs obtained from Harlan±Sprague±Dawley (Madison, IL). For behavioral testing, animals were maintained in isolation cabinets in plastic cages equipped with 17-cm running wheels connected to an interface and microcomputer that summed wheel revolutions in 10-min bins (Activity Count- Neuroscience Letters 282 (2000) 97±100 0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(00)00873-9 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 11-604-291-3462; fax: 11-604- 291-3427. E-mail address: mistlber@sfu.ca (R.E. Mistlberger)