Cannabinoids and hamster circadian activity rhythms

Abstract

Circadian activity rhythms in hamsters are entrained to the daily light:dark cycle by photic information arriving from the retina to the suprachiasmatic nucleus, the site of the master circadian pacemaker in mammals. The effects of light on adjusting the timing of the circadian pacemaker is modified, both positively and negatively, by a variety of transmitter systems, but the effects of endocannabinoids have not been reported. Therefore, in this study we evaluated cannabinoids specific for the cannabinoid type 1 receptor (CB₁) for their ability to modulate light-induced phase advances in hamster circadian activity rhythms. All compounds were administered intraperitoneally. The CB₁ agonist CP55940 potently inhibited light-induced phase shifts with near 90% inhibition achieved with a dose of 0.125 mg/kg. The inhibitory effect of CP55940 was partially reversed by the CB₁ antagonist LY320135 and completely reversed with 1 mg/kg of the CB₁ antagonist AM 251. Neither LY320135 nor AM 251 had any effect on light-induced phase shifts when administered alone. Further evidence for CB₁ involvement in hamster circadian rhythms was provided by immunohistochemical detection of CB₁ receptors in four separate nuclei comprising the principal components of the hamster circadian system: the suprachiasmatic nucleus, intergeniculate leaflet of the thalamus, and dorsal and median raphe nuclei. Altogether these data indicate that the endocannabinoid system has the capability to modulate circadian rhythms in the hamster and cannabis use should be evaluated for adverse effects on circadian rhythms in humans.

Introduction

Photic information from the retina entrains the mammalian circadian pacemaker in the suprachiasmatic nucleus (SCN) to the environmental light:dark cycle (Johnson et al., 1988). The SCN in turn functions as the body's master pacemaker as it synchronizes the timing of peripheral circadian pacemakers distributed in tissues and organs through direct and indirect mechanisms (Guo et al., 2005, Liu et al., 2007). In addition to afferent input from the retina, the SCN is also modulated by two other primary components of the circadian system, the intergeniculate leaflet of the thalamus (IGL) and the dorsal and median raphe nuclei of the midbrain (Meyer-Bernstein and Morin, 1996). The IGL is responsible for shifts in circadian timing in the absence of light (non-photic) and raphe input to the SCN inhibits the afferent signals from the retina (Mrosovsky, 1996, Morin, 1999). The ability of photic information from the retina to entrain the timing of the circadian pacemaker is known to be modulated by a variety of transmitter systems (for reviews see Sprouse, 2004, Morin and Allen, 2006), but any effects of the endocannabinoid system have not been reported.

There are a few reports that suggest endocannabinoid signaling may affect, or be affected by, circadian rhythms. The circadian rhythm of brain temperature became inverted following cessation of chronic Δ⁹-tetrahydrocannabinol administration in rats (Perron et al., 2001). Also, cannabinoid receptor 1 (CB₁) gene expression in the rat pons is modulated by sleep (Martinez-Vargas et al., 2003), and the CB₁ antagonist SR 1417161 has arousing properties in rats (Santucci et al., 1996); therefore both studies suggest that endocannabinoid signaling is involved in the circadian sleep/wake cycle. In addition, levels of the endocannabinoid anandamide and 2-arachidonoyl-glycerol also display diurnal variations in the rat brain (Valenti et al., 2004). CB₁ receptors have been suggested to be required for normal glucocorticoid feedback of the hypothalamic-pituitary-axis in mice (Cota et al., 2007), and glucocorticoid rhythms are known to be controlled by the SCN (Moore and Eichler, 1972). Further, corticotropin-releasing-factor receptor 1 antagonists are known to inhibit light-induced phase shifts of hamster circadian activity rhythms (Gannon and Millan, 2006b). Therefore, endocannabinoids likely interact indirectly with circadian rhythms through the HPA axis. Finally, CB₁ receptors have been identified in both the mouse SCN (Wittmann et al., 2007) and raphe nuclei (Haring et al., 2007).

There are two types of receptors for endocannabinoids, CB₁ and CB₂, with the former thought to be the major subtype located within the mammalian brain (Mackie, 2005). Therefore, in this study we sought to determine if the CB₁ agonist CP55940 (Wiley et al., 1995) and CB₁ antagonists AM 251 (Gatley et al., 1997), and LY320135 (Felder et al., 1998) had any effect on light-induced phase advances of hamster circadian wheel running activity. In addition, we sought to determine if CB₁ receptors were located in the SCN, IGL or raphe nuclei of the hamster using standard antibody-mediated immunohistochemical techniques.