Destruction of Serotonergic Neurons in the Median Raphe Nucleus Blocks Circadian Rhythm Phase Shifts to Triazolam but Not to Novel Wheel Access

Abstract

Systematic treatment of hamsters with triazolam (TRZ) or novel wheel (NW) access will yield PRCs similar to those for neuropeptide Y. Both TRZ and NW access require an intact intergeniculate leaflet (IGL) to modulate circadian rhythm phase. It is commonly suggested that both stimulus types influence rhythm phase response via a mechanism associated with drug-induced or wheel access-associated locomotion. Furthermore, there have been suggestions that one or both of these stimulus conditions require an intact serotonergic system for modulation of rhythm phase. The present study investigated these issues by making serotonin neuron-specific neurotoxic lesions of the median or dorsal raphe nuclei and evaluating phase response of the hamster circadian locomotor rhythm to TRZ treatment or NW access. The expected effect of TRZ injected at CT 6 h on the average phase advance was virtually eliminated by destruction of serotonin neurons in the median, but not the dorsal, raphe nucleus. No control or lesioned animal engaged in substantial wheel running in response to TRZ. By contrast, all median raphe-lesioned hamsters that engaged in substantial amounts of running when given access to a NW had phase shifts comparable to control or dorsal raphe-lesioned animals. The results demonstrate that serotonergic neurons in the median raphe nucleus contribute to the regulation of rhythm phase response to TRZ and that it is unlikely that these neurons are necessary for phase response to NW access. The data further suggest the presence of separate pathways mediating phase response to the two stimulus conditions. These pathways converge on the IGL, a nucleus afferent to the circadian clock, that is necessary for the expression of phase response to each stimulus type.

Midbrain Raphe Modulation of Nonphotic Circadian Clock Resetting and 5-HT Release in the Mammalian Suprachiasmatic Nucleus

“…Similar to other studies in the Syrian hamster (Bobrzynska and Mrosovsky, 1998;Meyer-Bernstein and Morin, 1998), there was a wide range of response to novel wheel access, with the majority of animals of the vehicle control and drug treatment groups running Ͻ400 revolutions over the 3 hr treatment period (Fig. 6).…”

“…The intrinsic pattern of activity of 5-HT neurons varies in accordance with behavioral state (Trulson and Jacobs, 1983;Jacobs and Fornal, 1997), and 5-HT release induced in the SCN by behavioral phase-resetting stimuli (Dudley et al, 1998;Grossman et al, 2000) could serve as an intrinsic clock-resetting feedback signal. Although a role for 5-HT is not certain (Bobrzynska et al, 1996;Mintz et al, 1997;Meyer-Bernstein and Morin, 1998), evidence supporting a phase-resetting action of 5-HT in the SCN includes reports that in vitro application of 5-HT receptor agonists to the SCN brain slice preparation during subjective midday advances the circadian rhythm of neuronal activity (Prosser et al, 1990Medanic and Gillette, 1992;Shibata et al, 1992). Also, in vivo administration of 8-OH-DPAT into the third ventricle upstream from the SCN in rats (Edgar et al, 1993) or bilateral microinjections of 8-OH-DPAT into the SCN region of hamsters (Challet et al, 1998) phase advances the circadian clock, and depletion of 5-HT in the SCN by neurotoxic lesioning prevents entrainment to daily schedules of activity in mice (Edgar et al, 1997;Marchant et al, 1997).…”

“…It is possible that the circadian phase-related actions of intra-DRN GABA involve the same GABA A -benzodiazepine receptor population responsible for mediating the phase-resetting effects of triazolam (Turek and Losee-Olson, 1986;Turek and Van Reeth, 1988;Van Reeth and Turek, 1989;Marchant and Morin, 1999). However, it is unlikely that the DRN is a target for the phase-shifting action of triazolam (Meyer-Bernstein and Morin, 1998;Marchant and Morin, 1999), and triazolam and behavioral stimuli do not act via the same phase-resetting mechanism (Meyer-Bernstein and Morin, 1998;Janik and Mrosovsky, 1992;Marchant and Morin, 1999).…”

“…This includes findings that 5-HT agonists applied to the SCN in vitro or in vivo reset circadian phase (Prosser et al, 1990Medanic and Gillette, 1992;Challet et al, 1998;Ehlen et al, 2001) and that behavioral phaseresetting manipulations evoke SCN 5-HT release (Dudley et al, 1998;Grossman et al, 2000). Conversely, reports that raphe lesions (Meyer-Bernstein and Morin, 1998), treatment with 5-HT antagonists (Antle et al, 1998), or depletion of 5-HT in the SCN (Bobrzynska et al, 1996) do not block behavioral phase shifting argue against a role of 5-HT in circadian clock resetting.…”

Expression of 5-HT7 receptor mRNA in the hamster brain: Effect of aging and association with calbindin-D28K expression

“…Because NPY projections from the IGL and serotonin projections from the median raphe are known to communicate nonphotic information to the suprachiasmatic nucleus (Meyer-Bernstein and Morin, 1998;Cutrera et al, 1994;Schuhler et al, 1999;Janik and Mrosovsky, 1994), it is possible that the calbindin subnucleus may be involved in mediating nonphotic resetting of the circadian pacemaker. One study investigating this possibility found that administration of calbindin antisense oligonucleotides did not affect phase shifts induced by injections of the 5-HT 1A/7/5 agonist, 8-OH-DPAT, although a small sample size (N=3) was used (Hamada et al, 2003).…”