Circadian Rhythm Phenotype of 5-HT₇ Receptor Knockout Mice: 5-HT and 8-OH-DPAT-Induced Phase Advances of SCN Neuronal Firing

Abstract

In vitro neuronal recordings in the SCN have clearly documented shifts in the peak of unit activity following the application of serotonergic agents, and yet selectivity issues with these very tools have limited progress in establishing the precise receptor mechanisms. As an alternative strategy, mice werebred (C57BL/6J) lacking 1 serotonin receptor, the 5-HT₇, to serve as a null background for this subtype; earlier work had documented the involvement of 5-HT₇ receptors in the phase advances elicited by 8-OH-DPAT, a mixed 5-HT_1A/7 agonist, in SCN slices prepared from rat donors. Single-unit recordings in sequential electrode passes revealed peaks of activity that occurred at nearly the same time in the knockout (KO; ZT4.2 ± 0.6) and wild-type animals (WT; ZT4.3 ± 0.1), where ZT0 marks the beginning of the light phase in a 12:12 LD cycle. Bath application of 8-OH-DPAT produced a phase advance in neuronal firing (2.1 ± 0.5 h) when applied 1 circadian cycle earlier at ZT6 (10 μM, 10 min), but surprisingly, the mean phase advance in slices prepared from KO mice (2.3 ± 0.1 h) was no different. Coapplication of 8-OH-DPAT with WAY-100,635 (10 μM), a highly selective 5-HT_1A antagonist, significantly reduced the phase advance, both in experiments with WT and KO mice, suggesting the greater importance of this serotonin sub-type independent of genetic modification. 5-HT itself (0.5 ±M, 10 min) at ZT6 also yielded phase advances that were indistinguishable in slices prepared from WT and KO mice (1.8 ± 0.4 h and 2.1 ± 0.2 h, respectively) and that were also sensitive to WAY-100,635. Unlike the pattern with 8-OH-DPAT, however, 5-HT-induced phase advances, in both WT and KO mice, were blocked by ritanserin, in this paradigm useful as a 5-HT_5A/7 antagonist (in addition to its more typical role as a 5-HT2A/2C antagonist). Serotonin antagonists when administered alone were without effect in slices from WT mice but produced significant phase shifts when administered to those from KO animals. Taken together, these results highlight the importance of the species used in establishing receptor mechanism. More provocatively, they support the involvement of multiple serotonin receptors in shifting the phase of circadian rhythms at ZT6.