The need for quantitative circannual as well as circadian rhythm assessment in murine chronopharmacology

Data on the sedative effects of tetrabenazine (.25% solution in a dose of 50 mg/kg body weight), studied after injection in adult male Sprague-Dawley rats at one of 8 different circadian stages, 3 hours apart, were subjected to chronobiometry. For 5 weeks prior to R_X, rats were standardized in L19³⁰-07³⁰D07³⁰-19³⁰, 2 per cage at 24±1 °C with food and water ad lib. Monitoring of spontaneous locomotor activity with an Animex-DS apparatus at 10-minute intervals for at least 27 hours prior to R_X and a total span of 7 days served to gauge susceptibility to tetrabenazine. Within 10 minutes after injection of tetrabenazine, the locomotion of the rats completely stopped. The duration of sedation was considered to have ended when the activity count reached 90 10 minutes. A circadian rhythm in locomotor activity monitored for 24h prior to r_x was highly significant (P <.001) by population-mean cosinor after log transformation of the activity counts, with an acrophase occurring around 14⁰⁰ or 18³⁰ hours after light onset (HALO). A statistically significant circadian rhythm in response to tetrabenazine (P = .045) gauged by the duration of sedation (expressed in minutes) showed an acrophase also occurring around 14⁰⁰: tetrabenazine seems most effective when Injected at the peak of locomotor activity. To account for the non-sinusoidality of the response, harmonic interpolation was applied, yielding a paraphase around 11⁴⁰ HALO (±01⁰⁰). Since rats were studied at different calendar dates, circannual variation was tentatively assessed for locomotor activity prior to R_X and for the response to tetrabenazine as gauged by the duration of sedation. A circannual variation in the latter was associated with a P = 0.16 and a circannual-to-circadian amplitude ratio of ~80% (with an acrophase in the second half of July), describing changes of large extent. A statistically significant circannual rhythm (P <.01) was found for the circadian mesors of locomotor activity, with an acrophase occurring in the second half of January. The limited sample originally collected for other purposes, here opportunistically analyzed, indicates the presence of circannual variation in the endpoint investigated, thus suggesting the need for sampling aimed systematically at a multifrequency rather than solely circadian focus in Chronopharmacology.