Quantitative differences in the circadian rhythm of locomotor activity and vasopressin and vasoactive intestinal peptide gene expression in the suprachiasmatic nucleus of tau mutant compared to wildtype hamsters

Abstract

The activity profiles of homozygous tau mutant hamsters bred in our colony exhibit several differences when compared to wildtype golden hamsters. In addition, tau mutant hamsters respond to saturating white light pulses presented between circadian time (CT) 11 and CT 16 with extremely large phase shifts (type 0 resetting) after prolonged time in constant darkness. We measured five parameters of the activity rhythm early during exposure to constant darkness (DD) (cycles 5–9), and after 44–48 cycles in DD, and we confirmed the tau mutants' unusual phase shifting response to light. Next we determined whether neurotransmitter peptide mRNA levels in the SCN differed between wildtype and tau mutant hamsters exhibiting these divergent activity patterns and responses to light. After 49 circadian cycles in DD, tau mutant hamsters responded to a 1 h light pulse at CT 15 with phase shifts averaging 10.19 ± 0.35 h. Among wildtype hamsters the mean phase shift was 1.22 ± 0.34 h and the largest phase shift observed was 3.67 h. Total wheel revolutions/circadian cycle were significantly lower in tau mutants (4022 ± 1103) vs. wildtypes (7528 ± 458) and there was a significant decrease in wheel-running activity after prolonged exposure to DD, particularly among the wildtype hamsters (tau = 3045 ± 972, wildtype = 4362 ± 388 rev/circadian cycle). When analyzed by 5 min segments throughout the circadian cycle, the highest intensity wheel-running activity did not differ between groups and there was no significant effect of length of time in DD on this measure (tau = 38.5 ± 6.3 and 38.4 ± 4.7 rev/min, wildtype = 46.8 ± 1.7 and 41.4 ± 2.7 rev/min early or late in DD, respectively). The precision of activity onset differed greatly between groups with tau mutants exhibiting a much higher daily deviation from mean τ (1.00 ± 0.24 h) than wildtypes (0.14 ± .02 h). Activity onset became significantly less precise with increased time in DD: tau = 1.66 ± 0.21 h, wildtype = 0.45 ± 0.14 h after 44–48 circadian cycles. The length of the active period, a, was significantly shorter in tau mutants than in wildtypes (7.2 ± 0.2 h vs. 8.0 ± 0.2 h) but a was a similar percentage of τ in the two groups (tau mutant = 36%, wildtype = 33%). After 48 circadian cycles in DD, a measured 7.2 ± 0.5 h in tau mutants and 8.9 ± 0.6 h in wildtypes, thus there was no significant effect of time in DD on this parameter. Activity records of tau mutant animals appear more fragmented to the eye and we quantitated this with a computer-aided analysis of the number of bouts of wheel-running per active period. Wildtype hamsters exhibited 2.8 ± 0.2 bouts of wheel-running activity early in DD and 3.1 ± 0.2 bouts per circadian cycle later in DD. The activity records of tau mutant hamsters were significantly more fragmented but this group actually showed some consolidation of bouts per circadian cycle after prolonged time in DD (4.7 ± 0.3 vs. 3.9 ± 0.3 bouts per cycle). Wildtype and tau mutant hamsters were killed after 66–71 cycles in DD at either CT 4 or CT 16 and in situ hybridization was performed for vasopressin (AVP) and vasoactive intestinal peptide (VIP). Levels of AVP and VIP mRNA were significantly lower in tau mutant than wildtype hamsters at CT 16. We conclude that the tau mutation causes these differences in gene expression and we speculate that differences in the peptidergic output of the clock may have some relevance for the differences in the quantitative aspects of the activity rhythm and the response to light pulses exhibited by these animals.