Research ReportImmunocytochemical evidence for different patterns in daily rhythms of VIP and AVP peptides in the suprachiasmatic nucleus of diurnal Funambulus palmarum

The suprachiasmatic nucleus (SCN) is the principal pacemaker that coordinates circadian rhythmicity in mammals. The studies on understanding the circadian system in diurnal rodents are limited. In this study, we have used the 3 striped South Indian Palm Squirrel (Funambulus palmarum). The locomotor activity showed a diurnal pattern of activity in LD 12:12, constant darkness (DD) and light (LL) conditions with circadian periods (τ) of 24.19 ± 0.1, 24.11 ± 0.03 and 24.92 ± 0.35 h respectively. Anatomical study of the brain revealed that this animal had short, thick and stout optic nerves with SCN elliptical in shape with a higher neuronal population as distinct from nocturnal rodents. Since the neuropeptides, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP) play important roles in photic entrainment and relay of information respectively in nocturnal rodents, we studied the distribution and daily rhythms of VIP-ir and AVP-ir in squirrel SCN. The VIP-ir and AVP-ir cells in the SCN showed a ventrolateral and dorsomedial distribution with daily rhythmicity in their levels. The peak time of VIP-ir rhythm was found ahead of AVP-ir. The VIP-ir levels were higher for longer duration than AVP-ir levels. The maximum and minimum VIP-ir levels were at ZT-6 and ZT-0 respectively and AVP-ir levels at ZT-12 and ZT-0 respectively. Thus, VIP and AVP maximum and minimum levels appeared 6 and 12 h apart respectively in squirrel, though 12 and 8 h apart in rat. These findings in the present report could be a step towards underpinning the mechanisms regulating diurnality.

Research Highlights►The locomotor activity showed a diurnal pattern of activity in LD 12:12, constant darkness (DD) and light (LL) conditions with circadian period (τ) of 24.19 ± 0.1, 24.11 ± 0.03 and 24.92 ± 0.35 h respectively. ►Diurnal squirrel has high neuronal count as distinct from nocturnal rodents. ►VIP and AVP-ir showed daily rhythmicity in their levels in SCN. ►VIP maximum and minimum levels delay by 6 and 12 h as compared to rat. ►AVP maximum and minimum levels delay by 8 and 12 h as compared to rat.