Pineal involvement in the diurnal rhythm of nociception in the rat

Male Wistar rats under cyclic lighting conditions (LD 12:12) were tested for tail flick latencies. A day-night rhythm of pain sensitivity was clearly demonstrated; response latencies were longest 2 hrs. before 'lights on' (-2 hrs.) and shortest 4 hours into the light phase (+4 hrs.). Hot plate data conformed to the tail flick results and supported the notion that the light-dark cycle cues were responsible for the observed diurnal rhythm of analgesia. The possible involvement of the pineal was studied on rats under LD 12:12 schedules, using two paradigms: (1) Functional pinealectomy by light induced suppression and (2) Surgical pinealectomy. The difference between hot plate response latencies measured at '-2 hrs.' and '+4 hrs.', was reduced when the analgesia tests were preceded by either functional pinealectomy or surgical removal of the pineal gland. The data indicates that the pineal gland is involved in modulation of the baseline diurnal rhythm of analgesia in the rat.