Is the pineal involved in the stimulatory influence of prolactin on tail regeneration in lizards? Studies with exogenous prolactin in lizards exposed to continuous darkness

Abstract

We have recently demonstrated that continuous light stimulates tail regeneration in the gekkonid lizard, Hemidactylus flaviviridis, whereas continuous darkness and pinealectomy depress regeneration. As a sequel to this observation, the effect of exogenous ovine prolactin (oPRL) on tail regeneration in normal (NL) and pinealectomized (PX) Hemidactylus exposed to continuous darkness (L:D 0:24) during the monsoon season was investigated. Once-daily intraperitoneal (ip) injections of 500 μg/kg oPRL were administered to a group of NL lizards, a group of PX lizards, and a group of sham-PX (sPX) lizards, 5 days prior to tail autotomy and 50 days afterward. Three groups—PX, sPX, and NL animals—served as the controls and received once-daily ip injections of 0.6% saline. Our observations show that initiation of regeneration, daily growth rate, total length of new growth (regenerate) at the end of regeneration, and total percentage replacement of lost (autotomized) tail were all significantly enhanced in oPRL-treated NL lizards as compared with their saline-treated (NL, PX, and sPX) and oPRL-treated (PX and sPX) counterparts. It is suggested that PRL may be the active factor that speeds up the rate of tail regeneration in lacertilians. The role of the pineal organ in vertebrate photoreception and the possibility that in Hemidactylus the presence of an intact pineal is somehow linked with the favorable influence of PRL on tail regeneration are discussed.