G. Tosini et M. Menaker, THE PINEAL COMPLEX AND MELATONIN AFFECT THE EXPRESSION OF THE DAILY RHYTHM OF BEHAVIORAL THERMOREGULATION IN THE GREEN IGUANA, Journal of comparative physiology. A, Sensory, neural, and behavioral physiology, 179(1), 1996, pp. 135-142
Daily variation in the body temperature of the green iguana (Iguana ig
uana) was studied by telemetry in laboratory photo-thermal enclosures
under a 12Light:12Dark (L:D) photoperiod. The lizards showed robust da
ily rhythms of thermoregulation maintaining their body temperatures (T
-b) at higher levels during the day than during the night. Some animal
s maintained rhythmicity when kept in constant darkness. On light:dark
cycles parietalectomy produced only a transient increase of median T-
b in the first or second night following the operation. Pinealectomize
d lizards on the other hand maintained their body temperatures at sign
ificantly lower levels during the day and at significantly higher leve
ls during the night than did sham-operated or intact lizards. This eff
ect was apparently permanent, since one month after pinealectomy lizar
ds still displayed the altered pattern. Plasma melatonin levels in int
act animals were high during the night and low during the day and were
unaffected by parietalectomy. Pinealectomized lizards showed low leve
ls of plasma melatonin during both the day and the night. A daily intr
aperitoneal injection of melatonin in pinealectomized animals given a
few minutes after the light to dark transition decreased the body temp
eratures selected by the lizards during the night and increased the bo
dy temperatures selected during the following day. Control injections
of saline solution had no effect. The significance of these results is
discussed in relation to the role of the pineal complex and melatonin
in the mediation of thermoregulatory behavior.