Effect of melatonin injection into the periaqueductal gray on antinociception and tonic immobility in male rats.

Melatonin (MLT) is a neurohormone with significant involvement in several biological functions, of which antinociception and tonic immobility (TI) may be the key neurobehavioral components to survive in adverse conditions such as a predator attack. TI-induced antinociception can be elicited, facilitated, or increased through opioid and γ-aminobutyric acid (GABA) among other chemical mediators at several levels of the central nervous system, mainly in the periaqueductal gray (PAG). The aim of this study was to assess the effect of the microinjection of MLT into the main PAG regions that are related to different integrated defensive responses, namely dorsal (D) and ventrolateral (VL), on both antinociception through the tail-flick (TF) test and TI duration as single behavioral response and on combined behavioral responses (TF/TI).