Involvement of prolactin, vasopressin and opioids in post-ictal antinociception induced by electroshock in rats

Abstract

The neurochemical mechanisms involved in post-ictal antinociception remain to be elucidated. Application of electroconvulsive shock (ECS) to rats results in post-ictal antinociception. The objective of this study was to identify endogenous substances that could participate in antinociception during post-ictal depression induced by ECS (70 mA, 60 Hz, 1 s). Antinociception was measured by the rat paw-pressure test, in which increased sensitivity is induced by intraplantar injection of carrageenan. This test proved to be efficient in detecting the electroshock-induced antinociception. Intense post-ictal antinociception was observed over a period of 30 min after the end of the seizure. It was used nonspecific opioid and specific vasopressin antagonists and the prolactin (PRL) release inhibitor to test the reversal of antinociception. Administration of naloxone (5, 7.5 and 10 mg/kg) blocked the post-ictal antinociception. The V 1 (125 μg/kg) and V 2 (250 μg/kg) vasopressin receptor antagonists ([β-mercapto-β,β-cyclopentamethylenepropionyl 1,O-Et-Tyr 2,Val 4,Arg 8]-vasopressin and [adamantaneacetyl 1,O-Et- d-Tyr 2,Val 4,Abu 6,Arg 8,9]-vasopressin) also inhibited the nociceptive response. The antinociception blockade was more intense after administration of the V 1 receptor antagonist. Bromocriptine (4, 8 and 12 mg/kg) was able to reverse antinociception behavior during the post-ictal period. Morphine (1, 2 and 4 mg/kg), vasopressin (12.5, 100 and 400 μg/kg) and prolactin (100, 200 and 400 μg/kg) administration promoted a higher nociceptive threshold. It was administered the three substances with their respective antagonists to verify the opioidergic pathway and vasopressin and prolactin release interactions, and as a positive control. We observed that the tested mediators were released in an independent manner, indicating no interference in which other.