Involvement of the dopaminergic system in two distinct regions of the hippocampus in the stress-induced antinociceptive responses in rats

Acute stress, as a protective mechanism to respond to an aversive stimulus, can be often accompanied by suppressing pain perception via promoting consistent burst firing of dopamine neurons. Besides, sensitive and advanced research techniques led to the recognition of the mesohippocampal dopaminergic terminals, particularly in the two hippocampal regions, CA1 and dentate gyrus (DG). Previous studies have shown that dopamine receptors within these two hippocampal regions play a critical role in antinociceptive responses induced by forced swim stress (FSS) in the presence of inflammatory pain. Since various mechanisms and transmitter systems can be triggered by different pain states, the present experiments were defined to investigate whether dopaminergic receptors within the CA1 and DG regions have the same role in the presence of acute thermal pain. Ninety-seven adult male albino Wistar rats underwent stereotaxic surgery, and a stainless steel guide cannula was unilaterally implanted 1mm above the CA1 and DG regions of the hippocampus. Different doses of SCH23390 or Sulpiride as D1- and D2-like dopamine receptor antagonists were microinjected into these two regions 5-10min before exposure to FSS, and 5 min after FSS exposure, the tail-flick test evaluated the effect of stress on the nociceptive response at the time-set intervals. The results demonstrated that exposure to FSS could significantly increase the acute pain perception threshold, while intra-DG and –CA1 administrations of SCH23390 and Sulpiride reduced the antinociceptive effect of FSS in the tail-flick test. It seems the dopaminergic receptors within these two regions play a significant role in FSS-induced analgesia.