Several lines of evidence have shown that stimulation of the lateral hypothalamus (LH) can induce antinociception. It has been indicated that hypothalamic orexinergic neurons send projections throughout the dopamine mesolimbic pathway. Functional interaction between the LH and the main area of the mesolimbic pathway such as the ventral tegmental area (VTA) and the nucleus accumbens (NAc) implicates in pain modulation. Thus, in this study, we investigated the role of D2-like dopamine receptors within the VTA and NAc in the LH stimulation-induced antinociception. Male Wistar rats weighing 230-280 g were unilaterally implanted with two separate cannulae into the LH and VTA or NAc. Animals received intra-VTA (0.25, 1 and 4 μg/0.3 μl DMSO) and intra-accumbal (0.125, 0.25, 1 and 4 μg/0.5 μl DMSO) infusions of sulpiride as a selective D2-like receptor antagonist, prior to intra-LH carbachol (125 nM/rat) administration. In the tail-flick test, the antinociceptive effects were measured using a tail-flick algesiometer and represented as maximal possible effect (%MPE) within 5, 15, 30, 45 and 60 min after injections. Our results showed that intra-VTA and intra-accumbal sulpiride dose-dependently attenuated the LH stimulation-induced antinociception. However, the blockade of D2-like receptors within the NAc was more significant than that of the VTA. These findings show that D2-like dopamine receptors in these regions play an important role in the LH-mediated modulation of nociceptive information in the acute model of pain in the rats. It seems that this pain modulating system is more relevant to D2-like receptors in the nucleus accumbens.
Keywords: Dopamine D2-like receptors; Lateral hypothalamus; Nucleus accumbens; Pain; Rat; Ventral tegmental area.
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