Microinjection of neurotensin (NT) into the rostral ventromedial medulla (RVM) produces dose-dependent antinociception. Here we show that antinociception produced by intra-RVM microinjection of neurotensin (NT) or the selective NT receptor subtype 1 (NTR1) agonist PD149163 can be partially blocked by intrathecal (i.t.) yohimbine, an α2-adrenoceptor antagonist and by methysergide, a serotonin receptor antagonist. Antinociception produced by the NTR2 agonist beta-lactotensin (β-LT) is blocked by intrathecal (i.t.) yohimbine, but not by methysergide i.t. It is not known which noradrenergic cell group is involved in this newly identified noradrenergic component of NTR-mediated antinociception.
These experiments provide the first evidence that selective activation of NTR2 in the RVM produces antinociception. These results also provide evidence that activation of NTR1 in the RVM produces antinociception through spinal release of norepinephrine (NE) and serotonin, and that activation of NTR2 in the RVM produces antinociception mediated by spinal release of NE.
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