The role of neurotransmitters in stress-induced antinociception (SIA).

The role of the neurotransmitters, norepinephrine, dopamine and serotonin in stress-induced antinociception (SIA) was examined by altering neurochemical tone with appropriate pharmacological tools. Quipazine (15.0 mg/kg, IP) a serotonin agonist, increased the peak and duration of antinociception following stress and BC-105 (3.0 mg/kg, IP), a serotonin antagonist, blocked the increase of tail-flick latency following stress. Clonidine (0.1 mg/kg, SC) an alpha 2 agonist, markedly decreased SIA whereas phenoxybenzamine (2.5 mg/kg, IP), an alpha 1 antagonist, increased the peak and duration of SIA. When dopaminergic tone was increased with apomorphine (0.05 mg/kg, SC) the increase of tail-flick latency after stress was markedly attenuated whereas blockage of dopamine receptors with haloperidol (2.5 mg/kg, IP) increased the peak and duration of SIA. Alterations of serotonergic, but not noradrenergic or dopaminergic, tone had similar effects on increased latency in tail-flick test produced by brain stimulation produced analgesia (SPA), morphine and SIA. These data support the hypothesis that alterations in tail-flick latency involves a serotonergic system.