Effects of coadministration of cannabinoids and morphine on nociceptive behaviour, brain monoamines and HPA axis activity in a rat model of persistent pain

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Abstract
The antinociceptive effects of Δ9‐tetrahydrocannabinol (Δ9‐THC) have been widely described; however, its therapeutic potential may be limited by secondary effects. We investigated whether coadministration of low doses of cannabinoids or cannabinoids and morphine produced antinociception in the absence of side‐effects. Effects of preadministration (i.p.) of Δ9‐THC (1 or 2.5 mg/kg), cannabidiol (5 mg/kg), morphine (2 mg/kg), Δ9‐THC + morphine, Δ9‐THC + cannabidiol or vehicle on formalin‐evoked nociceptive behaviour were studied over 60 min. Trunk blood and brains were collected 60 min after formalin injection and assayed for corticosterone and tissue levels of monoamines and metabolites, respectively. Drug effects on locomotor activity, core body temperature and grooming were assessed. Δ9‐THC reduced both phases of formalin‐evoked nociceptive behaviour, enhanced the formalin‐evoked corticosterone response and increased the 4‐hydroxy‐3‐methoxyphenylglycol : noradrenaline ratio in the hypothalamus. Cannabidiol alone had no effect on these indices and did not modulate the effects of Δ9‐THC. Morphine reduced both phases of formalin‐evoked nociceptive behaviour. Coadministration of Δ9‐THC and morphine reduced the second phase of formalin‐evoked nociceptive behaviour to a greater extent than either drug alone, and increased levels of thalamic 5‐hydroxytryptamine. While the antinociceptive effects of Δ9‐THC and morphine alone occurred at doses devoid of effects on locomotor activity, coadministration of Δ9‐THC and morphine inhibited locomotor activity. In conclusion, coadministration of a low dose of morphine, but not cannabidiol, with Δ9‐THC, increased antinociception and 5‐hydroxytryptamine levels in the thalamus in a model of persistent nociception. Nevertheless, these enhanced antinociceptive effects were associated with increased secondary effects on locomotor activity.

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