Abstract
The present experiments were performed to investigate the differential involvement of the opioid receptor subtypes in the antinociception of intrathecal (IT) butorphanol compared to IT morphine. A single dose (26 nmol) of IT nor-binaltorphimine (nor-BNI), β-funaltrexamine (β-FNA), and naltrindole (NTI) demonstrated a significant attenuation in the overall antinociception of IT butorphanol (52 nmol) or IT morphine (26 nmol). However, IT butorphanol elicits thermal antinociceptive effect through κ > δ ⩾ μ, whereas morphine acts on μ > δ >> κ. These results indicate that the antinociceptive effect of both IT butorphanol and IT morphine are mediated through μ, δ, and κ opioid receptors in different relative orders.
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