Antinociceptive action of morphine and pentazocine on unit activity in the nucleus centralis lateralis, nucleus ventralis lateralis and nearby structures of the cat

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Abstract
Single neuronal activity was recorded extracellularly from the nucleus centralis lateralis (CL), ventralis lateralis (VL) and medialis dorsalis (MD) of the cat thalamus. The majority of the CL, VL and MD neurons were excited by nociceptive stimulation such as pinching the skin with serrated forceps and/or intra-arterial injection of bradykinin. The nociceptive neurons were also driven by non-nociceptive stimulation such as tap of deep tissues, bending hairs with an air-puff and/or joint rotation, and their receptive fields were large. After i.v. administration of either morphine or pentazocine, most nociceptive neurons became unresponsive to nociceptive stimuli, although they were driven by non-nociceptive stimuli. Morphine and pentazocine apparently have a specific antinociceptive action on these nociceptive neurons. I.v. naloxone reversed the antinociceptive action of morphine, but failed to reduce the action of pentazocine. This differentiation has an important functional significance.