Intracerebroventricular self-injection of morphine in response to pain in the rat

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Abstract
Rats were chronically implanted with a lateral cerebral ventricular guide cannula. After recovery they were trained to self-inject morphine sulfate. A week after this training period, self-injection behavior was studied during nociceptive stimulation. In response to a nociceptive stimulation rats increased their intracerebroventricular self-administration of morphine. This effect is specific for morphine since under identical conditions rats did not increase NaCl administration. Naloxone pretreatment inhibits self-administration of morphine in 5 rats. However, two rats increased morphine self-intake. Apparently, rats can directly perceive a decrease of pain sensation induced by the morphine self-administration. This experimental situation is therefore similar to the therapeutic situation in humans, in which the criterion of efficacy of an analgesic is a reduction of the conscious pain sensation. Experimental self-administration during painful stimulation might be used for preclinical evaluation of the efficacy of an analgesic drug.