Morphine, administered in the first conditioning trial, did not enhance locomotor activity of GluA1−/− mice, whereas it enhanced the activity of GluA1+/+ mice. GluA1−/− mice displayed increased habituation compared to GluA1+/+ mice. It is possible that habituation to conditioning cages might have masked the locomotor-activating effect of the first morphine dose. In support to this, the lack of the morphine effect was transient and in the second conditioning trial the morphine effect was observed also in GluA1−/− mice. It is also possible that the lack of AMPA receptor adaptation in GluA1−/− mice might have increased the sedative effect of the first dose of morphine and thereby prevented opioid-induced hyperactivity.
