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Department of Pharmacology, University of Sydney, NSW, Australia.

1. Rat caudal periaqueductal grey (PAG) output neurones containing rhodamine microspheres, retrogradely transported from an injection site in the rostral ventromedial medulla (RVM), were visualized in brain slices and recorded from using whole-cell patch clamp techniques. 2. The specific GABAB receptor agonist baclofen (10 microM) produced an outward current or hyperpolarization in fifty out of fifty-six caudal PAG output neurones. In 44% of these baclofen-sensitive neurones, the opioid agonist methionine enkephalin (30 microM) also produced an outward current or hyperpolarization. The opioid current reversed polarity at -104 mV and could also be produced by DAMGO, an agonist selective for the mu-subtype of opioid receptor. 3. Opioid-responding output neurones were not distributed uniformly in the caudal PAG. In horizontal slices containing lateral PAG, 56% of output neurones were inhibited by opioids, as compared with only 14% of the output neurones in slices containing ventrolateral PAG. 4. These observations are consistent with opioid disinhibition of ventrolateral PAG neurones projecting to the RVM as the predominant mechanism underlying opioid-induced analgesia in the PAG. The role of opioid receptors found on a major proportion of the output neurones in the lateral PAG remains to be established, but is assumed not be related to modulation of nociceptive function.