Neurophysiology, Vol. 31, No. 5, 1999 Effects of a Delta Opioid Receptor Agonist and Inhibitors of Enkephalin Catabolism on Periaqueductal Gray Neurons in the Rat Midbrain: an in vitro Study A. I. Pilyavskii, 1 N. V. Bulgakova, 1 R. Maldonado, 2 B. P. Roques, 2 and V. A. Yakhnitsa 1 Neirofiziologiya/Neurophysiology,Vol. 31, No. 5, pp. 386-394, September-October, 1999. Received February 16, 1998. Effects of a selective agonist of delta-opioid receptors, BUBU, ~uad mixed inhibitors of enkephalin-degrading enzymes, RB-38A and RB-101, on the backgound and evoked activity of periaqueductal gray (PAG) neurons were studied on slices of the rat midbrain. The tested drugs could either activate or depress spontaneously active neurons, or exert no influence on these units. ':Silent" neurons generated no responses to application of the above compounds. In 80.5% of the studied neurons, 1 ~M BUBU evoked depression, while 6.5 ~tM RB-38A, 0.65 mM RB-38A, and 10 ~tM RB-101 depressed 68.8%, 73.7%, and 68.8% of the neurons, respectively. Facilitatory effects were induced by the above drugs in 5.0%, 5.9%, 15.8%, and 6.2% of the cells under study, respectively. The effects of RB-38A and RB-101 differed from the BUBU-evoked responses in a shorter latency and rise time and a higher intensity. Repeated application of RB-38A provided no significant changes in the duration and intensity of the inhibitory effects. Co-administration of RB-38A and BUBU, or BUBU and RB-38A intensified suppression of the neuronal discharges without considerable changing their duration. It is concluded that administration of inhibitors of enkephalin-degrading enzymes to in vitro midbrain preparations creates a pharmacologi- cally effective level of endogenous enkephalins, which exerts specific inhibitory influence on spontaneously active PAG neurons. The data are in agreement with the supposition on the existence of tonic release of enkephalins in midbrain slice preparations. INTRODUCTION It is now well documented that endogenous opioids (especially enkephalins) play a determining role in the mechanisms of pain control on the spinal and supraspinal levels [1-3]. Progress in synthesis of selective and mixed inhibitors of neutral endopeptidase 24.11 (NEP) and ami- nopeptidase N (APN) opened a new approach for the studies of the effects of endogenous enkephalins in nerve tissues [4, 5]. Intracerebral or intravenous administration of pepti- dase inhibitors produces analgesic effects [6] and attenuates the behavioral expression of the morphine withdrawal I Bogomolets Institute of Physiology National Academy of Sciences of Ukraine, Kyiv (Ukraine). 2 Universite Rene Descartes, Paris (France). 316 syndrome [7, 8]. As compared with the effects of applica- tions of p.-receptor and even 8-receptor agonists, administra- tion of inhibitors of enkephalin-degrading enzymes is accom- panied by only partial tolerance and moderate physical dependence [7]. Microinjection studies showed that exo- genous enkephalins exert naloxone-reversible inhibitory in- fluences on spinal [9], cortical [10], and brainstem [11] neurons; some excitatory effects of enkephalins were also reported [12]. Inhibition of spinal nociceptive neurons was observed after direct application of nonspecific inhibitors of APN and NEP (bestatin, thiorpharm, and kelatorpharm) to the lumbar segments of the rat spinal cord [13, 14]. The periaqueductal gray (PAG) region is known to contain immunoreactive enkephalin-containing neurons [15] mid is characterized by high densities of opiate-binding sites and receptor mRNA [16], as well as by high concentrations of NEP and APN [17]. Local injections of morphine [18, 00c,'0-2977/tr')/3104-0316522.00 91999Kluwer Academic/Plenum Publishers