ENHANCEMENT OF SPATIAL ATTENTION IN NOCICEPTIN ORPHANIN FQ RECEPTOR KNOCKOUT MICE

We isolated genes for the opioid receptor homologue MOR-C, namely noci ceptin receptor (designated alternatively as orphanin FQ receptor) and generated nociceptin receptor-knockout mice. Previously, we have repo rted that the nociceptin system appears to participate in the regulati on of the auditory system. However, the behavior of the nociceptin rec eptor-knockout mice has yet to be fully characterized. In the present study, we investigated changes in several behavioral performances in m ice which lack nociceptin receptor. Nociceptive thresholds of nocicept in receptor-knockout mice were unchanged in the hot-plate and electric foot-shock tests as well as tail-flick and acetic acid-induced writhi ng tests compared to those of wild-type mice. The nociceptin receptor- knockout mice did not show any behavioral changes in the elevated plus -maze task. Surprisingly, in the water-finding test, the nociceptin re ceptor-knockout mice showed an enhanced retention of spatial attention (latent learning) compared to wild-type mice. In a biochemical study, dopamine content in the frontal cortex was lower in nociceptin recept or-knockout mice than wild-type mice. These results suggest that nocic eptin receptor plays an important role in spatial attention by regulat ing the dopaminergic system in the brain. (C) 1998 Elsevier Science B. V.