Biochemical and behavioral anxiolytic-like effects of R(+)HA-966 at the level of the ventral tegmental area in rats.

R(+) HA-966, a weak partial agonist at the glycine/NMDA receptor complex, has been shown to have anxiolytic-like actions on restraint stress-induced mesoprefrontal dopamine metabolism. This study investigates the putative anxiolytic, R(+) HA-966, applied locally at the level of the mesocorticolimbic dopamine cell bodies in the ventral tegmental area (VTA), on the acquisition and expression of conditioned fear. Ten to 14 days after cannula implantation, rats were subjected to the acquisition session (10x5 s tone paired with 0.5 s, 0.8 mA footshock) followed about 24 h later by the expression session (ten tones only) of a conditioned fear protocol. Rats were treated with R(+) HA-966 (15 microg/VTA) or saline before either the acquisition or expression sessions. Other rats were injected with saline or R(+) HA-966 (10 microg/side), intra-medial prefrontal cortex, on the expression day. R(+)HA-966, intra-VTA, prevented stress-induced changes in mesoprefrontal, but not mesoaccumbal, dopamine metabolism and was associated with a reduction in fearful responses to physical (footshock) and psychological (conditioned fear) stressors. Additionally, rats treated with R(+)HA-966 intra-VTA before the acquisition session were less fearful at the beginning of the expression session. Local injection of R(+)HA-966 into medial prefrontal cortex did not have anxiolytic-like behavioral or biochemical actions but diminished the expression of exploratory behavior in non-stress, control rats. These studies indicate that the stress-induced activation of the mesoprefrontal dopamine neurons is necessary for the normal expression of fearful behaviors.