Morphine-induced modification of the functional properties of ventral tegmental area neurons in conscious rat

To elucidate the functional role of mesolimbocortical dopamine(DA)-containing and other mediator-specific neurons in mediation of different pharmacological effects of opiates, morphine (6 mg/kg, SC)-induced modification of spontaneous discharges of different types VTA-units and their changes during polymodal activating and aversive stimulation and spontaneous movement were studied in conscious, restrained rats. In presumed DA-containing neurons (D-type) prolonged increase of discharge rate, regularization of their pattern and decrease of firing changes during all types of external stimulation and spontaneous or stimulus-induced movement activity were found. This decrease of unit firing changes was connected with significant rising of atypical, compared with control conditions, firing inhibitions. Changes of presumed acetylcholine(ACh)-containing neurons (A-B type) properties were defined as prolonged and pronounced decrease of firing rate, intensification of bursting pattern and decrease of discharge changes during all types of stimulation used and animal movements. This decrease of discharge changes was associated with significant lowering of activations-typical for these units in control conscious animals and rising of firing inhibitions. In presumed ACh- or GABA-containing interneurons of C-type a significant decrease of firing rate, increase of activations and decrease of firing inhibitions, typical for these units in control conscious rats were found. The modifications of different VTA-unit functional properties are discussed in view of main pharmacological effects of opiates (sedation, analgesia, positive reinforcement, movement activation).