Effects of L-dopa on self-stimulation and brain biogenic amines in rats

Effects of various doses of L-DOPA were tested on self-stimulation behaviour in rats with electrodes implanted in the posterior hypothalamus (PH) or area ventralis tegmentum (A10 area) as well as on the concentration of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in the caudate nucleus (CN), diencephalon-midbrain (DM) and pons-medulla (PM). L-DOPA (50, 100 and 150 mg/kg. i.p.) showed a biphasic action on self-stimulation responding with an initial decrease for 40–60 min, followed by an increase for about an hour or more in rats with both PH and A10 electrodes. Both effects were more marked in rats with A10 electrodes. The effects of these doses on spontaneous motor activity were not significant. L-DOPA markedly increased DA levels in the CN and DM, and moderately decreased 5-HT, without significantly altering NE levels in the DM and PM. It appears that L-DOPA-induced changes in self-stimulation responding cannot be correlated with the increase of DA levels in the CN and DM. It is possible that this behaviour involves a system of several neurotransmitters, including biogenic amines and acetylcholine, and a sudden increase of one, such as DA, causes an imbalance in the multitransmitter system and disrupts the behaviour.