Striatal dopamine and motor activity changes observed shortly after lithium administration

Abstract

Lithium chloride was given to rats i.p. at single doses of 2 and 10 meq/kg, respectively. It produced a suppression of motor activity and an increase in the dopamine content of the striatum. The magnitude of these effects were dose- and time-dependent as well as transient in nature. After 60 min of injection, the higher dose (10 meq/kg) reduced motor activity by 67% and increased striatal dopamine content by 56% while the lower dose (2 meq/kg) reduced motor activity by 42% and elevated striatal dopamine by 36%. These effects vanished 24 h after administration regardless the dose employed. None of the two doses of LiCl altered either dopamine biosynthesis in vivo (measured as the accumulation of a precursor of synthesis after decarboxylase inhibition), or the activity of tyrosine hydroxylase ex vivo under subsaturating conditions (i.e. enzyme activity in the tissues obtained from the animals post mortem). An increased deamination of tyramine by monoamineoxidase (MAO) was found in striatal homogenates after 60 min of the injection of 2 or 10 meq/kg of LiCl. This was due to a lower Km for the substrate as revealed by kinetic studies. LiCl treatment did not change the proportion of MAO A:B. As neither dopamine synthesis was increased nor the activity of the catabolic enzyme MAO was reduced (but it was oppositely enhanced), the increment in striatal dopamine content might have likely resulted from a reduced release and/or an increased amine reuptake by the neurons. We postulate that the reduced motor activity observed shortly after injection of LiCl would be related to an interference with striatal dopaminergic neurotransmission.