Development of dopamine autoreceptors in the postnatal rat brain

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Abstract
The behavioural and biochemical effects of racemic 3-PPP (3-[3-hydroxyphenyl]-N-n-propyl-piperidine) and its enantiomers was studied in developing rats, aged 1–28 days. All three compounds exhibit dopamine (DA) autoreceptor-stimulating properties. Moreover, the (+)-enantiomer displays agonist and the (−)-enantiomer antagonist actions, respectively, on the postsynaptic DA receptor. This means that the racemate has a DA autoreceptor stimulatory action with slight or no effects on the postsynaptic receptor. Locomotor experiments demonstrated that (±)-3-PPP inhibited spontaneous locomotor activity dosedependently in the 28 days old rats. No effects were seen in the age groups 14 days and younger. While the racemate and the (−)-enantiomer inhibited spontaneous locomotor activity in 28 days old rats, the (+)-enantiomer had no effects compared to saline. Interestingly, the (+)-enantiomer increased locomotor activity in the 4 days old rats, while the (−)-enantiomer and the racemate did not induce any effects. In the biochemical experiments, after blockade of DA neurotransmission with gamma-butyrolactone (GBL), (±)-3-PPP inhibited the increase in tyrosine hydroxylase activity (DOPA accumulation after NSD 1015) after GBL in the DA rich striatum region of the 28 days but not of the 4 days old rats. From these experiments it may be concluded that functional postsynaptic but not presynaptic DA receptors exist in the brain af 4 days old rats. Presynaptic DA receptors do not seem to be functionally mature until 28 days postnatally in the rat, i.e. during adolescent age.