Dopamine-Dependent Postnatal Development of Enkephalin and Tachykinin Neurons of Rat Basal Ganglia

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Abstract
The influence of deprivation of the neurotransmitter dopamine (DA) on the development of [Met5]-enkephalin (ME) and substance P (SP) neuropeptide systems of the striatum was investigated in Sprague-Dawley rats. The neurotoxin 6-hydroxydopamine (6-OHDA) was used to induce DA deficiency on postnatal day 3 in rats, and the animals were killed at different postnatal time points until 35 days of age. The levels of ME and SP were determined by radioimmunoassay, and the abundance of preproenkephalin (PPE) and preprotachykinin (PPT) mRNA in the striatum was assessed by Northern blot hybridization analysis. The concentrations of DA, 5-hydroxytryptamine (5-HT), and their acid metabolites were determined by HPLC with electrochemical detection. The postnatal development of the PPE-derived peptide ME and the PPT-derived peptide SP closely paralleled the appearance of the respective mRNAs coding for these peptides. The dopaminergic lesion with 6-OHDA led to a marked depletion of DA and its metabolites but produced an increase in content of 5-HT and its metabolite in the striatum. The lesion did not affect the ME and PPE mRNA levels in the striatum up to 25 days but increased the levels at 35 days. In contrast, a decreased developmental expression in SP and PPT mRNA was observed throughout the observation period. The lesion failed to influence the development of the mRNA coding for the structural protein beta-actin. The results indicate that the normal development of enkephalin, tachykinin, and 5-HT systems of the striatum is dependent on the availability of DA, the integrity of dopaminergic neurons, or both. The studies provide evidence for an interrelationship and interdependence between the development of neurotransmitter and neuropeptide systems. It is suggested that an early developmental abnormality in the DA system could permanently alter the neuropeptide systems, which in turn could influence the progression and expression of the DA-deficiency state parkinsonism, Lesch-Nyhan disease, or both.