Studies on brain monoamine and neuropeptide systems after neonatal intracerebroventricular 6-hydroxydopamine treatment.

In order to study the effects of a neonatal dopamine lesion on dopaminergic, serotonergic and peptidergic systems, Sprague-Dawley rats were treated by intracerebroventricular administration of 6-hydroxydopamine (100 micrograms, days 3 and 6) following desipramine pretreatment (25 mg/kg s.c.). At 60-70 days postnatally a profound reduction of dopamine- and 3,4-dihydroxyphenylacetic acid levels was found in striatal and limbic forebrain regions concomitant with an extensive loss of tyrosine hydroxylase-immunoreactive fibers, while no significant alteration in noradrenaline levels was seen. A marked loss of tyrosine hydroxylase-immunoreactive cell profiles was also observed in the substantia nigra and ventral tegmental area in mesencephalon. In striatum, but not in other regions analysed, an almost 100% increase in serotonin levels and serotonin-immunoreactive fiber density was observed following 6-hydroxydopamine treatment. However, the number of serotonin-immunoreactive cell profiles in the median and dorsal raphe nuclei was not altered. The 6-hydroxydopamine treatment also led to reductions in substance P levels in striatum, nucleus accumbens and ventral mesencephalon. The cholecystokinin level in nucleus accumbens and neurotensin level in ventral mesencephalon were also reduced. A neonatal intracerebroventricular 6-hydroxydopamine treatment thus leads to a lesion of dopamine neurons in the mesencephalon with extensive loss of dopamine fibers in several forebrain areas, while localized serotonin fiber sprouting is induced in striatum. Furthermore, concomitant reductions of the levels of peptides related to the dopamine system occur following the 6-hydroxydopamine treatment. Behavioral disturbances such as hyperactivity and cognitive deficiencies occurring after a dopamine lesion early in life might therefore be due to plastic alterations in several different transmitter/neuromodulator systems as a direct or indirect consequence of the lesion.