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Operant behavioural and neurochemical effects after neonatal 6-hydroxydopamine treatment
Insights
Neonatal treatment with 6-hydroxydopamine (60HDA) in rats increased operant response rates on a fixed ratio schedule. This behavioral change may stem from hippocampal noradrenaline neuron destruction or pons-medulla neuron regeneration.
Area of Science:
- Neuroscience
- Neuropharmacology
- Behavioral Neuroscience
Background:
- Neonatal exposure to neurotoxins can induce long-lasting alterations in brain development and function.
- 6-hydroxydopamine (60HDA) is a neurotoxin that selectively damages catecholaminergic neurons, particularly dopaminergic and noradrenergic ones.
Purpose of the Study:
- To investigate the long-term effects of neonatal 6-hydroxydopamine (60HDA) administration on operant behavior and neurochemistry in rats.
- To determine if 60HDA-induced neurochemical changes correlate with specific behavioral alterations.
Main Methods:
- Newborn rats received subcutaneous injections of 60HDA (100 mg/kg) or vehicle on postnatal days 1 and 2.
- Operant behavioral testing (fixed ratio 30, extinction, progressive ratio, variable interval schedules) was conducted starting at 6 months of age.
- Biochemical analysis of noradrenaline (NA) levels and tyrosine hydroxylase activity in brain regions (cerebral cortex, hippocampus, pons-medulla) was performed at 9 months of age.
Main Results:
- Neonatal 60HDA treatment significantly increased response rates on a fixed ratio 30 schedule, primarily by reducing post-reinforcement pauses.
- Behavior on extinction, progressive ratio, and variable interval schedules remained unaffected by 60HDA treatment.
- Biochemical assays revealed decreased NA levels in the cerebral cortex and hippocampus, but doubled NA content in the pons-medulla. Tyrosine hydroxylase activity was largely unchanged, with a slight decrease in the hippocampus.
Conclusions:
- Neonatal 60HDA exposure induces specific alterations in operant behavior, particularly on fixed ratio schedules.
- The observed behavioral changes are likely linked to the neurochemical consequences of 60HDA, including noradrenaline (NA) depletion in certain brain areas and potential regeneration in others.
- The findings suggest a complex interplay between hippocampal NA neuron integrity and brainstem catecholamine systems in modulating operant response patterns.
Abstract:
Newborn rats were treated at 1 and 2 days after birth with 100 mg/kg 6-hydroxydopamine (60HDA), s.c. Testing on several operant behavioural tasks was begun at 6 months of age. On a fixed ratio 30 (FR 30) schedule of food reinforcement, the neonatal 60HDA treated rats responded at a significantly higher rate. Further analysis of the FR 30 response pattern indicated that the higher rate was due to a decrease in the amount of time spent pausing after the receipt of each reinforcer. The 60HDA treatment failed to alter the rat's behaviour during the extinction of the FR 30 response and on the progressive ratio or variable interval schedules of food reinforcement. Biochemical analysis of several brain areas at 9 months of age showed a decrease in noradrenaline (NA) levels in the cerebral cortex and hippocampus, while in the pons-medulla NA content was doubled. The tyrosine hydroxylase activity in these same brain areas was not significantly altered, but there appeared to be some decrease in the activity of this enzyme in the hippocampus. Comparison of the operant behavioural effects seen after various lesioning procedures in this and other studies, suggest the effects on FR performance are a result of destruction of NA neurons in the hippocampus and/or the apparent regeneration of neurons in the pons-medulla.