Choline acetyltransferase activity increases in the brain stem of rats treated at birth with 6-hydroxydopa

Perinatal Lesioning and Lifelong Effects of the Noradrenergic Neurotoxin 6-Hydroxydopa

6-hydroxydopa (6-OHDOPA) was synthesized with the expectation that it would be able to cross the blood-brain barrier to be enzymatically decarboxylated to 6-hydroxydopamine (6-OHDA), the newly discovered neurotoxin for noradrenergic and dopaminergic neurons. In part, 6-OHDOPA fulfilled these criteria. When administered experimentally to rodents, 6-OHDOPA destroyed peripheral sympathetic noradrenergic nerves and did exert neurotoxicity to noradrenergic nerves in brain-in large part, from its conversion to 6-OHDA. However, the efficacy of 6-OHDOPA was less than that of 6-OHDA; also, 6-OHDOPA was relatively selective for noradrenergic neurons; near-lethal doses of 6-OHDOPA were required to damage dopaminergic nerves; and ultimately, 6-OHDOPA was found to be an agonist at AMPA receptors, thus accounting for more non-specificity. Nevertheless, 6-OHDOPA was found to be a particularly valuable tool in uncovering processes and mechanisms associated with noradrenergic nerve regeneration and sprouting, particularly when administered to perinatal rodents. Also, 6-OHDOPA was a good tool for selective mapping of noradrenergic nerve tracts in brain, since dopaminergic tracts were unaffected and did not interfere with the histofluorescent methodology used for this purpose in the early 1970s. As an experimental research tool, 6-OHDOPA was valuable in a short time-window, but its utility is largely limited because of newer research technologies that provide better means today for nerve tract mapping, and for experimental approaches engaged toward study of processes and mechanisms attending nerve regeneration. AMPA actions of 6-OHDOPA have not been extensively studied, so this avenue may enliven use of 6-OHDOPA in the future.

Neonatal 6-hydroxydopa, but not DSP-4, elevates brainstem monoamines and impairs inhibitory avoidance learning in developing rats

The involvement of brain monoamines in learning and memory in developing rats was studied by comparing the effects of 3 different noradrenergic neurotoxin treatments. Two experimental groups of male Sprague-Dawley rat pups were injected systemically with 50 micrograms/g of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) either on the day of birth or on postnatal days 17-18. Rats in the third experimental group were injected systemically with 60 micrograms/g of 6-hydroxydopa (6-OHDOPA) on postnatal days 0 and 2. Control littermates received vehicle. The animals were trained on an inhibitory avoidance task on postnatal days 27-29 and tested for retention 24 h later. The drug treatments produced comparable depletion of norepinephrine in the hippocampus and frontal cortex. 6-OHDOPA, but neither DSP-4 treatment, significantly elevated brainstem concentrations of norepinephrine and serotonin. In addition, 6-OHDOPA, but not DSP-4, significantly impaired retention of the inhibitory avoidance task. The impairment did not reflect insensitivity to the footshock used in training: both neonatal drug treatments tended to lower, not raise, footshock thresholds, as measured by a flinch test. High affinity choline uptake was not affected by either neonatal drug treatment in any of the brain areas examined. Thus, the 6-OHDOPA-induced behavioral deficit did not involve altered acetylcholine function. The results implicate brainstem monoamines in the modulation of learning and memory during development.

Six-hydroxydopamine induced hyperactivity: neither sex differences nor caffeine stimulation are found

We investigated possible sex differences in the development of locomotor activity in rats treated neonatally with desmethylimipramine (DMI) followed by intraventricular 6-hydroxydopamine (6-HDA). In addition, the locomotor response to the stimulant caffeine was investigated in the male rats after they had reached adulthood. Both male and female 6-HDA-treated rats exhibited increased activity relative to controls. No sex differences were seen in either the development or magnitude of this effect. Male rats were used to determine the dose effects function for caffeine (0.5, 5, 15, 30 mg/kg) on locomotor activity. Control rats exhibited increased locomotor activity whereas 6-HDA-treated rats showed no increases with any dose of caffeine. Large decreases in the dopamine content of the olfactory tubercle (-88%, -82%), nucleus accumbens (-96%, -95%), and striatum (-99%, -99%) were found in both male and female rats. Choline acetyltransferase and glutamic acid decarboxylase activities were unchanged.

Different effects of neonatal vinblastine on peripheral and central noradrenaline neurons

The systemic injection of newborn rats of the mitotic inhibitor vinblastine sulfate (0.25 microgram/s.c. 48 h after birth), produces marked and persistent changes in peripheral sympathetic neurons. Approximately half the neuronal population of the superior cervical ganglia was destroyed already at 16 days of age and this was accompanied by a partial but persistent depletion of noradrenaline (NA) from peripheral organs receiving a rich sympathetic nerve supply such as the heart, salivary glands and spleen. After the systemic injection of vinblastine to newborn rats, the content of NA in several brain regions remained unaltered at 45-60 days of age. To overcome the obstacle that the blood-brain barrier could represent to vinblastine penetration into the brain, the compound was injected directly into the brain of rat pups at 2 days of age (0.25-1.0 microgram). When these animals were killed 45-60 days later, no changes were found in the concentration of NA in the cerebral cortex, the spinal cord or the cerebellum but NA levels were increased in the brain stem. Besides producing a partial but persistent peripheral sympathectomy, vinblastine injected either systemically or intracerebrally to newborn rats, provides a useful tool for the analysis of similarities and differences between the ontogenesis of central and peripheral NA neurons.

Muscarinic hyposensitivity in the developing rat pretreated with 6-hydroxydopa

Atropine-induced locomotor stimulation was investigated in the developing rat pretreated with 6-hydroxydopa at birth. The locomotor stimulation by atropine was first observed on day 20 and gradually increased with age. The treatment with 6-hydroxydopa potentiated atropine-induced locomotor stimulation on days later than day 20 and inhibited a pilocarpine-induced catalepsy on day 30. This suggests that central cholinergic neurons, probably in the neostriatum, reach functional maturity between 15 and 20 days, and that the pretreatment with 6-hydroxydopa induced muscarinic hyposensitivity in the developing rat.

Differential effect of various 6-hydroxydopa treatments on the development of central and peripheral noradrenergic neurons

6-Hydroxydopamine or 6-hydroxydopa injected systemically into newborn rats produced marked changes in the development of central and peripheral noradrenergic neurons. Noradrenaline concentration was elevated in the brain stem, particularly in the pons, and decreased in the cerebral cortex and the spinal cord while in the cerebellum, the effects were dependent on the mode of administration. The changes produced by 6-hydroxydopa in brain regional noradrenaline were related to the dose injected at birth. Similar modifications in the development of central noradrenergic neurons were found in the offspring of rats which had received 6-hydroxydopa at 16 days of gestation. The involvement of peripheral sympathetic neurons varied with the compound used and the form of its administration. Thus, 6-hydroxydopamine produced a permanent although partial peripheral sympathectomy, an effect which was less evident following multiple injections of 6-hydroxydopa after birth and almost minimal after a single injection. The prenatal administration of 6-hydroxydopa did not alter peripheral sympathetic neurons. It is concluded that with the appropriate treatment schedule, it is possible to lesion selectively the noradrenergic neurons in the central nervous system.