Unilateral and bilateral nucleus basalis lesions: differences in neurochemical and behavioural recovery

Pre- and post-synaptic cholinergic dysfunction in aged rodent brain regions: new findings and an interpretative review

Age-related impairment of dynamic aspects of central cholinergic neurotransmission has been indicated by many studies of aged rodents, but the regional distribution of cholinergic deficits and the relative contribution of presynaptic hypofunction and reduced acetylcholine release, loss of synaptic integrity or loss of muscarinic receptors remains unclear. This study therefore compared choline acetyltransferase activity (as a structural marker) and sodium-dependent high affinity choline uptake (which reflects both ongoing cholinergic neuronal activity and structural integrity) in the hippocampus, cortex and straitum of male C57BL mice at 3-4, 10-12 or 28-32 months of age. To evaluate the relationship of changes in muscarinic receptors to presynaptic alterations, binding of the antagonist 3H-quinuclidinyl benzilate was compared in membranes prepared from each of these brain regions. High affinity choline uptake was significantly reduced in all three brain regions by 28-32 months of age. This trend was already evident by 10-12 months of age, especially in hippocampus and cortex. By contrast, choline acetyltransferase activity was unchanged in striatum and actually increased in hippocampus and cortex of aged mice. Muscarinic binding was reduced significantly only in striatum and this effect was significant by 10-12 months of age. This decrease in antagonist binding was accompanied by a small but significant reduction in the relative proportion of high affinity agonist sites as defined by carbachol displacement. The impairment of high affinity choline uptake in the absence of a parallel reduction of choline acetyltransferase activity suggests a decline of ongoing cholinergic activity rather than loss of terminal integrity as the basis of presynaptic deficits in aging. This functional decline may be exacerbated by reduction of muscarinic receptors in striatum. Despite considerable literature support for the hypothesis that cholinergic mechanisms are impaired with age, several controversies leave important issues unresolved. Therefore, the present results are discussed in the context of a critical review with emphasis on dynamic properties of presynaptic function which require analysis in experimental animal models. The impact of normal aging on brain cholinergic systems is distinguished from the neurodegenerative changes in Alzheimer disease in that presynaptic function is compromised with a relative preservation of the integrity of innervation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of nerve growth factor and GM1 ganglioside on the number and size of cholinergic neurons in rats with unilateral lesion of the nucleus basalis

Four groups of rats with a unilateral ibotenic acid lesion of the nucleus basalis were treated with saline, nerve growth factor (NGF) 10 micrograms administered intracerebroventricularly twice per week, sialoganglioside GM1 30 m/kg daily i.p. and NGF twice per week plus GM1 10 mg/kg i.p. daily, respectively, beginning immediately after lesioning. Twenty-one days later the rats treated with saline showed a marked impairment in negotiating a 'step through' passive avoidance conditioned response, a 32% decrease in the number of choline acetyltransferase (ChAT)-positive neurons in the lesioned nucleus basalis and a 12% decrease in their areas. The rats treated with NGF and NGF plus GM1 showed no difference from sham-operated rats. In the GM1-treated rats a 12% decrease only in the number of ChAT-positive neurons was detected while performance and neuronal areas were normal. These findings indicate that NGF and GM1 prevent the cholinergic deficit by protecting the cholinergic neurons of the nucleus basalis from ibotenic acid neurotoxicity.

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.

Bilateral changes in neocortical [3H]pirenzepine and [3H]oxotremorine-M binding following unilateral lesions of the rat nucleus basalis magnocellularis: an autoradiographic study

Neocortical choline acetyltransferase (ChAT) activity and muscarinic [3H]pirenzepine, [3H]oxotremorine-M, [3H]N-methylscopolamine ([3H]NMS; both high- and low-affinity agonist (carbachol) sites) and nicotinic [3H]acetylcholine binding were assessed both ipsi- and contralaterally 1 week and 13 weeks after unilateral ibotenic acid lesions of the rat nucleus basalis magnocellularis (NBM). Ipsilateral ChAT activity was reduced to 49% of control values 1 week postlesion but by 13 weeks had recovered to 80% of control values. Contralateral ChAT activity did not change significantly at either 1 week or 13 weeks postlesion. At 1 week postlesion, [3H]oxotremorine-M binding was increased by 33% and 54% in ipsilateral and contralateral neocortex, respectively. By week 13, both ipsi- and contralateral [3H]oxotremorine-M binding had returned to normal but [3H]pirenzepine binding was significantly decreased by 31% and 39% in the ipsilateral and contralateral hemispheres, respectively. The binding of [3H]NMS and [3H]acetylcholine did not differ significantly from control values at either 1 week or 13 weeks postlesion. These data suggest that none of the cholinergic binding sites studied is preferentially localized presynaptically and that there may be interhemispheric regulation of neocortical cholinergic binding sites.

GM1 ganglioside counteracts cholinergic and behavioral deficits induced in the rat by intracerebral injection of vincristine

The intracerebroventricular injection of 0.5 mg of vincristine sulphate in adult male Wistar rats caused within 11 days the impairment of motor and reflexive behavior, evaluated by the elevated platform and hanging wire tests, a decrease in food consumption and loss of body weight, a 45% decrease in hippocampal choline acetyltransferase (ChAT) activity and a 35% decrease in the rate of high-affinity choline uptake (HACU) in the injected side. The latter effects are due to the death of neurons in the respective hemiseptum. Intrafimbrial injection of vincristine caused the same decrease in ChAT activity without behavioral alterations. Daily i.p. administration of GM1 ganglioside, beginning immediately after the vincristine injection, prevented dose dependently the decrease in ChAT activity and HACU rate. Prevention was complete with the 60 mg/kg dose. The same dose was equally active on ChAT activity when given s.c. but was inactive p.o. The ChAT decrease was also prevented when GM1 treatment began 5 days after vincristine. GM1 60 mg/kg i.p. also reduced the behavioral toxicity of vincristine. The possibility that GM1 might prevent vincristine toxicity by antagonizing its disruption of neurofilaments and axonal flow is discussed.

Interaction between nerve growth factor and GM1 monosialoganglioside in preventing cortical choline acetyltransferase and high affinity choline uptake decrease after lesion of the nucleus basalis

Monosialoganglioside GM1 and nerve growth factor (NGF) were administered alone or concomitantly to adult male rats with a unilateral ibotenic acid lesion of the nucleus basalis magnocellularis (NBM). High-affinity choline uptake (HACU) rate and choline acetyltransferase (ChAT) activity were measured, 4 and 21 days after surgery, respectively, in the frontal and parietal cortices of both hemispheres. A 33-34% decrease in HACU rate and a 43-39% decrease in ChAT activity was found in the ipsilateral cortices 4 and 21 days, respectively, after the lesion. If the lesioned rats received NGF (10 micrograms i.c.v.) twice a week or daily administrations of GM1 (30 mg/kg, i.p.), beginning immediately after surgery the decrease in HACU rate and ChAT activity was smaller. If NGF and GM1 were given concomitantly no decrease in HACU rate and ChAT activity was detected in the lesioned hemisphere and a slight increase occurred in the contralateral hemisphere. However, after the concurrent administration of NGF (10 micrograms i.c.v.) and the inactive dose of GM1 10 mg/kg i.p. no decrease in HACU and ChAT activity was also found in the lesioned rats. The latter finding indicates a potentiation by GM1 of NGF effects on the cholinergic neurons of the NBM. The two drugs may either antagonize the neurotoxic effects of ibotenic acid or stimulate a compensatory activity in the remaining neurons.

Functional damage of dopamine nerve terminals following intrastriatal kainic acid injection

The release of [3H]dopamine ([3H]DA) previously taken up into rat striatal slices was studied one week after a monolateral intrastriatal injection of kainic acid (KA). Different releasing stimuli (electrical pulses, veratrine, high-K+) were applied. The electrically evoked release in the KA-lesioned striata was drastically reduced with respect to the unlesioned contralateral striata. In contrast, KA had no effect on the release of [3H]DA evoked by veratrine or high-K+. In unlesioned striatal slices, depolarized with 15 mM KCl, apomorphine reduced and (-)sulpiride increased the release of [3H]DA. The effect of apomorphine was antagonized by (-)sulpiride indicating the presence of an autoreceptor system similar to that seen in unlesioned striata stimulated electrically. However, the effects of apomorphine and of (-)sulpiride were dramatically reduced in K+-depolarized slices prepared from KA-lesioned striata. The results suggest that the axon terminals in KA-treated areas remain intact in several of their properties but may be damaged in some critical processes.

Cholinergic and noradrenergic denervations decrease labelled purine release from electrically stimulated rat cortical slices

The origin of cortical purine release was investigated by measuring [3H]purine release from electrically stimulated cortical slices of rats after neurotoxic lesions of cholinergic, noradrenergic and serotoninergic pathways innervating the cortex. Purines were labelled by incubating the cortical slices with [3H]adenine. The 3H efflux at rest and during stimulation, analysed by high performance liquid chromatography, consisted of adenosine, inosine, hypoxanthine and a small amount of nucleotides. Twenty days after unilateral or bilateral lesion of the nucleus basalis a marked decrease in choline acetyltransferase activity was associated with a decrease in [3H]purine release. A linear relationship was found between the decrease in choline acetyltransferase activity and [3H]purine release. A partial recovery in both choline acetyltransferase activity and [3H]purine release was observed eight months after the lesion. Twenty days after intra-cerebroventricular injection of 6-hydroxydopamine a 59% decrease in cortical noradrenaline content was associated with a 44% decrease in [3H]purine release. Conversely, no change in [3H]purine release was found in rats in which a 89% decrease in cortical serotonin content was induced by intra-cerebroventricular injection of 5,7-dihydroxytryptamine. The decrease in [3H]purine release after the lesion of the cholinergic and noradrenergic pathways may depend on metabolic changes, a loss of a stimulating influence of acetylcholine and noradrenaline or may indicate a release of [3H]purine from cholinergic and noradrenergic fibres.

Local cerebral glucose utilization following unilateral and bilateral lesions of the nucleus basalis magnocellularis in the rat

To investigate to what extent the loss of cholinergic projections to the neocortex results in functional impairment in the target areas, local rates of cerebral glucose utilization were measured following excitotoxin lesions of the nucleus basalis magnocellularis (NBM) in the rat. Both unilateral and bilateral lesions of NBM resulted in reversible depression of cerebral metabolism. The effects of unilateral lesions were limited to the cortical areas which receive most of the cholinergic projections from NBM. The metabolic defect produced by bilateral lesions was spread to the whole brain. Within 4 months, however, normal metabolic values coexisted with marked changes of the presynaptic cholinergic markers and impairment of conditioned behavior.