Colchicine administered into the area of the nucleus basalis decreases cortical nicotinic cholinergic receptors labelled by [3H]-acetylcholine

Nicotinic receptors in dementia of Alzheimer, Lewy body and vascular types

OBJECTIVES

Comparisons were made of nicotinic receptors in 3 major forms of dementia in old age. Although it is well established the involvement of nicotinic receptors in Alzheimer's disease (AD), their status in the other two main causes of dementia in old age-dementia with Lewy bodies (DLB) and vascular dementia (VaD) is not widely reported.

METHODS

Temporal cortex was examined for epibatidine and alpha-bungarotoxin binding, and immunoreactivity of alpha4 and alpha7 nAChR subunits.

RESULTS

There were selective abnormalities in nicotinic receptor subtypes in the disorders examined. In AD there is a loss of high affinity receptor binding, reflecting a selective loss of alpha4 subunit, but no change in alpha7 subunits. Similar abnormalities in ligand binding are also apparent in DLB. In the VaD series, there was no overall loss of epibatidine binding or immunoreactivity for alpha4 or alpha7 subunits.

CONCLUSIONS

Loss of cortical receptor alpha4 subunit appears to be a characteristic feature of neurodegenerative dementia but not dementia of vascular origin. Since nicotinic receptors control cerebral vasodilation, the relative integrity of the receptors in VaD may auger well for nicotinic therapy in this disorder in which there is a cholinergic abnormality, to judge by the loss of the presynaptic enzyme.

Nicotinic receptor abnormalities in Alzheimer's disease

Loss of cortical nicotinic acetylcholine receptors with high affinity for agonists (20-50%) in patients with Alzheimer's disease is a common finding. Recent immunochemical analyses indicate that this deficit is predominantly associated with the loss of alpha4 subunits (30-50%), although modest reductions of alpha3 may occur in some individuals (25-29%). No reduction of beta2 subunit protein expression or levels of alpha3 and alpha4 messenger RNA has been reported. Decline in cortical [(125)I]alpha-bungarotoxin binding and alpha7 protein expression does not appear to be as extensive or widespread as the loss of alpha4 (0-40%), with no reduction in messenger RNA expression. In the thalamus, there was a trend for reduced [(3)H]nicotine binding in the majority of nuclei (0-20%) in Alzheimer's disease; however, there was a significant decline in [(125)I]alpha-bungarotoxin binding in the reticular nucleus. In the striatum [(3)H]nicotine binding was reduced in Alzheimer's disease, and although neuroleptic medication accentuated this change, it occurred in those free of neuroleptics. Changes in nicotinic acetylcholine receptors in Alzheimer's disease are distinct from those in normal aging and are likely to contribute to clinical features and possibly neuropathology.

Microtubule cytoskeleton involvement in muscarinic suppression of voltage-gated calcium channel current in guinea-pig ileal smooth muscle

1. Effects of agents, which affect microtubule polymerization-depolymerization cycle, on Ba2+ current (IBa) flowing through voltage-gated Ca2+ channels and carbachol (CCh)-induced sustained suppression of IBa were examined in whole-cell voltage-clamped smooth muscle cells of guinea-pig ileum. 2. offchicine (100 microM) and vinblastine (100 microM), microtubule depolymerizers, increased the ampitude of IBa. Lumicolchicine (100 microM), an inactive analogue of colchicine, had no effect on IBa. 3. Taxol (1 - 100 microM), a microtubule polymerizer, decreased IBa in a concentration-dependent manner and accelerated the rate of inactivation of IBa. Baccatin III (100 microM), an inactive analogue of taxol, had no effect on IBa. 4. Colchicine (100 microM) and vinblastine (100 microM), but not lumicolchicine (100 microM), decreased or abolished the sustained component of CCh (10 microM)-induced IBa suppression. 5. Pretreatment with taxol (10 - 100 microM) resulted in a concentration-dependent decrease in IBa and the action of CCh on IBa. The inhibitory effects of taxol and CCh on IBa were not additive. 6. Colchicine (100 microM) or taxol (100 microM) had no effect on voltage-gated K+ channel current or CCh-induced non-selective cationic channel current. 7. These results suggest that polymerization of microtubules leads to suppression of Ca2+ channel activity, and that muscarinic sustained suppression of Ca2+ channel current is mediated by a signal transduction element which involves microtubule cytoskeleton.

A population of nicotinic receptors is associated with thalamocortical afferents in the adult rat: laminal and areal analysis

In the adult rat brain, a prominent population of nicotinic cholinoceptors binds 3H-nicotine with nanomolar affinity. These receptors are abundant in most thalamic nuclei and in neocortical layers 3/4, which receive a major thalamic input. To test whether cortical nicotinic receptors are associated with thalamocortical afferents, unilateral excitotoxic (N-methyl-D-aspartate) lesions were made in one of four thalamic nuclear groups (anterior, ventral, medial geniculate, or dorsal lateral geniculate) or in temporal cortex. After 1 or 4 weeks of survival, cortical 3H-nicotine binding was quantified via autoradiography. Thalamic lesions resulted in a partial loss of 3H-nicotine binding in ipsilateral cerebral cortex. In each thalamic lesion group, the greatest decrease (35-45%) occurred within the cortical layers and area (i.e., cingulate, parietal, temporal, or occipital cortex) receiving the densest thalamocortical innervation. Binding of 3H-nicotine was also reduced within the thalamus local to the lesion, particularly at the longer survival time. Saturation analysis, performed in frontoparietal cortical tissue homogenates following ventral thalamic lesions, revealed a significant (34%) reduction in receptor density but not affinity. Direct excitotoxic lesions of the neocortex (temporal cortex) tended to preserve 3H-nicotine binding in layers 3/4, despite local neuronal loss. These results, taken with other published findings, suggest that some nicotinic cholinoceptors in adult rat cerebral cortex are located on thalamocortical terminals. This organizing principle appears to apply not only to sensory and motor relay projections but also to association nuclei that project to allocortical areas. These receptors may provide a local mechanism for nicotinic cholinergic modulation of thalamocortical input.

Sensitivity of rat frontal cortical neurones to nicotine is increased by chronic administration of nicotine and by lesions of the nucleus basalis magnocellularis: comparison with numbers of [3H]nicotine binding sites

The effects of chronic nicotine treatment and of unilateral AMPA lesion of the nucleus basalis magnocellularis (nbm) on the sensitivity of frontal cortical neurones to iontophoretically applied nicotine were studied. Chronic nicotine treatment increased the number of [3H]nicotine binding sites from 2.9 to 3.9 pmol g-1 wet weight, and increased the proportion of cortical neurones responding to nicotine from 32.3% to 60.0%. After unilateral nbm lesions, the densities of AChE-positive fibers and [3H]nicotine binding sites were reduced by approximately 97% and 55%, respectively, and the proportion of neurones responding to nicotine increased from 32.3% to 53.8%. The two treatments, chronic nicotine administration and nbm lesion, also increased the size of individual neuronal responses, prolonged their duration, and shortened the response latency. Responses to glutamate were unaffected by either procedures. The results show that the increase in [3H]nicotine binding produced by chronic nicotine administration is associated with an increased response to iontophoretically applied nicotine, suggesting that the receptor upregulation induced by the chronic treatment were functional. Less easily explained is the association between increased sensitivity of frontal cortical neurons to nicotine after nbm lesion with a decreased receptor density. It is suggested that a substantial proportion of nicotinic receptors are located presynaptically, and that their loss after lesion concealed an upregulation at postsynaptic sites.

Alteration in nicotine binding sites in Parkinson's disease, Lewy body dementia and Alzheimer's disease: possible index of early neuropathology

High-affinity nicotine binding, considered to primarily reflect the presence of CNS alpha 4 beta 2 nicotinic receptor subunits, was examined autoradiographically in brain regions most severely affected by Alzheimer and Parkinson types of pathology. In the midbrain, the high density of binding associated with the pars compacta of the substantia nigra was extensively reduced (65-75%, particularly in the lateral portion) in both Lewy body dementia and Parkinson's disease. Since loss of dopaminergic neurons in Lewy body dementia was only moderate (40%), loss or down-regulation of the nicotinic receptor may precede degeneration of dopaminergic neurons in this region. In the dorsolateral tegmentum, where diffuse cholinergic perikarya are located, nicotine binding was highly significantly decreased in both Lewy body dementia and Parkinson's disease with almost no overlap between the normal and disease groups, indicative of a major pathological involvement in or around the pedunculopontine cholinergic neurons. In the hippocampus, binding was decreased around the granular layer in Lewy body dementia and Alzheimer's disease, although unchanged in the stratum lacunosum moleculare, where binding was relatively higher. Dense bands of receptor binding in the presubiculum and parahippocampal gyrus--areas of highest binding in human cortex--were diminished in Alzheimer's disease but not Lewy body dementia. In temporal neocortex there were reductions in Alzheimer's disease throughout the cortical layers but in Lewy body dementia only in lower layers, in which Lewy bodies are concentrated. Abnormalities of the nicotinic receptor in the diseases examined appear to be closely associated with primary histopathological changes: dopaminergic cell loss in Parkinson's disease and Lewy body dementia, amyloid plaques and tangles in subicular and entorhinal areas in Alzheimer's disease. Loss or down-regulation of the receptor may precede neurodegeneration.

Comparison of intracranial infusions of colchicine and ibotenic acid as models of neurodegeneration in the basal forebrain

Colchicine and ibotenic acid were compared for their ability to produce neurodegeneration and cognitive deficit after bilateral infusions into the nucleus basalis magnocellularis of male Long-Evans rats. Four weeks post-lesion, there was no difference in locomotor activity following infusion of either neurotoxicant or vehicle. In a passive avoidance task, both treated groups had significantly shorter step-through latencies compared with vehicle. Five weeks post-lesion, rats were killed for neurochemistry or histochemistry. Choline acetyltransferase (ChAT) activity in both the frontal and parietal cortex was significantly decreased (25-35%) in the colchicine- and ibotenic acid-infused rats when compared to control. There was no effect of either neurotoxicant on ChAT activity in the hippocampus or striatum. Both neurotoxicants produced damage in the general area of the ventromedial pallidum, although ibotenic acid infusion consistently produced a larger area of damage as assessed in Nissl-stained sections. Analysis of the number of ChAT-immunoreactive cells in the nucleus basalis magnocellularis (NBM) showed an average 60% cell loss following colchicine infusion and a 75% cell loss after ibotenic acid infusion. Area of glutamic acid decarboxylase (GAD) staining was significantly decreased in several regions surrounding the NBM for ibotenic acid (51% average decrease), and showed non-significant decreases (28%) following colchicine infusion. Colchicine infusion decreased dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum; ibotenic acid had no effect on brain catechol of indoleamine levels. The results indicate that although similar cholinergic hypofunction and behavioral deficits were achieved, several non-cholinergic differences between the neurotoxicants were detected.

Dysfunction of the brain cholinergic system during aging and after lesions of the nucleus basalis of Meynert

In order to establish how closely the models mimic aging and Alzheimer's disease, a comparison was made, using the extensive literature available, between brain cholinergic dysfunction in aging animals and man, and between that in animals with lesions of the nucleus basalis of Meynert and in subjects affected by Alzheimer's disease. It is concluded that cholinergic dysfunction in the aging rat closely resembles that in aging man. A similarity can also be found between the cortical dysfunction induced by lesions of the nucleus basalis and that occurring in patients with Alzheimer's disease. However, cholinergic dysfunction only represents a limited aspect of the neorotransmitter deficits and neuropathological alterations of the disease.

Multiple binding sites for nicotine receptor antagonists in inhibiting [3H](-)-nicotine binding in rat cortex

The displacement of [3H](-)-nicotine from its binding site in rat cerebral cortex by a number of antagonists was investigated. [3H](-)-Nicotine appeared to bind to a single site with a dissociation constant (KD) of 5.5 nM; pancuronium, gallamine and trimetaphan displaced [3H](-)-nicotine with inhibition constants (KI) of 57, 99 and 621 microM, respectively, whereas mecamylamine only displaced 50% of nicotine binding in concentration > 1 mM. For hexamethonium and (+)-tubocurarine the displacement of [3H](-)-nicotine binding appeared to involve two sites; the higher affinity site comprising 30% of the total binding for hexamethonium but 76% of the sites for (+)-tubocurarine. In the presence of mecamylamine (100 microM) the displacement of [3H](-)-nicotine binding by (+)-tubocurarine appeared to involve only a single site with an affinity similar to that for the high affinity site in the absence of mecamylamine whereas that for hexamethonium still involved two sites. It is suggested that (+)-tubocurarine may act at both the binding site for [3H](-)-nicotine per se and also at an allosteric site. The failure of mecamylamine to influence the binding of hexamethonium suggests that there may be more than one allosteric site or that hexamethonium may distinguish between subtypes of nicotine receptors in the cortex.

Sustained cortical metabolic responsivity to physostigmine after nucleus basalis magnocellularis ablation in rats

Unilateral nucleus basalis magnocellularis (NBM) ablation, which causes partial cholinergic denervation of the ipsilateral anterior neocortex, results in an acute but transient depression of regional cerebral metabolic rates for glucose (rCMRglc) in deafferented areas; rCMRglc normalizes within 2 weeks. To seek possible compensatory changes in cholinergic mechanisms following NBM ablation that could lead to rapid metabolic normalization, we studied rCMRglc responses to the receptor agonists nicotine and arecoline and the cholinesterase inhibitor physostigmine in rats at 2 weeks after unilateral NBM destruction. Physostigmine increased rCMRglc in 10 of 30 cortical areas contralateral to the NBM lesion. Compared to the unlesioned side, rCMRglc after physostigmine in the lesioned cortex was significantly lower in 2, significantly higher in 1 and not different (P < 0.05) in 27 areas. Neither arecoline nor nicotine treatment produced rCMRglc asymmetry in lesioned rats. These results demonstrate that responsivity to physostigmine is maintained in most regions of the rat neocortex after extrinsic cholinergic denervation by NBM ablation. This adaptive response appears not to result from cholinergic receptor upregulation and may reflect instead reorganization of cholinergic synapses.

Long-term changes in phosphoinositide hydrolysis following colchicine lesions of the nucleus basalis magnocellularis

The effect of bilateral colchicine lesions of the nucleus basalis magnocellularis (NBM) on agonist-stimulated phosphoinositide (PI) hydrolysis was examined in cortical slices 1, 3, or 14 months after surgery. Colchicine lesions resulted in a loss of acetylcholinesterase staining in the cortex which recovered to control levels by 14 months. Choline acetyltransferase activity in the cortex was decreased by 43% one month after lesioning, but returned to control levels by 3 months. In vitro stimulation with carbachol produced a concentration-dependent increase in PI hydrolysis, which was enhanced 3 and 14 months after NBM lesions. Norepinephrine and quisqualate-stimulated PI hydrolysis was also enhanced 14 months after NBM lesions. These results suggest a slow up-regulation of postsynaptic receptor function following presynaptic loss of transmitter.

Impairment of radial-arm maze performance in rats following lesions involving the cholinergic medial pathway: reversal by arecoline and differential effects of muscarinic and nicotinic antagonists

Pharmacologic studies have indicated that accurate performance on the radial-arm maze depends upon the integrity of both nicotinic and muscarinic cholinergic neurotransmitter systems and that these systems interact in a complex fashion. Although numerous studies have suggested that pathways deriving from the basal nuclear complex of the forebrain are critical for the cholinergic modulation of learning and memory, most have focussed on the septohippocampal projection, and none have specifically targeted the medial or lateral systems. In Experiment 1, cortical knife cuts interrupting the medial cholinergic pathway were made at the level of the caudate-putamen nucleus. Such transections produced a robust but temporary disruption of choice accuracy performance in the radial-arm maze. Recovery of this behavior occurred within 10 days and before cholinergic fiber regeneration, suggesting that compensatory changes could have taken place in non-ablated neuronal circuits. In Experiment 2, daily postsurgical administration of arecoline, an agonist with predominantly muscarinic actions, was found to virtually eliminate the adverse behavioral effects of medial pathway transections, indicating that the deficit could be attributable, in part, to disruption of cholinergic projections. In Experiment 3, the effects of scopolamine, a muscarinic antagonist, and mecamylamine, a nicotinic antagonist, were examined in rats with medial cholinergic pathway transections after behavior had returned postsurgically to control levels. Although both drugs attenuated radial-arm maze performance before knife cuts, only scopolamine reduced choice accuracy following surgery. We conclude that the medial cholinergic pathway, particularly its nicotinic actions, plays an important role in cognitive function, at least as exemplified by radial-arm maze performance. Muscarinic mechanisms associated with other telencephalically projecting cholinergic networks, as well as possibly with the medial pathway itself, appear to operate interactively with nicotinic influences.

Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--II. Cholinergic drugs as probes to investigate lesion-induced deficits and transplant-induced functional recovery

The effects of two doses of muscarinic (arecoline and scopolamine) and nicotinic (nicotine and mecamylamine) cholinergic receptor agonists and antagonists on the radial maze errors of rats, performing poorly after ibotenate lesions to the nucleus basalis and medial septal brain regions, were assessed before and after transplantation of cholinergic-rich and -poor fetal grafts, using tasks which measured short- (working) and long-term (reference) spatial and associative memory. Lesioned rats showed improvement with the agonists, and impairment with the antagonists, at low doses which did not affect the performance of controls; these effects were more marked for working than reference memory, especially in the spatial task. The peripherally acting antagonists N-methylscopolamine and hexamethonium did not affect the performance of control or lesioned rats. Effects of the cholinergic probes were re-examined 16 weeks after grafting, in groups with cholinergic-rich grafts to cortex and/or hippocampus which showed functional recovery, and groups with cholinergic-rich grafts to basal forebrain, or cholinergic-poor grafts to basal forebrain, cortex, and hippocampus, which showed no improvement. All lesioned rats, regardless of site, type, or efficacy of transplant, continued to show marked impairment with the antagonists. Poorly performing grafted animals also showed improvement with the agonists. In rats with behaviourally effective cholinergic-rich grafts, arecoline had no effect, but nicotine substantially increased working and reference memory errors, particularly spatial working memory errors. Rats with grafts in both cortex and hippocampus showed the largest increases in errors after nicotine. These results show that lesioned rats were more sensitive to the bi-directional effects of cholinergic receptor ligands than controls, consistent with a role for acetylcholine in the lesion-induced deficits. The predominant effect of drugs on working memory may also be consistent with disruption of acquisition rather than of storage or retrieval processes in memory, and may be related to impairment of attention. The results further show that, despite behavioural recovery, supersensitive responses to cholinergic drugs were not normalized in rats with cholinergic-rich grafts, and that an additive interaction between graft and host may have occurred in response to nicotine.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.