Effects of ageing and long-term hormone replacement on cholinergic neurones in the medial septum and nucleus basalis magnocellularis of ovariectomized rats

Ovariectomized aged rats, some of which received long-term hormone replacement with oestrogen or oestrogen plus progesterone, were evaluated for the number and size of basal forebrain cholinergic neurones, as well as relative levels of choline acetyltransferase (ChAT) and trkA mRNA, in order to determine whether effects on basal forebrain cholinergic cell survival and function correspond with differences in cognitive performance previously described. The results show that ageing combined with long-term loss of ovarian function produced substantial reductions in the levels of ChAT and trkA mRNA in the medial septum and nucleus basalis magnocellularis, relative to much younger ovariectomized controls. In contrast, no significant effects on the number or size of the cholinergic cells were detected, indicating that loss of ovarian function does not cause a loss of cholinergic neurones with age. Long-term hormone replacement had no apparent effect on the number of ChAT-positive neurones detected, and did not prevent the reductions in ChAT and trkA mRNA associated with ovariectomy and ageing. Collectively, the data suggest that ageing combined with long-term loss of ovarian function has a severe negative impact on basal forebrain cholinergic function, but not on cholinergic cell survival per se.     

Impairment in T-maze reinforced alternation performance following nucleus basalis magnocellularis lesions in rats

Rats were trained on a reinforced alternation paradigm using an elevated T-maze. After pre-surgical training subjects received either ibotenic acid (4 micrograms/0.4 microliter) or vehicle (pH 7.4, 0.4 microliter) bilaterally into the region of the nucleus basalis magnocellularis--an important source of neocortical acetylcholine projections. Acetylcholinesterase staining of sectioned brains revealed a loss of neocortical, but not hippocampal staining in lesioned animals. On the T-maze task, lesioned rats showed significantly impaired choice performance relative to controls. They also demonstrated significant side biases, the degree of which was correlated with choice performance deficit.     

Neurochemical and behavioral recovery after colchicine lesions of the nucleus basalis magnocellularis in rats

Experimentally-induced lesions of the basal forebrain have been used to test the hypothesis that the cholinergic system plays a critical role in learning and memory. In the present study, a basal forebrain infusion of colchicine, a microtubule assembly inhibitor, was used to characterize the relationship between a cholinergic marker and behavioral function. Bilateral infusions were made in the nucleus basalis magnocellularis (NBM) of male Long-Evans rats. At 4 weeks post-lesion, behavioral assessments were made on half of the rats in each group. These rats were sacrificed 1 week later and regional choline acetyltransferase (ChAT) activity was measured. The remaining rats were behaviorally tested 11 weeks post-lesion and sacrificed 12 weeks post-lesion. The brains of additional rats were studied for Nissl-staining, ChAT-, GAD- and metEnk immunoreactivity (IR) and AChE histochemistry. At 5 weeks after colchicine infusion, there was a significant decrease in parietal and frontal cortical ChAT activity, impaired acquisition of a water maze spatial navigation task and decreased passive avoidance cross-over latency. At 12 weeks after colchicine infusion, ChAT activity was decreased in frontal but not parietal cortex; acquisition of the water maze task was not significantly different from vehicle-infused rats, and a significant deficit was observed in passive avoidance latency. ChAT-IR in the NBM showed a significant decrease at both time points, while changes in AChE-stained cortical fibers paralleled the ChAT activity. GAD- and metEnk-IR were decreased but were not different between the two time points. These data show task-specific behavioral recovery associated in time with recovery of regional cholinergic markers.     

Similar memory impairments found in medial septal-vertical diagonal band of Broca and nucleus basalis lesioned rats: are memory defects induced by nucleus basalis lesions related to the degree of non-specific subcortical cell loss?

The function of nucleus basalis (NB) and medial septal-vertical diagonal band of Broca (MS-VDBB) in a place navigation task requiring reference memory was investigated. Two subclasses of nucleus basalis ibotenic acid-lesioned rats could be identified: a group having both extensive non-specific subcortical damage and severely impaired learning behavior, and a less impaired group with correspondingly less subcortical damage. The depletion of cortical cholinergic enzymes was slightly higher in the group of NB-lesioned rats with extensive subcortical lesions than in the group with smaller lesioned areas. In the hippocampus of both of these NB-lesioned groups, cholinergic innervation remained unchanged. Ibotenic acid lesioning restricted to the MS-VDBB depleted hippocampal cholinergic innervation, but not the innervation of the frontal cortex, and also led to impaired learning behavior. Of all the lesioned rats, the most impaired were the NB-lesioned rats with large non-specific subcortical lesion.     

Lesions in nucleus basalis magnocellularis and medial septal area of rats produce qualitatively similar memory impairments

The functional contribution of nucleus basalis magnocellularis (NBM) and the medial septal area (MSA) to memory was evaluated in two different spatial discriminations. Preoperatively, rats were trained to a criterion level of performance in a simultaneous left/right discrimination on the stem of a T-maze (a trial-independent memory) and a discrete-trial, rewarded alternation discrimination on the arms of the T-maze (a trial-dependent memory). Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM, MSA, both NBM and MSA, or dorsal globus pallidus (DGP), and by radiofrequency current (RF) in the NBM and MSA. Control rats received operations in which either no current was passed or no neurotoxin was injected. Lesions in the NBM, MSA, or both the NBM and MSA produced a similar pattern of behavioral changes relative to the performance of controls; postoperative reacquisition of the arm discrimination was initially impaired but showed recovery to normal levels, whereas postoperative reacquisition and reversal of the stem discrimination was not impaired (except following the combined NBM and MSA lesion). Lesions of the DGP had no effect on choice accuracy in any discrimination. When the discrimination on the arms was made more difficult by increasing the delay interval during which the information had to be remembered, rats with combined NBM and MSA lesions were again impaired relative to controls and showed no signs of recovery of function. These results provide information about the behavioral functions of the basal forebrain cholinergic system and suggest that pathological changes in certain components of this system can cause disorders of memory.     

Neurochemical recovery in the neocortex after colchicine lesions of the nucleus basalis magnocellularis in rats

Neurochemical recovery was investigated in male, Fischer-344 rats up to 3 months after lesions of the nucleus basalis. Bilateral injections of colchicine (1.0 micrograms/site) into the nucleus basalis magnocellularis (NBM) resulted in a 30% decrease in choline acetyltransferase (ChAT) activity in frontal cortex 4 weeks after surgery, compared to unlesioned controls. ChAT activity in the frontal cortex gradually recovered to control levels by 12 weeks. The loss of ChAT-immunoreactive neurons in the NBM observed 4 weeks after surgery was still evident 12 weeks after surgery. These results suggest that surviving cholinergic neurons in the NBM contribute to recovery of ChAT activity in the neocortex.     

Nucleus basalis magnocellularis and medial septal area lesions differentially impair temporal memory

Functional dissociations between the medial septal area (MSA) and the nucleus basalis magnocellularis (NBM) were examined using the concepts and experimental procedures developed by scalar timing theory. Rats were tested in variations of a signalled discrete-trial peak-interval schedule of reinforcement in which the response rate functions identified the time when the rats expected reinforcement. The variations assessed aspects of both reference and working memory for information obtained from prior trials and from the current trial. A double dissociation was found in reference memory. Rats with NBM lesions, like those with frontal cortex (FC) lesions, remembered the time of reinforcement as having occurred later than it actually did; rats with MSA lesions, like those with fimbria-fornix (FF) lesions, remembered the time of reinforcement as having occurred earlier than it did. A single dissociation was found in working memory. MSA lesions and FF lesions impaired working memory, while NBM and FC lesions had no effect on it. These data begin to identify the brain mechanisms underlying temporal memory; they indicate that the frontal and hippocampal systems are both involved, but in complementary ways; and they provide information that helps specify more clearly the functions of the frontal and hippocampal systems.     

Unilateral AMPA lesions of nucleus basalis magnocellularis induce a sensorimotor deficit which is differentially altered by arecoline and nicotine

One week after unilateral -amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) lesions of nucleus basalis magnocellularis, rats showed significant lateralised bias in spontaneous turning and in turning induced by tail pinch or by placing the rat on a 45° grid. Turning was biased to the lesioned side and this side also showed increased responsiveness to pin-prick stimulation of the skin (somaesthesia), snout and whisker stimulation and ammonia olfaction. Arecoline (0.5 mg/kg), at a dose which did not affect responses to sensorimotor stimulation in sham-operated rats, corrected the lesion-induced biased turning to tail pinch and the 45° grid test and reduced the bias in the open field. In contrast, nicotine (0.05 mg/kg), at a dose which also did not substantially affect responses to sensorimotor stimulation in sham-operated rats, switched the lesion-induced turning bias towards the contralateral side. Neither cholinoceptor agonist reduced the lesion-induced increased sensory responsiveness. The effects of nicotine were blocked by the centrally acting nicotinic antagonist, mecamylamine (1.0 mg/kg), but not by hexamethonium (1.0 mg/kg), or ondansetron (0.01 mg/kg). Amphetamine (up to 1.0 mg/kg) did not affect the lesion-induced motor asymmetry. The results confirm that the basal forebrain cholinergic system plays a role in sensorimotor cortical functions, but suggest different functional roles for muscarinic and nicotinic receptors.     

Substantia nigra 6-hydroxydopamine lesions alter dopaminergic synaptic markers in the nucleus basalis magnocellularis and striatum of rats

Recent findings have demonstrated the existence of dopaminergic (DA) markers in the nbM of the human brain and a reduction of these markers in both the nbM and the striatum of patients suffering from Alzheimer's disease (AD). To investigate the source of the DA synaptic markers found in the nbM, rats received unilateral 6-OHDA lesions of the substantia nigra pars compacta (SNc). The SNc lesions caused significant reductions in DA and DOPAC but not HVA in the nbM and the striatum; 3H-sulpiride binding to D2 receptors ipsilateral to the SNc lesion was significantly increased in the striatum (16%), consistent with denervation supersensitivity, but single-point analysis showed no significant changes in the nbM. These data suggest that the decreases in DA and 3H-spiperone binding levels observed in the nbM of AD patients may be due to partial destruction of DA nbM afferent projections from the brainstem.     

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.     

The role of cortical sympathetic ingrowth in the behavioral effects of nucleus basalis magnocellularis lesions

Following cholinergic denervation of the neocortex by nucleus basalis magnocellularis (NBM) lesions, peripheral sympathetic fibers grow into the neocortex. Two experiments were performed to determine the behavioral effects of this neuronal rearrangement. Group I animals underwent training on a standard radial-8-arm maze task, while Group II animals learned a modified version (i.e. 4 arms baited). Following acquisition, NBM lesions were performed. Animals with lesions but without sympathetic ingrowth performed consistently better in both behavioral paradigms, than animals with NBM lesions and sympathetic ingrowth. These studies suggest that cortical sympathetic ingrowth can alter behavior and is detrimental to the learning of a spatial memory paradigm.     

Effects of lesions of the nucleus basalis magnocellularis on the acquisition of cocaine self‐administration in rats

The nucleus basalis magnocellularis (NBM) is one element in the limbic cortical-ventral striatal circuitry that has been implicated in reinforcement processes. The present study examined the involvement of the cholinergic neurons of the NBM in mediating aspects of cocaine reinforcement. Lesions of the NBM were made by injecting 0.01 m AMPA into the subpallidal basal forebrain. Following 4 days' recovery, rats were implanted chronically with catheters in the jugular vein. In three separate experiments, rats were trained to acquire cocaine self-administration under a FR1 schedule of reinforcement at doses of 0.25, 0.083 and 0.028 mg/injection. A dose–effect function was also determined at the end of the acquisition experiments using five different doses of cocaine (0.009, 0.028, 0.083, 0.25, 0.50 mg/injection) and saline which were presented once daily in a Latin square design. There were no significant differences between groups in the acquisition of cocaine self-administration at any of the three doses studied (0.028, 0.083 and 0.25 mg/injection), although at the lowest dose, lesioned animals responded at greater levels on both active and inactive levers. However, a shift to the left in the cocaine dose–response function was observed revealing that the lesioned group self-administered significantly higher amounts of low doses of cocaine than control rats. These data suggest that the integrity of the NBM is not a critical determinant of the reinforcing effects of cocaine during the acquisition of self-administration of the drug, but that NBM-dependent cholinergic mechanisms may nevertheless interact with the neural substrates mediating the reinforcing properties of cocaine. The data are relevant to recent hypotheses of functional interactions between the dopaminergic system and the cholinergic NBM.     

Cognitive effects of neurotoxic lesions of the nucleus basalis magnocellularis in rats: Differential roles for corticopetal versus amygdalopetal projections

The cholinergic hypothesis states that cholinergic neurons of the basal forebrain nucleus basalis magnocellularis (nbm) that project to cortical and amy-gdalar targets play an important role in memory. Biochemical studies have shown that these target areas are differentially sensitive to different excitotoxins (e.g., ibotenate vs. quisqualate). This observation might explain the finding from many behavioural studies of memory that different excito-toxins affect memory differentially even though they produce about the same level of depletion of cholinergic markers in the cortex and similar cortical electrophysiological effects. Thus, the magnitude of mnemonic impairment might be related to the extent of damage to cholinergic projections to the amygdala more than to the extent of damage to corticopetal cholinergic projections. This explanation might similarly apply to the observation that the immunotoxin 192 IgG-saporin produces mild effects on memory when injected into the nbm. This is because it damages cholinergic neurons projecting to the cortex but not those projecting to the amygdala. Studies comparing the effects on memory of ibotenic acid vs. quisqualic acid lesions of the nbm are reviewed as are studies of the mnemonic effects of 192 IgG-saporin. Results support the cholinergic hypothesis and suggest that amygdalopetal cholinergic neurons of the nbm play an important role in the control of memory.     

Deoxyglucose uptake and choline acetyltransferase activity in cerebral cortex following lesions of the nucleus basalis magnocellularis

The uptake of [3H]2-deoxyglucose (2-DG) into various brain regions of rats with unilateral or bilateral lesions of the nucleus basalis magnocellularis (nBM) was measured. The activity of choline acetyltransferase (ChAT) in these brain regions was also determined. Lesions of the nBM caused a significant decrease in cortical ChAT activity but had no effect on 2-DG accumulation. Pentobarbital treatment reduced 2-DG accumulation in all brain areas examined and these reductions were not influenced by the nBM lesions. The results indicate that a decrease in the cholinergic innervation of the cortex does not influence cortical glucose utilization. It appears unlikely, therefore, that the reported decrease in cortical glucose utilization in Alzheimer's disease is related to degeneration of the nBM-cortical cholinergic projection.     

Effect of neural transplantation on depressive behavior in rats with lesioned nucleus basalis magnocellularis

Recent data of our group have shown that bilateral electrolytic lesions of the nucleus basalis magnocellularis (NBM) in rats reduced the escape behavior deficit that occurs in the learned helplessness test. The present study was done to establish the effect of intracerebral neural transplantation on the change in escape behavior of NBM-lesioned adult male Wistar rats in the learned helplessness test. At 2 days (NBM-ET) or 10 days (NBM-DT) after bilateral electrolytic NBM-lesions, small fragments of fetal frontal cortex (18th day of gestation) were allotransplanted into the lesioned NBM. Ten days after neural transplantation, the learned helplessness test was performed. The number of shocks that animals received before making an escape response was significantly reduced in NBM-lesioned rats (p < .001, compared to intact control and sham-operated rats). In comparison to NBM-lesioned and sham-ET rats, the NBM-ET rats showed a marked (p < .001) increase in the number of shocks delivered before the animal made such an escape response. On the other hand, NBM-DT rats did not show this increase. These results indicate that neural transplantation performed at an early time after lesioning of NBM reversed the effect of this lesion in rats exposed to learned helplessness test.     

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.     

Comparison of quisqualic and ibotenic acid nucleus basalis magnocellularis lesions on water-maze and passive avoidance performance

The present study compares water-maze (WM) (reference and working memory) and passive avoidance (PA) (acquisition and retention) deficits induced by ibotenic (ibo) and quisqualic (quis) acid nucleus basalis magnocellularis (NBM) lesions. Ibo lesions produced a large subcortical cell loss and a decrease in frontal cortex (FR) choline acetyltransferase (ChAT) activity. Ibo lesions impaired WM acquisition and PA acquisition and retention performance. Quis NBM lesions were restricted to the ventromedial pallidum, but ChAT activity was decreased in FR. Quis NBM lesions impaired PA acquisition and retention, but had no effect on the reference or working memory WM performance.     

Enhanced detection of nucleus basalis magnocellularis lesion-induced spatial learning deficit in rats by modification of training regimen

Bilateral excitotoxic lesions of the nucleus basalis magnocellularis (NBM) in the rat cause deficits in the water maze, a spatial memory paradigm. Previous investigations aimed at reversing the water maze performance deficit with anticholinesterase treatments have been unable to demonstrate a consistent drug effect due to the relatively good acquisition of the task seen following NBM lesions. The present investigation tested three different water maze training regimens designed to separate the learning curves. F-344 rats received bilateral NBM injections of ibotenic acid; sham-operated rats served as controls. The animals were tested in three groups in the water maze as follows: (1) four trials per day with no intertrial interval (standard paradigm), (2) four trials per day with a 10-minute intertrial interval, and (3) two trials per day with no intertrial interval. Each group was tested in the water maze for five consecutive days, followed by two days of rest, and then tested for an additional five days. The two-trial per day paradigm was more difficult than the standard paradigm for both lesions and controls and yielded the most difference between lesions and controls as compared to the other two testing regimens. The 10-min intertrial interval schedule was more difficult than the standard paradigm for lesioned animals but acquisition was not affected in control rats. These data demonstrate that the nucleus basalis lesions cause a deficit in the water maze task regardless of training parameters. Further, while all rats showed some acquisition of the water maze task, training schedule affected the level of learning of both lesioned and control rats.(ABSTRACT TRUNCATED AT 250 WORDS)