Learning and memory deficits after lesions of the nucleus basalis magnocellularis: reversal by physostigmine

The role of the cholinergic nucleus basalis magnocellularis in spatial learning and memory was studied in the rat. Animals received bilateral injections of ibotenic acid (5 micrograms/microliters) into the region of the nucleus basalis magnocellularis. Six weeks postoperatively they were deprived of food and trained for 5 weeks in a 16-arm radial maze in which 9 of the arms were baited with food. The nucleus basalis magnocellularis-lesioned animals showed significant deficits in the acquisition of the task. Further analysis of the data indicated that this was due primarily to a deficit in reference (long-term) as opposed to working (short-term) memory. After the 5-week training period the nucleus basalis magnocellularis-lesioned animals received intraperitoneal injections of physostigmine sulphate (0.5 mg/kg) 30 min before each daily trial for 1 week. This treatment resulted in a significant improvement in the performance of the spatial memory task on all three measures. The ibotenate lesions reduced the activity of choline acetyltransferase by about 40% in the anterior cortex and by 15% in the posterior cortex. Hippocampal choline acetyltransferase activity was not affected, indicating that the septohippocampal cholinergic projection was spared by the lesions. The activity of glutamate decarboxylase was not affected in any of these regions. These results suggest that the cholinergic projections of the nucleus basalis magnocellularis play an important role in the acquisition of a spatial memory task.     

Impairments in negative patterning, but not simple discrimination learning, in rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis

Rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis (NBM) and sham-operated rats were trained in either a simple discrimination paradigm assessing simple association learning or a negative patterning paradigm assessing configural association learning. In the simple discrimination task, rats were reinforced for responding to a light but were not reinforced for responding to a tone. In the negative patterning discrimination task, rats were reinforced for responding to either a light or a tone presented alone but were not reinforced for responding to both stimuli presented simultaneously. Simple discrimination learning was not affected, whereas acquisition of negative patterning was impaired by NBM lesions. Impaired configural association learning may reflect a loss in the ability of rats with NBM lesions to attend to multiple sensory stimuli or to cope with conflicting response strategies.     

Effect of cholinesterase inhibitors on Morris water task behavior following lesions of the nucleus basalis magnocellularis

The effect of bilateral nucleus basalis magnocellularis (nBM) lesions on performance in the Morris water task was examined in the rat, and the ability of anticholinesterase inhibitors to reverse the behavioral deficit was evaluated. Lesions of nBM resulted in a prolongation of escape latency. A spatial probe trial revealed that animals with sham lesions swam a greater percentage of the distance in the platform quadrant; this finding was abolished by nBM lesions. Lesions of nBM produced a nonsignificant increase in both open-field activity and activity-box scores. In Experiment 1, administration of 0.32 mg/kg physostigmine on Day 3 only resulted in a decrease in escape latency. In Experiment 2, in which cholinesterase inhibitors were administered daily for 5 days, 0.32 mg/kg but not low-dose physostigmine or two substituted N,N-alkyl phenyl carbamate cholinesterase inhibitors (RA-6 and RA-7) again improved escape latency on Day 3. Thus it was concluded that nBM lesions impair behavior on the Morris water task and physostigmine shortens escape latency.     

Electrolytic and ibotenic acid lesions of the nucleus basalis magnocellularis interrupt long-term retention, but not acquisition of two-way active avoidance, in rats

Previous experiments on two-way active avoidance have shown conflicting results after nucleus basalis magnocellularis lesion: disrupting effects with electrolytic lesions and facilitative effects with excitotoxic lesions. To resolve this issue, in this experiment, Wistar rats received pre-training bilateral electrolytic or ibotenic acid lesions and were trained in a massed two-way active avoidance conditioning. In order to test the long-term retention of the learned response, one additional session was conducted 10 days after the acquisition. Results showed that whereas electrolytic lesions did not affect the acquisition, ibotenic acid lesions enhanced it. Retention of active avoidance response was impaired by both electrolytic and ibotenic lesions of the NBM. These results suggest a role of the NBM in the memory consolidation and/or retrieval of two-way active avoidance. Electronic Publication     

Medial septal and nucleus basalis magnocellularis lesions produce order memory deficits in rats which mimic symptomatology of Alzheimer's disease

Rats with electrolytic lesions of the medial septum or ibotenic acid lesions of the nucleus basalis magnocellularis (NBM) were tested in an order memory task for an 8-item list of varying spatial locations within an 8-arm radial maze. Results indicated that rats with small medial septal lesions resulting in small AchE depletion of dorsal hippocampal formation were impaired only for the first, but not the last choice orders of the list. Animals with large medial septal lesions resulting in large AchE depletion of the dorsal hippocampal formation displayed an order memory deficit for all the choice orders of the list. In contrast, rats with small NBM lesions resulting in small AchE depletion of parietal and part of frontal cortex were impaired only for the last, but not the first choice orders of the list. Animals with large NBM lesions resulting in large AchE depletion of parietal and part of frontal cortex displayed an order memory deficit for all the choice orders of the list. The relationship between these findings and mnemonic symptomatology of Alzheimer's disease was discussed, as was the possible meaning of these results in providing an animal model for studying certain aspects of the disease.     

Functional and neurochemical cortical cholinergic impairment following neurotoxic lesions of the nucleus basalis magnocellularis in the rat

The effect of kainic and quinolinic acid on cortical cholinergic function was examined following injections of these agents into the nucleus basalis magnocellularis (nbm) or into the frontoparietal cortex. The release of cortical ³H-acetylcholine (³H-ACh), high affinity choline uptake (HACU) and acetylcholinesterase was measured 7 days following injections of saline (control), kainic acid (4.7 nmoles) and quinolinic acid (60, 150 and 300 nmoles) into the nbm. These cortical cholinergic parameters were also examined after injections of saline (control), kainic acid (9.4 nmoles) and quinolinic acid (300 nmoles) into the fronto-parietal cortex. The release of ³H-ACh, HACU and AChE was significantly reduced in animals injected with kainic or quinolinic acid into the nbm. Histological examination of stained sections showed a loss of cell bodies in the region of the nbm and the globus pallidus. The size of the lesion produced by quinolinic acid was proportional to the dose injected into the nbm. In animals injected with kainic acid or quinolinic acid into the cerebral cortex, the release of ³H-ACh, HACU and AChE was not significantly reduced when compared with control animals, although histological examination of stained cortical sections showed a marked loss of cortical neurons. Th results show that quinolinic acid, an endogenous neuroexcitant, produces a deficit of cholinergic function similar to that described in the cortical tissue of patients with senile dementia of Alzheimer's type. The toxic effects of quinolinic acid on cortical cholinergic function are due to its action on cholinergic cell bodies in the nbm. The cortical slice preparation from quinolinic acid-treated animals showing impairment of ³H-ACh release, may be useful in assessing the action of drugs designed to improve cholinergic function.     

Functional and neurochemical cortical cholinergic impairment following neurotoxic lesions of the nucleus basalis magnocellularis in the rat

The effect of kainic and quinolinic acid on cortical cholinergic function was examined following injections of these agents into the nucleus basalis magnocellularis (nbm) or into the frontoparietal cortex. The release of cortical ³H-acetylcholine (³H-ACh), high affinity choline uptake (HACU) and acetylcholinesterase was measured 7 days following injections of saline (control), kainic acid (4.7 nmoles) and quinolinic acid (60, 150 and 300 nmoles) into the nbm. These cortical cholinergic parameters were also examined after injections of saline (control), kainic acid (9.4 nmoles) and quinolinic acid (300 nmoles) into the fronto-parietal cortex. The release of ³H-ACh, HACU and AChE was significantly reduced in animals injected with kainic or quinolinic acid into the nbm. Histological examination of stained sections showed a loss of cell bodies in the region of the nbm and the globus pallidus. The size of the lesion produced by quinolinic acid was proportional to the dose injected into the nbm. In animals injected with kainic acid or quinolinic acid into the cerebral cortex, the release of ³H-ACh, HACU and AChE was not significantly reduced when compared with control animals, although histological examination of stained cortical sections showed a marked loss of cortical neurons. Th results show that quinolinic acid, an endogenous neuroexcitant, produces a deficit of cholinergic function similar to that described in the cortical tissue of patients with senile dementia of Alzheimer's type. The toxic effects of quinolinic acid on cortical cholinergic function are due to its action on cholinergic cell bodies in the nbm. The cortical slice preparation from quinolinic acid-treated animals showing impairment of ³H-ACh release, may be useful in assessing the action of drugs designed to improve cholinergic function.     

17beta-estradiol enhances cortical cholinergic innervation and preserves synaptic density following excitotoxic lesions to the rat nucleus basalis magnocellularis

Estradiol exerts beneficial effects on neurodegenerative disorders associated with the decline of cognitive performance. The present study was designed to further investigate the effect of 17beta-estradiol on learning and memory, and to evaluate its neuroprotective action on cholinergic cells of the nucleus basalis magnocellularis, a neural substrate of cognitive performance. Female rats were ovariectomized at an age of 6 months. Three weeks later they received injections of either a mid-physiological dose of 17beta-estradiol or vehicle (oil), every other day for 2 weeks. The effect of estradiol on cognitive performance was tested in two associative learning paradigms. In the two-way active shock avoidance task estradiol-replaced animals learned significantly faster, while in the passive shock avoidance test no differences were observed between the experimental groups. Subsequent unilateral infusion of N-methyl-D-aspartate in the nucleus basalis magnocellularis resulted in a significant loss of cholinergic neurons concomitant with the loss of their fibers invading the somatosensory cortex. Estradiol treatment did not affect the total number of choline-acetyltransferase-immunoreactive neurons and their coexpression of the p75 low-affinity neurotrophin receptor either contralateral or ipsilateral to the lesion. In contrast, cholinergic fiber densities in estradiol-treated animals were greater both in the contralateral and ipsilateral somatosensory cortices as was detected by quantitative choline-acetyltransferase and vesicular acetylcholine transporter immunocytochemistry. However, estradiol treatment did not affect the lesion-induced relative percentage loss of cholinergic fibers. A significant decline of synaptophysin immunoreactivity paralleled the cholinergic damage in the somatosensory cortex of oil-treated animals, whereas an almost complete preservation of synaptic density was determined in estradiol-treated rats.Our results indicate that estradiol treatment enhances the cortical cholinergic innervation but has no rescuing effect on cholinergic nerve cells in the basal forebrain against excitotoxic damage. Nevertheless, estradiol may restore or maintain synaptic density in the cerebral cortex following cholinergic fiber loss. This estradiol effect may outweigh the lack of cellular protection on cholinergic cells at the functional level.     

Verapamil prevents, in a dose-dependent way, the loss of ChAT-immunoreactive neurons in the cerebral cortex following lesions of the rat nucleus basalis magnocellularis

In the present study we analysed the neuroprotective effect of the L-type voltage-dependent calcium channel antagonist verapamil on cholineacetyltransferase (ChAT)-immunoreactive neurons in the cerebral cortex of rats with bilateral electrolytic lesions of the nucleus basalis magnocellularis (NBM). Treatment with verapamil (1.0, 2.5, 5.0 and 10.0 mg/kg/12 h i.p.) started 24 h after NBM lesions and lasted 8 days. Animals were sacrificed on day 21 after NBM-lesions. The bilateral NBM-lesions produced significant loss of ChAT-immunoreactive neurons in frontal, parietal and temporal cortex. Although the number of ChAT-positive neurons was significantly higher in NBM-lesioned animals treated with verapamil at a dose of 2.5, 5.0 and 10.0 mg/kg than in saline treated ones, the most significant effect was obtained at a dose of 5 mg/kg. This is, to our knowledge, the first report showing an inverted U-shape mode of neuroprotective action of the calcium antagonist verapamil, at morphological level in this particular model of brain damage. The demonstrated beneficial effect of verapamil treatment suggests that the regulation of calcium homeostasis during the early period after NBM lesions might be a possible treatment to prevent neurodegenerative processes in the rat cerebral cortex.     

The effects of nucleus basalis magnocellularis lesions in Long-Evans hooded rats on two learning set formation tasks, delayed matching-to-sample learning, and open-field activity

Rats with quisqualic acid lesions of the nucleus basalis magnocellularis (nBM) and control rats were compared in discrimination reversal learning set (DRLS) and olfactory discrimination learning set (ODLS) tasks, a delayed matching-to-sample task (DMTS), and open-field activity. Evidence of learning set formation was seen in control rats but not in nBM-lesioned rats in both the DRLS and ODLS tasks. Better-than-chance performances were seen for both groups in DMTS, indicating no impairment after nBM lesions. There were no group differences in open-field activity. These findings suggest that the nBM is important for higher cognitive processing such as "learning to learn" and thus is important for a complex form of reference memory. In addition, perseverational, working memory, and attentional deficits could not explain learning set impairment after nBM lesions.     

Latent inhibition of conditioned taste aversion is not disrupted, but can be enhanced, by selective nucleus accumbens shell lesions in rats

Latent inhibition is a form of negative priming in which repeated non-reinforced pre-exposures to a stimulus retard subsequent learning about the predictive significance of that stimulus. The nucleus accumbens shell and the anatomical projection it receives from the hippocampal formation have been attributed a pivotal role in the control or regulation of latent inhibition expression. A number of studies in rats have demonstrated the efficacy of selective shell lesions to disrupt latent inhibition in different associative learning paradigms, including conditioned active avoidance and conditioned emotional response. Here, we extended the test to the conditioned taste aversion paradigm, in which the effect of direct hippocampal damage on latent inhibition remains controversial. We demonstrated the expected effect of selective shell lesions on latent inhibition of conditioned emotional response and of conditioned active avoidance, before evaluating in a separate cohort of rats the effect of comparable selective lesions on latent inhibition of conditioned taste aversion: a null effect of the lesions was first obtained using parameters known to be sensitive to amphetamine treatment, then an enhancement of latent inhibition was revealed with a modified conditioned taste aversion procedure. Our results show that depending on the associative learning paradigm chosen, shell lesions can disrupt or enhance the expression of latent inhibition; and the pattern is reminiscent of that seen following hippocampal damage.     

Nucleus basalis lesions attenuate aquisition,but not retention,of Pavlovian heart rate conditioning and have no effect on eyeblink conditioning

Summary Rabbits with lesions of the anterior nucleus basalis of Meynert (nBM) were compared with animals with sham lesions or unoperated control animals on a classical conditioning task in which heart rate (HR) and eyeblink (EB) conditioned responses (CRs) were as sessed. The nBM lesions impaired the magnitude of the decelerative HR CR, but had no effect on the EB CR. A second experiment, in which animals were lesioned af ter acquisition was complete, showed that anterior nBM lesions had no effect on retention of either the HR or EB CR. These data suggest that the anterior nBM may participate in the early stages of information processing in which stimuli are evaluated for their significance based on their association with a reinforcer. However, the ante rior nBM is apparently not involved in the selection of a somatomotor response to deal effectively with such changing stimulus contingencies.     

Zona incerta lesions: regulatory drinking deficits to intravenous NaCl, angiotensin, but not to salt in the food

Rats with bilateral damage to the anterior zona incerta (ZI) showed small and delayed drinking responses after IP hypertonic NaCl injection, but they normally excreted most of the salt load within 6 hr. The impaired drinking responses were also evident after nonpainful intravenous (IV) NaCl infusions. After nephrectomy, rats with complete ZI lesions did not drink within 24 hr of the NaCl infusion. Rats with less complete lesions showed reduced and delayed drinking. In contrast to these profound osmoregulatory drinking impairments, all of the lesioned rats increased their water to food ratio when fed a 3% NaCl-supplemented diet. ZI lesioned rats did not drink in response to IV infusions of angiotensin II. The role of the ZI in drinking behavior is discussed in terms of the paradigm-dependent nature of these results, and parallels with other findings are considered.     

Slower acquisition of positively reinforced behavior following medial, but not dorsolateral, septal lesions in rats

This study was conducted to examine effects of medial and dorsolateral septal lesions on acquisition by using a modified autoshaping procedure. The results showed that following medial septal lesions, lever-pressing responses had long latencies and were fewer than following either dorsolateral septal lesions or control operations. However, during later sessions, the rats with medial septal lesions started to press. The long response latencies and low response rates shown initially by rats with medial septal lesions may have been due to delayed classical conditioning. Following medial septal lesions, the acetylcholinesterase activities of the different subdivisions of the hippocampus correlated positively with the number of lever-press responses emitted during Session 1. The group with dorsolateral septal lesions acquired the task just as fast as the control group and showed higher response rates than the other two groups during the final sessions. The latter result may be ascribed to generally increased locomotor activity.     

Dopamine transporter, but not tyrosine hydroxylase, may be implicated in determining individual differences in behavioral sensitization to amphetamine

Repeated administration of psychostimulants, such as amphetamine and cocaine, results in a long-lasting enhancement of behavioral responses elicited by a subsequent challenge injection of these drugs. This phenomenon has been termed behavioral sensitization. A well established model of individual differences based on the locomotor response to a novel environment has been shown to reliably predict the degree of behavioral sensitization to amphetamine. Rats that have high locomotor response in a novel environment (high responders or HR) develop greater behavioral sensitization to psychostimulants when compared to rats that show low locomotor activity in the same novel environment (low Responders or LR). Therefore, this model is ideal to study genetic factors that may underlie behavioral sensitization to psychostimulants. In this study, adult Sprague-Dawley rats were daily injected with amphetamine (1 mg/kg, i.p.) or saline for 9 days. Locomotor activity was recorded every other day. Following a one week-withdrawal a subsequent challenge of a lower dose of amphetamine (0.5 mg/kg, i.p.) was given to all rats (amphetamine pretreated and saline pretreated) and their locomotor activity was recorded. Our results show that HR rats, but not LR rats, develop behavioral sensitization to the locomotor activating effects of amphetamine. Furthermore, only HR rats pretreated with amphetamine exhibited an increase in dopamine transporter mRNA in the ventral tegmental area (VTA) and substantia nigra (SN). Tyrosine hydroxylase mRNA in the VTA and SN was upregulated in both HR and LR rats pretreated with amphetamine when compared to HR and LR rats pretreated with saline. These results demonstrate the existence of individual differences in behavioral sensitization to amphetamine and suggest that dopamine transporter, but not tyrosine hydroxylase, may be a critical factor in the development and expression of behavioral sensitization to the locomotor activating effects of amphetamine.     

Eye movement deficits following ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys II. Pursuit, vestibular, and optokinetic responses

The eyes are moved by a combination of neural commands that code eye velocity and eye position. The eye position signal is supposed to be derived from velocity-coded command signals by mathematical integration via a single oculomotor neural integrator. For horizontal eye movements, the neural integrator is thought to reside in the rostral nucleus prepositus hypoglossi (nph) and project directly to the abducens nuclei. In a previous study, permanent, serial ibotenic acid lesions of the nph in three rhesus macaques compromised the neural integrator for fixation but saccades were not affected. In the present study, to determine further whether the nph is the neural substrate for a single oculomotor neural integrator, the effects of those lesions on smooth pursuit, the vestibulo-ocular reflex (VOR), vestibular nystagmus (VN), and optokinetic nystagmus (OKN) are documented. The lesions were correlated with long-lasting deficits in eye movements, indicated most clearly by the animals' inability to maintain steady gaze in the dark. However, smooth pursuit and sinusoidal VOR in the dark, like the saccades in the previous study, were affected minimally. The gain of horizontal smooth pursuit (eye movement/target movement) decreased slightly (<25%) and phase lead increased slightly for all frequencies (0.3-1.0 Hz, +/-10 degrees target tracking), most noticeably for higher frequencies (0.8-0.7 and approximately 20 degrees for 1.0-Hz tracking). Vertical smooth pursuit was not affected significantly. Surprisingly, horizontal sinusoidal VOR gain and phase also were not affected significantly. Lesions had complex effects on both VN and OKN. The plateau of per- and postrotatory VN was shortened substantially ( approximately 50%), whereas the initial response and the time constant of decay decreased slightly. The initial OKN response also decreased slightly, and the charging phase was prolonged transiently then recovered to below normal levels like the VN time constant. Maximum steady-state, slow eye velocity of OKN decreased progressively by approximately 30% over the course of the lesions. These results support the previous conclusion that the oculomotor neural integrator is not a single neural entity and that the mathematical integrative function for different oculomotor subsystems is most likely distributed among a number of nuclei. They also show that the nph apparently is not involved in integrating smooth pursuit signals and that lesions of the nph can fractionate the VOR and nystagmic responses to adequate stimuli.     

Hyperreactivity following temporary chemical lesions in the region ventral to the anterior septum but not in the anterior olfactory nucleus,the lateral olfactory tract,or the olfactory bulb

Bilateral temporary chemical lesions were produced by slow infusion of the general blocking agent lidocaine into several anterior forebrain sites which have been implicated in the increase in reactivity following lesions of the olfactory bulbs. There was an increase in reactivity with infusions of the blocking agent lidocaine into the region ventral to the anterior septum but not with infusions into the region of the anterior olfactory nucleus, or the olfactory bulbs. A control group in which lidocaine was infused into the vicinity of the lateral olfactory tract also showed no increase in reactivity to the experimenter. Infusions of 0.9% NaCl produced no change in reactivity at any of these sites. The results are consistent with previous evidence that lesions of the olfactory bulbs increase reactivity to the experimenter as a result of incidental damage to more caudal tissue. The present findings suggest that the important caudal region lies ventral to the anterior septum between the vertical arm of the diagonal band of Broca and the rostral limb of the anterior commissure.     

Environmental enrichment attenuates cognitive deficits,but does not alter neurotrophin gene expression in the hippocampus following lateral fluid percussion brain injury

Environmental enrichment attenuates neurological deficits associated with experimental brain injury. The molecular events that mediate these environmentally induced improvements in function after injury are largely unknown, but neurotrophins have been hypothesized to be a neural substrate because of their role in cell survival and neural plasticity. Furthermore, exposure to complex environments in normal animals increases neurotrophin gene expression. However, following an ischemic injury, environmental enrichment decreases neurotrophin mRNA levels. Whether these contrasting findings are attributable to differences between injured and uninjured animals or are dependent upon the specific type of brain injury has not been determined. We examined the effects of 14 days of environmental enrichment following a lateral fluid percussion brain injury on behavior and gene expression of brain-derived neurotrophic factor, its high-affinity receptor, TrkB, and neurotrophin-3 in the rat hippocampus. Environmental enrichment attenuated learning deficits in the injured animals, but neither the injury nor housing conditions influenced neurotrophin/receptor mRNA levels. From these data we suggest that following brain trauma, improvements in learning associated with environmental enrichment are not mediated by alterations in brain-derived neurotrophic factor, TrkB or neurotrophin-3 gene expression.     

Cell-body lesions of the posterodorsal preoptic nucleus or posterodorsal medial amygdala, but not the parvicellular subparafascicular thalamus, disrupt mating in male gerbils

In gerbils, the posterodorsal preoptic nucleus (PdPN) and the lateral part of the posterodorsal medial amygdala (MeApd) express Fos with ejaculation. In contrast, the medial/central part of the MeApd expresses Fos when a sexually experienced male reenters the environment associated with mating. The parvicellular part of the subparafascicular thalamic nucleus (SPFp) of gerbils expresses Fos under both conditions. To study the role of the PdPN and MeApd in male sex behavior, male gerbils were tested for mating before and after these areas were bilaterally lesioned by infusions of N-methyl-D-aspartate (NMDA). Controls received the vehicle or inactive isomer, NMLA. Lesions in either area reduced mounting, but MeApd lesions, which were more complete than PdPN lesions, also delayed ejaculation when males intromitted. To determine if the MeApd and PdPN affect mating via a common pathway, they were bilaterally disconnected by lesioning them unilaterally, contralateral to each other. Other groups received ipsilateral lesions, NMLA, or bilateral lesions of the PdPN or MeApd. In addition, the SPFp was studied using bilateral lesions. MeApd and PdPN lesions again decreased mounting, and this time both lesions, which were quite complete, delayed ejaculation when males intromitted. Contralateral lesions that bilaterally disconnected these cell groups from each other mimicked both effects. Thus, the MeApd and PdPN affect mounting and ejaculation, at least in part, via their connections with each other. In contrast, SPFp lesions did not affect mating. Thus, SPFp cells activated at ejaculation may react to ejaculation rather than trigger it, possibly initiating preparations for paternity.