Nimodipine enhances new learning after hippocampal damage

Verapamil attenuates scopolamine induced cognitive deficits by averting oxidative stress and mitochondrial injury - A potential therapeutic agent for Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial disorder where amyloid beta (Aβ) plaques, Ca2+ dysregulation, excessive oxidative stress, mitochondrial dysfunction and synaptic loss operate synergistically to bring about cholinergic deficits and dementia. New therapeutic interventions are gaining prominence as the morbidity and mortality of AD increases exponentially every year. Treating AD with antihypertensive drugs is thought to be a promising intervention; however, its mechanism of action of ameliorating AD needs further investigation. In this context, the present study explores the protective effect of verapamil, an antihypertensive agent of Ca2+ channel blocker (CCB) class against scopolamine-induced in vitro neurotoxicity and in vivo cognitive impairment. Supplementation of verapamil was found to attenuate oxidative stress by preventing mitochondrial injury, and augment the expression of genes involved in the cholinergic function (mACR1), synaptic plasticity (GAP43, SYP) and Ca2+-dependent memory-related genes (CREB1, CREBBP, BDNF). Further, verapamil treatment in mice attenuated the cognitive and behavioural deficits induced by scopolamine as measured by the elevated plus maze and passive avoidance test (P < 0.05). Thus, the present study demonstrates the neuroprotective effect of verapamil against the pathogenesis of AD such as oxidative stress, mitochondrial dysfunction and cognitive decline. These observations emphasize the importance of ‛Ca2+ dysregulation' and ‛mitochondrial dysfunction' theories in AD and recommends the supplementation of compounds that regulate Ca2+ homeostasis and mitochondrial function in susceptible AD individuals.

Influence of acute or chronic calcium channel antagonists on the acquisition and consolidation of memory and nicotine-induced cognitive effects in mice

Nicotinic cholinergic receptors (nAChRs) form a heterogeneous family of ligand-gated ion channels found in the nervous system. The main objective of our research was to investigate the interaction between cholinergic nicotinic system and calcium homeostasis in cognitive processes using the modified elevated plus maze memory model in mice. The time each mouse took to move from the open arm to either of the enclosed arms on the retention trial (transfer latency, TL2) was used as an index of memory. Our results showed that a single injection of nicotine (0.035 and 0.175 mg/kg) shortened TL2 values, improving memory-related processes. Similarly, L-type calcium channel antagonists (CCAs), i.e., flunarizine, verapamil, amlodipine, nimodipine, nifedipine, and nicardipine (at the range of dose 5–20 mg/kg) administered before or after training, decreased TL2 value improving memory acquisition and/or consolidation. Interestingly, at the subthresold doses, flunarizine, nicardipine, amlodipine, verapamil, and bupropion, a nAChR antagonist, significantly reversed the nicotine improvement of memory acquisition, while flunarizine, verapamil, and bupropion attenuated the improvement of memory consolidation provoked by an acute injection of nicotine (0.035 mg/kg, s.c.). After subchronic administration (14 days, i.p.) of verapamil and amlodipine, two CCAs with the highest affinity for nAChRs, only verapamil (5 mg/kg) impaired memory acquisition and consolidation while both verapamil and amlodipine, at the subthresold, ineffective dose (2.5 mg/kg), significantly reversed the improvement of memory provoked by an acute injection of nicotine (0.035 mg/kg, s.c.). Our findings can be useful to better understand the interaction between cholinergic nicotinic receptors and calcium-related mechanisms in memory-related processes.

Dietary nimodipine delays the onset of methylmercury neurotoxicity in mice

Adult-onset methylmercury (MeHg) exposure is thought to result primarily in sensory and motor deficits but effects on learning are poorly understood. One mechanism by which chronic MeHg may exert its neurotoxicity is via sustained disruption of intracellular calcium homeostasis, with a consequent increase of intracellular Ca(2+) ions in vulnerable neurons. A biochemically heterogeneous group of compounds, calcium channel blockers, have been shown in vitro to attenuate MeHg's toxicity. To evaluate the role of calcium antagonism in MeHg toxicity in vivo, adult BALB/c mice were exposed chronically to 0 or 15 ppm of Hg (as MeHg) via drinking water and to nimodipine, a dihydropryidine, L-type Ca(2+) channel blocker with action in the CNS. Nimodipine was administered orally in diets (0, 20, or 200 ppm, producing approximately 0, 2, or 20 mg/kg/day of nimodipine). An incremental repeated acquisition (IRA) of response chains procedure was used to detect MeHg-induced deficits in learning or motoric function and to evaluate possible neuroprotection by nimodipine. MeHg impaired performance on the IRA task, and this was partially or completely blocked by dietary nimodipine, depending on dose. Measures of learning co-varied with measures of motoric function as indicated by overall response rate. Nimodipine delayed or prevented the behavioral toxicity of MeHg exposure as evidenced by IRA performance; effects on learning seemed secondary to response rate decreases.

Dentate gyrus and spatial behaviour

This article reviews evidence from studies employing colchicine-induced granule cell loss in the adult rat brain, and irradiation-induced hypoplasia of the neonatal dentate gyrus, on the performance of spatial and non-spatial behavioral tasks. The general picture emerging from this analysis reveals that the dentate gyrus granule cells are critically involved in spatial behavior, particularly when this requires the adoption of place strategies. This notion also provides an explanation for the behavioral effects of dentate gyrus granule cell loss seen in apparently non-spatial tasks.

Dentate gyrus-selective colchicine lesion and performance in temporal and spatial tasks

The effects of multiple-site, intradentate, colchicine injections on the performance of a temporal, 'differential reinforcement of low rates of responding' (DRL-20s) task and a spatial, 'delayed non-matching-to-place' (DNMTP) task in a plus-maze were investigated in rats trained in both tasks prior to the lesion. Quantitative analysis revealed a greater than 86% reduction in the dentate gyrus (DG) of the colchicine-injected rats compared to the sham-operated controls. Dentate gyrus damage rendered rats less efficient than sham-operated controls in the performance of the DRL-20s task. The DRL inter-response time (IRT) distribution for the DG-lesioned rats and the sham-operated controls was similar; however, while the distribution peak for the control rats was 20s, it was 16s for the DG-lesioned rats, indicating that the latter rats underestimated time. Performance of the DG-lesioned rats was also disrupted in the DNMTP task. However, DG-lesioned rats recovered control levels of performance during repeated training with an intertrial interval equal to 3s. An increase in intertrial interval in lesioned and sham-operated controls disrupted performance in both groups; however, while DG-lesioned rats performed at chance levels when the intertrial interval was increased to 4min or longer, the sham-operated controls performed at chance levels only when the intertrial interval was increased to 16min. These results seem most parsimoniously interpreted following the cognitive map theory of hippocampal function.

L-type voltage-gated calcium channels are involved in the in vivo and in vitro expression of fear conditioning

Fear conditioning, a behavioral model of fear learning and cue-related anxiety, causes enhanced neuronal transmission in the thalamic to lateral amygdala pathway.(1,2) In the expression phase of learned fear, this increased transmission recorded in vitro is revealed in increased amplitudes of excitatory postsynaptic currents (EPSCs) and occlusion of paired-pulse facilitation (PPF) implicating a presynaptic increase in transmitter release. Here we examined the contribution of L-type calcium channels in fear conditioning. We measured the effect of nimodipine (Nim, 1.5-20 mg/kg), an L-type calcium channel antagonist, on fear-potentiated startle in which startle was assessed in animals receiving paired or unpaired tone and foot shock. Nim administered intraperitoneally blocked fear-potentiated startle but not baseline startle in a dose-dependent manner. We also analyzed the effect of Nim (10 micro M) in vitro on synaptic facilitation of EPSCs and PPF in slices from naïve control, unpaired control, and fear-conditioned animals. In neurons from naïve control animals, Nim had no effect on EPSC amplitude or PPF, but in slices from fear-conditioned rats, Nim reduced EPSC amplitude, suggesting the recruitment of L-type calcium channels within the fear-conditioning pathway. Nim increased PPF in slices from fear-conditioned animals, suggesting that L-type calcium channels may contribute to increased probability of release in fear conditioning. In slices from unpaired animals, Nim decreased synaptic transmission but had little effect on PPF, suggesting that stress or contextual fear learning may induce L-type channel activity in fear-conditioned and unpaired control animal groups. We also analyzed protein expression of the alpha(1C) and alpha(1D) L-type calcium channel subunits isolated from the amygdala and found that alpha(1C) protein was significantly increased in fear-conditioned animals. These findings suggest that L-type calcium channels play a role in the amygdala in cued fear conditioning and have important implications in the treatment of anxiety and in emotional learning and plasticity.

Nimodipine prevents scopolamine-induced impairments in object recognition

The effects of acute administration of the dihydropyridine calcium channel antagonist, nimodipine, were studied on the actions of scopolamine in the object recognition test. Scopolamine at 0.125 mg/kg decreased the difference in the time spent exploring novel and familiar objects when given either 15 min before, or immediately after, exposure to objects. Administration of nimodipine at 10 mg/kg, or 1 mg/kg, at the same time as the scopolamine completely prevented the deleterious effects on memory in this task. This effect was seen when nimodipine and/or scopolamine were given prior to the object exposure and also when the drugs were given after the experience of seeing the objects. Nimodipine had no effects on performance when given in the absence of scopolamine. This lack of change in total time spent exploring the objects indicated that the effects of scopolamine and nimodipine were not due to changes in motor coordination or alertness. The results are discussed in the light of the role of cholinergic transmission in memory and the known actions of dihydropyridines on brain function.

Nimodipine prevents the effects of ethanol in tests of memory

The effects of acute administration of the dihydropyridine calcium channel antagonist, nimodipine, were studied on the actions of ethanol in the radial arm maze and the object recognition test. In the former test, the effects of the drugs were examined on the performance in finding the four baited arms, after previous training in this task. Ethanol, at 1 g/kg, increased both the number of re-entries into baited arms (counted as errors of working memory) and the total number of arm choices required to complete the task. Administration of nimodipine, 10 mg/kg, with the ethanol, completely prevented the deleterious effects on memory in this task, but had no effects on the performance when given in the absence of ethanol. In the object recognition task, ethanol, 1 g/kg, significantly decreased the differences in the time spent exploring novel and familiar objects. Nimodipine, 10 mg/kg, given with the ethanol, completely prevented this effect, but nimodipine alone had no effects. The lack of changes in total exploration times indicated that the effects of ethanol in these tests were not due to loss of motor co-ordination or of alertness. The results are discussed in the light of the known actions of the drugs on brain function.

Calcium channel antagonists enhance retention of passive avoidance and maze learning in mice

Although a number of studies have shown that treatment with calcium channel antagonists (CCAs) can ameliorate impairments in learning and memory in aged animals, evidence for a general nootropic effect of CCAs in neurologically normal young adult animals is ambiguous. This study attempts to resolve some of this ambiguity by comparing the effects of several CCAs on retention of passive avoidance learning and acquisition and retention of appetitively motivated spatial discrimination learning in young adult mice. Animals were trained in a step through passive avoidance apparatus and, immediately after training, injected subcutaneously with different doses of nimodipine, nifedipine, amlodipine, flunarazine, diltiazem, or verapamil. Retention was tested 24 h after training. In the maze-learning task mice were treated with the same doses of the aforementioned CCAs immediately after a brief training session in a linear maze and retention was tested 24 h after training. The most effective dose of each agent in the maze-retention experiment was administered to additional groups of animals 1 h prior to training to determine the effects of CCAs on acquisition processes. The effects of central administration of CCAs were examined by intracerebroventricular injection of different doses of amlodipine immediately after passive avoidance training. Results showed (1) all peripherally administered drugs except verapamil facilitated retention of passive avoidance training in a dose-dependent manner, (2) all drugs dose dependently facilitated retention of linear maze learning, (3) all doses of the drugs (except verapamil) which facilitated maze retention also facilitated maze learning, and (4) central administration of the dihydropyridine amlodipine produced a dose-dependent facilitation of the retention of passive avoidance learning. These data indicate that drugs which block calcium channels can enhance retention of two different types of learning in mice.

Differential effects of a dihydropyridine calcium channel antagonist on the components of ethanol tolerance

The dihydropyridine calcium channel antagonist, nimodipine, was found to decrease the extent of tolerance that developed to the ataxic action of ethanol in experimental designs in which the tolerance was not context-specific, when ethanol was given by liquid diet. When ethanol was given by injection, so that cues were present for the effects of ethanol during the chronic treatment, tolerance to the ataxic actions of ethanol was unaffected. Nimodipine, however, decreased the tolerance to the hypothermic actions of ethanol, when the ethanol was given by injection. When the rats were given practice sessions on the motor task while under the influence of the ethanol, during the chronic treatment, nimodipine did not affect tolerance to the ataxic actions of ethanol. When nimodipine was given before the motor task learning and ethanol after the practice sessions, the tolerance to the ataxic effect of ethanol was increased. A similar schedule of drug treatment with the NMDA antagonist CGP37849 given before the practice sessions, and ethanol afterwards, resulted in decreased tolerance to ethanol. It is suggested that these changes in ethanol tolerance may be explained by dual actions of nimodipine in, firstly, decreasing the form of tolerance to ethanol that is not dependent on contextual cues and, secondarily, in increasing the learning of a motor task.

Spatial learning deficits in rats after injection of vincristine into the dorsal hippocampus

In the present study, performance in the Morris water escape task after bilateral lesioning of the dorsal hippocampus induced by the microtubule poison vincristine is discussed as a cognitive deficit model in rats. As we are especially interested in spontaneous or pharmacologically induced recovery processes after experimentally induced cognitive dysfunctions, the model should fulfil a number of criteria. Firstly, a clear dose-effect relationship between the dose of vincristine and the amount of spatial learning impairments should be present. Secondly, lesions must remain within the target area. Thirdly, there should be an observable behavioural recovery or compensation of the induced deficit. Two experiments evaluated the influence of the application volume (experiment 1) and the concentration of vincristine (experiment 2) on lesion location and size, and on spatial learning. The results of both experiments demonstrated that the effect of vincristine on the performance in the Morris water escape task seems to be characterized by an "all-or-none" relationship. Concentrations above a "threshold" value induced severe damage in the hippocampus and adjacent brain structures, whereas concentrations below the "threshold" value had marginal or no effects. The non-selective and highly toxic properties of vincristine make this neurotoxin an unsuitable tool for the establishment of a learning and memory deficit model.

Attenuation by nimodipine of amitriptyline-induced avoidance impairment in mice

The effects of the dihydropyridine calcium channel blocker nimodipine on avoidance impairment induced by the tricyclic antidepressant amitriptyline were assessed during shuttle-box training and in previously trained mice of the DBA/2 strain. Nimodipine (0, 0.5, 1, 2.5, or 5 mg/kg) had no effect alone, but attenuated the avoidance impairment induced by 5 mg/kg amitriptyline on avoidance acquisition, as well as on a previously learned avoidance response. The avoidance improving action of the calcium channel blocker was less evident in mice receiving a larger dose (7.5 mg/kg) of the antidepressant drug. The effect of nimodipine did not appear to be specifically related to the avoidance impairment induced by amitriptyline, because the calcium antagonist also attenuated the avoidance impairing action of the neuroleptic chlorpromazine. The avoidance impairment induced by amitriptyline and chlorpromazine, and the related ameliorating action of nimodipine, seem imputable to drug effects on the performance of the avoidance response, rather than to interferences with learning processes. The results suggest that, in the case of concomitant administration, nimodipine could alleviate adverse side effects of tricyclic antidepressant, i.e., psychomotor disturbances.

Nimodipine on shuttle-box avoidance learning in mice: no impairment but slight improvement

The dihydropyridine calcium channel antagonist nimodipine was tested in mice of CD-1, C57BL/6, and DBA/2 strains subjected to shuttle-box avoidance training. In contrast with some findings of other authors showing impairment of shuttle-box avoidance learning by low doses of the drug in rats, nimodipine given IP before each training session at doses of 0.25, 0.5, 1, 2.5, or 5 mg/kg never impaired avoidance acquisition in mice. On the contrary, one dose of nimodipine (1 mg/kg) significantly improved avoidance acquisition in mice of the DBA/2 strain. The drug failed to impair avoidance performance in DBA/2 mice even if given acutely in the middle (third session) or at the end (fifth session) of the training period. The results contradict studies showing cognitive impairment induced by calcium channel blockers, and may provide some limited evidence in support of improved cognitive function in normal animals, although this effect is much less evident than in aged or brain-damaged subjects.

Effects of nimodipine, felodipine and amlodipine on electroconvulsive shock-induced amnesia in the rat

The effects of various doses (0.03, 0.1, 0.3 or 1.0 mg/kg) of the Ca2+ channel blockers nimodipine, felodipine and amlodipine on the learning ability of rats exposed to electroconvulsive shock were examined. The animals were trained in a passive avoidance procedure. The drugs tested were injected 30 min before the learning trial started. The electroconvulsive shock was given immediately after the learning trial response had been acquired. A passive avoidance retention test was performed 24 h later. It was found that electroconvulsive shock strongly impaired the retention of the passive avoidance response. Nimodipine, felodipine and amlodipine did not influence the passive avoidance behavior in the sham electroconvulsive shock group, but significantly improved the retention deficits in the animals exposed to electroconvulsive shock. These findings support the hypothesis that perturbations in Ca2+ homeostasis can contribute to the memory deficits associated with electroconvulsive shock. The antiamnestic effects of the substances tested make them interesting candidates for clinical trials in patients with cognitive impairment caused by electroconvulsive shock therapy.

Recovery of function after brain damage: differential effects of blocking calcium uptake during the recovery of a learned behavior and the performance of the recovered behavior following neocortical brain injury

The recovery of a learned behavior following brain damage is typically considered to have occurred when the brain-injured individual reattains the performance criterion that defined original preoperative learning. While this is obviously correct from an operational point of view, it does not necessarily mean that the consequences of the brain injury have been reversed, particularly with regard to the sustained performance of the supposedly recovered behavior. The present research attempted a more comprehensive evaluation of the behavioral effects of localized neocortical injury by investigating how a calcium channel blocker would effect (a) the original preoperative acquisition, (b) the initial postoperative recovery, and (c) the subsequent long-term performance of a brightness discrimination learned by rats subjected to injuries of their visual neocortex. The results demonstrated that notwithstanding the brain-injured rat's ability to reattain the performance criterion used to define preoperative learning, its long-term performance of this recovered behavior was significantly inferior to that of a normal rat. More importantly, the present data suggest that there are important differences between the initial postoperative recovery of a behavior and its long-term performance since the same drug that will facilitate initial recovery has just the opposite effect with respect to the animals long-term postoperative performance of the behavior.

Nimodipine facilitates retention of the classically conditioned nictitating membrane response in aged rabbits over long retention intervals

Aged rabbits initially underwent 18 days of acquisition of the classically conditioned nictitating membrane response (NMR) using a tone conditioned stimulus (CS) and an air puff unconditioned stimulus (UCS). They were then treated with a low or high dose of nimodipine or a vehicle for 90 days. During this time no further CS-UCS pairings were presented. They underwent testing for retention of the conditioned response (CR) at 30 and 90 days. Retention testing consisted of 20 presentations of the CS alone. Rabbits in the control condition retained 46.4% of their predrug levels of conditioned responding and rabbits receiving the low dose of nimodipine retained 37.3% of their predrug levels after 30 days. After 90 days, retention in these animals declined to 8.1% and 14.1%, respectively. In contrast, rabbits receiving the high dose of nimodipine retained 85% of their predrug learning at 30 days with little decline at 90 days (77.1%). Nonassociative factors such as sensitivity to the CS or UCS could not explain these effects.

Nimodipine's functional benefits depend on lesion completeness in medial septal area

The effects of a 4-day nimodipine treatment (70 micrograms/kg IP beginning on the day of surgery) given to rats with lesions directed at the medial septal area were monitored for 120 days. Body weight, water intake, open-field activity, rearing, hole-poking, and repetitive motor acts were periodically measured through 120 postsurgical days. Although no differences were found in water intake between any of the groups, the body weights of rats with any medial septal damage, whether treated with nimodipine or not, were lower than rats with control operations by postsurgery day 120. Rats with any medial septal damage, whether treated with nimodipine or not, had lower rearing frequencies, rearing durations, and hole-poking frequencies than controls on all test days. However, rats with complete medial septal lesions treated with nimodipine exhibited movement in the open field and frequencies of stereotyped, species-typical acts similar to those of control rats by postsurgery day 60. This nimodipine effect was not observed in rats with partial lesions of the medial septal region. This study emphasizes that a brief administration of nimodipine shortly after brain damage can influence behavioral changes 40-60 days after surgery, but that this effect was not apparent in rats with only partial medial lesions.

Levemopamil reduces spatial learning deficit following transient occlusion of common carotid arteries in normotensive rats

The effect of the calcium channel blocker and 5-HT2 antagonist levemopamil on spatial learning impairment of rats subjected to transient bilateral clamping of carotid arteries (BCCA) was investigated. In addition, the acute effect of BCCA on local cerebral blood flow was measured in the presence and absence of levemopamil in a separate group of rats. Pretreatment with levemopamil prevented the BCCA-induced increase in escape latency during the test trials and ameliorated the BCCA-induced decrease in spatial bias during a probe trial. Under these experimental conditions, local cerebral blood flow fell to near ischaemic values in all mid- and forebrain regions during occlusion of the carotid arteries. However, pretreatment with levemopamil affected the BCCA-induced blood flow changes only in one brain area of 34 investigated. The present data suggest that pretreatment with levemopamil reduces impairment in spatial behaviour and that this effect seems not related to the compound's cerebral vasodilatory action, but to direct neuronal mechanisms.

Enhanced acquisition of reversal training in a spatial learning task in rats treated with chronic nimodipine

Nimodipine levels were measured in blood and brain of rats implanted SC with sustained-release pellets of nimodipine (0, 10, 20, or 40 mg). Dose-dependent levels of nimodipine were detected in both plasma and brain. These results indicated the possible usefulness of these pellets in behavioral studies where long-term treatment is required. Therefore, the effects of chronic nimodipine, using 40-mg pellets, were examined on the performance of young, middle-aged, and aged rats in the Morris water maze. Following implantation of either nimodipine or placebo pellets, rats were trained for 6 days (three sessions/day) followed by 6 days of reversal training. During both initial and reversal training, every sixth trial was a probe trial. During initial training, there were clear age-related acquisition deficits in place training, with no effects of chronic nimodipine. Nimodipine did, however, enhance performance of rats during the first and second probe trials of reversal training. Time spent in the training quadrant by nimodipine-treated rats was approximately 30% longer on the first reversal probe and 35% longer on the second reversal probe than time spent in this quadrant by placebo-treated animals. These results indicate that chronic nimodipine enhances the performance of normal animals in reversal training on a spatial learning task.

Effects of flunarizine on ocular motor and postural compensation following peripheral vestibular deafferentation in the guinea pig

The aim of the present study was to determine if the calcium channel antagonist flunarizine would affect the time course of vestibular compensation for unilateral labyrinthectomy (UL) in guinea pigs. Animals received either a single IP injection of flunarizine 1 h pre-UL or a series of IP injections every 6 h for 24 h post-UL, starting at 6 h post-UL. Flunarizine was dissolved in 50-100% DMSO or suspended in 10% Tween-80 and administered at a dose of 10 mg/kg in the pre-UL condition and 10 or 20 mg/kg in the post-UL condition. All injections were 1 ml/kg in volume. Spontaneous nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT) were measured using video analysis. When dissolved in DMSO and administered 1 h pre-UL, 10 mg/kg flunarizine had a small but significant effect on the rate of RHT compensation; otherwise, flunarizine had no significant effects on SN, YHT, or RHT when dissolved in DMSO. When suspended in Tween-80, 10 mg/kg flunarizine pre-UL resulted in a significant decrease in SN frequency and YHT relative to the control group, although the magnitude of the differences was small. When 20 mg/kg was given post-UL, both SN and YHT showed a small but significant change in the rate of compensation. No significant differences in RHT were observed. These results demonstrate that IP administration of flunarizine at a dose of 10-20 mg/kg IP has little effect on vestibular compensation compared to the effects obtained with low IM doses (0.8 mg/kg) of verapamil given 1 h pre-UL.

Facilitation of shuttle-box avoidance behaviour in mice treated with nifedipine in combination with amphetamine

The dihydropyridine calcium channel antagonist nifedipine, tested in mice of CD-1, C57BL/6 and DBA/2 strains, at doses of 2.5, 5 and 10 mg/kg IP, had no significant effect on shuttle-box avoidance acquisition. Nifedipine also failed to affect performance retention in CD-1 mice subjected to a one-trial passive avoidance task (step-through). While ineffective alone, nifedipine strongly enhanced the shuttle-box avoidance facilitating action of amphetamine (1 and 2 mg/kg IP) in low performing CD-1 mice. The results indicate that although calcium channel blockers do not affect learning in avoidance paradigms in normal animals, they can interfere with the effects of other centrally acting drugs. Calcium antagonists might interfere with neuronal changes induced by amphetamine, but at present it is difficult to explain the strong avoidance facilitation produced by combinations of nifedipine and amphetamine. A possibility that the action of nifedipine on cerebral circulation is involved in the amphetamine-nifedipine interaction cannot be excluded.

Pretreatment with MK-801 reduces spontaneous nystagmus following unilateral labyrinthectomy

Unilateral labyrinthectomy results in a syndrome of ocular motor and postural symptoms which abate over time in a process of behavioural recovery known as vestibular compensation. We have previously suggested that an increased Ca2+ influx in ipsilateral vestibular nucleus (VN) neurons at the time of the unilateral labyrinthectomy may exacerbate the depression of VN resting activity caused by the loss of excitatory input from the VIIIth nerve. In order to further test this hypothesis, we administered (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 1.0 or 2.5 mg/kg i.p.), which blocks Ca2+ influx via NMDA receptor-mediated ion channels, to guinea pigs 0.5 h before unilateral labyrinthectomy and examined the effects on three symptoms of unilateral labyrinthectomy: spontaneous ocular nystagmus, yaw head tilt and roll head tilt. Pretreatment with MK-801 significantly altered the time course of the vestibular compensation of spontaneous nystagmus and yaw head tilt but had no significant effect on roll head tilt; in particular, 2.5 mg/kg MK-801 depressed spontaneous nystagmus frequency at 10 and 20 h post-labyrinthectomy relative to saline controls (P less than 0.05, post-hoc Scheffé F-test). The reduction in spontaneous nystagmus frequency was not simply a result of extended anesthesia, since other control animals, which received additional injections of the general anesthetic in order to achieve equivalent sleep times to the MK-801 group, did not show reduced spontaneous nystagmus frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurological correlates of unilateral and bilateral "strokes" of the middle cerebral artery in the rat

Rats with unilateral lesions of the middle cerebral artery (MCA) were tested for the ability to detect (touch) and remove a square of adhesive tape from each forepaw, and for performance on a number of neurological tests (e.g., placing and hopping reflexes, activity). Rats with MCA damage showed deficits in both touching and removing the tape from the paw contralateral to the damage, but not ipsilateral to the damage, while performing within normal limits on the other tests. After scores of the MCA rats dropped into the control group range on the adhesive tape test, they sustained damage to the opposite MCA. This did not reinstate the original deficit, suggesting that the recovery seen after the unilateral lesion was not mediated by the opposite cortex. The second lesion, however, caused a deficit in removing the adhesive tape from the limb opposite the new stroke. Some of the rats that originally had sham operations received bilateral MCA lesions at this time. These animals showed much more severe deficits on the adhesive tape test than the rats with sequential strokes. Rats with bilateral MCA damage (simultaneous or sequential) also slipped on a long narrow plank more often than control animals. Nimodipine did not enhance recovery on any of the behavioral measures.

Nimodipine improves spatial working memory and elevates hippocampal acetylcholine in young rats

The calcium channel blocker nimodipine has been reported to improve cognitive performance in aged and brain-damaged animals. In the present study, the effects of nimodipine and placebo on spatial working memory and hippocampal acetylcholine were studied in young Fischer-344 rats. Nimodipine or placebo was administered via subcutaneously implanted, sustained-release pellets. Each active pellet contained 20 mg of nimodipine and released the drug over approximately 21 days. Two days after the drug or placebo pellets were implanted, training in the 8-arm radial maze started and continued for 12 days. Rats were required to learn a win-shift surgery. Nimodipine-treated animals learned the maze more rapidly than a placebo-treated group as indicated by the number of correct choices out of the first eight arms visited (p less than 0.001). Treated rats also made twice as many choices per unit time during the first week of training (p = 0.005). To assess hippocampal acetylcholine release, in vivo microdialysis was performed while animals were awake and unrestrained, 19-21 days after pellet implantation. A probe with a 3 mm semipermeable tip was placed in the hippocampus (CA1 and dentate gyrus), and individual microliters dialysate samples were collected at 2 microliters/min and immediately analyzed by high performance liquid chromatography with electrochemical detection. Significantly higher extracellular ACh levels were found in nimodipine-treated rats (71.4 +/- 3.6 nM; n = 4) compared to controls (52.5 +/- 2.5 nM; n = 5) (p = 0.003) and in another group of rats of the same age that received identical drug treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanisms of brainstem plasticity. The vestibular compensation model

Vestibular compensation is the process of behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (unilateral labyrinthectomy, UL). Since UL results in a permanent loss of vestibular input from the ipsilateral vestibular (VIIIth) nerve, vestibular compensation is attributed to CNS plasticity and has been used as a general model of lesion-induced CNS plasticity. Behavioral recovery from the ocular motor and postural symptoms of UL is correlated with a partial return of resting activity to neurons in the vestibular nucleus (VN) on the deafferented side (the "deafferented VN"), and lesions to the deafferented VN prevent compensation; therefore, the regeneration of resting activity within the deafferented VN is believed to have a causal role in vestibular compensation. The biochemical mechanisms responsible for the adaptive neuronal changes within the deafferented VN are poorly understood. Neuropeptide hormone fragments, such as adrenocorticotrophic hormone (ACTH)-4-10, have been shown to accelerate vestibular compensation and can act directly on some VN neurons in vitro. Antagonists for the N-methyl-D-aspartate (NMDA) receptor have been shown to inhibit vestibular compensation if administered early in the compensation process. Biochemical studies in frog indicate marked alterations in the phosphorylation patterns of several proteins during compensation, and the in vitro phosphorylation of some of these proteins is modulated by ACTH-(1-24), calcium (Ca2+), and calmodulin or protein kinase C. It is therefore possible that ACTH fragments and NMDA antagonists (via their effects on NMDA receptor-mediated Ca2+ channels) modulate vestibular compensation through their action on Ca(2+)-dependent pathways within VN neurons. Recent studies have shown that some Ca2+ channel antagonists and the Ca(2+)-dependent enzyme inhibitor calmidazolium chloride facilitate vestibular compensation. How the regulation of Ca2+ may be related to the neuronal changes responsible for vestibular compensation is unclear at present.

Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

This paper reviews the literature relating to the neurochemical basis of vestibular compensation, a process of behavioral recovery which occurs following the removal of afferent input from one labyrinth (unilateral labyrinthectomy, UL). Although vestibular compensation is known to be correlated with a return of resting activity to the vestibular nucleus (VN) ipsilateral to the UL (the deafferented VN), the neurochemical mechanisms by which this neuronal recovery occurs, are unknown. At present, there is little evidence to support the hypothesis that denervation supersensitivity of excitatory amino acid, dopamine, norepinephrine or acetylcholine receptors in the deafferented VN, is responsible for vestibular compensation: binding studies for glutamate or acetylcholine do not support an upregulation of these receptor types. However, changes in the affinity or efficacy of these receptor complexes cannot be ruled out. There are still many neurotransmitter systems, such as serotonergic and histaminergic systems, which have not been investigated in relation to vestibular compensation. In several species it has been shown that treatment with adrenocorticotropic hormone, fragment 4-10 (ACTH-(4-10], can accelerate vestibular compensation. It is not clear how these drugs exert their effects. In vitro electrophysiological studies have shown that VN neurons are capable of generating resting activity in the absence of their normal afferent inputs and it is possible that these neurons have pacemaker-like membrane characteristics which contribute to the regeneration of activity following UL. Recent biochemical studies have revealed changes in the phosphorylation patterns of a number of proteins during compensation. The possible relationship between these phosphorproteins and the synaptic or membrane changes which are responsible for vestibular compensation remains to be determined.

Sensorimotor cortical lesion effects and treatment with nimodipine

Rats with unilateral ablations of the sensorimotor cortex and others with control operations were tested for their ability to touch and remove adhesive tape applied to both forelimbs. Half of each group was administered a calcium channel antagonist (nimodipine) for two weeks following the lesions and the other half received vehicle. The rats with lesions showed a bias to remove the ipsilateral stimulus first and exhibited contralateral deficits relative to control animals. Nimodipine was shown to reduce the contralateral stimulus removal time when the animals began testing two weeks after surgery, but not when testing began 1 day after surgery and overlapped the period of drug administration. Lesion effects also appeared on tests for neurologic impairment and activity, but nimodipine did not reduce these deficits. These findings indicate that nimodipine has the potential to reduce some deficits after sensorimotor cortical lesions, but that the effects of this drug may be task specific.