Aging in the cerebellum and hippocampus and associated behaviors over the adult life span of CB6F1 mice.

In the present study we examined the effects of normal aging in the hippocampus and cerebellum, as well as behaviors associated with these substrates. A total of 67 CB6F1 hybrid mice were tested at one of five ages (4, 8, 12, 18 or 25 months) on the context pre-exposure facilitation effect (CPFE) modification of fear conditioning, rotorod, Barnes maze, acoustic startle, Morris water maze (MWM) and 500-ms trace eyeblink classical conditioning (EBCC). Behavioral tasks were chosen to increase the ability to detect age-related changes in learning, as trace EBCC is considered a more difficult paradigm (compared to delay EBCC) and the CPFE has been found to be more sensitive to hippocampus insults than standard contextual fear conditioning. To assess the effects of age on the brain, hippocampus volume was calculated and unbiased stereology was used to estimate the number of Purkinje neurons in the cerebellar cortex. A significant, age-related loss of Purkinje neurons was found-beginning at 12 months of age-and hippocampus volume remained stable over the adult life span. Age-related impairment was found, beginning at 12-18 months in the rotorod, and mice with fewer Purkinje neurons showed greater impairment in this task. CB6F1 mice retained auditory acuity across the life span and mice aged 25 months showed significant age-related impairment in the EBCC task; however, deficits were not associated with the loss of Purkinje neurons. Although the CPFE task is considered more sensitive to hippocampus insult, no age-related impairment was found. Spatial memory retention was impaired in the Barnes maze at 25 months, but no significant deficits were seen in the MWM. These results support the finding of differential aging in the hippocampus and cerebellum.

Stereological estimation of Purkinje neuron number in C57BL/6 mice and its relation to associative learning.

Cerebellar Purkinje neurons are among the most vulnerable neurons in the CNS. Impairment in Purkinje neurons has consequences for cerebellar cortical-dependent forms of behavior. The primary aim of this study was to evaluate Purkinje neuron number over the lifespan of C57BL/6 mice. Stereological estimates of the total number of Purkinje neurons in cerebellar cortex were made in 25 C57BL/6 mice aged 4, 8, 12, 18, and 24 months. Delay eyeblink classical conditioning to a white noise conditioned stimulus was also assessed for 10 daily sessions. Statistically significant age differences in Purkinje neuron number were observed beginning at 18 months. Delay eyeblink conditioning also showed significant age-related impairment, at least some of which resulted from age-related deficits in hearing. Eliminating the hearing-impaired 18- and 24-month-old mice from the analysis, the correlation between Purkinje neuron number and rate of conditioning was -0.435 (P=0.053) in 15 younger mice aged 4-12 months. Purkinje neurons are one of the few types of neurons showing significant age-associated loss. Results indicate that individual variation in Purkinje neuron number is related to eyeblink conditioning in young organisms suggesting that reserves of neuron numbers against which individuals draw are defined early in life.

Mecamylamine interactions with galantamine and donepezil: effects on learning, acetylcholinesterase, and nicotinic acetylcholine receptors.

Patch-clamp recordings of single ion channel activity demonstrated that donepezil, but not galantamine, could be blocked by the nicotinic cholinergic antagonist mecamylamine, suggesting that galantamine acted at a separate (allosteric) site. The aim of this experiment was to demonstrate at a whole organism, behavioral level that galantamine, but not donepezil, could reverse mecamylamine-induced learning impairment. Forty-four young female rabbits received 15 sessions in the 750-ms delay eyeblink classical conditioning procedure, after one of five drug treatments: 0.5 mg/kg mecamylamine, 3.0 mg/kg donepezil, 0.5 mg/kg mecamylamine plus 3.0 mg/kg galantamine, 0.5 mg/kg mecamylamine plus 3.0 mg/kg donepezil, or sterile saline vehicle. An additional 24 young female rabbits were tested in the explicitly unpaired condition after treatment with the same mecamylamine plus galantamine or donepezil combinations or with vehicle. In a previous study we demonstrated that 3.0 mg/kg galantamine facilitated learning in young rabbits. Donepezil (3.0 mg/kg) did not facilitate learning in this experiment. However, both galantamine and donepezil reversed the deleterious effects of mecamylamine on learning. Significant differences in plasma and brain acetylcholinesterase levels were detected among the drug treatment groups. Fifteen daily injections did not produce statistically significant changes in nicotinic receptor binding in any of the five treatment groups. One possible interpretation of these results is that donepezil affected nicotinic acetylcholine receptors by raising the synaptic level of acetylcholine and hence, the probability of receptor activation, whereas galantamine bound to distinct allosteric sites not blocked by mecamylamine.

MRI-assessed volume of cerebellum correlates with associative learning.

Richard F. Thompson's cerebellar model of classical eyeblink conditioning highlights Purkinje cells in cerebellar cortex and principal cells in the deep cerebellar nucleus as the integrating cells for acquisition of conditioned responses (CRs). CR acquisition is significantly slower in rabbits with lesions to cerebellar cortex and in Purkinje cell-deficient mice that lose all cerebellar cortical Purkinje cells. Purkinje cells are the largest neurons in the cerebellum and contribute significantly to cerebellar volume. Magnetic resonance imaging (MRI) was used to assess cerebellar volume in humans. Cerebellar volume was related to eyeblink conditioning (400-ms delay procedure) in 8 adults (21-35 years) and compared to 8 older adults (77-95 years) tested previously (Woodruff-Pak, Goldenberg, Downey-Lamb, Boyko, & Lemieux, 2000). In the young adult sample, there was a high correlation between percentage of CRs in a session and cerebellar volume (corrected for total intracranial volume [TIV], r =.58, p =.066). There were statistically significant age differences in cerebellar volume, t(14) = 8.96, p <.001, and percentage of CRs, t(14) = 3.85, p <.002, but no age difference in TIV. Combining the young and older adult sample, the correlation between percentage of CRs and cerebellar volume (corrected for TIV) was.832 (p <.001). Cerebellar volume showed age-related deficits likely due to Purkinje cell loss. Individual differences in classical eyeblink conditioning are associated with differences in cerebellar volume, supporting Thompson's model of a cerebellar cortical role in facilitating this form of associative learning.

Eyeblink classical conditioning differentiates normal aging from Alzheimer's disease.

Eyeblink classical conditioning is a useful paradigm for the study of the neurobiology of learning, memory, and aging, which also has application in the differential diagnosis of neurodegenerative diseases expressed in advancing age. Converging evidence from studies of eyeblink conditioning in neurological patients and brain imaging in normal adults document parallels in the neural substrates of this form of associative learning in humans and non-human mammals. Age differences in the short-delay procedure (400 ms CS-US interval) appear in middle age in humans and may be caused at least in part by cerebellar cortical changes such as loss of Purkinje cells. Whereas the hippocampus is not essential for conditioning in the delay procedure, disruption of hippocampal cholinergic neurotransmission impairs acquisition and slows the rate of learning. Alzheimer's disease (AD) profoundly disrupts the hippocampaL cholinergic system, and patients with AD consistently perform poorly in eyeblink conditioning. We hypothesize that disruption of hippocampal cholinergic pathways in AD in addition to age-associated Purkinje cell loss results in severely impaired eyeblink conditioning. The earliest pathology in AD occurs in entorhinal cortical input to hippocampus, and eyeblink conditioning may detect this early disruption before declarative learning and memory circuits become impaired. A case study is presented in which eyeblink conditioning detected impending dementia six years before changes on other screening tests indicated impairment. Because eyeblink conditioning is simple, non-threatening, and non-invasive, it may become a useful addition to test batteries designed to differentiate normal aging from mild cognitive impairment that progresses to AD and AD from other types of dementia.

Galantamine: effect on nicotinic receptor binding, acetylcholinesterase inhibition, and learning.

Classical eyeblink conditioning is a well-characterized model paradigm that engages the septohippocampal cholinergic system. This form of associative learning is impaired in normal aging and severely disrupted in Alzheimer's disease (AD). Some nicotinic cholinergic receptor subtypes are lost in AD, making the use of nicotinic allosterically potentiating ligands a promising therapeutic strategy. The allosterically potentiating ligand galantamine (Gal) modulates nicotinic cholinergic receptors to increase acetylcholine release as well as acting as an acetylcholinesterase (AChE) inhibitor. Gal was tested in two preclinical experiments. In Experiment 1 with 16 young and 16 older rabbits, Gal (3.0 mg/kg) was administered for 15 days during conditioning, and the drug significantly improved learning, reduced AChE levels, and increased nicotinic receptor binding. In Experiment 2, 53 retired breeder rabbits were tested over a 15-wk period in four conditions. Groups of rabbits received 0.0 (vehicle), 1.0, or 3.0 mg/kg Gal for the entire 15-wk period or 3.0 mg/kg Gal for 15 days and vehicle for the remainder of the experiment. Fifteen daily conditioning sessions and subsequent retention and relearning assessments were spaced at 1-month intervals. The dose of 3.0 mg/kg Gal ameliorated learning deficits significantly during acquisition and retention in the group receiving 3.0 mg/kg Gal continuously. Nicotinic receptor binding was significantly increased in rabbits treated for 15 days with 3.0 mg/kg Gal, and all Gal-treated rabbits had lower levels of brain AChE. The efficacy of Gal in a learning paradigm severely impaired in AD is consistent with outcomes in clinical studies.

A nicotinic cholinergic agonist (GTS-21) and eyeblink classical conditioning: acquisition, retention, and relearning in older rabbits.

Previously we demonstrated that GTS-21, a nicotinic cholinergic agonist, ameliorated eyeblink classical conditioning deficits in older rabbits. The present experiment was undertaken to replicate and extend these results by examining the effects of GTS-21 on retention and relearning. Retired breeder rabbits received 15 daily injections of 0.5 mg/kg GTS 21 (n = 8) or sterile saline vehicle (n = 8) during acquisition training, and no further injections occurred. Acquisition of conditioned responses (CRs) was significantly better in GTS-21-treated rabbits. During the first tone-alone retention session in week 6 of the experiment, rabbits initially treated with GTS-21 produced significantly more CRs than vehicle-treated rabbits. There were no group differences in retention at the 13-week retest. Differences in relearning were in the predicted direction but did not attain statistical significance. Results indicate that treatment with GTS-21 ameliorates learning beyond the period when the drug is actually administered.

Nicotinic modulation in an animal model of a form of associative learning impaired in Alzheimer's disease.

Eyeblink classical conditioning is a widely used associative learning paradigm that has striking behavioral and neurobiological parallels between humans and other mammals. Eyeblink conditioning is impaired in older organisms, and patients with Alzheimer's disease (AD) are impaired beyond the normal aging deficit. The cholinergic system is of demonstrated involvement in eyeblink conditioning. Blockade of nicotinic cholinergic receptors with mecamylamine prolonged acquisition of conditioned responses (CRs) in young adult rabbits, and the nicotinic agonist, GTS-21 ameliorated conditioning deficits in older rabbits. Galantamine induces allosteric modulation of nicotinic cholinergic receptors to increase acetylcholine release as well as acting as an acetylcholinesterase inhibitor. Galantamine doses of 0.0, 1.0, 2.0, 3.0, and 4.0 mg/kg were tested in ten daily sessions in 40 retired breeder rabbits (mean age = 29 months) in the 750 ms delay conditioning paradigm. A dose of 3 mg/kg galantamine was effective in improving conditioning in older rabbits, enabling them to achieve learning criterion rapidly and to produce a very high percentage of CRs. Control tests of rabbits in explicitly unpaired conditions demonstrated that non-associative factors could not account for the results. The efficacy of galantamine in a learning paradigm that shows severe impairment in AD indicates that the drug may be effective as a cognition-enhancer in AD.

Cerebellar volume in humans related to magnitude of classical conditioning.

Neural circuits in the cerebellum are essential for eyeblink classical conditioning, and hippocampal activation is also present during acquisition. Anatomical (volumetric) brain MRI, delay eyeblink conditioning and neuropsychological tests were administered to eight healthy older subjects. The correlation between cerebellar volume (corrected for total cerebral volume) and conditioned response percentage was 0.81 (p < 0.02), but neither hippocampal nor total cerebral volume correlated with conditioning or any neuropsychological test scores. There was no relationship between age and cerebellar volume, but the correlation between hippocampal volume and age was -0.80 (p < 0.02). These volumetric results add to the increasing evidence in humans demonstrating a relationship between the integrity of the cerebellum and eyeblink classical conditioning.

Eyeblink classical conditioning: hippocampal formation is for neutral stimulus associations as cerebellum is for association-response.

Extensive evidence has been amassed that the cerebellum, hippocampus, and associated circuitry are activated during classical conditioning of the nictitating membrane/eyeblink response. In this article, the authors argue that the cerebellum is essential to all eyeblink classical conditioning paradigms. In addition, the septohippocampal system plays a critical role when the classical conditioning paradigm requires the formation of associations in addition to the simple association between the conditioned and unconditioned stimuli. When only a simple conditioned stimulus--unconditioned stimulus association is needed, the septohippocampal system has a more limited, modulatory role. The neutral stimulus association versus simple association-response distinction is one of the ways in which declarative or relational memory can be separated from nondeclarative or nonrelational memory in classical conditioning paradigms.

Relationships among age, conditioned stimulus-unconditioned stimulus interval, and neuropsychological test performance.

To evaluate the effect of age at various conditioned stimulus (CS)-unconditioned stimulus (US) intervals, 144 young, middle-aged, and older adults were tested on eyeblink classical conditioning at CS-US intervals of 500, 1,000, or 1,500 ms. Reaction time, response timing, motor learning, declarative memory, and attention were assessed to identify correlates of conditioning at various CS-US intervals. Previously reported middle-aged and older adults were impaired at a 400-ms CS-US interval, but the addition of 100 ms to the CS-US interval in this study enabled equal conditioning in middle-aged and young adults. At a 1,000-ms CS-US interval, older adults remained significantly impaired. It was only at the 1,500-ms CS-US interval that conditioning was equal for the 3 age groups. Measures of reaction time, timing, and motor learning were not correlated systematically with conditioning. Whereas the results of age differences at various CS-US intervals were clear and striking, patterns of relationships among neuropsychological and conditioning variables were not consistent in indicating sources of age differences.

Predictors of eyeblink classical conditioning over the adult age span.

The major aim was to identify predictors of the large age differences that exist in eyeblink classical conditioning. Eyeblink conditioning was assessed in 190 participants over the age range of 20-89 years, with 150 trained in the paired condition and 40 trained in the explicitly unpaired control condition. Timed-interval tapping was used to assess cerebellar function. Blink reaction time and explicit learning and memory were also assessed. Stepwise multiple regression indicated that the effect of age accounted for the largest proportion of the variance, but the cerebellar measure also predicted eyeblink conditioning at a significant level. Reaction time and explicit memory measures did not account for a significant amount of the variance in eyeblink conditioning. Age-related effects in the cerebellum apparently affect timing and learning in normal adults.

Concurrent eyeblink classical conditioning and rotary pursuit performance: implications for independent nondeclarative memory systems.

The authors examined whether 2 nondeclarative tasks, simple eyeblink classical conditioning (EBCC) and rotary pursuit (RP), would interfere with each other when performed simultaneously. In Experiment 1,100 participants were assigned to 1 of 5 groups: paired EBCC/RP, unpaired EBCC/RP, paired EBCC as a single task, unpaired EBCC as a single task, and RP as a single task. Participants in the paired EBCC/RP group showed significantly greater acquisition of conditioned responses than did participants in the unpaired EBCC/RP group, and the unconditioned eyeblink response was similar in both groups. Comparisons of the paired EBCC/RP and paired EBCC-as-a-single-task groups indicated no differences in trials to criterion, but on some measures the single-task group conditioned better. Controls introduced in Experiment 2 did not change this pattern. Results provide some evidence for the lack of interference between EBCC and RP.

Mecamylamine- or scopolamine-induced learning impairment: ameliorated by nefiracetam.

Nefiracetam is undergoing preclinical and clinical tests as a cognition-enhancing drug in Alzheimer's disease (AD). Nicotinic cholinergic receptors are lost in AD, and nicotinic as well as muscarinic cholinergic receptors are involved in the modulation of eyeblink conditioning. Experiments were carried out using young rabbits to examine the effect of nefiracetam on cholinergic antagonists to nicotinic (mecamylamine) and muscarinic (scopolamine) receptors. Rabbits were tested for 15 days in the 750 ms delay eyeblink classical conditioning paradigm in paired and explicitly unpaired conditions. Nefiracetam at a dose of 15 mg/kg significantly ameliorated the effects of 0.5 mg/kg mecamylamine, and nefiracetam at a dose of 10 mg/kg significantly ameliorated the effect of 1.5 mg/kg scopolamine. The vehicle alone and nefiracetam alone groups performed similarly to the groups treated with mecamylamine or scopolamine and nefiracetam. Reversal by nefiracetam of a nicotinic as well as a muscarinic cholinergic antagonist indicates that the drug may affect deficits specific to AD.

Hippocampus in delay eyeblink classical conditioning: essential for nefiracetam amelioration of learning in older rabbits.

Rabbits acquire conditioned responses (CRs) normally with bilateral removal of the hippocampus, but alterations of the intact hippocampus can affect the rate of acquisition. The cognition-enhancing drug, nefiracetam ameliorated the acquisition of CRs in older rabbits, protected membrane dysfunction in hippocampal CA1 neurons following oxygen and glucose deprivation, and promoted the release of diverse neurotransmitters, including acetylcholine. Because the septo-hippocampal cholinergic system is demonstrated to be involved in eyeblink conditioning, this experiment was undertaken to explore whether nefiracetam ameliorates conditioning via the hippocampus. Data from 53 rabbits of a mean age of 28 months were tested under two drug conditions (10 or 0 mg/kg nefiracetam) and 4 lesion conditions (bilateral hippocampectomy, bilateral neocortical removal, sham surgery, no surgery). The three groups of nefiracetam-treated rabbits with intact hippocampus acquired CRs more rapidly than the vehicle-treated groups, but rabbits with bilateral hippocampectomy treated with nefiracetam learned like vehicle-treated rabbits. Results suggest that nefiracetam ameliorates learning via the hippocampus. Because of the parallels between conditioning in rabbits with disrupted hippocampal cholinergic systems and conditioning in Alzheimer's disease (AD), these results suggest that nefiracetam may ameliorate conditioning in AD as it ameliorates conditioning in older rabbits.

Nefiracetam ameliorates learning deficits in older rabbits and may act via the hippocampus.

Behavioral and neurobiological parallels between rabbits and humans in eyeblink classical conditioning (EBCC) make this paradigm potentially useful for preclinical and clinical trials. The known involvement of hippocampal pyramidal cells in EBCC in rabbits led us to evaluate the nootropic compound, nefiracetam. In Expt. 1, 56 rabbits of a mean age of 27.9 months received daily doses of vehicle, 1, 3, 10, or 15 mg/kg nefiracetam 15 min before testing in the 750 ms delay EBCC paradigm. Trials to learning criterion (T/C) was significantly better with 10 mg/kg. To investigate whether nefiracetam ameliorated EBCC via the hippocampus, 20 older rabbits received bilateral hippocampal aspirations in Expt. 2. Eleven rabbits of a mean age of 26.8 months had histologically confirmed bilateral hippocampectomy and were dosed with vehicle or 10 mg/kg nefiracetam and tested with EBCC. Hippocampectomized rabbits treated with vehicle and 10 mg/kg nefiracetam had 718 and 996 T/C, respectively, that were not different from 845 T/C of 8 intact older rabbits and vehicle mg/kg nefiracetam, but significantly worse than 473 T/C for 8 intact older rabbits and 10 mg/kg nefiracetam. Nefiracetam ameliorated EBCC in older rabbits only when the hippocampus was intact.

Classical conditioning.

Evidence has amassed from research in humans indicating that the cerebellar circuitry serving as the substrate for eyeblink classical conditioning is similar to that in nonhuman primates. In patients with bilateral cerebellar lesions or neurodegenerative cerebellar disease, few conditioned eyeblink responses are produced with either the ipsilesional or the contralesional eye. Cerebellar patients with lateralized lesions, like rabbits with experimentally produced unilateral cerebellar lesions, produce relatively normal conditioned responses (CRs) with the contralesional eye and few or no CRs with the ipsilesional eye. Age-related deficits in eyeblink classical conditioning appear in humans and rabbits in middle age. In normal aging in many species, including humans, there is Purkinje cell loss in cerebellar cortex. In rabbits, the Purkinje cell number correlates highly with the rate of learning, regardless of age. Positron emission tomography imaging of normal young adults during eyeblink conditioning reveals changes in activity in the cerebellum. Timed interval tapping, a task that assesses cerebellar function, also predicts performance on eyeblink conditioning. In dual-task conditions involving simultaneous performance of eyeblink conditioning and timed interval tapping, eyeblink conditioning is impaired. Investigations of patients with lesions or neurodegenerative disease not involving the cerebellum demonstrate that acquisition of CRs is possible, although prolonged in the case of hippocampal cholinergic disruption. Evidence to date suggests that the human analogue of the rabbit interpositus nucleus, the globose nucleus, is essential for the production of the conditioned eyeblink response and that cerebellar cortical Purkinje cells play a role in normal acquisition.

Nimodipine ameliorates impaired eyeblink classical conditioning in older rabbits in the long-delay paradigm.

Research using the hippocampally dependent short-conditioned stimulus trace conditioning paradigm demonstrated that nimodipine ameliorated learning deficits in older rabbits. Evidence from in vitro and in vivo measures indicated that the site of drug action was hippocampal pyramidal cells. Acquisition occurs in the long (750 ms) delay conditioning paradigm in the absence of the hippocampus. This experiment with 40 older rabbits was undertaken to determine if nimodipine ameliorates impaired acquisition in a conditioning paradigm not dependent on the hippocampus. Fifteen 90-trial sessions of paired conditioning stimuli were presented to 3 groups receiving daily injections of 0, 1, or 5 mg/kg nimodipine. Explicitly unpaired control groups received 0 or 5 mg/kg nimodipine. Acquisition with the 5 mg/kg dose was significantly faster. Existing evidence suggested that nimodipine acted in the hippocampus, but we could not rule out the possibility that the drug also affected conditioning via the cerebellum.

Eyeblink classical conditioning in Alzheimer's disease and cerebrovascular dementia.

Eyeblink classical conditioning (EBCC) is severely and consistently impaired in probable Alzheimer's disease (AD), presumably due to normal age related changes in the cerebellum and AD-related hippocampal cholinergic disruption. Less consistent impairment and more variable EBCC performance was predicted in patients with cerebrovascular dementia (CVD) because some CVD patients should have impairment in EBCC when their lesions affect the EBCC circuitry, whereas others with lesions in noncritical regions should have normal EBCC. As predicted, variability in EBCC performance was greater in patients with CVD than in probable AD patients. Acquisition of conditioned responses in the group of CVD patients was better than in the probable AD group. These data show in another sample of normal control subjects and probable AD patients that EBCC has a high sensitivity for probable AD.

Huntington's disease and eyeblink classical conditioning: normal learning but abnormal timing.

On the basis of what is known about the neural circuitry essential or normally involved in eyeblink classical conditioning (EBCC), the pattern of neurodegeneration in Huntington's disease (HD) would not appear to interfere with this type of learning. HD causes severe atrophy of the basal ganglia and thinning and shrinkage of the cerebral cortex. However, the hippocampus and hippocampal cholinergic system remain relatively intact, as does the cerebellum. Because the brain circuitry engaged in EBCC is neither lesioned nor disrupted in HD, it was predicted that HD patients would perform like normal control subjects in the 400-ms delay EBCC paradigm. Performance of seven patients with HD was compared to age-matched normals, with two control subjects matched to each HD patient. There were no differences in production of conditioned responses (CRs) between HD patients and normal control subjects, but the timing of the CR was abnormal in HD. Comparisons of HD patients to patients with other neurodegenerative diseases (probable Alzheimer's disease (pAD) and Down syndrome (DS) over the age of 35 with presumed Alzheimer-like neuropathology) and to patients with cerebellar lesions demonstrated significantly better EBCC performance in HD. Results suggest that the ability to acquire CRs is normal in HD, but the striatum may have some role in optimizing the timing of the CR.