Diana S. Woodruff-Pak

Classical conditioning of the eyeblink response in the delay paradigm in adults aged 18-83 years.

To determine if age differences in classical conditioning of the eyelid response begin to appear in middle age in humans as they do in animals, adult subjects aged 18-83 years were trained in the delay conditioning paradigm. Large age effects occurred. Statistically significant differences first appeared in the decade of the 40s. Within-age-group variability was large. To reduce variability, subjects were classified by the magnitude of their unconditioned response (UR). Regardless of age, subjects with low amplitude URs conditioned poorly. In the normal UR amplitude group, the correlation between age and total percentage conditioned responses (CRs) was -.58. Eyeblink rate and voluntary responding did not account for age differences in conditioning, and it was unlikely that hearing acuity or corneal sensitivity caused the differences. Parallels between human and animal eyelid conditioning are considered, and it is suggested that age changes in the cerebellum may affect conditioning in aging mammals, including humans.

Cerebellar involvement in eyeblink classical conditioning in humans.

The role of the ipsilateral cerebellum in human eyeblink conditioning was investigated using the 400-ms delay paradigm and testing 14 cerebellar patients (7 with unilateral lesions and 7 with bilateral lesions) and 20 control participants. Patients performed significantly worse with the ipsilesional eye than control participants but showed no difference when tested with (he contralesional eye. Conditioned responses (CRs) totaled 14% for all patients in comparison with 60% for control participants. Data on timed-interval tapping for 6 patients and 14 control participants showed that clock variability was greater with Ihe ipsilesional hand in patients. Only clock variability correlated significantly with percentage of CRs in control participants. Comparisons of paired associate learning and memory for 8 patients and 14 control participants revealed no significant differences. Results confirm that the ipsilateral cerebellum plays a role in eyeblink classical conditioning.

Where is the trace in trace conditioning

Intensive mapping of the essential cerebellar brain circuits for Pavlovian eyeblink conditioning appeared relatively complete by 2000, but new data indicate the need for additional differentiation of cerebellar regions and mechanisms coding delay and trace conditioning. This is especially important, as trace conditioning is an experimentally tractable model of declarative learning. The temporal gap in trace eyeblink conditioning may be bridged by forebrain regions through pontine-cerebellar nuclear connections that can bypass cerebellar cortex, whereas a cerebellar cortical long-term-depression-like process appears to be required to support normal delay conditioning. Experiments focusing on the role of cerebellar cortex and deep nuclei in delay versus trace conditioning add perspective on brain substrates of these seemingly similar paradigms, which differ only by a brief stimulus-free time gap between conditioned and unconditioned stimuli. This temporal gap appears to impose forebrain dependencies and differentially engage different cerebellar circuitry during acquisition of conditioned responses.

Eyeblink classical conditioning in H. M.: Delay and trace paradigms.

H.M., a well-known subject with bilateral removal of medial-temporal-lobe structures and profound amnesia, performed eyeblink classical conditioning (EBCC) for 21 90-trial sessions in the 400-ms delay and 900-ms trace paradigms. A 2nd amnesic subject with temporal lobe lesions and 2 normal control subjects (NCSs) were also conditioned. Acquisition occurred in both paradigms for all subjects. Acquisition in the delay paradigm was prolonged in H.M. (perhaps because of his cerebellar degeneration in the vermis and hemispheres), but not in the 2nd amnesic subject. Amnesic subjects and NCSs showed more rapid acquisition in the trace than in the delay paradigm. Two years after initial EBCC, H.M. attained learning criterion in the trace paradigm in 1/10th as many trials. No recollection of the experimenters, apparatus, instructions, or procedure was manifested by H.M. Results suggest that humans can condition in the 400-ms delay and 900-ms trace EBCC paradigms with the hippocampus radically excised.

A nicotinic agonist (GTS-21), eyeblink classical conditioning and nicotinic receptor binding in rabbit brain

The septo-hippocampal cholinergic system is of demonstrated involvement in eyeblink classical conditioning (EBCC). To determine if a nicotinic cholinergic agonist, GTS-21, would facilitate acquisition of EBCC in older rabbits, three doses (0.1, 0.5, 1.0 mg/kg) in sterile saline vehicle and vehicle alone were administered to older rabbits (n = 48; mean age = 29.8 months). A control group of vehicle-treated young rabbits (n = 12; mean age = 3.5 months) was included. Rabbits were conditioned for fifteen 90-trial sessions in the 750 ms delay paradigm with a tone conditioned stimulus and corneal airpuff unconditioned stimulus. Dependent measures of trials to learning criterion, percentage of conditioned responses (CRs) and CR amplitude consistently showed significant improvement in older rabbits treated with 0.5 and 1.0 mg/kg of GTS-21. Acquisition was similar in vehicle-treated young and GTS-treated older rabbits. Vehicle-treated older rabbits conditioned more poorly than vehicle-treated young rabbits. No non-associative learning effects were observed in GTS-21 treated animals. Nicotinic receptor binding was similar in all groups of older rabbits, indicating that GTS-21 administration over a 15-day period did not affect nicotinic receptors. Alzheimers disease (AD) has been associated with significant loss of nicotinic cholinergic receptors, and patients diagnosed with probable AD are seriously impaired on EBCC. These results demonstrating that the nicotinic agonist, GTS-21, facilitated EBCC in older rabbits suggest that the compound should receive additional investigation for its potential to affect cognition in AD.

Animal models of Alzheimer's disease: therapeutic implications.

This Special Issue of the Journal of Alzheimers Disease (JAD) provides an overview of animal models of Alzheimers disease (AD). Very few species spontaneously develop the cognitive, behavioral, and neuropathological symptoms of AD, yet AD research must progress at a more rapid pace than the rate of human aging. In recent years, a variety of models have been created--from tiny invertebrates with life spans measurable in months to huge mammals that live several decades. The fruit fly, Drosophila melanogaster, is a powerful genetic tool that has recently emerged as a model of AD with neural features and assessable learning and memory. Transgenic mice are the most widely used animal models of AD and have yielded significant research breakthroughs. Accelerated aging seen in the SAMP8 mouse is a non-transgenic model with great utility. Rat models provided early evidence about the deleterious impact of amyloid-beta (Abeta) on neurons and continue to provide insights. Rabbits, as langomorphs, are more closely related to primates than are rodents and have conserved the sequence of Abeta in humans (as have canines and non-human primates). The hypercholesterolemic rabbit is an excellent AD model. The aging canine develops AD neuropathology spontaneously and is especially suitable for tests of therapeutics. Non-human primates are invaluable for the development of therapeutics translating to humans. Each animal model has limitations and strengths, but used together in complementary fashion, animal models for research on AD are essential for rapid progress toward a cure.

Eyeblink conditioning discriminates Alzheimer's patients from non-demented aged

Classical conditioning of the eyeblink response in rabbits is a model system useful in research on the neurobiology of learning, memory and aging, and it has implications for Alzheimers disease (AD). The hippocampus and cerebellum are brain structures of demonstrated involvement in eyeblink conditioning. AD profoundly impairs the septo-hippocampal cholinergic system; thus, AD patients should show greater impairment of eyeblink conditioning than non-demented, age-matched subjects. Twenty probable AD patients and 20 non-demented age-matched subjects were classically conditioned in the delay paradigm. While control subjects showed clear evidence of acquisition (31.54% conditioned responses [CRs]), probable AD patients showed significant impairment (10.77% CRs). Eyeblink classical conditioning may be useful for AD research and assessment.

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.

Differential effects and rates of normal aging in cerebellum and hippocampus

Cognitive functions show many alternative outcomes and great individual variation during normal aging. We examined learning over the adult life span in CBA mice, along with morphological and electrophysiological substrates. Our aim was to compare cerebellum-dependent delay eyeblink classical conditioning and hippocampus-dependent contextual fear conditioning in the same animals using the same conditioned and unconditioned stimuli for eyeblink and fear conditioning. In a subset of the behaviorally tested mice, we used unbiased stereology to estimate the total number of Purkinje neurons in cerebellar cortex and pyramidal neurons in the hippocampus. Several forms of synaptic plasticity were assessed at different ages in CBA mice: long-term depression (LTD) in both cerebellum and hippocampus and NMDA-mediated long-term potentiation (LTP) and voltage-dependent calcium channel LTP in hippocampus. Forty-four CBA mice tested at one of five ages (4, 8, 12, 18, or 24 months) demonstrated statistically significant age differences in cerebellum-dependent delay eyeblink conditioning, with 24-month mice showing impairment in comparison with younger mice. These same CBA mice showed no significant differences in contextual or cued fear conditioning. Stereology indicated significant loss of Purkinje neurons in the 18- and 24-month groups, whereas pyramidal neuron numbers were stable across age. Slice electrophysiology recorded from an additional 48 CBA mice indicated significant deficits in LTD appearing in cerebellum between 4 and 8 months, whereas 4- to 12-month mice demonstrated similar hippocampal LTD and LTP values. Our results demonstrate that processes of aging impact brain structures and associated behaviors differentially, with cerebellum showing earlier senescence than hippocampus.

Amyloid plaques in cerebellar cortex and the integrity of purkinje cell dendrites

We probed serial and near serial sections of cerebellum from 13 Alzheimers disease (AD), 10 older Downs syndrome (DS) patients, and 9 age-matched, non-AD controls, using single and double labeling immunohistochemistry to investigate the pathologic consequences of beta-amyloid or A4 (A beta) deposits in cerebellum and their relationship to Purkinje cells (PCs). Our data showed that A beta deposits in cerebellum of AD and older DS adults only form diffuse or preamyloid plaques and the density of A beta lesions per unit area of molecular layer correlated with the number of PCs per unit length of the subjacent PC layer in double immunostained sections (r = 0.85; p < 0.001). About 65% of these cerebellar A beta deposits were in physical contact with PC dendrites. No A beta plaques were found in the cerebellum of controls. Despite the abundance of A beta deposits in the cerebellar cortex of AD and older DS patients, neither PC bodies nor PC dendrites in physical contact with A beta lesions showed evidence of structural abnormalities.