Effect of hippocampal lesions on rat shuttle responses in four different behavioral tests

Exercise reduces diet-induced cognitive decline and increases hippocampal brain-derived neurotrophic factor in CA3 neurons

BACKGROUND

Previous studies have shown that a western diet impairs, whereas physical exercise enhances hippocampus-dependent learning and memory. Both diet and exercise influence expression of hippocampal brain-derived neurotrophic factor (BDNF), which is associated with improved cognition. We hypothesized that exercise reverses diet-induced cognitive decline while increasing hippocampal BDNF.

METHODS

To test the effects of exercise on hippocampal-dependent memory, we compared cognitive scores of Sprague-Dawley rats exercised by voluntary running wheel (RW) access or forced treadmill (TM) to sedentary (Sed) animals. Memory was tested by two-way active avoidance test (TWAA), in which animals are exposed to a brief shock in a specific chamber area. When an animal avoids, escapes or has reduced latency to do either, this is considered a measure of memory. In a second experiment, rats were fed either a high-fat diet or control diet for 16 weeks, then randomly assigned to running wheel access or sedentary condition, and TWAA memory was tested once a week for 7 weeks of exercise intervention.

RESULTS

Both groups of exercised animals had improved memory as indicated by reduced latency to avoid and escape shock, and increased avoid and escape episodes (p<0.05). Exposure to a high-fat diet resulted in poor performance during both the acquisition and retrieval phases of the memory test as compared to controls. Exercise reversed high-fat diet-induced memory impairment, and increased brain-derived neurotrophic factor (BDNF) in neurons of the hippocampal CA3 region.

CONCLUSIONS

These data suggest that exercise improves memory retrieval, particularly with respect to avoiding aversive stimuli, and may be beneficial in protecting against diet induced cognitive decline, likely via elevated BDNF in neurons of the CA3 region.

Précis ofThe neuropsychology of anxiety: An enquiry into the functions of the septo-hippocampal system

A model of the neuropsychology of anxiety is proposed. The model is based in the first instance upon an analysis of the behavioural effects of the antianxiety drugs (benzodiazepines, barbiturates, and alcohol) in animals. From such psychopharmacologi-cal experiments the concept of a “behavioural inhibition system” (BIS) has been developed. This system responds to novel stimuli or to those associated with punishment or nonreward by inhibiting ongoing behaviour and increasing arousal and attention to the environment. It is activity in the BIS that constitutes anxiety and that is reduced by antianxiety drugs. The effects of the antianxiety drugs in the brain also suggest hypotheses concerning the neural substrate of anxiety. Although the benzodiazepines and barbiturates facilitate the effects of γ-aminobutyrate, this is insufficient to explain their highly specific behavioural effects. Because of similarities between the behavioural effects of certain lesions and those of the antianxiety drugs, it is proposed that these drugs reduce anxiety by impairing the functioning of a widespread neural system including the septo-hippocampal system (SHS), the Papez circuit, the prefrontal cortex, and ascending monoaminergic and cholinergic pathways which innervate these forebrain structures. Analysis of the functions of this system (based on anatomical, physiological, and behavioural data) suggests that it acts as a comparator: it compares predicted to actual sensory events and activates the outputs of the BIS when there is a mismatch or when the predicted event is aversive. Suggestions are made as to the functions of particular pathways within this overall brain system. The resulting theory is applied to the symptoms and treatment of anxiety in man, its relations to depression, and the personality of individuals who are susceptible to anxiety or depression.

Pontine-wave generator activation-dependent memory processing of avoidance learning involves the dorsal hippocampus in the rat

The aim of this study was to test the hypothesis that the dorsal hippocampus plays a critical role in pontine-wave (P-wave) generator activation-dependent memory processing of two-way active avoidance (TWAA) learning. To achieve this objective, rats were given small bilateral lesions in the CA1, dentate gyrus (DG), or CA3 region of the dorsal hippocampus by microinjecting ibotenic acid. After recovery, lesioned and sham-lesioned rats were trained on a TWAA learning paradigm, allowed a 6-hr period of undisturbed sleep, and then were tested on the same TWAA paradigm. It was found that lesions in the CA3 region impaired retention of avoidance learning. Conversely, lesions in the CA1 and DG regions had no effect on TWAA learning retention. None of the groups showed any changes in the baseline sleep-wake cycle or in the acquisition of TWAA learning. All rats showed increased rapid eye movement (REM) sleep and increased REM sleep P-wave density during the subsequent 6-hr recording period. Impaired retention in the CA3 group occurred despite an increase in REM sleep and P-wave density, suggesting that during REM sleep, the P-wave generator interacts with the CA3 region of the dorsal hippocampus to aid in consolidation of TWAA learning. The results of the present study thus demonstrate that P-wave generator activation-dependent consolidation of memory requires an intact CA3 subfield of the dorsal hippocampus. The results also provide evidence that under mnemonic pressure, the dorsal hippocampus may not be involved directly in regulating the sleep-wake cycle.

Time-dependent neurobiological effects of colchicine administered directly into the hippocampus of rats

Rats were given bilateral injections of colchicine into the dorsal and ventral hippocampus. Behavioral, neurochemical and histopathological measurements were taken, up to 12 weeks after surgery. Colchicine produced a consistent increase in spontaneous motor activity, enhanced acoustic startle reactivity, and accelerated acquisition of two-way shuttle box avoidance, but did not affect reactivity to a noxious thermal stimulus. Measurement of dynorphin in the hippocampus indicated that colchicine rapidly depleted this neuropeptide, which is thought to be contained preferentially in the mossy fibers of granule cells of the hippocampus. Colchicine also decreased Met-enkephalin in the hippocampus, but the magnitude of the change (22%) was less than that (89% depletion) observed for hippocampal dynorphin. Examination of hippocampal morphology using light microscopic techniques indicated that colchicine caused approximately 60% degeneration of granule cells in the hippocampus. Although the length of the pyramidal cells was decreased (12-16%), the width of the CA1 and CA3 region of the hippocampus was not affected. These data underscore the importance of the granule cells in the mediation of behavioral processes such as motor activity, startle reactivity and performance of shuttle box avoidance.

Comparison between the behavioural effects of septal and hippocampal lesions: a review

The literature on the behavioural effects of septal and hippocampal lesions is classified according to behavioural paradigm. The effects of the two kinds of lesion are summarized and compared to each other. A 'septo-hippocampal syndrome,' consisting of the effects common to both lesions, is delineated, and divergences between the effects of the two lesions are noted.

Effect of various forms of training and stimulation on the incorporation of 32P into the nuclear phosphoproteins of the rat brain

Incorporation of 32P into acid-extractable nuclear proteins was measured in the hippocampus, caudate nucleus, rest of the brain, and liver of rats submitted to various different behavioral treatments in a shuttle-box. After 5 min of classical conditioning, of avoidance without CS-US pairing, and of avoidance with CS-US pairing (standard shuttle avoidance), there was an increased 32P uptake by acid-extractable nuclear proteins in the hippocampus and caudate nucleus. The effect disappeared between 5 and 25 min of training. After 25 min of buzzers alone, or of footshocks alone, a similar 32P uptake change was noted in the same brain structure, which raises doubts as to the specificity of the phenomenon in terms of learning mechanisms.

Effect of brain lesions on rat shuttle behavior in four different tests

Rats were trained to perform shuttle responses to a buzzer in four different situations: pseudoconditioning or D test (buzzers and footshocks presented at random), classical conditioning or DP test (buzzers and footshocks paired on every trial), avoidance without stimulus pairing or DC test (buzzer-shock intervals varied at random, shocks contingent upon non-emission of a shuttle response to the preceding buzzer), and standard two-way avoidance or DPC test (buzzers paired to shocks, but the latter omitted every time there was shuttling to the buzzer). The letters in each test disignate the factors involved in the emission of responses in each one, which were shown in previous papers to be: a non-associative factor or "drive" (D), the Pavlovian or stimulus-stimulus relation ("pairing", P), and the shuttle-no shock or main avoidance contingency (C). The effects of various brain lesions on these behaviors were studied. Ventral caudate and amygdala lesions depress both the Pavlovian (P) and the avoidance (C) component. Dorsal caudate lesions have an opposite influence on these two factors. Septal (n.medialis + lateralis, and n.accumbens) and tuberculum olfactorium lesions enhance the non-associative component (D); accumbens lesions, in addition, impair operation of the C factor. The effect of the diverse lesions on jumping responses to the buzzer or on performance of intertrial crossings does not correlate with the effect on shuttle responses.

A near-lethal dose of ethanol, given intraperitoneally, does not affect memory consolidation of two different avoidance tasks

Ethanol (3 g/kg, given i.p. as a 33% w/v solution), was given to adult Wistar rats 1 min after training in (1) a 50-trial session of shuttle avoidance, using buzzers and shocks, and (2) an inhibitory avoidance experience in a step-through apparatus. The animals were retested 7 days after training in condition (1), and 3 days after training in condition (2). There was no difference in retention scores between the ethanol-treated, saline-treated, or untreated animals. In addition, there was no evidence of an aversive effect of ethanol per se under any of the two training conditions, in spite of the fact that the dose of ethanol used caused a very profound ataxia, and was lethal for 14.3% of the animals (a slightly higher dose, 4 g/kg, is lethal for about 80% of our rats). These data do not favor the hypothesis that an acute administration of ethanol may influence memory consolidation.