Footshock Avoidance Research Articles

The Memory Enhancing Effects of Cholecystokinin Octapeptide Are Dependent on an Intact Stria Terminalis

Cholecystokinin is a gastrointestinal peptide which is released from the duodenum during eating. Cholecystokinin octapeptide (CCK-8) has been found to improve retention when administered intraperitoneally in intact mice but not in vagotomized mice. This suggested that CCK-8 improves retention by stimulating vagal afferents to the nucleus of the solitary tract in the brainstem. In this study, we tested whether nerve fibers in the stria terminalis which project from the NTS to the amygdala need to be intact for CCK-8 to enhance retention. Three groups of mice were used: nonoperated, bilateral cortical control lesioned, and bilateral stria terminalis lesioned. The lesions were performed 1 week prior to footshock avoidance training in a T-maze. Saline, CCK-8 (0.5 μg/kg, ip), epinephrine (100 μg/kg, sc), or arecoline (1.5 mg/kg, sc) were administered immediately after training. Retention was tested 1 week later. Neither bilateral stria terminalis lesions nor cortical control lesions significantly altered acquisition of the task compared to the nonoperated group. Whether the groups received a saline injection after training or received no injection did not affect retention test performance. CCK-8 and epinephrine enhanced retention in the mice with cortical lesions but not in mice with stria terminalis lesions. Arecoline enhanced retention in both groups. Possible pathways and neurotransmitters mediating the effect are discussed.

Localized injections of various compounds effecting neurotransmitter activity in the mammillary complex enhance (T-maze) avoidance retention

The mammillary complex is implicated in the amnesic syndrome associated clinically with Korsakoff's syndrome, Alzheimer's disease and experimentally with lesions in animals. There is however no direct evidence that the mammillary bodies are involved in long term memory processing. Mice were partially trained on a footshock avoidance task. Immediately after training drugs were injected into the mammillary complex. Retention was tested 1 week later by continuing training until each mouse made five avoidance responses in six trials. The results indicated that muscarine, nicotine, dopamine, glutamine and adrenoceptor agonists as well as GABA and 5-HT receptor antagonists and neuropeptide Y improved retention test performance relative to the control. Injection of the same drugs 1 mm above the injection site for the mammillary complex failed to significantly improve retention test performance. It is concluded that the mammillary complex, with its important connections to other areas of the limbic system, is involved in memory processing events that occur shortly after training.

Effects of an exogenous β-amyloid peptide on retention for spatial learning

Three experiments assessed the effects of beta-amyloid 1-40 (beta A4) on spatial learning in Sprague-Dawley rats. In Experiment 1, rats were trained on a signaled footshock avoidance in a Y-maze. Rats received a single injection of beta A4 or vehicle in both sides of the hippocampus immediately after the fifth trial. The beta A4 group took significantly longer than the vehicle group to learn to avoid the shock when trained to criterion 1 week later, suggesting a detrimental effect of beta A4 on memory consolidation. Experiment 2 used a food reinforcer rather than shock relief under procedures similar to Experiment 1. Again, the beta A4 group took longer to learn the maze to criterion. This shocks that the effect in Experiment 1 was not specific to shock-maintained learning. In Experiment 3, rats were trained to retrieve a food pellet from each arm of an eight-arm radial maze. After training to criterion, beta A4 or vehicle was administered intrahippocampally 30 min before the daily session for 26 sessions. There were no acute or chronic effects of beta A4 injection on radial maze performance, and no aggregation of beta A4 or significant necrosis was observed upon postmortem histological analysis. These experiments suggest that single injections of beta A4 impair memory consolidation, but repeated injections of beta A4 over an extended period do not affect well-learned behavior.

Age-related changes in footshock avoidance acquisition and retention in senescence accelerated mouse (SAM)

An inbred strain has been developed which exhibits some characteristics of aging in mice as young 6 months of age. The strain was designated “Senescence Accelerated Mouse” (SAM). A subline of the SAM strain, P/8, has an early onset of impaired learning in appetitive and aversive training tasks. Simple aversive and appetitive tasks were learned normally up to 12 months of age, whereas more difficult tasks detected impairment as early as 6 months of age while the mice still had good general health. Comparable impairment of learning and memory in C57BL/6Nnia mice was not detected until about 24 months of age which is near the end of the strain's lifespan when general health declines. In the present study, we report age-dependent impairment of acquisition and retention for footshock avoidance conditioning. Long-term (1 week) but not short-term (1 h) retention was adversely effected by aging. In 12-month-old P/8 mice, practicing the avoidance response (overtraining) did not alleviate the impairment of long-term memory processing.

Effects of thermal peptides on retention of footshock avoidance training in mice

Some hydrophobic polypeptides known as thermal proteins have been found to have neurotrophic effects. Thermal proteins were synthesized from aspartic acid, glutamic acid, proline, and tryptophan. Two hydrophobic and one nonhydrophobic polymers were injected intracerebroventricularly into brains of mice after partial training on footshock avoidance run in a T-maze. When retention was tested 1 week later, the hydrophobic polymers enhanced retention while the nonhydrophobic polymer did not. Thermal proteins exhibiting hydrophobicity and having neurotrophic effects may aid in altering synaptic connections by facilitating cell recognition.

Effect on memory processing by D-cycloserine, an agonist of the NMDA/glycine receptor

Glycine has been shown to modulate N-methyl-D-aspartate (NMDA) subclass of acidic amino acid receptors which have been implicated in learning and memory. We report that d-cycloserinc (DCS) which has a high affinity for the glycine modulatory site in the NMDA receptor complex modulated memory processing in a dose-dependent manner. Mice were trained on a footshock avoidance task. Immediately after training DCS was administered (2.5 to 50 mg/kg s.c.). When retention was tested a week later, 20 mg/kg facilitated retention the best with lower and higher doses be less effective in weakly trained young mice. DCS also facilitated retention in ‘senescence-accelerated mice’ in which impairment of learning and memory increases with age. DCS had to be administered at higher doses to improve retention as impairment of learning and memory increased.

Differential effects of amylin on memory processing using peripheral and central routes of administration

Amylin is a peptide hormone secreted from the beta cells of the pancreatic islets. Amylin was administered peripherally or centrally following weak or strong training on footshock avoidance conditioning in a T-maze. Under conditions of weak training, amylin improved memory retention in a dose-dependent manner. Under conditions of strong training, it impaired retention over the same dose range. Central administration of amylin in mice given strong training impaired retention but had no effect on the retention of mice given weak training. These findings suggest that the mechanisms of action by which amylin altered memory processing are different for peripheral and central administration. Peripherally secreted amylin may play a role in the amnesia seen in diabetes and the memory enhancement following glucose administration.

Early Onset of Age-Related Impairment of Aversive and Appetitive Learning in the SAM-P/8 Mouse

SAM-P/8 (P/8if) mice, trained on aversive and appetitive training tasks, showed age-related impairment of learning on more difficult tasks. Using aversive training tasks, acquisition of step-down passive avoidance did not differ between 4- and 12-month-old mice. T-maze footshock avoidance acquisition detected impairment from 9 to 12 months of age. On the appetitive tasks, 12-month-old mice required significantly more trails than 4-month-old mice to learn to press a lever for milk reinforcement or to learn a specific sequence of 10 black-white, left-right discriminations. However, learning a simple black-white discrimination in the same 10-unit maze did not discriminate between the performance of 4- and 12-month-old mice. A lack of age-related changes in food, water, and milk consumption suggests that changes in motivation do not account for the decline in learning in older P/8if mice. The results suggest an early onset of impaired learning relative to the 17.2-month median life span of the P/8if mice.

Vasoactive intestinal peptide (VIP): An amnestic neuropeptide

Vasoactive intestinal peptide (VIP) is a neuropeptide present in high concentrations in the hippocampus. The studies reported here demonstrate that VIP administered into the third ventricle of the brain caused amnesia in mice trained on a left-right footshock avoidance task in a T-maze. VIP resulted in amnesia when administered directly into the rostral portion of the hippocampus at a 10-fold lower dose than was needed to produce amnesia when VIP was administered intracerebroventricularly. When VIP was administered 24 hr after training, it failed to impair retention measured a week later. VIP receptor antagonist ([4-Cl-D-Phe 6,Leu 17]VIP) enhanced retention when administered into the rostral portion of the hippocampus, suggesting that VIP plays a physiological role in memory modulation. VIP receptor antagonist administered 24 hr after training did not facilitate retention. To gain some insight as to how VIP may be affecting memory processing, we determined if some memory-improving compounds showed a selective ability to block amnesia induced by VIP. The amnestic effect of VIP was blocked by peripheral administration of the memory-enhancing agents, arecoline, naloxone and ST 587 (a noradrenergic receptor agonist) but not by cholecystokinin octapeptide. Central administration of arecoline, but not neuropepide Y, blocked the amnestic effect of VIP. It is concluded that VIP is a potent amnestic peptide.

Modulation of memory retention by neuropeptide K

Neuropeptide K (NPK) is one of the structures in β-preprotachykinin which also inclueds substance P. NPK, a 36 amino acid peptide, contains the sequence of neurokinin A as amino acids 27–36 of its C-terminus. Neurokinin A is also contained separately in the γ-preprotachykinin precursor. Both NPK (2.5–10 μg) and neurokinin A administered intracerebroventricularly after footshock avoidance training in the T-maze enhanced memory retention in CD-1 male mice. Local microinjections of NPK enhanced memory retention when injected into the rostral and caudal portions of the hippocampus (0.25 and 0.50 μg) and the amygdala (1.0 μg), but were without effect when injected into the septum and the caudate. The differential effects of NPK on memory retention across brains regions differed from those previously reported for substance P and neuropeptide Y. These studies suggest that NPK, acting through discrete anatomical areas, modulates memory processing. The functional significance of co-localization of neuropeptides with classical neurotransmitters and other transmitter peptides in the same neurons is not well understood, but recent studies have indicated that the neuropetides modulate the release of the primary transmitter. Since NPK occurs in the same precursor molecule as substance P, NPK may be co-released with the putative neurotransmitter substance P and act with it, in a synergistic manner, to enhance memory processing. These studies provide further evidence that the hippocampus is an anatomical structure involved in memory processing that occurs shortly after training.

Modulation of memory processing by neuropeptide Y varies with brain injection site

Neuropeptide Y (NPY) is a 36 amino acid peptide which was shown to enhance memory retention, recall and prevent amnesia induced by either scopolamine or anisomycin. In this study, we examined the effects of NPY administration into 6 areas of the mouse brain on memory retention for footshock avoidance training in a T-maze. NPY was injected into the rostral and caudal hippocampus, amygdala, caudate, septum and thalamus shortly after training. NPY improved retention when injected into the rostral portion of the hippocampus and septum, impaired retention in the caudal portion of the hippocampus and amygdala and had no effect in the thalamus and caudate. NPY was ineffective at either improving or impairing retention when injected 24 h after training, thus demonstrating that the effects of NPY or retention were time-dependent and not due to proactive effects on retention test performance per se. In addition, NPY had no effect on retention when injected into overlying cortical areas. NPY antibody impaired retention when administered into the rostral hippocampus and septum; it improved retention in the caudal hippocampus and amygdala. Thus NPY antibody had the opposite effect to that of NPY on memory retention suggesting that NPY has a physiological role as a modulator of memory processing within specific anatomical areas of the central nervous system.

Cholecystokinin receptors mediate enhanced memory retention produced by feeding and gastrointestinal peptides

Ingestion of food in mice following training on T-maze footshock avoidance enhanced memory retention when tested 7 days later. This eating-induced improvement of retention was blocked by a specific cholecystokinin antagonist, L-364,718. The cholecystokinin antagonist prevented enhancement of memory retention resulting from posttraining administration of the gastrointestinal hormones, cholecystokinin, bombesin or gastrin releasing peptide. L-364,718 neither impaired or improved retention when given alone. Specificity of the effect of L-364,718 was demonstrated by the failure of L-364,718 to block improved memory retention resulting from administration of arecoline and D-amphetamine. The studies provide evidence that activation of cholecystokinin receptors plays a physiological role in the mediation of meal-induced enhancement of memory retention.

Effects of systemic pancreastatin on memory retention.

Pancreastatin, a peptide isolated from the pancreas, was shown to enhance memory retention after peripheral administration in mice when administration following T-maze footshock avoidance training. The effect of pancreastatin on memory retention, one week after training, was time dependent showing enhancement of retention when pancreastatin was administered 0 and 30 min but not 60 min after training. Pancreastatin reversed the amnesia produced by scopolamine. The pancreastatin fragment (33-49) also enhanced memory. Pancreastatin did not increase glucose in vivo. We conclude that peripherally administered pancreastatin modulates memory processing.

Two-drug combinations of memory enhancers: Effect of dose ratio upon potency and therapeutic window, in mice

Two-drug combinations have been reported to enhance retention more effectively than when either drug was administered alone at the same dose. Some combinations of cholinergic drugs enhance retention even though the total drug dosage is reduced by as much as 97% compared to the dose needed to improve retention when the same drugs are administered singly. The choice of dose ratio is usually arbitrary or based on empirical results. The study systematically varied the ratio of two drugs in a combination and at the same time varied the dosage of each drug. The drug combinations were administered to mice immediately after training on T-maze footshock avoidance task. Retention was tested one week later. Three two-drug combinations were selected for presentation because they differed considerably as to (a) the lowest effective total dose that improved memory-retention, (b) the optimal ratio that improved retention and (c) the width of the therapeutic window. The effect of a drug combination on retention was found to be dependent on the particular drugs in the combination, the ratio and the dose administered.

Effect of acute arecoline, tacrine and arecoline + tacrine post-training administration on retention in old mice

The amnesias characteristic of Alzheimer's disease and other age-related dementias are refractory to conventional pharmacotherapy. A recent treatment strategy is to combine drugs to improve their memory enhancing effect. We previously reported that in young weakly trained mice, the combination of arecoline and tacrine was more effective on a mg/kg basis than either drug administered alone. This was true whether the route of administration was intracerebroventricular, subcutaneous or oral. We now report that 24 month old mice trained to avoid footshock in a T-maze show poor retention when tested one week later. Subcutaneous administration of arecoline, tacrine (also referred to as tetrahydroaminoacridine, THA) and arecoline plus tacrine administered immediately after T-maze footshock avoidance training enhanced retention of 24 month old mice compared to the saline-injected control. Since the combination was as effective as the single drug treatments even though 96% less arecoline and 99.7% less tacrine was administered, the combination showed marked potentiation of drug action of memory processing.

Fluoxetine enhances memory processing in mice.

Fluoxetine (FLU) increases brain concentrations of serotonin by blocking its uptake, without appreciably affecting the dopamine or norepinephrine systems. The present experiments provide evidence that a subcutaneous injection of FLU enhanced post-memory processing ("consolidation") and retrieval, but not acquisition in young adult mice. FLU (15 mg/kg) enhanced 1-week memory retention when injected 2 min post-training. Similar enhancement was obtained with intracerebroventricular injection (20 micrograms per mouse). FLU enhanced retention when administered prior to training (1-5 mg/kg). FLU (2.5 mg/kg) enhanced recall scores when injected 1 h before the 1-week retention test, indicating an enhancing effect on memory retrieval. Neither the pre-training nor pre-testing effects depended on improved acquisition, since FLU did not improve acquisition of T-maze foot-shock avoidance over the dose range 0.5-35 mg/kg. The sensitive period for post-training enhancement by FLU (15 mg/kg) was less than 90 min, as shown by the temporal gradient typical of memory-enhancing drugs. The amnesia induced by a protein synthesis inhibitor anisomycin, or by an anticholinergic drug scopolamine, was blocked by FLU (15 mg/kg) injected post-training. Finally, FLU (15 mg/kg) injected after one-trial passive avoidance training enhanced 1-week retention, demonstrating effectiveness in this task as well as in the active avoidance task.

Modulation of memory processing by neuropeptide Y

Neuropeptide Y (NPY) is a 36 amino acid peptide which occurs in high concentrations in the amygdala and the hippocampus. The studies reported here demonstrate that administration of porcine NPY into the third ventricle of the brain enhanced memory retention for T-maze footshock avoidance and step-down passive avoidance training in mice. Human NPY at 5 μg enhanced retention but the inactive free acid form for NPY did not. NPY at 5 μg administered subcutaneously did not enhance retention. Post-training administration of NPY produced a dose-dependent, inverted U-shaped dose-response curve for retention of both passive and active avoidance conditioning. NPY enhanced retention in a time-dependent manner. NPY was also found to alleviate the amnesia caused by anisomycin, a protein synthesis inhibitor, and scopolamine, an anticholinergic. Pre-test administration of NPY improved recall but did not affect acquisition. These data support the concept that NPY is a modulator of memory processes.

Differences in learning between hyperprolinemic mice and their congenic controls

These experiments expanded earlier work on hyperprolinemic mice which showed learning deficits. The following behavioral tasks were used: step-through, passive avoidance; T-maze acquisition; shuttlebox acquisition, and radial-arm maze. Mouse species included PRO/Re-bb (genetically hyperprolinemic mice) and PRO/Re-aa (congenic nonhyperprolinemic controls) obtained from the Jackson Breeding Laboratories. Hyperprolinemic mice were impaired in acquiring T-maze and shuttlebox footshock avoidance behavior. One-trial passive avoidance behavior did not clearly differentiate between the groups. Radial maze performance was poor in both groups due possibly to observed acrophobia and lack of exploratory behavior. The results of this study combined with previously published work suggest that high-brain proline in conjunction with other amino acid changes account for the learning deficits.

Effects of Undernutrition during Suckling on Phosphoryl-Serine Levels in Brain Nuclear Proteins of Adult Rats

The effect of undernutrition during suckling on neurochemical and behavioral parameters were investigated in adult rats. Young rats were undernourished by suckling their lactating mothers fed an 8% (by wt) protein diet. Mothers of normal rats were fed a 20% protein diet. After weaning (d 21), normal and undernourished rats were fed a 20% protein diet until 90–120 d of age, when the rats were subjected to a short shuttle avoidance session and/or killed for neurochemical analyses. During shuttle avoidance sessions, footshock escape and footshock avoidance responses were analyzed simultaneously using 0.2- or 0.8-mA footshock intensity. In addition, the effect of undernutrition during suckling on phosphoryl-serine (P-Ser) level in brain nuclear proteins and the response of this level to a shuttle avoidance session were studied. We observed that undernutrition during suckling had no effect on footshock avoidance responses and decreased the latencies of footshock escape responses using 0.2-mA footshock intensity. Undernutrition decreased the basal level of P-Ser in brain nuclear proteins and abolished the decrease of this level observed in a shuttle avoidance session in normal rats using 0.2- or 0.8-mA footshock intensity.

Memory retention: Effect of prolonged cholinergic stimulation in mice

The fundamental hypothesis that drugs may affect memory processing by prolonging transmitter action was tested by extending the time of drug action, using repeated administrations of the cholinergic agonist, arecoline hydrobromide (ARE). The ARE was injected intracerebroventicularly into mice immediately after training (T-maze footshock avoidance) and at 90-min intervals thereafter, for a total of 1, 2, or 3 injections. The results indicate that 1 injection had no effect whereas 3 successive injections significantly improved memory retention test performance. The results confirm the hypothesis being tested; six control groups ruled out other plausible interpretations of the results.