James F. Flood

Memory facilitating and anti-amnesic effects of corticosteroids

Abstract The effects of corticosterone, hydrocortisone and dexamethasone on retention of active and passive avoidance training were studied in male mice. Posttraining administration of any of the hormones facilitated subsequent retention test performance of poorly trained mice when tested one week after training and drug administration. The optimum dose of dexamethasone was 4 mg/kg, while corticosterone and hydrocortisone were effective at 30 and 40 mg/kg, respectively. Dexamethasone significantly facilitated retention when administered up to 150 min but not at 210 min after training. It was further determined that dexamethasone blocked the amnesic effect of two but not four successive injections of anisomycin in both active and passive avoidance tasks. Corticosterone and dexamethasone when administered to anisomycin-injected mice caused only a small, transient increase in the protein synthesis inhibition. In saline-injected control mice, the hormones also caused a small inhibition of protein synthesis which disappeared quickly. Plasma corticosterone levels were measured in mice trained and given anisomycin, cycloheximide or saline. Plasma corticosterone levels were reduced 43% by anisomycin and 89% by cycloheximide. In both cases the corticosterone levels subsequently increased rapidly after the inhibitor injection and were elevated by about 5 times above control levels at 130 min after the inhibitor injection. The results are discussed in terms of the effect of central stimulant action of corticosteroids on memory formation.

Cholinergic receptor interactions and their effects on long-term memory processing

Mice were treated on active avoidance to determine the effect of cholinergic and anticholinergic drugs on retention. All drugs were administered intraventricularly after training and one week prior to testing retention. A dose-response curve was determined for each drug. The results indicate that each of 6 anticholinergics impaired retention. The 8 cholinergic drugs tested all yielded U-shaped dose-response curves with controls showing poor retention, low doses of the drugs yielding good retention and high doses resulting in poor retention.

Effects of ACTH peptide fragments on memory formation

The effects of peptides derived from ACTH on the formation of long-term memory have been investigated in male mice. Post-training administration of ACTH 4-10-L-Phe-7 (ACTH-L) improved retention for both passive and active avoidance tasks. Administration of ACTH 4-10-D-Phe-7 (ACTH-D) impaired retention for both tasks. The optimum dose for ACTH-L was about 0.3 mg/kg; the optimum dose for ACTH-D was in the range of 1.0-3.0 mg/kg. Using the passive avoidance task, it was shown that either drug had to be administered within 60 min of training to be highly effective. Amnesia produced by anisomycin (Ani), an inhibitor of protein synthesis, was lessened by ACTH-L and increased by ACTH-D, ACTH-D opposed the memory facilitating effects of ACTH-L. Using intact mice, ACTH-L or ACTH-D did not significantly change the incorporation of valine into protein, nor did these peptides influence the inhibition of protein synthesis caused by anisomycin. The results show that ACTH may play a major role in memory processing, perhaps by facilitating essential protein synthesis at sites specific for the memory being established.

An investigation of tolerance to the actions of leptogenic and anorexigenic drugs in mice

This study compared the effects of chronic administration of anorexigenic drugs on weight loss in mice. Tolerance to the effects of peripheral anorexigenic peptides, viz. cholecystokinin-octapeptide and bombesin, developed rapidly. Morphine, cocaine and dehydroepiandrosterone-sulfate caused weight loss and appeared similar to d-amphetamine in mechanisms of action. A high dose of fluoxetine (25 mg/kg) proved to be a potent leptogenic agent but was also associated with death in some animals. A lower dose of fluoxetine (5 mg/kg) was associated with the development of tolerance. Calcitonin, a potent anorexigenic agent, did not produce weight loss and tolerance to its anorectic effect had developed by 10 days. Animals varied widely in their individual responsiveness to a given drug. Peripheral administration of peptide YY caused weight loss. We conclude that acute or chronic effects of agents on food intake do not necessarily predict effects on body weight. However, neurotransmitters that enhance feeding centrally appear to cause weight loss when administered peripherally.

The effect of stimulants, depressants, and protein synthesis- inhibition on retention.

This study tested the hypothesis that the level of arousal is an important determinant of memory trace formation. The amnesia was caused in mice by inhibition of cerebral protein synthesis with anisomycin. The level of arousal was modified by the use of excitant and depressant drugs. Post-training administration of a stimulant (d-amphetamine, strychnine, or picrotoxin) counteracts the amnesic effects of protein synthesis inhibition, so that amnesia does not occur unless the duration of inhibition is lengthened. Stimulants show a time dependency, since they are less effective when administered at longer intervals after training. Depressants enhance the amnesia resulting from protein synthesis inhibition. Biochemical experiments showed that depressants alone had only slight effects on the rate of protein synthesis. In combination with anisomycin, the depressant did not markedly prolong the duration or increase the degree of inhibition. Stimulants, either by themselves or in combination with the inhibitors, had little or no effect on protein synthesis. The results are all consistent with the hypothesis that the level of arousal following acquisition plays an important role in determining the length of time over which the biosynthetic phase of memory formation will last.

A comparison of the effects of localized brain administration of catecholamine and protein synthesis inhibitors on memory processing

Protein synthesis inhibitors disrupt biosynthetic processes thought to control the formation of long-term memory. While the agents used (i.e. puromycin, acetoxycycloheximide, cycloheximide and anisomycin) do not selectively inhibit the synthesis of any particular class of protein, it has generally been hypothesized or assumed that the critical proteins(s) is structural and necessary for modification and/or growth of synapses. Recent reports indicated that all of the protein synthesis inhibitors causing amnesia inhibited tyrosine hydroxylase activity. Tyrosine hydroxylase is needed for the conversion of tyrosine to dopamine (DA) and norpinephrine (NE); altering the level of this enzyme could affect catecholamine (CA) turnover. Since drugs known to inhibit CA synthesis cause amnesia, it is of considerable interest whether amnesia induced by protein synthesis inhibitors depends basically on inhibition of CA synthesis.

Protein synthesis inhibition and memory for pole jump active avoidance and extinction

This study utilizes a pole jump active avoidance task to investigate the effects of protein synthesis on memory formation. An extinction training procedure for this task is also described. Amnesia for extinction is produced by inhibition of protein synthesis and is also demonstrated by active responding, so it is clear that there is no general impairment sufficient to disrupt motor skill, motivation, or retrieval of stored memories. It was found that while inhibition of protein synthesis in brain for 2 hr did not produce amnesia, inhibition for 6 to 8 hr did. These results demonstrate that for both shock-motivated learning and non-shock motivated extinction learning, the duration of inhibition of protein synthesis is important in determining whether amnesia occurs. We conclude that inhibition of cerebral protein synthesis can best account for amnesia induced by anisomycin, cycloheximide, and acetoxycycloheximide.

Protein synthesis dependent gradient of ECS retrograde amnesia.

The interacting amnesic effect of a protein synthesis inhibitor, anisomycin, and ECS was studied in active and passive avoidance tasks. By giving one to three injections of anisomycin, the duration of inhibition of protein synthesis was from 2 to 6 hr at 80% or greater. ECS was administered at various times after training (1 min to 9 hr) to both inhibited and uninhibited mice. The ECS gradient in uninhibited mice was never greater than 30 min. The ECS gradient in anisomycin-treated mice ranged from 3 to 8 hr, depending on the training strength and the number of anisomycin injections. The ECS gradient of retrograde amnesia consistently developed at about 1 hr after the recovery of protein synthesis began, and this displaced the ECS amnesic gradients by as much as 8–9 hr. The study also determined that ECS caused only a transient, low-percentage inhibition of protein synthesis in uninhibited mice. The ECS given to anisomycin-treated mice had only a slight additional effect on inhibition of protein synthesis and did not seem to increase the inhibition enough to account for the amnesia observed. The results are discussed in terms of the ECS amnesic gradients being dependent on memory-related protein synthesis that precedes ECS administration.

Neurochemical and behavioral effects of catecholamine and protein synthesis inhibitors in mice

A series of biochemical and behavioral experiments tested the hypothesis that anisomycin (ANI), a protein synthesis inhibitor, produced decrements in long-term memory by raising free tyrosine levels and by the accumulation of catecholamines (CAs) rather than by its primary effect on protein synthesis. We compared the effects of ANI and three catecholamine synthesis inhibitors (CAIs)--diethyldithiocarbamic acid, alpha-methyl-p-tyrosine, and tetrabenazine--on cerebral concentrations of tyrosine and CAs and on the rate of accumulation of CAs. ANI had a relatively small effect, whereas the CAIs resulted in large reductions. When ANI and a CAI were used in combination, effects on CA levels were determined mainly by the CAI. The amnestic effects of ANI and the CAIs were also compared across seven experimental paradigms. Pretraining administration of any of the four drugs could result in amnesia for passive avoidance training, but only when training was weak. With an increase in training strength, a series of three injections of ANI (one pre- and two post-training) caused amnesia, but a similar series of CAI injections did not. Substituting one CAI injection for the second of three successive ANI injections did not cause amnesia, but substituting cycloheximide, another protein synthesis inhibitor, resulted in amnesia. With an active avoidance test, ANI caused amnesia while AMPT did not; d-amphetamine blocked the amnestic effect of ANI but caused amnesia in AMPT injected mice. Whereas ANI lengthened the temporal gradient over which electroconvulsive shock produced amnesia, AMPT or DDC did not. DDC caused only transient amnesia for passive avoidance training, while the amnestic effect of ANI remained constant at 24-hr and 1-week retention tests. We conclude that ANI and CAIs have distinctly different abilities to produce amnesia. These experiments provide additional support for the hypothesis that protein synthesis is required for formation of long-term memory.

Memory retention test performance in mice: Improvement by chronic oral cholinergic drug treatment

Mice consumed solutions containing 0, 0.025, 0.050 or 0.075 mg/ml of arecoline hydrobromide (ARE) one week prior to training (T-maze, footshock, active avoidance) and a total of two weeks prior to testing memory retention. The mean daily doses of ARE were estimated to be 0, 157, 302, or 500 micrograms per mouse, respectively. An inverted-U dose-response curve was obtained; the best retention test performance was by the group receiving 0.050 mg/ml of ARE. Measures of activity and weight taken over the experiment indicated no significant differences between ARE groups and the control group; thus no apparent toxicity. Separate groups of mice consumed 0 or 0.050 mg/ml of ARE for one week, then were trained to a criterion of 5 avoidances in 6 training trials. There were no significant differences in trials to first avoidance response or to criterion. Thus the enhanced retention test performance of the 0.050 mg/ml ARE group reflected improved memory processing rather than better learning.