Persistence of retrieval enhancement by amphetamine following scopolamine-induced amnesia

Amphetamine increases errors during episodic memory retrieval

Moderate doses of stimulant drugs are known to enhance memory encoding and consolidation, but their effects on memory retrieval have not been explored in depth. In laboratory animals, stimulants seem to improve retrieval of emotional memories, but comparable studies have not been carried out in humans. In the present study, we examined the effects of dextroamphetamine (AMP) on retrieval of emotional and unemotional stimuli in healthy young adults, using doses that enhanced memory formation when administered before encoding in our previous study. During 3 sessions, healthy volunteers (n = 31) received 2 doses of AMP (10 and 20 mg) and placebo in counterbalanced order under double-blind conditions. During each session, they first viewed emotional and unemotional pictures and words in a drug-free state, and then 2 days later their memory was tested, 1 hour after AMP or placebo administration. Dextroamphetamine did not affect the number of emotional or unemotional stimuli remembered, but both doses increased recall intrusions and false recognition. Dextroamphetamine (20 mg) also increased the number of positively rated picture descriptions and words generated during free recall. These data provide the first evidence that therapeutic range doses of stimulant drugs can increase memory retrieval errors. The ability of AMP to positively bias recollection of prior events could contribute to its potential for abuse.

Combined effects of acute stress and amphetamine on serial memory retrieval pattern in mice

INTRODUCTION

This study investigated the dose-effect of amphetamine on contextual serial (contextual serial discrimination (CSD)) and serial (serial discrimination (SD)) memory in acutely stressed versus nonstressed C57 Bl/6 Jico mice.

MATERIALS AND METHODS

Memory was first evaluated in nonstress condition. Mice learned two consecutive discriminations (D1 and D2) in a four-hole board involving either distinct (CSD) or identical (SD) internal contextual cues. All mice received i.p. injections of vehicle before acquisition and vehicle or amphetamine 20 min before the memory retrieval phase occurring 24 h after acquisition.

RESULTS

Results showed that: (1) vehicle group expressed in both tasks a similar memory retrieval pattern, D2 being better retrieved than D1; (2) 2 mg/kg amphetamine significantly enhanced D1 but not D2 performance in both tasks, whereas 4 mg/kg amphetamine enhanced D2 but not D1 retrieval. Thus, amphetamine more specifically modulates serial order memory retrieval in a context-independent manner. In a further step, we studied the effect of an acute stress (electric foot shocks 5 min before retrieval) specifically on D1 performance of the CSD task in 2 mg/kg amphetamine-treated mice. Immediately after testing, blood was sampled to measure plasma corticosterone levels. Results showed that acute stress significantly improved D1 performance in vehicles but blocked the memory-enhancing effect of 2 mg/kg amphetamine, as compared to the nonstress condition. However, statistical analysis failed to evidence a significant interaction between treatments and conditions (stress vs nonstress) on corticosterone levels, contrary to another vigilance-enhancing drug, modafinil (Béracochéa, Psychopharmacology 196:1-13, 2008).

Stress modulation of the memory retrograde-enhancing effects of the awakening drug modafinil in mice

PURPOSE

This study investigated the dose-effect relationship of modafinil administration on contextual memory processes, in parallel with the measurements of plasma corticosterone levels in acutely stressed mice.

MATERIALS AND METHODS

Memory was first evaluated in normal (nonstressed) mice either in contextual (CSD) or spatial (SSD) tasks. Thus, C57 Bl/6 Jico mice learned two consecutive discriminations (D1 and D2) in a four-hole board. The discriminations occurred on either distinct (CSD) or identical (SSD) floors (internal contextual cues). All mice received a vehicle intraperitoneal injection before learning and were injected 24 h later (20 min before the test session) either with vehicle or modafinil.

RESULTS

Results showed that modafinil-treated mice behaved similarly as vehicles in the spatial SSD task, whereas in contrast, memory of the first-learned discrimination (D1) in the CSD task was enhanced by a 32- but not a 16-mg/kg modafinil dose. Hence, we studied the effect of a pretest acute stress (electric footshocks) specifically on D1 performance in modafinil-treated subjects. Immediately after behavioral testing, blood was sampled to measure plasma corticosterone levels.

CONCLUSIONS

Results showed that: (1) stress significantly improved performance in vehicles, (2) stress decreased the efficiency threshold of modafinil, as performance was enhanced at the low dose (16 mg/kg), whereas this enhancement was obtained for the high dose (32 mg/kg) under nonstress conditions, (3) the performance was impaired at the high (32 mg/kg) dose, and (4) modafinil significantly reduced the magnitude of the stress-induced corticosterone secretion, mainly at the dose of 32 mg/kg.

Dimaprit, a histamine H2-agonist, inhibits anaphylactic histamine release from mast cells and the decreased release is restored by thioperamide (H3-antagonist), but not by cimetidine (H2-antagonist)

Whether anaphylactic histamine release from rat peritoneal mast cells is influenced by betahistine, a histamine H1-receptor agonist/H3-antagonist, and dimaprit, an H2-agonist, was examined. Treatment with dimaprit at 6 and 60 microM for 20 min significantly inhibited the anaphylactic histamine release, whereas betahistine at up to 80 microM under the same conditions did not affect it. Treatment with dimaprit at 6 and 60 microM for 1 to 20 min and for 5 to 20 min, respectively, caused a time-dependent inhibition of the release, but up to 30 min treatment with 8 and 80 microM betahistine had no effect. The decreased histamine release induced by dimaprit was recovered by neither mepyramine nor cimetidine. However, thioperamide, an H3-selective antagonist, dose-dependently restored the diminished release. From these results, the inhibition of anaphylactic histamine release by dimaprit is not produced by the stimulation of H2-receptors, but involves the stimulation of H3-like receptors or H3-subtype receptors, which are distinct from the H3-receptors located in brain, and suggests that the receptor plays an important role in the negative feedback regulation of histamine release.

A threshold for the protective effect of over-reinforced passive avoidance against scopolamine-induced amnesia

Acetylcholine-receptor blockers produce amnesia of aversively motivated behaviors. However, when animals are submitted to relatively high intensities of footshock (over-reinforcement), anticholinergic treatment does not induce memory impairments. The aim of this work was to determine whether the antiamnesic effect produced by increasing the magnitude of the negative reinforcer is gradually established or if a threshold should be reached to obtain such an effect. Wistar rats were trained in passive avoidance using 2.5, 2.6, 2.7, 2.8, 2.9 or 3.0 mA; 5 min after training they were given one systemic injection of scopolamine (8 mg/kg). An amnesic state was produced in the groups that were trained with the lower intensities (2.5-2.7 mA); with the three higher intensities near-perfect retention was evident. These results suggest that acetylcholine is critically involved in memory consolidation, and that by increasing the magnitude of the negative reinforcer, a threshold is reached where cholinergic activity of the nervous system is not necessary for the development of the consolidation process.

Memory modulation with peripherally acting cholinergic drugs

Physostigmine (PHYSO), in doses as low as 0.003 mg/kg IP, antagonized scopolamine (SCOP, 3 mg/kg) induced amnesia of step-through passive avoidance in mice. The peripherally acting acetylcholinesterase (AChE) inhibitor neostigmine (NEO) was also found to reliably, though less strongly, antagonize the SCOP induced amnesia at a dose of 0.03 mg/kg. The NEO antagonism of the SCOP amnesia could be reversed with SCOP (0.3, 1, and 3 mg/kg) and mecamylamine (MECA, 1, 3, and 10 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active both peripherally and centrally, as well as with M-SCOP (0.3 and 1 mg/kg) and hexamethonium (HEX, 1 and 3 mg/kg), muscarinic and nicotinic antagonists, respectively, which are active only in the periphery. In contrast to the ability of these four compounds to attenuate the SCOP amnesia, only the centrally acting compounds SCOP (3 mg/kg) and MECA (10 mg/kg) induced an amnesia when administered alone. These findings suggest that the induction of amnesia of passive avoidance involves central cholinergic systems, whereas the NEO, and possibly PHYSO, reversal of the SCOP induced amnesia is mediated peripherally by both muscarinic and nicotinic receptors. It is hypothesized that the release of adrenal catecholamines, the influence of which on memory processes is well known, and secondarily glucose, may be responsible for the NEO antagonism of the SCOP amnesia.