Effects of glucose and fructose on recently reactivated and recently acquired memories.

1. The effects of glucose and fructose on memory reactivation were investigated. 2. Rats were trained originally on a brightness discrimination passive avoidance task. 3. Memory reactivation treatment consisted of re-exposing the rats 24 hr later to the footshock unconditioned stimulus in the experimental room. Glucose or fructose (32, 100, 320, 1000, or 2000 mg/kg) was administered immediately after reactivation. 4. Twenty-four hr after reactivation (48 hr after training) the rats were tested for their ability to acquire an active avoidance (reversal) task. 5. The dose-response functions for the effects of both glucose and fructose on the reactivated memory followed identical cubic trends. However, a combined dose of glucose and fructose was significantly less effective at modulating memory than was an equimolar dose of either sugar alone. 6. We compared analytically the effects of combined glucose and fructose treatment on new versus old memories. The dose-response functions for both types of memories follow cubic trends, suggesting that similar multiple interacting mechanisms operate when memories are originally stored and when they are later re-encoded.

Time-dependent effects of fructose on the modulation of a reactivated memory.

1. A passive-avoidance-to-active-avoidance negative transfer paradigm was used to investigate systematically the time-dependent effects of fructose on reactivated memories in rats. 2. Memory reactivation consisted of re-exposing the rats 24 hr after passive-avoidance conditioning to environmental and learning cues present during training; post-reactivation injections of fructose (100 mg/kg, sc) or saline were followed 24 hr later by active-avoidance (discrimination reversal) conditioning. Fructose or saline was administered in the experimental room 0, 2, 5, or 30 min, or in the colony room 60 min, after reactivation. 3. The results showed a time-dependent decrease in the ability of fructose to modulate a reactivated memory. 4. These results suggest that the time-dependent effects for the modulation of a reactivated memory by fructose (a hexose that does not readily pass the blood-brain barrier) and glucose (a hexose that readily passes the blood-brain barrier) follow similar trends.

Cocaine impairs acquisition of an autoshaped lever-touch response.

The effects of daily peripheral (IP) post-session injection of cocaine on the development of an autoshaped lever-touch response in rats were investigated. Male Sprague-Dawley rats received ten daily pairings of a retractable lever (conditioned stimulus; CS) and food delivery (unconditioned stimulus; UCS). Food delivery occurred if the subjects contacted the extended lever within 10 s, or, if the subjects failed to contact the lever, at the end of the 10-s stimulus interval. These contingencies resulted in increased lever-touch responses over 10 days of conditioning. Cocaine (5.6-19.0 mg/kg) impaired development of the lever-touch response, as compared to saline-treated control subjects. Because the injections were given immediately after each conditioning session, we suggest that cocaine affects the neural processes involved in consolidation. Three additional control experiments support this suggestion. The effect of cocaine on lever-touch acquisition was time-dependent as daily injection of cocaine (5.6 mg/kg) 3 h after each conditioning session did not affect lever-touch acquisition. In addition, the effect of cocaine was dependent upon the explicit pairing of lever extension (CS) and food delivery (UCS) as immediate post-session cocaine (5.6 mg/kg) administration did not alter responding when the presentation of both the CS and the UCS was uncorrelated. Cocaine (5.6 mg/kg) administered to subjects previously trained to a performance criterion did not affect lever-touch responding, indicating that cocaine administration (5.6 mg/kg) impairs the development, but not the maintenance, of autoshaped lever-touch responding. In contrast, the highest dose of cocaine tested, 19.0 mg/kg, did decrease lever-touch responding in well-trained subjects, indicating that post-session administration of higher doses of cocaine can produce aversive effects that may affect both the acquisition and maintenance of appetitively motivated behavior in the rat. The relative contributions of the instrumental and classical associations inherent in the autoshaping procedure were investigated by altering response contingencies. Rats showed no evidence of learning the lever-touch response when lever insertion and food delivery were positively correlated, and no explicit response contingency was present (classical conditioning); further, cocaine-treated subjects did not differ from saline-treated subjects. However, cocaine did impair lever-touch responding in the instrumental version of the task. Taken together, these results show that the post-session administration of cocaine can impair the acquisition of a multi-trial, multi-session appetitively motivated response.

Protein kinase inhibitors disrupt memory formation in two chick brain regions.

The amnesic effects of protein kinase inhibitors (H-7, HA-156, TFP, W-9, and W-13) on memory formation for a one trial peck-avoidance task in chicks were investigated with bilateral and unilateral injections into either the left or the right intermediate medial hyperstriatum ventrale (IMHV) or the left or right lobus parolfactorius (LPO). All five inhibitors injected bilaterally 5 min pretraining into either the IMHV or LPO or unilaterally into the left IMHV produced amnesia. Unilateral injections into the right IMHV did not produce amnesia. Unilateral injections of W-9 or W-13 into the left but not the right LPO produced amnesia, H-7, HA-156, or TFP did not produce amnesia when injected unilaterally into either the left or right LPO. The time of onset of amnesia produced by injecting TFP and W-13 into the LPO occurred 45 min after training, whereas H-7 produced amnesia significantly later (90 min after training). Amnesia induced by TFP, W-13, and H-7 injected into the LPO occurred significantly later than amnesia produced when these agents were injected into the IMHV. Together these data suggest that the IMHV and LPO process memory sequentially using various protein kinase activities.

Protein kinase C inhibitor chelerythrine disrupts memory formation in chicks.

Chelerythrine (CHELE), a specific, potent protein kinase C (PKC) inhibitor, disrupts memory formation for a one-trial peck-avoidance task. Three predictions were made about how CHELE, injected into chick brain near the time of training, would affect memory formation, based on previous work with two classes of protein kinase inhibitors (M. R. Rosenzweig et al., 1992; P. A. Serrano et al., 1994) and the in vitro inhibition of PKC by CHELE: (a) CHELE, injected into the intermediate medial hyperstriatum ventrale, would significantly impair memory formation; (b) the amnestic dose would be approximately 10 nmol; (c) CHELE would not produce amnesia for about 45 min after training, but significantly impair memory by 60 min. Experimental tests confirmed each prediction. This study adds to evidence that PKC activity is part of a cascade of neurochemical events initiated by learning and that PKC activity shortly after training is necessary for long-term memory.

(+/-) CPP, an NMDA receptor antagonist, blocks the induction of commissural-CA3 LTP in the anesthetized rat.

Commissural CA3-CA3 (cCA3) long-term potentiation (LTP) was investigated in the anesthetized rat treated with the highly selective NMDA-receptor antagonist D,L-3[(+/-)-2-carboxypiperazin-4-yl]- propyl-1-phosphonic acid (CPP). Intraperitoneal injections of CPP did not significantly affect baseline test responses for either field EPSP slope or amplitude measures but did reduce LTP in a dose-dependent manner, with 3.2 mg/kg as the lowest effective dose. EPSP variability following tetanization was also significantly reduced in both the 3.2 mg/kg and 10.0 mg/kg groups. We interpret these results to suggest that a 3.2 mg/kg dose of CPP may be sufficient for studying the behavioral effects of this NMDA receptor antagonist.

Comparable dose-response functions for the effects of glucose and fructose on memory.

A passive avoidance-to-active avoidance negative transfer paradigm was used to investigate in rats the effects of glucose and fructose on recently acquired memories. Immediate post-passive avoidance conditioning injections of glucose, fructose, or saline were followed 48 h later by active avoidance conditioning. Equimolar 10, 32, 100, and 2000 mg/kg sc doses of the two sugars significantly impaired acquisition of the reversal task, whereas 3.2 mg/kg doses of both sugars were without significant effect on subsequent performance and 320 mg/kg doses of both sugars significantly enhanced subsequent performance. The cubic trends for both dose-response functions were statistically significant and did not differ from each other. This is the first demonstration that glucose and fructose affect recently acquired memories in accord with comparable cubic dose-response functions, and that there are doses of both sugars that can enhance memory (as indicated by an increase in the number of trials required to reach criterion on the reversal task) and doses of both sugars that can impair memory (as indicated by a decrease in the number of trials required to reach criterion on the reversal task), compared to saline treatment. The similar cubic dose-response functions for glucose and fructose suggest that their mechanisms of action when they are injected peripherally are similar. In addition, because fructose does not readily pass the blood-brain barrier, the results suggest that these two monosaccharides may act through a common peripheral pathway.

Differential effects of protein kinase inhibitors and activators on memory formation in the 2-day-old chick.

Thirteen protein kinase inhibitors (PKIs) were investigated in chicks for their in vitro effects on PKC activity and for their in vivo effects on memory formation for a peak-avoidance task. Amnesia occurred by 15-30 min post-training when agents that inhibit primarily Ca2+/calmodulin were injected into brain. Amnesia occurred by 60 min post-training when agents that inhibit PKC-, PKA-, and/or PKG-dependent protein kinases, but not Ca2+/calmodulin, were injected. Enhancement of memory formation was accomplished by injecting bradykinin, but not forskolin. Both of these agents, however, attenuated the amnesia produced by H-7. These results are discussed as relevant neural processes involved in memory and synaptic plasticity.

Post-reactivation cocaine administration facilitates later acquisition of an avoidance response in rats.

We previously demonstrated that cocaine administered immediately prior to a reactivation episode comprised of re-exposure to selected features of the original fear-conditioning session alters subsequent memory retrieval or reconsolidation. In the present study we determined that, similar to pre-reactivation administration, post-reactivation administration of cocaine also alters memory retrieval or reorganization, as measured by subsequent conditioned performance. The dose-response function for this effect of cocaine was U-shaped; maximal enhancement of subsequent avoidance performance was produced by a 7.5 mg/kg i.p. dose of cocaine. Because a dose of lidocaine equimolar to the effective cocaine dose was found not to alter subsequent conditioned performance, the effect of cocaine on memory processing most likely is not attributable to its local anesthetic properties.

Individual variability in Tyr-Gly-Gly formation from enkephalin hydrolysis in mouse plasma.

We examined the effects of captopril (an angiotensin-converting enzyme inhibitor) and phosphoramidon (a selective enkephalinase inhibitor) on Tyr-Gly-Gly production during Met-enkephalin hydrolysis in plasma samples taken from individual outbred Swiss-Webster and inbred C57BL/6J and DBA/2J mice. Discriminant analysis procedures identified three distinct plasma profiles of Tyr-Gly-Gly production in all strains of mice: a captopril-sensitive/phosphoramidon-insensitive profile, a captopril-insensitive/phosphoramidon-sensitive profile, and a moderate captopril and phosphoramidon sensitivity profile. The abilities of captopril and phosphoramidon to inhibit Tyr-Gly-Gly production in the same mouse plasma sample were highly inversely correlated (r = -0.938). Plasma of Swiss-Webster mice whose cages and bedding had been changed 24 h prior to sample collection was significantly more likely to exhibit the captopril-sensitive/phosphoramidon-insensitive profile than the plasma of mice whose cages/bedding had not been changed for at least 5 days. The results suggest that environmental novelty may dramatically alter the activity of a plasma dipeptidyl carboxypeptidase system, and thereby regulate behavioral and physiological responses to novel experiences.

Cocaine administration prior to reactivation facilitates later acquisition of an avoidance response in rats.

The effect of cocaine administered prior to memory reactivation on the subsequent acquisition of an avoidance response was investigated. Two noncontingent footshocks were administered to rats in the black compartment of a one-way avoidance chamber. Twenty-four hours later, cocaine or saline was administered 5 min prior to a 30-s reactivation treatment consisting of re-exposure to selected stimuli present during the initial conditioning. Subjects were trained 24 h later to move from the chamber's black compartment to its white compartment in order to avoid a footshock. Intermediate (5.0 or 7.5 mg/kg IP), but not low (3.3 mg/kg IP) or high (11.25 or 16.88 mg/kg IP), doses of cocaine given prior to the reactivation treatment enhanced later acquisition of the one-way avoidance response. These results suggest that cocaine administered prior to the reintroduction of cues associated with a conditioning episode can modulate memory processes, and that the dose-response function for this effect is U-shaped. The avoidance performance of rats that received cocaine (5.0 mg/kg IP) 3 h after the reactivation treatment did not differ from that of saline-treated control subjects, suggesting that the conjoint neural activity elicited by cocaine and exogenous retrieval cues is necessary for potentiation of memory retrieval or reconstruction processes.