Plasma catecholamines: effects of footshock level and hormonal modulators of memory storage

Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in male Sprague-Dawley rats before and at several times after training injections of agents known to enhance or to impair later retention performance for a one-trial inhibitory (passive) avoidance task. Two days before testing, each animal was surgically prepared with a chronic tail artery catheter that allows for repeated blood sampling in unhandled rats. Exposure to a single, intense training footshock (3.0 mA, 2.0 sec duration) resulted in an immediate but transient increase in plasma levels of EPI and to a lesser extent NE. Plasma levels of both catecholamines did not differ between unshocked controls and animals that received a weak training footshock (0.6 mA, 0.5 sec duration). An injection of EPI at a dose that enhances retention performance (0.1 mg/kg, sc) resulted in increments in plasma EPI levels of 0.8-1.9 ng/ml from 5 to 40 min after injection. An injection of EPI (0.5 mg/kg, sc) at a dose that produces retrograde amnesia resulted in increments in plasma EPI ranging from 3.7 to 4.5 ng/ml during the 40 min after injection. Plasma NE levels were not significantly altered following an EPI injection. A single injection of adrenocorticotropin (ACTH, 0.3 or 3.0 IU per rat) did not alter the plasma catecholamine responses to training with a weak footshock. Similarly, the synthetic ACTH analog, Organon 2766 (125 or 250 mg/Kg) did not affect plasma catecholamines in untrained (unshocked) rats.These results demonstrate that significant increments in plasma levels of NE and EPI occur shortly after inhibitory avoidance training. Furthermore, an injection of EPI that enhances retention of an inhibitory avoidance task mimics the magnitude, though not the temporal characteristics, of the endogenous adrenal medullary response to a training footshock. Other hormonal treatments (ACTH and Organon 2766) which enhance memory storage do not affect plasma levels of NE and EPI.     

Effects of posttrial hormone injections on memory processes

This experiment examined the effect on memory of posttrial injections of epinephrine, norepinephrine, ACTH, growth hormone, vasopressin and corticosterone. Rats were trained with a weak footshock (0.7 mA, 0.35 sec) in a one-trial inhibitory (passive) avoidance task. The animals received subcutaneous injections of one of the above hormones or saline immediately after training. On a retention test 24 hr after training, animals which received ACTH (0.03 or 0.3 IU/rat), epinephrine (0.1 mg/kg) or norepinephrine (0.1, 0.3 or 1.0 mg/kg) had retention performance which was significantly better than that of saline control animals. A higher posttrial ACTH dose (3.0 I.U./animal) impaired later retention performance. ACTH (0.3 I.U./animal) and norepinephrine (0.3 mg/kg) injections administered 2 hr after training had no significant effect on retention. Immediate posttrial injections of vasopressin (dose range 0.001–1.0 I.U./animal), growth hormone (0.5–1.0 mg/kg), or corticosterone (0.01–4 mg/kg) did not significantly enhance retention. These findings indicate that epinephrine, norepinephrine, and ACTH injections can enhance memory processes if the hormones are injected shortly after training. Such results are consistent with the view that hormonal consequences of an experience, particularly epinephrine, norepinephrine and ACTH release, may normally have a modulatory influence on memory processes in untreated animals. In addition, it is therefore possible that other posttrial treatments which enhance or impair later retention performance may act through hormonal mechanisms.     

Effects of leptin on memory processing

Leptin is a peptide hormone secreted by adipose tissue. Studies have shown that leptin crosses the blood-brain barrier (BBB) by a saturable transport system where it acts within the hypothalamus to regulate food intake and energy expenditure. Leptin also acts in the hippocampus where it facilitates the induction of long-term potentiation and enhances NMDA receptor-mediated transmission. This suggests that leptin plays a role in learning and memory. Obese mice and rats, which have leptin receptor deficiency, have impaired spatial learning. In disease states such as diabetes, humans and animals develop leptin resistance at the BBB. This suggests that low leptin levels in the brain may be involved in cognitive deficits associated with diabetes. In the current study, the effects of leptin on post-training memory processing in CD-1 mice were examined. Mice were trained in T-maze footshock avoidance and step down inhibitory avoidance. Immediately after training, mice received bilateral injections of leptin into the hippocampus. Retention was tested 1 week later in the T-maze and 1 day later in step down inhibitory avoidance. Leptin administration improved retention of T-maze footshock avoidance and step down inhibitory avoidance. Leptin administered 24 h after T-maze training did not improve retention when tested 1 week after training. SAMP8 mice at 12 months of age have elevated amyloid-beta protein and impaired learning and memory. We examined the effect of leptin on memory processing in the hippocampus of 4 and 12 months old SAMP8 mice. Leptin improved retention in both 4 and 12 months old SAMP8 mice; 12 month SAMP8 mice required a lower dose to improve memory compared to 4 months SAMP8 mice. The current results indicate that leptin in the hippocampus is involved in memory processing and suggests that low levels of leptin may be involved in cognitive deficits seen in disease states where leptin transport into the CNS is compromised.     

Dynorphin1-17 can enhance or impair retention of an inhibitory avoidance response in rats

The possible effects of subcutaneous administration of dynorphin1-17 on retention of an inhibitory avoidance behavior have been studied in rats. Post-training or pre-test administration of dynorphin1-17 in doses of 25 or 50 micrograms/kg facilitated retention performance in rats subjected to a footshock of 0.2 mA n the acquisition trial. However, the same doses of the opioid peptide exerted a deleterious effect on retention performance when a footshock of 0.4 mA was used after either post-training or pre-test administration. Post-training injection of the kappa-receptor antagonist MR-2266 in doses of 0.5, 1 or 2.5 mg/kg failed to affect retention behavior. However, the previous administration of 2.5 mg/kg of MR-2266 prevented the facilitatory effect exerted by dynorphin1-17 after post-training, as well as after pre-test administration. Our results suggest that dynorphin1-17 may be involved in modulating the consolidation, as well as the retrieval, of recently acquired information.     

Levels of arginine-vasopressin in cerebrospinal fluid during passive avoidance behavior in rats

The concentration of immunoreactive arginine-vasopressin (IR-AVP) was measured in the cerebrospinal fluid (CSF) during acquisition and retention of passive avoidance behavior. IR-AVP level in CSF of male Wistar rats immediately after the learning trial was increased; the rate of which was related to the intensity of the electric footshock during the learning trial and the avoidance latency as measured 1 day after the learning trial. Immediately after the 24 h retention test IR-AVP levels were significantly increased in rats subjected to the low (0.25 mA) shock intensity during the learning trial, but IR-AVP levels of rats exposed to the high shock (1.0 mA) were under the limit of detection. If the retention test was postponed till 5 days after the learning trial, the increase of IR-AVP level in the CSF was related to avoidance latencies which reflect the intensity of aversive stimulation (electric footshock). The results suggest an association between central AVP release and passive avoidance behavior and may be indicative of the role of this peptide in neuronal mechanisms underlying learning and memory processes.     

Moderate prenatal carbon monoxide exposure produces persistent, and apparently permanent, memory deficits in rats

The effects of moderate (150 +/- 2 ppm) prenatal carbon monoxide (CO) exposure (maternal HbCO concentrations of 15.6 +/- 1.1%) on learning and memory were assessed in young and aged adult rats using a two-way active avoidance paradigm. In experiment 1, the prenatal CO-exposed rats at 120 days of age acquired a conditioned avoidance response equally well as control animals in a 100-trial session. However, following a 24-hr interval the CO-exposed rats failed to demonstrate significant retention of the task as indicated by the absence of significant improvement in performance over the indicated by the absence of significant improvement in performance over the previous day; control subjects did show significant retention. In experiment 2, in which 120-day-old animals received 50 training trials per day until a criterion of ten consecutive avoidance responses was met, the prenatal CO-exposed subjects again acquired the task as well as control animals. When tested for retention 28 days later, a significant memory impairment was again observed in terms of trials required to reattain the avoidance criterion as well as in total percent avoidance responding. In neither experiment did an analysis of initial or average latency to escape the footshock stimulus reveal any significant alterations. These latter results suggest that the observed performance impairment reflected a memory deficit and not a disruption of sensory, motor, or motivational factors. In experiment 3, prenatal CO-exposed rats approximately 1 year of age (300-360 days of age) showed impairment relative to air-exposed controls in both the original learning and retention of the two-way avoidance response. Again, however, there was no evidence for alterations in performance factors per se. Collectively these data indicate that while young adult rats prenatally exposed to 150 ppm CO demonstrate an associative deficit restricted to memory impairment, aged adults similarly exposed during the prenatal period display a more pronounced deficit similar to that recently reported for animals tested as juveniles. The importance of parametric manipulations in uncovering long-term toxicity is also discussed.     

Testosterone modulates performance on a spatial working memory task in male rats

Gonadal hormones have been shown to modulate memory retention in female rats. The current experiments examine the role of testicular hormones in modulating the performance of male rats on two spatial water maze tasks. In the first study, castrated and intact rats were trained on the visible platform and hidden platform versions of the Morris water maze task. Castration did not affect performance on either version of this reference memory task with castrated and intact rats demonstrating similar performance both during acquisition and on post-training probe trials. In the second experiment, castrated and intact rats were tested on a delayed-matching-to-place version of the water maze. Rats received a series of trial pairs in the maze with a hidden platform located in the same pool location on the exposure and retention trials of each pair; between pairs of trials, however, the platform was repositioned to a novel pool location. The interval between trials was either 10- or 60-min and memory retention, taken as the difference between the pathlengths on the exposure and retention trials, declined as the interval increased. Relative to intact males, castrated males demonstrated impaired working memory retention at 60-min but not at 10-min retention intervals. This interval-dependent impairment in working memory retention was reversed by physiologic levels of testosterone replacement. These findings indicate that castration does not significantly affect acquisition or probe trial performance on a classic reference memory task but does impair spatial working memory retention, an effect that is reversed by exogenous testosterone.     

Pretraining but not Preexposure to the Task Apparatus Prevents the Memory Impairment Induced by Blockade of Protein Synthesis,PKA or MAP Kinase in Rats

Adult male Wistar rats were trained and tested in a step-down inhibitory avoidance task (0.4 mA footshock, 24 h training-test interval). Fifteen minutes before or 0, 1.5 or 3 hours after training, animals received a 0.8 l intrahippocampal infusion of the protein synthesis inhibitor anisomycin (80 g), the PKA inhibitor Rp-cAMP (0.05 g), the MAPK kinase inhibitor PD 098059 (50 M solution) or vehicle (phosphate buffer in saline, pH 7.4). Anisomycin, Rp-cAMP and PD 098059 impaired retention test performance in animals injected at different times, prior and after training. Pretraining with a low footshock intensity (0.2 mA) 24 h before training prevented the amnestic effect of all drugs studied. However, simple preexposure to the inhibitory avoidance apparatus did not alter the amnestic effects of all drugs. The results suggest that memory processing requires hippocampal mechanisms dependent on protein synthesis, PKA and MAPK kinase at different times after training. These findings suggest that weak training must be sufficient to produce some lasting cellular expression of the experience so that the enhancement of consolidation of a previously acquired memory is not dependent on protein synthesis, PKA or MAPK.     

Effect of sildenafil (Viagra) on memory retention of a passive avoidance response in rats

The effects of an intraperitoneal (i.p.) injection of different doses of sildenafil, a cyclic guanosin monophosphate (cGMP) specific phosphodiesterase type 5 (PDE 5) inhibitor, on memory retention of young (2-month-old) and middle aged (12-month-old) male Wistar rats were investigated. Passive avoidance behaviour was studied in a one trial learning, step--through type, passive avoidance task utilizing the natural preference of rats for a dark environment. In each category (young or middle-aged) different groups of rats received vehicle or sildenafil (1, 3, 10, 20 mg*kg(-1), i.p.) immediately after training and one group remained uninjected serwing as control. Retention latencies were measured 48 h later. To asses a possible non-specific proactive effect of sildenafil, the response latencies in a group of rats not receiving foot shock were also tested. The results showed that the post-training i.p. administration of sildenafil did not facilitate retention performance of a passive avoidance response in both young and middle aged rats compared to control or vehicle groups. Also, sildenafil did not affect response latencies in rats not having received the footshock on the training trial, indicating that sildenafil does not show a non-specific proactive affect on retention performance. The comparison of retention time between young and middle aged rats showed that the memory of the latter had been significantly reduced. In conclusion, this study suggests that sildenafil has no effects on memory retention in Wistar rats.     

Prior exposure to a single stress session facilitates subsequent contextual fear conditioning in rats. Evidence for a role of corticosterone

Previous studies showed that exposure of rats to chronic restraint stress for 21 days enhances subsequent contextual fear conditioning. Since recent evidence suggest that this effect is not dependent on stress-induced neurodegenerative processes, but to elevated training-elicited glucocorticoid release in chronically stressed animals, we aimed to explore here whether a single exposure to restraint stress, which is not expected to induce neuronal damage, would also affect contextual fear conditioning. We also questioned whether post-training corticosterone levels might be associated with any potential effect of stress on fear conditioning. Adult male Wistar rats were exposed to acute restraint stress for 2 h and, two days later, trained in the contextual fear conditioning task, under training conditions involving either moderate (0.4 mA shock) or high (1 mA shock) stress levels. The results showed that acute stress enhanced conditioned freezing at both training conditions, although data from the 1 mA shock intensity experiment only approached significance. Stressed animals were shown to display higher post-training corticosterone levels. Furthermore, the facilitating effect of prior stress was not evident when animals were trained in the hippocampal-independent auditory-cued conditioning task. Therefore, these findings support the idea that stress experiences preceding exposure to new types of stressors facilitate the development of contextual fear conditioning. They also indicate that not only repeated, but also a single exposure to aversive stimulation is sufficient to facilitate context-dependent fear conditioning, and suggest that increased glucocorticoid release at training might be implicated in the mechanisms mediating the memory facilitating effects induced by prior stress experiences.     

Infusion of methylphenidate into the basolateral nucleus of amygdala or anterior cingulate cortex enhances fear memory consolidation in rats

The psychostimulant methylphenidate (MPD; also called Ritalin) is a blocker of dopamine and norepi-nephrine transporter. It has been clinically used for treatment of Attention Deficit and Hyperactivity Disorder (ADHD). There have been inconsistent reports regarding the effects of systemically adminis-tered MPD on learning and memory, either in animals or humans. In the present study, we investigated the effect of direct infusion of MPD into the basolateral nucleus of amygdala (BLA) or the anterior cin-gulate cortex (ACC) on conditioned fear memory. Rats were trained on a one-trial step-through inhibi-tory avoidance task. MPD was infused bilaterally into the BLA or the ACC, either at ‘0’ or 6 h post-training. Saline was administered as control. Memory retention was tested 48 h post-training. In-tra-BLA or intra-ACC infusion of MPD ‘0’ h but not 6 h post-training significantly improved 48-h memory retention: the MPD-treated rats had significant longer step-through latency than controls. The present results indicate that action of MPD in the BLA or the ACC produces a beneficial effect on the consoli-dation of inhibitory avoidance memory.     

Effect of hypophysectomy on responsiveness of rat adrenal slices to ACTH--pregnenolone formation from endogenous cholesterol.

Responsiveness of rat adrenal slices to ACTH was studied by measuring pregnenolone formation from endogenous cholesterol. The yield of pregnenolone by adrenal slices of hypophysectomized for 48 hrs rats was markedly higher as compared to intact animals.     

Electroconvulsive shock and brain muscarinic receptors: relationship to anterograde amnesia

Rats were administered one electroconvulsive shock daily for 7 days (ECS X 7) and were killed 24 hours after the last treatment. Muscarinic cholinergic receptor number, as determined by [3H] quinuclidinyl benzilate [( 3H]QNB) binding, was significantly reduced in the cerebral cortex. A parallel group of rats was trained on a passive avoidance task 24 hours following the last ECS and tested for retention of the original avoidance response 24 hours later; these animals exhibited a profound amnesia. Animals tested 1 hour following training were not amnestic, indicating that learning was unimpaired. Animals trained 7 days following ECS X 7 were not amnestic and [3H] QNB binding changes were not demonstrable at this time. A single ECS which does not significantly affect cortical [3H] QNB binding, did not induce amnesia in rats trained 24 hours after the treatment and tested 24 hours later. The parallel, cumulative nature of ECS-induced muscarinic receptor down-regulation and ECS-induced anterograde amnesia suggests a possible causative relationship.     

Aversive Learning under Different Training Conditions: Effects of NMDA Receptor Blockade in Area CA1 of the Hippocampus

Adult male Wistar rats were given either a single training trial or one training trial per day during 3 days followed by a retention test trial in an inhibitory avoidance (IA) task. In animals given a single training trial, pretraining, but not pretest bilateral infusion of the NMDA glutamate receptor antagonist d,l-2-amino-5-phosphonopentanoic acid (AP5) (5.0 μg) into the CA1 hippocampal area blocked IA retention. In animals given three training trials, infusions of AP5 given prior to each of the three training trials severely impaired, but did not block retention. The results indicate that NMDA receptors in the hippocampus are involved in the formation, but not in expression, of aversive memory. In addition, rats given repeated training were able to show a mild improvement of performance across training trials, possibly through mechanisms that do not depend on NMDA receptor activation in the dorsal hippocampus.     

The expression of the c-fos gene in the brain of mice in the dynamic acquisition of defensive behavioral habits

Levels of c-fos mRNA expression in mouse cerebral cortex and hippocampus at different stages of footshock escape and avoidance learning were studied by Northern hydridization. In the first series of experiments a mouse was presented with 30 electric footshock daily in a chamber where it could escape from the floor by jumping on the safe platform attached to the wall. A large increase in c-fos mRNA level in the cerebral cortex and hippocampus was observed during the first day of training. Mice that were trained for 9 consecutive days and acquired a footshock escape reaction showed no elevation of c-fos expression in the brain as compared to the quiet control group. In the second series of experiments the levels of c-fos expression were compared in individual mice trained to avoid the footshock by jumping on the platform in response to an auditory conditioned stimulus. Mice which acquired avoidance behavior more rapidly had lower c-fos mRNA levels than slow learners. There was no such to difference between the corresponding yoked control groups which consisted of animals matched the rapid and slow learners by the number of footshocks received. It is concluded that achievement of adaptive results in the course of learning leads to a suppression of further c-fos induction by motivational excitation.     

Mirex-induced liver enlargement in rats is dependent upon an intact pituitary-adrenalcortical axis

The effect of prior hypophysectomy upon mirex-induced liver hypertrophy in male Sprague-Dawley rats was examined. Mirex had no effect upon adrenal weight, liver weight, plasma glucose or plasma corticosterone in hypophysectomized rats. However, daily corticosterone supplements (20 mg/kg body weight, sc) given to mirex-treated hypophysectomized animals yielded a 52% increase in liver weight to body weight ratios over those observed in mirex-treated hypophysectomized animals not receiving supplement. In intact rats, both liver weight to body weight ratios and plasma ACTH were significantly increased over controls 2 days after mirex treatment. These results indicate that mirex-induced liver enlargement not only requires corticosterone, but that the response is dependent upon an intact pituitary-adrenalcortical axis.     

Ornithine decarboxylase and thymidine kinase activity in tissues of prolactin-treated rats: effect of hypophysectomy

The activities of ornithine decarboxylase and thymidine kinase were determined in tissues of young intact and hypophysectomized rats at various times after treatment with prolactin. In both types of animals, ornithine decarboxylase activity increased in liver, kidney, spleen and adrenal of prolactin treated rats. Thymidine kinase activity increased only in liver and spleen of intact rats. Increase in the kinase activity was smaller, and occurred later than the change in ornithine decarboxylase. In hypophysectomized animals, thymidine kinase activity increased in spleen, but not in liver, following prolactin treatment.     

Tolerance to the anti-avoidance properties of cholecystokinin-octapeptide

Previous investigations have suggested a neuroleptic-like action of cholecystokinin-octapeptide (CCK8) on conditioned-avoidance behavior. This study was initiated to test tolerance to this effect. Rats were trained to avoid electric shock in a shuttle box under a free-operant (Sidman) avoidance paradigm. Each shuttle response postponed a 0.2 sec, 1 mA shock for 20 sec. If the rat failed to respond, shock was delivered every 5 sec until a response occurred. After avoidance training, half of the rats received two daily injections of CCK8 (0.320 mg/kg, IP) and half received saline for 7 days. Rats were then tested on the Sidman avoidance 1 min after receiving CCK8 (0.640 mg/kg, IP) or saline. CCK8 depressed avoidance responding if rats received saline for 7 days prior to the test. Rats pretreated with CCK8 for 7 days were not significantly affected by CCK8 during the avoidance test. Thus, repeated injections of CCK8 result in tolerance to its anti-avoidance properties.     

Incorporation of methionine-35S into the structure of the brain at different pituitary-adrenal system hormone levels

The rate of 35S-methionine incorporation into the anterior and posterior hypothalamus, tonsil and hippocamp was studied in experiments on intact, adrenalectomized, and hypophysectomized rats as well as in those exposed to ACTH or dexamethasone. A conclusion is drawn that ACTH and glucocorticoids exert a dissimilar and non-equivalent effect on the brain structures responsible for the activity displayed by the hypophyseal-adrenal system.     

One trial learning in the American cockroach, Periplaneta americana

A new and simple method of training intact cockroaches was designed. (1) Cockroaches were maintained before and during the experiment with a high motivation to look for sugar. They were kept in a chamber with water and yeast extract ad libitum but without any sugar. (2) Cockroaches were individually trained to associate two artificial scent signals, i.e. menthol and vanilla, with sugar and salt solutions. Discriminatory learning performance was measured by testing the scent preference before and after training. Care was taken to exclude other than olfactory cues in the discriminatory paradigm. (3) Cockroaches exhibited a spontaneous initial preference for vanilla over menthol. This preference could be modified by training. The insect showed fast learning and long retention. One training trial was enough to reverse the initial preference with a significant retention after 7 days. Reversal of trained preference was accomplished by retraining. (4) The method provides an opportunity to study sensory performance and memory consolidation in cockroaches.