Excessive serotonin release, not depletion, leads to memory impairments in rats.

Eight experiments compared and contrasted the effects of serotonin release and depletion on performance by rats in two tests of memory. Most experiments (Experiments 1-5) examined the effects of the serotonergic releasing/depleting agent p-chloroamphetamine on passive avoidance performance. Additional experiments explored p-chloroamphetamine's effects on retention performance by animals trained in an 8-arm radial maze (Experiment 6), and the effects of dorsal raphe nucleus lesions on passive avoidance in animals treated with (Experiment 8) or not treated with (Experiment 7) p-chloroamphetamine. In general, acute increases in serotonin release produced consistent and extensive retention performance deficits in both passive avoidance and radial arm maze. Results from an ancillary control experiment indicated that the p-chloroamphetamine-induced passive avoidance impairment was not related to drug-induced alterations in pain sensitivity. Other experiments ruled out the possibility that p-chloroamphetamine was disrupting passive avoidance retention performance by affecting post-trial consolidation processes, producing state-dependent retention, having direct effects at postsynaptic receptors, or indirectly by affecting non-serotonergic neurotransmitter systems. Depletion of serotonin resulting from either the long-term residual effects of p-chloroamphetamine or lesions of the dorsal raphe nucleus failed to alter passive avoidance retention scores although it produced extensive depletion (45-85%) of serotonin and 5-hydroxyindoleacetic acid in the cortex and hippocampus. These data contribute to the growing body of literature indicating an important role of serotonin in cognitive processes by demonstrating that excessive release, but not depletion, of serotonin produces profound retention performance impairment.

Transient enhancement of cholinergic neurochemical markers induced by NGF in aged F344 rats.

Two experiments were conducted to assess the durability of nerve growth factor (NGF) effects on cholinergic neurochemical markers. Artificial CSF or NGF was infused via osmotic pumps for 2 weeks into the lateral ventricles of young adult (3- to 6-month-old) and aged (22- to 26-month-old) Fischer 344 rats. Assessment of choline acetyltransferase (CAT) and acetylcholinesterase (AChE) within the cortex, striatum, and hippocampus was conducted either approximately 3 (experiment 1) or 12 (experiment 2) weeks after termination of treatment. A variety of age-related deficiencies were found in the two experiments with decreased marker levels within the dorsal hippocampus and striatum being most consistent. NGF increased cholinergic marker enzyme activity in experiment 1 only. Specifically, NGF (a) attenuated age-related CAT and AChE deficits within the dorsal hippocampus and striatum, (b) enhanced CAT activity within the frontal cortex and ventral hippocampus in aged animals, and (c) increased CAT activity within the dorsal hippocampus in young subjects. It is concluded that NGF may be beneficial in enhancing cholinergic neurochemical parameters, especially in aged animals, but such effects are most likely transient.

Effects of scopolamine on spatial working memory in rats pretreated with the serotonergic depleter p-chloroamphetamine.

The present study investigated the mnemonic consequences of muscarinic blockade in rats pretreated with the serotonergic depleter p-chloroamphetamine (PCA). Subjects were initially injected with either PCA (2.5 mg/kg) or saline (SAL) approximately 1 week prior to testing on a working memory version of the Morris water maze (four daily trials for 4 days). Fifteen minutes before each daily session, PCA- and SAL-pretreated subjects were injected either with SAL or with one of two doses of scopolamine (SCOP; 0.5, 1.0 mg/kg). Latency to find the platform on each trial served as the major dependent measure. The results indicated that SCOP increased escape latencies. However, the dose needed to produce this effect was dependent upon whether subjects were pretreated with PCA. Significant increases in escape latencies in SAL-pretreated rats were induced with 1.0 mg/kg SCOP (on Day 1), while 0.5 mg/kg SCOP produced the same effect in PCA-pretreated subjects (on Days 1-4). Thus, although it was clearly demonstrated that muscarinic blockade did impair spatial working memory, the present data indicated that the dose necessary to produce this effect was dependent upon the status of the serotonergic system. More generally, these results imply that the cholinergic and serotonergic systems interact in a functionally important, as yet undetermined manner.

Induction of memory and cortical cholinergic neurochemical recovery with combine fetal transplantation and GM1 treatments in rats with lesions of the NBM.

Sixty-four Sprague-Dawley rats received ibotenic-acid-induced unilateral nucleus basalis of Meynert (nbM) lesions; 10 additional animals served as sham controls. Eight to ten days later, subjects with lesions received either fetal cholinergic transplants implanted within the ipsilateral (relative to the lesion) frontal cortex or control transplant surgeries. Lesioned animals with and without transplants were then treated with GM1 (20 mg/kg, i.p.) for either 0, 1 or 10 days and were then trained and tested for 72-hour retention of passive avoidance. Results indicated that the lesion produced a significant impairment on this task. Transplant therapy combined with GM1 for 10 days yielded a significant reversal of this deficit. GM1 injections continued once per week for 4 weeks for half the lesioned animals in the transplant and no-transplant 10-day conditions. During a 6-month period, all subjects were assessed on two additional memory tasks (complex spatial discrimination and delayed spatial alternation). In general, there was no indication of a lesion, transplant, GM1, or transplant X GM1 effect on these tasks. Approximately 7.5 months after transplants, subjects were sacrificed and their frontal cortices examined for choline acetyltransferase (CAT) and acetylcholinesterase (AChE) activity. Only lesioned subjects with transplants which were given sustained GM1 treatment (i.e., 10 days plus weekly injections for 4 weeks) showed significant attenuations of lesion-induced CAT and AChE depletions. These data suggest that a combined treatment strategy of fetal transplant plus GM1 is capable of reversing nbM lesion-induced memory and neurochemical deficits in an animal model of the cholinergic deficits in Alzheimer's disease.

Cholinergic marker deficits induced by lesions of the nucleus basalis of Meynert are attenuated by nerve growth factor in young, but not in aged, F344 rats.

To investigate the efficacy of nerve growth factor (NGF) in promoting recovery from cholinergic damage, young (3-4 month old) and aged (22-23 month old) Fischer 344 rats received NMDA-induced unilateral lesions of the nucleus basalis of Meynert and subcutaneous osmotic pumps (2-week duration) connected to permanently implanted cannulas directed at the lateral ventricle ipsilateral to the lesion. Pumps were filled with either artificial CSF/rat serum albumin (the vehicle) or 5.0 micrograms of angiotensin-free, beta-NGF. Fourteen days after surgery, all subjects were sacrificed and their brains regionally dissected (frontal and occipital cortices, striatum, and dorsal and ventral hippocampi) and assayed for choline acetyltransferase (CAT) and acetylcholinesterase (AChE). Results indicated that the lesion decreased CAT and AChE levels within the frontal cortex of both young (29.8% and 39.4% depletion, respectively) and aged (30.5% and 34.8% depletion, respectively) animals. Only in young animals did NGF reduce these lesion-induced CAT (by 34.2%) and AChE deficits (by 65.5%). In fact, NGF exacerbated frontal cortical CAT depletions in aged animals in that percent depletion was 11.3% more following treatment (30.5% vs. 41.8% depletion in Aged/CSF and Aged/NGF groups, respectively). Lower CAT and AChE levels were found in the striatum of aged animals, an effect not reversed by NGF treatment. In contrast, NGF in young animals enhanced striatal CAT activity on the non-lesioned side by 22.2%.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical mechanisms of stress-induced impairment of rat T cell mitogenesis.

Splenic mononuclear cells isolated from rats exposed to two brief stressors (5 min of restraint or 2 min of footshock) showed a diminished response to the T cell mitogens, concanavalin A and phytohemagglutinin. Cells from these stressed animals also exhibited a diminished response to stimulation with the phorbol ester, tetradecanoylphorbol acetate (TPA), and/or the calcium ionophore, ionomycin. Since stimulation with these latter two agents mimics early signals generated by mitogen surface receptor binding including increased intracellular calcium and protein kinase C activation, the data indicate that stress-related defects in T cell proliferation occur at sites other than or in addition to these early events in cellular activation.

Increased biosynthesis of Alzheimer amyloid precursor protein in the cerebral cortex of rats with lesions of the nucleus basalis of Meynert.

The nucleus basalis of Meynert was lesioned by infusion of N-methyl-D-aspartate (NMDA) unilaterally in adult rat brain. Seven days post lesion we observed that polysomes isolated from the cerebral cortex affected by the lesion synthesized 2.6-fold greater amounts of the Alzheimer amyloid precursor protein (AAPP) compared to the nonlesioned side of the same rat brain. This increase exhibited specificity to AAPP in that overall protein synthesis was not altered by the lesion. The increase of AAPP did not alter the ratio of AAPP isotypes in rat brain (in which AAPP 695, which is lacking the protease inhibitor insert remains the predominant form). The increased synthesis did not result in the apparent accumulation of mature AAPP. These results indicate that a cholinergic lesion which models many of the neurochemical changes observed in Alzheimer's disease induces the expression of AAPP in a major projection region, the cerebral cortex.

Pharmacological alleviation of combined cholinergic/noradrenergic lesion-induced memory deficits in rats.

Data derived from a number of preclinical studies examining the effects of combined cholinergic and noradrenergic lesions in a rat model of Alzheimer's disease are reviewed. Results from these studies indicated that a nucleus basalis of Meynert (nbM) lesion combined with a lesion of the ascending noradrenergic bundle (ANB) did not exacerbate 72-h passive avoidance retention deficits beyond the degree of impairment produced by nbM lesions alone. However, the addition of an ANB lesion did block the efficacy of two choiinomimetics (physostigmine and oxotremorine) to reverse the lesion-induced memory impairment. Memory in combined lesioned rats was restored when cholinomimetic therapy was administered in combination with low doses of clonidine. Studies investigating a number of Hoechst-Roussel Pharmaceuticals compounds have produced memory-enhancing effects in animals prepared with combined nbM/ANB lesions without the need for clonidine supplementation. These compounds include P128, P86-7493, and P87-8184. Moreover, these compounds have also been shown to be effective in reversing passive avoidance memory deficits in animals with nbM lesions and treated with the noradrenergic toxin DSP-4. Implications for pharmacotherapeutic approaches for the treatment of Alzheimer's disease are discussed.

Fetal transplant-induced restoration of spatial memory in rats with lesions of the nucleus basalis of Meynert.

Bilateral lesions of the nucleus basalis of Meynert (nbM) in rats produced mnemonic deficits when subjects were tested on tests of spatial memory over a period of 3 to 7.5 months postoperatively. The transplantation of cholinergic-rich, fetal ventral forebrain tissue to either two or four frontoparietal cortical sites normalized performance on the spatial memory tasks. However, which transplant condition yielded recovery depended upon the nature of the task and/or posttransplantation interval. When assessed 8 months following transplant surgery, cortical choline acetyltransferase and acetylcholinesterase activity levels in both transplant groups were comparable to those values found in sham-operated animals. These data indicate that fetal transplants can reverse the mnemonic deficits and restore cortical cholinergic neurochemical activity to near-normal levels in rats with nbM lesions.

Homeostatic disruption and memory: effect of insulin administration in rats.

The present study examined the effect of insulin-induced hypoglycemia on 24-h retention of passive avoidance in rats. In the initial experiment, rats received either insulin (50 U/kg) or saline injections 30 min prior to training and testing. Impairments in retention were observed when animals were trained with insulin and tested with saline. This anterograde memory loss was attenuated, however, when insulin was administered prior to both training and testing. A subsequent experiment further explored the disruptive effect of hypoglycemia on memory. Data from this study indicated that lower doses of insulin at training (5 and 10 U/rat) yielded impairments in 24-h retention of passive avoidance. It is concluded that disruption of glucoregulation can produce state-dependent anterograde memory losses in rats. Possible implications for the effects of hypoglycemia on cognitive functioning in humans are discussed.

Role of ACTH in recovery from retrograde amnesia induced by hypothermia in rats.

The present investigation assessed whether increased congruency between ACTH state present shortly after training and that at testing contributed to memory recovery. If recovery were related to an increased correspondence between internal state present after training and that at testing, then suppressing ACTH release should block memory recovery. This was the hypothesis that was examined in the present investigation. Specifically, animals were trained on a passive avoidance task, administered hypothermia (the amnestic agent) and, shortly prior to testing, given treatments known to be effective in reversing memory loss induced by hypothermia. Before training (Experiment 1) or testing (Experiment 2) animals were injected with either dexamethasone (an agent that suppresses ACTH release) or saline. Results, in general, indicated that when ACTH release was suppressed, a blunted recovery effect was obtained. This reduction in the extent of memory recovery was observed when ACTH was suppressed either at training or at testing. These data are interpreted as providing support for an ACTH-related, state-dependent retention mechanism contributing to recovery from hypothermia-induced retrograde amnesia in rats.

Infusion of NMDA into the nucleus basalis of Meynert, frontal cortex or lateral ventricle in rats: effect on memory and cholinergic brain neurochemistry.

The present study's aim was to examine the behavioral and neurochemical effects of damage limited to intrinsic neurons of the frontal cortex in rats. Specifically, it was of interest to evaluate the effects of N-methyl-D-aspartic acid-induced lesions of discrete frontal cortical loci on passive avoidance memory and on cortical cholinergic neurochemical markers (choline acetyltransferase--CAT and acetylcholinesterase--ACHE). The present study also compared the behavioral and neurochemical effects produced by frontal cortical damage with those effects produced by lesions of the nucleus basalis of Meynert (nbM). Results indicated that nbM lesions and lesions to a rostral frontal cortical site produced severe passive avoidance memory impairments when subjects were tested 72 hours after training. Cortical, but not hippocampal, levels of CAT and ACHE were depleted in nbM animals only. These data were interpreted as providing support for the view that intrinsic frontal cortical neurons contribute to memory.

Nucleus basalis lesions impair memory in rats trained on nonspatial and spatial discrimination tasks.

Ibotenic acid-induced lesions of the nucleus basalis of Meynert (nbM) in rats produced severe memory impairments when subjects were required to acquire a brightness discrimination and learn its reversal (Experiment 1). Lesion-induced impairments were also observed when a complex (30-choice) spatial discrimination task served as the assessment instrument (Experiment 2). The memory deficits observed in Experiment 2 were evident despite an approximate 4-month postoperative recovery period and prior brightness discrimination training. Identical nbM lesions failed to produce memory decrements when rats were trained and tested 24 hr later on a simple (2-choice) right vs. left spatial discrimination task. It is concluded that nbM lesions impair memory for both nonspatial and spatial tasks and that, at least with tests of spatial memory, task difficulty may be an important determinant of this impairment.

Effect of physostigmine on memory consolidation and retrieval processes in intact and nucleus basalis-lesioned rats.

The efficacy of physostigmine on memory enhancement in rats trained on a passive avoidance task was investigated. In experiment 1, the effect of posttraining injections of physostigmine (0.03 mg/kg) in subjects tested at either short (1.25 h or 72 h) or long (3 weeks or 5 weeks) retention intervals was explored. Results indicated drug-induced enhancement of memory at only the two short intervals. Experiment 2 examined the mnemonic consequences of pretest administration of physostigmine. Administration of physostigmine (0.015 mg/kg and 0.03 mg/kg) shortly prior to testing led to a significant potentiation of memory retrieval. The aim of experiment 3 was to determine whether or not an intact cholinergic system is necessary for pretest physostigmine to enhance memory retrieval. Results from this experiment indicated that physostigmine (0.03 mg/kg) was effective in enhancing memory in rats prepared with ibotenic acid-induced lesions of the nucleus basalis of Meynert. Together, these data provide further empirical support for the importance of the cholinergic system in memory processes.

Hypothermia-induced anterograde amnesia: is memory loss attributable to impaired acquisition?

The present investigation examined whether the poor test performance observed in studies of anterograde amnesia reflects a memory deficit or is a by-product of weaker initial learning resulting from impaired sensory, motivational, or associative processes. Two experiments were performed which utilized latent extinction (Experiment 1) and delay of punishment (Experiment 2) manipulations in order to assess the nature of original learning in rats trained under either hypothermic (29 degrees C) or normothermic conditions. Results from both experiments provided evidence that hypothermia treatment administered prior to training had relatively little influence on the animal's ability to acquire a passive avoidance response. Therefore, the rapid forgetting observed in hypothermia-induced anterograde amnesia is most likely due to memory deficits rather than an artifact of poorer acquisition.

Hypothermia-induced anterograde amnesia and its reversal in rats trained on a T-maze escape task.

Previous studies have demonstrated anterograde amnesia (AA) and its reversal in rats trained on passive avoidance tasks. The present investigation was conducted to determine whether induction and/or reversal of AA is limited to inhibitory learning tasks or whether these phenomena can be illustrated in behavioral situations involving choice. Accordingly, in Experiment 1, rats were trained on a T-maze escape task as either hypothermic (28 degrees C) or normothermic. Twenty-four hours later half of each acquisition group was tested as either hypothermic or normothermic. Results indicated a stern retention decrement for animals trained at a lower body temperature and tested as normothermic. However, this prograde memory deficit was attenuated when animals were recooled shortly prior to testing. In an attempt to extend the phenomenon of memory recovery observed in Experiment 1, Experiment 2 examined whether pretest injections of d-amphetamine (0.5 mg/kg), a purported amnesia-attenuating agent, could lessen the AA induced by hypothermia. Amphetamine, at least at the dose used, did not reduce the memory impairment. Results are interpreted in terms of the state dependent nature of memory.