Effects of long-term corn consumption on brain serotonin and the response to electric shock.

Rats fed tryptophan-poor corn diets have reduced levels of brain serotonin and show increased responsiveness to electric shock. This diet-induced hyperalgesia can be reversed by feeding the animals diets with adequate amounts of tryptophan, or by systemic injections of the amino acid.

Memory consolidation in senescence: effects of CO2, amphetamine and morphine.

Alterations in memory storage processes that occur in senescence were investigated by challenging young and old female "small Wistar" rats with posttraining administration of CO2, amphetamine or morphine, and measuring retention performance. Neither duration of CO2 immersion, nor the time of CO2 immersion after training had a differential amnestic effect with age on retention of a one-trial, shock-motivated inhibitory avoidance task. These results indicate that the times during which memory is susceptible to disruption for old and young rats are similar. Challenge with drugs, however, did reveal age-related alterations in memory storage processes. Amphetamine attenuated CO2-induced amnesia in young rats, but had no effect in old rats. This could not be attributed to a general decline in response to amphetamine in old rats because amphetamine increased open field activity of both young and old animals. Morphine also had a differential effect on memory with age: it caused amnesia in old rats trained in a one-trial hot plate escape task, while having no effect on retention performance of young rats. Thus, the modulatory influence of catecholamine and opioid systems on memory processes is probably altered in senescence.

Greater task difficulty amplifies the facilitatory effect of des-glycinamide arginine vasopressin on appetitively motivated learning.

Rats were trained in a discrete-trial forward autoshaping paradigm to touch an extended lever to earn food pellets. Reinforcement was delivered either simultaneously with or 6 s after lever retraction which occurred either non-contingently after 15 s or when the animal touched the lever. Treatment with des-glycinamide arginine vasopressin (DGAVP; 15 micrograms/kg, sc) 1 hr before sessions increased the rate of acquisition of the extended-lever-touch response and also facilitated development of intertrial (adjunctive) nose poking. Effects of the peptide were more robust in the more difficult, delayed reinforcement task. DGAVP lacks the classical peripheral activity of vasopressin. In both experiments, peptide treatment was terminated before asymptotic levels of performance were attained; the continued facilitation of acquisition in treated groups suggests a specific enhancement of learning and/or enhanced memory retrieval.

Facilitation of hot-plate response learning by pre- and posttraining naltrexone administration.

Retention performance for shock-motivated learning is generally enhanced by opiate antagonists. To test the hypothesis that opioid systems mediate learning and memory in nonshock-motivated tasks, the effects of naltrexone on hot-plate response learning were investigated. Naltrexone (0.1 or 1.0 mg/kg IP) 10 min before hot-plate exposure produced hyperalgesia, as measured by decreased latencies of naltrexone-injected mice to escape. Jump latencies, however, were not significantly decreased by naltrexone given 10 min before initial testing. Nevertheless, jump latencies of naltrexone-injected mice, but not saline-injected mice, decreased further on subsequent test trials 1 and 4 days later although treatment was discontinued, suggesting that the opiate antagonist also influenced learning or memory. Mice given naltrexone (0.3-10.0 mg/kg IP) immediately after their first hot-plate exposure also exhibited learning of the jump response, on a test trial 48 h later. The decreases in jump latencies on test trials were smaller after posttraining, than pretraining naltrexone administration. The results indicate that the effect of naltrexone on learning and memory can be at least partially separated from its hyperalgesic activity.

Long-term effects of neonatal exposure to isobutylmethylxanthine. II. Attenuation of acute morphine withdrawal in mature rats.

An acute model of morphine withdrawal was used to determine if neonatal exposure to 3-isobutyl-1-methylxanthine (IBMX) would cause alterations in the expression of withdrawal in the adult rat. IBMX induces a quasi-morphine withdrawal syndrome (QMWS), which is almost identical to true morphine withdrawal both behaviorally and neurochemically. Transient IBMX treatment during infancy (on days 7-10 of life) caused an attenuated suppression of fixed ratio (FR) responding during acute morphine withdrawal in adulthood; however, there appeared to be no attenuation of withdrawal-induced hypothermia. The attenuated behavioral response was not due to an altered ability to express withdrawal, as these rats did not react differently to various doses of IBMX plus naloxone (i.e., varying severities of quasi-morphine withdrawal) in adulthood. Coadministration of the serotonin (5-HT) antagonist mianserin with IBMX in the neonate prevented the effects of IBMX. Both the mianserin-treated and the IBMX plus mianserin-treated groups had increased levels of [3H]naloxone binding in brainstem, while IBMX treatment alone apparently had no significant effect. None of the neonatal drug treatments affected [3H]naloxone binding in frontal cortex. Thus, the long-term effects of IBMX on the opioid withdrawal response cannot be explained by changes in the number of opioid binding sites (labelled with [3H]naloxone) within the brain. The results indicate that exposure to a methylxanthine, and thus quasi-morphine withdrawal, during development results in long-lasting alterations of a system which is involved in opioid withdrawal. Because coadministration of mianserin prevented the effects of IBMX, 5-HT and 5-HT2 receptors are implicated in these effects.

Selective learning impairment of delayed reinforcement autoshaped behavior caused by low doses of trimethyltin.

The organometal neurotoxin trimethyltin (TMT), induces impaired learning and memory for various tasks. However, administration is also associated with other "non-specific" behavioral changes which may be responsible for effects on conditioned behaviors. To determine if TMT treatment causes a specific learning impairment, three experiments were done using variations of a delay of reinforcement autoshaping task in which rats learn to associate the presentation and retraction of a lever with the delivery of a food pellet reinforcer. No significant effects of TMT treatment were found with a short (4 s) delay of reinforcement, indicating that rats were motivated and had the sensorimotor capacity for learning. When the delay was increased to 6 s, 3.0 or 6.0 mg TMT/kg produced dose-related reductions in behaviors directed towards the lever. Performance of a group given 7.5 mg TMT/kg, while still impaired relative to controls, appeared to be better than the performance of groups given lower doses. This paradoxical effect was investigated with a latent inhibition paradigm, in which rats were pre-exposed to the Skinner boxes for several sessions without delivery of food reinforcement. Control rats showed retardation of autoshaping when food reinforcement was subsequently introduced. Rats given 7.5 mg TMT/kg exhibited elevated levels of lever responding during pre-exposure and autoshaping sessions. The results indicate that 7.5 mg TMT/kg produces learning impairments which are confounded by hyperreactivity to the environment and an inability to suppress behavior toward irrelevant stimuli. In contrast, low doses of TMT cause learning impairments which are not confounded by hyperreactivity, and may prove to be useful models for studying specific associational dysfunctions.

Post-training amygdaloid lesions impair retention of an inhibitory avoidance response.

The study examined the effect of pre- and post-training bilateral amygdaloid lesions on retention of a one-trial inhibitory avoidance response. Groups of rats, including unimplanted controls and implanted controls, were trained and tested for retention at 4, 7 or 12 days following training. The lesions were made at one of several intervals before or after training: 2 days before, immediately after, or 2, 5 or 10 days after. At all retention intervals the retention of implanted controls was poorer than that of unimplanted controls and, in comparison with both control groups, the retention of animals lesioned before training was impaired. Retention was also impaired by the post-training lesions. The degree of impairment varied with the interval between the training and the lesion: lesions made within 2 days following training impaired retention, while lesions made 10 days following training had no impairing effect. These findings suggest that post-training lesions of the amygdala affect retention by impairing time-dependent processes involved in memory storage. With a sufficiently long training-lesion interval (10 days) an intact amygdala is not essential for retention.

Naloxone attenuates amnesia caused by amygdaloid stimulation: the involvement of a central opioid system.

This study investigated the effect of naloxone on amnesia produced by subseizure amygdaloid stimulation. Animals were trained in an inhibitory avoidance task, and given amygdaloid stimulation following training. Immediately after training, prior to stimulation, naloxone was injected either peripherally (i.p.) or into the bed nucleus of the stria terminalis (BNST) where the Met-enkephalin-containing fibers from the amygdala terminate. Amygdaloid stimulation caused retention deficits. The deficits were attenuated by 3.0 mg/kg naloxone given peripherally or by 1.0 microgram or 0.3 microgram naloxone injected bilaterally into the BNST. The attenuative effect was anatomically and receptor specific: 0.3 microgram of naloxone injected into the caudate nucleus was ineffective; the attenuative effect of naloxone was antagonized by simultaneous injection of 1.5 or 4.5 micrograms levorphanol into the BNST. These results suggest that endogenous opioids, possibly the enkephalins of the stria terminalis released into the BNST following amygdaloid stimulation, are at least partially involved in mediating the effect of amygdaloid stimulation on memory.

Parallel changes in operant behavioral adaptation and hippocampal corticosterone binding in rats treated with trimethyltin.

Rats were given water vehicle or trimethyltin (TMT; 3.0, 6.0 or 7.5 mg/kg, p.o.). Lever responding for food was measured 3 months later, in a test in which the fixed ratio requirement was doubled daily (FR1-128). Response rates for all groups were inverted U-shaped functions of FR values. However, the effect of increasing ratio values was attenuated in the 6.0 mg/kg group, which responded less than controls when control rates were maximal (at FR16 and FR32). In contrast, rats given the high dose responded at higher rates (at FR4 and FR64). [3H]Corticosterone binding to hippocampal cytosolic protein was maximally reduced for the group given 6.0 mg TMT/kg. The greatest reduction in hippocampal weight resulted from injection of 7.5 mg TMT/kg, but a smaller reduction in [3H]corticosterone binding (i.e. 22%) was observed for this group. In the absence of an effect of 3.0 mg TMT/kg upon weight of hippocampus, there also was a reduction in steroid binding, indicating the sensitivity of this parameter for TMT toxicity. The results support the notion that hippocampal corticosteroid receptors are important for behavioral adaptation, and rats given moderate doses of TMT may be useful for studying functions of corticosterone receptors.

Attenuation of amphetamine-induced enhancement of learning by adrenal demedullation.

These experiments investigated the effect of immediate posttrial administration of peripherally acting DL-4-hydroxyamphetamine on retention of a one-trial inhibitory avoidance response in intact, adrenal medullectomized, sympathectomized, and medullectomized and sympathectomized rats. In intact rats, 0.82 mg/kg of DL-4-OH-amphetamine enhanced retention performance. In rats sympathectomized by peripheral 6-hydroxydopamine, 24 h prior to training, a lower dose of 4-OH-amphetamine (0.21 mg/kg) was most effective in enhancing retention. Adrenal demedullation abolished the memory enhancing effects of DL-4-OH-amphetamine and also D-amphetamine. These findings suggest that the memory enhancing effects of DL-4-OH-amphetamine and D-amphetamine involve adrenal medullary catecholamines.

Central and peripheral actions of amphetamine on memory storage.

These experiments investigated the effects of central (intracerebroventricular) and peripheral (i.p.) posttraining administration of D-amphetamine on rat's retention of a one-trial inhibitory avoidance response. While retention was enhanced by i.p. administration (1.0 mg/kg) the central administration (dose range 50-500 microgram) did not affect retention. In rats given peripheral 6-OHDA 24 h prior to training a lower dose (i.p.) of amphetamine (0.25 mg/kg) was most effective in enhancing retention. These findings suggest that the mrmory enhancing effects of D-amphetamine are mediated at least in part through peripheral systems.

Des-Gly-vasopressin improves acquisition and slows extinction of autoshaped behavior.

The effects of a vasopressin analog (DGAVP) with minimal endocrinological activity, were assayed on acquisition and extinction of a discrete trial, food reinforced, autoshaped lever touch response. Magazine-trained rats, maintained at 80-85% of free-feeding body weights, were injected s.c. with saline, 5 or 10 micrograms/kg of DGAVP 1 h before each of two sessions in which they learned to touch a retractable lever, presented on a 45 s random interval (RI 45) schedule. Retracted lever contacts (nose-pokes) and unconditioned rearing activity were simultaneously monitored. After acquisition of the extended lever touch response, rats were reassigned to treatment groups, and again injected with saline, 5 or 10 micrograms/kg of DGAVP 1 h before each of two extinction sessions. DGAVP facilitated acquisition (5 micrograms/kg) and slowed extinction (5 and 10 micrograms/kg) of conditioned behavior, while having no effects on the other behaviors, thus demonstrating the specificity of the effect of a vasopressin-like compound on both tasks (enhanced acquisition and retarded extinction) used to study learning.

Tryptophan and 5-hydroxyindoles in different CNS regions following acute morphine.

Morphine induces naloxone-reversible increases of tryptophan and 5-hydroxyindoles in rat cerebral hemispheres, thalamus and cerebellum, but does not do so in striatum, hypothalamus, hippocampus and brain stem. Morphine also induces a rise in spinal cord 5-HIAA which is antagonized by naloxone, but there is no parallel change in tryptophan. Increases in brain 5-hydroxyindoles may be related to greater availability of tryptophan, but the elevation in spinal cord 5-HIAA appears to be unrelated to precursor availability.

Morphine induced increases in the incorporation of 3H-thymidine into brain striatal DNA.

3H-thymidine uptake into DNA fractions of rat brain regions was measured following in vivo administration of (methyl-3H)-thymidine and morphine. Acute morphine administration (10 mg/kg; 30 min prior to 3H-thymidine) increased incorporation of 3H-thymidine into DNA of rat striatum. This effect was antagonized by naloxone (1 mg/kg). Further, the observed change in incorporation of 3H-thymidine into DNA in striatum could not be accounted for by differences in the local availability of the label in morphinized rats. An autoradiographic study revealed that the 3H-thymidine was localized in nuclei in cells of the sub-ependymal layer lining the lateral ventricles, an area of glial cell proliferation in adult rats. No change in 3H-thymidine incorporation into DNA was observed in any area of the brain in morphine-addicted rats or in rats undergoing naloxone-precipitated withdrawal. The results indicate that opiates may induce permanent anatomical changes in the brain, including alterations of neuroglial interactions.

Neonatal opiate withdrawal alters the reactivity of adult rats to the hot-plate.

Prenatal exposure of rats to 0.2 mg LAAM/kg/day but not to 0.05 mg LAAM/kg/day resulted in faster hot-plate escape latencies in 6 mo old offspring. No differences in tail-flick latencies were observed at 7 mo of age in offspring exposed to either dose of LAAM prenatally. Subsequent testing of littermates at 16 mo of age revealed that the greater sensitivity to the hot-plate observed in rats prenatally exposed to LAAM is apparently a result of neonatal withdrawal rather than a primary consequence of the drug. The data are discussed in relation to possible effects of drug or withdrawal on central nervous system development.

Long lasting behavioral effects of dimethyl sulfoxide and the "peripheral" toxicant p-bromophenylacetylurea.

Behavioral toxic effects caused by a relatively small dose of the "peripheral" neurotoxin, p-bromophenylacetylurea (BPAU), and of its vehicle, dimethyl sulfoxide (DMSO) were investigated. BPAU induces, in rats, a central-peripheral distal axonopathy similar to that produced in humans by toxic organophosphorus-containing compounds, and has been proposed as a model to study this type of toxicity in a convenient experimental mammal. Rats were injected with BPAU (50 or 100 mg/kg) in DMSO (1 ml/kg), with DMSO alone, or with saline. 100 mg BPAU/kg produced permanent weight loss and hind limb paresis; the low dose did not. Behavioral testing, 2 days to 4 mo post-treatment, indicated that DMSO and/or 50 mg/kg of BPAU retarded habituation of spontaneous exploratory activity, impaired acquisition of conditioned (auto-shaped) behavior, and changed the dose-response relationship ford-amphetamine-induced suppression of operant (fixed ratio 32) responding. BPAU-treated animals were also impaired in initial performance of operant behavior maintained by a fixed ratio schedule of reinforcement, at high (greater than or equal to FR 16) ratio values. Thus, neurobehavioral toxicity may occur at doses too low to induce organophosphorus-type sensorimotor impairment or pathology. Further, DMSO may also exert effects on neurobehavioral function, suggesting it too may be potentially toxic within this domain.

Learning enhancement and behavioral arousal induced by yohimbine.

Learning of a food motivated delayed reinforcement autoshaping task was investigated in rats treated with water vehicle or the prototypical anxiogenic agent and alpha 2-adrenergic antagonist yohimbine (0.5 or 1.5 mg/kg, i.p. 30 min before behavioral testing). Unconditioned exploratory rearing activity was monitored concomitantly with acquisition of a lever touch response. The low dose of yohimbine enhanced learning, but it also increased unconditioned behavioral arousal. The high dose retarded acquisition, but when it was withdrawn the animals learned but exploratory activity increased beyond control levels prior to acquisition. Learning thus appeared to be related to the behavioral arousal produced by yohimbine, suggesting that learning enhancement by anxiogenic substances is not due to a direct effect on processes intrinsic to information storage and retrieval; rather, anxiogenic substances may be important modulators of vigilance and performance variables.

Dose-specific effects of trimethyltin poisoning on learning and hippocampal corticosterone binding.

The organometal neurotoxin trimethyltin (TMT) damages limbic forebrain, and impairs acquisition of lever-directed behaviors in an autoshaping task, in which a lever is presented according to a random time schedule, and rats learn to associate its presentation/retraction with food delivery (Cohen et al., 1987). This impairment is evident only if a sufficiently long delay of reinforcement is interposed between lever retraction (which occurs either automatically after 15 sec, or immediately upon a touch response) and food pellet delivery. Paradoxically, rats given a higher (7.5 mg/kg) dose show a smaller acquisition impairment, perhaps because they are generally more reactive to the lever than controls. These rats sustain a larger hippocampal lesion (measured by wet weight of the structure). The experiment reported here was done to investigate (1) an autoshaping deficit related to hippocampal weight loss, and (2) biochemical changes in hippocampus which might be related to behavioral impairments. Rats were treated with water vehicle or TMT four weeks before autoshaping using a 6 sec reinforcement delay. In addition to lever touching, touches of the food trough were measured. The timing of trough-touching behaviors within a trial was used as an indication of the strength of the association formed between the lever and the site of food delivery. Following autoshaping rats were adrenalectomized and killed for measurement of cytosolic [3H]corticosterone binding in hippocampus. As before, rats treated with 6.0 mg TMT/kg showed a deficit in acquisition of lever-directed behaviors. Also, as hypothesized, the proportion of total trough-directed behaviors made during the 6 sec reinforcement delay intervals (when reinforcement probability was high) diverged significantly from control values as learning progressed. These rats also showed a reduction in hippocampal weight compared with controls, but significant decreases in hippocampal steroid binding were observed only in groups given the low and median dose of TMT. Further, steroid binding was correlated with lever-directed behaviors. It thus appears that lever and trough behaviors can be used to simultaneously assess different aspects of impairment in associative learning which are accompanied by differential cell loss and biochemical deficit.