Endocrine and behavioral traits in low-avoidance Sprague-Dawley rats

In the present series of experiments, we have examined the endocrine profile of two stable colonies of Sprague-Dawley rats, here labeled Stock A, and Stock B, differing markedly in their ability to acquire a conditioned avoidance response. On separate occasions, the animals were subjected to five daily sessions (approximately 20 trials per 15 min session) of conditioned avoidance training, measurements of startle reactivity to an auditory stimulation and open-field spontaneous locomotor activity observations. The experiments were concluded by taking blood samples for later analysis of plasma glucose and plasma levels of the following hormones: insulin, gastrin, CCK, glucagon, somatostatin, oxytocin and corticosterone. The low-performing Stock B animals were characterized by [1] being more reactive to sensory stimulation: higher startle amplitude and shorter startle latency; [2] having higher plasma insulin and corticosterone levels, whereas plasma gastrin and oxytocin were significantly lowered and a strong tendency for a decrease also in plasma CCK. There were no differences in spontaneous locomotor activity between the two substrains. Taking total variability in avoidance performance into account, there was a statistically significant positive correlation between plasma oxytocin, as well as gastrin, levels and avoidance performance. The evidence obtained here, and in other laboratories, suggests that the Stock B animals display hormonal changes indicative of a submissive-defensive reaction pattern. Thus, the avoidance acquisition deficits displayed by the present Sprague-Dawley stocks A and B, are in all probability caused by emotional reactions when challenged with external stimuli requiring active responding.     

Level of neuroactive steroids in the brain, and gender-related differences in formation and decrease of conditioned reflex in rats

We investigated gender-related differences in dynamics of the brain sexual steroids during learning and the decrease of conditioned reflex in the modeling experiment. It was revealed that, before training to conditioned reflexes, females manifested a greater anxiety than males. Significant differences between males and females in formation of the conditioned reflex of passive avoidance were not revealed, whereas the conditioned response decrease were by 2-3 days faster in male rats than in females. It was revealed that there was an increase of testosterone content in various structures of the brain, especially in hippocampus and frontal cortex with its constant level in the blood plasma in learning conditioned response of passive avoidance in male adult rats. Also an increased estradiol concentration was found in females' amygdale, while increased value of estradiol was detected in hippocampus and the singular cortex in decrease of the conditioned response of passive avoidance. In blood plasma, the testosterone level was not changed, and the estradiol concentration was reduced significantly after the decreased conditioned reflex. Different dynamics of changes in the levels of sex steroids in the brain and the blood plasma can indicate a probability of their formation in nervous tissue. The correlation analysis confirms the conception about selective involvement of the brain's testosterone and estradiol in the specific structures in realization ofthe learning and memory processes in adult male and female rats.     

Influence of acoustic signal with ultrasonic components on the development of defensive conditioned reflex in Wistar rats

Short-term (90 s) effects of the complex acoustic signal (CAS) with ultrasonic components on the development of defensive conditioned reflex of two-way active avoidance in a shuttle-box were studied in female Wistar rats. The learning ability of rats was measured on a scale designed in our laboratory. It was shown that CAS stimulation triggered an audiogenic seizure of different strength in 59% of animals. The CAS was extremely stressful for Wistar rats: it prevented the active avoidance learning in early terms after its application (the first training session in 4 or 6 days). This effect did not depend on the presence or intensity of audiogenic seizures during CAS. In the second training session in 9 days (the first session was in 4 days), learning was impaired as compared to control without CAS. However, during repeated training procedure 1.5 months after the CAS (the first session in 6 days), rats rapidly reached the criterion of learning (10 consecutive avoidance reactions). On the other hand, if the CAS was presented with different time lags (immediately, in 3 or in 45 days) after the first training session, the ability of animals to learn during the second session was not impaired both in early and late terms after exposure to the stressor. The results suggest that exposure to CAS prevents development of short-term memory but does not affect consolidation process and long-term memory.     

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.     

Level of neuroactive steroids in the brain and sex-related peculiarities of formation and extinction of conditioned reflex in rats

Sex-related peculiarities of dynamics of brain sex steroids in the process of learning and extinction of the conditioned reflex of passive avoidance have been studied in model experiment. Prior to learning of the conditioned reflex, female rats were found to be distinguished by manifestation of anxiety and fear as compared with male rats. At formation of the conditioned reflex, no significant sex-related differences were detected between males and females, whereas extinction of the conditioned reaction of passive avoidance in males occurred by 2–3 days faster than in females. At learning of conditioned reaction of passive avoidance, in sexually mature male rats there was revealed an increase of the testosterone content in various brain structures, especially in hippocampus and frontal cortex, while its level in blood plasma remained unchanged. Also shown was an elevation of estradiol concentration in female amygdale, whereas at extinction of the conditioned reaction of passive avoidance, a rise of estradiol values was noted in hippocampus and cingular cortex. At the same time, the testosterone level in blood plasma did not change, whereas after extinction of the conditioned reflex the estradiol concentration decreased statistically significantly. Different dynamics of changes of the sex steroid levels in brain and blood plasma can indicate a possibility of their formation in the nervous tissue. The performed correlation analysis confirms the concept of selective involvement of testosterone and estradiol of individual brain structures in realization of processes of learning and memory in sexually mature male and female rats.     

Effect of adaptation to altitude hypoxia in early ontogeny on the indices of higher nervous activity]

Newborn male and female Wistar rats were adapted to hypoxia in a pressure chamber. Adaptation was started from the "altitude" of 1000 m, 1 hour daily, and then the time and the intensity of hypoxic actions were gradually increased: beginning from the 17th day the animals were subjected to adaptation to the "altitude" of 5000 m, for 5 hours, 5 days a week. The defence conditioned reflex of active avoidance was provoked in the animals after 2-month adaptation. A tendency to a more rapid elaboration of the reflex and a markedly enhanced degree of its retention in comparison with control was revealed in the adapted males. No significant changes in the elaboration and rentention of the reflex were found in the female animals adapted to hypoxia under analogous conditions.     

Changes in the learning ability of epiphysectomized rats

The experiments on rats have shown changes in the learning ability of animals. The learning of locomotor reactions of escape and avoidance conditioned reflex was accelerated in animals after pinealectomy. Training of rats in U-shaped and 16-door mazes was significantly affected after pinealectomy. The index of reflex retention (1 and 7 days later) was not changed in the operated animals. Epithalamin (0.5 mg/kg for 5 days) selectively facilitated learning of visual and space discrimination.     

Changes in the Content of Glial Fibrillary Acidic Protein in the Frontal Cortex of Rats during Conditioned Active Avoidance Training

We studied the dynamics of expression of glial fibrillary acidic protein (GFAP) in the frontal neocortex of rats during training for conditioned active avoidance reaction (CAAR). The GFAP content in a cytoskeletal fraction of the frontal cortex tissue was quantitatively estimated with the use of solid-phase immunoenzyme analysis, 2 h and 1, 3, 7, 14, 21, and 30 days after the beginning of training of the animals. It was found that the changes in expression of the filamentous form of GFAP in the neocortex are clearly pronounced and possess a three-phase pattern: the level of GFAP significantly increased within the first hours after the beginning of CAAR training, within 7-14 days this index decreased and became smaller than that in the initial state, and then again rose (with a maximum after 21 days). The obtained data indicate that the neurospecific protein under study is involved in the processes of learning and formation of conditioned memory engrams.     

Changes in brain serotonin metabolism and [3H]-serotonin receptor binding during recall of conditioned passive avoidance in rats

The content of serotonin and its metabolite 5-hydroxyindoleacetic acid, monoamine oxidase activity, and [3H]-serotonin radioligand receptor binding were examined in the prefrontal cortex, striatum, amygdala, hippocampus and periaqueductal gray matter at different time after one-trial passive avoidance training of rats. Changes in the serotonergic activity were observed only in rats, which showed retrieval of conditioned passive avoidance response. No serotonergic changes were found immediately and one day after training. Also, there were no changes in trained rats without retrieval of conditioned passive avoidance response or rats with experimental amnesia. The pattern of the involvement of brain structures in the retrieval process was also revealed. [3H]-serotonin binding was decreased in the amygdala, periaqueductal gray matter and striatum, whereas it did not change in the prefrontal cortex and hippocampus. At the same time, the serotonin content in these structures did not differ from that of intact rats. Deamination of serotonin by monoamine oxidase and active transport of 5-hydroxyindoleacetic acid from nerve terminals were increased in the amygdala and periaqueductal gray matter, whereas in the striatum serotonin catabolism was decreased. The obtained differences in serotonin catabo- lism suggest that the decrease in receptor binding of serotonin in these brain structures is provided by different synaptic processes: presynaptic changes in the striatum and postsynaptic receptor changes in the amygdala and periaqueductal gray matter. It is concluded that the decrease in the serotonergic activity in the amygdala and periaqueductal gray matter represents one of the mechanisms activating the emotiogenic system mediating the memory trace retrieval in inhibitory avoidance learning.     

Analysis of the process of learning in the formation of the conditioned reflex of avoidance in rats

During acquisition of avoidance conditioned reflex (CR) in shuttle box by electric shock it is shown that the performance of that defence reaction is induced by the joint action of two factors: general arousal of animals and motivation. Motor activity of rats in an "open field", the number of short-latency (2s) and intersignal responses during formation of the avoidance CR are in index of the general arousal of rats. An artificial increase in the general arousal of animals by caffeine induces acceleration of CR performance. A constant level of the general arousal of rats is one of the reasons that oxytocin does not influence the rate and dynamics of the avoidance CR performance in rats.     

Function of the adrenal cortex during elaboration of the passive avoidance conditioned reflex in immature rats

Plasma 11-hydroxycorticosteroid (11-OHCS) levels were measured in 30-day-old rats by fluorometry during passive avoidance (PA) learning by means of a single electric footshock. In contrast to the data obtained in adult animals, pre-exposure of young rats for 7 days to the experimental environment (over 3 min daily) resulted in elevation of the basal 11-OHCS levels and in the lack of distinct changes in the hormonal background after placing the young rats into a chamber. As in previous experiments on adult rats, one day after PA learning the 11-OHCS levels were significantly lower in young rats displaying PA than in the animals which did not exhibit PA behavior. Five days after PA training these differences in adrenocortical reactivity disappeared, as was the case in adult animals.     

Changes in serum corticosterone and testosterone during induced maternal behavior in rats

Serum corticosterone (Cpd B) and testosterone (T) concentrations were studied during the avoidance phase of artificially induced parental behavior in male rats. Adult rats were exposed to the presence of standard size foster pups for 60 min daily. On day 1 the avoidance behavior characterized by typical burying reaction was accompanied by an elevation of Cpd B and T. The behavioral responses and hormonal changes diminished during the days of repeated exposure. It was found that neither pup-killing nor spontaneous retrieval were dependent on circulating hormones in the male rat. Subcutaneous injection of ACTH 4-10 inhibited extinction of the avoidance and the humoral responses in both female and male animals. Oxytocin failed to exert any influence on the behavioral and hormonal components of the aversive reaction.     

Analysis of the participation of rat brain noradrenergic terminals in elaboration of conditioned passive avoidance response]

A change in a noradrenaline concentration in the noradrenergic terminals was investigated by the fluorescent histochemical method in pars ventralis nuc. interstitialis striae terminalis, nuc. septi lateralis and in nuc. preopticus medialis of male Wistar rats in the process of reproduction of conditioned passive avoidance response. In all the investigated structures of the preliminarily trained animals the reproduction of a conditioned response (without any electrical stimulation) led to a statistically reliable decrease in the noradrenaline concentration. The data obtained testified to the participation of the noradrenergic brain system in the reproductive process of the conditioned passive avoidance response.     

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.     

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.     

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.     

The reactivity of the hypophyseal-adrenal system in rats with different forms of adaptive behavioral pathology]

The dexamethasone suppression test (DST) was applied to male Wistar rats with different models of depression: group with the learned helplessness, group with informational neurosis provided by time-deficit conditioned avoidance training, as well as groups of rats of two strains selected for low (KLA)--and high (KHA) avoidance learning. The pre-dexamethasone basal and stress-induced corticosterone levels were similar in intact rats and those exposed to inescapable shock. The dexamethasone administration (5 mkg/kg) failed to decrease the serum corticosterone level in rats with learned helplessness. The informational neurosis increased significantly the basal corticosterone level and decreased the stress response. Serum corticosterone levels were similar in KLA and KHA rats. These results give evidence that two stress-induced rat models of depression with similar behavioural disturbances (reduction of escape/avoidance reactions) exhibit marked differences in the activity of hypothalamo-pituitary-adrenal (HPA) axis.     

Influences de la castration et du propionate de testosterone sur le comportement des rats conditionnes par paires

Four groups of 10 male rats (Group A: normal rats, Group B: castrated rats, Group C: castrated rats + 1 mg P.T./day, Group D: castrated rats + 1 dose of 50 mg/kg of P.T. cyclohexane) were avoidance conditioned during nine sessions of 50 trials. They were conditioned individually during sessions 4, 5 and 6 and by pairs during sessions 1, 2, 3, 7, 8 and 9. During these last sessions, all groups presented a low level of acquisition and aggressive behaviour. These phenomena were worsened in the castrated rats whose performances stayed low even when they were conditioned individually. On the other hand, the frequency of their aggressive behaviour was often higher than that of the other groups. The injection of P.T. cyclo-hexane improves the performance of the castrated animals but does not entirely compensate for the differences between control and experimental rats.     

Modulation of Masheri- and benzo[a]pyrene-inducible carcinogen-metabolizing enzymes by dietary vitamin A

The modulatory role of dietary vitamin A on the carcinogen metabolizing enzymes was studied in masheri extract and benzo[a]pyrene-treated rats. Weanling male Sprague-Dawley rats were fed vitamin A deficient (SR-) and vitamin A sufficient (SR+) semisynthetic diets for 12 weeks. ME/B[a]P treatment significantly increased the phase I activating enzymes in both SR- and SR+ groups. However, a higher percentage increase in enzyme activities was observed in both liver and lung of the SR- animals compared to the SR+ groups. Glutathione content and activity of glutathione S-transferase were decreased in both liver and lung of SR- animals on treatment with either ME or B[a]P. In the SR+ group, an increase in GSH content and GST activity was observed following the ME/B[a]P treatment. The hepatic pool of vitamin A was depleted while that of vitamin C was increased after ME or B[a]P treatment in both SR- and SR+ groups.     

The acceleration of the acquisition of a conditioned reflex to time in rats with destruction of the suprachiasmatic nuclei of the hypothalamus]

In male rats sound conditioned reflex with water reinforcement was gradually (within 14-16 daily training sessions) was replaced by a conditioned reflex to time. After the bilateral lesions of the suprachiasmatic nuclei of hypothalamus, the necessary level of timing performance was reached sooner than in the animals with a partial destruction of the nuclei.