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

Delaying reinforcement in an autoshaping task generates adjunctive and superstitious behaviors

Rats were autoshaped to touch a lever upon its insertion into an operant chamber on a 45 s random time schedule. Occurrence of a reinforced touch on each of 12 lever insertions per session and nose-pokes at the retracted lever were monitored, as was exploratory rearing activity. Delays of 2, 4 or 8 s interposed between the retraction of the lever, which occurred either after 15 s or after a touch response, and delivery of the food pellet reward, resulted in progressively slower acquisition of the extended lever touch response. However, if rats had already acquired the response under immediate reinforcement conditions, the delays subsequently introduced did not cause a decline in autoshaped touch responding. Nose-pokes at the retracted lever occurred during both intertrial and reinforcement delay intervals over the course of autoshaping. The appearance and frequency of these (adjunctive or superstitious) behaviors depended upon the reinforcement delay and behavioral history. These interval behaviors offer measurements of learning not based on arbitrary criteria; they thus provide information about effects on endpoints, or a more global approach to learning.

Differential increase in Fos immunoreactivity in hypothalamic and septal nuclei by arginine8-vasopressin and desglycinamide9-arginine8-vasopressin

Subcutaneous or intracerebroventricular injection of either arginine8-vasopressin or desglycinamide9-arginine8-vasopressin has been shown to facilitate memory, reduce or reverse the effects of amnesic drugs, and maintain tolerance to some effects of ethanol. These actions of vasopressin (and, by inference, of desglycinamide9-arginine8-vasopressin) are mediated by vasopressin V1 receptors in brain, via a c-fos-dependent mechanism, but the receptors at which the desglycinamide analog acts have not been identified. The precise central sites are also not known, but evidence of several types suggested the anterior hypothalamus and septum as probable loci of vasopressin action. In the present work, this question was studied by immunocytochemistry, using antibodies against Fos and Fos-like proteins. The numbers of Fos-immunoreactive nuclei were counted in several related brain regions and structures, after administration of arginine8-vasopressin, des-Gly9-[Arg8]-vasopressin or saline. A subcutaneous injection of vasopressin, but not of saline, enhanced Fos expression in the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus, but the desglycinamide analog stimulated Fos expression only in the suprachiasmatic nucleus. Vasopressin injection significantly increased the number of Fos-immunoreactive cells in the intermediate lateral septum, medial septum, and dorsal and ventral divisions of the lateral septum. In contrast, the desglycinamide analog increased the numbers of Fos-immunoreactive cells in the dorsal and intermediate portions of the lateral septum, but caused no change in the medial septum, and a decrease in the ventral portion of the lateral septum. Increased Fos expression was also found in the subfornical organ after subcutaneous injection of either vasopressin or the desglycinamide analog. Double labeling with antibodies against Fos protein and against vasopressin revealed that most of the vasopressin-induced Fos-immunoreactive cells in the supraoptic, paraventricular and suprachiasmatic hypothalamic nuclei are also vasopressin immunoreactive, i.e. they are vasopressin-producing neurons. These findings suggest that a circuit involving V1 receptors in the subfornical organ, connecting fibres to the suprachiasmatic nucleus, and vasopressinergic projections from the suprachiasmatic nucleus to the lateral septum, may play a central role in mediating the actions of both vasopressin and its desglycinamide analog in the maintenance of ethanol tolerance.

Food-restriction interacts with vasopressin modulation of activity

The effects of peripheral injection of various doses of lysine-vasopressin (LVP), administered 30 min before a 5-min session in a hole-board apparatus, were compared as a function of food restriction. Comparison of performance for various indices of general activity clearly showed that the food-restricted rats were more active and exhibited less photophobia than normally fed ones. The differences between the two groups were maintained in a second session 24 hours later. There was no sign of behavioral habituation to the apparatus among restricted animals. Different doses (0.2, 1, 2 micrograms of LVP) affected food-restricted animals differently from the rats fed ad lib. Only the highest dose reduced activity in both groups. A posttest injection of the smallest dose (0.2 micrograms) had an opposite effect on the activity in the hole-board, measured 24 hours after the injection. A second experiment showed that plasma and adrenal corticosterone were higher in deprived rats. The administration of 0.2 micrograms of LVP was followed by an increase in corticosterone. In food-restricted rats this increase was bigger and was still observed 24 hours after the injection. There is an interaction between feeding conditions and LVP injections which affects both the internal hormonal state and spontaneous reactivity to environment. These findings are of relevance to the effect of vasopressin on behavior.

Effects of vasopressin on spontaneous black-white choices in a T-maze

The effects of subcutaneous administration of vasopressin on spontaneous black-white choice were investigated to determine whether they could account for modifications of performances during learning. First, pretrial injections of 0.2 microgram of lysine-vasopressin (LVP) were given to rats fed ad lib submitted after the last injection to ten consecutive choices in the T-maze. Rats received one or five injections and were tested after either 30 minutes or 24 hours. Treatment with LVP reduced natural photophobia and modified the activity. A more striking effect was observed after one injection and with an injection-test interval of 30 min. Second, the influence of repeated injections of LVP on free choices was studied in food-motivated rats. The effect of repeated injections was marked, suggesting greater sensitivity to treatment in such rats. Third, we compared the action of posttrial administration of the peptide on the behavior of food-restricted rats submitted to appetitive learning in the T-maze, or to free choices. The treatment slightly disturbed the learning of the white arm and altered the preference for black in free choices condition. The two actions were different, showing that the effect on spontaneous behavior cannot account for the effect on learning.

Microinjection of anti-vasopressin serum into hippocampus in mice: effects on appetitively reinforced task after intraventricular administration of Arg-vasopressin

Antiserum to [Arg8]vasopressin (anti-AVP) was bilaterally administered into dorsal hippocampus at 1:50 or 1:10 dilution 20 min before the 24-day retention session of a visual discrimination task. This treatment by itself did not affect the retention performance by comparison with the respective control group, whatever the dilution of anti-AVP, suggesting that hippocampal endogenous AVP is not involved in our behavioral paradigm. On the other hand, intracerebroventricular (i.c.v.) administration of AVP 10 min before the retention session improved retention performance of the visual discrimination task. When anti-AVP was injected at the 1:10 dilution into the dorsal hippocampus 10 min before the i.c.v. administration of AVP, the retention performance was not improved. These data suggest the involvement of the hippocampus in the behavioral expression of AVP following an i.c.v. treatment.

Naloxone administration impairs autoshaped learning

Effects of naloxone on acquisition of autoshaped behavior were investigated. Rats deprived to 85% of free-feeding weights were trained to touch a retractable lever; delivery of a food pellet occurred on every trial following lever retraction. The lever was retracted immediately if a touch occurred within 15 s, or automatically after 15 s. Analyses were conducted on number and latencies of touches of the extended lever, nose-pokes (touches) directed at the retracted lever during intertrial intervals (a measure less constrained by ceiling effects than extended lever touches), and unconditioned exploratory rearing activity, measured as touches of a metal strip mounted above the grid floor of the apparatus. In an initial experiment, male Sprague-Dawley rats were given saline or naloxone (2.0 mg/kg, ip) 5 min before a training session of 12 trials. Two days later they were tested, in the absence of drug, in a session of 36 (three blocks of 12) trials. Naloxone depressed training levels of lever responding, in addition to slowing acquisition rate. No effect of naloxone was observed on rearing activity. Previous work showed that injection of saline 5 min before behavioral testing increases the rate of autoshaping compared to injections 30 min before (Messing & Sparber, 1984). Thus, effects of naloxone on acquisition of lever-directed behaviors may have been confounded by behavioral depressant effects and/or by an injection effect such a short time before testing. In a second experiment naloxone (0.5 or 2.0 mg/kg) was injected after five of seven training sessions (12 trials each) to male and female rats. A 6-s delay of reinforcement was inserted between lever retraction and food delivery, slowing acquisition rates and providing the opportunity to test the effects of naloxone throughout a multiple-session task. The low dose retarded acquisition of extended lever touching in both sexes; both doses retarded acquisition of interim lever touching in males. Thus, in some circumstances, post-training naloxone administration may impair learning. The results support the notion that low doses of naloxone may have agonist activity.

Reversal of a trimethyltin-induced learning deficit by desglycinamide-8-arginine vasopressin

Trimethyltin (TMT) is an organometal neurotoxin which produces lesions primarily in the limbic system. Selectivity seems to depend upon the dose, but the hippocampus and related entorhinal cortical structures, of importance for learning and memory, are most often described as target sites. We have previously demonstrated that subjects treated with a moderate dose of TMT prior to acquisition sessions, are unable to learn a forward autoshaping task with a 6 sec delay of reinforcement, but are capable of acquiring the same task when no delay of reinforcement is used. These data suggested that the performance deficit is one of learning (i.e. consolidation) rather than of memory (i.e. storage), retrieval, or sensorimotor impairment. To more rigorously test this hypothesis, we determined if performance of a task already learned would be impaired by the neurotoxin. Adult male Long Evans rats were given 10 acquisition sessions of 24 trials, following which TMT (6.0 mg/kg, p.o.) was administered. One month later, these rats performed the lever-touching behavior as well as controls, despite the fact that the same dose of TMT interfered with learning if given one month prior to acquisition sessions, thus confirming our hypothesis. In a second experiment we determined if the peptide analog of vasopressin, desglycinamide-8-arginine vasopressin (DGAVP), could reverse a learning deficit in a population of non-learners. Rats were treated with TMT or water vehicle one month prior to autoshaping. TMT significantly retarded acquisition. After 10 sessions of 12 trials each, non-learners (i.e. rats treated with TMT that failed to associate the lever with delivery of a reinforcer) were administered saline or DGAVP (7.5 micrograms/kg, s.c.) 1 hr before sessions 11-13; treatment was discontinued prior to sessions 14 and 15. Peptide treated subjects showed evidence of acquisition and exhibited higher levels of lever-directed behavior than saline treated nonlearners. Performance was maintained after DGAVP treatment was discontinued, indicating that the learning-enhancing action of DGAVP was not transient or state-dependent.

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.

Studies on desglycinamide arginine vasopressin and scopolamine in a modified/lever-touch autoshaping model of learning/memory in rats

Vasopressin administration has been reported to improve acquisition and retard extinction of both conditioned avoidance and food-reinforced behavioral tasks. In the present experiment the effects of a vasopressin analog (DGAVP) and scopolamine (SCOP) were tested in an autoshaped lever-touch model of learning and memory. Rats were food-deprived to 80% of original body weights and tested in modular cages which contained a retractable lever that was presented on a random interval 48 sec schedule. The lever retracted after 15 sec or when it was touched, at which time one 45 mg food pellet was delivered. Subcutaneous injection of 10 micrograms/kg DGAVP 1 hr prior to acquisition and extinction sessions did not alter responding compared to saline controls. DGAVP at doses of 10, 20, and 30 micrograms/kg also failed to affect responding in a more difficult task which included an 8 sec delay between lever retraction and reinforcement. Homozygous Brattleboro rats, which are deficient in vasopressin, did not differ from normal heterozygous littermates in the acquisition of the lever-touch response. Intraperitoneal injection of SCOP (0.1-0.8 mg/kg) 30 min prior to testing caused a dose-related impairment of acquisition compared to saline controls, but did not alter responding in animals which had previously acquired the lever-touch response. These data suggest that manipulations of vasopressin do not affect, while SCOP impairs, the acquisition of a positively reinforced lever-touch response in rats.

A mnemonic role for vasopressin: the evidence for and against

This review critically evaluates the animal and human research concerning vasopressin's putative mnemonic role. Weaknesses in the interpretations of the early animal experiments as well as the implications of the later inconsistent findings are discussed. It is concluded that both the initial enthusiasm and the subsequent skepticism concerning this hypothesized role were premature. This conclusion applies equally to the human research. A review of these studies reveals that almost all of the negative reports involved cognitively-impaired individuals. The relatively few studies that have been conducted concerning vasopressin's effects in unimpaired human subjects are consistent with the hypothesis that vasopressin does affect cognition, though both the mechanism of action and the specific cognitive processes which are altered have yet to be elucidated.

Effects of peripherally injected vasopressin and des-glycinamide vasopressin on the extinction of a spatial learning task in rats

An elevated eight-arm radial maze was employed to study the effects of neuropeptide administration on the spatial learning abilities of food-deprived rats. Following 18 days of reinforced training, each animal was briefly exposed to the maze with no food available in any of the eight food-cups. Immediately after this preliminary trial, animals were injected with a single subcutaneous dose of either saline, arginine vasopressin (AVP: 1.0 or 5.0 micrograms/kg), or an AVP analog with only weak endocrinological activity, des-gly-arginine vasopressin (DG-AVP: 1.0, 5.0 or 10.0 micrograms/kg). Additional extinction trials were conducted at 2, 4, 6 and 8 h post-injection. These tests consisted of individually placing an animal on the empty maze and recording the number of arms chosen in a 5-min period. In this situation, animals learn that food is no longer present in the maze and, consequently, extinguish responding. Vasopressin potentiated this radial maze extinction behavior while DG-AVP produced behavioral results directionally opposite to those predicted by a memory facilitation hypothesis. In a subsequent experiment, vasopressin had no effects on unconditioned locomotor activity measured 2 and 4 h post-injection. These results suggest that: vasopressin improved the learning that occurred during extinction of conditioned appetitive behaviors, these vasopressin effects on conditioned behavior were independent of any unconditioned, sedative or non-specific actions of the peptide, and peripheral endocrinological responses may be necessary to demonstrate memory-enhancing effects following peripherally administered AVP.

Intracerebroventricular application of a vasopressin antagonist peptide prevents the behavioral actions of vasopressin

Three experiments were conducted to test the hypothesis that the memory-improving properties of peripherally-applied vasopressin (AVP) were related to its aversive (i.e. arousing) actions. The memory effects of AVP were observed in a one-trial food-finding task where non-deprived rats were briefly exposed to a large open field that contained an alcove in which a high-incentive familiar food reward (sweetened milk) was freely available. AVP injections immediately upon removal from the open-field produced faster latencies to refind the alcove (compared to vehicle controls) when tested 48 h later. The aversive actions of AVP were demonstrated in two behavioral assays: (1) a conditioned taste aversion test in which rats learned to avoid a preferred saccharin solution after it had been paired with injections of AVP; and (2) a conditioned place test in which rats learned to avoid a distinctive environment associated with AVP administration. Both the memory and aversive responses to AVP were prevented, in a dose-dependent manner, by immediate pretreatment with intracerebroventricular infusions of the pressor antagonist analog 1-deaminopenicillamine-2-(O-methyl)-tyrosine AVP. The large antagonist doses required to block AVP's behavioral effects suggest that the critical site of action may be far removed from the lateral ventricles. The possibility that AVP-induced improvements in memory result from peripheral arousing actions is discussed.

Vasopressin effects on food-rewarded learning tasks might be due to its action on carbohydrate/lipid metabolism, not memory

Vasopressin (VP) has been implicated in memory processes on the basis of effects observed in aversively motivated learning situations. Therefore researchers have tried to confirm this role by using food-motivated learning tasks. However, the well-established physiological influences of VP on carbohydrate and lipid metabolism were not taken into consideration. At various times following administration, VP might act as a feeding stimulant or as a satiating agent. Experimental designs should allow for these effects when food-rewarded learning paradigms are used to determine whether VP acts on memory.

Amphetamine cumulation and tolerance development: concurrent and opposing phenomena

Whether diminished or augmented behavioral effects are observed after repeated amphetamine administration may reflect the relative balance between tolerance and drug cumulation. To investigate this, we measured the distribution of d-amphetamine in various tissues and its effects on performance of a conditioned behavior after acute or chronic treatment. Rats trained to lever press under a fixed ratio 5 schedule for food-reinforcement were tested daily for 4 min epochs in each of 6 consecutive hours. After responding was stable, animals were injected for 16 days with saline or 1.0, 2.5 or 5.0 mg 3H-d-amphetamine sulfate/kg IP 15 min before the second daily behavioral epoch. On the 17th day, animals which had been receiving 3H-d-amphetamine were given their usual dose and those which had been receiving saline were given one of the doses of 3H-d-amphetamine; all animals were decapitated approximately 2 1/4 hours after this final injection, immediately after the 4th behavioral epoch. Brain, heart, muscle, epididymal fat, and kidney were removed for subsequent analysis of unchanged 3H-d-amphetamine. The experiment was carried out in two phases, 3 1/2 months apart, which inadvertently resulted in shipment of rats from different buildings on the supplier's campus. Acute treatment produced dose-related effects on operant responding, the lowest dose increasing responding and the highest dose suppressing it. Chronic injection of the highest dose of d-amphetamine resulted in significant attenuation of its acute suppressant effect. Additionally, chronic treatment suppressed responding of rats 23 1/4 hours after injection (i.e., before the subsequent daily injection). Tissue levels of d-amphetamine were dose related and d-amphetamine cumulated after chronic treatment with the highest dose. When d-amphetamine was administered acutely, the behavioural effect immediately before decapitation was highly correlated with the concentration of d-amphetamine in brain and in heart. This was not the case after chronic treatment, since rats given the higher doses showed less behavioural effect than would have been predicted from the concentrations of d-amphetamine in their tissues. Besides evidence of tolerance and cumulation of drug in one or more tissues, a significant phase or colony difference emerged, which could have been due to seasonal or other factors. Additional, different experiments, performed concurrently on a new shipment of rats from each colony, allowed us to conclude that the original observations of phase differences were not due to seasonal differences or chance.(ABSTRACT TRUNCATED AT 400 WORDS)