Does vasopressin really facilitate memory processes?

The behavioral neurobiology of learning and memory: a conceptual reorientation

Research on the neurobiology of learning and memory has been guided by two major theories: (i) memory as a psychological process and (ii) memory as a change in synaptic neural connectivity. It is not widely recognised that not only are these theories different but, moreover, they are fundamentally incompatible. Confusion concerning basic concepts in the learning and memory field in mammals has lead to the creation of an extensive but often inconclusive experimental literature. However, one important conclusion suggested by recent work in this field is that experience-dependent changes in neural connectivity occur in many different brain systems. Particular brain structures, such as the hippocampus, do not play any uniquely important role in experience-dependent behavior. Research in learning and memory can be best pursued on the basis of biological studies of animal behavior and a cellular approach to brain function.

Effects of DDAVP on movement planning and execution processes in the healthy elderly

Effects of DDAVP on speed and consistency of planning and executing simple and complex movements in healthy older adults were studied. A simple reaction time (SRT) task, a single-plane movement task, and two tasks involving multiplane movements of distal upper extremities were performed with and/or without a 0.6 ml intranasal dose (60 micrograms) of DDAVP or placebo. Results indicated that DDAVP had no significant effect on speed or consistency of SRT processes, or the speed with which simple or complex movements were planned or executed. There was also no effect on retention of motor responses.

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 The lever was retracted immediately if a touch occurred within 15 s, or automatically after 15 s. Analyses were conducted on number and latencies of touches of the extended lever,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.

Behavioral effects of angiotensin II and angiotensin II-(4-8)-pentapeptide in rats

One nM of angiotensin II (AII) or angiotensin II-(4-8)-pentapeptide [AII(4-8)] given intracerebroventricularly did not affect locomotor and exploratory behavior of rats in open field. AII significantly increased and AII(4-8) did not affect vertical activity of animals in electromagnetic motimeter. Neither of the peptides influenced horizontal activity in the motimeter. Both peptides intensified stereotypy produced by apomorphine and amphetamine. AII significantly improved, while AII(4-8) did not affect, consolidation of memory of the correct way to food in T-maze. Similarly, AII increased and AII(4-8) did not change the rate of acquisition of conditioned avoidance responses in a shuttle-box. Of the two examined peptides only AII significantly improved retrieval of memory of the passive avoidance behavior. The results show that AII(4-8) influences central dopaminergic system but, unlike its parent peptide AII, has no apparent effect on memory.

Vasopressin injections impair working memory in a delayed matching to sample task in rats

Effects of vasopressin were measured in a nonspatial working memory task: food-reinforced, delayed matching to sample. Subcutaneous injections of 0.2 microgram of lysine vasopressin (LVP) or saline were alternately administered to Sprague-Dawley rats after the presentation of the sample and compared to the effect of the same treatments given to a yoked control group of rats. Different durations of sample presentation (5 and 30 min) and various retention intervals (10 min, 3 h, 24 h) were selected. The results showed that LVP never facilitated retention performance: there was no improvement under conditions of weak memory (short presentation of the sample and long retention interval); moreover LVP abolished the facilitation normally obtained when either the length of the sample presentation was longer or the duration of the retention interval was shorter. The performance after the injection of the peptide was differentially impaired, according to brightness of the sample which had been presented: after the presentation of the white box, LVP injections lead to more errors and after that of the black one the treatment induced an increase in latencies of response. All these data may suggest that the physiological consequences of hormonal modifications triggered by the LVP injection are processed along with the stimuli of the learning episode and interfere with the learned positive value of the sample.

Neonatal administration of vasopressin antiserum induces long-term deficits on active and passive avoidance behaviour in rats

Two-day-old male rats received a subcutaneous injection of arginine-vasopressin (AVP) antiserum and avoidance behaviour was studied 3 months later. Rats treated with the antiserum showed a clear retention deficit in a one-trial learning, step-through passive avoidance situation. Anti-AVP treatment also induced an impairment on the acquisition of a two-way active avoidance task. Systolic blood pressure was lower than normal in these animals. The results obtained appear to be indicative of the high vulnerability of the developing nervous system, and are discussed in the context of the different hypothesis on the role of central or peripheral mechanisms in the behavioural effects of AVP. Although no definite conclusions may be drawn in this regard, the present data strongly suggest that neonatal administration of AVP antiserum exerts long-lasting effects upon the functionality of several physiological mechanisms related to the behavioural adaptation of the organism.

Behavioral variability within the Brattleboro and Long-Evans rat strains

Samples of genetically diabetes insipidus (DI) and normal (NO) rats were obtained from American suppliers (Rochester (RO)/DI and NO) and from the colony maintained at Charing Cross Hospital in London (Charing Cross (CC)/DI and NO) to test the hypothesis that the behavior of vasopressin-deficient Brattleboro (DI) and possibly normal Long-Evans rats may vary significantly between different colonies. DI rats of both colonies exhibit longer latencies to emerge into an open field than do NO rats. RO/DI and CC/DI rats acquire goal-approach behavior in a straight runway at similar rates. Following shock in the runway goal box, however, RO/DI rats exhibit marked recovery of running behavior relative to CC/DI rats over the ten post-shock sessions. All DI animals show reductions in goal-approach speed on the first post-shock trial, indicating that the aversive experience is remembered. CC/NO rats acquire goal-approach behavior more slowly than RO/NO rats, but neither NO group shows substantial recovery of goal approach behavior following shock. CC/DI rats showed impaired acquisition of a delayed non-match to sample task relative to RO/DI rats. All groups demonstrated the ability to utilize representational memory to solve the delayed non-match to sample problem once the contingency was learned. The results indicate that DI and normal Long-Evans rats from different colonies show marked differences in behavior. Since differences between DI and normal rats on tests indicating memory are not consistent across colonies, it is unlikely that vasopressin deficit is solely responsible for memory deficiencies. However, vasopressin deficiency may result in changes in temperament.

Preventive effect of cholecystokinin octapeptide on experimental amnesia in rats

The effect of intracerebroventricular (ICV) administration of cholecystokinin octapeptide (CCK-8) on electroconvulsive shock (ECS)-induced amnesia in passive avoidance response was studied in rats. In normal rats, CCK-8 in doses from 1 ng to 1 microgram had no effect on the response when injected before the training trials, immediately after foot shock or before the first retention test. However, proglumide, a CCK-8 receptor blocker, induced marked amnesia when injected in doses from 0.1 to 10 micrograms before the training trials and in doses of 1 and 10 micrograms before the first retention test, though not subsequent to foot shock. ECS given immediately after the foot shock caused amnesia in the 24 hr and 48 hr retention tests, which could have been prevented by CCK-8 injected in doses of 10 ng to 1 microgram prior to the training trials, of 10 ng to 1 microgram following ECS and of 0.1 and 1 microgram before the first retention test. In addition, the effects of CCK-8 and proglumide became pronounced following chronic ICV infusion, using an osmotic minipump, for 7 days at a dose of 1 ng/day and 10 ng/day, respectively. The amnesia induced by proglumide was not affected by arginine vasopressin (AVP), while AVP in doses of 10 ng and 100 ng given 30 min before the training trials prevented ECS-induced amnesia. The antiamnesic effect of AVP was abolished by simultaneous administration of proglumide. On the other hand, AVP-antiserum produced marked amnesia which could be antagonized by CCK-8. However, the antiamnesic effect of CCK-8 was not suppressed by AVP-antiserum.(ABSTRACT TRUNCATED AT 250 WORDS)

Administered peptides inhibit the degradation of endogenous peptides. The dilemma of distinguishing direct from indirect effects

Virtually all peptides are biologically active following central administration as a consequence of both direct and indirect cellular actions. Direct effects are mainly interactions with specific membrane receptors but may include unions with other components of the receptor/effector complex. Significant indirect biological effects of exogenous peptides, including apparent secretagogue effects on endogenous peptides largely overlooked in practice, result from extensive competition with endogenous peptides for degradative enzymes (peptidases). A consequence of this competition is enhancement of tonic or intermittent activity of endogenous peptides. The pharmacological profile of any peptide reflects or includes, therefore, the spectrum of endogenous peptides that is protected from peptidase action. It is likely that certain pharmacologically active peptides, including a large number of di-, tri- and oligo-peptides, elicit responses mainly or exclusively by competing for peptidases. Therefore, reliable estimates of the relative contributions of direct and indirect actions of exogenous peptides may be difficult, if not impossible, to obtain.

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.