ADH and related peptides: effect of pre- or posttraining treatment on puromycin amnesia

Protein synthesis inhibition and memory: formation vs amnesia

Studies using protein synthesis inhibitors have provided key support for the prevalent view that memory formation requires the initiation of protein synthesis as a primary element of the molecular biology of memory. However, many other interpretations of the amnesia data have received far less attention. These include: (a) protein synthesis may play a constitutive role in memory formation, providing proteins prior to an experience that can be activated by training; (b) protein synthesis may be needed to replace proteins available prior to learning but 'consumed' by learning; (c) inhibition of protein synthesis impairs the well-being of neurons, leading to an inability to deliver resources needed for memory formation; and (d) inhibition of protein synthesis results in abnormal neural functions that interfere with memory. One of these, abnormal release of neurotransmitters after inhibition of protein synthesis, is detailed here, along with a review of many circumstances in which it appears that protein synthesis at the time of training is not required for the formation of new memories. Evidence of activation of cell signaling molecules and transcription factors is another form of support for a role of training-initiated protein synthesis in memory. However, recent findings suggest that many of these molecules are activated by training and remain activated for days after training, i.e. activated for times well beyond those typically invoked for memory consolidation processes. Reviewing these results, this paper suggests that the long-lasting molecular changes may be the basis of a form of intracellular memory, one responsible for up-regulating the probability that a neuron, once activated in this manner, will engage in future plasticity. This view melds ideas of modulation of memory with those of consolidation of memory.

Pharmacological characterization of a novel AVP(4-9) binding site in rat hippocampus

pGlu-Asn-Cys (Cys)-Pro-Arg-Gly-NH(2) (AVP(4-9)), a major metabolite C-terminal fragment of Arginine(8)-vasopressin (AVP), improves the disruption of the learning and memory, and is a far more potent in the mnemonic function than AVP. In this study, we pharmacologically characterized its putative binding site and mechanism of intracellular signaling. Radioligand binding assay showed that [35S]AVP(4-9) could detect specific binding sites in the rat hippocampus membrane preparations, and the binding site was specifically displaced by AVP(4-9) but not by either V(1) or V(2) antagonists. Furthermore, [35S]AVP(4-9) could not detect the cloned rat V(1a), V(1b) and V(2) vasopressin receptors. Even at a low doses (10-100 pM), AVP(4-9) caused an increase in both inositol(1,4, 5)-trisphosphate (Ins(1,4,5)P(3)) and intracellular calcium concentrations ([Ca(2+)](i)) in rat hippocampal cells. The AVP(4-9)-induced [Ca(2+)](i) increase was partially inhibited by the absence of Ca(2+) or by Ca(2+)-channel blocker, suggesting that AVP(4-9) caused the [Ca(2+)](i) increase via release from intracellular calcium store as well as influx from extracellular calcium. For the first time, this study provides evidence to show that AVP(4-9) activates Ins(1,4,5)P(3)/[Ca(2+)](i) pathway through a novel type of receptor in rat hippocampus, which might be potentially important in improving the mnemonic function.

Antiamnesic effects of D-pipecolic acid and analogues of Pro-Leu-Gly-NH2 in rats

The antiamnesic effects of prolyl-leucyl-glycinamide (PLG) and analogues of this tripeptide were investigated in rats. Retrograde amnesia was induced by electroconvulsive shock treatment and the degree of amnesia was characterized by the attenuation of one-trial learning passive avoidance response. PLG resulted in dose-dependent attenuation of retrograde amnesia. Structural modifications included N-terminal protection, substitution of the C-terminal NH2 group, replacement of the N-terminal amino acid, and replacement of the second amino acid of the tripeptide. Some tripeptides, all of them containing D-pipecolic acid instead of the N-terminal proline, were more effective than PLG. Therefore, D-pipecolic acid, D-pipecolamide and their N-terminally protected analogues were also investigated, and were found to have powerful antiamnesic effects.

The effect of AVP and DGAVP on the exploratory activity of rats

The effects of [8-L-arginine] vasopressin (AVP) and desglycinamide [8-L-arginine] vasopressin (DGAVP) were tested on the exploratory activity of adult male rats in a novel environment. The inherited individual differences in the non-specific excitability level of the animals were ascertained prior to the drug administration and the rats were then distributed evenly into the experimental groups. One half of each groups contained the less excitable and the other the more excitable animals. The peptides or saline were injected every other day--altogether 4 times--in a dose of 5 micrograms/kg/ml subcutaneously, 40 min before starting the experiments. The exploratory activity in the novel environment was observed for 15 min. AVP and DGAVP, which differ in their peripheral endocrine activities, had opposite effects on the behavior in a novel environment: AVP, with its wide spectrum of peripheral effects, decreased the exploratory activity, whereas DGAVP, with minimal peripheral effects, increased the exploratory activity slightly. This basic response to the administration of peptides was influenced by the type of inherent non-specific excitability level. The depressive action of AVP was more pronounced in the more excitable rats, whereas DGAVP significantly stimulated the less excitable animals. It is concluded that the inhibitory effect of AVP is mainly due to its peripheral endocrine, especially hemodynamic, effects, whereas DGAVP is supposed to increase arousal, which is responsible for differences in the animals' performance with regard to their inherited non-specific excitability levels.

Conditioned taste aversion in vasopressin-deficient rats (Brattleboro strain)

Brattleboro rats are homozygous for diabetes insipidus (DI), lacking the ability to synthesize vasopressin. Previous studies reported learning deficits in DI rats on passive avoidance tasks using footshock. Other studies, however, could not replicate these results. In two experiments, we studied the learning of DI and control Long Evans (LE) rats in a different avoidance paradigm: conditioned taste aversion (CTA). In the first experiment a mild CTA to saccharin was established gradually using low levels of an illness-inducing agent (lithium chloride). In the second experiment a strong CTA was established in one acquisition trial and the extinction of the conditioned aversion was followed for 12 trials. The two experiments found no differences between the DI and LE rats in either the magnitude or the rate of acquisition and extinction of the CTA. These results suggest that vasopressin is not involved in the acquisition and retention of CTA, and support previous studies indicating that vasopressin may not be involved in avoidance learning.

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.

Vasopressin antagonists block peripheral as well as central vasopressin receptors

The aim of the present study was to differentiate between the postulated central behavioral effects of vasopressin and its pressor response, which is mainly mediated by peripheral vascular receptors. Thus, the interaction between the vasopressor antagonists dPTyr(Me)AVP (AAVPa) and d(CH2)5Tyr(Me)AVP (AAVPb) with the effects of [Arg8]vasopressin (AVP-(1-9)) and [pGlu4,Cyt6]AVP-(4-8) (referred to as AVP-(4-8)) was examined using passive avoidance behavior and the pressor response as parameters. AVP-(4-8) was approximately 4 and 200 times more potent than AVP-(1-9) in facilitating passive avoidance behavior after subcutaneous (SC) or intracerebroventricular (ICV) administration respectively. This effect of SC injected AVP-(1-9) and AVP-(4-8) could be prevented by both vasopressor antagonists following SC treatment. A similar antagonistic action was found when AVP-(1-9) or AVP-(4-8) and the antagonist AAVPb were administered ICV. SC injection of AAVPb prevented the behavioral effect of ICV administered AVP-(1-9) while ICV treatment with the antagonist blocked the behavioral action of systemically injected AVP-(1-9) and AVP-(4-8). In contrast to SC injected AVP-(1-9) which dose-dependently increased blood pressure and decreased heart rate, AVP-(4-8) injected SC in identical doses did not affect blood pressure and heart rate, neither did AVP-(1-9) and AVP-(4-8) when injected ICV in behaviorally active doses. A SC, but not an ICV injection of the antagonist AAVPb could prevent the blood pressure increase and bradycardia induced by SC AVP-(1-9).(ABSTRACT TRUNCATED AT 250 WORDS)

A major metabolite of arginine vasopressin in the brain is a highly potent neuropeptide

A peptide that accumulated as the major product during the proteolysis of arginine vasopressin by rat brain synaptic membranes was isolated and its structure was shown to be the hexapeptide pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH2. When administered intracerebroventricularly in extremely low doses, this vasopressin fragment and its desglycinamide derivative facilitated memory consolidation in a passive avoidance situation. These vasopressin metabolites, which are devoid of pressor activity, constitute highly potent neuropeptides with selective effects on memory and related processes; they are activated via proteolytic processing of vasopressin by brain peptidases.

Centrally mediated effects of neurohypophyseal hormones

The neurohypophyseal hormones oxytocin and vasopressin cause a variety of biological effects in animals which are mediated by central nervous system mechanisms. Among the best studied of these effects is the modulation of both memory processes and the development of drug tolerance and dependence. Neurohypophyseal hormones have also been shown to alter various physiological parameters such as heart rate and body temperature following central administration. In addition, these peptides can profoundly alter spontaneous, unlearned behavior in several rodent species. Many of the centrally mediated effects of neurohypophyseal hormones have been shown to be elicited at sites within the brain stem and the limbic system where vasopressin and oxytocin occur in cell bodies, axons and nerve terminals, suggesting a physiological role for these peptide effects. The various central effects of neurohypophyseal hormones involve different mechanisms which can be distinguished from one another on the basis of required dose, time-course of action, and structure-activity relationships. Thus, alterations of spontaneous behavior are mediated by putative receptors closely related to vasopressin receptors in blood vessels responsible for the peripheral pressor response while the effects on memory processes are mediated by a mechanism which is not closely related to those involved in the peripheral hormonal effects of the peptides. The influence of neurohypophyseal hormones on memory and attention may be useful clinically. A potential role for these peptides in mental disorders is discussed.

Lysine vasopressin fails to alter (3H)-noradrenaline uptake or release from hippocampal tissue in vitro

Lysine vasopressin (5.2 and 10.4 microM) failed to significantly alter the spontaneous release of (3H)-noradrenaline from hippocampal slices in vitro or the release observed after stimulation with potassium chloride (15 and 25 mM). Uptake of (3H)-noradrenaline by synaptosomes prepared from hippocampal tissue was not significantly changed by incubation with vasopressin (5.2 and 10.4 mu M). Our results fail to support the hypothesis, derived from behavioural and in vivo biochemical studies, that vasopressin modulates the activity of noradrenergic nerve terminals in this part of the limbic system.

Temporal and pharmacological parameters of puromycin-induced amnesia

Mice were trained in step-down passive avoidance behavior. Bitemporal injections of puromycin (PM) were given either immediately or delayed until 24 hrs after training. PM produced a marked amnesia in both cases during retention testing 3 days later. The amnesia persisted during a second retention test 6 days after training. Of all the antibiotics, only PM is effective as an amnestic agent when injections are delayed 24 or more hours after training. cycloheximide (CXM) was also injected bitemporally immediately after training. However, CXM produced a weaker amnestic effect even though it produced a much greater inhibition of cerebral protein synthesis, more rapidly, and of longer duration. In an effort to attenuate the amnesia produced by PM, in separate experiments, the mice were injected with combined injections of PM and and CXM (bitemporally): mice were also given combined injections of PM (bitemporally) and amphetamine (subcutaneously). The amnesia produced by immediate injections of PM wa not attenuated by either CXM or amphetamine. However, the amnesia produced by delayed injections of PM was attenuated by both CXM and amphetamine. These results suggest that delayed injections of PM (25 hours after training) block the expression or retrieval of memory. This study also supports the contention that puromycin has two separate effects on memory with different temporal parameters depending on when the drug is injected relative to initial training.

Vasopressin potentiates the stimulation of cyclic AMP accumulation by norepinephrine

Vasopressin, a peptide that appears to enhance the consolidation process of memory was studied for its ability to stimulate the accumulation of cyclic AMP in slices of the mouse hippocampus. While vasopressin alone exhibited no effects in this system, it substantially potentiated the effects of norepinephrine. Such actions are discussed in reference to an endogenous brain vasopressin system, and they suggest a possible neuromodulator role for vasopressin.

Neuropeptide effects on memory in aged monkeys

The effects of several neuropeptides were evaluated using a non-human primate model of age-related memory impairments. Several doses of ACTH4-10, lysine vasopressin, arginine vasopressin, oxytocin and somatostatin were each tested in several aged monkeys. Because data from a large number of non-drug control sessions was collected before, during and after this study, it was possible to define the normal range of control performance for each monkey and statistically determine whether a change in performance under any single dose of drug reflected a significant change from the particular monkey's normal baseline performance. Although none of the neuropeptides produced consistent group effects, evaluations of individual subjects against their own baseline performance revealed reliable changes at certain doses. Arginine vasopressin appeared to produce the best overall effects with three of the five monkeys exhibiting reliable changes in performance from baseline. These same three monkeys also responded positively to the lysine form. Oxytocin impaired memory in three of the six aged monkeys tested over a wide range of doses. Three of six aged monkeys performed better under ACTH4-10 compared to baseline; however, in two of these cases only a single dose was effective. The performance of only one subject was improved under somatostatin, and this was at a single dose only. The data reported here provide evidence for neuropeptides producing behavioral improvement in non-human primates using an appetitive task, eliminating a popular criticism that the data in this literature has depended too heavily on the testing of rodents in shock-motivated tasks. Additionally, the improvements observed in this study involve a behavior that it naturally impaired by age and one which has many operational similarities and some empirical relevance to measures of recent memory in humans. However, these positive findings must be tempered by the lack of robust effects and high individual variation observed.

Time-dependency of neurohypophyseal peptide attenuation of puromycin amnesia in mice

[Ile3, Arg8]vasopressin (arginine-vasotocin), as well as the C-terminal tripeptides of the neurohypophyseal hormones arginine and lysine vasopressin, Pro-Arg-Gly-NH2 and Pro-Lys-Gly-NH2, were protective against puromycin-induced amnesia in mice when administered 24h before training. The N-protected tripeptide derivative, Z-Pro-Lys-Gly-NH2, was effective when given 5 days before training. The effectiveness of all peptides to attenuate puromycin-induced amnesia decreased as the interval between training and peptide treatment increased, indicating that the peptides influence memory processes, rather than general arousal. Z-Pro-Lys-Gly-NH2 was active at 24h after training, when the other peptides were no longer effective. Although it seems clear that neurohypophyseal hormones per se can attenuate puromycin-induced amnesia, these results are in line with the possibility that some portion of hormone action may be mediated via formation of longer-lived hormone fragments in the CNS.

Lysine vasopressin attenuation of diethyldithiocarbamate-induced amnesia

The effect of lysine vasopressin on diethyldithiocarbamate-induced amnesia for a step-through passive avoidance task was studied in rats. It was determined that although the hormone attenuates the amnesia when it is administered prior to retrieval testing, it fails to do so when it is injected prior to training. These results are consistent with the reports of others which demonstrate the reversal of diethyldithiocarbamate-induced amnesia by catecholamine agonists.

Distribution, survival and biological effects in mice of a behaviorally active, enzymatically stable peptide: pharmacokinetics of cyclo(Leu-Gly) and puromycin-induced amnesia

Cyclo(Leu-Gly), the enzymatically resistant diketopiperazine formally derived from the C-terminal dipeptide sequence of oxytocin, exhibits activity in several behavioral systems. The distribution of cyclo(Leu-14C(U)Gly) in brain, and the time course of the disappearance of this labeled peptide from brain and plasma after subcutaneous injection into mice have been studied. The intact peptide was distributed equally in the five cerebral areas studied, for up to 96 hours after injection. Two exponential components were determined for peptide disappearance rates in plasma and brain; peptide half-lives in plasma up to 10 hr and from 24--96 hr after injection were, respectively, 0.8 and 33 hr; in brain, 1.0 and 42 hr. The peptide was found to accumulate in brain intracellular space to some degree. The time course of distribution of labeled cyclo(Leu-Gly) in subcellular fractions of mouse brain was also examined, and the concentration of peptide in the synaptosomal fraction was significantly correlated with the degree of protection against puromycin-induced amnesia of a maze-learning test. The results obtained not only confirm that cyclo(Leu-Gly) penetrates brain tissue intact and remains intact after peripheral administration in order to exert its behavioral effects, but, moreover, suggest an intriguing dynamic relationship between peptide concentration in the synaptosomal fraction and behavioral activity.