Modulation of memory by pituitary hormones and related peptides

On neurodegenerative diseases, models, and treatment strategies: lessons learned and lessons forgotten a generation following the cholinergic hypothesis

Life's Journey If life is indeed a journey, then poetry must be the map that reveals all its topographic possibilitiesellipsis while science is the compass that keeps us from getting lost. -R. T. Bartus, Simple Words for Complex Lives, (c) 1998 In the nearly 20 years since the cholinergic hypothesis was initially formulated, significant progress has been achieved. Initial palliative treatments for Alzheimer's disease (AD) have proven beneficial and have gained FDA approval, the use of animal models for studying AD and other neurodegenerative diseases has achieved wider acceptance, and important insight into the potential causes and pathogenic variables associated with various neurodegenerative diseases continues to increase. This paper reviews the current status of the cholinergic hypothesis in the context of continuing efforts to improve upon existing treatments for AD and explores the role that animal models might continue to play. Using the benefit of hindsight, particular emphasis is placed on an analysis of the approaches, strategies, and assumptions regarding animal models that proved useful in developing the initial treatments and those that did not. Additionally, contemporary issues of AD are discussed within the context of the cholinergic hypothesis, with particular attention given to how they may impact the further refinement of animal models, and the development of even more effective treatments. Finally, arguments are presented that, despite the deserved enthusiasm and optimism for identifying means of halting the pathogenesis of AD, a clear need for more effective palliative treatments will continue, long after successful pathogenic treatments are available. This review, therefore, focuses on issues and experiences intended to: (a) facilitate further development and use of animal models for AD and other neurodegenerative diseases, and (b) accelerate the identification of newer, even more effective treatments.

The enhancement of retention induced by vasopressin in mice may be mediated by an activation of central nicotinic cholinergic mechanisms

Immediate post-training subcutaneous administration of lysine vasopressin (LVP, 0.003-1.00 microgram/kg) enhanced retention, whereas the vasopressin antagonist AAVP (0.01-0.30 microgram/kg) impaired it, in male Swiss mice tested 48 h after training in an inhibitory avoidance task. Both effects were dose-dependent. Neither LVP nor AAVP affected response latencies in mice not given the footshock on the training trial. The simultaneous administration of AAVP at a dose (0.01 microgram/kg) which had no effect on retention shifted the dose-response curve of LVP to the right. Nicotine (1.0-30.0 micrograms/kg, sc), a central nicotinic cholinergic agonist, also facilitated retention in a dose-related manner without affecting the retention performance of unshocked mice. The effect of nicotine was prevented by the central acting nicotinic cholinergic receptor antagonist mecamylamine (5 mg/kg, sc.). In contrast, neither hexamethonium (5 mg/kg, sc), a peripheral acting nicotinic receptor blocker, nor atropine (0.5 mg/kg, sc) or methylatropine (0.5 mg/kg, sc), two anticholinergic drugs which are known to act on muscarinic cholinergic receptors, prevented the effect of post-training nicotine. The effects of LVP and nicotine were time-dependent, suggesting that both treatments enhanced retention by influencing post-training processes involved in memory storage. Low doses of nicotine (1.50 microgram/kg, sc) or the central anticholinesterase physostigmine (35 micrograms/kg, sc) and LVP (0.003 microgram/kg, sc), which had no effect on retention when administered alone, produced a synergistic interaction when given together following training. The influence of LVP (0.03 microgram/kg, sc) on retention was prevented not only by AAVP (0.01 microgram/kg, sc) but also by mecamylamine (5 mg/kg, sc), whereas the effects of nicotine (10.0 micrograms/kg, sc) were prevented only by mecamylamine. These results suggest that the enhancement of retention induced by vasopressin is probably due to an activation of central nicotinic cholinergic mechanisms which are critical for memory formation.

Brain ACTH prevents stress gastric lesions in rats

This study evaluated the effect of ACTH and several ACTH fragments on the development of gastric glandular lesions induced by cold-restraint stress in rats. Intracerebroventricular administration of ACTH1-39 dose-dependently (0.1-10 micrograms) inhibited stress gastric lesion formation. Studies with smaller molecular weight forms of ACTH (in a dose equimolar to 10 micrograms of ACTH1-39) revealed that ACTH1-13 and ACTH1-10 were also protective. The ACTH fragments ACTH5-10, ACTH34-39 and ACTH1-17 were without effect. Immunoneutralization of endogenous brain ACTH1-39 significantly increased stress gastric lesion severity. Antisera raised against synthetic somatostatin, gonadotropin-releasing hormone, and L-enkephalin were ineffective. These results with ACTH coupled with our previous demonstration of a protective effect of beta-endorphin suggest that specific brain pro-opiomelanocortin gene products modulate gastric mucosal integrity in response to stress.

Nicotine interactions with ethanol tolerance

Nicotine (N) administration (0.05 mg/kg SC) was paired with ethanol (E, 2.5 g/kg, 15% v/v, IP) to determine if N alters either the acquisition of extinction of tolerance to the hypothermic and sedative effects of E. During tolerance acquisition the following groups were tested: E + N (N = 16), E + NaCl vehicle (V) (N = 16), V + N (N = 4) and V + V (N = 4). For 11 days a colonic temperature was taken, both drugs were injected and the rats were tested for locomotor activity for 45 min, after which a final colonic temperature was taken. N significantly enhanced the rate of tolerance development to the hypothermic effects of E and blocked a degree of the sedative effects. On Days 12 to 17 rats in all groups received V injections to extinguish tolerance. On Days 18 to 24 rats in the E + N group were tested with either E + N or E + V and rats in the E + V group were similarly divided. Previous treatment with N significantly attenuated the extinction process which in turn enhanced the reacquisition of tolerance.

Memory enhancement in mice: role of drug dose and training-testing interval

Pharmacologic probes are useful for studying memory mechanisms. For eight drug treatments affecting a variety of transmitter systems [arecoline, piribedil, clonidine, fluoxetine, naloxone, ACTH (4-10)], we determined how long memory retention would remain improved with a dose sufficient to improve 3-hour retention. While all 6 treatments enhanced 3-hour retention test performance at p less than 0.05, only 5 treatments significantly enhanced retention 24 hour after training and none of the treatments significantly affected retention at 168 hours. A detailed analysis of the dose and retention interval interaction for arecoline indicated that at low doses retention decreased as the retention interval increased while higher doses improved retention up to 3 hours and only the highest dose tested enhanced retention at 3 and 24 hours. Drug doses that enhance short-term retention (3 hours) were not adequate to enhance long term retention (168 hours). The 6 drug treatments had no significant or systematic effect on activity or on acquisition. We conclude that short-term retention performance was better because of enhanced memory processing or recall and not because of performance effects per se.

The role of arginine vasopressin in the development of tolerance to ethanol in normal and Brattleboro rats

Administration of AVP and related peptide fragments following ethanol (EtOH) administration has been shown to enhance retention of tolerance to ethanol. The present studies were designed specifically to: (1) examine the influence of AVP given concurrently with EtOH on the development of tolerance to the ataxic and hypothermic effects of EtOH in Long-Evans rats, and (2) to determine if tolerance to these effects develops in Brattleboro rats which are deficient in AVP. In Experiment 1, EtOH (2.5 g/kg, 15% v/v) was administered IP to 2 groups of rats in combination with a SC injection of either AVP (6 micrograms/kg) or an equal volume of saline. Two additional control groups received IP saline injections in combination with either saline or AVP. After 13 days, EtOH-treated rats were significantly more tolerant than saline-treated animals. AVP significantly increased the hypothermic and ataxic effects of EtOH and failed to enhance tolerance development. AVP delayed the extinction of tolerance to the hypothermic (but not the ataxic) effects of ethanol when administered during the extinction phase to rats previously treated with EtOH. In Experiment 2, Brattleboro rats were injected with EtOH or an equivalent volume of saline and tested for ataxia and hypothermia. Rats receiving EtOH failed to demonstrate significant tolerance to either effect of ethanol after 12 treatment days.

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.

Specific antagonists of the acute behavioral response to centrally-administered vasopressin in mice

Vasopressin and related peptides cause behavioral excitation after intracerebroventricular injection in mice. This behavioral excitation is characterized by excessive scratching and grooming behavior in the unrestrained animal and enhanced escape-directed activity in stressful situations. These effects of vasopressin were found to be blocked by the administration of analogs which act as competitive antagonists of the pressor-activity of vasopressin. The potencies of these analogs in suppressing the behavioral effect paralleled the pressor antagonist potencies. The antagonists did not cause the characteristic behavioral alterations by themselves, nor did they block grooming and/or scratching behavior induced by the structurally-unrelated substances, mescaline, bombesin and substance P. It is suggested that these antagonists provide useful tools for studying the role of endogenous vasopressin in behavior.

ACTH 4-9 analogue (Org 2766) in depressed elderly patients. II. Effect on memory and vigilance

In a double-blind interindividual comparison 80 mg of an ACTH 4-9 analogue (Org 2766) or placebo was administered daily to 49 elderly depressed in- and out-patients for 4 weeks. 20 patients then changed to the opposite treatment in a cross-over study for a further 2 weeks. All patients were tested for memory on days 0 and 29 with a battery consisting of the 30 World-Pair Test, the 30 Figure-Test and the 30 Personal-Facta Test. Three scores were obtained from each test, immediate memory score (IMS), delayed memory score (DMS) and their difference, forgetting score (FS). Cross-over patients were tested for vigilance on days 0, 29 and 43 in an apparatus testing ability to rapidly detect and react to specific minor changes at random intervals. Org 2766 had no better effect than placebo on learning, consolidation, or retrieval of memorized material in elderly depressed patients. Statistically significant fewer target misses in the test of vigilance suggest a higher degree of sustained attention after treatment with Org 2766.

Vasopressin and ethanol preference. I. Effects of vasopressin and the fragment DGAVP on altered ethanol preference in Brattleboro diabetes insipidus rats

Preference for concentrations of ethanol between 2.2 and 10 percent versus tap water was studied in Brattleboro rats homozygous for diabetes insipidus (di/di), heterozygous (di/+) or normal (+/+). The di/di rats, totally lacking in vasopressin, had greatly reduced preference scores for all concentrations of ethanol. Their intake of ethanol (g/day) was higher than heterozygotes or normals, but only when 2.2 percent ethanol was offered as a choice. Administration of lysine vasopressin or the vasopressin fragment des-9-Glycinamide-[Arginine8] vasopressin (DGAVP) using osmotic minipumps enhanced ethanol preference scores, reduced ethanol (g/day) intake, and restored total daily fluid intake in di/di rats. When di/di and di/+ rats were first allowed to develop stable ethanol preference before treatment with DGAVP, the peptide had no effect on preference scores. Thus, no treatment was effective in dissociating polydipsia from reduced ethanol preference and increased ethanol intake. While these results cannot exclude a possible regulatory role for endogenous vasopressin in ethanol preference drinking, they more strongly suggest that reduced preference for ethanol and increased ethanol intake are epiphenomena secondary to a polydipsic state.

Site of interaction of serotonin and desglycinamide-arginine-vasopressin in maintenance of ethanol tolerance

Rats trained to walk in a moving belt apparatus were subjected to a partial (fornix-fimbria (FF] or total (fornix-fimbria + cingulum bundles (FF + CB] chemical denervation of the dorsal serotonergic afferent pathways to the hippocampus. After chronic alcohol treatment that resulted in tolerance development to the motor-impairing effects of ethanol, desglycinamide-arginine8-vasopressin (DGAVP) or saline treatment was started and the residual tolerance measured at several intervals after ethanol withdrawal. DGAVP administration resulted in a virtually complete retention of ethanol tolerance when given to sham-operated controls or FF-lesioned rats. The peptide treatment failed, however, to prolong tolerance in rats bearing a complete FF + CB lesion, that reduced serotonin (5-HT) levels in the hippocampus and overlying parietal cortex to 10 and 45% of controls respectively. These results suggest that the serotonergic innervation of these areas is necessary for the action of DGAVP in the maintenance of ethanol tolerance.

The effect of vasopressin treatment on learning in Down's syndrome

Vasopressin is reported to enhance learning and memory in animal models. In 9 Down's syndrome patients DDAVP, a vasopressin derivative, was administered for 10 days, 40 micrograms per day, in a double-blind randomized crossover design. A visual-verbal paired associate learning test showed a not significant tendency for benefit with DDAVP. Word List memory was not improved with DDAVP treatment.

Growth hormone, vasopressin, cortisol, and catecholamine responses to insulin hypoglycemia in alcoholics

The hormonal responses to insulin-induced hypoglycemia were studied in 15 abstinent alcoholics with varying degrees of central and peripheral nerve damage and in six normal controls. Blood samples were taken at intervals after the injection of soluble insulin (0.1 U/kg of body weight). Growth hormone responses were significantly depressed (p less than 0.05) in nine alcoholics with severe central nerve damage (Korsakoff's psychosis) as compared to other alcoholic subjects. The alcoholic subjects with Korsakoff's psychosis also showed significant depression (p less than 0.01) of glucose recovery from hypoglycemia as compared with controls. However, responses of vasopressin, cortisol, and catecholamines (epinephrine and norepinephrine) were generally normal in the Korsakoff patients. Our results do not support previous suggestions that impairment of memory in alcoholism may be related to altered vasopressin secretion, even though the reduced growth hormone secretion in brain-damaged alcoholics does indicate some hypothalamic-pituitary dysfunction.

Human memory and neurohypophyseal hormones: opposite effects of vasopressin and oxytocin

A classical task of experimental psychology, the retention of lists of words, was given twice to three groups of subjects treated with lysine vasopressin (LVP), oxytocin or saline. From a baseline session (no treatment) to a second session with treatment, the LVP and placebo groups showed an enhancement of the number of words remembered correctly, whereas the oxytocin group did not. Rather, oxytocin impaired memory performance. However, we cannot claim a memory enhancing effect of LVP, because placebo treatment enhanced memory performance to the same extent.

L-prolyl-L-arginyl-glycineamide induces memory enhancement in chicks

Two-day-old chicks were injected either intraventricularly or intraperitoneally with saline or a L-prolyl-L-arginyl-glycineamide solution. This C-terminal tripeptide of arginine vasopressin produced dose dependent enhancement effects when injected centrally but not peripherally. Physical debilitation and/or aversive effects of the peptide were eliminated as the cause of the decreased responding noted in memory enhancement studies using this avoidance paradigm. Possible memory mechanisms are discussed in light of this peptide's relationship to vasopressin, vasotocin, and L-propyl-L-leucyl-glycineamide.

Hyperreflexive behavior in Brattleboro rats

In two experiments we compared the size of the startle response elicited by shock and acoustic stimuli in animals having a congenital absence of vasopressin (Brattleboro rats) with closely related controls, which do synthesize vasopressin. The rats lacking vasopressin were hyperresponsive to both shock and acoustic stimuli. Inhibition of the acoustic startle by shock prestimuli also was greater in these animals. These results are consistent with reports which indicate that vasopressin attenuates responses to noxious stimuli.

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.

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.

High performance liquid chromatography of pharmacologically active amines and peptides in biological materials

Precise and quantitative reversed-phase high performance liquid chromatographic (HPLC) procedures are described which can be used in biogenic amine and neuropeptide research. The amine procedure was applied to various pharmacological matrices including plasma, heart tissue and brain. The use of peptide HPLC as an analytical tool for various neuropeptides is illustrated by studies on des-tyrosine-gamma-endorphin (DT gamma E) metabolism in the brain and the stability of an ACTH (ORG-2766) analogue during a chronic infusion in rats. The power of HPLC as a research tool in peptide pharmacology is described, discussed and demonstrated as an aid in the understanding of the pharmacological effects of exogenous peptides and the function of the brain.

Analysis of behavioural responses to an ACTH analog in CXB/By recombinant inbred mice

Male mice of the C57BL/6By and BALB/cBy inbred strains, their reciprocal F1 hybrids, and 7 recombinant inbred strains, were tested for open-field activity, a shock-motivated successive reversal position discrimination problem in a T-maze, and a toggle box exploration task. The test battery was repeated one month later. Finally, mice were tested for the acquisition and extinction of a taste aversion conditioned by ethanol injection. Mice of each strain were tested after injection with saline or one of 3 doses of an ACTH analogue. Highly significant genotypic differences were found for all measures, an expected result. The strain distribution pattern seen in the toggle box suggested single gene mediation of exploratory activity after habituation. One aspect of avoidance responding and extinction of conditioned taste aversion also yielded strain distribution pattern consistent with single gene control. Peptide treatment reduced internal field crossings in the open field. This effect was not strain dependent. Peptide treatment had no effect on T-maze learning, conditioned taste aversion, or toggle-box exploration.

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.

A comparison of the ability of opioid peptides and opiates to affect active avoidance conditioning in rats

Enkephalins reduce acquisition of an active avoidance response when administered intraperitoneally shortly before training. The present study examined whether microgram or delta opiate receptors are involved in this enkephalin effect. This was done by comparing the efficacy of micro- and delta-receptor agonists; by attempting to block the enkephalin effect with micro- and delta-receptor antagonists; and by comparing the characteristics of the effects of Met-enkephalin and Leu-enkephalin. In addition, the efficacy of kappa-agonists in reducing acquisition was assessed. It was found that micro-agonists are inactive in this assay; several delta- and kappa-agonists are active. However, not all of the data are consistent with the adequacy of this receptor classification. The micro-receptor antagonist naloxone did not readily block the effect of Met- or Leu-enkephalin but neither did the micro/delta-antagonist, diprenorphine. An additional complexity is the emergence of differences in behavioral activity of Met- snd Leu-enkephalin.

Behavioral characteristics of vasopressin-deficient rats (Brattleboro strain)

A group of vasopressin-deficient rats (Brattleboro strain--DI) and a group of normal Long-Evans rats were successively evaluated on visual discrimination, olfactory discrimination, delayed alternation at short and long intertrial intervals (ITIs), approach-avoidance conflict in a straight runway, and open-field behavior. It was found that DI rats adapted more slowly than normal rats in the T-maze, in the straight runway, and they were slower to emerge into the open field. The DI rats were impaired relative to normal animals on the discrimination tasks (visual and olfactory), but they were not impaired on delayed alternation (at least for short ITIs). DI rats also showed better retention of the punishment effect in the approach-avoidance conflict test than normal animals. It is suggested that DI rats have defective reference-memory mechanisms, fairly intact working-memory processes and altered adaptability (timidity or cautiousness).

Effects of peptides related to neurohypophyseal hormones on ethanol tolerance

Mice were rendered tolerant to the hypothermic effect of ethanol by forcing them to inhale ethanol vapor for 3 days. One day after withdrawal, tolerance was assessed by determining the response of the mice to an acute 3 g/kg IP challenge dose of ethanol. Thirty minutes before the injection of ethanol, saline or peptide solution was SC injected. The peptides studied were des-Gly9-Arg8-vasopressin (a peptide with reduced peripheral endocrine activities), oxytocin, and analogs and fragments of these peptides. None of the peptides, with the possible exception of oxytocin, affected body temperature in naive animals or the acute hypothermic response to ethanol in non-tolerant mice. Des-Gly9-Arg8-vasopressin enhanced the expression of tolerance to ethanol hypothermia; shorter fragments of vasopressin did not share this effect. Oxytocin attenuated the expression of tolerance but this may been due to an interaction with the acute effects of ethanol.