Peptide mimetics of thyrotropin-releasing hormone based on a cyclohexane framework: design, synthesis, and cognition-enhancing properties.

The design and synthesis of peptide mimetics of thyrotropin-releasing hormone (TRH) in which the peptide backbone is entirely replaced by a cyclohexane framework are described. The cis-1,3,5-trisubstituted ring was expected to permit key pharmacophoric groups to adopt conformations consistent with proposed bioactive conformations of the peptide. Compounds were synthesized by a stereoselective synthesis starting from L-glutamic acid. In a behavioral model of cognition in which TRH is active, the mimetics are potent, active compounds, exhibiting oral activity. One analog (26, (1S,3R,5(2S),5S)-5-[[5-oxo-1-(phenylmethyl)-2-pyrrolidinyl]-methyl]-5- [(1H-imidazol-5-yl)methyl]cyclohexaneacetamide) was radiolabeled for binding studies and evaluated in other binding assays and pharmacological tests. Competition binding of 26 vs [3H]MeTRH to rat brain slices suggests a two-site model for ligand binding with IC50's of 1 microM and 3 mM. Direct binding of [3H]-26 shows a biphasic curve with IC50's of 80 and 49 microM, respectively. Further studies would be needed to establish a link between the novel binding site(s) and the behavioral activity of 26 and TRH analogs.

AF102B, a novel M1 agonist, enhanced spatial learning in C57BL/10 mice with a long duration of action.

Orally administered AF102B, a selective muscarinic M1 cholinergic agonist, improved spatial learning in C57BL/10 mice in the Morris water maze. In four experiments in which all drug-treated animals received only one single administration of AF102B, improvement of acquisition depended on two factors: pretreatment time (tp) and dose. When a standard tp of 1 h was used, AF102B exhibited a U-shaped dose-response curve that is characteristic of many nootropic agents: learning was significantly improved by dose levels ranging from 0.1 to 1 mg/kg p.o. When the tp was extended out to as long as 8 days, two new effects emerged: (a) 1 mg/kg, the dose that had been the peak active dose at 1 h, exhibited a biphasic time course of action, being active at 1 h or at all tp intervals from 3 h to 5 days, but not at 1.5 h; (b) 0.03 mg/kg, a dose that had been inactive at a tp of 1 h, was active at all tp intervals from 3 h to 5 days, but not at shorter (1 and 2 h) or longer (6-8 days) tp intervals. In another experiment, animals received 0.03 mg/kg for 1-5 consecutive days: this dose level was active if the tp interval between the last dose and the learning session was 24-120 h, but not if it was only 1 h. Thus AF102B enhanced cognition in mice with a longer duration of action than reported for traditional muscarinic agonists.

TRH protection against memory retrieval deficits is independent of endocrine effects.

An electrobrainshock (EBS)-induced memory retrieval deficit was produced in normal and hypophysectomized mice. In normal mice, thyrotropin-releasing hormone (TRH) (0.1 to 30 mg/kg) protected against this EBS disruption of memory after intraperitoneal but not oral (1.0 to 100 mg/kg) administration. In hypophysectomized mice, TRH (0.3 and 3.0 mg/kg) also protected against the retrieval deficit induced by EBS. The memory protection afforded by TRH was unrelated to its ability to elevate plasma levels of triiodothyronine (T3) and thyroxine (T4), nor was TRH's memory protection mediated through an anticonvulsive mechanism. These results support the notion that TRH may play an important role in memory modulation and may have therapeutic value in certain disease states in humans.

Effect of centrally and peripherally administered beta-endorphin on food intake in rats.

A role for beta-EP in the regulation of food intake has been suggested as a contributory factor in the obesity of some genetically obese animal models. Studies undertaken to determine whether continuous administration of beta-EP could alter food intake in normal rats are described. The present studies demonstrated that continuous subcutaneous infusion with beta-EP was ineffective in modulating food intake, but that acute intraperitoneal or intracerebroventricular administration stimulated food intake in previously food deprived or satiated animals, respectively. These results suggest that beta-EP is not involved in the long-term regulation of food intake, but under certain conditions it may play some role in the regulation of individual meals. It is speculated that the latter activity may result from the action of other appetitive regulatory hormones.

Stomach acid secretion in submissive and aggressive rats.

These studies were conducted to determine whether aggressive and submissive behavior are related to either an increase or a decrease in gastric secretion. In Experiment 1, intruder rats placed in an established male-female colony and attacked by a dominant alpha male secreted less acid than intruders exposed to nonaggressive males and females. In Experiment 2, intruders exposed to attack and subsequently returned to the encounter site, but protected from physical attack, still demonstrated a gastric hyposecretion. Rats with chronic gastric cannulas in Experiment 3 also revealed an acid inhibition when attacked and later when exposed to, but protected from, attack. Both intruders and attacking males were prepared with gastric cannulas in Experiment 4. Both demonstrated secretory inhibition following attack and attack-protected sessions. The inhibitory effect was greater and more persistent for intruder rats than for aggressive rats. The inhibition occurring during the attack-protected sessions may have been mediated by some conditioning processes. Other associative mechanisms are discussed, and the present results are also compared with those of relevant clinical reports.

Daily feeding schedule and housing on incidence of activity-stress ulcer.

In Experiment 1 the number and size of ulcers resulting from exposure to an activity-stress procedure were inversely related to the length of the pre-stress experience with either 1 hr or 2 hr daily feeding schedules. In Experiment 2, rats housed in group cages during the pre-stress period were more vulnerable to the ulcerogenic effects of the activity-stress procedure. A pellet food or powdered food treatment condition failed to provide significant group differences. Vulnerability to stress-ulcer is discussed in terms of the disparity of the environmental conditions between the acclimation period and the activity-stress period.

Cimetidine and stress ulcer in aged rats.

Male Sprague-Dawley rats, 2, 6, 11, 18, and 26 months old, were subdivided into four drug treatment groups and received either 25, 50, 100 mg/kg of cimetidine or a placebo control before being exposed to restraint plus cold stress for 3 hr. Cimetidine, at all dose levels, significantly reduced ulcer severity in all age groups except the 18- and 26-month-old rats. Senescent rats were not more susceptible to restraint ulcer and cimetidine had the least anti-ulcer effect with the senescent 26-month-old rats.

Aggression, body temperature, and stress ulcer.

Female rats which were exposed to supine restraint plus cold for 3 hr and were able to bite a passing nylon brush, developed fewer gastric lesions as compared to control rats which were similarly restrained but did not have access to the aggressive biting response. A second study, wherein rats were exposed to two restraint sessions, replicated the results obtained from the first experiment. Core body temperature measures revealed that rats with access to the biting response were more successful in maintaining body temperature. The protective effect of aggression may thus be due to the reduction in restraint hypothermia and not necessarily the affective qualities of the aggressive response per se.

Post-stress development and healing of supine-restraint induced stomach lesions in the rat.

In Experiment 1, rats were subjected to 3 hr of supine restraint and sacrificed either immediately, 30, 60, 90, 120 or 180 min following restraint. Rats sacrificed 90 min after restraint revealed significantly more stomach lesions as compared to other treatment conditions. The healing rate for supine-restraint ulcers was observed in Experiment 2 and comparisons with conventional restraint procedures, as reported in other publications, would suggest a slower healing rate for lesions induced with supine restraint. Experiment 3 indicated that cimetidine significantly accelerated the rate of healing for supine-restraint lesions.

Comparison of pyloric ligation and pyloric cuff techniques for collecting gastric secretion in the rat.

A comparison was made between two methods for collecting gastric secretion in the rat. The two included a pyloric ligation technique and a snare-type pyloric cuff which was surgically implanted 2 weeks before the collection was initiated. It was hypothesized that the surgical trauma associated with pyloric ligation would inhibit gastric secretion and thus yield smaller gastric samples with this procedure as compared to the cuff technique. No differences in volume of gastric secretion or total acid output were observed between the two methods.

Age differences and stress ulcer in the rat.

Rats 2-, 7- and 12-mo. old were exposed to supine body restraint plus cold (5 degrees C) for 3 hr. The mean cumulative length of lesions for the 2-, 7-, and 12 mo. old rats was 22.0, 43.0 and 16.0 mm. respectively. The same experimental design was used in a second study, but the pylorus was ligated prior to restraint. Total acid output/hr. was 134.6, 178.2, and 64.7 muEq/60 min. respectively for the three age groups. Older rats were not more susceptible to stress-ulcer and gastric acid was not significantly related to degree of ulceration.

Ascorbic acid and stress ulcer in the rat.

Rats were orally administered ascorbic acid at a dose of 30 g/liter during either total starvation, partial starvation, the activity-stress ulcer procedure, or the restraint-cold procedure. In four experiments, ascorbic acid failed to exert significant protective action against stomach ulcer formation and, in fact, may have potentiated the ulcerogenic process.

Effects of cimetidine on stress ulcer and gastric acid secretion in the rat.

Cimetidine at 25, 50, and 100 mg/kg significantly inhibited gastric acid secretion in rats with chronic gastric cannulas. Rats receiving either 50 or 100 mg/kg of cimetidine secreted significantly less gastric acid 3 hr after injection. Cimetidine failed to reduce the number or size of gastric lesions in rats exposed to the activity-stress procedure, but cimetidine at 100 mg/kg significantly reduced the number and size of gastric lesions in rats subjected to a supine restraint procedure.

Vendor differences in starvation-induced gastric ulceration.

The incidence of starvation-induced gastric lesions was observed in Sprague-Dawley rats obtained from four different vendors (ARS/Sprague-Dawley, Hilltop, Holtzman, and Charles River). Food was withheld for 5 days from rats weighing 150, 200, 260 or 330 g. Glandular lesions occurred in 150 g rats; rumenal lesions occurred in 200+ rats. ARS/Sprague-Dawley rats developed more glandular lesions, whereas Hilltop rats developed more rumenal lesions.

Preparation of a chronic gastric fistula in the rat.

A stainless steel gastric cannula was surgically implanted in the rat forestomach to allow gastric secretion to be collected continuously without restraining the animal. When gastric collections were not being made, the cannula was closed with a plugging screw. Rats with these cannulas were easily maintained without special care or complications for 12 months.