Appetitive acquisition and extinction effects with exogenous ACTH

Alzheimer's disease drugs: an application of the hormetic dose-response model

This article provides an evaluation of the dose-response features of drugs that are intended to improve memory, some of which have been used in the treatment of Alzheimer's disease (AD). A common feature of these drugs is that they act via an inverted U-shaped dose response, consistent with the hormetic dose response model. This article assesses historical foundations that lead to the development of AD drugs, their dose-response features and how the quantitative features of such dose responses affected drug discovery and development, and the successes and possible failures of such agents in preclinical and clinical settings. This story begins about 150 years ago with the discovery of an active agent in the Calabar bean plant called physostigmine, its unfolding medical applications, and its implications for dose-response relationships, memory enhancement, and improved drug discovery activities. The article also demonstrates the occurrence of U-shaped dose responses for memory with numerous endogenous agonists including neurosteroids, various peptides (e.g., vasopressin, CCK-8, neuropeptide Y), and other agents (e.g., epinephrine, antagonists for platelet activity factor and nicotinic receptors), supporting the generalizability of the hormetic biphasic dose response. Finally, the significance of the U-shaped dose response is critical for successful clinical application, since it defines the therapeutic window.

Effects of ACTH and ACTH 4-10 on aversive memory retrieval in rats

The aim of the study was to examine whether ACTH and ACTH-fragment 4-10, given before the test would produce a selectively enhanced retrieval of aversive memories, in the same way as preexposure to inescapable footshocks, in rats. For this purpose animals conditioned in a T-maze with appetitive (10% sucrose) and aversive (2.0 mA footshock) events were administered (s.c.) a single dose of 10, 20 or 40 ug/rat of ACTH or 5, 10 or 20 ug/rat of ACTH-fragment 4-10, 20-min before testing. The retention test conducted in the same training apparatus 72-hrs after conditioning showed a dose-dependent increase in latencies to enter the previously shocked goalarm with the absence of such a difference in responding to the nonshocked goalarm, in ACTH and ACTH 4-10 treated groups. This differential response was not observed in saline treated rats. This effect of peptides on memory retrieval was similar to that seen following inescapable footshock in rats. The results suggest the possible involvement of ACTH in the differential enhancement of memory of helplessness condition.

Effects of central alpha-MSH injections on performance in a cued discrimination task

Alpha-MSH has been implicated in changing attention behavior following peripheral injections, but no brain sites were studied. In the present report, alpha-MSH was injected directly into specific sites in the medial anterior hypothalamic/preoptic area (MAHPOA) while measuring performance in a visually cued discrimination task. Alpha-MSH injections resulted in reduced errors, indicated by decreased responding during noncued intervals, but no change in responding to correct cues. The improved error rate was consistent with attentional changes in a variety of paradigms. Attentional and motivational parameters were differentiated. The injected alpha-MSH appears to act on an inhibitory component of an attentional mechanism.

Free-choice ethanol consumption by rats: effects of ACTH4-10

The individual and interactive effects of immobilization stress, ACTH11-24 and ACTH4-10 on the free-choice consumption of ethanol in rats were studied. Stress and ethanol both result in activation of the HPA axis and release of ACTH1-39. The animals were offered a two-bottle choice consumption of 0.2% saccharin and 10% ethanol. They were exposed to immobilization stress or IP injections of ACTH4-10 or ACTH11-24 on an irregular, unpredictable schedule. Stress resulted in a decrease in ethanol consumption during the stress period while ACTH11-24 was devoid of any effect. The ACTH4-10 fragment produced an almost complete block of ethanol consumption during the injection period. Saccharin consumption was not affected by any of the above treatments. These results suggest an important role for ACTH4-10 (produced from ACTH1-39) in ethanol consummatory behavior in rats.

Arginine vasopressin enhances retention of morphine tolerance

The hot plate method was used to assess tolerance in rats following daily injections of morphine. Following analgesia assessment, or a time equated rest period, rats were injected with either saline or a pituitary peptide. Arginine vasopressin, but not ACTH 4-10, prolonged the retention of morphine tolerance when assessed five weeks after the last injection. Neither the rate nor the degree of tolerance development were influenced by either peptide. These hormones had no effect on retention of tolerance development were influenced by either peptide. These hormones had no effect on retention of tolerance in rats not assessed for analgesia during the period of tolerance development. The effects of pituitary peptides on morphine tolerance are analogous to the effects they have on learning and memory processes, suggesting that similar adaptational processes are occurring in both phenomena.

The neurochemistry and pharmacology of extinction behavior

The role of various neurotransmitter systems in the brain in extinction behavior is examined. An attempt is made to suggest psychological mechanisms (such as attention, secondary reinforcement or internal inhibition) by which the neurotransmitter systems or drugs act to produce the observed alteration in extinction behavior. The putative neurotransmitters acetylcholine, noradrenaline, dopamine, serotonin, endorphins and the peptides are reviewed, as are pharmacological agents such as the benzodiazepines, the barbiturates, the psychodelics, the neuroleptics, the psychomotor stimulants and cannabinoids. Other treatments and factors are considered such as peripheral hormones and the adrenal-pituitary axis. It is suggested that the noradrenergic system may be involved in the expression of extinction behavior by a role in selective attention, the dopamine system via an involvement with secondary reinforcement, the cholinergic system by a mechanism of response inhibition and the barbiturates and benzodiazepines by a block of nonreward.

Influence of MSH and corticosterone on stimulus control over an operant response

Several previous papers have indicated that MSH/ACTH-like neuropeptides facilitate the reversal of brightness discrimination in a runway task. The present experiment was undertaken to explore the development during both acquisition and reversal of stimulus control over an operant response. Male albino adult rats were treated daily with either 10 micrograms of MSH, 500 micrograms of corticosterone or a placebo in a double blind procedure during the establishment of an on-light control over availability of reinforcement. Having completed this regime the conditions were reversed so that reinforcement was present only when the chamber-light was off. The results indicated that treatment with MSH during acquisition produced a significant decrease in % correct responses compared to animals treated with either the placebo or corticosterone with neither of these latter groups differing from the other. During the reversal phase of training there was a trend for both MSH and corticosterone to lower % correct responses. The discussion focuses on other literature which has shown opposite effects of corticosterone on aversive and appetitively motivated tasks.

The modulation of the dorsal immobility response in the adult male Wistar rat: the opposite effects of ACTH4-10 and [D-Phe7]ACTH4-10

A series of experiments were conducted to determine the behavioral effect of ACTH4-10 and [D-Phe7]ACTH4-10 on the dorsal immobility response (DIR) in adult male Wistar rats. Both peripheral and central injections of ACTH4-10 attenuated the duration of DIR, whereas, peripherally injected [D-Phe7]ACTH4-10 resulted in the potentiation of this response. The effect of peripherally administered ACTH4-10 was dose dependent in the range of 0.2 to 20 micrograms. These results, in part, extend the prior findings on the modulation of various learned and species-typical behaviors by ACTH-related peptides to a complex state of behavioral inhibition and immobility.

Alpha-melanocyte stimulating hormone facilitates learning of visual but not auditory discriminations

The neuropeptide alpha-MSH has been proposed to influence learning and memory by increasing visual attention. To test the possibility that MSH selectively affects visual learning, rats were tested in learning tasks in which the cues were either visual or auditory. Maze and bar-press tasks were used. MSH administration increased the rate of learning of the visual tasks, regardless of the task difficulty or the type of response required of the rat. MSH had no effect on the rate of learning of the auditory tasks. These results support the hypothesis that MSH facilitates learning by influencing some aspect of visual information processing.

Social and exploratory behaviour in the rat after septal administration of ORG 2766 and ACTH4-10

The time spent in active social interaction by pairs of male rats in a dimly-lit, familiar test arena, was decreased significantly after intraseptal injection of ACTH4-10 (250-500 ng). In contrast, the time spent in active social interaction was increased significantly after intraseptal injection of the tri-substituted ACTH4-9 analogue ORG 2766 (250-500 pg). Neither ACTH4-10 (250 ng) nor ORG 2766 (250 pg) affected exploration or locomotor activity measured in a holeboard, after intraseptal injection. Intraseptally administered ACTH4-10 (250 ng) increased aggression measured in a colony intruder model, but ORG 2766 (250 pg) was without effect.

Effect of vasopressin-like peptides on consumption of ethanol by the rat

Rats were trained to accept ethanol in their drinking water, by successive small increments or decrements in alcohol concentration in response to the individual consumption of each rat. Those injected with desglycinamide9-lysine8-vasopressin (DGLVP), 1--4 microgram SC every second day, attained almost twice as high a final acceptance concentration (FAC) and mean daily ethanol intake (g/kg) as vehicle-treated controls. Hypophysectomized animals initially accepted the same alcohol concentrations as intact rats, but drank much larger volumes and correspondingly higher daily g/kg intakes. However, this was rapidly succeeded by rejection of all but very low concentrations, which was unaffected by DGLVP. During a subsequent free-choice period (water vs. ethanol at individual FACs), the groups maintained their relative positions with respect to ethanol intake. This was not altered by injection of vasopressin in the hypophysectomized rats, but was overcome by raising the alcohol concentration. The results suggest that vasopressin-like peptides facilitate acquisition of alcohol drinking behavior.

Behavioural effects of naloxone in rats

Naloxone in rats induces a behavioural syndrome closely resembling that induced by intraliquorally injected ACTH peptides. This effect is probably due to a displacement of the ACTH peptides from other receptors (e.g. opiate receptors).

Effect of ACTH 4-10 on copulatory bahavior and on the response for socio-sexual motivation in the female rat

Female copulatory behavior and the urge of a female rat to seek contact with a sexually active male was studied after treatment with the peptide ACTH 4-10. No effects of this peptide were seen on the copulatory response of ovariectomized females treated with estradiol benzoate (EB) alone or EB and progesterone combined. An increasing barrier technique was used to measure how much of an average stimulus (cresssing an electrified grid) a female was willing to endure to gain contact with sexually active male. It has been shown in previous investigations (15) with ovariectomized rats that after a certain time of adaptation and training in this apparatus the response attained remains at a certain basal level from which it is increased by treatment with EB. Animals treated with ACTH 4-10 during the phase of adaptation and training attained a significantly higher basal response level than controls. This effect was only observed when the peptide treatment covered the period of adaptation and was not seen when the treatment was started after this critical period. The EB-induced response was not influenced by ACTH 4-10 given during the period of adaptation or given in connection with the EB treatment. It is concluded that ACTH 4-10 influenced the acquired response level but had no effect on steroid induced responses, which are probably controlled by more innate mechanisms.

Reversal of amnesia by an orally active ACTH 4-9 analog (Org 2766)

The ACTH 4-9 analog, H-Met((O2)-Glu-His-Ph-D-Lys-Phe-OH (Or 2766), attenuates in rats CO2-induced amnesia for a one-trial passive avoidance step-through response when administered prior to the retrieval test but not when given prior to acquisition. Even a dose of 0.001 mug/rat Org 2766 yields an anti-amnesic effect. In this respect Org 2766 is more active than the ACTH fragment ACTH 4-10. An anti-amnesic effect was also obtained when Org 2766 was administered orally. ACTH 4-10 (100 mug/rat) has to be given SC within 8 hr of the retrieval test in order to be effective. A similar time span of effectiveness was observed when Org 2766 was SC injected in a dose of 0.1 mug/rat. The anti-amnesic effect of ACTH 4-10 remains when the time interval between acquisition and retrieval is extended beyond the usual 24 hr. The same appeared to be true for SC ADMINISTERED Org 2766. It is suggested that ACTH-like peptides, and particularly the orally active Org 2766, may be helpful in the treatment of deficient mental performance.