María A. Carrasco

Beta-endorphin causes retrograde amnesia and is released from the rat brain by various forms of training and stimulation

The endogenous opiate peptide, beta-endorphin (0.4, 1.0, 2.0, and 10.0 μg/kg) was injected IP into rats immediately after training in a shuttle avoidance task, and its effect on memory retention was evaluated in test sessions carried out 24 h later. The drug was found to cause retrograde amnesia, the ED50 being 1.0 μg/kg. Beta-endorphin immunoreactivity was measured in the hypothalamus and rest of the brain of rats submitted to training, or test sessions of shuttle avoidance learning, pseudoconditioning in the shuttle-box, tones alone, or foot-shocks alone. After training in any of the four paradigms, there was a marked (46–60%) depletion of beta-endorphin immunoreactivity in the rest of the brain. No changes were detected in the hypothalamus or after test sessions. The loss of beta-endorphin immunoreactivity may be attributed to release of this substance caused by the stimuli used for training. From the present findings, as well as previous observations on the memory-facilitating influence of the opiate receptor antagonist, naloxone, it is concluded that there is a physiological amnesic mechanism mediated by beta-endorphin (and perhaps other opoid peptides as well), which is triggered by the non-associative factors present in the various forms of learning.

Effect of various behavioral training and testing procedures on brain β-endorphin-like immunoreactivity and the possible role of β-endorphin in behavioral regulation

Beta-Endorphin-like immunoreactivity is reduced in the rat diencephalon after the animals are exposed for the first time to any of the following behavioral situations: 50 tones (habituation), 50 tone-footshock shuttle avoidance trials, one step-down inhibitory avoidance trial, simple exposure to the avoidance apparatus with no footshocks, or inescapable shock. The effect is not observed when animals are exposed to any of these situations for a second time. The reduction of brain beta-endorphin-like immunoreactivity is attributable to release and subsequent metabolism of the substance, and correlates with the novelty inherent in the diverse training or test situations. The role of beta-endorphin in behavior is discussed in the light of these and previous results which showed that it causes both retrograde amnesia and a facilitation of retrieval. The substance would appear to serve an adaptive function when animals are exposed to a new experience, by inducing a temporary forgetting of the experience together with (or leading to) a state of alertness or preparedness for what may happen next.

The role of opioid peptides in memory and learning

Evidence is discussed which points to the existence of a physiologic amnesic mechanism mediated by beta-endorphin and perhaps by other opioid peptides as well. This mechanism is triggered by various forms of training and by either painful or painless stimulation. It may operate through the inhibition of central dopaminergic and beta-adrenergic systems that modulate the memory consolidation process. This amnesic mechanism in unrelated to the regulation of pain perception, and operates at opioid peptide levels several orders of magnitude below those that are needed to cause analgesia or other effects. In addition, shuttle avoidance and habituation learning seem to be dependent on a state induced by the release of beta-endorphin. It is possible that this may be related to the amnesic properties of this substance. Therefore, it appears that the endogenous opioid peptides may exert their primary function in the modulation of memory processes.

Naloxone reverses retrograde amnesia induced by electroconvulsive shock

Transcorneal electroconvulsive shock (ECS) (15.0 mA, 60 Hz, 2 sec) caused retrograde amnesia for a shuttle avoidance task in rats. The effect was completely reversed by the concomitant posttraining administration of naloxone (0.2 or 0.4 mg/kg, ip). It had been shown previously that ECS releases β-endorphin and Met-enkephalin in the rat brain. The present findings are consistent with the hypothesis that the amnestic effect of ECS may be mediated, at least in part, by the release of endogenous opioid peptides; however, actions upon other neurochemical parameters, particularly upon other neurotransmitter systems, cannot be excluded from that effect.

Effect of morphine, ACTH, epinephrine, Met-, Leu- and des-Tyr-Met-enkephalin on β-endorphin-like immunoreactivity of rat brain☆

Morphine (1,0 mg/kg), ACTH1-24 (10.0 micrograms/kg), epinephrine (12.0 micrograms/kg), Met-enkephalin (2.0 and 5.0 micrograms/kg), Leu-enkephalin (2.0 micrograms/kg) and des-Tyr-Met-enkephalin (2.0 micrograms/kg) all produced marked reductions of beta-endorphin-like immunoreactivity in the rat diencephalon. At a dose of 0.4 mg/kg, naloxone had no effect of its own and was unable to reverse the depleting effect of the other substances. The depletion of beta-endorphin-like immunoreactivity caused by the various treatments is attributable to release and subsequent degradation of beta-endorphin and/or of its precursors. The various behavioral effects of morphine, ACTH, epinephrine and the enkephalins may be explained by the release of endogenous beta-endorphin.

Effect of electroconvulsive shock on β-endorphin immunoreactivity of rat brain, pituitary gland, and plasma

Ten or thirty minutes after electroconvulsive shock (ECS) (15 mA, 60 Hz, 2 sec administered through transcorneal electrodes) there was a 30 to 53% reduction of β -endorphin immunoreactivity in the hypothalamus and in the rest of the brain of rats. The reduction is attributable to release and subsequent metabolization of the substance. No changes were detected in the pituitary gland or plasma. These results may mean that the amnestic or other effects of ECS are due to the release of β -endorphin in the brain.

Response of the rat brain β-endorphin system to novelty: Importance of the fornix connection

In control rats, a step-down inhibitory avoidance training trial using a 0.8 mA footshock, or simple exposure to the training apparatus without footshock, was followed by a decrease of beta-endorphin-like immunoreactivity measured in the hypothalamus and ventral thalamus. The effect of inhibitory avoidance training was also measured in rats submitted to a brain sham operation, to bilateral transection of the dorsal fornix, to anterior or to posterior hypothalamic deafferentation, to adrenal medullectomy, to an adrenal sham operation, to 16 daily ip injections of 0.2 mg/kg dexamethasone, or to 16 daily ip injections of 1 ml/kg saline. The diencephalic beta-endorphin-like immunoreactivity response to training was abolished by fornix transection and was unaffected by all other treatments. This suggests that the response is not mediated by anterior or posterior neural afferents to the hypothalamus, or by a hypersecretion of epinephrine by the adrenal medullae, or of ACTH by the pituitary gland. The response, instead, appears to require the integrity of the pathway that sends projections from the septo-hippocampal system to the hypothalamus. Previous evidence had suggested that the diencephalic beta-endorphin-like immunoreactivity response to training is a result of novelty, and the septo-hippocampal system has been postulated to play a role in the registration of novelty.

Effect of tones, footshocks, shuttle avoidance, and electroconvulsive shock on met-encephalin immunoreactivity of rat brain

Met-encephalin immunoreactivity is measured in the amygdala, hypothalamus, and rest of the brain of rats submitted to a 25-min session of 50 footshocks (1.0 mA, 2 sec, 60 Hz), or 50 tones (1 kHz, 5 sec, 70 db), or 50 tone-footshock shuttle-avoidance trials; or sacrificed 10 or 30 min after ECS (15.0 mA, 2 sec, transcorneal). Footshock stimulation and electroconvulsive shock (ECS) cause a reduction of Met-encephalin immunoreactivity in the amygdala and hypothalamus, attributable to release and subsequent degradation of the substance. The other treatments have no effect. The data are consistent with hypotheses that relate endogenous Met-encephalin to pain and/or stress, and with a previously advanced speculation that the amnesic effect of ECS might be mediated by endogenous opioid peptide release.

β-Endorphin like immunoreactivity of brain, pituitary gland and plasma of rats submitted to postnatal protein malnutrition: Effect of behavioral training

Wistar-derived rats were raised and maintained either on a normal- (25% casein) or on a low-protein (8% casein) diet until the age of 100 to 114 days. Both diets were isocaloric and contained an adequate supply of salts and vitamins. There were gross differences in body, brain and pituitary weight between the two groups. In addition, the brain and pituitary content of beta-endorphin like immunoreactivity was lower in the protein malnourished rats, and three different forms of training (50 tone-footshock shuttle avoidance trials; 50 tones alone (habituation); 50 footshocks alone) caused a depletion of brain beta-endorphin like immunoreactivity in the normal, but not in the malnourished rats. Footshock stimulation caused, in addition, a pituitary decrease and a plasma increase of beta-endorphin like immunoreactivity, also restricted to the normal diet group. Performance in the habituation and in the shuttle avoidance tasks was similar in the two groups, despite the different responsiveness of their brain and pituitary beta-endorphin systems to training and/or stimulation. In view of the possible involvement of these systems in learning suggested by these and by previous data, it seems likely that the neurohumoral regulation of habituation and avoidance learning may be different in rats submitted to protein malnutrition when compared to controls.

Effect of naloxone, haloperidol and propranolol on cyclic 3′,5′-adenosine monophosphate content of rat amygdala

Naloxone (0.4 mg/kg, i.p.) causes an increase of cyclic adenosine monophosphate levels in the amygdala, but not in the hippocampus, caudate, or hypothalamus, of rats. The effect is antagonized by haloperidol (0.5 mg/kg, i.p.) and by propranolol (0.5 mg/kg, i.p.). This is consistent with the hypothesis of a tonic inhibitory influence of endogenous opiates on central dopaminergic and beta-noradrenergic systems. Haloperidol had an effect of its own on amygdala cyclic adenosine monophosphate levels which was blocked by propranolol. This suggests the possibility of an antagonistic interaction between dopaminergic and beta-noradrenergic innervation on this structure.