Delayed onset of the amnestic effect of posttraining beta-endorphin: effects of propranolol administered prior to retention testing

Revisiting and revising memory consolidation: Personal reflections on the research legacy of Ivan Izquierdo

Two important themes in Ivan Izquierdo's research each offered both answers and questions about the topic of memory formation and maintenance. The first theme provided evidence supporting the view that short- and long-term memory were distinct processes and could be selectively modulated by several treatments, with some affecting only short-term, others only affecting long-term memory, and still others affecting both. Over many years, Izquierdo's laboratory documented molecular responses across time after training obtaining results that showed differences as well as similarities in the biochemical changes during the first 1-2 hours and the next 4-6 hours after training, i.e., during the transition from short- to long-term memory. This work clarified the biological underpinnings of the memory processes. The second theme described waves of susceptibility of memory to enhancing and impairing treatments after time, a biphasic profile that contrasted with earlier monotonic decreases in the efficacy of memory modulating treatments as a function of time between training and treatment. Remarkably, these waves of susceptibility to modification were accompanied by biphasic changes in molecular measures at similar times after training. Remarkably, some of the molecular players exhibited persistent changes after training, with increases in levels lasting days following the training experience. These persistent molecular changes may reveal a biological basis for the dynamic nature of memories seen long after the initial memory is consolidated.

Corticotropin-releasing factor and noradrenergic signalling exert reciprocal control over startle reactivity

Corticotropin-releasing factor (CRF) and norepinephrine (NE) levels are altered in post-traumatic stress disorder and may be related to symptoms of hyperarousal, including exaggerated startle, in these patients. In animals, activation of both systems modulates anxiety behaviours including startle plasticity; however, it is unknown if they exert their actions orthogonally or dependently. We tested the hypothesis that NE receptor activation is required for CRF effects on startle and that CRF1 receptor activation is required for NE effects on startle. The study examined the effects of: (1) α2 agonist clonidine (0.18 mg/kg i.p.), α1 antagonist prazosin (0.8 mg/kg), and β1/2 antagonist propranolol (0.8, 8.0 mg/kg) pretreatment on ovine-CRF (oCRF)- (0.6 nmol) induced increases in startle reactivity and disruption of prepulse inhibition (PPI); (2) α2 antagonist atipamezole (1-30 mg/kg) and α1 agonist cirazoline (0.025-1.0 mg/kg) treatment on startle; (3) CRF1 antagonist (antalarmin, 14 mg/kg) pretreatment on atipamezole- (10.0 mg/kg) induced increases in startle. oCRF robustly increased startle and reduced PPI. Pretreatment with clonidine or prazosin, but not propranolol, blocked oCRF-induced increases in startle but had no effect on oCRF-induced disruptions in PPI. Atipamezole treatment increased startle, which was partially attenuated by CRF1 antagonist pretreatment. Cirazoline treatment did not increase startle. These findings suggest that CRF modulation of startle, but not PPI, requires activation of α1 adrenergic receptors, while CRF1 activation also contributes to NE modulation of startle. These data support a bi-directional model of CRF-NE modulation of stress responses and suggest that both systems must be activated to induce stress effects on startle reactivity.

Orphanin FQ/nociceptin interacts with the basolateral amygdala noradrenergic system in memory consolidation

Extensive evidence indicates that the basolateral complex of the amygdala (BLA) mediates hormonal and neurotransmitter effects on the consolidation of emotionally influenced memory and that such modulatory influences involve noradrenergic activation of the BLA. As the BLA also expresses a high density of receptors for orphanin FQ/nociceptin (OFQ/N), an opioid-like peptide with anxiolytic and amnestic properties, the present experiments investigated whether the BLA is involved in mediating OFQ/N effects on memory consolidation and whether such effects require noradrenergic activity. OFQ/N (0.01-100 pmol in 0.2 microL) administered bilaterally into the BLA of male Sprague-Dawley rats immediately after aversively motivated inhibitory avoidance training induced dose-dependent impairment on a 48-h retention trial. The beta(1)-adrenoceptor antagonist atenolol (2.0 nmol) administered concurrently into the BLA potentiated the dose-response effects of OFQ/N. In contrast, immediate post-training infusions of the peptidergic OFQ/N receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2) (1-100 pmol in 0.2 microL) into the BLA enhanced 48-h retention of inhibitory avoidance training, an effect that was blocked by coadministration of atenolol. Delayed infusions of OFQ/N or [Nphe(1)]nociceptin(1-13)NH(2) into the BLA administered either 6 or 3 h after training, respectively, or immediate post-training infusions of OFQ/N into the adjacent central amygdala did not significantly alter retention performance. These findings indicate that endogenously released OFQ/N interacts with noradrenergic activity within the BLA in modulating memory consolidation.

Effect of naltrexone upon self-injurious behavior, learning and activity: a case study

Naltrexone significantly attenuated self-injurious behavior in a 20-year-old mildly retarded autistic male patient. The patient was videotaped daily and behavior was evaluated with a time-sampling procedure. Behavioral ratings of SIB frequency, SIB severity, and activity were collected automatically with a computerized system. Learning and memory were tested on a weekly basis with a modification of a paired associate learning test (PALT). Treatment with naltrexone resulted in (a) attenuation of SIB in the unstructured setting and (b) improvements in learning and memory without influencing activity levels.

Chronic opioids impair acquisition of both radial maze and Y-maze choice escape

Chronic morphine impaired acquisition of two dissimilar behavioral tasks. In the radial maze, the performance of saline-treated and morphine-treated groups diverged with the latter failing to improve despite extensive training. In contrast, rats treated with naltrexone became skilled in the procedure 2-4 times as rapidly as saline controls. Withdrawal of treatment significantly improved performance of morphine-treated rats, with no change for rats treated with saline or naltrexone. When a second group of rats was extensively trained prior to instituting chronic morphine treatment, performance scores were not affected, suggesting that morphine does not impair spatial working memory despite subjective evidence of other gross behavioral effects, such as ataxia. In the Y-maze choice escape task, acquisition of a response strategy was significantly impaired in rats that had been previously treated with morphine for 17-21 days, despite clear indications that morphine-treated rats were sensitive to the aversive stimulus.

Adrenoreceptor antagonist treatment influences recovery of learning following medial septal lesions and hippocampal sympathetic ingrowth

Previous studies have demonstrated that in male rats hippocampal sympathetic ingrowth (HSI), which is induced by medial septal lesions (MS), is detrimental to recovery of spatial learning. The present study was performed in an attempt to determine if this effect was mediated through adrenergic receptor activity. Adult male Sprague-Dawley rats underwent training on a modified version (i.e., 4 arms baited) of a radial-8-arm maze task. Following attainment of learning criterion animals underwent one of three surgical procedures: CON (sham surgeries); MSGx (MS + superior cervical ganglionectomy--to prevent HSI); MS (MS + sham ganglionectomy). Reacquisition trials were performed in the same manner as initial acquisition except animals were treated with vehicle, propranolol (20 mg/kg), or phentolamine (20 mg/kg) 30 minutes prior to testing. As expected, vehicle-treated MS animals took longer to reacquire the task than MSGx animals, who were in turn more impaired than CON animals. Propranolol (beta-adrenergic antagonist) treatment impaired performance of both the MS and MSGx group, but did not alter the CON group. Phentolamine (alpha-adrenergic antagonist) increased the number of trials to reattain criterion in the CON group, had no effect in the MSGx group, and markedly improved performance in the MS group. The results suggest that HSI mediates its detrimental effects through alpha-receptors, while beta-blockade, in the setting of brain injury, is detrimental to performance regardless of the presence or absence of HSI.

Different forms of post-training memory processing

Memories are not acquired in their definitive form, but can be considerably modified in the period that follows after acquisition, both quali- and quantitatively. This may happen either by a process of consolidation or strengthening of each memory trace or by the incorporation of further information to each experience. This further information may be provided by the action of drugs, including that of endogenous substances released by each training experience, and by the addition of information provided by other tasks or events. Evidence in favor of the existence of these various post-training processes, and of their importance for memory formation, is discussed. All of these processes are time dependent, all may depend on stimulus aftereffects, each is differently affected by drugs given in the post-training period, and all may interact with each other.

Antagonism of endogenous opioids modulates memory processing

The studies reported here demonstrate that opioid antagonism enhances memory in two classes of animals viz. Aves and Mammalia. In mice, immediate posttraining administration of naloxone produces a time-dependent improvement in retention tested one week later. This effect is stereospecific. As naloxone was approximately 1000-fold more potent when administered intracerebroventricularly compared to subcutaneously, it appears that it produces its effect within the central nervous system. Pretest administration of naloxone, at a dose that failed to alter acquisition, also improved test performance, suggesting that naloxone also improved recall. Similar improvement in retention was demonstrated with the more potent opioid antagonist, nalmefene, at a 500-fold lower dose. The dose response to naloxone in both the mouse and the chick and to nalmefene in the mouse had the characteristics of an inverted U, with high doses either being ineffective or suppressing memory retention. In mice, naloxone demonstrated anti-amnestic properties against both anisomycin, a protein synthesis inhibitor, and scopolamine, an acetylcholine receptor blocker. Administration of beta-funaltrexamine (B-FNA) 72 h prior to training did not alter acquisition but did enhance retention. In studies where the mu-opioid receptor was blocked with B-FNA, naloxone was unable to enhance retention. B-FNA failed to impair the memory enhancing properties of arecoline, fluoxetine or clonidine. This demonstrates specificity of the B-FNA ability to prevent naloxone from enhancing memory and suggests that the opioid antagonist effects on memory are mediated by the mu-receptor.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.

Differential effect of posttraining naloxone, beta-endorphin, leu-enkephalin and electroconvulsive shock administration upon memory of an open-field habituation and of a water-finding task

Rats were trained and tested in an open field for habituation of rearing responses, for a water-finding task, or for both tasks simultaneously. Training-test interval was 24 hr. The water-finding task consisted of locating a metal tube in one of the walls of the box, which was attached to a water bottle on the outside; animals were water deprived between training and testing. Retention was estimated by measuring the latency to lick from the tube on the test session. Rats learned this task either with or without water deprivation, also prior to training. Habituation learning (reduction of the number of rearings between the training and test session) occurred either simultaneously with the water-finding task or in animals trained without the water tube, so that they could not learn the water-finding task. As happens with many other tasks, training in the open field was followed by a large decrease of hypothalamic beta-endorphin immunoreactivity, attributable to a release of this substance. Posttraining IP naloxone (1.6 mg/kg) administration facilitated, and posttraining beta-endorphin (2.0 micrograms/kg), leu-enkephalin (5.0 micrograms/kg), or electroconvulsive shock (15 mA, 60 Hz, 2 sec) depressed the retention of habituation; this occurred regardless of whether the animals were trained and/or tested with or without water deprivation, and whether the task was acquired alone or simultaneously with the water-finding task. By contrast, none of these treatments had any effect on retention of the water finding task, acquired either with or without prior water deprivation. Thus, habituation was, and water-finding was not, sensitive to posttraining treatments known to affect endogenous opioids: the opioids themselves, their antagonist, naloxone, and electroconvulsive shock which releases brain opioids and causes naloxone-reversible retrograde amnesia. Learning of the water-finding task was merely incidental to exploration of the open field; it took place even when the animals were trained without the water tube. This suggests that the posttraining treatments that affect endogenous opioid function affect memory only of the task(s) that actually cause the release of brain beta-endorphin (in this case, probably habituation), and not of others that may occur simultaneously but are merely incidental (water-finding). A feature apparently common to the former is that they must directly involve either the recognition of novelty, or the initiation of an interaction with a new environment, or perhaps the habituation of such interaction.

The brain beta-endorphin system and behavior: the modulation of consecutively and simultaneously processed memories

This article shows how new memories may be acquired, or old ones retrieved, in animals who have recently released brain beta-endorphin and why some memories may be insensitive to beta-endorphin modulation even if they are processed in parallel to others that are sensitive to that system. In addition, one example is given of an interaction between tasks that is possibly independent from the brain beta-endorphin system. The data examined here represent a new approach to memory modulation in that they apply findings previously obtained in isolated tasks to the analysis of interactions between training and test sessions of the same or different tasks. This may be viewed as a step toward understanding the organization of memory mechanisms in everyday behavior, which consists of consecutive and simultaneous, rather than isolated, behavioral training and test paradigms. The present approach, however, still relies on the individual analysis of separate behaviors, and is therefore no substitute for studies on complex behaviors per se. Indeed, both approaches may be complementary for a full understanding of the organization of memory processes, along with further investigations studying isolated tasks.

Effect of a novel experience prior to training or testing on retention of an inhibitory avoidance response in mice: involvement of an opioid system

These experiments examined the effects of a novel experience prior to training or retention testing on 24-h retention of an inhibitory avoidance response in mice. The experiments were based on previous evidence that novel training experiences release hypothalamic beta-endorphin. When given 1 h prior to training, the novel experience (clinging to the wire-mesh ceiling and exploring a small box) attenuated the memory-enhancing effects of post-training administration of naloxone as well as the enhancing effects of beta-endorphin administered prior to the retention test. The novel experience given prior to training did not block the enhancing effects of post-training administration of epinephrine. beta-Endorphin and the novel experience both enhanced retention performance when administered 1 h (as well as 3 but not 6 h) prior to the retention test. The enhancement found with both treatments was blocked by simultaneous administration of naloxone or by administration of propranolol a few minutes prior to the retention test. The findings of these experiments are consistent with the view that the effects of the novel experience are due to a release of endogenous beta-endorphin and provide additional evidence that the effects, on retention, of naloxone given post-training and beta-endorphin given prior to a retention test, are based on training-induced release of beta-endorphin.