Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

Adolescent morphine exposure impairs dark avoidance memory and synaptic potentiation of ventral hippocampal CA1 during adulthood in rats

Adolescence is a neurobiological critical period for neurodevelopmental processes. Adolescent opioid exposure can affect cognitive abilities via regional-specific lasting changes in brain structure and function. The current study was therefore designed to assess the long-term effects of adolescent morphine exposure on dark avoidance memory and synaptic plasticity of the ventral hippocampal CA1. Adolescent Wistar rats received escalating doses of morphine for 10 days. Morphine injections were started with an incremental dose of 2.5 mg/kg to reach a dose of 25 mg/kg. 30 days after the last injection, inhibitory memory and in vitro field potential recording were evaluated. Also, the weight of the animals was measured during drug and post-drug exposure. We found that adolescent morphine exposure decreased weight gain during morphine and post-morphine exposure. Passive avoidance memory was impaired in the morphine group. Moreover, adolescent morphine exposure caused an increase in baseline synaptic responsiveness and failed long-term potentiation (LTP) in the ventral hippocampal CA1 during adulthood. In the morphine group, the mean values of the field excitatory postsynaptic potential (fEPSP) slopes required to elicit a half-maximal population spike (PS) amplitude were significantly greater than that of the saline group. Therefore, adolescent morphine exposure has a durable effect on memory functions, synaptic activity, and plasticity of ventral hippocampal CA1. Adults with adolescent morphine exposures may experience maladaptive behaviors and cognitive disabilities.

Mutual interactions between cognition and welfare: The horse as an animal model

Research in cognitive psychology has repeatedly shown how much cognition and emotions are mutually related to one another. Psychological disorders are associated with cognitive (attention, memory and judgment) biases and chronic pain may affect attention, learning or memory. Laboratory studies have provided useful insights about the processes involved but observations about spontaneous animal models, living in different stress/welfare conditions may help understand further how cognition and welfare are interrelated in the « real world ». Domestic horses constitute such a model as they live in a variety of conditions that impact differently their welfare state. In the present review, we try and provide an overview of the scientific literature on cognition and welfare of domestic horses and their interrelationship. We address how emotions and welfare may affect cognitive processes in horses and impact the way they perceive their environment (including work). We propose new methods for assessing the relationship between welfare and cognition and open up the discussion on the evolution of the brain and the part domestication may have played.

μ-Opioid receptor in the CA1 involves in tramadol and morphine cross state-dependent memory

In the present study, the effect of tramadol - an opioid painkiller drug with abuse potential- on amnesia and state-dependent memory and its interaction with the opioidergic system was investigated in male Wistar rats. Intra CA-1 administration of tramadol (0.5, 1, and 2 μg/rat) before training, dose-dependently decreased the learning ability in passive avoidance task. Amnesia induced by pre-train tramadol administration was significantly reversed by pre-test administration of tramadol (1 μg/rat). Pre-test administration of naltrexone (a μ-opioid receptor (MOR) antagonist) inhibited the effect of tramadol on memory retrieval. In addition, the pre-test administration of morphine (1 μg/rat, intra-CA1) also reversed memory impairment induced by pre-train tramadol administration. Although, pre-train morphine administration (1 μg/rat, intra-CA1), induced memory impairment reversed by pre-test tramadol administration (1 μg/rat, intra-CA1). In addition, the level of MOR in the hippocampus decreased in animals with memory impairment due to using tramadol in the training day. However, state-dependent retrieval using tramadol or cross state-dependent retrieval using morphine enhanced the MOR level in the hippocampus. The results of the study suggested that intra-CA1 tramadol administration induced memory impairment, improved by pre-test administration of either tramadol or morphine (MOR agonist). It could be concluded that tramadol is capable to induced state-dependent memory and also, it has a cross state-dependent memory with morphine in the hippocampus, done possibly through MOR.

Cross state-dependency of learning between 5-HT1A and/or 5-HT7 receptor agonists and muscimol in the mouse dorsal hippocampus

BACKGROUND

Dysfunction of the serotonergic and GABAergic systems in cognitive disorders has been revealed. Understanding the neurobiological mechanisms of drug-associated learning and memory formation may help treatment of cognitive disorders.

AIMS

The aim of the present study was to investigate: 1) 8-OH-DPAT (5-HT1A agonist), AS19 (5-HT7 agonist) and muscimol (GABA-A agonist) on memory retrieval and state of memory, 2) cross state-dependent learning between 8-OH-DPAT and/or AS19 and muscimol.

METHODS

The dorsal hippocampal CA1 regions of adult male NMRI mice were bilaterally cannulated, and all drugs were microinjected into the intended sites of injection. A single-trial step-down inhibitory avoidance task was used for the evaluation of memory retrieval and state of memory.

RESULTS

Post-training and/or pre-test 8-OH-DPAT, AS19 and muscimol induced amnesia. Pre-test microinjection of the same doses of 8-OH-DPAT, AS19 and muscimol reversed the post-training 8-OH-DPAT-, AS19- and muscimol-induced amnesia, respectively. This event has been named state-dependent learning (SDL). The amnesia induced by 8-OH-DPAT was reversed by muscimol and induced 8-OH-DPAT SDL. The amnesia induced by muscimol was reversed by 8-OH-DPAT and induced muscimol SDL. The amnesia induced by AS19 was reversed by muscimol and induced AS19 SDL. The amnesia induced by muscimol was reversed by AS19 and induced muscimol SDL. Pre-test administration of a selective GABA-A receptor antagonist, bicuculline, 5 min before muscimol, 8-OH-DPAT and AS19 dose-dependently inhibited muscimol-, 8-OH-DPAT- and AS19-induced SDL, respectively.

CONCLUSIONS

The results strongly revealed a cross SDL among 8-OH-DPAT and/or AS19 and muscimol in the dorsal hippocampal CA1 regions.

The role of nucleus accumbens shell on acquisition and retrieval stages of morphine state dependent learning

AIM

In the present study, the effect of transient inactivation of the shell subregion of the nucleus accumbens (NAC shell) by lidocaine on the acquisition and retrieval stages of passive avoidance learning (PAL) and memory and morphine state-dependent learning (SDL) in male wistar rats was investigated.

METHODOLOGY

Adult male wistar rats weighing (220-250 g) were used. Lidocaine hydrochloride was bilaterally injected into the shell area of the nucleus accumbens 5 min before of subcutaneous morphine administration.

RESULTS

pre-training and pre-test infusion of lidocaine into the NAC shell significantly impaired PAL and memory. Furthermore, Pre-training administration of morphine (5 mg/kg, s.c.) in a step-through passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after pre-test administration of the same dose of morphine. This phenomenon has been named as morphine state dependent learning (SDL). Moreover, Pre-training and pre-test inactivation of the NAC shell impaired morphine SDL.

CONCLUSIONS

The results suggest the role of NAC shell as a common structure in the PAL and morphine SDL. It is suggested that NAC shell as a common area plays a critical role in the acquisition and retrieval stages of PAL and also morphine SDL.

Blockade of dorsal hippocampal orexin-1 receptors impaired morphine-induced state-dependent learning

Behavioral abnormalities associated with opiate addiction include memory and learning deficits, which are the result of some alterations in the neuromodulatory systems. Recently, orexin has shown to influence drug addiction neural circuitry, specifically in mediating reward-related perception and memory. To explore the possible interaction of orexinergic and opioidergic system on modulation of learning and memory, we have investigated the effects of intra-dorsal hippocampal (intra-CA1) administration of orexin-1 receptor agonist and the competitive orexin-1 antagonist, SB-334867, on morphine-induced memory impairment by using step-down passive avoidance task in mice. Pre-training injection of morphine (5mg/kg, i.p.) impaired memory, which was restored when 24h later the same dose of the drug was administered. Pre-test administration of orexin-1 (0.5, 5 and 50pmol, intra-CA1) had not a significant effect on the retention latency compared to the saline-treated animals, but it restored the memory impairment induced by pre-training morphine (5mg/kg, i.p.). Pre-test administration of SB-334867 (10, 20 and 40nmol, intra-CA1) by itself decreased the retention latencies of passive avoidance task. Co-administration of orexin-1 (0.5, 5 and 50pmol, intra-CA1) and morphine (1mg/kg, i.p.) on the test day induced morphine state-dependent memory. Conversely, pre-test injection of SB-334867 (10, 20 and 40nmol, intra-CA1) inhibited the orexin-1-induced potentiation of morphine state-dependent learning on the test day. It is concluded that dorsal hippocampal orexin-1 receptors may be involved, at least in part, in morphine state-dependent learning in mice.

Stress and opioids: role of opioids in modulating stress-related behavior and effect of stress on morphine conditioned place preference

Research studies have defined the important role of endogenous opioids in modulating stress-associated behavior. The release of β-endorphins in the amygdala in response to stress helps to cope with a stressor by inhibiting the over-activation of HPA axis. Administration of mu opioid agonists reduces the risk of developing post-traumatic stress disorder (PTSD) following a traumatic event by inhibiting fear-related memory consolidation. Similarly, the release of endogenous enkephalin and nociceptin in the basolateral amygdala and the nucleus accumbens tends to produce the anti-stress effects. An increase in dynorphin levels during prolonged exposure to stress may produce learned helplessness, dysphoria and depression. Stress also influences morphine-induced conditioned place preference (CPP) depending upon the intensity and duration of the stressor. Acute stress inhibits morphine CPP, while chronic stress potentiates CPP. The development of dysphoria due to increased dynorphin levels may contribute to chronic stress-induced potentiation of morphine CPP. The activation of ERK/cyclic AMP responsive element-binding (CREB) signaling in the mesocorticolimbic area, glucocorticoid receptors in the basolateral amygdala, and norepinephrine and galanin system in the nucleus accumbens may decrease the acute stress-induced inhibition of morphine CPP. The increase in dopamine levels in the nucleus accumbens and augmentation of GABAergic transmission in the median prefrontal cortex may contribute in potentiating morphine CPP. Stress exposure reinstates the extinct morphine CPP by activating the orexin receptors in the nucleus accumbens, decreasing the oxytocin levels in the lateral septum and amygdala, and altering the GABAergic transmission (activation of GABAA and inactivation of GABAB receptors). The present review describes these varied interactions between opioids and stress along with the possible mechanism.

Early life inflammatory pain induces long-lasting deficits in hippocampal-dependent spatial memory in male and female rats

The present experiment tested the hypothesis that neonatal injury disrupts adult hippocampal functioning and that normal aging or chronic stress during adulthood, which are known to have a negative impact on hippocampal function, exacerbate these effects. Male and female Sprague-Dawley rats were given an intraplantar injection of the inflammatory agent carrageenan (1%) on the day of birth and their memory was tested in the hippocampal-dependent spatial water maze in adulthood and again in middle age. We found that neonatal injury impaired hippocampal-dependent memory in adulthood, that the effects of injury on memory were more pronounced in middle-aged male rats, and that chronic stress accelerated the onset of these memory deficits. Neonatal injury also decreased glucocorticoid receptor mRNA in the dorsal CA1 area of middle-aged rats, a brain region critical for spatial memory. Morphine administration at the time of injury completely reversed injury-induced memory deficits, but neonatal morphine treatments in the absence of injury produced significant memory impairments in adulthood. Collectively, these findings are consistent with our hypothesis that neonatal injury produces long-lasting disruption in adult hippocampal functioning.

Effect of repeated morphine withdrawal on spatial learning, memory and serum cortisol level in mice

BACKGROUND

One of the serious problems that opioid addicted people are facing is repeated withdrawal syndrome that is accompanying with a significant stress load for addicts. Therefore, the aim of this study was to evaluate the effects of repeated withdrawal on spatial learning, memory and serum cortisol levels in morphine-dependent mice.

MATERIALS AND METHODS

Male NMRI mice received morphine as daily increasing doses for 3 days. After that, the mice underwent one time or repeated spontaneous or pharmacologic (naloxone-precipitated) withdrawal. Then spatial learning and memory were investigated by morris water maze test, and at the end trunk blood samples were collected for measurement of serum cortisol levels.

RESULTS

The results showed that only repeated spontaneous withdrawal significantly increases escape latency (P < 0.05), and in other models of withdrawal, spatial learning and memory were intact. The results of probe trial were intact in all groups. Radioimmunoassay showed that serum cortisol levels were increased significantly in all models of withdrawal (P < 0.05 and P < 0.01) except the repeated spontaneous withdrawal.

CONCLUSION

The results showed that short periods of withdrawal syndrome can increase serum cortisol levels; however they do not affect spatial learning and memory. Nevertheless, repeated spontaneous withdrawal can make learning slow.

Preclinical evaluation of reconsolidation blockade by clonidine as a potential novel treatment for posttraumatic stress disorder

Exposure to traumatic events can lead to posttraumatic stress disorder (PTSD). Current PTSD treatments typically only produce partial improvement. Hence, there is a need for preclinical research to identify new candidate drugs and to develop novel therapeutic approaches. Animal studies have indicated that fear memories can be weakened by blocking restabilization after retrieval, a process known as reconsolidation. Furthermore, evidence suggests that there are important alterations of the noradrenergic system in PTSD, and hence it may be of interest to study drugs that target this pathway. Here, we investigated the efficacy of clonidine, an α₂-adrenoreceptor agonist, to block reconsolidation in an animal model of persistent traumatic memories. Using an auditory fear conditioning paradigm in rats, we tested the efficacy of clonidine to weaken fear memory retention when administered systemically after retrieval. We evaluated dosage, number of treatments, and specificity in reconsolidation blockade. We found that postretrieval administration of clonidine disrupts fear-related memories in a dose-dependent manner and that two treatments are sufficient for maximal memory impairment. Furthermore, we determined that this effect is long lasting and specific to reconsolidation processes as shown by the selectivity to affect reactivated memories and the absence of spontaneous recovery and of postreactivation short-term memory impairment. Our results demonstrate the efficacy of systemic administration of clonidine following retrieval to persistently disrupt fear memory retention through reconsolidation blockade. This study provides important preclinical parameters for future therapeutic strategies involving clonidine to block reconsolidation as a novel treatment for PTSD symptoms.

Adjuvant therapy with intrathecal clonidine improves postoperative pain in patients undergoing coronary artery bypass graft

BACKGROUND

Alpha2 adrenergic agonists have long been employed as analgesics and to sedate patients undergoing surgical procedures. In addition, their therapeutic response synergizes that elicited by opioids. Although this response is well known, the role of alpha2 agonists, such as clonidine, during various painful surgical procedures remains to be elucidated. The goal of our study was to evaluate the effects of the intrathecal administration of clonidine on postoperative pain control and time to extubation in patients undergoing coronary artery bypass grafting.

METHODS

Eighty-five patients undergoing coronary artery bypass grafting randomly received either an intrathecal injection of preservative free morphine 0.5 mg (MOR) or a combination of morphine 0.5 mg and clonidine 100 microg (CMC) before induction of anesthesia. Anesthesia was induced and maintained using a balanced anesthesia technique. Patients were transferred to the intensive care unit while intubated and weaned from mechanical ventilation following an established weaning protocol. Postoperative pain, opioid use within the first 24 hours, and time to extubation were used as primary outcome variables. Data were analyzed by a 2-tailed t test for continuous variables and Fisher exact test for nonparametric variables.

RESULTS

There were no demographic differences between the CMC and MOR groups. Postoperative pain, as assessed by a visual analog scale, was milder in the CMC group when compared with that of the MOR group (2.2+/-0.36 vs. 3.4+/-0.33, P<0.05). Similarly, patients in the CMC group required lower doses of morphine within 24 hours compared with the MOR group (2.02+/-0.36 vs. 6.47+/-0.49 mg, P<0.0001). Time to extubation was significantly shorter in patients receiving CMC than in those who received MOR (592+/-52 vs. 887+/-75 min, P<0.05). There was no mortality in either group. There was a trend for increased vasopressin use in the CMC group compared with the MOR group, although this was not statistically significant (P=0.07).

CONCLUSIONS

Addition of clonidine to neuraxial opioids improves the quality of analgesia postoperatively and expedites the process of weaning from mechanical ventilation. There were no serious adverse events in the cohort of the patients studied. However, the safety profile of this medication remains to be examined with a larger group of patients.

Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.

Involvement of dorsal hippocampal nicotinic receptors in the effect of morphine on memory retrieval in passive avoidance task

The present study evaluated the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus on morphine-induced amnesia and morphine state-dependent memory in adult male Wistar rats. The animals were bilaterally implanted with chronic cannulas in the CA1 regions of the dorsal hippocampi, trained in a step-through type passive avoidance task, and tested 24 h after training to measure step-through latency. Results indicate that post-training subcutaneous (s.c.) administration of morphine (2.5-7.5 mg/kg) dose-dependently reduced the step-through latency, showing an amnestic response. Post-training intra-CA1 microinjection of nicotine (0.5-1 microg/rat) decreased significantly the amnesia induced by post-training morphine (7.5 mg/kg). Moreover, co-treatment of mecamylamine (0.5 and 1 microg/rat, intra-CA1) with an ineffective dose of morphine (2.5 mg/kg), immediately after training, caused inhibition of memory retrieval. On the other hand, amnesia produced by post-training morphine (7.5 mg/kg) was reversed by pre-test administration of the opioid that is due to a state-dependent effect. Interestingly, pre-test intra-CA1 microinjection of nicotine (0.25 and 0.5 microg/rat) improved post-training morphine (7.5 mg/kg)-induced retrieval impairment. Moreover, pre-test administration of the same doses of nicotine in combination with a lower dose of morphine (0.5 mg/kg), which had no effects alone, synergistically improved memory performance impaired by post-training morphine. Pre-test injection of mecamylamine (0.5-2 microg/rat) prevented the restoration of memory by pre-test morphine. It is important to note that post-training or pre-test intra-CA1 administration of the same doses of nicotine or mecamylamine, alone did not affect memory retrieval. These results suggest that nicotinic acetylcholine receptors of the hippocampal CA1 regions may play an important role in morphine-induced amnesia and morphine state-dependent memory.

Dorsal hippocampal dopamine receptors are involved in mediating ethanol state-dependent memory

In the present study, the effects of bilateral injections of dopaminergic agents into the hippocampal CA1 regions (intra-CA1) on ethanol (EtOH) state-dependent memory were examined in mice. A single-trial step-down passive avoidance task was used for the assessment of memory retention in adult male NMRI mice. Pre-training intra-peritoneal (i.p.) administration of EtOH (0.25, 0.5 and 1 g/kg) dose dependently induced impairment of memory retention. Pre-test administration of EtOH (0.5 g/kg)-induced state-dependent retrieval of the memory acquired under pre-training EtOH (0.5 g/kg) influence. Intra-CA1 administration of the dopamine D(1) receptor agonist, SKF 38393 (0.5, 1 and 2 g/mouse) or the dopamine D(2) receptor agonist, quinpirole (0.25, 0.5 and 1 microg/mouse) alone cannot affect memory retention. While, pre-test intra-CA1 injection of SKF 38393 (2 microg/mouse, intra-CA1) or quinpirole (0.25, 0.5 and 1 microg/mouse, intra-CA1) improved pre-training EtOH (0.5 g/kg)-induced retrieval impairment. Moreover, pre-test administration of SKF 38393 (0.5, 1 and 2 microg/mouse, intra-CA1) or quinpirole (0.5 and 1 microg/mouse, intra-CA1) with an ineffective dose of EtOH (0.25 g/kg) significantly restored the retrieval and induced EtOH state-dependent memory. Furthermore, pre-training injection of the dopamine D(1) receptor antagonist, SCH 23390 (4 microg/mouse), but not the dopamine D(2) receptor antagonist, sulpiride, into the CA1 regions suppressed the learning of a single-trial passive avoidance task. Pre-test intra-CA1 injection of SCH 23390 (2 and 4 microg/mouse, intra-CA1) or sulpiride (2.5 and 5 microg/mouse, intra-CA1) 5 min before the administration of EtOH (0.5 g/kg, i.p.) dose dependently inhibited EtOH state-dependent memory. These findings implicate the involvement of a dorsal hippocampal dopaminergic mechanism in EtOH state-dependent memory and also it can be concluded that there may be a cross-state dependency between EtOH and dopamine.

Effects of Clozapine and Sulpiride on Morphine State-Dependent Memory in the Step-Down Passive Avoidance Test

The effects of antipsychotic drugs sulpiride and clozapine on morphine state-dependent memory of passive avoidance task were examined in mice. Post-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory retrieval on the test day which was reversed by pre-test administration of the same dose of the opioid (5 mg/kg). In animals where memory was impaired by post-training morphine, the administration of either sulpiride or clozapine before pre-test morphine reduced the improvement of memory produced by the opioid. Co-administration of sulpiride with clozapine did not potentiate their antagonistic response. In conclusion, the inhibition of improvement of memory retrieval by morphine treatment on the test day by the two dopamine receptor antagonists seems to be induced through two different receptor mechanisms.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

Influence of cholinergic system modulators on morphine state-dependent memory of passive avoidance in mice

Memories are shown to be impaired in mice during step-down passive avoidance tasks with substantial residual effects lasting as long as 24 h after the pre-training administration of morphine. Administration of the same dose of morphine as a pre-test treatment restored memory. Since the cholinergic system has been reported to be involved in several actions of morphine, e.g.: modulation of memory and analgesia, we have investigated the part played by cholinergic modulator drugs, on the memory recall in mice. The locomotor activity of animals was studied as well. Administration of either atropine, a peripheral-central muscarinic antagonist, or mecamylamine, a peripheral-central nicotinic antagonist, failed to alter memory themselves, but significantly prevented morphine-induced memory recall following co-administration with morphine. Neither hexamethonium, a peripheral nicotinic antagonist, nor neostigmine, a peripheral anticholinesterase, showed intrinsic activity or a significant change in morphine-induced memory recall. Finally, physostigmine, a peripheral-central anticholinesterase, not only induced memory recall itself, but also increased morphine-induced retrieval. Memory recall of the step-down passive avoidance task following drug combinations was not related to locomotor activity changes. Thus, morphine-induced memory recall appears to be influenced by central cholinergic activity.

Influence of intracerebroventricular administration of cannabinergic drugs on morphine state-dependent memory in the step-down passive avoidance test

The effects of cannabinergic drugs on morphine state-dependent memory of passive avoidance task were examined in mice. Pre-training (0.25, 0.5 and 5 mg/kg) and post-training (5 mg/kg) administration of morphine impaired memory retrieval on the test day. Impairment of memory retrieval by morphine (5 mg/kg) on the test day was reversed by pre-test administration of the same dose of the opioid. The pre-test intracerebroventricular administration of the cannabinoid CB1/CB2 receptor agonist (WIN55,212-2) (0.75 and 1 microg/mouse) not only mimicked the effect of pre-test morphine treatment, but also increased this action of the opioid. Furthermore, the pre-test intracerebroventricular administration of CB1 receptor antagonist (AM251) (20 and 100 ng/mouse) prevented the restoration of memory by morphine. Pre-training administration of WIN55,212-2 (1 microg/mouse) led to state-dependent learning with impaired memory retrieval on the test day as well, which was reversed by pre-test administration of the drug (0.5, 0.75 and 1 microg/mouse) or morphine (1 and 5 mg/kg). Restoration of impairment induced by WIN55,212-2 was decreased by both the opioid receptor antagonists, naloxone (0.01 microg/mouse) and AM251 (20 and 100 ng/mouse). In conclusion, the improvement of memory retrieval by morphine treatment on the test day seems to be induced, at least in part, by the cannabinoid CB1 receptors.

Effect of the GABAergic System on Memory Formation and State-Dependent Learning Induced by Morphine in Rats

In the present study, the effects of intraperitoneal injections of GABA(A) receptor agonist and antagonist on memory formation and morphine state-dependent learning were investigated in rats. Pre-training administration of morphine (1-15 mg/kg) in a step-down passive avoidance task induced state-dependent learning with impaired memory retrieval on the test day. The impairment of memory was restored after the pre-test administration of the same dose of morphine. The pre-test administration of the GABA(A) receptor agonist, muscimol (0.01, 0.05 and 0.1 mg/kg), significantly decreased state-dependent retrieval induced by pre-test morphine (5 mg/kg). The state-dependency effect of morphine (1 mg/kg) was significantly potentiated by the pre-test administration of the GABA(A) receptor antagonist, bicuculline (0.125, 0.25 and 0.5 mg/kg). Furthermore, the pre-training injection of muscimol (0.01 mg/kg) impaired memory retrieval which was restored by pre-test morphine (1, 3 and 5 mg/kg) administration. However, the pre-training administration of bicuculline did not affect retention by itself. In addition, amnesia induced by pre-training morphine (5 mg/kg) was significantly reversed in rats which had received pre-test injections of muscimol (0.01, 0.05 and 0.1 mg/kg). Pre-test injections of bicuculline (0.125, 0.25 and 0.5 mg/kg) significantly decreased morphine-induced amnesia. It is concluded that the GABA(A) receptor mechanisms may be involved in the memory formation and it is postulated that these receptors may play an important role in morphine state-dependent learning.

The influence of NMDA receptor agonist and antagonist on morphine state-dependent memory of passive avoidance in mice

The measurement of step-down latency in passive avoidance has been used to study memory in laboratory animals. The pre-training injection of 5 mg/kg morphine impaired memory, which was restored when 24 h later the same dose of the drug was administered. To explore the possible involvement of NMDA modulators on morphine-induced memory impairment, we have investigated the effects of intracerebroventricular (i.c.v.) administration of NMDA and the competitive NMDA antagonist, DL-AP5, on morphine-induced memory impairment or recall, on the test day. Morphine (5 mg/kg, s.c.) was administered 30 min before training to induce impairment of memory and 24 h later, 30 min before test to improve it. Pre-test administration of NMDA (0.00001, 0.0001 and 0.001 microg/mouse, i.c.v.) did not alter the retention latency compared to the saline-treated animals. But restored the memory impairment induced by pre-training morphine (5 mg/kg, s.c.). Pre-test administration of DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) by itself decreased the retention latencies. The same doses of DL-AP5 increased pre-training morphine-induced memory impairment. Co-administration of NMDA (0.0001 and 0.001 microg/mouse, i.c.v.) and morphine (5 mg/kg, s.c.) on the test day increased morphine memory improvement. Conversely, DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) inhibited morphine-induced memory recall. It is concluded that NMDA receptors may be involved, at least in part, in morphine state-dependent learning in mice.

Influence of Intracerebroventricular Administration of Histaminergic Drugs on Morphine State-Dependent Memory in the Step-Down Passive Avoidance Test

The effects of histaminergic drugs on morphine state-dependent memory of a passive avoidance task were examined in mice. Pre-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory recall on the test day which was reversed by pre-test administration of the same dose of the opioid. The pre-test intracerebroventricular (i.c.v.) administration of the H(1) blocker (pyrilamine) prevented the restoration of memory by morphine. The H(2) blocker (ranitidine) was ineffective in this regard and the H(3) blocker (clobenpropit) potentiated the effect of morphine on memory recall. The pre-test i.c.v. administration of histamine alone (5, 10, and 20 microg/mouse) not only mimicked the effect of pre-test morphine treatment, but also increased this action of the opioid. The effect of histamine on memory recall was not changed by the pre-test administration of mu-opioid receptor antagonist, naloxone. In conclusion, the improvement of memory recall by morphine treatment, on the test day, seems to be, at least in part, through the release of histamine followed by the stimulation of H(1) receptors. Histamine by itself, when administered on the test day, mimicked morphine-induced memory improvement by a mechanism independent of the mu-opioid receptors.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Alpha-2 agonist-induced memory impairment is mediated by the alpha-2A-adrenoceptor subtype

The activation of alpha2-adrenoceptors has been reported to impair memory functions in both rats and humans. The alpha2-adrenoceptor subtype responsible for this detrimental effect is still unknown. The effect of the alpha2-agonists clonidine and guanabenz on memory processes, in dependence to the time of administration, was evaluated in the mouse passive avoidance test. Clonidine (0.02-0.2 mg kg(-1) i.p.) and guanabenz (0.1-0.3 mg kg(-1) i.p.) induced amnesia in a dose-dependent manner. From time-course experiments emerged that the impairment of memory function was detectable only when clonidine and guanabenz were administered 60 min before or immediately after the training test, respectively. This detrimental effect was prevented by pretreatment with the alpha2-antagonist yohimbine (1-3 mg kg(-1) i.p.) and by the alpha2A-antagonist BRL-44408 (0.3-1 mg kg(-1) i.p.). By contrast, the alpha(2B,C) antagonists ARC-239 (10 mg kg(-1) i.p.) and prazosin (1 mg kg(-1) i.p.) did not revert the amnesia induced by both clonidine and guanabenz. At the highest effective doses, clonidine and guanabenz were devoid of behavioral side-effects as well as maintained unaltered the motor coordination, as revealed by the rota-rod test. Furthermore, none of the compounds used modified the spontaneous motility as indicated by the Animex apparatus. These results indicate that clonidine and guanabenz impaired memory processes in a mouse passive avoidance paradigm through the selective activation of the alpha2A-adrenoceptor subtype.

Morphine state-dependent learning: sensitization and interactions with dopamine receptors

In the present study, the effects of morphine sensitization on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (0.5, 2.5 and 5 mg/kg) dose dependently decreased the learning of a one-trial passive avoidance task. Pretest administration of morphine (0.5, 2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under pretraining morphine influence. Pretraining or pretest administration of naloxone (0.25, 0.5 and 1 mg/kg) reversed both responses to morphine (5 mg/kg). Amnesia induced by pretraining morphine was significantly reversed in morphine-sensitized mice which had previously received once daily injections of morphine [20 and 30 mg/kg, subcutaneously (s.c.)] for 3 days. Morphine sensitization tended to reverse but did not significantly affect morphine state-dependent memory. The inhibition of morphine-induced amnesia in morphine-sensitized mice was decreased by once daily administration of naloxone (0.5, 1 and 2 mg/kg) 30 min prior to injection of morphine (20 mg/kg/day x 3 days). Three-days administration of 1-phenyl-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (SKF 38393; 8, 16 and 32 mg/kg) or SCH 23390; R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCL (0.01, 0.05 and 0.1 mg/kg) before morphine (for 3 days) and during morphine-sensitization, decreased and increased, the amnesia induced by pretraining morphine, respectively. Similar administration of quinpirole (0.5, 1 and 2 mg/kg) or sulpiride (25, 50 and 100 mg/kg) before morphine also decreased and increased the amnesia induced by pretraining morphine, respectively. The results suggest that morphine sensitization affects the impairment of memory formation, but not the facilitation of retrieval induced by morphine and thus it is postulated that dopamine receptors may play an important role in this effect.

EFFECT OF ETHANOL ON MORPHINE STATE-DEPENDENT LEARNING IN THE MOUSE: INVOLVEMENT OF GABAERGIC, OPIOIDERGIC AND CHOLINERGIC SYSTEMS

AIMS

We have studied the effect of acute administration of ethanol when it replaced morphine in step-down passive avoidance task on the test day and the effects of antagonists of GABAergic, opioidergic and cholinergic systems on ethanol actions.

METHODS

Morphine (5 mg/kg, s.c.) was administered as pre-training and 24 h later as pre-test drug, and the latencies were measured in mice. Ethanol (0.125, 0.25, 1 and 2 g/kg, i.p.) was administered instead of pre-test morphine. Antagonists of GABAergic (bicuculline 0.5, 1 and 2 mg/kg, i.p.), opioidergic (naloxone 0.06, 0.25 and 1 mg/kg, i.p.) and cholinergic (atropine 0.625 and 1.25 mg/kg, i.p. and mecamylamine 0.5, 1 and 2 mg/kg, i.p.) systems were co-administered with ethanol (0.25 g/kg, i.p.) on the test day. Locomotor activity was measured as well.

RESULTS

Pre-training morphine impaired the memory on the test day which was restored when the same dose of morphine was used as pre-test drug. All four doses of ethanol replaced pre-test morphine and enhanced the memory. This effect was prevented by all of the above antagonists. No significant changes were seen in the locomotor activity of the animals treated with ethanol or antagonists compared to the proper controls.

CONCLUSIONS

GABAergic, endogenous opioidergic and cholinergic systems are involved in the memory recall improvement by ethanol when it replaced morphine on the test day. A review of the literature suggests other possibilities such as the release of intermediate neurotransmitters.