Involvement of the ventral tegmental area (VTA) in morphine-induced memory retention in morphine-sensitized rats

Pentylenetetrazol and Morphine Interaction in a State-dependent Memory Model: Role of CREB Signaling

Introduction:

State-dependent (STD) memory is a process, in which the learned information can be optimally retrieved only when the subject is in the state similar to the encoding phase. This phenomenon has been widely studied with morphine. Several studies have reported that Pentylenetetrazole (PTZ) impairs memory in experimental animal models. Due to certain mechanistic interactions between morphine and PTZ, it is hypothesized that PTZ may interfere with the morphine-STD. The cyclic adenosine monophosphate Response Element-Binding (CREB) is considered as the main downstream marker for long-term memory. This study was designed to determine the possible interaction between PTZ and morphine STD and the presumable changes in CREB mRNA.

Methods:

In an Inhibitory Avoidance (IA) model, posttraining morphine (2.5, 5, and 7.5 mg/ kg-i.p.) was used. The pre-test morphine was evaluated for morphine-induced STD memory. Moreover, the effect of a pre-test PTZ (60 mg/kg-i.p.) was studied along with morphine STD. Locomotion testing was carried out using open-field. Eventually, using real-time-PCR, the CREB mRNA changes in the hippocampus were evaluated.

Results:

Posttraining MOR (7.5 mg/kg-i.p.) impaired IA memory (P<0.001). The pre-test injection of similar doses of morphine recovered the morphine-induced memory impairment (P<0.001). The pre-test PTZ impaired the IA memory recall (P<0.001); however, the pre-test PTZ along with morphine STD potentiated the morphine-induced STD (P<0.001). Alterations in CREB mRNA were observed in all groups. No difference was seen in the locomotor activity.

Conclusion:

Presumably, the certain interactive effect of PTZ on morphine-induced STD is mediated through gamma-aminobutyric acid and opioid systems via CREB signaling.

The effect of alpha-2 adrenergic receptors on memory retention deficit induced by rapid eye movement sleep deprivation

Objective(s):

Evidence shows that sleep deprivation (SD) disrupts the formation of hippocampus-related memories. Moreover, α2 adrenergic receptors that are wildly expressed in the CA1 hippocampal region have a significant role in modulating both sleep and memory formation. In the present research, we wanted to investigate the effect of stimulation and blockage of CA1 α2 adrenergic receptors by clonidine (an agonist of α2 adrenergic receptor) and yohimbine (an antagonist of α2 adrenergic receptor), respectively, on memory retention impairment induced by REM SD (RSD) in rats.

Materials and Methods:

Multiple platform apparatus were used to induce RSD, and the passive avoidance task was used to assess memory consolidation. Clonidine and yohimbine were injected intra-CA1.

Results:

The results showed that RSD (for 24 and 36, but not 12 hr) decreased memory retention, with no effect on locomotion. Post-training intra-CA1 infusion of a subthreshold dose of yohimbine (0.001 μg/rat) did not alter, while clonidine (0.1 μg/rat) restored memory retention impairment induced by RSD (24 and 36 hr). Also, none of the interventions did not influence locomotor activity.

Conclusion:

Our data strongly showed that CA1 α2 adrenergic receptors have a critical role in RSD-induced memory retention impairment.

Combination therapy of the granulocyte colony stimulating factor and intravenous lipid emulsion protect the hippocampus after global ischemia in rat: focusing on CA1 region

Brain stroke is one of the causes of human death and disability worldwide. Global ischemia results in the accumulation of free radicals in the neurons. It leads to histologically brain damage. The CA1 region of the hippocampus is a sensitive area for free radicals. This study investigated the combined therapy of the Granulocyte colony stimulating factor (G-CSF) and the Intravenous lipid emulsion (ILE). These neuroprotective agents play a role in the regeneration of neurons. They improve the learning ability and memory in rats induced global ischemia. We divided 35 rats into five groups. The groups were sham group, ischemia group, G-CSF group, ILE group, and G-CSF plus ILE group. Ischemia was induced by occlusion of the bilateral common carotid about 10 min. The drugs applied on days 1, 3 and 7. The treated groups received subcutaneous injection of 20 μg/kg G-CSF and intravenous injection of 5 ml/kg ILE. After two weeks, the memory and learning ability of the rats was evaluated by the shuttle box. Hematoxylin and Eosin and Nissl and TUNEL stainings were used to determine the necrosis, normal and apoptotic cells. The combined therapy increased normal cells compared to the ischemia group. They decreased the number of necrotic and apoptosis cells in other groups. The combined group improved the passive avoidance test compared to the other groups. The combination therapy of G-CSF plus ILE is more effective than each alone.

Members of the same pharmacological family are not alike: Different opioids, different consequences, hope for the opioid crisis?

Pain management is the specialized medical practice of modulating pain perception and thus easing the suffering and improving the life quality of individuals suffering from painful conditions. Since this requires the modulation of the activity of endogenous systems involved in pain perception, and given the large role that the opioidergic system plays in pain perception, opioids are currently the most effective pain treatment available and are likely to remain relevant for the foreseeable future. This contributes to the rise in opioid use, misuse, and overdose death, which is currently characterized by public health officials in the United States as an epidemic. Historically, the majority of preclinical rodent studies were focused on morphine. This has resulted in our understanding of opioids in general being highly biased by our knowledge of morphine specifically. However, recent in vitro studies suggest that direct extrapolation of research findings from morphine to other opioids is likely to be flawed. Notably, these studies suggest that different opioid analgesics (opioid agonists) engage different downstream signaling effects within the cell, despite binding to and activating the same receptors. This recognition implies that, in contrast to the historical status quo, different opioids cannot be made equivalent by merely dose adjustment. Notably, even at equianalgesic doses, different opioids could result in different beneficial and risk outcomes. In order to foster further translational research regarding drug-specific differences among opioids, here we review basic research elucidating differences among opioids in pharmacokinetics, pharmacodynamics, their capacity for second messenger pathway activation, and their interactions with the immune system and the dopamine D2 receptors.

Environmental novelty modulates the induction and expression of single injection-induced behavioral sensitization to morphine

Opioid addiction is a growing public health problem, being currently considered an epidemic in the United States. Investigating the behavioral effects of opioids and the factors influencing their development becomes of major importance. In animals, the effects of drugs of abuse can be assessed using the behavioral sensitization model, which shares similar neuronal substrates with drug craving in humans. Importantly, novelty plays a critical role on the development of behavioral sensitization. The aim of the present study was to investigate the influence of a new environment on both the induction and expression phases of morphine (Mor)-induced behavioral sensitization in the two-injection protocol. Mice were initially treated with saline, 15 or 30 mg/kg Mor (induction phase), and subsequently challenged 7 days later with 15 mg/Kg Mor (expression phase). Locomotor frequency was evaluated during behavioral sessions, performed as follow: induction session on a novel environment and expression on a familiar open-filed apparatus; induction session on animals' home-cage (familiar environment) and expression session on an unknown open-filed apparatus; both sessions on novel environments; and both sessions on familiar contexts. Mor-induced behavioral sensitization was only observed when animals were exclusively exposed to novelty during the induction phase, not being observed when both the induction and expression sessions were performed on similar (novel or familiar) environments. Our results suggest that the development of behavioral sensitization to Mor depends on the exposure to novelty during the induction phase and absence of novelty during the expression phase, indicating a complex relationship between novelty and Mor-induced behavioral effects.

Sex differences in the generalization of fear as a function of retention intervals

In previous studies using male rodents, context change disrupted a fear response at a short, but not a long, retention interval. Here, we examined the effects of context changes on fear responses as a function of time in male and female rats. Males displayed context discrimination at all intervals, whereas females exhibited generalization by 5 d. Ovariectomized females with no hormone replacement displayed context discrimination at 5 d, whereas those receiving 17β-estradiol generalized their fear response to a neutral context. These results demonstrate that fear generalization for contextual cues occurs faster in female rats and is mediated, in part, by estrogens.

Importance of GluA1 subunit-containing AMPA glutamate receptors for morphine state-dependency

In state-dependency, information retrieval is most efficient when the animal is in the same state as it was during the information acquisition. State-dependency has been implicated in a variety of learning and memory processes, but its mechanisms remain to be resolved. Here, mice deficient in AMPA-type glutamate receptor GluA1 subunits were first conditioned to morphine (10 or 20 mg/kg s.c. during eight sessions over four days) using an unbiased procedure, followed by testing for conditioned place preference at morphine states that were the same as or different from the one the mice were conditioned to. In GluA1 wildtype littermate mice the same-state morphine dose produced the greatest expression of place preference, while in the knockout mice no place preference was then detected. Both wildtype and knockout mice expressed moderate morphine-induced place preference when not at the morphine state (saline treatment at the test); in this case, place preference was weaker than that in the same-state test in wildtype mice. No correlation between place preference scores and locomotor activity during testing was found. Additionally, as compared to the controls, the knockout mice showed unchanged sensitization to morphine, morphine drug discrimination and brain regional μ-opioid receptor signal transduction at the G-protein level. However, the knockout mice failed to show increased AMPA/NMDA receptor current ratios in the ventral tegmental area dopamine neurons of midbrain slices after a single injection of morphine (10 mg/kg, s.c., sliced prepared 24 h afterwards), in contrast to the wildtype mice. The results indicate impaired drug-induced state-dependency in GluA1 knockout mice, correlating with impaired opioid-induced glutamate receptor neuroplasticity.

Withdrawal from repeated administration of morphine alters histamine-induced anxiogenic effects produced by intra-ventral hippocampal microinjection

In the present study, the influence of withdrawal from repeated administration of morphine on intra-ventral hippocampal microinjection of histamine-induced anxiety-like behavior was investigated in male Wistar rats. Three days subcutaneous administration of morphine (5-10 mg/kg) followed by five days free of the drug decreased the percentage open arm time and the percentage open arm entries. Intra-ventral hippocampal administration of histamine (2.5-7.5 microg/rat) decreased percentage open arm time and percentage open arm entries. Intra-ventral hippocampal histamine-induced anxiogenic effect was reversed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal administration of pyrilamine (5-20 microg/rat) or ranitidine (10-40 microg/rat) decreased percentage open arm time and percentage open arm entries. Pyrilamine- or ranitidine-induced anxiogenic effect was not changed in animals that had previously received the three days morphine (7.5 mg/kg) followed by five days free of the drug. Intra-ventral hippocampal injections of clobenpropit increased percentage open arm time. The percentage open arm time and percentage open arm entries were decreased in the morphine-treated animals compared with non-morphine-treated controls. Percentage open arm entries and locomotor activity was reduced with some doses of clobenpropit. It can be concluded that the histamine system is involved in anxiety-like behavior, and repeated injections of morphine followed by five days free of the drugs interact with histamine receptor mechanism.

Morphine-induced behavioral sensitization increased the mRNA expression of NMDA receptor subunits in the rat amygdala

This study was designed to evaluate the effect of repeated morphine treatment on rat behavioral responses. In the genetic section, the mRNA expression of NMDA receptor subunits (NR1 and NR2A) was measured in certain areas of the male rat brain (striatum, prefrontal cortex, hippocampus, hypothalamus and amygdala). In the behavioral section, the effect of repeated morphine treatment on animal models such as locomotion, oral stereotypy, and state-dependent memory in a passive avoidance test was evaluated in the presence or absence of MK801 (NMDA receptor antagonist). Our results showed that chronic morphine treatment, followed by a 7-day (but not 24-hour) washout period, potentiated the effect of test doses of morphine, which is referred to as behavioral sensitization. Meanwhile, pretreatment of animals with MK801 (0.1 and 0.25 mg/kg), 30 min before a test dose of morphine (5 mg/kg), failed to attenuate the locomotion and oral stereotypy in the behavioral sensitization state. Interestingly, a higher dose of MK801 (0.25 mg/kg) decreased memory retrieval induced by morphine (2.5 mg/kg) in state-dependent memory. This effect may be due to the intrinsic motor enhancer property of higher doses of MK801, rather than the blockade of NMDA receptors. It can be concluded that MK801 does not affect morphine-induced behavioral sensitization in the expression phase. In the genetic section of the study, results of quantitative real-time RT-PCR clearly indicated that morphine sensitization increased the expression of NMDA receptor subunits mRNA in the amygdala (NR1 by 104% and NR2A by 85%), while the other areas of the brain were unaffected. Maenwhile, no change in the mRNA levels was observed in non-sensitized animals (chronic morphine treatment followed by a 24-hour washout period). In summary, the present study indicates that repeated morphine treatment followed by long-term (7-day washout) induces behavioral sensitization and causes a delayed increase in mRNA levels of NMDA receptor subunits in the rat amygdala. Meanwhile, it has previously been reported that the amygdala is involved in behavioral sensitization. Thus, it can be concluded that the increase in NMDA receptor expression is associated with morphine-induced behavioral sensitization.

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.

N-Methyl-d-aspartate receptors in the ventral tegmental area are involved in retrieval of inhibitory avoidance memory by nicotine

The interaction of opiate, cholinergic, glutamatergic and (possibly) dopaminergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is certainly a topic of great interest. In the present study, the effect of intra-VTA administration of N-methyl-d-aspartate (NMDA) receptor agents on nicotine's effect in morphine state-dependent learning was investigated. An inhibitory avoidance (IA) task was used for memory assessment in male Wistar rats. Subcutaneous (s.c.) administration of morphine (5 and 7.5mg/kg) immediately after training decreased IA response on the test day, which was reinstated by pre-test administration of the same doses of the opioid; this is known as state-dependency. Moreover, pre-test administration of nicotine (0.2, 0.4 and 0.6 mg/kg, s.c.) also reversed the decrease in IA response because of post-training morphine (5mg/kg). Here, we also show that when infused into the VTA before testing, NMDA (0.01 and 0.1 microg/rat) reverse the post-training morphine effect on memory. In addition, the sub-effective doses of NMDA (0.0001 and 0.001 microg/rat) in combination with a low dose of nicotine (0.1mg/kg) which had no effects by themselves, synergistically improved retrieval of IA memory on the test day. In contrast, pre-test administration of a competitive NMDA receptor antagonist D-AP5 (0.5, 1 and 2 microg/rat) which had no effect alone prevented the nicotine reversal of morphine effect on memory. Our data indicate that NMDA receptors in the VTA are involved in the reversing effect of nicotine on morphine induced state-dependency.

Nicotine improves morphine-induced impairment of memory: Possible involvement ofN-methyl-D-aspartate receptors in the nucleus accumbens

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in the nucleus accumbens (NAc) in nicotine's effect on impairment of memory by morphine was investigated. A passive avoidance task was used for memory assessment in male Wistar rats. Subcutaneous (s.c.) administration of morphine (5 and 10 mg/kg) after training impaired memory performance in the animals when tested 24 h later. Pretest administration of the same doses of morphine reversed impairment of memory because of post-training administration of the opioid. Moreover, administration of nicotine (0.2 and 0.4 mg/kg, s.c.) before the test prevented impairment of memory by morphine (5 mg/kg) given after training. Impairment of memory performance in the animals because of post-training administration of morphine (5 mg/kg) was also prevented by pretest administration of a noncompetitive NMDA receptor antagonist, MK-801 (0.75 and 1 microg/rat). Interestingly, an ineffective dose of MK-801 (0.5 microg/rat) in combination with low doses (0.075 and 0.1 mg/kg) of nicotine, which had no effects alone, synergistically improved memory performance impaired by morphine given after training. On the other hand, pretest administration of NMDA (0.1 and 0.5 microg/rat), which had no effect alone, in combination with an effective dose (0.4 mg/kg, s.c.) of nicotine prevented the improving effect of nicotine on memory impaired by pretreatment morphine. The results suggest a possible role for NMDA receptors of the NAc in the improving effect of nicotine on the morphine-induced amnesia.

Endogenous opiates and behavior: 2005

This paper is the 28th 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 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Dopamine receptor mechanism(s) and antinociception and tolerance induced by swim stress in formalin test

In the present study, involvement of D1 and D2 dopamine receptors in the antinociception and tolerance induced by water swim stress in the formalin test has been investigated. Water swim stress at 20 degrees C temperature induced antinociception in both phases of the formalin test. Intraperitoneal administration of the D2 dopamine receptor antagonist, sulpiride (25 and 50 mg/kg) reduced swim stress-induced antinociception in the second phase of the formalin test. A higher dose of the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, intraperitoneal) also reduced swim stress-induced antinociception in both phases of the test. Exposure to 3 min water swimming stress, once daily for 3 days, induced tolerance to swim stress-induced antinociception in the second phase of the formalin test. Administration of sulpiride (12.5, 25 and 50 mg/kg), during exposure to water swimming stress (once daily for 3 days), decreased tolerance in the second phase, whereas the antagonist (12.5 and 50 mg/kg) increased pain scores in the first phase of the formalin test. Sulpiride (25 mg/kg) treatment however, once daily for 3 days with no water swimming stress, did not alter swim stress-induced antinociception (0.5, 1 and 3 min tests). Similarly, repeated treatment with SCH23390 (0.05 mg/kg) and water swimming stress did not alter tolerance induced by water swimming stress. Repeated administration of the antagonist in the absence of water swimming stress also did not change swim stress-induced antinociception. The results may indicate a possible involvement of both dopamine D1 and D2 receptors in the antinociception induced by swim stress and D2 receptor mechanism in the tolerance induced by repeated swim stress.