Distribution of opiate receptor subtypes and enkephalin and dynorphin immunoreactivity in the hippocampus of squirrel, guinea pig, rat, and hamster

The Kappa Opioid Receptor System in Temporal Lobe Epilepsy

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients frequently develop cognitive deficits and emotional blunting along progression of the disease. The high incidence of refractoriness to antiepileptic drugs and a frequent lack of admissibility to surgery pose an unmet medical challenge. In the urgent quest for novel treatment strategies, neuropeptides and their receptors are interesting candidates. However, their therapeutic potential has not yet been fully exploited. This chapter focuses on the functional role of the dynorphins (Dyns) and the kappa opioid receptor (KOR) system in temporal lobe epilepsy and the hippocampus.Genetic polymorphisms in the prepro-dynorphin (pDyn) gene causing lower levels of Dyns in humans and pDyn gene knockout in mice increase the risk to develop epilepsy. This suggests a role of Dyns and KOR as modulators of neuronal excitability. Indeed, KOR agonists induce inhibition of presynaptic neurotransmitter release, as well as postsynaptic hyperpolarization in glutamatergic neurons, both producing anticonvulsant effects.The development of new approaches to modulate the complex KOR signalling cascade (e.g. biased agonism and gene therapy) opens up new exciting therapeutic opportunities with regard to seizure control and epilepsy. Potential adverse side effects of KOR agonists may be minimized through functional selectivity or locally restricted treatment. Preclinical data suggest a high potential of such approaches to control seizures.

Perinatal exposure to a glyphosate-based herbicide causes dysregulation of dynorphins and an increase of neural precursor cells in the brain of adult male rats

Glyphosate, the most used herbicide worldwide, has been suggested to induce neurotoxicity and behavioral changes in rats after developmental exposure. Studies of human glyphosate intoxication have reported adverse effects on the nervous system, particularly in substantia nigra (SN). Here we used matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) to study persistent changes in peptide expression in the SN of 90-day-old adult male Wistar rats. The animals were perinatally exposed to 3 % GBH (glyphosate-based herbicide) in drinking water (corresponding to 0.36 % of glyphosate) starting at gestational day 5 and continued up to postnatal day 15 (PND15). Peptides are present in the central nervous system before birth and play a critical role in the development and survival of neurons, therefore, observed neuropeptide changes could provide better understanding of the GBH-induced long term effects on SN. The results revealed 188 significantly altered mass peaks in SN of animals perinatally exposed to GBH. A significant reduction of the peak intensity (P < 0.05) of several peptides from the opioid-related dynorphin family such as dynorphin B (57 %), alpha-neoendorphin (50 %), and its endogenous metabolite des-tyrosine alpha-neoendorphin (39 %) was detected in the GBH group. Immunohistochemical analysis confirmed a decreased dynorphin expression and showed a reduction of the total area of dynorphin immunoreactive fibers in the SN of the GBH group. In addition, a small reduction of dynorphin immunoreactivity associated with non-neuronal cells was seen in the hilus of the hippocampal dentate gyrus. Perinatal exposure to GBH also induced an increase in the number of nestin-positive cells in the subgranular zone of the dentate gyrus. In conclusion, the results demonstrate long-term changes in the adult male rat SN and hippocampus following a perinatal GBH exposure suggesting that this glyphosate-based formulation may perturb critical neurodevelopmental processes.

Acute Delta 9-tetrahydrocannabinol administration differentially alters the hippocampal opioid system in adult female and male rats

Our prior studies demonstrated that the rat hippocampal opioid system can undergo sex-specific adaptations to external stimuli that can influence opioid-associated learning processes. This opioid system extensively overlaps with the cannabinoid system. Moreover, acute administration of Δ⁹ Tetrahydrocannabinoid (THC), the primary psychoactive constituent of cannabis, can alter cognitive behaviors that involve the hippocampus. Here, we use light and electron microscopic immunocytochemical methods to examine the effects of acute THC (5 mg/kg, i.p., 1 h) on mossy fiber Leu-Enkephalin (LEnk) levels and the distribution and phosphorylation levels of delta and mu opioid receptors (DORs and MORs, respectively) in CA3 pyramidal cells and parvalbumin dentate hilar interneurons of adult female and male Sprague-Dawley rats. In females with elevated estrogen states (proestrus/estrus stage), acute THC altered the opioid system so that it resembled that seen in vehicle-injected females with low estrogen states (diestrus) and males: (1) mossy fiber LEnk levels in CA2/3a decreased; (2) phosphorylated-DOR levels in CA2/3a pyramidal cells increased; and (3) phosphorylated-MOR levels increased in most CA3b laminae. In males, acute THC resulted in the internalization of MORs in parvalbumin-containing interneuron dendrites which would decrease disinhibition of granule cells. In both sexes, acute THC redistributed DORs to the near plasma membrane of CA3 pyramidal cell dendrites, however, the dendritic region varied with sex. Additionally, acute THC also resulted in a sex-specific redistribution of DORs within CA3 pyramidal cell dendrites which could differentially promote synaptic plasticity and/or opioid-associated learning processes in both females and males.

DARK Classics in Chemical Neuroscience: Etonitazene and Related Benzimidazoles

Etonitazene and related 2-benzylbenzimidazoles are potent analgetics invented in the research laboratories of the Swiss pharmaceutical giant CIBA in the late 1950s. Though the unprecedented structure distinguishes this class of compounds from poppy-derived and other synthetic analgetics, a range of studies indicate that these drugs are selective μ opioid receptor agonists possessing morphine-like pharmacotoxicological properties in animals as well as humans. Several unscheduled members of this synthetically readily accessible class of opioids that are not controlled under the international and national drug control systems have recently emerged on the illicit drug market. Among them, isotonitazene has been implicated in at least 200 fatalities in Europe and North America. None of the 2-benzylbenzimidazole derivatives have been developed into medicines, but etonitazene and some of its derivatives have been used as receptor probes and in addiction behavior studies in animals. The unique structure has inspired research on such benzimidazoles and related benzimidazolones of which "brorphine" made its debut as one of the newest psychoactive substance to emerge on the illicit opioid drug market in mid-2019. This in-depth review provides a historical introduction, an overview on the chemistry, pharmacological profiles, adverse effects, addiction liability, regulatory status, and the impact on chemical neuroscience of the 2-benzylbenzimidazoles. Structurally related benzimidazoles with opioid and/or analgesic properties are also discussed briefly.

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Harmaline potentiates morphine-induced antinociception via affecting the ventral hippocampal GABA-A receptors in mice

Morphine is one of the most effective medications for treatment of pain, but its side effects limit its use. Therefore, identification of new strategies that can enhance morphine-induced antinociception and/or reduce its side effects will help to develop therapeutic approaches for pain relief. Considering antinociceptive efficacy of harmaline and also highlighted the important role of GABA-A receptors in the pain perception, this research aimed to determine whether the ventral hippocampal (vHip) GABA-A receptors are involved in the possible harmaline-induced enhancement of morphine antinociception. To achieve this, vHip regions of adult male mice were bilaterally cannulated and pain sensitivity was measured in a tail-flick apparatus. Intraperitoneally administration of morphine (0, 2, 4 and 6 mg/kg) or harmaline (0, 1.25, 5 and 10 mg/kg) increased the percentage of maximal possible effect (%MPE) and induced antinociception. Interestingly, co-administration of sub-effective doses of harmaline (5 mg/kg) and morphine (2 mg/kg) induced antinociception. Intra-vHip microinjection of muscimol (0, 200 and 300 ng/mice), a GABA-A receptor agonist, enhanced the anti-nociceptive effects of harmaline (2.5 mg/kg)+morphine (2 mg/kg) combination. Microinjection of the same doses of muscimol into the vHip by itself did not alter tail-flick latency. Intra-vHip microinjection of bicuculline (100 ng/mouse), a GABA-A receptor antagonist, did not cause a significant change in MPE%. Bicuculline (60 and 100 ng/mouse, intra-vHip) was administered with the harmaline (5 mg/kg)+morphine (2 mg/kg), and inhibited the potentiating effect of harmaline on morphine response. These findings favor the notion that GABAergic mechanisms in the vHip facilitate harmaline-induced potentiation of morphine response in the tail-flick test in part through GABA-A receptors. These findings shall provide insights and strategies into the development of pain suppressing drugs.

Five Decades of Research on Opioid Peptides: Current Knowledge and Unanswered Questions

In the mid-1970s, an intense race to identify endogenous substances that activated the same receptors as opiates resulted in the identification of the first endogenous opioid peptides. Since then, >20 peptides with opioid receptor activity have been discovered, all of which are generated from three precursors, proenkephalin, prodynorphin, and proopiomelanocortin, by sequential proteolytic processing by prohormone convertases and carboxypeptidase E. Each of these peptides binds to all three of the opioid receptor types (μ, δ, or κ), albeit with differing affinities. Peptides derived from proenkephalin and prodynorphin are broadly distributed in the brain, and mRNA encoding all three precursors are highly expressed in some peripheral tissues. Various approaches have been used to explore the functions of the opioid peptides in specific behaviors and brain circuits. These methods include directly administering the peptides ex vivo (i.e., to excised tissue) or in vivo (in animals), using antagonists of opioid receptors to infer endogenous peptide activity, and genetic knockout of opioid peptide precursors. Collectively, these studies add to our current understanding of the function of endogenous opioids, especially when similar results are found using different approaches. We briefly review the history of identification of opioid peptides, highlight the major findings, address several myths that are widely accepted but not supported by recent data, and discuss unanswered questions and future directions for research. SIGNIFICANCE STATEMENT: Activation of the opioid receptors by opiates and synthetic drugs leads to central and peripheral biological effects, including analgesia and respiratory depression, but these may not be the primary functions of the endogenous opioid peptides. Instead, the opioid peptides play complex and overlapping roles in a variety of systems, including reward pathways, and an important direction for research is the delineation of the role of individual peptides.

Oxycodone injections not paired with conditioned place preference have little effect on the hippocampal opioid system in female and male rats

Oxycodone (Oxy) conditioned place preference (CPP) in Sprague Dawley rats results in sex-specific alterations in hippocampal opioid circuits in a manner that facilitates opioid-associative learning processes, particularly in females. Here, we examined if Oxy (3 mg/kg, I.P.) or saline (Sal) injections not paired with behavioral testing similarly affect the hippocampal opioid system. Sal-injected females compared to Sal-injected males had: (1) higher densities of cytoplasmic delta opioid receptors (DOR) in GABAergic hilar dendrites suggesting higher baseline reserve DOR pools and (2) elevated phosphorylated DOR levels, but lower phosphorylated mu opioid receptor (MOR) levels in CA3a suggesting that the baseline pools of activated opioid receptors vary in females and males. In contrast to CPP studies, Oxy-injections in the absence of behavioral tests resulted in few changes in the hippocampal opioid system in either females or males. Specifically, Oxy-injected males compared to Sal-injected males had fewer DORs near the plasma membrane of CA3 pyramidal cell dendrites and in CA3 dendritic spines contacted by mossy fibers, and lower pMOR levels in CA3a. Oxy-injected females compared to Sal-injected females had higher total DORs in GABAergic dendrites and lower total MORs in parvalbumin-containing dendrites. Thus, unlike Oxy CPP, Oxy-injections redistributed opioid receptors in hippocampal neurons in a manner that would either decrease (males) or not alter (females) excitability and plasticity processes. These results indicate that the majority of changes within hippocampal opioid circuits that would promote opioid-associative learning processes in both females and males do not occur with Oxy administration alone, and instead must be paired with CPP.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

Orexin-1 receptors in the rostral ventromedial medulla are involved in the modulation of capsaicin evoked pulpal nociception and impairment of learning and memory

AIM

To investigate the role of rostral ventromedial medulla orexin-1 receptors in the modulation of orofacial nociception as well as nociception-induced learning and memory impairment in adult male rats.

METHODOLOGY

Pulpal nociception was induced by intradental application of capsaicin (100 μg) into the incisors of rats. Orexin-1 receptors agonist (orexin-A, 10, 25 and 50 pmol L^-1  rat^-1 ) and antagonist (SB-334867-A, 40 and 80 nmol L^-1  rat^-1 ) were microinjected into the rostral ventromedial medulla prior to capsaicin administration. Total time spent on nocifensive behaviour was recorded by direct visualization of freely moving rats whilst learning and memory were evaluated by the Morris water maze test. One-way analysis of variance and repeated-measures were used for the statistical analysis.

RESULTS

Capsaicin-treated rats had a significant increase of nocifensive behaviours (P < 0.001), as well as learning and memory impairment (P < 0.001). However, intraventromedial medulla prior micro-injection of orexin-A (50 pmol L^-1  rat^-1 ) significantly reduced the nociceptive behaviour (P < 0.001). This effect was blocked by pre-treatment with SB334867-A (80 nmol L^-1  rat^-1 ). Orexin-A (50 pmol L^-1  rat^-1 ) also inhibited nociception-induced learning and memory deficits. Moreover, administration of SB-334867-A (80 nmol L^-1  rat^-1 ) plus orexin-A (50 pmol L^-1  rat^-1 ) had no effect on learning and memory deficits induced by capsaicin.

CONCLUSIONS

The data suggest that rostral ventromedial medulla orexin-A receptors are involved in pulpal nociceptive modulation and improvement of learning and memory deficits induced by intradental application of capsaicin.

Kappa Opioid Receptors Mediate Heterosynaptic Suppression of Hippocampal Inputs in the Rat Ventral Striatum

Kappa opioid receptors (KORs) are highly enriched within the ventral striatum (VS) and are thought to modulate striatal neurotransmission. This includes presynaptic inhibition of local glutamatergic release from excitatory inputs to the VS. However, it is not known which inputs drive this modulation and what impact they have on the local circuit dynamics within the VS. Individual medium spiny neurons (MSNs) within the VS serve as a site of convergence for glutamatergic inputs arising from the PFC and limbic regions, such as the hippocampus (HP). Recent data suggest that competition can arise between these inputs with robust cortical activation leading to a reduction in ongoing HP-evoked MSN responses. Here, we investigated the contribution of KOR signaling in PFC-driven heterosynaptic suppression of HP inputs onto MSNs using whole-cell patch-clamp recordings in slices from adult rats. Optogenetically evoked HP EPSPs were greatly attenuated after a short latency (50 ms) following burst-like PFC electrical stimulation, and the magnitude of this suppression was partially reversed following blockade of GABA_ARs (GABA Type A receptors), but not GABA_BRs (GABA Type B receptors). A similar reduction in suppression was observed in the presence of the KOR antagonist, norBNI. Combined blockade of local GABA_ARs and KORs resulted in complete blockade of PFC-induced heterosynaptic suppression of less salient HP inputs. These findings highlight a mechanism by which strong, transient PFC activity can take precedence over other excitatory inputs to the VS.SIGNIFICANCE STATEMENT Emerging evidence suggests that kappa opioid receptor (KOR) activation can selectively modulate striatal glutamatergic inputs onto medium spiny neurons (MSNs). In this study, we found that robust cortical stimulation leads to a reduction in ongoing hippocampal-evoked MSNs responses through the combined recruitment of local inhibitory mechanisms and activation of presynaptic KORs in the ventral striatum (VS). These processes are likely to facilitate the efficient transfer of cortical information through the VS during critical decision making by dampening competing information from less salient excitatory inputs. These data provide a novel mechanism through which VS information processing could influence decision making, a function thought to occur primarily in the PFC.

Sex differences in subcellular distribution of delta opioid receptors in the rat hippocampus in response to acute and chronic stress

Drug addiction requires associative learning processes that critically involve hippocampal circuits, including the opioid system. We recently found that acute and chronic stress, important regulators of addictive processes, affect hippocampal opioid levels and mu opioid receptor trafficking in a sexually dimorphic manner. Here, we examined whether acute and chronic stress similarly alters the levels and trafficking of hippocampal delta opioid receptors (DORs). Immediately after acute immobilization stress (AIS) or one-day after chronic immobilization stress (CIS), the brains of adult female and male rats were perfusion-fixed with aldehydes. The CA3b region and the dentate hilus of the dorsal hippocampus were quantitatively analyzed by light microscopy using DOR immunoperoxidase or dual label electron microscopy for DOR using silver intensified immunogold particles (SIG) and GABA using immunoperoxidase. At baseline, females compared to males had more DORs near the plasmalemma of pyramidal cell dendrites and about 3 times more DOR-labeled CA3 dendritic spines contacted by mossy fibers. In AIS females, near-plasmalemmal DOR-SIGs decreased in GABAergic hilar dendrites. However, in AIS males, near-plasmalemmal DOR-SIGs increased in CA3 pyramidal cell and hilar GABAergic dendrites and the percentage of CA3 dendritic spines contacted by mossy fibers increased to about half that seen in unstressed females. Conversely, after CIS, near-plasmalemmal DOR-SIGs increased in hilar GABA-labeled dendrites of females whereas in males plasmalemmal DOR-SIGs decreased in CA3 pyramidal cell dendrites and near-plasmalemmal DOR-SIGs decreased hilar GABA-labeled dendrites. As CIS in females, but not males, redistributed DOR-SIGs near the plasmalemmal of hilar GABAergic dendrites, a subsequent experiment examined the acute affect of oxycodone on the redistribution of DOR-SIGs in a separate cohort of CIS females. Plasmalemmal DOR-SIGs were significantly elevated on hilar interneuron dendrites one-hour after oxycodone (3 mg/kg, I.P.) administration compared to saline administration in CIS females. These data indicate that DORs redistribute within CA3 pyramidal cells and dentate hilar GABAergic interneurons in a sexually dimorphic manner that would promote activation and drug related learning in males after AIS and in females after CIS.

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Females have more near-plasmalemmal DORs in pyramidal CA3 dendrites than males.

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Acute stress in males relocates DORs in CA3 & GABA dendrites to promote activation.

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Chronic stress in females relocates DORs in GABA dendrites in females to promote activation.

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Chronic stress in males relocates DORs in GABA dendrites opposite of females.

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DOR-stress relocation may contribute to sexually dimorphic effects on drug related learning.

Is Hippocampus Susceptible to Antinociceptive Tolerance to NSAIDs Like the Periaqueductal Grey?

Emotional distress is the most undesirable feature of painful experience. Numerous studies have demonstrated the important role of the limbic system in the affective-motivational component of pain. The purpose of this paper was to examine whether microinjection of nonsteroidal anti-inflammatory drugs (NSAIDs), Clodifen, Ketorolac, and Xefocam, into the dorsal hippocampus (DH) leads to the development of antinociceptive tolerance in male rats. We found that microinjection of these NSAIDs into the DH induces antinociception as revealed by a latency increase in the tail-flick (TF) and hot plate (HP) tests compared to controls treated with saline into the DH. Subsequent tests on consecutive three days, however, showed that the antinociceptive effect of NSAIDs progressively decreased, suggesting tolerance developed to this effect of NSAIDs. Both pretreatment and posttreatment with the opioid antagonist naloxone into the DH significantly reduced the antinociceptive effect of NSAIDs in both pain models. Our data indicate that microinjection of NSAIDs into the DH induces antinociception which is mediated via the opioid system and exhibits tolerance.

Antinociceptive tolerance to NSAIDs microinjected into dorsal hippocampus

Background

Pain is characterized as a complex experience, dependent not only on the regulation of nociceptive sensory systems, but also on the activation of mechanisms that control emotional processes in limbic brain areas such as the amygdala and the hippocampus. Several lines of investigations have shown that in some brain areas, particularly the midbrain periaqueductal gray matter, rostral ventro-medial medulla, central nucleus of amygdala and nucleus raphe magnus, microinjections of non-steroidal anti-inflammatory drugs (NSAIDs) induce antinociception with distinct development of tolerance. The present study was designed to examine whether microinjection of NSAIDs, clodifen, ketorolac and xefocam into the dorsal hippocampus (DH) leads to the development of antinociceptive tolerance in male rats.

Methods

The experiments were carried out on experimental and control (with saline) white male rats. Animals were implanted with a guide cannula in the DH and tested for antinociception following microinjection of NSAIDs into the DH in the tail-flick (TF) and hot plate (HP) tests. Repeated measures of analysis of variance with post-hoc Tukey-Kramer multiple comparison tests were used for statistical evaluations.

Results

We found that microinjection of these NSAIDs into the DH induces antinociception as revealed by a latency increase in the TF and HP tests compared to controls treated with saline into the DH. Subsequent tests on days 2 and 3, however, showed that the antinociceptive effect of NSAIDs progressively decreased, suggesting tolerance developed to this effect of NSAIDs. Both pretreatment and post-treatment with the opioid antagonist naloxone into the DH significantly reduced the antinociceptive effect of NSAIDs in both pain models.

Conclusions

Our results indicate that microinjection of NSAIDs into the DH induces antinociception which is mediated via the opioid system and exhibits tolerance.

The effect of chronic thienorphine administration on long-term potentiation and synaptic structure in rat hippocampus

Thienorphine is a new nonselective partial agonist of opioid receptors, which is currently under a Phase II clinical trial in China as a new treatment for opioid dependence. In this study, we compared the effect of thienorphine with morphine on long-term potentiation (LTP) in the lateral perforant path (LPP)-granule cell synapse of the rat dentate gyrus (DG). Furthermore, the effect of thienorphine on the synaptic structure of the CA1 hippocampal region and the expression of synaptophysin was investigated. Results indicated interesting differences between thienorphine and morphine on the modulation of hippocampal synaptic plasticity. Chronic thienorphine treatment facilitated LTP in the LPP-DG cell synapses more than chronic morphine treatment. Morphometric measurement and analysis showed that chronic thienorphine administration decreased the length of the active zone and reduced the thickness of CA1 postsynaptic densities compared with the saline group (control), but were elevated compared with the morphine group. Furthermore, the expression of hippocampal synaptophysin was increased with chronic thienorphine administration but reduced with chronic morphine treatment. Taken together, our study clearly demonstrates that chronic thienorphine treatment enhances LTP, modulates hippocampal synaptic structure, and increases the expression of hippocampal synaptophysin. Therefore, further study is warranted to investigate thienorphine as a new treatment for opioid dependence.

The long-term effects of neonatal morphine administration on the pentylenetetrazol seizure model in rats: the role of hippocampal cholinergic receptors in adulthood

Early life exposure to opiates may affect neuropathological conditions, such as epilepsy, during adulthood. We investigated whether neonatal morphine exposure affects pentylenetetrazol (PTZ)-induced seizures in adulthood. Male rats were subcutaneously injected with morphine or saline on postnatal days 8-14. During adulthood, each rat was assigned to 1 of the following 10 sub-groups: saline, nicotine (0.1, 0.5, or 1 μg), atropine (0.25 or 1 μg), oxotremorine M (0.1 or 1 μg), or mecamylamine (2 or 8 μg). An intrahippocampal infusion of the indicated compound was administered 30 min before seizure induction (80 mg/kg PTZ). Compared with the saline/oxotremorine (1 μg), saline/saline, and morphine/saline groups, the morphine/oxotremorine (1 μg) group showed a significantly increased latency to the first epileptic behavior. The duration of tonic-clonic seizures was significantly lower in the morphine/oxotremorine (1 μg) group compared to the saline/saline and morphine/saline groups. The severity of seizure was significantly decreased in the morphine/atropine (1 μg) group than in the saline/atropine (1 μg). Seizure severity was also decreased in the morphine/mecamylamine (2 μg) group than in the saline/mecamylamine (2 μg) group. Latency for death was significantly lower in the morphine/mecamylamine (2 μg) group compared with the saline/mecamylamine (2 μg) group. Mortality rates in the morphine/atropine (1 μg) and morphine/mecamylamine (2 μg) groups were significantly lower than those in the saline/atropine (1 μg) and saline/mecamylamine (2 μg) groups, respectively. Chronic neonatal morphine administration attenuated PTZ-induced seizures, reduced the mortality rate, and decreased the impact of the hippocampal cholinergic system on seizures and mortality rate in adult rats. Neonatal morphine exposure induces changes to μ-receptors that may lead to activation of GABAergic neurons in the hippocampus. This pathway may explain the anti-convulsant effects of morphine observed in our study.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

A role for the ventral hippocampal endocannabinoid system in fear-conditioned analgesia and fear responding in the presence of nociceptive tone in rats

The endogenous cannabinoid (endocannabinoid) system plays an important role in fear-conditioned analgesia (FCA) and expression and extinction of conditioned fear. The hippocampus has an established role in both pain and conditioned fear and is a substrate for endocannabinoid activity. This study aimed to investigate the role of the endocannabinoid system in the ventral hippocampus (vHip) in FCA and in fear responding in the presence of nociceptive tone. Fear-conditioned rats displayed significantly increased freezing and 22-kHz ultrasonic vocalisation and a reduction in formalin-evoked nociceptive behaviour (ie, FCA) upon re-exposure to a context previously paired with footshock. Tissue levels of the endocannabinoids, anandamide, and 2-arachidonoylglycerol, as well as the fatty acid amide, palmitoylethanolamide, were significantly higher in the vHip of fear-conditioned rats compared with non-fear-conditioned controls. URB597 (inhibitor of fatty acid amide hydrolase [FAAH]), administered bilaterally into the vHip, significantly enhanced FCA during the entire trial and increased fear responding in formalin-treated rats early in the trial. The URB597-induced enhancement of FCA was blocked by intra-vHip administration of the cannabinoid(1) (CB(1)) receptor antagonist/inverse agonist rimonabant. Intra-vHip rimonabant alone had no effect on the expression of FCA, and URB597 did not significantly alter formalin-evoked nociceptive behaviour in non-fear-conditioned rats. These data suggest an important role for the endocannabinoid system in the vHip in FCA, whereby levels of 2-arachidonoylglycerol and the FAAH substrates palmitoylethanolamide and anandamide are increased in rats expressing FCA, and pharmacological inhibition of FAAH in the vHip enhances this form of endogenous analgesia via a CB(1) receptor-dependent mechanism.

Activation of κ opioid receptors increases intrinsic excitability of dentate gyrus granule cells

The dentate gyrus of the hippocampus is thought to control information flow into the rest of the hippocampus. Under pathological conditions, such as epilepsy, this protective feature is circumvented and uninhibited activity flows throughout the hippocampus. Many factors can modulate excitability of the dentate gyrus and ultimately, the hippocampus. It is therefore of critical importance to understand the mechanisms involved in regulating excitability in the dentate gyrus. Dynorphin, the endogenous ligand for the kappa (κ) opioid receptor (KOR), is thought to be involved in neuromodulation in the dentate gyrus. Both dynorphin and its receptor are widely expressed in the dentate gyrus and have been implicated in epilepsy and other complex behaviours such as stress-induced deficits in learning and stress-induced depression-like behaviours. Administration of KOR agonists can prevent both the behavioural and electroencephalographic measures of seizures in several different models of epilepsy. Antagonism of the KORs also prevents stress-induced behaviours. This evidence suggests the KORs as possible therapeutic targets for various pathological conditions. In addition, KOR agonists prevent the induction of LTP. Although there are several mechanisms through which dynorphin could mediate these effects, no studies to date investigated the effects of KOR activation on intrinsic membrane properties and cell excitability. We used whole-cell, patch-clamp recordings from acute mouse hippocampus slices to investigate the effect of KOR activation on dentate gyrus granule cell excitability. The agonist U69,593 (U6, 1 μM) resulted in a lower spike threshold, a decreased latency to first spike, an increased spike half-width, and an overall increase in spike number with current injections ranging from 15 to 45 pA. There was also a reduction in the interspike interval (ISI) both early and late in the spike train, with no change in membrane potential or input resistance. Preincubation of the slice with the selective KOR antagonist, nor-binalthorphimine (BNI, 1 μM) inhibited the effect of U6 on the latency to first spike and spike half-width suggesting that these effects are mediated through KORs. The inclusion of GDP-βS (1 mM) in the recording pipette prevented all of the U6 effects, suggesting that all effects are mediated via a G-protein-dependent mechanism. Inclusion of the A-type K+ current blocker, 4-aminopyridine (4-AP, 5 mM) in the pipette also antagonised the effects of U6. Kv4.2 is one of the channel α subunits thought to be responsible for carrying the A-type K+ current. Incubation of hippocampus slices with U6 resulted in a decrease in the Kv4.2 subunit protein at the cell surface. These results are consistent with an increase in cell excitability in response to KOR activation and may reflect new possibilities for additional opioid functions.

Kappa opioid receptors mediate where fear is expressed following extinction training

Six experiments used a within-subjects renewal design to examine the involvement of kappa opioid receptors (KORs) in regulating the expression and recovery of extinguished fear. Rats were trained to fear a tone conditioned stimulus (CS) via pairings with foot shock in a distinctive context (A). This was followed by extinction training of the CS in a second context (B). Finally, all rats were tested for fear to the tone in the extinction context (ABB) and the training (ABA) or a novel (ABC) context. Intracerebroventricular (ICV) infusion of the KOR antagonist nor-binaltorphimine dihydrochloride (nor-BNI) dose-dependently prevented ABA renewal of fear, but had no effect on the expression of ABC renewal, the expression of extinction, or the expression of nonextinguished fear. Conversely, pretest infusion of the KOR agonist U50,488 hydrochloride (U50,488) selectively facilitated the expression of ABA renewal and had no effect on the expression of extinction. Pretest infusion of nor-BNI had no effect on the expression of context-specific latent inhibition. Collectively, these results suggest that KORs gate the expression of fear following extinction training and may comprise a critical neuropeptide component of the circuitry underlying context-dependent expression of fear.

Hippocampal dynorphin immunoreactivity increases in response to gonadal steroids and is positioned for direct modulation by ovarian steroid receptors

The hippocampal formation (HF) is involved in modulating learning related to drug abuse. While HF-dependent learning is regulated by both endogenous opioids and estrogen, the interaction between these two systems is not well understood. The mossy fiber (MF) pathway formed by dentate gyrus (DG) granule cell axons is involved in some aspects of learning and contains abundant amounts of the endogenous opioid peptide dynorphin (DYN). To examine the influence of ovarian steroids on DYN expression, we used quantitative light microscopic immunocytochemistry to measure DYN levels in normal cycling rats as well as in two established models of hormone-treated ovariectomized (OVX) rats. Rats in estrus had increased levels of DYN-immunoreactivity (ir) in the DG and certain CA3 lamina compared with rats in proestrus or diestrus. OVX rats exposed to estradiol for 24 h showed increased DYN-ir in the DG and CA3, while those with 72 h estradiol exposure showed increases only in the DG. Six hours of estradiol exposure produced no change in DYN-ir. OVX rats chronically implanted with medroxyprogesterone also showed increased DYN-ir in the DG and CA3. Next, dual-labeling electron microscopy (EM) was used to evaluate the subcellular relationships of estrogen receptor (ER) alpha-, ERbeta and progestin receptor (PR) with DYN-labeled MFs. ERbeta-ir was in some DYN-labeled MF terminals and smaller terminals, and had a subcellular association with the plasmalemma and small synaptic vesicles. In contrast, ERalpha-ir was not in DYN-labeled terminals, although some DYN-labeled small terminals synapsed on ERalpha-labeled dendritic spines. PR labeling was mostly in CA3 axons, some of which were continuous with DYN-labeled terminals. These studies indicate that ovarian hormones can modulate DYN in the MF pathway in a time-dependent manner, and suggest that hormonal effects on the DYN-containing MF pathway may be directly mediated by ERbeta and/or PR activation.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Theory of active antidepressants: A nonsynaptic approach to the treatment of depression

Although depression is one of the major neuropsychiatric disorders, the success rate of medication for any drug is about 60%, which means that approximately 40% of the patients does not respond to the initial treatment. The major aim of this review is to provide a possible explanation for the relative inefficacy of currently used antidepressants and to propose a novel mechanism of action, which might improve the success rate of clinical treatment. According to the monoamine theory the most important neurochemical process in depression is the impairment of monoaminergic neurotransmission and the concomitant decrease of extracellular concentration of noradrenaline and/or serotonin. Since the vast majority of monoaminergic varicosities makes no synaptic contact but is able to release transmitters directly into the extrasynaptic space, the monoaminergic neurotransmission is predominantly nonsynaptic in nature. Depression can be regarded, therefore, as a disease, which is developed (at least in part) on the basis of the impairment of nonsynaptic interactions and the effective treatment has to improve this non-conventional communication in the nervous system. The currently used antidepressants (reuptake inhibitors, negative feedback inhibitors, monoamino oxidase inhibitors) can increase the monoamine levels in the extracellular space only if the monoaminergic cells are electrically active and without an action potential-induced vesicular exocytosis these compounds are ineffective. It is proposed that a selective and moderate induction of the carrier-mediated release of NA and 5-HT might be a better therapeutic approach to the treatment of depression, since this new class of antidepressants, the so-called 'active antidepressants' have a mechanism of action, which is independent from the electrical activity of monoaminergic cells, therefore the extrasynaptic concentration of monoamines and thereby the nonsynaptic communication can be enhanced more efficiently.

Layer selective presynaptic modulation of excitatory inputs to hippocampal cornu Ammon 1 by mu-opioid receptor activation

Chronic and acute activation of mu-opioid receptors (MOR) in hippocampal cornu Ammon 1 (CA1) disrupts rhythmic activity, alters activity-dependent synaptic plasticity and impairs spatial memory formation. In CA1, MORs act by hyperpolarizing inhibitory interneurons and suppressing inhibitory synaptic transmission. MOR modulation of inhibitory synaptic function translates into an increase in excitatory activity in all layers of CA1. However, the exact anatomical sites for MOR actions are not completely known. Therefore, we used voltage-sensitive dye imaging, whole cell patch clamping, photolysis of alpha-carboxy-2-nitrobenzyl ester, trifluoroacetic acid salt (CNB) -caged GABA, and micro-sectioned slices of rat hippocampus to investigate the effect of MOR activation in CA1. First, we investigated the effect of MOR activation using a MOR agonist [d-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO) on the direct activation of GABA receptors by photolysis of CNB-caged GABA in all layers of CA1. MOR activation did not affect hyperpolarizations due to direct GABA receptor activation in any layer of CA1, but MOR activation did suppress GABAergic inhibitory postsynaptic potentials suggesting that MOR activation acts by presynaptically inhibiting interneuron function. We next examined whether MOR activation was equivalently effective in all anatomical layers of CA1. To do this, cuts were made between anatomical layers of CA1 and isolated layers were stimulated electrically (five pulses at 20 Hz) to produce excitatory postsynaptic potentials (EPSPs). Under these conditions, MOR activation significantly increased EPSP areas in stratum radiatum (SR), stratum pyramidale (SP) and stratum oriens (SO) relative to stratum lacunosum-moleculare (SLM). When compared with the effect of GABA(A) and GABA(B) receptor antagonists on EPSP areas, the effect of DAMGO was proportionately larger in SR, SP and SO than in SLM. We conclude that MOR activation is more effective at directly modulating activity in SR, SP and SO, and the smaller effect in SLM is likely due to a smaller MOR inhibition of GABA release in SLM.

Effects of mu-opioid receptor modulation on GABAB receptor synaptic function in hippocampal CA1

Activation of mu-opioid receptors (MORs) alters information coding, synaptic plasticity, and spatial memory in hippocampal CA1. In CA1, MORs act by inhibiting GABA release onto both GABA(A) and GABA(B) receptors exclusively. MOR activation can facilitate excitatory inputs in CA1 dendritic layers by inhibiting synaptic activation of GABA(A) receptors. In this study, we use voltage-sensitive dye imaging to show that MOR activation by the MOR agonist DAMGO suppressed GABA(B) inhibitory postsynaptic potentials in all layers of CA1. When stimulating excitatory input in stratum oriens (SO), stratum radiatum (SR), or stratum lacunosum-moleculare (SLM) with five pulses at 20 Hz in the presence of bicuculline (50 microM), DAMGO (1 muM) was most effective at increasing the amplitude of the last excitatory event. This effect was reversed by the MOR antagonist CTOP (1 muM) and occluded by the GABA(B) receptor agonist CGP 55845 (10 microM). DAMGO was less effective at increasing the amplitude of later excitatory events compared with the effect of CGP 55845. DAMGO was relatively ineffective at increasing the amplitude of excitatory inputs in SLM but had significantly greater effects on excitatory events as they propagated to stratum pyramidale (SP). When stimulating in SR, DAMGO was least effective at increasing excitatory amplitudes in SLM and most effective in SP and SO. Finally, DAMGO was equally effective at increasing excitatory activity amplitudes in all layers of CA1 after stimulating in SO. Therefore MOR suppresses GABA(B) synaptic hyperpolarizations in all layers of CA1 and most effectively facilitates excitatory activity in CA1 output layers.

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Opioid systems in the dentate gyrus

Opiate drugs alter cognitive performance and influence hippocampal excitability, including long-term potentiation (LTP) and seizure activity. The dentate gyrus (DG) contains two major opioid peptides, enkephalins and dynorphins, which have opposing effects on excitability. Enkephalins preferentially bind to delta- and mu-opioid receptors (DORs and MORs) while dynorphins preferentially bind to kappa-opioid receptors (KORs). Opioid receptors can also be activated by exogenous opiate drugs such as the MOR agonist morphine. Enkephalins are contained in the mossy fiber pathway, in the lateral perforant path (PP) and in scattered GABAergic interneurons. MORs and DORs are predominantly in distinct subpopulations of GABAergic interneurons known to inhibit granule cells, and are present at low levels within granule cells. MOR and DOR agonists increase excitability and facilitate LTP in the molecular layer. Anatomical and physiological evidence is consistent with somatodendritic and axon terminal targeting of both MORs and DORs. Dynorphins are in the granule cells, most abundantly in mossy fibers but also in dendrites. KORs have been localized to granule cell mossy fibers, supramammillary afferents to granule cells, and PP terminals. KOR agonists, including endogenous dynorphins, diminish the induction of LTP. Recent evidence indicates that opiates and opioids also modulate other processes in the hippocampal formation, including adult neurogenesis, the actions of gonadal hormones, and development of neonatal transmitter systems.

The distribution of dopamine D1 receptor and μ-opioid receptor 1 receptor immunoreactivities in the amygdala and interstitial nucleus of the posterior limb of the anterior commissure: Relationships to tyrosine hydroxylase and opioid peptide terminal systems

Mismatches between dopamine innervation and dopamine D1 receptor (D1) distribution have previously been demonstrated in the intercalated cell masses of the rat amygdala. Here the distribution of enkephalin and beta-endorphin immunoreactive (IR) nerve terminals with respect to their mu-opioid receptors is examined in the intercalated cell masses, along with a further immunohistochemical analysis of the dopamine/D1 mismatches. A similar analysis is also made within the extended amygdala. A spatial mismatch in distribution patterns was found between the mu-opioid receptor-1 immunoreactivity and enkephalin IR in the main intercalated island of the amygdala. Discrete cell patches of dopamine D1 receptor and mu-opioid receptor-1 IR were also identified in a distinct region of the extended amygdala, the interstitial nucleus of the posterior limb of the anterior commissure, medial division (IPACM), which displayed sparse tyrosine hydroxylase or enkephalin/beta-endorphin IR nerve terminals. Furthermore, distinct regions of the main intercalated island that showed dopamine/D1 receptor matches (the rostral and rostrolateral parts) were associated with strong dopamine and cyclic AMP regulated phosphoprotein, 32 kDa-IR in several D1 IR neuronal cell bodies and dendrites, whereas this was not the case for the dopamine/D1 mismatch areas (the rostromedial and caudal parts) of the main intercalated island. The lack of correlation between the terminal/receptor distribution patterns suggests a role for volume transmission for mu-opioid receptor- and dopamine D1 receptor-mediated transmission in distinct regions of the amygdala and extended amygdala. This may have implications for amygdaloid function, where slow long lasting responses may develop as a result of volume transmission operating in opioid peptide and dopaminergic communication.

Mifepristone (RU486) inhibits lateral perforant path long‐term potentiation in hippocampal slices from prenatally morphine‐exposed female rats

In brain slices from prenatally saline-exposed female rats during proestrus and diestrus, long-term potentiation (LTP) can be induced in the lateral perforant pathway (LPP). Prenatal morphine exposure suppresses LTP induction in the LPP during proestrus. Here we studied synaptic plasticity in the LPP in slices from female rats prenatally exposed to morphine. Two additional factors were investigated: the role of the estrous cycle and role of glucocorticoid receptors. Hippocampal slices were prepared from adult, prenatally saline- or morphine-exposed female rats. One hour prior to decapitation, vaginal smears were obtained and the rats either in proestrus or diestrus were treated with a non-specific glucocorticoid receptor antagonist mifepristone (RU486) or with a vehicle. LPP was stimulated with high-frequency stimulation. Short-tem plasticity (STP) and the induction and maintenance of long-term potentiation (LTP) were assessed. In all groups of prenatally saline-exposed rats, LTP was induced and maintained with the exception of RU486-treated rats during proestrus where the LTP was induced but not maintained. In prenatally morphine-exposed females in diestrus, both STP and LTP were induced after postnatal vehicle treatment. In morphine-exposed, proestrous females, neither STP nor LTP were induced irrespective of the postnatal treatment. Thus, prenatal morphine exposure suppresses the induction of LTP in the LPP, except during diestrus. Data indicate that the induction and maintenance of LTP in the LPP in hippocampal slices from female rats is multifactorial: ovarian steroids and functionality of glucocorticoid receptors cooperation are necessary for induction and maintenance of the LTP, prenatal morphine exposure interferes with this process possibly by its long-term effects on synaptic plasticity.

Phase-specific central regulatory systems of hibernation in Syrian hamsters

The central body temperature (T(b)) regulation system during hibernation was investigated in Syrian hamsters of either sex. Hibernation induced in Syrian hamsters by housing them in a cold room under short day-light/dark cycle was confirmed by marked reductions in the heart rate, T(b) and respiratory rate. The hibernation of hamsters was classified into (i) entrance, (ii) maintenance and (iii) arousal phases according to T(b) changes. In hibernating hamsters, T(b) elevations were phase-selectively elicited by intracerebroventricular (ICV) injection of 8-cyclopenthyltheophylline (CPT; a selective A1-adenosine receptor antagonist) and naloxone (a non-selective opioid receptor antagonist) during the entrance and maintenance phases, respectively. Moreover, a similar T(b) elevation tendency during the maintenance phase was also induced by ICV naloxonazine, (a selective mu1-opioid receptor antagonist), although such was not the case for naltrindole (a selective delta-opioid receptor antagonist) or nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist). Furthermore, T(b) elevations in hibernating hamsters were similarly induced with ICV thyrotropin-releasing hormone (TRH) during the entrance and maintenance phases. Furthermore, ICV injection of the anti-TRH antibody ameliorated the T(b) elevations induced by tactile stimulation. These results suggest that activation of the A1-receptor by adenosine is important for the generation of hypothermia in the entrance phase, and that activation of the mu1-opioid receptor by opioid peptides is required for perpetuation of hypothermia in the maintenance phase. In addition, TRH is a key endogenous substance involved in T(b) elevations during the arousal phase of hibernating hamsters.

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Epilepsy, CNS viral injury and dynorphin

Epilepsy is a significant health problem. Despite the widespread use of both classic and newer pharmacological agents that target ion channels, amino acid transmission or receptors, there are numerous examples of mono- or polytherapy being ineffective. Seizures that are secondary to CNS infections are among the most refractory medically, and thus insult-specific agents are desirable. Recently, the study of the neuropharmacological actions of dynorphin in CNS viral injury has yielded new insights into epileptogenesis and epilepsy treatment. The opioid neuropeptide dynorphin modulates neuronal excitability in vitro in hippocampal slices and potentiates endogenous anti-ictal (i.e. protective) processes in animal models and humans. This work has renewed interest in the role of dysregulation of dynorphin in the pathogenesis of refractory seizures, including encephalitic seizures. The important role of dynorphin in epilepsy is also supported by new models of symptomatic epilepsies based on viral-induced seizures.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

Kappa-opioid receptor in the rodent hippocampus: a comparative immunocytochemical study in the rat, guinea pig, hamster and gerbil

Pre-embedding light microscopic immunocytochemistry, using a monoclonal antibody (mAb-KA8) raised against a frog brain kappa receptor preparation, recognising selectively the kappa-opioid receptor, was used for studying the occurrence, distribution, and species-specificity of the kappa-opioid receptor in the hippocampal formation of four rodent species (rat, guinea pig, hamster and gerbil). MAb-KA8 immunoreactivity was detectable in the rat, hamster and gerbil hippocampus, however the distribution of the labelled structures was heterogeneous. In the rat and hamster the hilus of dentate gyrus and the stratum oriens of the CA1 area contained immunoreactive cell bodies and proximal dendrites. In the gerbil mAb-KA8 immunopositive cell bodies were recognisable in the stratum radiatum of the CA1 and CA3 areas and in the subiculum. In the hamster varicose axon-like elements were also detected in the CA3 pyramidal layer. With the mAb-KA8 antibody there was no detectable kappa opioid receptor labelling in the hippocampus of the guinea pig. The results confirm the high degree of species-specific heterogeneity characterising the distribution of opioid peptides and their receptors in the hippocampal formation. The receptor was found in most cases postsynaptically, however in the hamster the immunopositive axons may refer to a presynaptic localisation.

Mu-opioid receptors facilitate the propagation of excitatory activity in rat hippocampal area CA1 by disinhibition of all anatomical layers

Hippocampal mu-opioid receptors (MORs) have been implicated in memory formation associated with opiate drug abuse. MORs modulate hippocampal synaptic plasticity acutely, when chronically activated, and during drug withdrawal. At the network level, MORs increase excitability in area CA1 by disinhibiting pyramidal cells. The precise inhibitory interneuron subtypes affected by MOR activation are unknown; however, not all subtypes are inhibited, and specific interneuron subtypes have been shown to preferentially express MORs. Here we investigate, using voltage-sensitive dye imaging in brain slices, the effect of MOR activation on the patterns of inhibition and on the propagation of excitatory activity in rat hippocampal CA1. MOR activation augments excitatory activity evoked by stimulating inputs in stratum oriens [i.e., Schaffer collateral and commissural pathway (SCC) and antidromic], stratum radiatum (i.e., SCC), and stratum lacunosum-moleculare (SLM; i.e., perforant path and thalamus). The augmented excitatory activity is further facilitated as it propagates through the CA1 network. This was observed as a proportionately larger increase in amplitudes of excitatory activity at sites distal from where the activity was evoked. This facilitation was observed for excitatory activity propagating from all three stimulation sites. The augmentation and facilitation were prevented by GABAA receptor antagonists (bicuculline, 30 microM), but not by GABAB receptor antagonists (CGP 55845, 10 microM). Furthermore, MOR activation inhibited IPSPs in all layers of area CA1. These findings suggest that MOR-induced suppression of GABA release onto GABAA receptors augments all inputs to CA1 pyramidal cells and facilitates the propagation of excitatory activity through the network of area CA1.

Opioid-induced proliferation through the MAPK pathway in cultures of adult hippocampal progenitors

Administration of opioid agonists or antagonists has been reported to regulate proliferation or survival of neural progenitors in vivo. Here we report that beta-endorphin and selective mu-opioid receptor (MOR) and delta-opioid receptor (DOR) agonists stimulate proliferation of isolated rat adult hippocampal progenitors (AHPs). The AHPs were found to express DORs and MORs, but not kappa-opioid receptors. Incubation with beta-endorphin for 48 h increased the number of AHPs found in mitosis, the total DNA content, and the expression of proliferating cell nuclear antigen. This proliferative effect from beta-endorphin on AHPs was antagonized by naloxone. The beta-endorphin-induced proliferation was mediated through phosphorylation of extracellular signal-regulated kinases 1 and 2 and dependent on phosphatidylinositol 3-kinase and both intra- and extracellular calcium. These data suggest a role for the opioid system in the regulation of proliferation in progenitors from the adult hippocampus.

Differential involvement of the mu and kappa opioid receptors in spatial learning

In order to test the role of mu and kappa opioid receptors (Mu opioid receptor (MOR) and Kappa opioid receptor (KOR)) in hippocampal-dependent spatial learning, we analyzed genetically engineered null mutant mice missing the functional MOR or KOR gene. Compared to wild-type mice, the homozygous MOR null mutants exhibited an impairment in the ultimate level of spatial learning as shown in two distinct tasks, the 8-arm radial-maze and the Morris water-maze. Control behaviors were normal. The learning impairment could be associated with the impairment we found in the maintenance of long-term potentiation in mossy fibers in CA3. In comparison, there was no impairment in spatial learning in our KOR mutants or in mossy fibers (mf) in CA3 region long-term potentiation (LTP). Our work suggests that the MOR may play a positive role in learning and memory by increasing LTP in CA3 neurons.

Mu- and delta-opioid receptor antagonists decrease proliferation and increase neurogenesis in cultures of rat adult hippocampal progenitors

Opioids have previously been shown to affect proliferation and differentiation in various neural cell types. In the present study, cultured rat adult hippocampal progenitors (AHPs) were shown to release beta-endorphin. Membrane preparations of AHPs were found to bind [125I]beta-endorphin, and immunoreactivity for mu- and delta-opioid receptors (MORs and DORs), but not for kappa-opioid receptors (KORs), was found on cells in culture. Both DNA content and [3H]thymidine incorporation were reduced after a 48-h incubation with 100 microM naloxone, 10 micro m naltrindole or 10 microM beta-funaltrexamine, but not nor-binaltorphimine, suggesting proliferative actions of endogenous opioids against MORs and DORs on AHPs. Furthermore, analysis of gene and protein expression after incubation with MOR and DOR antagonists for 48 h using RT-PCR and Western blotting suggested decreased signalling through the mitogen-activated protein kinase (MAPK) pathway and lowered levels of genes and proteins that are important in cell cycling. Cultures were incubated with naloxone (10 or 100 microM) for 10 days to study the effects on differentiation. This resulted in an approximately threefold increase in neurogenesis, a threefold decrease in astrogliogenesis and a 50% decrease in oligodendrogenesis. In conclusion, this study suggests that reduced signalling through MORs and DORs decreases proliferation in rat AHPs, increases the number of in vitro-generated neurons and reduces the number of astrocytes and oligodendrocytes in culture.

Monoclonal Antibodies to Late-differentiating Epitopes Identify Mossy Fibre Terminals Innervating Normal and Transplanted Hippocampal CA3 Pyramidal Cells

We have derived two monoclonal antibodies, MF-1 and MF-2, which both recognize the same 58-kD antigen. Light and electron microscopic immunocytochemistry showed that this antigen is highly expressed in the large mossy fibre terminals innervating the proximal portion of the apical dendrites of pyramidal neurons in hippocampal field CA3. Staining was seen in the adult hippocampus in rats and mice, and in a post mortem human sample. Comparison with the Timm stain showed that the antibodies recognize mossy fibres from all parts of the adult dendate gyrus except for the tip of the infrapyramidal blade (the latest part of the dentate gyrus to develop). The MF antigen is expressed by mature terminals, and is not detected immunohistochemically in developing hippocampal mossy terminals until the end of the first postnatal week (i.e. later than the Timm-positive material). It was also found in host mossy fibre terminals innervating embryonic CA3 pyramids transplanted into adult hosts, but not in areas of the graft containing transplanted CA1 pyramids. These results indicate that this previously undescribed, late-developing antigen provides a useful specific marker for the mossy fibre projection in both the normal hippocampus and in situations of experimentally manipulated connectivity.

Neurons with mu opioid receptors interact indirectly with enkephalin-containing neurons in the rat dentate gyrus

In the dentate gyrus, mu opioid receptors (MORs) and their enkephalin agonists have overlapping distributions and influence excitability and plasticity. Released endogenous enkephalins can activate at least some of these MORs; however, whether these interactions involve synaptically associated profiles or more distant associations and whether some subcellular compartments (e.g., terminals or dendrites) are more likely to be targeted than others are not known. To elucidate the relationships between potential sites of enkephalin release and MORs, MOR1 and leucine-enkephalin (LE) immunoreactivities were localized in the hilus by electron microscopy, using immunoperoxidase and immunogold markers. Of the 573 MOR-immunoreactive (ir) profiles analyzed, most were axons and terminals (51 and 30%, respectively), and fewer were dendrites (12%), glia (3%), or unclassifiable (4%). Most MOR-ir profiles resembled interneuron processes, while most LE-ir terminals resembled mossy fibers. One third of MOR-ir profiles were within 3 microm and approximately half were within 4 microm of the nearest LE-ir profile. In contrast, few (3%) MOR-ir profiles contacted LE-ir profiles; only 16% of these contacts included observable synapses, and very few profiles (0.5%) colocalized MOR and LE immunoreactivity. MOR-ir axons, terminals, and dendrites were not distributed differently relative to LE-ir profiles. These results suggest that activation of hilar MORs by LE usually involves short-range volume transmission and that dendritic MORs are as likely as axonal and terminal MORs to be activated by released LE. However, the greater abundance of MOR-ir axons and terminals compared to dendrites indicates that presynaptic profiles are a more prominent target for enkephalins and exogenous MOR agonists such as morphine.

Chronic morphine treatment alters endogenous opioid control of hippocampal mossy fiber synaptic transmission

Synaptic adaptations are thought to be an important component of the consequences of drug abuse. One such adaptation is an up-regulation of adenylyl cyclase that has been shown to increase transmitter release at several inhibitory synapses. In this study the effects of chronic morphine treatment were studied on mossy fiber synapses in the guinea pig hippocampus using extracellular field potential recordings. This opioid-sensitive synapse was chosen because of the known role of the adenylyl cyclase cascade in the regulation of glutamate release. Long-term potentiation (LTP) at the mossy fiber synapse was enhanced after chronic morphine treatment. In control animals, opioid antagonists increased LTP but had no effect in morphine-treated guinea pigs. In contrast, the long-lasting depression of transmission induced by a mGluR agonist and CA1 LTP were not altered. Chronic morphine treatment neither caused tolerance to mu- and kappa-receptor-mediated inhibition at the mossy fiber synapse nor modified total hippocampal dynorphin levels. The results suggest that the phasic inhibition of glutamate transmission mediated by endogenous opioids is reduced after chronic exposure to morphine.

Long-term potentiation in direct perforant path projections to the hippocampal CA3 region in vivo

The perforant path constitutes the primary projection system relaying information from the neocortex to the hippocampal formation. Long-term synaptic potentiation (LTP) in the perforant path projections to the dentate gyrus is well characterized. However, surprisingly few studies have addressed the mechanisms underlying LTP induction in the direct perforant path projections to the hippocampus. Here we investigate the role of N-methyl-D-aspartate (NMDA) and opioid receptors in the induction of LTP in monosynaptic medial and lateral perforant path projections to the CA3 region in adult pentobarbital sodium-anesthetized rats. Similar to LTP observed at the medial perforant path-dentate gyrus synapse, medial perforant path-CA3 synapses display LTP that is blocked by both local and systemic administration of the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid [(+/-)-CPP]. By contrast, LTP induced at the lateral perforant path-CA3 synapses is not blocked by either local or systemic administration of this NMDA receptor antagonist. The induction of LTP at lateral perforant path-CA3 synapses, which is blocked by the opioid receptor antagonist naloxone, is also blocked by the selective mu opioid receptor antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-amide (CTOP), but not the selective delta opioid receptor antagonist naltrindole (NTI). CTOP was without effect on the induction of medial perforant path-CA3 LTP. The selective sensitivity of lateral perforant path-CA3 LTP to mu-opioid receptor antagonists corresponds with the distribution of mu-opioid receptors within the stratum lacunosum-moleculare of area CA3 where perforant path projections to CA3 terminate. These data indicate that both lateral and medial perforant path projections to the CA3 region display LTP, and that LTP induction in medial and lateral perforant path-CA3 synapses are differentially sensitive to NMDA receptor and mu-opioid receptor antagonists. This suggests a role for opioid, but not NMDA receptors in the induction of LTP at lateral perforant path projections to the hippocampal formation.

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.