Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition

Contributions of site- and sex-specific LTPs to everyday memory

Commentaries about long-term potentiation (LTP) generally proceed with an implicit assumption that largely the same physiological effect is sampled across different experiments. However, this is clearly not the case. We illustrate the point by comparing LTP in the CA3 projections to CA1 with the different forms of potentiation in the dentate gyrus. These studies lead to the hypothesis that specialized properties of CA1-LTP are adaptations for encoding unsupervised learning and episodic memory, whereas the dentate gyrus variants subserve learning that requires multiple trials and separation of overlapping bodies of information. Recent work has added sex as a second and somewhat surprising dimension along which LTP is also differentiated. Triggering events for CA1-LTP differ between the sexes and the adult induction threshold is significantly higher in females; these findings help explain why males have an advantage in spatial learning. Remarkably, the converse is true before puberty: Females have the lower LTP threshold and are better at spatial memory problems. A mechanism has been identified for the loss-of-function in females but not for the gain-of-function in males. We propose that the many and disparate demands of natural environments, with different processing requirements across ages and between sexes, led to the emergence of multiple LTPs. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Divergent opioid-mediated suppression of inhibition between hippocampus and neocortex across species and development

Opioid receptors within the CNS regulate pain sensation and mood and are key targets for drugs of abuse. Within the adult rodent hippocampus (HPC), μ-opioid receptor agonists suppress inhibitory parvalbumin-expressing interneurons (PV-INs), thus disinhibiting the circuit. However, it is uncertain if this disinhibitory motif is conserved in other cortical regions, species, or across development. We observed that PV-IN mediated inhibition is robustly suppressed by opioids in HPC but not neocortex in mice and nonhuman primates, with spontaneous inhibitory tone in resected human tissue also following a consistent dichotomy. This hippocampal disinhibitory motif was established in early development when immature PV-INs and opioids already influence primordial network rhythmogenesis. Acute opioid-mediated modulation was partially occluded with morphine pretreatment, with implications for the effects of opioids on hippocampal network activity during circuit maturation as well as learning and memory. Together, these findings demonstrate that PV-INs exhibit a divergence in opioid sensitivity across brain regions that is remarkably conserved across evolution and highlights the underappreciated role of opioids acting through immature PV-INs in shaping hippocampal development.

Effects of prenatal opioid exposure on synaptic adaptations and behaviors across development

In this review, we focus on prenatal opioid exposure (POE) given the significant concern for the mental health outcomes of children with parents affected by opioid use disorder (OUD) in the view of the current opioid crisis. We highlight some of the less explored interactions between developmental age and sex on synaptic plasticity and associated behavioral outcomes in preclinical POE research. We begin with an overview of the rich literature on hippocampal related behaviors and plasticity across POE exposure paradigms. We then discuss recent work on reward circuit dysregulation following POE. Additional risk factors such as early life stress (ELS) could further influence synaptic and behavioral outcomes of POE. Therefore, we include an overview on the use of preclinical ELS models where ELS exposure during key critical developmental periods confers considerable vulnerability to addiction and stress psychopathology. Here, we hope to highlight the similarity between POE and ELS on development and maintenance of opioid-induced plasticity and altered opioid-related behaviors where similar enduring plasticity in reward circuits may occur. We conclude the review with some of the limitations that should be considered in future investigations. This article is part of the Special Issue on 'Opioid-induced addiction'.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Cortical Modulation of Nociception

Nociception is the neuronal process of encoding noxious stimuli and could be modulated at peripheral, spinal, brainstem, and cortical levels. At cortical levels, several areas including the anterior cingulate cortex (ACC), prefrontal cortex (PFC), ventrolateral orbital cortex (VLO), insular cortex (IC), motor cortex (MC), and somatosensory cortices are involved in nociception modulation through two main mechanisms: (i) a descending modulatory effect at spinal level by direct corticospinal projections or mostly by activation of brainstem structures (i.e. periaqueductal grey matter (PAG), locus coeruleus (LC), the nucleus of raphe (RM) and rostroventral medulla (RVM)); and by (ii) cortico-cortical or cortico-subcortical interactions. This review summarizes evidence related to the participation of the aforementioned cortical areas in nociception modulation and different neurotransmitters or neuromodulators that have been studied in each area. Besides, we point out the importance of considering intracortical neuronal populations and receptors expression, as well as, nociception-induced cortical changes, both functional and connectional, to better understand this modulatory effect. Finally, we discuss the possible mechanisms that could potentiate the use of cortical stimulation as a promising procedure in pain alleviation.

The role of peptidase neurolysin in neuroprotection and neural repair after stroke

Current experimental stroke research has evolved to focus on detailed understanding of the brain’s self-protective and restorative mechanisms, and harness this knowledge for development of new therapies. In this context, the role of peptidases and neuropeptides is of growing interest. In this focused review, peptidase neurolysin (Nln) and its extracellular peptide substrates are briefly discussed in relation to pathophysiology of ischemic stroke. Upregulation of Nln following stroke is viewed as a compensatory cerebroprotective mechanism in the acute phase of stroke, because the main neuropeptides inactivated by Nln are neuro/cerebrotoxic (bradykinin, substance P, neurotensin, angiotensin II, hemopressin), whereas the peptides generated by Nln are neuro/cerebroprotective (angiotensin-(1–7), Leu-/Met-enkephalins). This notion is confirmed by experimental studies documenting aggravation of stroke outcomes in mice after inhibition of Nln following stroke, and dramatic improvement of stroke outcomes in mice overexpressing Nln in the brain. The role of Nln in the (sub)chronic phase of stroke is less clear and it is likely, that this peptidase does not have a major role in neural repair mechanisms. This is because, the substrates of Nln are less uniform in modulating neurorestorative mechanisms in one direction, some appearing to have neural repair enhancing/stimulating potential, whereas others doing the opposite. Future studies focusing on the role of Nln in pathophysiology of stroke should determine its potential as a cerebroprotective target for stroke therapy, because its unique ability to modulate multiple neuropeptide systems critically involved in brain injury mechanisms is likely advantageous over modulation of one pathogenic pathway for stroke pharmacotherapy.

Attention to breath sensations does not engage endogenous opioids to reduce pain

The endogenous opioidergic system is critically involved in the cognitive modulation of pain. Slow-breathing-based techniques are widely used nonpharmacological approaches to reduce pain. Yet, the active mechanisms of actions supporting these practices are poorly characterized. Growing evidence suggest that mindfulness-meditation, a slow-breathing technique practiced by nonreactively attending to breathing sensations, engages multiple unique neural mechanisms that bypass opioidergically mediated descending pathways to reduce pain. However, it is unknown whether endogenous opioids contribute to pain reductions produced by slow breathing. The present double-blind, placebo-controlled crossover study examined behavioral pain responses during mindfulness-meditation (n = 19), sham-mindfulness meditation (n = 20), and slow-paced breathing (n = 20) in response to noxious heat (49°C) and intravenous administration (0.15 mg/kg bolus + 0.1 mg/kg/hour maintenance infusion) of the opioid antagonist, naloxone, and placebo saline. Mindfulness significantly reduced pain unpleasantness ratings across both infusion sessions when compared to rest, but not pain intensity. Slow-paced breathing significantly reduced pain intensity and unpleasantness ratings during naloxone but not saline infusion. Pain reductions produced by mindfulness-meditation and slow-paced breathing were insensitive to naloxone when compared to saline administration. By contrast, sham-mindfulness meditation produced pain unpleasantness reductions during saline infusion but this effect was reversed by opioidergic antagonism. Sham-mindfulness did not lower pain intensity ratings. Self-reported "focusing on the breath" was identified as the operational feature particularly unique to the mindfulness-meditation and slow paced-breathing, but not sham-mindfulness meditation. Across all individuals, attending to the breath was associated with naloxone insensitive pain-relief. These findings provide evidence that slow breathing combined with attention to breath reduces pain independent of endogenous opioids.

Aberrant Epigenetic Gene Regulation in GABAergic Interneuron Subpopulations in the Hippocampal Dentate Gyrus of Mouse Offspring Following Developmental Exposure to Hexachlorophene

Maternal hexachlorophene (HCP) exposure causes transient disruption of hippocampal neurogenesis in mouse offspring. We examined epigenetically hypermethylated and downregulated genes related to this HCP-induced disrupted neurogenesis. Mated female mice were dietary exposed to 0 or 100 ppm HCP from gestational day 6 to postnatal day (PND) 21 on weaning. The hippocampal dentate gyrus of male offspring was subjected to methyl-capture sequencing and real-time reverse transcription-polymerase chain reaction analyses on PND 21. Validation analyses on methylation identified three genes, Dlx4, Dmrt1, and Plcb4, showing promoter-region hypermethylation. Immunohistochemically, DLX4⁺, DMRT1⁺, and PLCB4⁺ cells in the dentate hilus co-expressed GAD67, a γ-aminobutyric acid (GABA)ergic neuron marker. HCP decreased all of three subpopulations as well as GAD67⁺ cells on PND 21. PLCB4⁺ cells also co-expressed the metabotropic glutamate receptor, GRM1. HCP also decreased transcript level of synaptic plasticity-related genes in the dentate gyrus and immunoreactive granule cells for synaptic plasticity-related ARC. On PND 77, all immunohistochemical cellular density changes were reversed, whereas the transcript expression of the synaptic plasticity-related genes fluctuated. Thus, HCP-exposed offspring transiently reduced the number of GABAergic interneurons. Among them, subpopulations expressing DLX4, DMRT1, or PLCB4 were transiently reduced in number through an epigenetic mechanism. Considering the role of the Dlx gene family in GABAergic interneuron migration and differentiation, the decreased number of DLX4⁺ cells may be responsible for reducing those GABAergic interneurons regulating neurogenesis. The effect on granule cell synaptic plasticity was sustained until the adult stage, and reduced GABAergic interneurons active in GRM1–PLCB4 signaling may be responsible for the suppression on weaning.

The Nociceptin/Orphanin FQ System and the Regulation of Memory

Nociceptin/orphanin FQ (N/OFQ) is an endogenous neuropeptide of 17 amino acids, related to opioid peptides but with its own receptor, distinct from conventional opioid receptors, the ORL1 or NOP receptor. The NOP receptor is a G protein-coupled receptor which activates Gi/o proteins and thus induces an inhibition of neuronal activity. The peptide and its receptor are widely expressed in the central nervous system with a high density of receptors in regions involved in learning and memory. This review describes the consequences of the pharmacological manipulation of the N/OFQ system by NOP receptor ligands on learning processes and on the consolidation of various types of long-term memory. We also discuss the role of endogenous N/OFQ release in the modulation of learning and memory. Finally we propose several putative neuronal mechanisms taking place at the level of the hippocampus and amygdala and possibly underlying the behavioral amnestic or promnesic effects of NOP ligands.

Treating a novel plasticity defect rescues episodic memory in Fragile X model mice

Episodic memory, a fundamental component of human cognition, is significantly impaired in autism. We believe we report the first evidence for this problem in the Fmr1-knockout (KO) mouse model of Fragile X syndrome and describe potentially treatable underlying causes. The hippocampus is critical for the formation and use of episodes, with semantic (cue identity) information relayed to the structure via the lateral perforant path (LPP). The unusual form of synaptic plasticity expressed by the LPP (lppLTP) was profoundly impaired in Fmr1-KOs relative to wild-type mice. Two factors contributed to this defect: (i) reduced GluN1 subunit levels in synaptic NMDA receptors and related currents, and (ii) impaired retrograde synaptic signaling by the endocannabinoid 2-arachidonoylglycerol (2-AG). Studies using a novel serial cue paradigm showed that episodic encoding is dependent on both the LPP and the endocannabinoid receptor CB₁, and is strikingly impaired in Fmr1-KOs. Enhancing 2-AG signaling rescued both lppLTP and learning in the mutants. Thus, two consequences of the Fragile-X mutation converge on plasticity at one site in hippocampus to prevent encoding of a basic element of cognitive memory. Collectively, the results suggest a clinically plausible approach to treatment.

Synaptic Plasticity and Excitation-Inhibition Balance in the Dentate Gyrus: Insights from In Vivo Recordings in Neuroligin-1, Neuroligin-2, and Collybistin Knockouts

The hippocampal dentate gyrus plays a role in spatial learning and memory and is thought to encode differences between similar environments. The integrity of excitatory and inhibitory transmission and a fine balance between them is essential for efficient processing of information. Therefore, identification and functional characterization of crucial molecular players at excitatory and inhibitory inputs is critical for understanding the dentate gyrus function. In this minireview, we discuss recent studies unraveling molecular mechanisms of excitatory/inhibitory synaptic transmission, long-term synaptic plasticity, and dentate granule cell excitability in the hippocampus of live animals. We focus on the role of three major postsynaptic proteins localized at excitatory (neuroligin-1) and inhibitory synapses (neuroligin-2 and collybistin). In vivo recordings of field potentials have the advantage of characterizing the effects of the loss of these proteins on the input-output function of granule cells embedded in a network with intact connectivity. The lack of neuroligin-1 leads to deficient synaptic plasticity and reduced excitation but normal granule cell output, suggesting unaltered excitation-inhibition ratio. In contrast, the lack of neuroligin-2 and collybistin reduces inhibition resulting in a shift towards excitation of the dentate circuitry.

Hippocampal GABAergic Inhibitory Interneurons

In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10-15% of the total neuronal population; however, their remarkable anatomical and physiological diversity allows them to regulate virtually all aspects of cellular and circuit function. Here we provide an overview of the current state of the field of interneuron research, focusing largely on the hippocampus. We discuss recent advances related to the various cell types, including their development and maturation, expression of subtype-specific voltage- and ligand-gated channels, and their roles in network oscillations. We also discuss recent technological advances and approaches that have permitted high-resolution, subtype-specific examination of their roles in numerous neural circuit disorders and the emerging therapeutic strategies to ameliorate such pathophysiological conditions. The ultimate goal of this review is not only to provide a touchstone for the current state of the field, but to help pave the way for future research by highlighting where gaps in our knowledge exist and how a complete appreciation of their roles will aid in future therapeutic strategies.

Role of DOR in neuronal plasticity changes promoted by food-seeking behaviour

Several lines of evidence support that food overconsumption may be related to the role of the endogenous opioid system in the control of food palatability. The opioid system, and particularly the delta opioid receptor (DOR), plays a crucial role in the regulation of food rewarding properties. In our study, we used operant conditioning maintained by chocolate-flavoured pellets to investigate the role of DOR in the motivation for palatable food and the structural plasticity changes promoted by this behaviour. For this purpose, we evaluated the specific role of this receptor in the behavioural and neuroplastic changes induced by palatable food in the prefrontal cortex (PFC), hippocampus (HCP) and nucleus accumbens (NAc) in constitutive knockout (KO) mice deficient in DOR. Mutant mice and their wild-type littermates were trained to obtain chocolate-flavoured pellets on fixed ratio 1 (FR1), FR5 and progressive ratio (PR) schedule of reinforcement. No significant differences between genotypes were revealed on operant behaviour acquisition in FR1. DOR knockout mice displayed lower number of active lever-presses than wild-type mice on FR5, and a similar decrease was revealed in DOR KO mice in the breaking point during the PR. This operant training to obtain palatable food increased dendritic spine density in the PFC, HCP and NAc shell of wild-type, but these plasticity changes were abolished in DOR KO mice. Our results support the hypothesis that DOR regulates the reinforcing effects and motivation for palatable food through neuroplastic changes in specific brain reward areas.

Mutation of neuron-specific chromatin remodeling subunit BAF53b: rescue of plasticity and memory by manipulating actin remodeling

Recent human exome-sequencing studies have implicated polymorphic Brg1-associated factor (BAF) complexes (mammalian SWI/SNF chromatin remodeling complexes) in several intellectual disabilities and cognitive disorders, including autism. However, it remains unclear how mutations in BAF complexes result in impaired cognitive function. Post-mitotic neurons express a neuron-specific assembly, nBAF, characterized by the neuron-specific subunit BAF53b. Subdomain 2 of BAF53b is essential for the differentiation of neuronal precursor cells into neurons. We generated transgenic mice lacking subdomain 2 of Baf53b (BAF53bΔSB2). Long-term synaptic potentiation (LTP) and long-term memory, both of which are associated with phosphorylation of the actin severing protein cofilin, were assessed in these animals. A phosphorylation mimic of cofilin was stereotaxically delivered into the hippocampus of BAF53bΔSB2 mice in an effort to rescue LTP and memory. BAF53bΔSB2 mutant mice show impairments in phosphorylation of synaptic cofilin, LTP, and memory. Both the synaptic plasticity and memory deficits are rescued by overexpression of a phosphorylation mimetic of cofilin. Baseline physiology and behavior were not affected by the mutation or the experimental treatment. This study suggests a potential link between nBAF function, actin cytoskeletal remodeling at the dendritic spine, and memory formation. This work shows that a targeted manipulation of synaptic function can rescue adult plasticity and memory deficits caused by manipulations of nBAF, and thereby provides potential novel avenues for therapeutic development for multiple intellectual disability disorders.

Effect of acetylcholine receptors on the pain-related electrical activities in the hippocampal CA3 region of morphine-addicted rats

OBJECTIVES

To determine the effect of acetylcholine (ACh), pilocarpine, and atropine on pain evoked responses of pain excited neurons (PEN) and pain inhibited neurons (PIN) in hippocampal CA3 region of morphine addicted rats.

MATERIALS AND METHODS

Female Wistar rats, weighing between 230-260 g were used in this study. Morphine addicted rats were generated by subcutaneous injection of increasing concentrations of morphine hydrochloride for six days. Trains of electrical impulses applied to the sciatic nerve were used as noxious stimulation and the evoked electrical activities of PEN or PIN in hippocampal CA3 area were recorded using extracellular electrophysiological recording techniques in hippocampal slices. The effect of acetylcholine receptor stimulation by ACh, the muscarinic agonist pilocarpine, and the muscarinic antagonist atropine on the pain evoked responses of pain related electrical activities was analyzed in hippocampal CA3 area of morphine addicted rats.

RESULTS

Intra-CA3 microinjection of ACh (2 μg/1 μl) or pilocarpine (2 μg/1 μl) decreased the discharge frequency and prolonged the firing latency of PEN, but increased the discharge frequency and shortened the firing inhibitory duration (ID) of PIN. The intra-CA3 administration of atropine (0.5 μg/1 μl) produced opposite effect. The peak activity of cholinergic modulators was 2 to 4 min later in morphine addicted rats compared to peak activity previously observed in normal rats.

CONCLUSION

ACh dependent modulation of noxious stimulation exists in hippocampal CA3 area of morphine addicted rats. Morphine treatment may shift the sensitivity of pain related neurons towards a delayed response to muscarinergic neurotransmission in hippocampal CA3 region.

Pronounced differences in signal processing and synaptic plasticity between piriform-hippocampal network stages: a prominent role for adenosine

KEY POINTS

Extended trains of theta rhythm afferent activity lead to a biphasic response facilitation in field CA1 but not in the lateral perforant path input to the dentate gyrus. Processes that reverse long-term potentiation in field CA1 are not operative in the lateral perforant path: multiple lines of evidence indicate that this reflects differences in adenosine signalling. Adenosine A1 receptors modulate baseline synaptic transmission in the lateral olfactory tract but not the associational afferents of the piriform cortex. Levels of ecto-5'-nucleotidase (CD73), an enzyme that converts extracellular ATP into adenosine, are markedly different between regions and correlate with adenosine signalling and the efficacy of theta pulse stimulation in reversing long-term potentiation. Variations in transmitter mobilization, CD73 levels, and afferent divergence result in multivariate differences in signal processing through nodes in the cortico-hippocampal network.

ABSTRACT

The present study evaluated learning-related synaptic operations across the serial stages of the olfactory cortex-hippocampus network. Theta frequency stimulation produced very different time-varying responses in the Schaffer-commissural projections than in the lateral perforant path (LPP), an effect associated with distinctions in transmitter mobilization. Long-term potentiation (LTP) had a higher threshold in LPP field potential studies but not in voltage clamped neurons; coupled with input/output relationships, these results suggest that LTP threshold differences reflect the degree of input divergence. Theta pulse stimulation erased LTP in CA1 but not in the dentate gyrus (DG), although adenosine eliminated potentiation in both areas, suggesting that theta increases extracellular adenosine to a greater degree in CA1. Moreover, adenosine A1 receptor antagonism had larger effects on theta responses in CA1 than in the DG, and concentrations of ecto-5'-nucleotidase (CD73) were much higher in CA1. Input/output curves for two connections in the piriform cortex were similar to those for the LPP, whereas adenosine modulation again correlated with levels of CD73. In sum, multiple relays in a network extending from the piriform cortex through the hippocampus can be differentiated along three dimensions (input divergence, transmitter mobilization, adenosine modulation) that potently influence throughput and plasticity. A model that incorporates the regional differences, supplemented with data for three additional links, suggests that network output goes through three transitions during the processing of theta input. It is proposed that individuated relays allow the circuit to deal with different types of behavioural problems.

Sucrose-induced analgesia during early life modulates adulthood learning and memory formation

This study is aimed at examining the long-term effects of chronic pain during early life (postnatal day 0 to 8weeks), and intervention using sucrose, on cognitive functions during adulthood in rats. Pain was induced in rat pups via needle pricks of the paws. Sucrose solution or paracetamol was administered for analgesia before the paw prick. Control groups include tactile stimulation to account for handling and touching the paws, and sucrose alone was used. All treatments were started on day one of birth and continued for 8weeks. At the end of the treatments, behavioral studies were conducted to test the spatial learning and memory using radial arm water maze (RAWM), as well as pain threshold via foot-withdrawal response to a hot plate apparatus. Additionally, the hippocampus was dissected, and blood was collected. Levels of neurotrophins (BDNF, IGF-1 and NT-3) and endorphins were assessed using ELISA. The results show that chronic noxious stimulation resulted in comparable foot-withdrawal latency between noxious and tactile groups. On the other hand, pretreatment with sucrose or paracetamol increased pain threshold significantly both in naive rats and noxiously stimulated rats (P<0.05). Chronic pain during early life impaired short-term memory, and sucrose treatment prevented such impairment (P<0.05). Sucrose significantly increased serum levels of endorphin and enkephalin. Chronic pain decreased levels of BDNF in the hippocampus and this decrease was prevented by sucrose and paracetamol treatments. Hippocampal levels of NT-3 and IGF-1 were not affected by any treatment. In conclusion, chronic pain induction during early life induced short memory impairment, and pretreatment with sucrose prevented this impairment via mechanisms that seem to involve BDNF. As evident in the results, sucrose, whether alone or in the presence of pre-noxious stimulation, increases pain threshold in such circumstances; most likely via a mechanism that involves an increase in endogenous opioids.

The role of δ-opioid receptors in learning and memory underlying the development of addiction

Opioids are important endogenous ligands that exist in both invertebrates and vertebrates and signal by activation of opioid receptors to produce analgesia and reward or pleasure. The μ-opioid receptor is the best known of the opioid receptors and mediates the acute analgesic effects of opiates, while the δ-opioid receptor (DOR) has been less well studied and has been linked to effects that follow from chronic use of opiates such as stress, inflammation and anxiety. Recently, DORs have been shown to play an essential role in emotions and increasing evidence points to a role in learning actions and outcomes. The process of learning and memory in addiction has been proposed to involve strengthening of specific brain circuits when a drug is paired with a context or environment. The DOR is highly expressed in the hippocampus, amygdala, striatum and other basal ganglia structures known to participate in learning and memory. In this review, we will focus on the role of the DOR and its potential role in learning and memory underlying the development of addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Hippocampal mossy fiber leu-enkephalin immunoreactivity in female rats is significantly altered following both acute and chronic stress

Research indicates that responses to stress are sexually dimorphic, particularly in regard to learning and memory processes: while males display impaired cognitive performance and hippocampal CA3 pyramidal cell dendritic remodeling following chronic stress, females exhibit enhanced performance and no remodeling. Leu-enkephalin, an endogenous opioid peptide found in the hippocampal mossy fiber pathway, plays a critical role in mediating synaptic plasticity at the mossy fiber-CA3 pyramidal cell synapse. Estrogen is known to influence the expression of leu-enkephalin in the mossy fibers of females, with leu-enkephalin levels being highest at proestrus and estrus, when estrogen levels are elevated. Since stress is also known to alter the expression of leu-enkephalin in various brain regions, this study was designed to determine whether acute or chronic stress had an effect on mossy fiber leu-enkephalin levels in females or males, through the application of correlated quantitative light and electron microscopic immunocytochemistry. Both acute and chronic stress eliminated the estrogen-dependence of leu-enkephalin levels across the estrous cycle in females, but had no effect on male levels. However, following acute stress leu-enkephalin levels in females were consistently lowered to values comparable to the lowest control values, while following chronic stress they were consistently elevated to values comparable to the highest control values. Ultrastructural changes in leu-enkephalin labeled dense core vesicles paralleled light microscopic observations, with acute stress inducing a decrease in leu-enkephalin labeled dense core vesicles, and chronic stress inducing an increase in leu-enkephalin labeled dense-core vesicles in females. These findings suggest that alterations in leu-enkephalin levels following stress could play an important role in the sex-specific responses that females display in learning processes, including those important in addiction.

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.

Cannabinoid-1 receptor gene deletion has a compartment-specific affect on the dendritic and axonal availability of μ-opioid receptors and on dopamine axons in the mouse nucleus accumbens

Cannabinoid-type 1 (CB1) receptors are implicated in μ-opioid receptor (μ-OR)-dependent reward ascribed partially to mesolimbic dopamine release in the nucleus accumbens (Acb) shell. Thus, CB1 receptor gene deletion may preferentially alter the availability of μ-ORs and/or dopamine innervation in this brain region, which is functionally distinct from the motor-associated Acb core. To test this hypothesis, we examined the electron microscopic immunolabeling of the μ-OR and the dopamine-synthesizing enzyme, tyrosine hydroxylase (TH) in Acb shell, and core of adult C57BL/6J wild-type (WT) and CB1-knock-out (KO) mice. The μ-OR-immunogold particles were observed in the cytoplasm and on the plasmalemma in dendrites, dendritic spines, and axon terminals throughout the Acb. Compared to WT, the Acb shell of CB1-KO mice showed a lower cytoplasmic density of μ-ORs in dendrites and fewer μ-OR labeled, but not unlabeled, dendritic spines. In this region, the CB1-KO's had a significantly enhanced plasmalemmal density of μ-OR-immunogold in axon terminals, 70% of which formed excitatory-type synapses. However, the number of both μ-OR-labeled terminals and TH-labeled small varicosities was significantly reduced in the Acb shell of CB1-KO's. These adaptations were not seen in the Acb core, where CB1-KO's had a preferentially lower dendritic plasmalemmal and total spine density of μ-OR immunogold. Our results indicate that constitutive deletion of the CB1 receptor gene has a major impact on the pre and postsynaptic availability of μ-ORs at axospinous synapses and on the dopamine innervation of the Acb shell as well as the dendritic surface expression of μ-ORs in Acb core of mature rodents.

Endogenous opioid peptides contribute to associative LTP in the hippocampal CA3 region

The medial and lateral perforant path projections to the hippocampal CA3 region display distinct mechanisms of long-term potentiation (LTP) induction, N-methyl-d-aspartate (NMDA) and opioid receptor dependent, respectively. However, medial and lateral perforant path projections to the CA3 region display associative LTP with coactivation, suggesting that while they differ in receptors involved in LTP induction they may share common downstream mechanisms of LTP induction. Here we address this interaction of LTP induction mechanisms by evaluating the contribution of opioid receptors to the induction of associative LTP among the medial and lateral perforant path projections to the CA3 region in vivo. Local application of the opioid receptor antagonists naloxone or Cys2-Tyr3-Orn5-Pen7-amide (CTOP) normally block induction of lateral perforant path-CA3 LTP. However, these opioid receptor antagonists failed to block associative LTP in lateral perforant path-CA3 synapses when it was induced by strong coactivation of the medial perforant pathway which displays NMDAR-dependent LTP. Thus strong activation of non-opioidergic afferents can substitute for the opioid receptor activation required for lateral perforant path LTP induction. Conversely, medial perforant path-CA3 associative LTP was blocked by opioid receptor antagonists when induced by strong coactivation of the opioidergic lateral perforant path. These data indicate endogenous opioid peptides contribute to associative LTP at coactive synapses when induced by strong coactivation of an opioidergic afferent system. These data further suggest that associative LTP induction is regulated by the receptor mechanisms of the strongly stimulated pathway. Thus, while medial and lateral perforant path synapses differ in their mechanisms of LTP induction, associative LTP at these synapses share common downstream mechanisms of induction.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Hormonal regulation of delta opioid receptor immunoreactivity in interneurons and pyramidal cells in the rat hippocampus

Clinical and preclinical studies indicate that women and men differ in relapse vulnerability to drug-seeking behavior during abstinence periods. As relapse is frequently triggered by exposure of the recovered addict to objects previously associated with drug use and the formation of these associations requires memory systems engaged by the hippocampal formation (HF), studies exploring ovarian hormone modulation of hippocampal function are warranted. Previous studies revealed that ovarian steroids alter endogenous opioid peptide levels and trafficking of mu opioid receptors in the HF, suggesting cooperative interaction between opioids and estrogens in modulating hippocampal excitability. However, whether ovarian steroids affect the levels or trafficking of delta opioid receptors (DORs) in the HF is unknown. Here, hippocampal sections of adult male and normal cycling female Sprague-Dawley rats were processed for quantitative immunoperoxidase light microscopy and dual label fluorescence or immunoelectron microscopy using antisera directed against the DOR and neuropeptide Y (NPY). Consistent with previous studies in males, DOR-immunoreactivity (-ir) localized to select interneurons and principal cells in the female HF. In comparison to males, females, regardless of estrous cycle phase, show reduced DOR-ir in the granule cell layer of the dentate gyrus and proestrus (high estrogen) females, in particular, display reduced DOR-ir in the CA1 pyramidal cell layer. Ultrastructural analysis of DOR-labeled profiles in CA1 revealed that while females generally show fewer DORs in the distal apical dendrites of pyramidal cells, proestrus females, in particular, exhibit DOR internalization and trafficking towards the soma. Dual label studies revealed that DORs are found in NPY-labeled interneurons in the hilus, CA3, and CA1. While DOR colocalization frequency in NPY-labeled neuron somata was similar between animals in the hilus, proestrus females had fewer NPY-labeled neurons that co-labeled with DOR in stratum oriens of CA1 and CA3 when compared to males. Ultrastructural analysis of NPY-labeled axon terminals within stratum radiatum of CA1 revealed that NPY-labeled axon terminals contain DORs that are frequently found at or near the plasma membrane. As no differences were noted by sex or estrous cycle phase, DOR activation on NPY-labeled axon terminals would inhibit GABA release probability equally in males and females. Taken together, these findings suggest that ovarian steroids can impact hippocampal function through direct effects on DOR levels and trafficking in principal cells and broad indirect effects through reductions in DOR-ir in NPY-labeled interneurons, particularly in CA1.

NCS-1 in the dentate gyrus promotes exploration, synaptic plasticity, and rapid acquisition of spatial memory

The molecular underpinnings of exploration and its link to learning and memory remain poorly understood. Here we show that inducible, modest overexpression of neuronal calcium sensor 1 (Ncs1) selectively in the adult murine dentate gyrus (DG) promotes a specific form of exploratory behavior. The mice also display a selective facilitation of long-term potentiation (LTP) in the medial perforant path and a selective enhancement in rapid-acquisition spatial memory, phenotypes that are reversed by direct application of a cell-permeant peptide (DNIP) designed to interfere with NCS-1 binding to the dopamine type-2 receptor (D2R). Moreover, the DNIP and the D2R-selective antagonist L-741,626 attenuated exploratory behavior, DG LTP, and spatial memory in control mice. These data demonstrate a role for NCS-1 and D2R in DG plasticity and provide insight for understanding how the DG contributes to the origin of exploration and spatial memory acquisition.

COMPLEX MAGNETIC FIELDS ENABLE STATIC MAGNETIC FIELD CUE USE FOR RATS IN RADIAL MAZE TASKS

Male Wistar rats were trained in an eight-arm radial maze task (two sessions per day, delayed-non-matching-to-sample) that included an intramaze static magnetic field "cue" (185 microT) specific to the entrance point of one of the arms. Rats were exposed daily for 60 min to a complex magnetic field waveform (theta-burst pattern, 200-500 nT), presented with several different interstimulus intervals (ISIs), either immediately following training sessions or immediately preceding testing sessions. Application of the theta-burst stimulus with a 4000 ms ISI significantly improved the rats' memory for the arm of the radial maze whose position was indicated by the presence of a static magnetic field cue. Reference memory errors were homogeneously distributed among all eight arms of the maze for sham-exposed rats, and among the other seven arms of the maze for complex magnetic field-treated rats. These results suggest that static magnetic field cues may be salient orienting cues even in a microenvironment such as a radial maze, but their use as a cue during maze learning in rats is dependent on whole-body application of a specific time-varying complex magnetic field.

Ovarian steroids alter mu opioid receptor trafficking in hippocampal parvalbumin GABAergic interneurons

The endogenous hippocampal opioid systems are implicated in learning associated with drug use. Recently, we showed that ovarian hormones regulate enkephalin levels in the mossy fiber pathway. This pathway overlaps with parvalbumin (PARV)-basket interneurons that contain the enkephalin-activated mu opioid receptors (MORs) and are important for controlling the "temporal timing" of granule cells. Here, we evaluated the influence of ovarian steroids on the trafficking of MORs in PARV interneurons. Two groups of female rats were analyzed: cycling rats in proestrus (relatively high estrogens) or diestrus; and ovariectomized rats euthanized 6, 24 or 72 h after estradiol benzoate (10 microg, s.c.) administration. Dorsal hippocampal sections were dually immunolabeled for MOR and PARV and examined by light and electron microscopy. As in males, in females MOR-immunoreactivity (-ir) was in numerous PARV-labeled perikarya, dendrites and terminals in the dentate hilar region. Variation in ovarian steroid levels altered the subcellular distribution of MORs in PARV-labeled dendrites but not terminals. In normal cycling rats, MOR-gold particles on the plasma membrane of small PARV-labeled dendrites (area <1 microm2) had higher density in proestrus rats than in diestrus rats. Likewise, in ovariectomized rats MORs showed higher density on the plasma membrane of small PARV-labeled dendrites 72 h after estradiol exposure. The number of PARV-labeled cells was not affected by estrous cycle phase or estrogen levels. These results demonstrate that estrogen levels positively regulate the availability of MORs on GABAergic interneurons in the dentate gyrus, suggesting cooperative interaction between opioids and estrogens in modulating principal cell excitability.

Prenatal morphine exposure attenuates the maintenance of late LTP in lateral perforant path projections to the dentate gyrus and the CA3 region in vivo

Previously we reported that prenatal exposure to morphine twice daily during gestation decreases proenkephalin levels in adult progeny within the brain, including the dentate gyrus, and alters mu and delta opioid receptors in the hippocampal CA3 region. The lateral aspect of the perforant path contains and releases enkephalin-derived opioid peptides, and induction of long-term potentiation (LTP) in lateral perforant path projections to both the dentate gyrus and the hippocampal CA3 region is blocked by antagonists of opioid receptors. Thus LTP induction at these synapses involves opioid receptor activation mediated by the release of proenkephalin-derived opioid peptides with lateral perforant path activation. Here we show in adult behaving animals, neither LTP induction nor the early phase of LTP (E-LTP) maintenance is altered by prenatal morphine exposure in the lateral perforant path projections to the dentate gyrus and the CA3 region. However, maintenance and longevity of late LTP (L-LTP), as reflected in the magnitude of LTP over days, was attenuated in animals prenatally exposed to morphine. In contrast, in medial perforant path projections to the dentate gyrus and CA3 region, both LTP induction and the maintenance of E- and L-LTP were unaffected by prenatal morphine treatment. Thus a brief prenatal exposure to the opiate morphine produces sustained, and possibly permanent, alterations in L-LTP in the opioidergic lateral perforant path projection. This suggests that prenatal morphine exposure disrupts LTP via disruption of opioid mechanisms involved in LTP maintenance or via disruption of opioid receptor activation during LTP induction, which can subsequently alter LTP maintenance.

Distinct patterns of neural activation associated with ethanol seeking: effects of naltrexone

BACKGROUND

Alcoholism, like other substance abuse disorders, is a chronically relapsing condition. Compared with other abused drugs, however, little is known about the neural mechanisms mediating ethanol (EtOH)-craving and -seeking behavior leading to relapse. This study, therefore, was conducted to identify candidate brain regions that are recruited by an EtOH-associated contextual stimulus (S(+)). A secondary objective was to determine whether EtOH S(+)-elicited neural recruitment patterns are modified by the opiate antagonist naltrexone (NTX), a compound that reduces cue-induced craving in alcoholics and attenuates ethanol seeking in animal models of relapse.

METHODS

Rats were tested in a conditioned reinstatement model of relapse with subsequent examination of brain c-fos expression patterns elicited by an EtOH S(+) versus a cue associated with nonreward (S(-)). In addition, modification of these expression patterns by NTX was examined.

RESULTS

The EtOH S(+) reinstated extinguished responding and increased c-fos expression within the prefrontal cortex, hippocampus, nucleus accumbens, and hypothalamic paraventricular nucleus (PVN). Naltrexone suppressed the S(+)-induced reinstatement and attenuated hippocampal CA3 c-fos expression, while increasing neural activity in the extended amygdala and PVN.

CONCLUSIONS

Ethanol-associated contextual stimuli recruit key brain regions that regulate associative learning, goal-directed behavior, and Pavlovian conditioning of emotional significance to previously neutral stimuli. In addition, the data implicate the hippocampus, amygdala, and PVN as potential substrates for the inhibitory effects of NTX on conditioned reinstatement.

Blocking effects of intra-hippocampal naltrexone microinjections on glucocorticoid-induced impairment of spatial memory retrieval in rats

Previous studies have indicated that stress levels of glucocorticoid hormones induce impairment of long-term memory retrieval. In a recent study, we have found that peripheral injections of naloxone blocked stress or glucocorticoid-induced deficit in memory retrieval, but the anatomical sites of such an interaction were not known. The present study examined whether the opioid receptors in the hippocampus interact with glucocorticoid effects on memory retrieval in a water maze (WM). Young rats carrying bilateral cannulae aimed at the hippocampus were trained in a WM task with six trials per day for six consecutive days. Retention of the spatial training was assessed 24h after the last training session with a 60-s probe trial. Corticosterone (1mg/kg) was injected 30 min before retention testing with or without prior bilateral intra-hippocampal injections of naltrexone (5, 10 or 20 microg/mul per site) as a classical opioid antagonist. The results show that corticosterone-induced impairment of memory retrieval was blocked by intra-hippocampal infusions of naltrexone in a dose-dependent manner. Moreover, even a higher dose of corticosterone (3 mg/kg) was ineffective in impairing memory retrieval in the animals that received 20 microg of naltrexone. These findings provide evidence for the view that glucocorticoids interact with the hippocampal opioid receptors in influencing long-term memory retrieval.

Delta opioids reduce the neurotransmitter release probability by enhancing transient (KV4) K+-currents in corticostriatal synapses as evaluated by the paired pulse protocol

Field recordings were used to determine the influence of delta-opioid receptor activation on corticostriatal synaptic transmission. Application of the selective delta-opioid receptor agonist, [Tyr-D-Pen-Gly-Phe-D-Pen]-enkephalin (DPDPE, 1 microM), decreased the amplitude of the field-excitatory synaptic potential and at the same time increased the paired pulse ratio (PPR) suggesting a presynaptic site of action. This response reversed rapidly when DPDPE was washed and blocked by 1 nM of the selective delta-receptor antagonist naltrindole. Neither omega-conotoxin GVIA (1 microM) nor omega-agatoxin TK (400 nM), blockers of N- and P/Q-type Ca2+-channels, respectively, nor TEA (1 mM), blocker of some classes of K+-channels, occluded the effects of DPDPE. Instead, 1 mM 4-AP or 400 microM Ba2+ occluded completely the effects of DPDPE. Therefore, the results suggest that the modulation by delta opioids at corticostriatal terminals is mediated by transient (KV4) K+-conductances.

Plastic processes in the dentate gyrus: a computational perspective

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Control of synaptic consolidation in the dentate gyrus: mechanisms, functions, and therapeutic implications

Synaptic consolidation refers to the development and stabilization of protein synthesis-dependent modifications of synaptic strength as observed during long-term potentiation (LTP) and long-term depression (LTD). Activity-dependent changes in synaptic strength are thought to underlie memory storage and other adaptive responses of the nervous systems of importance in mood stability, reward behavior, and pain control. This chapter focuses on the mechanisms and functions of synaptic consolidation in the dentate gyrus, a critical structure not only in hippocampal memory function, but also in regulation of stress responses and cognitive aspects of depression. Recent evidence suggests that synaptic consolidation at excitatory medial perforant path-granule cell synapses requires brain-derived neurotrophic factor (BDNF) signaling and induction of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is strongly induced and transported to dendritic processes following high-frequency stimulation (HFS) that induces LTP in the rat dentate gyrus in vivo. Sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Furthermore, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has revealed a panel of BDNF-regulated genes that may cooperate with Arc during synaptic consolidation. In addition to regulating gene expression, BDNF signaling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively-expressed mRNA in dendrites, BDNF may effectively control the window of synaptic consolidation. Dysregulation of BDNF synthesis and Arc function, specifically within the dentate gyrus, is linked to behavioral symptoms and cognitive deficits in animal models of depression and Alzheimer's disease. Therapeutics strategies targeting synaptic consolidation hold promise for the future.

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.

Trafficking of delta-opioid receptors and other G-protein-coupled receptors: implications for pain and analgesia

A cell can regulate how it interacts with its external environment by controlling the number of plasma membrane receptors that are accessible for ligand stimulation. G-protein-coupled receptors (GPCRs) are the largest superfamily of cell surface receptors and have a significant role in physiological and pathological processes. Much research effort is now focused on understanding how GPCRs are delivered to the cell surface to enhance the number of 'bioavailable' receptors accessible for activation. Knowing how such processes are triggered or modified following induction of various pathological states will inevitably identify new therapeutic strategies for treating various diseases, including chronic pain. Here, we highlight recent advances in this field, and provide examples of the importance of such trafficking events in pain.

Mu opioid receptors are extensively co-localized with parvalbumin, but not somatostatin, in the dentate gyrus

In the rat dentate gyrus, mu opioid receptor (MOR) agonists disinhibit principal cells, promoting excitation, but whether MOR protein is differentially distributed to interneuron subtypes is unknown. Here, the distribution of MOR immunoreactivity was semi-quantitatively examined in neurochemically identified interneurons using fluorescence microscopy. We find that MOR- and parvalbumin-immunoreactivities are frequently co-localized, while MOR- and somatostatin-immunoreactivities are less commonly co-localized. This suggests that MORs are most frequently on interneurons specialized to inhibit granule cell output, and are on a limited number of interneurons that inhibit granule cell distal dendrites.

NT-3 facilitates hippocampal plasticity and learning and memory by regulating neurogenesis

In the adult brain, the expression of NT-3 is largely confined to the hippocampal dentate gyrus (DG), an area exhibiting significant neurogenesis. Using a conditional mutant line in which the NT-3 gene is deleted in the brain, we investigated the role of NT-3 in adult neurogenesis, hippocampal plasticity, and memory. Bromodeoxyuridine (BrdU)-labeling experiments demonstrated that differentiation, rather than proliferation, of the neuronal precursor cells (NPCs) was significantly impaired in DG lacking NT-3. Triple labeling for BrdU, the neuronal marker NeuN, and the glial marker GFAP indicated that NT-3 affects the number of newly differentiated neurons, but not glia, in DG. Field recordings revealed a selective impairment in long-term potentiation (LTP) in the lateral, but not medial perforant path-granule neuron synapses. In parallel, the NT-3 mutant mice exhibited deficits in spatial memory tasks. In addition to identifying a novel role for NT-3 in adult NPC differentiation in vivo, our study provides a potential link between neurogenesis, dentate LTP, and spatial memory.

Epileptogenesis Is Associated With Enhanced Glutamatergic Transmission in the Perforant Path

The perforant path provides the main excitatory input into the hippocampus and has been proposed to play a critical role in the generation of temporal lobe seizures. It has been hypothesized that changes in glutamatergic transmission in this pathway promote the epileptogenic process and seizure generation. We therefore asked whether epileptogenesis is associated with enhanced glutamatergic transmission from the perforant path to dentate granule cells. We used a rat model of temporal lobe epilepsy in which spontaneous seizures occur after an episode of pilocarpine-induced status epilepticus. Whole cell patch-clamp recordings were obtained from dentate granule cells in hippocampal slices from control and epileptic animals 3 wk after pilocarpine-induced status epilepticus. The paired pulse ratio of perforant path-evoked AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) was reduced in tissue obtained from epileptic rats. This is consistent with an increase in release probability. N-methyl-d-aspartate (NMDA) receptor-mediated EPSCs were also prolonged. This prolongation could not be accounted for by decreased activity of glutamate transporters or by a change in NMDA receptor subunit composition in dentate granule cells, implying a change in NMDA receptor kinetics. This change in NMDA receptor kinetics was associated with the emergence of significant synaptic cross-talk, detected as a use-dependent block of receptors activated by medial perforant path synapses after lateral perforant path stimulation in MK-801. Enhanced glutamatergic transmission and the emergence of cross-talk among perforant path-dentate granule cell synapses may contribute to lowering seizure threshold.

Intracerebroventricular d-Pen2, d-Pen5-enkephalin administration soon after stressor imposition influences behavioral responsivity to a subsequent stressor encounter in CD-1 mice

Endogenous opioid peptide systems diminish stress-induced autonomic nervous system, neuroendocrine (hypothalamic-pituitary-adrenal axis) and behavioral responses, attenuating a collection of physiological symptoms basic to emotional and affective states. Neurogenic stressors may incite specific central changes in opioid peptide availability as well as changes in mu and delta-opioid receptor function. The present investigation evaluated the proactive influence of an intracerebroventricular injection of the opioid receptor agonist D-Pen2, D-Pen5-enkephalin (DPDPE) (0 microg, 0.005 microg, 1.0 microg or 2.5 microg) on locomotor behavior of mice following uncontrollable footshock (Shock) or novel shock chamber exposure (No Shock). It was expected that DPDPE administration following Shock on Day 1 would restore locomotor activity up to 1 week and prevent shock-associated behavior of mice encountering a brief session of footshock 18 days later. Exposure to Shock reduced horizontal locomotor and vertical locomotor (rearing) activity of mice while 2.5 microg DPDPE restored behavior. Eighteen days following Shock and DPDPE challenge, mice were exposed to either an abbreviated session of footshock (Mild Stress) or the shock chamber (Cues). Mice in the No Shock and Shock groups administered 2.5 microg DPDPE on Day 1 did not exhibit any locomotor deficits in response to Mild Stress on Day 18. Mice in the Shock group administered 0.005 microg DPDPE on Day 1, did not exhibit exaggerated rearing deficits following ensuing Mild Stressor encounter relative to mice reexposed to Cues on Day 18. Taken together, these data show that (a) footshock differentially affects rearing and locomotor activity, (b) DPDPE administration increases locomotor activity for up to 1 week following footshock and DPDPE administration, (c) reexposure to Mild Stress affects rearing and locomotor performance differently depending on previous stressor history and DPDPE dose, (d) DPDPE affords long-lasting protection to previously non-stressed mice against the deleterious effects of subsequent mild stress on locomotor activity, while a low dose of DPDE is sufficient to prevent shock-induced sensitization of rearing deficits, 18 days following original stressor and drug presentation. Finally, our investigation demonstrates that DPDPE administration alters the behavioral impact of future stressful encounters and emphasizes the importance of investigating opioid mechanisms in chronic stress disorders.

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.

Expression of the mu-opioid receptor is induced in dentate gyrus granule cells after focal cerebrocortical ischaemia and stimulation of entorhinal afferents

Focal ischaemia in the cerebral cortex affects the inducibility of long-term potentiation (LTP) in the hippocampus. This impairment of hippocampal function may result from excessive activation of cortico-hippocampal afferents and subsequent perturbation of hippocampal LTP-relevant transmitter systems, which include opioids. Here, we tested if permanent focal ischaemia and electrical afferent stimulation influence the expression of the mu-opioid receptor (MOR) in the rat hippocampus. In the applied ischaemia model, the entire ipsilateral cortical hemisphere and hippocampus experienced sustained excitation as indicated by a long-lasting increase in the expression of arg 3.1/arc (ARG) mRNA, a marker for neuronal activity. Expression of MOR mRNA and protein was strongly increased in granule cells, which contain very low MOR levels under normal conditions, but not in gamma-aminobutyric acid (GABA)ergic neurons, which express the MOR constitutively. In the molecular layer, which contains the dendrites of granule cells, focal ischaemia caused a redistribution of MOR-like immunoreactivity. In contrast to the dentate gyrus, MOR expression was unaltered in the hippocampus proper and in non-infarcted cortical areas. Repetitive high-frequency stimulation of cortico-hippocampal perforant path afferents induced strong MOR mRNA expression throughout the granular layer. However, weak tetanization sufficient to induce LTP and ARG expression did not influence MOR mRNA levels. Taken together, we provide direct evidence for the induction of MOR expression in granule cells experiencing sustained excitation by cortical afferents. In activated, MOR-expressing granule cells, inhibitory opioids may counter-regulate glutamatergic excitation by the perforant path.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Selective involvement of opioids in the mechanisms of synapse-specific plasticity in the common snail during the acquisition of sensitization

Studies on defensive behavior command neurons LP11 and RP11 in semi-intact common snail preparations were performed to investigate the effects of the opioid peptide met-enkephalin and the opioid antagonist naloxone on the effects of nociceptive sensitization. Application of nociceptive stimuli to the snail's head elicited marked reversible membrane depolarization along with depression of neuron responses to sensory stimulation during the short-term stage of sensitization and facilitation of responses in the long-term stage. Met-enkephalin at a dose of 10 microM but not at a dose of 0.1 microM partially suppressed responses to nociceptive stimuli. Acquisition of sensitization during exposure to met-enkephalin at doses of 10 and 0.1 microM led to complete suppression of the facilitation of responses to tactile stimulation of the head. Facilitation of responses to chemical stimulation of the head and tactile stimulation of the foot in these conditions was similar to that of neurons in control sensitized animals. Acquisition of sensitization during exposure to met-enkephalin and/or naloxone elicited selective suppression of facilitation of responses to chemical stimulation of the head but had no effect on facilitation of responses to tactile stimulation of the head and foot. Met-enkephalin and naloxone had no effect on the depression of neuron responses evoked by sensory stimulation in the short-term stage of sensitization. It is suggested that during the acquisition of sensitization in the common snail, opioids are involved in controlling the mechanism of nociception and in the mechanisms of selective induction of long-term plasticity of the synaptic inputs to command neurons activated by tactile and chemical stimulation of the animal's head.

NMDA receptor antagonists block heterosynaptic long-term depression (LTD) but not long-term potentiation (LTP) in the CA3 region following lateral perforant path stimulation

High-frequency stimulation of lateral perforant path is accompanied by a heterosynaptic long-term depression (LTD) of medial perforant path synaptic responses in both the dentate gyrus and the CA3 region of the hippocampus. We reported previously that LTP induction at lateral perforant path-CA3 synapses is unaffected by NMDA antagonists. However, it is not known if heterosynaptic LTD that is observed in the CA3 region following lateral perforant path stimulation also is independent from NMDA receptors. We address this question in anesthetized adult rats using systemic administration of the competitive NMDA receptor antagonist CPP. Induction of lateral perforant path-CA3 LTP produced a sustained heterosynaptic depression of medial perforant path-CA3 responses. Systemic administration of CPP (10 mg/kg) was ineffective in blocking the induction of LTP at lateral perforant path-CA3 responses. However, heterosynaptic LTD of medial perforant path-CA3 responses was blocked completely by CPP. These data indicate that NMDA receptors are not required for the induction of lateral perforant path-CA3 LTP, but are involved in the induction of heterosynaptic LTD that accompanies lateral perforant path activity. The requirement for NMDA receptors for heterosynaptic LTD suggests one functional role of NMDA receptors at termination fields of the lateral perforant path.