Delta-opioid receptor is present in presynaptic axon terminals in the rat nucleus locus coeruleus: relationships with methionine5-enkephalin

Cannabinoid and opioid interactions: implications for opiate dependence and withdrawal

Withdrawal from opiates, such as heroin or oral narcotics, is characterized by a host of aversive physical and emotional symptoms. High rates of relapse and limited treatment success rates for opiate addiction have prompted a search for new approaches. For many opiate addicts, achieving abstinence may be further complicated by poly-drug use and co-morbid mental disorders. Research over the past decade has shed light on the influence of endocannabinoids (ECs) on the opioid system. Evidence from both animal and clinical studies point toward an interaction between these two systems, and suggest that targeting the EC system may provide novel interventions for managing opiate dependence and withdrawal. This review will summarize the literature surrounding the molecular effects of cannabinoids and opioids on the locus coeruleus-norepinephrine system, a key circuit implicated in the negative sequelae of opiate addiction. A consideration of the trends and effects of marijuana use in those seeking treatment to abstain from opiates in the clinical setting will also be presented. In summary, the present review details how cannabinoid-opioid interactions may inform novel interventions in the management of opiate dependence and withdrawal.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Subcellular targeting of kappa-opioid receptors in the rat nucleus locus coeruleus

The dynorphin (DYN)-kappa opioid receptor (kappaOR) system has been implicated in stress modulation, depression, and relapse to drug-seeking behaviors. Previous anatomical and physiological data have indicated that the noradrenergic nucleus locus coeruleus (LC) is one site at which DYN may contribute to these effects. Using light microscopy, immunofluorescence, and electron microscopy, the present study investigated the cellular substrates for pre- and postsynaptic interactions of kappaOR in the LC. Dual immunocytochemical labeling for kappaOR and tyrosine hydroxylase (TH) or kappaOR and preprodynorphin (ppDYN) was examined in the same section of tissue. Light microscopic analysis revealed prominent kappaOR immunoreactivity in the nuclear core of the LC and in the peri-coerulear region where noradrenergic dendrites extend. Fluorescence and electron microscopy revealed kappaOR immunoreactivity within TH-immunoreactive somata and dendrites in the LC as well as localized to ppDYN-immunoreactive processes. In sections processed for kappaOR and TH, approximately 29% (200/688) of the kappaOR-containing axon terminals identified targeted TH-containing profiles. Approximately 49% (98/200) of the kappaOR-labeled axon terminals formed asymmetric synapses with TH-labeled dendrites. Sections processed for kappaOR and ppDYN showed that, of the axon terminals exhibiting kappaOR, 47% (223/477) also exhibited ppDYN. These findings indicate that kappaORs are poised to modulate LC activity by their localization to somata and dendrites. Furthermore, kappaORs are strategically localized to presynaptically modulate DYN afferent input to catecholamine-containing neurons in the LC. These data add to the growing literature showing that kappaORs can modulate diverse afferent signaling to the LC.

Dynorphin-containing axons directly innervate noradrenergic neurons in the rat nucleus locus coeruleus

Stress causes increased dynorphin (DYN) expression in limbic brain regions and antagonism of kappa-opioid receptors may offer therapeutic potential for the treatment of depression. A potential site of DYN action relevant to stress and related neuropsychiatric disorders is the locus coeruleus (LC), the primary source of forebrain norepinephrine. Therefore, using immunofluorescence and immunoelectron microscopic analyses, we characterized the cellular substrates for interactions between DYN and tyrosine hydroxylase (TH), a catecholamine synthesizing enzyme in single sections through the rat LC. Light microscopic analysis of DYN immunoreactivity indicated that DYN fibers are distributed within the core and pericoerulear subregions of the LC. Using electron microscopy, immunoperoxidase labeling for DYN was primarily found in axon terminals, although in some cases was diffusely localized to somatodendritic processes. When DYN-containing axons formed synaptic contacts, they typically (89%) exhibited an asymmetric morphology. Almost a third (28%) of the postsynaptic targets of DYN-containing axons contained immunogold labeling for TH. These findings reveal some diversity as to the localization of DYN in the LC within axons that contact both TH and non-TH containing dendrites. However, the present data provide the first ultrastructural evidence that DYN-containing axon terminals directly innervate catecholaminergic LC dendrites. Moreover, DYN axon terminals targeting catecholaminergic LC dendrites via asymmetric synapses are consistent with localization within excitatory type afferents to the LC. Therefore, direct modulation of catacholaminergic LC neurons maybe an important site of action for DYN relevant to stress and stress-related disorders.

Morphine and pain-related stimuli enhance cell surface availability of somatic delta-opioid receptors in rat dorsal root ganglia

The present study demonstrates that perikaryaldelta-opioid receptors (deltaORs) in rat dorsal root ganglion (DRG) neurons bind and internalize opioid ligands circulating in the CSF. Using confocal and electron microscopy, we found that prolonged morphine treatment increased the cell surface density of these perikaryal deltaORs and, by way of consequence, receptor-mediated internalization of the fluorescent deltorphin (DLT) analog omega-Bodipy 576/589 deltorphin-I 5-aminopentylamide (Fluo-DLT) in all three types of DRG neurons (small, medium, and large). In contrast, chronic inflammatory pain induced by the injection of complete Freund's adjuvant (CFA) into one hindpaw selectively increased Fluo-DLT internalization in small and medium-sized DRG neurons ipsilateral to the inflammation. Based on our previous studies in the spinal cord of mu-opioid receptor (muOR) knock-out mice, it may be assumed that the enhanced membrane recruitment of deltaORs observed after sustained morphine is attributable to stimulation of muORs. However, the selectivity of the effect induced by inflammatory pain suggests that it involves a different mechanism, namely a modality-specific and pain-related activation of C and Adelta fibers. Indeed, stimulation by capsaicin of transient receptor potential vanilloid 1 receptors, which are selectively expressed by small diameter (< 600 microm2) DRG neurons, increased Fluo-DLT internalization exclusively in this cell population. The present results, therefore, demonstrate that DRG neurons express perikaryal deltaORs accessible to CSF-circulating ligands and that the density and, hence, presumably also the responsiveness, of these receptors may be modulated by both pain-related stimuli and sustained exposure to muOR agonists.

Continued morphine modulation of calcium channel currents in acutely isolated locus coeruleus neurons from morphine-dependent rats

1. The actions of the opioid agonists morphine and methionine-enkephalin (met-enkephalin) on the calcium channel currents (IBa) of acutely isolated locus coeruleus (LC) neurons from morphine-dependent and vehicle-treated rats were examined using whole cell patch clamp techniques. 2. In LC neurons maintained in 5 microM morphine, co-superfusion of naloxone (1 microM) or the mu-opioid receptor antagonist CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 1 microM) with morphine resulted in a significant increase in the amplitude of IBa. The increases in IBa were not different in neurons from morphine-dependent or vehicle rats. The increase in IBa was mimicked by washing off morphine, but not by co-superfusion of the kappa-receptor antagonist norbinaltorphimine (300 nM) or the delta-receptor antagonist ICI-174864 (1 microM). 3. In spontaneously withdrawn LC neurons from morphine-dependent rats, met-enkephalin (pD2 7.1, maximum inhibition 49%) and morphine (pD2 6.5, maximum inhibition 33%), inhibited IBa in all cells. In cells from vehicle rats the pD2 for met-enkephalin was 7.3, maximum inhibition 52%, while the pD2 for morphine was 6.6 and the maximum inhibition 43% (P<0.05 versus cells from morphine-dependent rats). 4. IBa in LC neurons was mostly comprised of omega-conotoxin GVIA- (N-type) and omega-agatoxin IVA- (P/Q-type) sensitive components, with lesser amounts of nimodipine-sensitive current and current resistant to all three blockers. Neither the density of IBa nor the proportion of any of the components of IBa differed between neurons from morphine-dependent or vehicle-treated rats. 5. This study demonstrates that in morphine-dependent rats, morphine and met-enkephalin modulation of somatic IBa in LC neurons displays modest tolerance compared with untreated rats. Further, chronic morphine treatment does not alter the type or density of IBa in LC neurons. These results provide more evidence that functional mu-opioid receptor coupling is not dramatically altered in the LC in morphine-dependent rats.

delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

Endogenous enkephalins and delta opiates affect sensory function and pain sensation by inhibiting synaptic transmission in sensory circuits via delta opioid receptors (DORs). DORs have long been suspected of mediating these effects by modulating voltage-dependent Ca(2+) entry in primary sensory neurons. However, not only has this hypothesis never been validated in these cells, but in fact several previous studies have only turned up negative results. By using whole-cell current recordings, we show that the delta enkephalin analog [D-Ala(2), D-Leu(5)]-enkephalin (DADLE) inhibits, via DORs, L-, N-, P-, and Q-high voltage-activated Ca(2+) channel currents in cultured rat dorsal root ganglion (DRG) neurons. The percentage of responding cells was remarkably high (75%) within a novel subpopulation of substance P-containing neurons compared with the other cells (18-35%). DADLE (1 microM) inhibited 32% of the total barium current through calcium channels (I(Ba)). A delta (naltrindole, 1 microM), but not a mu (beta-funaltrexamine, 5 microM), antagonist prevented the DADLE response, whereas a DOR-2 subtype (deltorphin-II, 100 nM), but not a DOR-1 (DPDPE, 1 microM), agonist mimicked the response. L-, N-, P-, and Q-type currents contributed, on average, 18, 48, 14, and 16% to the total I(Ba) and 19, 50, 26, and 20% to the DADLE-sensitive current, respectively. The drug-insensitive R-type current component was not affected by the agonist. This work represents the first demonstration that DORs modulate Ca(2+) entry in sensory neurons and suggests that delta opioids could affect diverse Ca(2+)-dependent processes linked to Ca(2+) influx through different high-voltage-activated channel types.