Microinjections Of Naltrexone Research Articles

The possible mechanisms of analgesia produced by microinjection of morphine into the lateral habenula in the acute model of trigeminal pain in rats.

This study aimed to assess the effect of intra-habenular injection of morphine on acute trigeminal pain in rats. Also here, we examined the involvement of raphe nucleus opioid and 5HT3 receptors on the antinociceptive activity of intra habenular morphine to explore the possibility of existence of descending antinociceptive relay between the habenula and raphe nucleus. The numbers of eye wiping response elicited by applying a drop (40 μL) of NaCl (5 M) solution on the corneal surface were taken as an index of acute trigeminal nociception. Intra habenular microinjection of morphine at a dose of 2 μg was without effect, whereas at doses of 5 and 8 μg significantly produced antinociception. Microinjection of naltrexone (4 μg) and ondansetron (1 μg) into the dorsal raphe nucleus prior to intra-habenular saline did not produce any significant effect on corneal pain perception. Pretreatment of the raphe nucleus with ondansetron but not naltrexone prevented intra habenular morphine (8 μg) induced antinociception. Also, intra habenular injection of lidocaine (2%, 0.5 μL reduced corneal pain response. Moreover, intra-habenular microinjection of L-glutamic acid (1 and 2 μg/site) did not produce any analgesic activity in this model of pain. In conclusion, the present results suggest that the activation of the habenular μ opioid receptor by microinjection of morphine or inhibition of habenular neurons by microinjection of lidocaine produced an analgesic effect in the acute trigeminal model of pain in rats. The analgesic effect of intra habenular morphine was blocked by intra-dorsal raphe injection of serotonin 5-HT3 antagonist.

µ1 -Opioid receptors in the dorsomedial and ventrolateral columns of the periaqueductal grey matter are critical for the enhancement of post-ictal antinociception.

Generalised tonic and tonic-clonic seizures are followed by significant increase in nociceptive thresholds in both laboratory animals and humans. The endogenous opioid peptides play a role in antinociceptive signalling, and the periaqueductal grey matter (PAG) is recruited to induce analgesia. Thus, the aim of this investigation was to evaluate the role of µ1 -opioid receptors in the dorsomedial (dm) and ventrolateral (vl) columns of PAG in post-ictal antinociception. Pentylenetetrazole (PTZ; 64 mg/kg), which is an ionotropic GABA-mediated Cl- influx antagonist, was intraperitoneally (IP) administered to induce tonic-clonic seizures in Wistar rats. The tail-flick test was used to measure the nociceptive threshold. Microinjections of naltrexone (5.0 µg/0.2 µL), which is a non-selective opioid receptor antagonist, in both dmPAG and vlPAG decreased the tonic-clonic seizure-induced antinociception in seizing animals from 10 to 120 min after seizures. Furthermore, microinjections of the µ1 -opioid receptor-selective antagonist naloxonazine (5.0 µg/0.2 µL) into the dmPAG decreased post-ictal antinociception immediately after convulsive reactions and from 10 to 90 min after seizures. However, vlPAG-pretreatment with naloxonazine at the same concentration decreased the post-ictal antinociception 30 min after the onset of tonic-clonic seizures and the nociceptive threshold returned to basal values 120 min after seizures. These findings indicate that µ1 -opioid receptor-signalling mechanisms in both dmPAG and vlPAG play a relevant role in the organisation of post-ictal antinociception. In addition, µ1 -opioid receptors in the dmPAG rather than in vlPAG seem to be more critically recruited during the antinociception induced by generalised tonic-clonic seizures.

Chemical neuroanatomical and psychopharmacological evidence that κ receptor-mediated endogenous opioid peptide neurotransmission in the dorsal and ventral mesencephalon modulates panic-like behaviour

The chemical neuroanatomy and the effects of central administration of opioid antagonists on the innate fear-induced responses elicited by electrical (at escape behaviour threshold) stimulation of the midbrain tectum were determined. The aim of the present work was to investigate the interaction between the tecto-nigral endogenous opioid peptide-mediated disinhibitory pathways and nigro-tectal inhibitory links in the control of panic-like behaviour and their organisation in the continuum comprised by the deep layers of the superior colliculus (dlSC) and the dorsolateral columns of the periaqueductal grey matter (dlPAG). Beta-endorphin-labelled neurons and fibres were found in the dorsal midbrain and also in the substantia nigra. Opioid varicose fibres and terminal buttons were widely distributed in PAG columns and in all substantia nigra subdivisions. Microinjections of naltrexone (a non-selective opioid receptor antagonist; 5.0μg/0.2μl) or nor-binaltorphimine (a selective κ-opioid receptor antagonist; 5.0μg/0.2μl) in the dlSC/dlPAG continuum, in independent groups of animals, induced significant increases in the escape thresholds for midbrain tectum electrical stimulation. The microinjection of naltrexone or nor-binaltorphimine into the SNpr also increased the escape behaviour threshold for electrical stimulation of dlSC/dlPAG. These morphological and neuropharmacological findings support previous evidence from our team for the role played by the interaction between opioidergic and GABAergic mechanisms in the modulation of innate fear-induced responses. The present data offer a neuroanatomical basis for both intratectal axo-axonic/pre-synaptic and tecto-nigral axo-somatic opioid inhibition of GABAergic nigro-tectal neurons that modulate the dorsal midbrain neurons related to the organisation of fear-related emotional responses.

Systemic opioid receptor antagonism blocks swim stress-induced retention impairment independently from CA1 and BLA opioidergic pathways

Aims The basolateral amygdale (BLA) and dorsal hippocampus (CA1) are critical sites in mediating the effects of stress hormones on memory formation. Severe stress exposure induces memory impairment probably through activating inhibitory modulatory mechanisms such as opioidergic pathways. However, the detailed mechanisms and effect sites are not well understood. The present study was designed to investigate whether opioidergic agents, specifically in the BLA and CA1, are implicated in memory impairment induced by stressful situations. Main methods Rats carrying bilateral cannulae aimed at the BLA or CA1 were trained in a step-through passive-avoidance task and immediately were exposed to a forced swim (SW) stress (10 min, 20 °C). Retention was assessed 24 h after the training session after the animals had received a systemic or microinjection of naltrexone (a classical opioid antagonist) into the BLA/CA1 before training. Key findings The systemic administration of naltrexone (0.5, 1.5, and 3 mg/kg) blocked SW stress-induced memory impairment in a dose-dependent manner, while the microinjection into the BLA (5, 10, and 20 μg/rat) or CA1 (5, 10, and 20 μg/site) exhibited no effect on the disruptive pattern. Significance These findings, albeit confirming the role of opioidergic agents in stress-induced memory impairment, rule out the involvement of the BLA and CA1 in this effect. Future studies are needed to evaluate the opioidergic sites which mediate the disruptive effects of stress exposure on memory formation.

Disruption of maternal behaviour by acute conspecific interaction induces selective activation of the lateral periaqueductal grey

Maternal behaviour is sensitive to stress and opioidergic activation. The periaqueductal grey (PAG) is involved in coping strategies to stress, whereas morphine inhibition of maternal behaviour depends on the activation of the PAG. The aim of this study was to investigate whether the PAG is activated by disrupting maternal behaviour. Lactating Wistar rats were assigned to four groups: C (control); E1 (acute exposure to a male rat); E2 (daily 2-h exposure to another lactating female and a male rat from Day 3 to 6 of lactation); and E1 + 2 (treated first as E2 and, on Day 9, as E1). Maternal behaviour was recorded on Day 9 of lactation and analysed for 1 h. The E1 group spent more time retrieving their pups, took longer to initiate nursing, had shorter nursing bouts and spent more time in non-maternal activities compared with control. Rats submitted to E2 or E1 + 2 did not differ from the control. In another experiment, lactating rats were treated as above, except that 90 min after the end of the observation period the rats were killed and their brains were processed for immunohistochemical detection of Fos protein in the PAG. Fos increased in the lateral PAG only in the E1 group. We also observed that neurons activated by acute conspecific interaction in the PAG could be responsible for an opioid-dependent decrease in maternal behaviour as this effect was reversed by a microinjection of naltrexone, nor-binaltorphimine or naloxonazine into the lateral PAG. Chronic conspecific interaction alters the way this circuitry responds to acute conspecific interaction.

Microinjection of Naltrexone into the Central, but not the Basolateral, Amygdala Blocks the Anxiolytic Effects of Diazepam in the Plus Maze

The amygdala is involved in behavioral and physiological responses to fear, and the anxiolytic properties of several drugs are localized to this region. Activation of endogenous opioid systems is known to occur in response to stress and a growing body of literature suggests that opioid systems regulate the properties of anxiolytic drugs. These experiments sought to elucidate the role of opioid receptors in the central (CeA) and basolateral (BLA) nuclei of the amygdala in regulating the anxiolytic properties of ethanol and diazepam. Male rats fitted with cannula received bilateral microinjections of the nonselective opioid receptor antagonist naltrexone (NAL) immediately followed by systemic delivery of either ethanol (1 g/kg) or diazepam (2 mg/kg) in the elevated plus maze. Both diazepam and ethanol decreased anxiety-like behavior. Delivery of NAL into the CeA blocked the anxiolytic properties of diazepam. Delivery of NAL into the BLA slightly increased open arm avoidance, but had no effect on the anxiolytic properties of diazepam. Microinjection of NAL into either nucleus failed to block the effects of ethanol. These results were specific to the anxiolytic properties of diazepam, since baseline behaviors were unaffected by microinjection of NAL. Microinjection of lidocaine produced results distinct from NAL and failed to block the anxiolytic actions of diazepam. These studies indicate distinct roles for opioid receptor systems in the CeA and BLA in regulating the anxiolytic properties of diazepam in the elevated plus maze. Further, opioid receptor systems in the CeA and BLA do not regulate the anxiolytic properties of ethanol in this test.

Systemic and intra-accumbens microinjections of naltrexone interfere with tolerance to ethanol in rats

Evidence suggests a role for the opioid system in the control of ethanol reinforcement and drinking. Previous findings have shown that naltrexone, an opioid antagonist that decreases ethanol consumption in humans and experimental animals, reduces the acquisition of acute ethanol tolerance in rats. However, there are few data regarding the role of the opioid system in the acquisition of ethanol tolerance, particularly in brain areas involved in the rewarding actions of ethanol. This study investigates the effects of systemic and of intra-accumbens injections of naltrexone on the development of rapid tolerance to ethanol. Wistar rats received intraperitoneal injections of naltrexone (0.1-3.0 mg/kg) or microinjections into the core or shell portions of the nucleus accumbens (5-20 microg) before ethanol (2.7 g/kg i.p.). The animals were tested for motor coordination on the tilting plane apparatus. Tolerance was assessed 24 h later by administering the same dose of ethanol to all animals and retesting them on the tilting plane. The second injection of ethanol resulted in less motor incoordination on Day 2, suggesting the development of rapid tolerance. Pretreatment with naltrexone, either i.p. (0.3 and 0.6 mg/kg) or intra-accumbens (5-20 microg), on Day 1, blocked the development of rapid tolerance to the motor-incoordinating effects of ethanol on Day 2 without affecting the motor performance of the animals on Day 1. The results suggest that the opioid system may be involved in the development of ethanol tolerance, and that the nucleus accumbens may play a role in this phenomenon.

Opioid-dependent effects of inescapable shock on escape behavior and conditioned fear responding are mediated by the dorsal raphe nucleus

Manipulations of the dorsal raphe nucleus (DRN) modulate the behavioral effects of exposure to inescapable shock (IS). Opiate agonists and antagonists also influence the impact of IS, but the role of the DRN in mediating these effects is unknown. The opiate antagonist naltrexone micro-injected into the region of the DRN immediately prior to IS prevented both the escape deficit and the enhancement of fear conditioning that occur 24 h later. Intra-DRN naltrexone administered at the time of later behavioral testing reduced, but did not eliminate, these effects of prior IS. Conversely, the opiate agonist morphine, in combination with a subthreshold number of 20 IS trials, induced an escape deficit and enhanced conditioned fear 24 h later. Microinjections of naltrexone into the dorsolateral periaqueductal gray area did not alter the effects of IS and electrolytic lesions of the DRN prevented the effect of the morphine-20 IS trial combination. The role of opioids in mediating the behavioral effects of IS is discussed.

Endogenous opioids acting at a medullary μ-opioid receptor contribute to the behavioral antinociception produced by GABA antagonism in the midbrain periaqueductal gray

This study examined the contribution of endogenous opioids to the antinociception produced by microinjection of the GABA A receptor antagonist, bicuculline, into the rat midbrain ventrolateral periaqueductal gray region. Microinjection of bicuculline (40 ng/0.4 μl) into the periaqueductal gray produced robust antinociception as measured by the tail-flick latency to noxious heat. This antinociception was partially reversed by intravenous administration of the non-selective opioid antagonist naloxone hydrochloride (1 and 5 mg/kg), indicating that endogenous opioid release is necessary for this effect. To determine whether opioid release in the rostral ventromedial medulla, a major projection target of the periaqueductal gray, contributes to this effect, we microinjected another opioid antagonist, naltrexone, into the rostral ventromedial medulla. Naltrexone in the rostral ventromedial medulla (5 and 10 μg/μl) significantly attenuated bicuculline antinociception elicited from the periaqueductal gray. Cys 2,tyr 3,orn 5,pen 7-amide (26.5 nmol), a selective μ-opioid receptor antagonist, also reversed the antinociception when microinjected into the rostral ventromedial medulla. Microinjections of naltrexone (10 μg/μl) or cys 2,tyr 3,orn 5,pen 7-amide at sites in the medulla dorsal to the rostral ventromedial medulla were ineffective. None of the antagonists altered baseline tail-flick latencies. These results support the hypothesis that a population of periaqueductal gray neurons produces antinociception through a μ-opioid receptor-mediated action of endogenous opioids in the rostral ventromedial medulla. Thus, two opioid-sensitive pain-modulating brainstem sites are linked by an endogenous opioid synapse in the rostral ventromedial medulla.