Blockade effect of mu and kappa opioid antagonists on the anti-nociception induced by intra-periaqueductal grey injection of oxytocin in rats

Intra-periaqueductal grey (PAG) injection of 1 nmol of oxytocin induced significant increases in hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation in rats. The anti-nociceptive effect of oxytocin was attenuated significantly by subsequent intra-PAG injection of the mu opioid antagonist beta-funaltrexamine (beta-FNA) and the kappa opioid antagonist nor-binaltorphimine (nor-BNI), but not by the delta antagonist naltrindole. The results demonstrated that mu and kappa opioid receptors, not delta receptors, were involved in the oxytocin-induced anti-nociception in PAG of rats.     

Effects of lesions of amygdaloid nuclei and substantia nigra on aversive responses induced by electrical stimulation of the inferior colliculus

Stimulation of the central nucleus of the inferior colliculus causes defensive behavior. In this work we examined the influence of lesions of brain structures involved in the expression of fear, such as periaqueductal gray matter, amygdala, and substantia nigra pars reticulata (SNpr), on these aversive responses. Thus, rats were implanted with an electrode in the central nucleus of the inferior colliculus, for the determination of the thresholds of alertness, freezing, and escape responses. Each rat also bore a cannula implanted in the periaqueductal, amygdala or Snpr for injection of the neurotoxin N-methyl-d-aspartate (8 μg/0.8 μl). The data obtained show that lesion of the central nucleus of the amygdala increases the thresholds of aversive responses whereas lesion of the basolateral complex decreases the threshold of these responses. Lesion of the Snpr increased the aversive consequences of the electrical stimulation of the inferior colliculus whereas periaqueductal gray lesions, either dorsal or ventral regions, did not change these responses. From the evidences obtained in this work, it is suggested that the expression of the defensive behavior induced by activation of the neural substrates of the inferior colliculus does not seem to depend on the integrity of the periaqueductal gray. On the contrary, the basolateral complex inhibits and the central nucleus amplifies the aversive responses integrated in the inferior colliculus. Furthermore, SNpr seems also to be an important motor output for the defensive behavior induced by stimulation of the inferior colliculus, in agreement with what has been suggested for other brain structures implicated in the expression of fear.     

A model of chronic pain in the rat: high-resolution neuroanatomical approach identifies alterations in multiple opioid systems in the periaqueductal grey

Inoculation of the tail base of rats with Mycobacterium butyricum led to an arthritic swelling and inflammation of the limbs which displayed a hyperalgesia to noxious pressure: these effects peaked at 3 weeks postinoculation. In vitro autoradiography of coronal sections of rat brain was used for a parallel determination of binding to mu-, delta- and kappa-opioid binding sites. In only two regions, the dorsomedial and dorsolateral parts of the periaqueductal grey (PAG), was a significant change seen: this comprised an increase in binding to kappa-sites, whereas mu- and delta-sites therein were unaffected. This region was analysed for opioid peptides derived from each of the three opioid peptide families known. While no change was seen in levels of immunoreactive (ir)-dynorphin1-17 A (DYN) and ir-Met-enkephalin, a decrease was detected in those of ir-beta-endorphin (beta-EP): this change was restricted to the PAG. These data demonstrate a highly localized and selective influence of chronic arthritic pain upon multiple opioid systems in the PAG of the rat, a structure playing a key role in the control of pain and in the expression of the antinociceptive actions of opioids. The data suggest a possible significance of PAG pools of beta-EP and kappa-receptors in the response to and modulation of chronic pain.     

An enkephalinergic mechanism involved in amygdaloid suppression of affective defence behavior elicited from the midbrain periaqueductal gray in the cat

A series of recent studies in our laboratory have provided evidence that opioid peptides powerfully suppress feline affective defense behavior at the level of the midbrain periaqueductal gray (PAG). In the present study, we tested the hypothesis that the central (CE) nucleus of the amygdala constitutes a significant inhibitory input to the PAG which utilizes enkephalins as its neurotransmitter or neuromodulator. Cannula-electrodes were implanted into the PAG for the elicitation of affective defense behavior as well as for infusion of opioid antagonists. Monopolar stimulating electrodes were also implanted into the central, lateral and medial amygdaloid nuclei from which suppression or facilitation of affective defense behavior could be obtained. Initially, 4 trials of concurrent, subseizure stimulation of the CE or lateral amygdala at very low (100 microA, 60 Hz) currents and PAG resulted in an immediate suppression of this response which displayed a time dependent decline after 30 min. In the next stage of the experiment, naloxone (2.7, 18.9 and 27.5 nM) was microinjected through the cannula-electrode into the PAG affective defense site and the experimental procedures noted above were repeated. Naloxone treatment (at 27.5 and 18.9 nM) blocked the suppressive effects of CE and lateral amygdaloid stimulation in a dose and time dependent manner. Further analysis revealed that this effect is likely mediated via the mu receptor since the suppressive effects of amygdaloid stimulation were blocked by the selective mu antagonist, beta-Funaltrexamine (0.05 and 0.2 nM) but not by the selective delta-antagonist, ICI 174,864 (0.7 nM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Multiple opioid receptors mediate feeding elicited by mu and delta opioid receptor subtype agonists in the nucleus accumbens shell in rats

The nucleus accumbens, and particularly its shell region, is a critical site at which feeding responses can be elicited following direct administration of opiate drugs as well as micro-selective and delta-selective, but not kappa-selective opioid receptor subtype agonists. In contrast to observations of selective and receptor-specific opioid antagonist effects upon corresponding agonist-induced actions in analgesic studies, ventricular administration of opioid receptor subtype antagonists blocks feeding induced by multiple opioid receptor subtype agonists. The present study examined whether feeding responses elicited by either putative mu ([D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO)), delta(1) ([D-Pen(2), D-Pen(5)]-enkephalin (DPDPE)) or delta(2) ([D-Ala(2), Glu(4)]-deltorphin (Deltorphin)) opioid receptor subtype agonists administered into the nucleus accumbens shell were altered by accumbens pretreatment with either selective mu (beta-funaltrexamine), mu(1) (naloxonazine), delta(1) ([D-Ala(2), Leu(5), Cys(6)]-enkephalin (DALCE)), delta(2) (naltrindole isothiocyanate) or kappa(1) (nor-binaltorphamine) opioid receptor subtype antagonists. Similar magnitudes and durations of feeding responses were elicited by bilateral accumbens administration of either DAMGO (2.5 microg), DPDPE (5 microg) or Deltorphin (5 microg). DAMGO-induced feeding in the nucleus accumbens shell was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. DPDPE-induced feeding in the accumbens was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. Deltorphin-induced feeding in the accumbens was largely unaffected by accumbens delta(2) antagonist pretreatment, and was significantly enhanced by accumbens mu or kappa(1) antagonist pretreatment. These data indicate different opioid pharmacological profiles for feeding induced by putative mu, delta(1) and delta(2) opioid agonists in the nucleus accumbens shell, as well as the participation of multiple opioid receptor subtypes in the elicitation and maintenance of feeding by these agonists in the nucleus accumbens shell.     

III. Comparison of the antinociceptive action of Mu and delta opioid receptor ligands in the periaqueductal gray matter, medial and paramedial ventral medulla in the rat as studied by the microinjection technique

In rats stereotaxically implanted with microinjection cannula in either the periaqueductal gray matter (PAG) or the medial/paramedial medullary reticular formation (MRF), microinjection of morphine, sufentanil,d-Ala²-d-Leu⁵-enkephalin (DADL) ord-Ser²- Thr⁶-leucine enkephalin (DSTLE) produced dose-dependent elevations in the response latency on tail-flick and hot plate tests. These effects were reversed by naloxone administered by microinjection into the same intracerebral site. Both mu (morphine and sufentanil) and delta (DADL and DSTLE) opioid receptor ligands produced a maximal elevation in the supraspinally mediated hot plate response when administered into either the PAG or the MRF. Similarly, mu and delta receptor ligands produced maximum elevations in the spinally mediated tail-flick response when microinjected into the PAG. In contrast, delta, but not mu, receptor agonists produced a total blockade of the tail-flick response following administraion into the MRF. Microinjection of mu (morphine) or delta (DADL) agonists into the PAG or the MRF also resulted in a naloxone-reversible inhibition of the visceral chemical evoked writhing response. These observations suggest that mu and delta opioid receptor linked systems within the MRF but not the PAG produce their antinociceptive effects by discriminable mechanisms with a differential action on spinopetal vs supraspinal modulatory systems.     

Origins of endomorphin-immunoreactive fibers and terminals in different columns of the periaqueductal gray in the rat

Endomorphin 1 (EM1) and endomorphin 2 (EM2) are endogenous ligands for mu-opioid receptors (MOR). In the central nervous system, EM-immunoreactive (IR) neuronal cell bodies are located mainly in the hypothalamus and the nucleus tractus solitarius (NTS). EM-IR fibers and terminals are found widely distributed in many brain areas, including the different columns of the periaqueductal gray (PAG). The hypothalamus, NTS, and PAG are closely involved in modulation of vocalization, autonomic and neuroendocrine functions, pain, and defensive behavior through endogenous opioid peptides that bind to the MOR in these regions. Projections exist from both the hypothalamus and the NTS to the PAG. In order to examine whether there are EM1- and/or EM2-ergic projections from the hypothalamus and NTS to the PAG, immunofluorescence histochemistry for EM1 and/or EM2 was combined with fluorescent retrograde tracing. In rats that had Fluoro-Gold (FG) injected into different columns of the PAG, some of the EM1- or EM2-IR neurons in the hypothalamus, but none in the NTS, were labeled retrogradely with FG. The majority of the EM1/FG and EM2/FG double-labeled neurons in the hypothalamus were distributed in the dorsomedial nucleus, areas between the dorsomedial and ventromedial nucleus, and arcuate nucleus; a few were also seen in the ventromedial, periventricular, and posterior nucleus. The present results indicate that the EM-IR fibers and terminals in the PAG originate principally from the hypothalamus. They also suggest that EMs released from hypothalamus-PAG projecting neurons might mediate or modulate various functions of the PAG through binding to the MOR.     

Opioid antagonists in the periaqueductal gray inhibit morphine and β-endorphin analgesia elicited from the amygdala of rats

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and μ-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or β-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the μ-selective antagonist, β-funaltrexamine (BFNA) or theδ₂ antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5–5 μg) and β-endorphin (2.5–5 jig) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1–5 μg) in the PAG significantly reduced both morphine (tail-flick: 70–75%; jump: 60–81%) and β-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not β-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and β-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15–24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAGδ₂ and to a lesser degree, μ opioid receptors are necessary for the full expression of morphine and β-endorphin analgesia elicited from the amygdala.     

Presence of kappa-opioid tone at the onset of the ovulatory luteinizing hormone surge in the proestrous rat

A decrease in endogenous opioid peptide inhibitory tone on the afternoon of proestrus is one event underlying generation of the ovulatory luteinizing hormone (LH) surge. Whether this disinhibition involves a complete loss of opioid suppression at the time of the LH surge is controversial. The objective of the present study was to determine whether a total loss specifically of the kappa-opioid inhibitory component suppressing LH secretion occurs on proestrus at the onset of the LH surge. Proestrous rats were infused intraventricularly with either artificial cerebrospinal fluid (aCSF) or aCSF containing nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, from 15:30 or 16:30 h (the approximate onset time of the spontaneous LH surge) to 18:50 h. The LH surge in rats treated with nor-BNI beginning at 15:30 h started 0.5 h earlier than the spontaneous surge in aCSF controls, and had significantly higher plasma LH levels from 16:30 to 17:30 h. Nor-BNI administration begun at 16:30 h also produced an LH surge with more elevated plasma LH levels at 17:30 and 18:00 h than in aCSF-treated controls. These results demonstrate that significant amounts of kappa-opioid tone are still present during the hours when the LH surge is initiated. Thus, a complete loss of kappa-opioid inhibition is not required for the onset of the LH surge on proestrus.     

Butorphanol dependence and withdrawal decrease hippocampal kappa 2-opioid receptor binding

The present study examines the degree and distribution of alterations in the expression of kappa-opioid receptor subtypes using a model of chronic intracerebroventricular (i.c.v.) infusion of butorphanol. Autoradiographic characterization of binding for brain kappa(1) ([3H]CI-977)-, kappa(2) ([3H]bremazocine in the presence of DAMGO, DPDPE, and U-69,593)- and total kappa ([3H]bremazocine in the presence of only DAMGO and DPDPE)-opioid receptors was performed. Dependence was induced by a 72 h i.c.v. infusion with butorphanol (26 nmol/microl per hour) (butorphanol-dependent). Butorphanol withdrawal was produced by terminating the infusion of butorphanol in dependent animals. Responses were studied 7 h following termination (butorphanol-withdrawal). During both dependence and withdrawal phases, the binding signals for both kappa(1)- and kappa(2)-opioid receptors were significantly increased in certain regions, with especially marked increases in the frontal cortex, nucleus accumbens, parietal cortex, dorsomedial hypothalamus, ventral tegmental area and locus coeruleus. In contrast, a highly specific decrease in kappa(2)-, but increase in kappa(1)-, opioid receptor binding was noted in the hippocampus of rats in both butorphanol-dependent and-withdrawal groups. Therefore, alterations in kappa(1)- and kappa(2)-opioid receptors in the hippocampus may be differently involved in both adaptation to and recovery from chronic exposure to a mixed agonist/antagonist opioid analgesic. These results further illustrate the regional distribution of changes in binding characteristics of rat brain kappa(1)- and kappa(2)-opioid receptor subtypes in an established model of butorphanol dependence and withdrawal.     

Opioid tolerance induced by metamizol (dipyrone) microinjections into the periaqueductal grey of rats

Non-opioid analgesics have been shown to elicit antinociception by an action upon central nervous system structures, in addition to their well known action upon peripheral tissues. Microinjection of metamizol (dipyrone), a widely used nonopioid analgesic, into the periaqueductal grey matter (PAG) of rats activates pain-modulating systems in the nucleus raphe magnus and inhibits spinal nociceptive neurons and the tail-flick reflex. Since these effects involve an activation of endogenous opioidergic systems, the possibility that metamizol induces opioid tolerance was investigated. Microinjection of metamizol into the ventrolateral PAG in awake rats induced antinociception, as demonstrated in the heat-elicited tail flick and hot plate tests. When microinjected into the ventrolateral PAG twice daily for 2 days, metamizol induced tolerance, i.e. a progressive loss of its antinociceptive effect. In contrast to rats repeatedly microinjected with saline, metamizol-tolerant rats were also tolerant to morphine microinjection into the same PAG site, and displayed signs of opioid withdrawal upon systemic administration of naloxone. These and other results suggest that metamizol activates endogenous opioid systems and that nonopioid analgesics may, by an action upon the central nervous system, lead to opioid tolerance and the risk of opioid withdrawal.     

Relationship of opioid receptors with GABAergic neurons in the rat inferior colliculus

The inferior colliculus is a critical structure for processing auditory information and receives ascending and descending synaptic auditory projections. In addition to GABAergic and glutamatergic innervations, other neurotransmitter systems are also reported in the inferior colliculus, including opioid peptides. In the present study, the relative distribution of each type of opioid receptor, mu (MOR), delta (DOR) and kappa (KOR) within GABAergic neurons in the inferior colliculus was examined. GABA immunoreactivity was expressed by small, medium and large neurons and distributed in the central nucleus and the pericentral nucleus of the inferior colliculus. Immunostaining for MOR, DOR and KOR receptors was found in both disc-shaped cells and stellate cells. Punctiform beta-endorphin immunolabelling was observed in the proximity of GABA-positive neurons. Co-localization of GABA and MOR receptors was observed in neurons and nerve terminals in the central nucleus, dorsal cortex and external cortex of the inferior colliculus. Quantification of the co-localization patterns determined that a higher proportion of GABA neurons was associated with MOR receptors compared with KOR or DOR receptors.     

Dorsal mesencephalic projections to pons, medulla, and spinal cord in the cat: limbic and non-limbic components.

The vertebrate dorsal mesencephalon consists of the superior colliculus, the dorsal portion of the periaqueductal gray, and the mesencephalic trigeminal neurons in between. These structures, via their descending pathways, take part in various behavioral responses to environmental stimuli. This study was undertaken to compare the origins and trajectories of these pathways in the cat. Injections of horseradish peroxidase into the cervical spinal cord and upper medullary medial tegmentum retrogradely labeled cells mainly in the contralateral intermediate and deep superior colliculus, and in the ipsilateral dorsal and lateral periaqueductal gray and adjacent tegmentum. Only injections in the medullary lateral tegmental field labeled mesencephalic trigeminal neurons ipsilaterally. Autoradiographic tracing results, based on injections across the dorsal mesencephalon, revealed three efferent fiberstreams. A massive first fiberstream (limbic pathway), consisting of thin fibers, descended ipsilaterally from the dorsal and lateral periaqueductal gray and adjacent superior colliculus through the mesencephalic and pontine lateral tegmentum, terminating in these areas as well as in the ventral third of the caudal pontine and medullary medial tegmentum. A few fibers from the dorsal periaqueductal gray matter (PAG) were distributed bilaterally to the dorsal vagal, solitary, and retroambiguus nuclei. The second fiberstream (the predorsal bundle) descended contralaterally from the superior colliculus (SC) and consisted of both thick and thin labeled fibers. The thin fibers terminated bilaterally in the dorsomedial nucleus reticularis tegmenti pontis and the medial half of the caudal medial accessory inferior olive. The thick fibers targeted the contralateral dorsal two thirds of the caudal pontine and medullary medial tegmental fields, and the facial, abducens, lateral reticular, subtrigeminal, and prepositus hypoglossi nuclei. A few fibers recrossed the midline to terminate in the ipsilateral medial tegmentum. Caudal to the obex, fibers terminated laterally in the tegmentum and upper cervical intermediate zone. From the lateral SC, fibers terminated bilaterally in the lateral tegmental fields of the pons and medulla and lateral facial subnuclei. The third fiberstream (mesencephalic trigeminal or Probst tract) terminated in the supratrigeminal and motor trigeminal nuclei, and laterally in the tegmentum and upper cervical intermediate zone. In summary, neurons in the PAG and in the deep layers of the SC give rise to a massive ipsilateral descending pathway, in which a medial-to-lateral organization exists. A similar topographical pattern occurs in the crossed SC projections. The possibility that these completely different descending systems cooperate in producing specific defensive behaviors is discussed.     

Afferents of vocalization-controlling periaqueductal regions in the squirrel monkey

In order to determine the input of vocalization-controlling regions of the midbrain periaqueductal gray (PAG), wheat germ agglutinin-horseradish peroxidase was injected in six squirrel monkeys (Saimiri sciureus) at PAG sites yielding vocalization when injected with the glutamate agonist homocysteic acid. Brains were scanned for retrogradely labeled areas common to all six animals. The results show that the vocalization-eliciting sites receive a widespread input, with the heaviest projections coming from the surrounding PAG, dorsomedial and ventromedial hypothalamus, medial preoptic region, substantia nigra pars diffusa, zona incerta and reticular formation of the mesencephalon, pons, and medulla. The heaviest cortical input reaches the PAG from the mediofrontal cortex. Moderate to weak projections come from the insula, lateral prefrontal, and premotor cortex as well as the superior and middle temporal cortex. Subcortical moderate to weak projections reach the PAG from the central and medial amygdala, nucleus of the stria terminalis, septum, nucleus accumbens, lateral preoptic region, lateral and posterior hypothalamus, globus pallidus, pretectal area, deep layers of the superior colliculus, the pericentral inferior colliculus, mesencephalic trigeminal nucleus, locus coeruleus, substantia nigra pars compacta, dorsal and ventral raphe, vestibular nuclei, spinal trigeminal nucleus, solitary tract nucleus, and nucleus gracilis. The input of the periaqueductal vocalization-eliciting regions thus is dominated by limbic, motivation-controlling afferents; input, however, also comes from sensory, motor, arousal-controlling, and cognitive brain areas.     

Effects of microinjections of neurotoxin AvTx8, isolated from the social wasp Agelaia vicina (Hymenoptera, Vespidae) venom, on GABAergic nigrotectal pathways

Several investigations have provided information that defensive behaviors evoked by stimulation of deep layers of the superior colliculus (dlSC) are subjected to inhibitory nigral modulation. This inhibition is made mainly through GABAergic neurons from substantia nigra, pars reticulata (SNpr), that sends outputs toward neural networks of the deep layers of the superior colliculus and dorsal periaqueductal gray matter involved with the organization of fear-like responses. In this work, we compared the effects of two GABAergic agonists, muscimol and baclofen, with the effect of neurotoxin AvTx8 (1567 Da), isolated from the venom of the social wasp Agelaia vicina, microinjected into SNpr of Rattus norvegicus (Wistar rats) prior to dlSC saline or bicuculline microinjections, considering that wasp venom has some influence on the uptake of GABA and/or glutamate neurotransmitters. GABA(A) receptor blockade in the dlSC evoked a vigorous escape behavior, expressed by rapid running, jumps and turns, as compared to control. These defensive reactions were maximized after the intranigral GABA(A) agonism with muscimol, but not after in situ GABA(B) agonism. Nigral microinjection of AvTx8 induced similar effects to those of baclofen, decreasing the intensity of behavioral defensive reactions caused by GABA(A) receptor blockade in the dorsal mesencephalon. These findings suggest that AvTx8 has some effects on GABAergic neurotransmission, increasing the activity of the inhibitory nigro-collicular pathways, causing an anti-panic (antiaversive) effect. Therefore, our work suggests AvTx8 as a novel pharmacological tool to study differences between the two types of GABAergic receptors and excitatory amino acid-mediated mechanisms in the brain and brainstem networks.     

Predatory hunting and exposure to a live predator induce opposite patterns of Fos immunoreactivity in the PAG

Considering the periaqueductal gray's (PAG) general roles in mediating motivational responses, in the present study, we compared the Fos expression pattern in the PAG induced by innate behaviors underlain by opposite motivational drivers, in rats, namely, insect predation and defensive behavior evoked by the confrontation with a live predator (a cat). Exposure to the predator was associated with a striking Fos expression in the PAG, where, at rostral levels, an intense Fos expression was found largely distributed in the dorsomedial and dorsolateral regions, whereas, at caudal levels, Fos-labeled cells tended to be mostly found in the lateral and ventrolateral columns, as well as in the dorsal raphe nucleus. Quite the opposite, insect predation was associated with increased Fos expression predominantly in the rostral two thirds of the lateral PAG, where the majority of the Fos-immunoreactive cells were found at the oculomotor nucleus levels. Remarkably, both exposure to the cat and insect predation upregulated Fos expression in the supraoculomotor region and the laterodorsal tegmental nucleus. Overall, the present results clearly suggest that the PAG activation pattern appears to reflect, at least partly, the animal's motivational status. It is well established that the PAG is critical for the expression of defensive responses, and, considering the present findings, it will be important to investigate how the PAG contributes to the expression of the predatory behavior, as well.     

Projections of the medial cerebellar nucleus to oculomotor-related midbrain areas in the rat: an anterograde and retrograde HRP study.

The mesencephalic projections of the medial cerebellar nucleus (MCN) were studied in the rat by using the method of anterograde transport of wheat germ agglutinin/horseradish peroxidase to establish connections of the nucleus with oculomotor-related nuclei as a basis for its proposed role in eye movement. The principal targets of projections were the supraoculomotor ventral periaqueductal gray (PAG) and lateral PAG, and paraoculomotor cell groups (nucleus of Darkschewitsch and medial accessory nucleus of Bechterew). Lesser projections were observed to the intermediate layer of the superior colliculus, nucleus of the posterior commissure, and prerubral field. Following transcannular HRP gel implants into the oculomotor complex that included adjacent paraoculomotor nuclei, the largest number of retrogradely labeled cells was found in the caudal MCN. The findings suggest that the caudal MCN in the rat, like the primate fastigial nucleus, is involved in the control of eye movement.     

大鼠中脑导水管周围灰质和中缝核簇内P物质受体定位分布的免疫细胞化学研究

本研究用免疫细胞化学染色技术对大鼠中脑导水管周围灰质（ＰＡＧ）和中缝核簇内Ｐ物质受体的精确定位分布进行了系统地观察。Ｐ物质受体阳性的神经元胞体和纤维主要密集地分布于ＰＡＧ吻段的背侧部、中段的外侧部和尾段的腹侧部。在线形核、中缝背核、中缝正中核、中缝桥核、中缝大核和中缝隐核可见中等密度至高密度的阳性神经元胞体和纤维；在中央下核和中缝苍白核内仅见少量阳性纤维。     

Kappa-Opioid Receptor Involvement in the Regulation of Pulsatile Luteinizing Hormone Release During Early Pregnancy in the Rat

The object of this study was to gain further insight into endogenous opioid peptide suppression of pulsatile luteinizing hormone (LH) release in early gestation in the rat by examining whether selective blockade of mu -, delta -, or kappa-opioid receptor(s) results in stimulation of pulsatile LH secretion at this time. Previous reports demonstrated stimulation of pulsatile LH release during early gestation by intravenous infusions of naloxone, an endogenous opioid peptide receptor antagonist whose binding is not specific to a single class of opioid peptide receptors. In the present study, naloxone infused intraventricularly similarly stimulated an increase in pulsatile LH release on Days 7 to 8 of gestation. Antagonists of specific opioid peptide receptor subtypes were thus given by this route. Administration of nor-binaltorphimine, an antagonist of kappa-opioid receptors, but not β-funaltrexamine or ICI 174, 864, antagonists of mu- and delta-opioid receptors, respectively, exerted a stimulatory action on both LH pulse amplitude and frequency similar to that of naloxone, indicating involvement of this opioid peptide receptor subtype in the endogenous opioid peptide suppression of pulsatile LH release in early gestation in the rat.     

Morphine withdrawal precipitated by specific mu,delta or kappa opioid receptor antagonists: a c-Fos protein study in the rat central nervous system

We have recently shown concurrent changes in behavioural responses and c-Fos protein expression in the central nervous system in both naive and morphine-dependent rats after systemic administration of the opioid antagonist naloxone. However, because naloxone acts on the three major types of opioid receptors, the present study aimed at determining, in the same animals, both changes in behaviour and c-Fos-like immunoreactivity after intravenous injection of selective opioid antagonists, such as mu (beta-funaltrexamine, 10 mg/kg), delta (naltrindole, 4 mg/kg) or kappa (nor-binaltorphimine, 5 mg/kg) opioid receptor antagonists, in naive or morphine-dependent rats. In a first experimental series, only beta-funaltrexamine increased c-Fos expression in the eight central nervous system structures examined, whereas no effect was seen after naltrindole or nor-binaltorphimine administration in naive rats. These results suggest a tonic activity in the endogenous opioid peptides acting on mu opioid receptors in normal rats. A second experimental series in morphine-dependent rats showed that beta-funaltrexamine had the highest potency in the induction of classical signs of morphine withdrawal syndrome, as well as the increase in c-Fos expression in the 22 central nervous system structures studied, suggesting a major role of mu opioid receptors in opioid dependence. However, our results also demonstrated that naltrindole and, to a lesser extent, nor-binaltorphimine were able to induce moderate signs of morphine withdrawal and relatively weak c-Fos protein expression in restricted central nervous system structures. Therefore, delta and kappa opioid receptors may also contribute slightly to opioid dependence.