Localization of the antinociceptive and antianalgesic effects of neurotensin within the rostral ventromedial medulla

Systemically administered neurotensin receptor agonist produces antinociception through activation of spinally projecting serotonergic neurons in the rostral ventromedial medulla

Background

Supraspinal delivery of neurotensin (NTS), which may contribute to the effect of a systemically administered agonist, has been reported to be either pronociceptive or antinociceptive. Here, we evaluated the effects of systemically administered NTSR1 agonist in a rat model of neuropathic pain and elucidated the underlying supraspinal mechanism.

Methods

Neuropathic pain was induced by L5 and L6 spinal nerve ligation in male Sprague-Dawley rats. The effects of intraperitoneally administered NTSR1 agonist PD 149163 was assessed using von Frey filaments. To examine the role of 5-HT neurotransmission, a serotonin (5-HT) receptor antagonist dihydroergocristine was pretreated intrathecally, and spinal microdialysis studies were performed to measure the change in extracellular level of 5-HT in response to PD 149163 administration. To investigate the supraspinal mechanism, NTSR1 antagonist 48692 was microinjected into the rostral ventromedial medulla (RVM) prior to systemic PD 149163. Additionally, the effect of intrathecal DHE on intra-RVM PD 149163 was assessed.

Results

Intraperitoneally administered PD 149163 exhibited a dose-dependent attenuation of mechanical allodynia. This effect was partially reversed by intrathecal pretreatment with dihydroergocristine and was accompanied by an increased extracellular level of 5-HT in the spinal cord. The PD 149163-produced antinociception was also blocked by intra-RVM SB 48692. Direct injection of PD 149163 into the RVM mimicked the maximum effect of the same drug delivered intraperitoneally, which was reversed by intrathecal dihydroergocristine.

Conclusions

These observations indicate that systemically administered NTSR1 agonist produces antinociception through the NTSR1 in the RVM, activating descending serotonergic projection to release 5-HT into the spinal dorsal horn.

Divergent Modulation of Nociception by Glutamatergic and GABAergic Neuronal Subpopulations in the Periaqueductal Gray

The ventrolateral periaqueductal gray (vlPAG) constitutes a major descending pain modulatory system and is a crucial site for opioid-induced analgesia. A number of previous studies have demonstrated that glutamate and GABA play critical opposing roles in nociceptive processing in the vlPAG. It has been suggested that glutamatergic neurotransmission exerts antinociceptive effects, whereas GABAergic neurotransmission exert pronociceptive effects on pain transmission, through descending pathways. The inability to exclusively manipulate subpopulations of neurons in the PAG has prevented direct testing of this hypothesis. Here, we demonstrate the different contributions of genetically defined glutamatergic and GABAergic vlPAG neurons in nociceptive processing by employing cell type-specific chemogenetic approaches in mice. Global chemogenetic manipulation of vlPAG neuronal activity suggests that vlPAG neural circuits exert tonic suppression of nociception, consistent with previous pharmacological and electrophysiological studies. However, selective modulation of GABAergic or glutamatergic neurons demonstrates an inverse regulation of nociceptive behaviors by these cell populations. Selective chemogenetic activation of glutamatergic neurons, or inhibition of GABAergic neurons, in vlPAG suppresses nociception. In contrast, inhibition of glutamatergic neurons, or activation of GABAergic neurons, in vlPAG facilitates nociception. Our findings provide direct experimental support for a model in which excitatory and inhibitory neurons in the PAG bidirectionally modulate nociception.

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

Distribution of glycine/GABA neurons in the ventromedial medulla with descending spinal projections and evidence for an ascending glycine/GABA projection

The ventromedial medulla (VM), subdivided in a rostral (RVM) and a caudal (CVM) part, has a powerful influence on the spinal cord. In this study, we have identified the distribution of glycine and GABA containing neurons in the VM with projections to the cervical spinal cord, the lumbar dorsal horn, and the lumbar ventral horn. For this purpose, we have combined retrograde tracing using fluorescent microspheres with fluorescent in situ hybridization (FISH) for glycine transporter 2 (GlyT2) and GAD67 mRNAs to identify glycinergic and/or GABAergic (Gly/GABA) neurons. Since the results obtained with FISH for GlyT2, GAD67, or GlyT2 + GAD67 mRNAs were not significantly different, we concluded that glycine and GABA coexisted in the various projection neurons. After injections in the cervical cord, we found that 29% ± 1 (SEM) of the retrogradely labeled neurons in the VM were Gly/GABA (RVM: 43%; CVM: 21%). After lumbar dorsal horn injections 31% ± 3 of the VM neurons were Gly/GABA (RVM: 45%; CVM: 12%), and after lumbar ventral horn injections 25% ± 2 were Gly/GABA (RVM: 35%; CVM: 17%). In addition, we have identified a novel ascending Gly/GABA pathway originating from neurons in the area around the central canal (CC) throughout the spinal cord and projecting to the RVM, emphasizing the interaction between the ventromedial medulla and the spinal cord. The present study has now firmly established that GABA and glycine are present in many VM neurons that project to the spinal cord. These neurons strongly influence spinal processing, most notably the inhibition of nociceptive transmission.

Post-ictal analgesia in genetically epilepsy-prone rats is induced by audiogenic seizures and involves cannabinoid receptors in the periaqueductal gray

Post-ictal depression of consciousness occurs after generalized convulsive seizures, and includes analgesia, lasting for hours after electrically or chemically induced seizures in animals. The brain sites and mechanisms, mediating post-ictal analgesia, are unclear. The ventrolateral periaqueductal gray (PAG) is an important neuronal network site for mediating analgesia and also in generalized seizures, particularly in genetically epilepsy-prone rats (GEPRs). Endocannabinoids are implicated in mediating analgesia in several brain sites, including the PAG, and generalized seizures result in endocannabinoid release. This study evaluated if post-ictal analgesia occurs in GEPRs, following audiogenic seizures (AGS), and whether this analgesia involves endocannabinoid actions in PAG. Analgesia was evaluated, using thermal stimulation to evoke nociception, measuring changes in paw withdrawal latencies (PWLs) induced by AGS. Endocannabinoid involvement in post-ictal analgesia in GEPRs was evaluated, using focal bilateral microinjection of a cannabinoid (CB1) receptor antagonist (AM251) into PAG. AGS induced a significant increase in PWLs, lasting for ≥120min. Microinjection of AM251 (100 and 200, but not 50 pmol/side) into PAG significantly decreased post-ictal analgesia in GEPRs. Endocannabinoids are also known to activate transient receptor potential vanilloid (TRPV1) receptors, but PAG microinjection of a TRPV1 receptor antagonist (capsazepine) did not affect post-ictal analgesia in GEPRs. These results indicate that AGS in GEPRs induce post-ictal analgesia, which is the first observation of this phenomenon in a genetic epilepsy model. These findings suggest an important role of PAG in post-ictal analgesia. The results also suggest that CB1 receptors in PAG are critical for mediating post-ictal analgesia in GEPRs.

Descending facilitatory pathways from the rostroventromedial medulla mediate naloxone-precipitated withdrawal in morphine-dependent rats

Opioids produce analgesic effects, and extended use can produce physical dependence in both humans and animals. Dependence to opiates can be demonstrated by either termination of drug administration or through precipitation of the withdrawal syndrome by opiate antagonists. Key features of the opiate withdrawal syndrome include hyperalgesia, anxiety, and autonomic signs such as diarrhea. The rostral ventromedial medulla (RVM) plays an important role in the modulation of pain and for this reason, may influence withdrawal-induced hyperalgesia. The mechanisms that drive opiate withdrawal-induced hyperalgesia have not been elucidated. Here, rats made dependent upon morphine received naloxone to precipitate withdrawal. RVM microinjection of lidocaine, kynurenic acid (excitatory amino acid antagonist) or YM022 (CCK2 receptor antagonist) blocked withdrawal-induced hyperalgesia. Additionally, these treatments reduced both somatic and autonomic signs of naloxone-induced withdrawal. Spinal application of ondansetron, a 5HT3 receptor antagonist thought to ultimately be engaged by descending pain facilitatory drive, also blocked hyperalgesia and somatic and autonomic features of the withdrawal syndrome. These results indicate that the RVM plays a critical role in mediating components of opioid withdrawal that may contribute to opioid dependence.

PERSPECTIVE

Manipulations targeting these descending pathways from the RVM may diminish the consequences of prolonged opioid administration-induced dependence and be useful adjunct strategies in reducing the risk of opioid addiction.

Altered morphine-induced analgesia in neurotensin type 1 receptor null mice

Both neurotensin (NT) and opioid agonists have been shown to induce antinociception in rodents after central administration. Besides, previous studies have revealed the existence of functional interactions between NT and opioid systems in the regulation of pain processing. We recently demonstrated that NTS1 receptors play a key role in the mediation of the analgesic effects of NT in long-lasting pain. In the present study, we therefore investigated whether NTS1 gene deletion affected the antinociceptive action of mu opioid drugs. To this end, pain behavioral responses to formalin were determined following systemic administration of morphine in both male and female NTS1 knockout mice. Acute injection of morphine (2 or 5 mg/kg) produced strong antinociceptive effects in both male and female wild-type littermates, with no significant sex differences. On the other hand, morphine analgesia was considerably reduced in NTS1-deficient mice of both sexes compared to their respective controls, indicating that the NTS1 receptor actively participates in mu opioid alleviating pain. By examining specifically the flinching, licking and biting nociceptive behaviors, we also showed that the functional crosstalk between NTS1 and mu opioid receptors influences the supraspinally-mediated behaviors. Interestingly, sexual dimorphic action of morphine-induced pain inhibition was found in NTS1 null mice in the formalin test, suggesting that the endogenous NT system interacts differently with the opioid network in male and female mice. Altogether, these results demonstrated that NTS1 receptor activation operates downstream to the opioidergic transmission and that NTS1-selective agonists combined with morphine may act synergistically to reduce persistent pain.

Descending control of nociception: Specificity, recruitment and plasticity

The dorsal horn of the spinal cord is the location of the first synapse in pain pathways, and as such, offers a very powerful target for regulation of nociceptive transmission by both local segmental and supraspinal mechanisms. Descending control of spinal nociception originates from many brain regions and plays a critical role in determining the experience of both acute and chronic pain. The earlier concept of descending control as an "analgesia system" is now being replaced with a more nuanced model in which pain input is prioritized relative to other competing behavioral needs and homeostatic demands. Descending control arises from a number of supraspinal sites, including the midline periaqueductal gray-rostral ventromedial medulla (PAG-RVM) system, and the more lateral and caudal dorsal reticular nucleus (DRt) and ventrolateral medulla (VLM). Inhibitory control from the PAG-RVM system preferentially suppresses nociceptive inputs mediated by C-fibers, preserving sensory-discriminative information conveyed by more rapidly conducting A-fibers. Analysis of the circuitry within the RVM reveals that the neural basis for bidirectional control from the midline system is two populations of neurons, ON-cells and OFF-cells, that are differentially recruited by higher structures important in fear, illness and psychological stress to enhance or inhibit pain. Dynamic shifts in the balance between pain inhibiting and facilitating outflows from the brainstem play a role in setting the gain of nociceptive processing as dictated by behavioral priorities, but are also likely to contribute to pathological pain states.

Neurotensin and pain modulation

Neurotensin (NT) can produce a profound analgesia or enhance pain responses, depending on the circumstances. Recent evidence suggests that this may be due to a dose-dependent recruitment of distinct populations of pain modulatory neurons. NT knockout mice display defects in both basal nociceptive responses and stress-induced analgesia. Stress-induced antinociception is absent in these mice and instead stress induces a hyperalgesic response, suggesting that NT plays a key role in the stress-induced suppression of pain. Cold water swim stress results in increased NT mRNA expression in hypothalamic regions known to project to periaqueductal gray, a key region involved in pain modulation. Thus, stress-induced increases in NT signaling in pain modulatory regions may be responsible for the transition from pain facilitation to analgesia. This review focuses on recent advances that have provided insights into the role of NT in pain modulation.

Potent spinal analgesia elicited through stimulation of NTS2 neurotensin receptors

Intrathecal administration of the neuropeptide neurotensin (NT) was shown previously to exert antinociceptive effects in a variety of acute spinal pain paradigms including hotplate, tail-flick, and writhing tests. In the present study, we sought to determine whether some of these antinociceptive effects might be elicited via stimulation of low-affinity NTS2 receptors. We first established, using immunoblotting and immunohistochemical techniques, that NTS2 receptors were extensively associated with putative spinal nociceptive pathways, both at the level of the dorsal root ganglia and of the superficial layers of the dorsal horn of the spinal cord. We then examined the effects of intrathecal administration of NT or selective NTS2 agonists on acute thermal pain. Both NT and NTS2 agonists, levocabastine and Boc-Arg-Arg-Pro-Tyrpsi(CH2NH)Ile-Leu-OH (JMV-431), induced dose-dependent antinociceptive responses in the tail-flick test. The effects of levocabastine and of JMV-431 were unaffected by coadministration of the NTS1-specific antagonist 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxy-phenyl)pyrazol-3-yl)carboxylamino]tricyclo)3.3.1.1.(3.7))-decan-2-carboxylic acid (SR48692), confirming that they were NTS2 mediated. In contrast, the antinociceptive effects of NT were partly abolished by coadministration of SR48692, indicating that NTS1 and NTS2 receptors were both involved. These results suggest that NTS2 receptors play a role in the regulation of spinal nociceptive inputs and that selective NTS2 agonists may offer new avenues for the treatment of acute pain.

Neurotensin activation of the NTR1 on spinally-projecting serotonergic neurons in the rostral ventromedial medulla is antinociceptive

Microinjection of neurotensin (NT) in the rostral ventromedial medulla (RVM) produces dose-dependent antinociception. The NTR1 (Neurotensin Receptor Subtype 1) may mediate part of this response, however definitive evidence is lacking, and the spinal mediators of NTR1-induced antinociception are unknown. In the present study, we used immunohistochemical techniques to show that the NTR1, but not the NTR2 is expressed by spinally projecting serotonergic neurons of the RVM. We also show that microinjection of NT or the NTR1-selective agonist PD149163 in the RVM both produce dose-dependent antinociception in the tail-flick test that is blocked by the NTR1-selective antagonist SR48692. The antinociception produced by NT or PD149163 is also blocked by intrathecal administration of the non-selective serotonergic receptor antagonist methysergide. The results of these experiments provide anatomical and behavioral evidence that activation of NTR1-expressing spinally projecting neurons in the RVM produces antinociception through release of serotonin in the spinal dorsal horn. These results support the conclusion that the NTR1 plays an important role in the central modulation of nociception.

Endogenous neurotensin facilitates visceral nociception and is required for stress-induced antinociception in mice and rats

Central neurotensin (NT) administration can both facilitate and inhibit somatic and visceral nociception, depending on the dose and administration site. NT microinjection in the rostroventral medulla facilitates nociception at low doses, while NT antagonist microinjection can markedly attenuate nociception, supporting the hypothesis that endogenous NT facilitates nociception. However, higher doses of NT produce a mu-opioid receptor-independent analgesia, similar to that resulting from various intense stressors. Furthermore, intense stress results in increased NT expression in several hypothalamic nuclei that have been implicated in stress-induced antinociception (SIAN); however, there is little direct evidence that endogenous NT is required for SIAN. We have investigated the role of endogenous NT in both basal visceral nociception and SIAN using both NT knockout mice and pharmacological approaches in rats. Visceral nociception was monitored by measuring visceromotor responses during colorectal distension both prior to and following water avoidance stress. Visceral nociception was significantly attenuated in both NT knockout mice and rats pre-treated with the NT antagonist SR 48692. Disruption of NT signaling also blocked SIAN, revealing a novel stress-induced hyperalgesic response that was significantly greater in female than in male rats. NT was also required for acetic acid-induced hyperalgesia. These results indicate that endogenous NT normally facilitates visceral pain responses, is required for irritant-induced hyperalgesia, and plays a critical role in SIAN.

Peptides and hormesis

The article provides a broad assessment of the occurrence of hormetic-like biphasic dose-response relationships by over 30 peptides representing many major peptide classes. These peptide-induced biphasic dose responses were observed to occur in a extensive range of tissues, affecting an diverse range of biological endpoints. Despite diversity of peptides, models and endpoints, the quantitative features of the biphasic dose responses are remarkably similar with respect to the amplitude and width of the stimulatory response. These findings strongly suggest that hormetic-like biphasic dose responses represent a broadly generalizable biological phenomenon.

Reciprocal interactions between the amygdala and ventrolateral periaqueductal gray in mediating of Q/N(1-17)-induced analgesia in the rat

The opioid peptide, Orphanin FQ/nociceptin (OFQ/N(1-17))(,) its active fragments, and a related precursor peptide each produce analgesia following microinjection into the amygdala of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala is blocked by amygdala pretreatment of either general, mu, kappa, or delta-opioid antagonists even though OFQ/N(1-17) binds poorly to these receptor subtypes, and the antagonists bind poorly to the ORL-1/KOR-3 receptor. Agonists at mu and kappa opioid receptors as well as beta-endorphin each produce analgesia elicited from the amygdala that is blocked by opioid antagonist pretreatment in the ventrolateral periaqueductal gray (vlPAG) of rats. The present study examined whether pretreatment of general and selective opioid antagonists in the vlPAG blocked OFQ/N(1-17)-induced analgesia on the tail-flick test elicited from the amygdala, and whether pretreatment of general and selective opioid antagonists in the amygdala blocked OFQ/N(1-17)-induced analgesia elicited from the vlPAG of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala was significantly and markedly reduced following vlPAG pretreatment with a dose range of either naltrexone, beta-funaltrexamine (beta-FNA, mu), nor-binaltorphamine (NBNI, kappa) or naltrindole (NTI, delta). In contrast, opioid antagonists administered into misplaced mesencephalic control placements ventral and lateral to the vlPAG actually enhanced OFQ/N(1-17)-induced analgesia elicited from the amygdala. OFQ/N(1-17)-induced analgesia elicited from the vlPAG was significantly and markedly reduced following amygdala pretreatment with naltrexone and NBNI, to a lesser degree by NTI, and was unaffected by beta-FNA. Yet, opioid antagonists administered into misplaced amygdala control placements were generally ineffective in altering OFQ/N(1-17)-induced analgesia elicited from the vlPAG. Latencies were transiently increased by general, but not selective opioid antagonist treatment alone in the amygdala, but not the vlPAG. These data indicate reciprocal and regional interactions between the amygdala and vlPAG in the mediation of OFQ/N(1-17) by classic opioid receptor subtype antagonists in rats.

Modulation of noxious and non-noxious spinal mechanical transmission from the rostral medial medulla in the rat

Modulatory influences on spinal mechanical transmission from the rostral medial medulla (RMM) were studied. Noxious stimulation, produced by von Frey-like monofilaments, and non-noxious stimulation, produced by a soft brush, was applied to the glabrous skin of the hind foot. At 28 sites in RMM, electrical stimulation facilitated responses to noxious mechanical stimulation at low intensities (5-25 microA) and inhibited responses of the same neurons at greater intensities (50-100 microA) of stimulation. At 24 and 9 other sites in RMM, stimulation at all intensities only inhibited or only facilitated, respectively, responses to noxious mechanical stimulation of the hind foot. Stimulus-response functions to mechanical stimulation were shifted leftward by low intensities and decreased by high intensities of stimulation. Inhibitory influences were found to descend in the dorsolateral funiculi; facilitatory effects were contained in the ventral spinal cord. Descending modulation of non-noxious brush stimulation revealed biphasic facilitatory-inhibitory effects (9 sites in RMM), only inhibitory effects (14 sites) and only facilitatory effects (8 sites). The effects of electrical stimulation were replicated by intra-RMM administration of glutamate; a low concentration (0.25 nmol) facilitated and a greater concentration (2.5 nmol) inhibited spinal mechanical transmission, providing evidence that cells in RMM are sufficient to engage descending influences. Descending modulatory effects were specific for the site of stimulation, not for the spinal neuron, because modulation of the same neuron was different from different sites in RMM. These results show that spinal mechanical transmission, both noxious and non-noxious, is subject to descending influences, including facilitatory influences that may contribute to exaggerated responses to peripheral stimuli in some chronic pain states.

Biphasic modulation of spinal visceral nociceptive transmission from the rostroventral medial medulla in the rat

Descending inhibitory and facilitatory influences from the rostroventral medulla (RVM) on responses of lumbosacral spinal neurons to noxious colorectal distension (CRD, 80 mmHg, 20 s) were studied. At 25 sites in the RVM, electrical stimulation produced biphasic effects, facilitating responses of spinal neurons to CRD at lesser intensities of stimulation (5-25 microA) and inhibiting responses of the same neurons at greater intensities of stimulation (50-100 microA). At 38 other sites in the RVM, electrical stimulation produced only intensity-dependent inhibition of neuron responses to CRD. At another 13 sites in the RVM, electrical stimulation (5-100 microA) produced only facilitatory effects on responses to CRD. Descending modulatory effects were selective for distension-evoked activity; spontaneous activities of the same spinal neurons were not significantly affected by electrical stimulation that either facilitated or inhibited neuron responses to CRD. Neuron responses to graded CRD (20-100 mmHg) were positively accelerating functions that were shifted leftward or rightward, respectively, by lesser, facilitatory intensities or greater, inhibitory intensities of RVM stimulation. L-glutamate microinjection into the RVM replicated the effects of electrical stimulation, producing similar biphasic modulatory effects as produced by electrical stimulation. Microinjection of glutamate into the RVM at a low dose (5 nmoles) facilitated responses of spinal neurons to CRD and inhibited responses of the same neurons at a greater dose (50 nmoles). In some experiments, microinjection of lidocaine (0.5 microl of 4% solution) or the neurotoxin ibotenic acid (0.5 microl, 10 microg) into the RVM produced reversible or long-lasting, respectively, decreases in spontaneous activity and responses of spinal neurons to CRD. These results reveal that spinal visceral nociceptive transmission is subject to a tonic descending excitatory influence from the RVM and that descending modulatory effects from the RVM on visceral nociceptive transmission are qualitatively similar to modulation of cutaneous nociceptive transmission.

Supraspinal circuitry mediating opioid antinociception: antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.

Actions of NMDA and cholinergic receptor antagonists in the rostral ventromedial medulla upon beta-endorphin analgesia elicited from the ventrolateral periaqueductal gray

Analgesia elicited by morphine in the ventrolateral periaqueductal gray is mediated in part by NMDA and cholinergic receptors in the rostral ventromedial medulla because selective receptor antagonists applied to the latter structure reduced morphine analgesia elicited from the former structure. Previous studies have demonstrated that morphine and beta-endorphin employ different anatomical and neurochemical pathways in exerting their supraspinal analgesic effects. The present study evaluated whether pretreatment with either competitive (AP7, 3-10 microg) or non-competitive (MK-801, 3-10 microg) NMDA antagonists, or muscarinic (scopolamine, 5 microg) or nicotinic (mecamylamine, 1 microg) cholinergic antagonists administered into the rostral ventromedial medulla altered beta-endorphin (15 microg) analgesia elicited from the ventrolateral periaqueductal gray as measured by the tail-flick and jump tests in rats. Whereas AP7 produced minimal (11%) and transient (30 min) reductions in beta-endorphin analgesia on the jump test, MK-801 produced minimal (9%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Whereas mecamylamine failed to reduce beta-endorphin analgesia on either measure, scopolamine produced small (23%) and transient (30 min) reductions in beta-endorphin analgesia on the tail-flick test. Each of these antagonists administered into the rostral ventromedial medulla at comparable or lower doses virtually eliminated morphine analgesia elicited from the ventrolateral periaqueductal gray. The opioid mediation of beta-endorphin analgesia in the ventrolateral periaqueductal gray was confirmed by its sensitivity to naltrexone (1-20 microg) pretreatment into the same structure. These data provide further evidence for dissociations between the descending neuroanatomical and neurochemical circuitry mediating the supraspinal analgesic responses induced by morphine and beta-endorphin, and indicate that the latter response is mediated by either non-cholinergic and non-NMDA synapses within the rostral ventromedial medulla, and/or by brainstem sites outside of the rostral ventromedial medulla.

Supraspinal neurotensin-induced antianalgesia in mice is mediated by spinal cholecystokinin

Intracerebral injection of neurotensin into specific brain loci in rats produces hyperalgesia due to the release of cholecystokinin (CCK) in the spinal cord. The present purpose was to show in another species that neurotensin can antagonize the antinociceptive action of morphine through the spinal CCK mechanism in mice. Neurotensin given intracerebroventricularly (i.c.v.) at doses higher than 100 ng produced antinociception in the tail flick test. However, at lower doses between 1 pg to 25 ng, neurotensin antagonized the antinociceptive action of morphine given intrathecally (i.t.), thus demonstrating the antianalgesic activity of neurotensin. The rightward shift in the morphine dose-response curve produced by i.c.v. neurotensin was eliminated by an i.t. pretreatment with CCK8 antibody (5 microl of antiserum solution diluted 1:1000). I.t. administration of lorglumide, a CCK(A)-receptor antagonist (10-1000 ng), and PD135,158, a CCK(B)-receptor antagonist (250-500 ng), also eliminated the antianalgesic action of neurotensin. Thus, the mechanism of the antianalgesic action of neurotensin given i.c.v. involved spinal CCK. This mode of action is similar to that for the antianalgesic action of supraspinal pentobarbital which also involves spinal CCK.

Alterations in swim stress-induced analgesia and hypothermia following serotonergic or NMDA antagonists in the rostral ventromedial medulla of rats

Serotonergic, NMDA, or opioid antagonists in the rostral ventromedial medulla (RVM) reduce morphine analgesia elicited from the periaqueductal gray (PAG). Continuous (CCWS) and intermittent (ICWS) cold-water swims elicit respective naltrexone-insensitive and naltrexone-sensitive analgesic responses. CCWS analgesia is reduced by systemic NMDA receptor antagonism and by systemic, but not intrathecal serotonergic antagonism. ICWS analgesia is reduced by both systemic and intrathecal serotonergic antagonism, but unaffected by systemic NMDA antagonism. The present study evaluated whether serotonergic (methysergide: 5-10 microg) or competitive [AP7 (2-amino-7-phosphonoheptanoic acid): 0.01-0.1 microg] or non-competitive [MK-801 (dizocilipine maleate): 0.3-3 microg] NMDA antagonists in the RVM altered CCWS and ICWS analgesia and hypothermia as well as basal nociceptive latencies. Methysergide in the RVM significantly potentiated CCWS, but not ICWS analgesia. In contrast, AP7 in the RVM significantly potentiated ICWS analgesia. Antagonist-induced changes in either hypothermia or basal nociception failed to account for any alterations in stress-induced analgesia. These data suggest that serotonergic, but not NMDA, receptors in the RVM may mediate collateral inhibition between mesencephalic morphine analgesia and naltrexone-insensitive CCWS analgesia.

In vivo studies with low doses of levocabastine and diphenhydramine, but not pyrilamine, antagonize neurotensin-mediated antinociception

The present study describes in vivo experiments in the rat addressing the role of levocabastine, and two other specific histamine H1 antagonists, diphenhydramine and pyrilamine, at neurotensin (NT)-mediated hypothermia and antinociception (hotplate). Levocabastine given i.p. or microinjected directly into the periaqueductal gray (PAG) did not cause antinociception or hypothermia. This indicates that despite the results with the recently-cloned levocabastine-sensitive NT receptors (NTR) in the rat (NTR-2) and mouse (NTRL), levocabastine by itself does not mediate either hypothermia or antinociception at NT receptors. However, pretreatment with 5 or 50 microg/kg of levocabastine or 5 microg/kg diphenhydramine all caused over a three-fold reduction in NT-mediated antinociception. Higher doses (500 or 5000 microg/kg) of levocabastine did not cause any antagonism of NT-mediated antinociception. All three antihistamines did not affect NT-mediated hypothermia. In addition, histamine H1 pathways are not involved in NT-mediated antinociception, as pretreatment with the much more potent histamine H1 antagonist pyrilamine did not affect antinociception mediated by NT. Therefore, these data may suggest the presence of yet unidentified NTR subtypes responsible for NT-mediated hypothermia and antinociception.

Characterization of biphasic modulation of spinal nociceptive transmission by neurotensin in the rat rostral ventromedial medulla

Modulation of spinal nociceptive transmission by neurotensin microinjected in the rostral ventromedial medulla (RVM) was examined in anesthetized, paralyzed rats. Forty-three spinal dorsal horn neurons in the L3-L5 spinal segments responding to mechanical and noxious thermal stimulation (50 degrees C) of the plantar surface of the ipsilateral hind foot were studied. Spinal units were classified as either wide dynamic range or nociceptive specific and were located in spinal laminae I-V. Microinjection of neurotensin (0.03 pmol/0.2 microl) into the RVM produced a significant facilitation (135% of control) of spinal unit responses to noxious thermal stimulation (50 degrees C) that lasted approximately 12 min. In contrast, injection of greater doses of neurotensin (300 or 3,000 pmol) produced an inhibition of spinal unit responses to noxious heat (51.7 and 10.6% of control, respectively) that had a longer duration (60-120 min). The effects of neurotensin on wide-dynamic-range and nociceptive-specific neuron responses to noxious heat were qualitatively and quantitatively similar. Spinal unit responses to graded heating of the skin (42-50 degrees C) were completely inhibited after microinjection of 3,000 pmol of neurotensin into the RVM. Injection of a lesser dose of neurotensin (300 pmol), however, resulted in a partial inhibition of spinal unit responses and significantly reduced the slope of the stimulus-response function to graded heating of the skin. Transection of either the ipsilateral or contralateral dorsolateral funiculus (DLF) significantly reduced the inhibition of spinal nociceptive transmission produced by neurotensin (3,000 pmol) in the RVM, whereas bilateral transection of the DLFs completely blocked the effect. In contrast, bilateral transection of the DLFs had no effect on facilitation of spinal nociception by neurotensin (0.03 pmol) in the RVM. The inhibition of spinal nociceptive transmission by neurotensin (3,000 pmol) in the RVM was completely blocked by injection of the nonpeptide neurotensin receptor antagonist SR48692 (30 fmol) into the RVM 10 min before neurotensin. To confirm a specific block of neurotensin-receptor-mediated effects by the antagonist, a subsequent injection of L-glutamate into the RVM was performed. L-Glutamate (100 nmol) was found to inhibit the nociceptive responses of those spinal units whose responses were no longer inhibited by neurotensin. In contrast, injection of SR48692 (30 fmol) into the RVM failed to block the facilitation of spinal unit responses to noxious heat produced by a subsequent injection of neurotensin (0.03 pmol) into the same site. The present series of experiments demonstrate a specific role for neurotensin in the RVM in the modulation of spinal nociceptive transmission, because the peptide was found to both facilitate and inhibit spinal neuron responses to noxious thermal stimulation. Additionally, the facilitatory and inhibitory effects of neurotensin appear to occur via interaction with multiple neurotensin receptors in the RVM that activate independent systems that descend in the ventrolateral funiculi and DLFs, respectively. The results from these experiments are consistent with prior studies demonstrating that the RVM both facilitates and inhibits spinal nociceptive transmission, and they complement previous work showing that neurotensin in the RVM modulates spinal nociceptive behavioral responses.

Biphasic modulation of spinal nociceptive transmission from the medullary raphe nuclei in the rat

The modulatory effects of electrical and chemical (glutamate) stimulation in the rostral ventromedial medulla (RVM) on spinal nociceptive transmission and a spinal nociceptive reflex were studied in rats. Electrical stimulation at a total 86 sites in the RVM in the medial raphe nuclei (n = 54) and adjacent gigantocellular areas (n = 32) produced biphasic (facilitatory and inhibitory, n = 43) or only inhibitory (n = 43) modulation of the tail-flick (TF) reflex. At these 43 biphasic sites in the RVM, facilitation of the TF reflex was produced at low intensities of stimulation (5-25 microA) and inhibition was produced at greater intensities of stimulation (50-200 microA). At 43 sites in the RVM, electrical stimulation only produced intensity-dependent inhibition of the TF reflex. Activation of cell bodies in the RVM by glutamate microinjection reproduced the biphasic modulatory effects of electrical stimulation. At biphasic sites previously characterized by electrical stimulation, glutamate at a low concentration (5 nmol) produced facilitation of the TF reflex; a greater concentration (50 nmol) only inhibited the TF reflex. In electrophysiological experiments, electrical stimulation at 62 sites in the RVM produced biphasic (n = 26), only inhibitory (n = 26), or only facilitatory (n = 10) modulation of responses of lumbar spinal dorsal horn neurons to noxious cutaneous thermal (50 degrees C) or mechanical (75.9 g) stimulation. Facilitatory effects were produced at lesser intensities of stimulation and inhibitory effects were produced at greater intensities of stimulation. The apparent latencies to stimulation-produced facilitation and inhibition, determined with the use of a cumulative sum method and bin-by-bin analysis of spinal neuron responses to noxious thermal stimulation of the skin, were 231 and 90 ms, respectively. The spinal pathways conveying descending facilitatory and inhibitory influences were found to be different. Descending facilitatory influences on the TF reflex were conveyed in ventral/ventrolateral funiculi, whereas inhibitory influences were conveyed in dorsolateral funiculi. The results indicate that descending inhibitory and facilitatory influences can be simultaneously engaged throughout the RVM, including nucleus raphe magnus, and that such influences are conveyed in different spinal funiculi.

Comparison of the effects of nucleus tractus solitarius and ventral medial medulla lesions on illness-induced and subcutaneous formalin-induced hyperalgesias

We have previously demonstrated that illness-inducing agents (lipopolysaccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formalin) induce hyperalgesia by similar pathways. The present series of experiments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocellularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). Illness-induced hyperalgesia is abolished by unilateral (left) NTS lesions, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias induced by illness agents and by inflammatory agents are mediated by similar but not identical pathways. They also illustrate that neural structures have the capacity for opposed actions, in that both the NTS and NRM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structures involved in pain modulation.

Participation of central descending nociceptive facilitatory systems in secondary hyperalgesia produced by mustard oil

The present series of experiments were designed to examine a potential role for central descending pain facilitatory systems in mediating secondary hyperalgesia produced by topical application of mustard oil and measuring the nociceptive tail-flick reflex in awake rats. Topical application of mustard oil (100%) to the lateral surface of the hind leg produced a facilitation of the tail-flick reflex that was significantly reduced in spinal transected animals. Mustard oil hyperalgesia was also inhibited in animals that had received electrolytic lesions in the rostral ventromedial medulla (RVM). Intrathecal (i.t.) administration of the non-selective cholecystokinin (CCK) receptor antagonist proglumide (10 micrograms) prior to mustard oil application completely blocked both the lesser and greater hyperalgesic responses observed in spinal transected and normal animals, respectively, and produced an inhibition of the tail-flick reflex in normal animals. Administration of the selective CCKB receptor antagonist L-365260 i.t. dose-dependently inhibited mustard oil hyperalgesia (ID50 = 364 ng) at doses approximately 5-fold less than the CCKA receptor antagonist devazepide (ID50 = 1760 ng). Similar to spinal proglumide, microinjection of the neurotensin antagonist SR48692 (3.5 micrograms) into the RVM blocked mustard oil hyperalgesia and inhibited the tail-flick reflex. These data suggest that secondary hyperalgesia produced by mustard oil is mediated largely by a central, centrifugal descending pain facilitatory system which involves neurotensin in the RVM and spinal CCK (via CCKB receptors). The inhibition of the tail-flick reflex produced by mustard oil following spinal or supraspinal administration of receptor antagonists suggests concurrent activation of central descending facilitatory and inhibitory systems.

Neuropharmacological profile of non-peptide neurotensin antagonists

Neurotensin, an endogenous peptide widely distributed throughout the brain, fulfils neurotransmitter criteria. When administered centrally, neurotensin induces various effects and modulates the activity of the mesolimbic dopamine system. It antagonizes the behavioural action of dopamine in a manner similar, but not identical, to antipsychotic drugs. Neurotensin is even considered to be an endogenous neuroleptic. In fact, microinjection of neurotensin elicits different effects depending on both the dose and the cerebral structures into which the injection is made. Our work on the development of orally-active neurotensin antagonists has led to the identification of SR 48692, the first non-peptide antagonist of the neurotensin receptor, and some analogues. This small molecule reveals a surprising neuropharmacological profile. It antagonizes turning behaviour induced in mice and rats (after striatal or ventral tegmental area administration of neurotensin, respectively), hypolocomotion induced by intracerebroventricular injection of neurotensin in rats, and reverses the inhibitory effect of neurotensin (nucleus accumbens injection) on amphetamine-induced hyperlocomotion in rats. However, SR 48692 cannot reverse either dopamine release in the nucleus accumbens evoked by neurotensin injection in ventral tegmental area, or hypothermia and analgesia induced by intracerebroventricular injection of neurotensin. As direct and indirect dopamine agonists have been reported to promote neurotensin release in the cortex, behavioural studies were performed using injection of apomorphine. In these experiments, SR 48692 inhibited only turning and yawning. It did not antagonize other apomorphine-dependent effects such as climbing, hypothermia, hypo- or hyperlocomotion, penile erection and stereotypies. All together, these data raise the question of the existence of neurotensin receptor subtypes and confirm that the nature of neurotensin and dopamine interactions depends on the brain structures considered.