Spinal Outflow Research Articles

Investigating the Role of the Locus Coeruleus in Periaqueductal Gray-mediated Antinociception

Activity in the ventrolateral periaqueductal gray (vlPAG) has been shown to be antinociceptive and is considered to be a major component in the expression of opioid drug-induced analgesia. Canonically, opioids disinhibit vlPAG activity, which modulates neuronal responses in the rostroventral medulla (RVM) and ultimately gates spinal outflow of nociceptive information via serotonergic and opioidergic mechanisms in the spinal dorsal horn. Fundamental pharmacological studies, however, indicate that spinal noradrenergic signaling is consistently implicated in opioid antinociception, suggesting that there is a gap in our knowledge of how the vlPAG may control the release of antinociceptive neuromodulators in the dorsal horn through different output regions. Although the locus coeruleus (LC) has been shown to modulate nociception, its role in and recruitment by the canonical descending circuitry is unclear. Using genetically-encoded and anatomically-targeted viral tools, immunohistochemistry, and acute pain behavior assays in conjunction with intrathecal pharmacology in mice, we show that activity in the vlPAG is necessary for the expression of systemic morphine analgesia at low doses, and that both systemic morphine-induced and PAG-driven antinociception require opioidergic and noradrenergic signaling in the spinal cord. Additionally, we find that vlPAG projection neurons send branching axons to both the RVM and LC, which may account for the LC activation we observed in both opioid-induced and PAG-driven antinociception. These findings reveal a previously underappreciated architecture of the descending pain modulatory circuitry and advance our understanding of the circuit mechanisms by which morphine produces analgesia. Grant support from NIDA R00 DA034648, Rita Allen Foundation (non-profit), Klingenstein-Simons Foundation (non-profit).

Effect of electrical vs. chemical deep brain stimulation at midbrain sites on micturition in anaesthetized rats.

To understand how deep brain stimulation of the midbrain influences control of the urinary bladder. In urethane-anaesthetized male rats, saline was infused continuously into the bladder to evoke cycles of filling and voiding. The effect of electrical (0.1-2.0 ms pulses, 5-180 Hz, 0.5-2.5 V) compared to chemical stimulation (microinjection of D,L-homocysteic acid, 50 nL 0.1 M solution) at the same midbrain sites was tested. Electrical stimulation of the periaqueductal grey matter and surrounding midbrain disrupted normal coordinated voiding activity in detrusor and sphincters muscles and suppressed urine output. The effect occurred within seconds was reversible and not secondary to cardiorespiratory changes. Bladder compliance remained unchanged. Chemical stimulation over the same area using microinjection of D,L-homocysteic acid (DLH) to preferentially activate somatodendritic receptors decreased the frequency of micturition but did not disrupt the coordinated pattern of voiding. In contrast, chemical stimulation within the caudal ventrolateral periaqueductal grey, in the area where critical synapses in the micturition reflex pathway are located, increased the frequency of micturition. Electrical deep brain stimulation within the midbrain can inhibit reflex micturition. We suggest that the applied stimulus entrained activity in the neural circuitry locally, thereby imposing an unphysiological pattern of activity. In a way similar to the use of electrical signals to 'jam' radio transmission, this may prevent a synchronized pattern of efferent activity being transmitted to the spinal outflows to orchestrate a coordinated voiding response. Further experiments to record neuronal firing in the midbrain during the deep brain stimulation will be necessary to test this hypothesis.

Sacro-lumbar Intersegmental Spinal Reflex in Autonomic Pathways Mediating Female Sexual Function

Autonomic neurons in paracervical ganglia mediating vasodilation in the female reproductive tract receive inputs from both midlumbar and sacral spinal levels. However, it is not known how the lumbar pathways are activated. This study tested whether stimulation of pudendal sensory nerve could activate lumbar spinal outflows to paracervical ganglia via a spinal reflex pathway. Isolated spinal cords with attached peripheral nerves were removed from urethane-anesthetized female guinea pigs and perfused via the aorta with physiological salt solution. Spinal pathways to midlumbar preganglionic neurons were tested by recording extracellular compound action potentials (CAPs) in lumbar splanchnic or distal hypogastric nerves after electrical stimulation of thoracic spinal cord or the pudendal nerve. CAPs also were recorded from pelvic nerves after pudendal nerve stimulation. Sensory neurons were retrogradely traced from the pudendal nerve and characterized immunohistochemically. Activation of preganglionic neurons projecting from midlumbar spinal cord to paracervical ganglia following stimulation of pudendal sensory nerves in isolated preparations. Thoracic spinal cord stimulation produced CAPs in hypogastric nerves that were abolished by transection of L3 lumbar splanchnic nerves. Pudendal nerve stimulation produced CAPs in L3 lumbar splanchnic, hypogastric, and pelvic nerves, demonstrating an ascending intersegmental spinal circuit to midlumbar levels in addition to the sacral spinal circuit. These CAPs in hypogastric nerves were enhanced by bicuculline (10 µM), blocked by tetrodotoxin (1 µM) but were not affected by hexamethonium (200 µM). Retrograde axonal tracing revealed four groups of sensory neurons in S3 dorsal root ganglia that were distinguished immunohistochemically. Midlumbar preganglionic neurons projecting to paracervical ganglia regulating blood flow and motility in the female reproductive tract can be activated by an ascending intersegmental spinal pathway from pudendal sacral inputs, which is inhibited by local spinal circuits. This pathway will help understand pathological conditions affecting reproductive function.

Mexiletine and Lidocaine Suppress the Excitability of Dorsal Horn Neurons

Spinal sensitization and facilitatory processes in dorsal horn neurons after nerve injury alter spinal outflow leading to enhanced pain perception and chronic pain syndromes. Clinically used Na+ channel blockers at doses which do not block conduction can relieve such chronic pain. Although much attention has been paid to their effect upon afferents, less work has been done with their effect on the excitability of central sensory neurons. Thus, we investigated the effects of the Na+ channel blockers mexiletine and lidocaine on sensory spinal dorsal horn neurons. Patch-clamp recordings were directly performed in visualized neurons of the substantia gelatinosa in the spinal cord of young rats to investigate the effect of mexiletine and lidocaine in different types of dorsal horn neurons (tonically firing, adapting-firing, and single spike neurons). All three different types of neurons responded dose-dependently to mexiletine and lidocaine. Both local anesthetics reversibly inhibited Na+ and K+ currents. The half-maximal inhibitory concentration for Na+ conductance block was 89 +/- 2 or 54 +/- 6 microM and for delayed-rectifier K+ conductance block was 582 +/- 36 or 398 +/- 14 microM for lidocaine and mexiletine, respectively. The inhibition of Na+ and K+ currents consecutively altered the properties of single action potentials and reduced the firing rate of tonically firing and adapting-firing neurons. In clinically relevant concentrations, lidocaine and mexiletine reduced the excitability of sensory dorsal horn neurons via a blockade of Na+ and K+ channels. Our work confirms that, in addition to the peripheral effects of lidocaine and mexiletine, modulation of voltage-gated ion channels in the central nervous system contributes to the antinociceptive effects of these drugs used in pain therapy.

Neonates have a spinal alpha receptor too, as do adults

British Journal of Pharmacology (2007) 151, 1139–1140; doi:10.1038/sj.bjp.0707291

The integrative role of the rat medullary subnucleus reticularis dorsalis in nociception.

Neurons within the medullary subnucleus reticularis dorsalis (SRD) of the rat convey selectively nociceptive information from all parts of the body. We have sought to define the neuronal networks that convey information from widespread noxious stimuli to the diffuse thalamocortical system and also modulate spinal outflow. The experiments, which were performed in rats, were designed to determine whether efferents from the SRD issue collaterals to the thalamus and spinal cord. Injections of the tracers fluorogold and tetramethylrhodamine-labelled dextran were centred stereotaxically in two areas that receive dense projections from the SRD: the cervical spinal cord and the lateral ventromedial thalamus (VMl), respectively. In other experimental series, SRD neurons were characterized electrophysiologically and individually labelled in a Golgi-like manner following juxtacellular iontophoresis of biotin-dextran. More than half reticulothalamic neurons within the SRD provided monosynaptic connections to the spinal cord. SRD neurons that responded to Adelta- or Adelta- and C-fibre activation from any area of the body had axons that gave both ascending and descending collaterals. Because the SRD innervates several areas involved in motor processing and receives strong, direct influences from several cortical regions, it could provide a structural basis for the processing of nociceptive and motor activities.

Influences of pelvic floor structures and sacral innervation on the response to distension of the cat rectum.

The contributions to the rectal response to distension of the pelvic floor structures surrounding the rectum and of the sacral spinal innervation have never been studied. Using paralysed intercollicularly decerebrate, anaesthesia-free cats, we studied pressure-volume relationships during slow ramp distensions of the rectum. Results obtained from animals with intact pelvic cavities were compared with those following mobilization of the rectum from the pelvic floor musculature. To assess the influences of spinal outflow and afferent input, rectal pressure-volume relationships were measured in the mobilized rectum following bilateral sequential transection of the spinal roots S1 to S3, first dorsal, then ventral. Isolation of the rectum from the pelvic floor structures resulted in a decrease in balloon volume in the lower range of distension pressure but did not affect volumes at higher pressures. The only afferent effect was seen after sectioning of dorsal roots S1, which resulted in a decrease in balloon volume. The only efferent effect was seen after sectioning of ventral roots S3, which decreased balloon volume further. In conclusion, the rectal response to distension depends on the properties of the rectal wall. It may be influenced by somatic inputs, inputs from the myenteric nervous plexus, and from the parasympathetic and sympathetic nervous systems. Afferent inputs and spinal autonomic reflexes may decrease the tone of the rectal musculature during distension.

The neuropeptide FF analogue, 1DMe, reduces in vivo dynorphin release from the rat spinal cord.

Intrathecal infusion of the neuropeptide FF analogue, [D-Tyr1, (NMe)Phe3]neuropeptide FF (1DMe; 0.1 microm-0.1 mm) in anaesthetized rats produced a concentration-dependent decrease in the spinal outflow of dynorphin A (1-8)-like material, which persisted for at least 90 min after treatment with 10 microm-0.1 mm of the compound. Co-administration of d-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; 1 microm) to block spinal micro-opioid receptors did not modify this effect, whereas naltrindole (10 microm) totally prevented it and nor-binaltorphimine (10 microm) reduced the post-effect. These data suggest that 1DMe triggers the release of endogenous opioids that stimulate mainly delta-opioid receptors, and secondarily kappa-opioid receptors, thereby exerting a negative influence on dynorphin A (1-8)-like material outflow. Because dynorphin has pronociceptive properties, such a decrease in spinal dynorphin A (1-8)-like material release might underlie the long-lasting antinociceptive effects of intrathecally administered neuropeptide FF and analogues.

The novel analgesic, cizolirtine, inhibits the spinal release of substance P and CGRP in rats

Although previous studies have established that cizolirtine (5-{[( N, N-dimethylaminoethoxy)phenyl]methyl}-1-methyl-1 H-pyrazol citrate) is a potent analgesic in rodents, its mechanism(s) of action remain(s) unclear. In vitro and in vivo approaches were used to assess whether cizolirtine could affect the spinal release of two pain-related neuropeptides, substance P (SP) and calcitonin gene-related peptide (CGRP), in rats. Cizolirtine significantly reduced the K +-evoked overflow of both the SP-like material (SPLM; −25% at 0.1 μM–0.1 mM) and CGRPLM (–20% at 0.1–1.0 μM) from slices of the dorsal half of the lumbar enlargement of the spinal cord. Intrathecal perfusion in halothane-anaesthetized rats showed that local application of cizolirtine markedly diminished the spinal outflow of SPLM (up to −50% at 0.1 mM) but only marginally that of CGRPLM. Systemic administration of cizolirtine at an analgesic dose (80 mg/kg i.p.) also reduced spinal SPLM outflow (−50%) but not that of CGRPLM. Under both in vitro and in vivo conditions, idazoxan (10 μM) antagonized the effects of cizolirtine on SPLM and CGRPLM release, suggesting their mediation through α 2 adrenoceptors.

Pelvic floor structures and sacral innervation influence rectal compliance

Structural anatomy and the surrounding pelvic floor musculature influence the compliance of the rectum. Additionally, afferent inputs and spinal reflexes from sacral spinal roots SI-S3 modulate rectal compliance. The individual contribution of these mechanisms has not been clearly established. Aims. The current study sought to evaluate the influence of pelvic floor muscles and sacral innervation, both afferent and efferent, on rectal compliance in the cat. Methods. Following surgical preparation, animals were decerebrated and ventilated. After a lumbo-sacral laminectomy, the sacral roots were exposed. All measures were obtained after stabilization of respiration and arterial pressure. Pressure-volume curves were determined during slow ramp distention (0 to 50 mmHg) in animals with intact pelvic floor (n= 10) and following isolation of the rectum from the pelvic floor musculature (n= 10). To assess the influence of spinal outflow and afferent inputs, rectal compliance was measured in the isolated rectum following transection of the dorsal and ventral sacral roots sequentially from SI to S3 and from dorsal to ventral (n=4). Results. Isolation of the rectum from the pelvic floor musculature resulted in a significant interaction effect (F(9,189)=6.12, p<O.OOI) due to a rightward shift of the pressure-volume curve in the lower range of distention (0-25 mmHg) without affecting compliance at higher pressure. Bilateral section of dorsal root SI further shifted the pressure-volume curve to the right (F(l,3)= 13.69, p=O.03). Subsequent sections to ventral root S3 failed to significantly influence rectal compliance. Section of ventral root S3 resulted in an additional rightward shift (F(l,3)=41.35, p=0.008) of the compliance curve. Conclusion. The rightward shift of the compliance curve in the lower distention range after isolation of the rectum from its pelvic floor anchors may have resulted from an alteration in adaptive relaxation of the longitudinal muscle. Section of sacral spinal afferents in the dorsal root S I and efferent inputs from ventral root S3 further shifted the compliance curve to the right. These observations suggest that afferent inputs and spinal reflexes may exert inhibitory influences on the rectum during distention, thereby initiating adaptive responses to rectal filling (Supported by DFG).

Altered opioid-mediated control of the spinal release of dynorphin and met-enkephalin in polyarthritic rats

Previous studies showed that spinal opioidergic neurotransmission is markedly altered in the polyarthritic rat, a model of chronic inflammatory pain. Present investigations aimed at assessing possible changes in opioid-mediated control of the spinal outflow of met-enkephalin (ME) and dynorphin (DYN) in these animals. Intrathecal (i.t.) perfusion under halothane anesthesia showed that polyarthritis was associated with both a 40% decrease in the spinal outflow of ME-like material (MELM) and a 90% increase in that of DYNLM. Local treatment with the mu-opioid agonist DAGO (10 microM i.t.) inhibited equally (-30%) the MELM outflow in polyarthritic and control rats, whereas the delta agonist DTLET (10 microM i.t.) also reduced the peptide outflow in controls (-27%) but enhanced it in polyarthritic animals (+56%). On the other hand, both DAGO (10 microM i.t.) and DTLET (10 microM i.t.) decreased (-40 and -49%) DYNLM outflow in polyarthritic rats, but were inactive in controls. Finally, neither MELM outflow nor that of DYNLM were affected by the kappa-agonist U50488H (10 microM i.t.) in both groups of rats. In all cases, the changes due to active agonists could be prevented by specific antagonists which were inactive on their own except the kappa antagonist nor-binaltorphimine (10 microM i.t.) that decreased (-38%) DYNLM outflow in polyarthritic rats. These data indicate that functional changes in spinal opioid receptors may promote enkephalinergic neurotransmission and reduce dynorphinergic neurotransmission in polyarthritic rats, thereby contributing to the analgesic efficacy of opioids in inflammatory pain.

The neuropeptide FF analogue, 1DME, enhances in vivo met-enkephalin release from the rat spinal cord

Behavioural studies have suggested that endogenous opioids mediate the antinociceptive action of neuropeptide FF (FLFQPQRF–NH 2) at the spinal level in the rat. This hypothesis was directly assessed by investigating the effects of a NPFF analogue, 1DMe ([D-Tyr 1,(NMe)Phe 3]NPFF), on the spinal outflow of met-enkephalin-like material (MELM) in halothane-anaesthetised rats. Intrathecal infusion (0.1 ml/min) of 1DMe (0.1 μM–0.1 mM, for 45 min) produced a concentration-dependent increase in spinal MELM outflow which persisted for at least 90 min at the highest concentration tested. Intrathecal coadministration of the μ-opioid receptor antagonist CTOP (1 μM) did not significantly affect the spinal MELM overflow due to 0.1 mM 1DMe. In contrast, both naltrindole and nor-binaltorphimine, at concentrations (10 μM) that allow the selective blockade of ∂- and κ-opioid receptors, respectively, significantly reduced the stimulatory effect of 1DMe on spinal MELM outflow. These data provide the first direct demonstration that met-enkephalin (among other opioid peptides) can mediate the antinociceptive action of NPFF at the spinal level in rats. In addition, they suggest that reciprocal excitatory interactions between opioids and opioid-modulatory factors (such as NPFF) participate in the physiological control of nociception.

Polyarthritis-associated changes in the opioid control of spinal CGRP release in the rat

As a model of chronic inflammatory pain, Freund's adjuvant-induced polyarthritis has been shown to be associated with marked alterations in the activity of opioid- and calcitonin gene-related peptide (CGRP)-containing neurons in the dorsal horn of the spinal cord in rats. Possible changes in the interactions between these two peptidergic systems in chronic inflammatory pain were investigated by comparing the effects of various opioid receptor ligands on the spinal outflow of CGRP-like material (CGRPLM) in polyarthritic and age-paired control rats. Intrathecal perfusion of an artificial cerebrospinal fluid in halothane-anaesthetized animals allowed the collection of CGRPLM released from the spinal cord and the application of opioid receptor ligands. The blockade of κ-opioid receptors similarly increased CGRPLM release in both groups of rats as expected of a κ-mediated tonic inhibitory control of CGRP-containing fibres in control, as well as in polyarthritic rats. In contrast, the higher increase in CGRPLM outflow due to the preferential blockade of μ opioid receptors by naloxone in polyarthritic rats as compared to non-suffering animals supports the idea of a reinforced μ opioid receptor-mediated tonic inhibitory control of CGRP-containing fibres in rats suffering from chronic pain. Even more strikingly, the differences observed in the effects of ∂-opioid receptor ligands on CGRPLM outflow suggest that ∂ receptors are functionally shifted from a participation in a phasic excitatory control in non-suffering rats to a tonic inhibitory control in polyarthritic rats. These data indicate that agonists acting at the three types of opioid receptors all exert a tonic inhibitory influence on CGRP-containing nociceptive primary afferent fibres within the spinal cord of polyarthritic rats. Such a convergence probably explains why morphine and other opioids are especially potent to reduce pain in subjects suffering from chronic inflammatory diseases.

Preferential reduction in vascular responses to endothelin-1 in rats with streptozotocin-induced diabetes.

The effects of vasodepressor (acetylcholine and bradykinin) and pressor [electrical stimulation of the spinal sympathetic outflow, norepinephrine and endothelin-1 (ET-1)] stimuli were determined in rats with 2- and 5-week untreated streptozotocin-induced diabetes (blood glucose 400 and >500 mg/dl, respectively). In pentobarbital-anesthetized animals, the hypotensive response to an intravenous dose of acetylcholine or bradykinin was unaffected in animals treated for 2 weeks with streptozotocin but was significantly reduced (22% and 48%, respectively) after 5 weeks. However, the pressor responses to ET-1 were significantly decreased in animals that had been given streptozotocin 2 (38%) and 5 (45%) weeks previously. In contrast, the vasoconstrictor effects of electrical stimulation of the spinal cord outflow and norepinephrine were significantly inhibited (47% and 30%, respectively) at 5 weeks, but not at 2 weeks, after streptozotocin administration. These results indicate that, in untreated streptozotocin diabetes, a substantial impairment of vascular reactivity to ET-1 appears more rapidly than inhibition of the pressor responses to endogenous and exogenous norepinephrine or to vasodilator substances that require integrity of vascular endothelial cell function to produce their normal effects.

The medullary subnucleus reticularis dorsalis (SRD) as a key link in both the transmission and modulation of pain signals

The involvement of the dorsal part of the caudal medulla in both the transmission and modulation of pain is supported by recent electrophysiological and anatomical data. In this review, we analyse the features of a well-delimited area within the caudal-most aspect of the medulla, the subnucleus reticularis dorsalis (SRD) which plays a specific role in processing cutaneous and visceral nociceptive inputs. From a general viewpoint, the reciprocal connections between the caudal medulla and spinal cord suggest that this area is an important link in feedback loops which regulate spinal outflow. Moreover, the existence of SRD-thalamic connections put a new light on the role of spino-reticulo-thalamic circuits in pain transmission.

Distribution of spinal cord projections from the medullary subnucleus reticularis dorsalis and the adjacent cuneate nucleus: A phaseolus vulgaris‐ leucoagglutinin study in the rat

The distribution and organization of descending spinal projections from the dorsal part of the caudal medulla were studied in the rat following injections of Phaseolus vulgaris-leucoagglutinin into small areas of the subnucleus reticularis dorsalis (SRD) and the adjacent cuneate nucleus (Cu). The caudal aspect of the Cu projected only to the dorsal horn of the ipsilateral cervical cord via the dorsal funiculus. These projections were mainly to laminae I, IV, and V. More ventrally located reticular structures projected to the full length of the cord. Fibers originating from the SRD travelled through the ipsilateral dorsolateral funiculus and terminated within the deep dorsal horn and upper layers of the ventral horn, mainly in laminae V-VII. Fibers originating from subnucleus reticularis ventralis (SRV) travelled ipsilaterally through the lateral and ventrolateral funiculi and bilaterally through the ventromedial funiculus. These fibers terminated within the ventral horn. The density of labeling within the gray matter varied at different levels of the cord was as follows: cervical > sacral > thoracic > lumbar. The reciprocal connections between the caudal medulla and the spinal cord suggest that the former is an important link in feedback loops that regulate spinal outflow.

Differential effects of the novel analgesic, S 12813-4, on the spinal release of substance P- and calcitonin gene-related peptide-like materials in the rat.

The possible inhibitory control by the novel analgesic S 12813-4 (3-(2-(4-phenylpiperazine-1-yl)-ethyl)-2-oxo-2,3- dihydrooxazolo(b)pyridine) of spinal neurones containing substance P (SP) and/or calcitonin gene-related peptide (CGRP) was assessed in vitro and in vivo in the rat. S 12813-4 (10 nM-0.1 mM) did not affect the spinal release of CGRP-like material (CGRPLM) but inhibited in a concentration dependent manner the K(+)-evoked overflow of SP-like material (SPLM) from slices of the dorsal half of the rat lumbar enlargement. The inhibitory effect of 10 microM S 12813-4 on SPLM release was not additive with that of Na (0.1 mM), and could be prevented by the alpha 2-adrenoceptor antagonist idazoxan (10 microM). Similarly, idazoxan (10 microM) suppressed the inhibition by intrathecally administered S 12813-4 (10 microM) of the spinal outflow of SPLM in halothane anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. These data suggest that the analgesic effect of S 12813-4 might involve some alpha 2-adrenoreceptor-mediated control of SPLM release within the spinal cord. Whether this control concerns SP-containing primary afferent fibres (presynaptic inhibition) or SP-containing interneurones and/or bulbo-spinal SP-ergic pathways (postsynaptic inhibition) deserves further investigations.

Kappa 1/mu interactions in the control of the in vivo release of neuropeptides from the rat spinal cord

The possible modulation by κ 1 opioid receptor stimulation of the effects of μ receptor stimulation on the spinal release of met-enkephalin- (ME), substance P- (SP) μ calcitonin gene-related peptide- (CGRP) like materials (LM) was investigated in vivo in the rat. The stimulation of κ 1 receptors by 10 μM U 50488 H exerted no effect per se on the release processes. The stimulation of μ opioid receptors by 10 UM DAGO significantly reduced (-40%) the spinal outflow of MELM, enhanced (+44%) that of SPLM and did not affect CGRPLM outflow. In the presence of both DAGO and U 50488 H, the spinal release of MELM

Increased in vivo release of calcitonin gene-related peptide-like material from the spinal cord in arthritic rats

Possible alterations in spinal systems containing calcitonin gene-related peptide (CGRP) due to polyarthritis were assessed in rats 3–4 weeks after an intradermal injection of Freund's adjuvant in the low back. The tissue levels of CGRP-like material (CGRPLM) were ~ 50% higher in the dorsal zone of the spinal cord and dorsal root ganglia at both the cervical and lumbar (but not thoracic) segments in polyarthritic rats than in age-paired control animals. In addition the rate of the spinal release of CGRPLM determined through an intrathecal perfusion procedure in halothane-anaesthetized animals was ~ 15-fold higher in polyarthritic rats than in controls. The blockade of μ-opioid receptors by intrathecal perfusion with 10 μ M naloxone produced a larger increase in the spontaneous CGRPLM outflow in polyarthritic rats than in age-paired controls. Furthermore, the stimulation of μ-opioid receptors by intrathecal perfusion with 10 μ M DAGO significantly inhibited the spinal outflow of CGRPLM only in polyarthritic rats. These data indicate that CGRP-containing primary afferent fibres are markedly activated in chronic suffering polyarthritic rats. This activation occurs in spite of an increased tonic inhibitory control by endogenous opioids acting at μ receptors.

Opioid action on spinal cord reflexes due to dorsolateral pontine tegmentum stimulation

Electrical stimulation of the dorsolateral pontine tegmentum (DLPT) produces phasic facilitatory and inhibitory actions on the lumbar spinal monosynaptic reflexes (MSRs) of both flexor and extensor muscle nerves in the decerebrate cat. Naloxone, an opioid receptor antagonist, given intravenously or intraspinally enhanced the DLPT-induced potentiation of MSRs in most of the reflexes studied. However, systemic naloxone had no significant effect on the unconditioned MSR of the spinal cord. Intraspinal microinjections of naloxone significantly attenuated the DLPT-induced inhibition of MSRs of both flexors and extensors, similar to the action of systemic injection of naloxone, indicating a direct opioid action at the spinal ventral horn level upon DLPT stimulation. Results of the present experiment further support the anatomical finding that there are pontospinal enkephalinergic pathways in the cat, and indicate that these descending pathways modulate spinal motor outflow.