Descending projections from the medullary dorsal reticular nucleus make synaptic contacts with spinal cord lamina I cells projecting to that nucleus: an electron microscopic tracer study in the rat

Opioid and Cannabinoid Systems in Pain: Emerging Molecular Mechanisms and Use in Clinical Practice, Health, and Fitness

Pain is an unpleasant sensory and emotional experience. Adequate pain control is often challenging, particularly in patients with chronic pain. Despite advances in pain management, drug addiction, overtreatment, or substance use disorders are not rare. Hence the need for further studies in the field. The substantial progress made over the last decade has revealed genes, signalling pathways, molecules, and neuronal networks in pain control thus opening new clinical perspectives in pain management. In this respect, data on the epigenetic modulation of opioid and cannabinoid receptors, key actors in the modulation of pain, offered new perspectives to preserve the activity of opioid and endocannabinoid systems to increase the analgesic efficacy of opioid- and cannabinoid-based drugs. Similarly, upcoming data on cannabidiol (CBD), a non-psychoactive cannabinoid in the marijuana plant Cannabis sativa, suggests analgesic, anti-inflammatory, antioxidant, anticonvulsivant and ansiolitic effects and supports its potential application in clinical contexts such as cancer, neurodegeneration, and autoimmune diseases but also in health and fitness with potential use in athletes. Hence, in this review article, we summarize the emerging epigenetic modifications of opioid and cannabinoid receptors and focus on CBD as an emerging non-psychoactive cannabinoid in pain management in clinical practice, health, and fitness.

A study of cortical and brainstem mechanisms of diffuse noxious inhibitory controls in anaesthetised normal and neuropathic rats

Diffuse noxious inhibitory controls (DNIC) are a mechanism of endogenous descending pain modulation and are deficient in a large proportion of chronic pain patients. However, the pathways involved remain only partially determined with several cortical and brainstem structures implicated. This study examined the role of the dorsal reticular nucleus (DRt) and infralimbic (ILC) region of the medial prefrontal cortex in DNIC. In vivo electrophysiology was performed to record from dorsal horn lamina V/VI wide dynamic range neurones with left hind paw receptive fields in anaesthetised sham‐operated and L5/L6 spinal nerve‐ligated (SNL) rats. Evoked neuronal responses were quantified in the presence and absence of a conditioning stimulus (left ear clamp). In sham rats, DNIC were reproducibly recruited by a heterotopically applied conditioning stimulus, an effect that was absent in neuropathic rats. Intra‐DRt naloxone had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. In addition, intra‐DRt naloxone blocked DNIC in sham rats, but had no effect in SNL rats. Intra‐ILC lidocaine had no effect on spinal neuronal responses to dynamic brush, punctate mechanical, evaporative cooling and heat stimuli in sham and SNL rats. However, differential effects were observed in relation to the expression of DNIC; intra‐ILC lidocaine blocked activation of DNIC in sham rats but restored DNIC in SNL rats. These data suggest that the ILC is not directly involved in mediating DNIC but can modulate its activation and that DRt involvement in DNIC requires opioidergic signalling.

A Brainstem-Spinal Circuit Controlling Nocifensive Behavior

Response to danger needs to be rapid and appropriate. In humans, nocifensive behaviors often precede conscious pain perception. Much is known about local spinal cord circuits for simple reflexive responses, but the mechanisms underlying more complex behaviors remain poorly understood. We now describe a brainstem circuit that controls escape responses to select noxious stimuli. Tracing experiments characterized a highly interconnected excitatory circuit involving the dorsal spinal cord, parabrachial nucleus (PBNl), and reticular formation (MdD). A combination of chemogenetic, optogenetic, and genetic ablation approaches revealed that PBNl^Tac1 neurons are activated by noxious stimuli and trigger robust escape responses to heat through connections to the MdD. Remarkably, MdD^Tac1 neurons receive excitatory input from the PBN and target both the spinal cord and PBN; activation of these neurons phenocopies the behavioral effects of PBNl^Tac1 neuron stimulation. These findings identify a substrate for controlling appropriate behavioral responses to painful stimuli.

Electroacupuncture Inhibits Visceral Nociception via Somatovisceral Interaction at Subnucleus Reticularis Dorsalis Neurons in the Rat Medulla

Electroacupuncture (EA) is an efficacious treatment for alleviating visceral pain, but the underlining mechanisms are not fully understood. This study investigated the role of medullary subnucleus reticularis dorsalis (SRD) neurons in the effects of EA on visceral pain. We recorded the discharges of SRD neurons extracellularly by glass micropipettes on anesthetized rats. The responses characteristics of SRD neurons to different intensities of EA (0.5, 1, 2, 4, 6, and 8 mA, 0.5 ms, and 2 Hz) on acupoints “Zusanli” (ST 36) and “Shangjuxu” (ST 37) before and during noxious colorectal distension (CRD) were analyzed. Our results indicated that SRD neurons responded to either a noxious EA stimulation ranging from 2 to 8 mA or to noxious CRD at 30 and 60 mmHg by increasing their discharge frequency at an intensity-dependent manner. However, during the stimulation of both CRD and EA, the increasing discharges of SRD neurons induced by CRD were significantly inhibited by 2–8 mA of EA. Furthermore, SRD neurons can encode the strength of EA, where a positive correlation between current intensity and the magnitude of neuronal responses to EA was observed within 2–6 mA. Yet, the responses of SRD neurons to EA stimulation reached a plateau when EA exceeded 6 mA. In addition, 0.5–1 mA of EA had no effect on CRD-induced nociceptive responses of SRD neurons. In conclusion, EA produced an inhibiting effect on visceral nociception in an intensity-dependent manner, which probably is due to the somatovisceral interaction at SRD neurons.

Reticular Formation and Pain: The Past and the Future

The involvement of the reticular formation (RF) in the transmission and modulation of nociceptive information has been extensively studied. The brainstem RF contains several areas which are targeted by spinal cord afferents conveying nociceptive input. The arrival of nociceptive input to the RF may trigger alert reactions which generate a protective/defense reaction to pain. RF neurons located at the medulla oblongata and targeted by ascending nociceptive information are also involved in the control of vital functions that can be affected by pain, namely cardiovascular control. The RF contains centers that belong to the pain modulatory system, namely areas involved in bidirectional balance (decrease or enhancement) of pain responses. It is currently accepted that the imbalance of pain modulation towards pain facilitation accounts for chronic pain. The medullary RF has the peculiarity of harboring areas involved in bidirectional pain control namely by the existence of specific neuronal populations involved in antinociceptive or pronociceptive behavioral responses, namely at the rostroventromedial medulla (RVM) and the caudal ventrolateral medulla (VLM). Furthermore the dorsal reticular nucleus (also known as subnucleus reticularis dorsalis; DRt) may enhance nociceptive responses, through a reverberative circuit established with spinal lamina I neurons and inhibit wide-dynamic range (WDR) neurons of the deep dorsal horn. The components of the triad RVM-VLM-DRt are reciprocally connected and represent a key gateway for top-down pain modulation. The RVM-VLM-DRt triad also represents the neurobiological substrate for the emotional and cognitive modulation of pain, through pathways that involve the periaqueductal gray (PAG)-RVM connection. Collectively, we propose that the RVM-VLM-DRt triad represents a key component of the “dynamic pain connectome” with special features to provide integrated and rapid responses in situations which are life-threatening and involve pain. The new available techniques in neurobiological studies both in animal and human studies are producing new and fascinating data which allow to understand the complex role of the RF in pain modulation and its integration with several body functions and also how the RF accounts for chronic pain.

The medullary dorsal reticular nucleus as a relay for descending pronociception induced by the mGluR5 in the rat infralimbic cortex

Metabotropic glutamate receptor 5 (mGluR5) activation in the infralimbic cortex (IL) induces pronociceptive behavior in healthy and monoarthritic rats. Here we studied whether the medullary dorsal reticular nucleus (DRt) and the spinal TRPV1 are mediating the IL/mGluR5-induced spinal pronociception and whether the facilitation of pain behavior is correlated with changes in spinal dorsal horn neuron activity. For drug administrations, all animals had a cannula in the IL as well as a cannula in the DRt or an intrathecal catheter. Heat-evoked paw withdrawal was used to assess pain behavior in awake animals. Spontaneous and heat-evoked discharge rates of single DRt neurons or spinal dorsal horn wide-dynamic range (WDR) and nociceptive-specific (NS) neurons were evaluated in lightly anesthetized animals. Activation of the IL/mGluR5 facilitated nociceptive behavior in both healthy and monoarthritic animals, and this effect was blocked by lidocaine or GABA receptor agonists in the DRt. IL/mGluR5 activation increased spontaneous and heat-evoked DRt discharge rates in healthy but not monoarthritic rats. In the spinal dorsal horn, IL/mGluR5 activation increased spontaneous activity of WDR neurons in healthy animals only, whereas heat-evoked responses of WDR and NS neurons were increased in both experimental groups. Intrathecally administered TRPV1 antagonist prevented the IL/mGluR5-induced pronociception in both healthy and monoarthritic rats. The results suggest that the DRt is involved in relaying the IL/mGluR5-induced spinal pronociception in healthy control but not monoarthritic animals. Spinally, the IL/mGluR5-induced behavioral heat hyperalgesia is mediated by TRPV1 and associated with facilitated heat-evoked responses of WDR and NS neurons.

Cat's medullary reticulospinal and subnucleus reticularis dorsalis noxious neurons form a coupled neural circuit through collaterals of descending axons

Animals and human beings sense and react to real/potential dangerous stimuli. However, the supraspinal mechanisms relating noxious sensing and nocifensive behavior are mostly unknown. The collateralization and spatial organization of interrelated neurons are important determinants of coordinated network function. Here we electrophysiologically studied medial medullary reticulospinal neurons (mMRF-RSNs) antidromically identified from the cervical cord of anesthetized cats and found that 1) more than 40% (79/183) of the sampled mMRF-RSNs emitted bifurcating axons running within the dorsolateral (DLF) and ventromedial (VMF) ipsilateral fascicles; 2) more than 50% (78/151) of the tested mMRF-RSNs with axons running in the VMF collateralized to the subnucleus reticularis dorsalis (SRD) that also sent ipsilateral descending fibers bifurcating within the DLF and the VMF. This percentage of mMRF collateralization to the SRD increased to more than 81% (53/65) when considering the subpopulation of mMRF-RSNs responsive to noxiously heating the skin; 3) reciprocal monosynaptic excitatory relationships were electrophysiologically demonstrated between noxious sensitive mMRF-RSNs and SRD cells; and 4) injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin evidenced mMRF to SRD and SRD to mMRF projections contacting the soma and proximal dendrites. The data demonstrated a SRD-mMRF network interconnected mainly through collaterals of descending axons running within the VMF, with the subset of noxious sensitive cells forming a reverberating circuit probably amplifying mutual outputs simultaneously regulating motor activity and spinal noxious afferent input. The results provide evidence that noxious stimulation positively engages a reticular SRD-mMRF-SRD network involved in pain-sensory-to-motor transformation and modulation.

The pronociceptive dorsal reticular nucleus contains mostly tonic neurons and shows a high prevalence of spontaneous activity in block preparation

Despite the importance and significant clinical impact of understanding information processing in the nociceptive system, the functional properties of neurons in many parts of this system are still unknown. In this work we performed whole cell patch-clamp recording in rat brain stem blocks to characterize the electrophysiological properties of neurons in the dorsal reticular nucleus (DRt), a region known to be involved in pronociceptive modulation. We also compared properties of DRt neurons with those in the adjacent parvicellular reticular nucleus and in neighboring regions outside the reticular formation. We found that neurons in the DRt and parvicellular reticular nucleus had similar electrophysiological properties and exhibited mostly toniclike firing patterns, whereas neurons outside the reticular formation showed a larger diversity of firing patterns. Interestingly, more than one-half of the neurons also showed spontaneous activity. While the general view of the reticular formation, being a loosely associated mesh of groups of neurons with diverse function, and earlier reports suggests more electrophysiological heterogeneity, we showed that this is indeed not the case. Our results indicate that functional difference of neurons in the reticular formation may mostly be determined by their connectivity profiles and not by their intrinsic electrophysiological properties. The dominance of tonic neurons in the DRt supports previous conclusions that these neurons encode stimulus intensity through their firing frequency, while the high prevalence of spontaneous activity most likely shapes nociceptive modulation by this brain stem region.

Role of glutamate receptors in the dorsal reticular nucleus in formalin-induced secondary allodynia

The role of glutamate receptors present in the medullary dorsal reticular nucleus (DRt) in the formalin test and formalin-induced secondary nociception was studied in rats. Secondary mechanical allodynia was assessed with von Frey filaments applied to the rat's hindpaw, and secondary thermal hyperalgesia was evaluated with the tail-immersion test. The selective glutamate receptor antagonists MK801 (N-methyl-D-aspartate receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (AMPA/KA receptor antagonist) and A841720 (metabotropic glutamate 1 receptor antagonist) were injected into the DRt before or 6 days after formalin injection in the rat. In the formalin test, the three antagonists significantly reduced the number of flinches in both phases of the test. DRt microinjection of MK801 or A841720, but not of CNQX, reduced both secondary nociceptive behaviors. Moreover, pre-treatment with the three antagonists injected into the DRt prevented the development of secondary mechanical allodynia and secondary thermal hyperalgesia. Similarly, in these rats, the number of c-Fos-like immunoreactive neurons were markedly reduced in both the superficial and deep lamina of the dorsal horn. Our findings support the role of DRt as a pain facilitator in acute and chronic pain states, and suggest a key role of glutamate receptors during the development and maintenance of formalin-induced secondary allodynia.

Electrophysiological study of supraspinal input and spinal output of cat's subnucleus reticularis dorsalis (SRD) neurons

This work addressed the study of subnucleus reticularis dorsalis (SRD) neurons in relation to their supraspinal input and the spinal terminating sites of their descending axons. SRD extracellular unitary recordings from anesthetized cats aimed to specifically test, 1) the rostrocaudal segmental level reached by axons of spinally projecting (SPr) neurons collateralizing or not to or through the ipsilateral nucleus reticularis gigantocellularis (NRGc), 2) whether SPr fibers bifurcate to the thalamus, and 3) the effects exerted on SRD cells by electrically stimulating the locus coeruleus, the periaqueductal grey, the nucleus raphe magnus, and the mesencephalic locomotor region. From a total of 191 SPr fibers tested to cervical 2 (Ce2), thoracic 5 (Th5) and lumbar5 (Lu5) stimulation, 81 ended between Ce2 and Th5 with 39 of them branching to or through the NRGc; 21/49 terminating between Th5 and Lu5 collateralized to or through the same nucleus, as did 34/61 reaching Lu5. The mean antidromic conduction velocity of SPr fibers slowed in the more proximal segments and increased with terminating distance along the cord. None of the 110 axons tested sent collaterals to the thalamus; instead thalamic stimulation induced long-latency polysynaptic responses in most cells but also short-latency, presumed monosynaptic, in 7.9% of the tested neurons (18/227). Antidromic and orthodromic spikes were elicited from the locus coeruleus and nucleus raphe magnus, but exclusively orthodromic responses were observed following stimulation of the periaqueductal gray or mesencephalic locomotor region. The results suggest that information from pain-and-motor-related supraspinal structures converge on SRD cells that through SPr axons having conduction velocities tuned to their length may affect rostral and caudal spinal cord neurons at fixed delays, both directly and in parallel through different descending systems. The SRD will thus play a dual functional role by simultaneously regulating dorsal horn ascending noxious information and pain-related motor responses.

The perception and endogenous modulation of pain

Pain is often perceived an unpleasant experience that includes sensory and emotional/motivational responses. Accordingly, pain serves as a powerful teaching signal enabling an organism to avoid injury, and is critical to survival. However, maladaptive pain, such as neuropathic or idiopathic pain, serves no survival function. Genomic studies of individuals with congenital insensitivity to pain or paroxysmal pain syndromes considerable increased our understanding of the function of peripheral nociceptors, and especially of the roles of voltage-gated sodium channels and of nerve growth factor (NGF)/TrkA receptors in nociceptive transduction and transmission. Brain imaging studies revealed a "pain matrix," consisting of cortical and subcortical regions that respond to noxious inputs and can positively or negatively modulate pain through activation of descending pain modulatory systems. Projections from the periaqueductal grey (PAG) and the rostroventromedial medulla (RVM) to the trigeminal and spinal dorsal horns can inhibit or promote further nociceptive inputs. The "pain matrix" can explain such varied phenomena as stress-induced analgesia, placebo effect and the role of expectation on pain perception. Disruptions in these systems may account for the existence idiopathic pan states such as fibromyalgia. Increased understanding of pain modulatory systems will lead to development of more effective therapeutics for chronic pain.

Anatomical basis for the dynamic processing of nociceptive input

The involvement of the lamina I, or marginal zone, of the spinal cord dorsal horn in pain processing has been extensively demonstrated. Its neuronal population comprises four structurally distinct neuronal groups, which were shown to be present in species as different as the rat, cat and pigeon. In the rat, each neuronal type presents a particular neurochemical nature and supraspinal projection pattern. In addition, there is recent evidence that supraspinal modulatory actions may affect differentially cells of different types. By combining retrograde tracing with noxious-evoked induction of the c fos proto-oncogene, it was verified that activation of different neuronal groups varies as a function of the brain site they target and the nature of the stimulus. These data suggest that the responsiveness of lamina I neurones is under controlling mechanisms which reflect the stimulus characteristics. Accordingly, both the spinal GABAergic and opioidergic anti-nociceptive actions appear to differ when different kinds df input are being processed. Moreover, supraspinal pain-modulatory circuits are also likely to control the excitability of each cell group. Not only reciprocal connections between the spinal cord and various pain-control centres have been identified, but also a correlation between the number of cfos positive projecting cells and the pain-inhibitory or facilitating role of their target has been observed. It is proposed that the compound nature of the nociceptive system serves a dynamic interplay between the spinal cord and the brain, which, by controlling the activation of distinct projection neurones, modulates the nociceptive signal while integrating nociceptive processing with several brain functions.

Intracellular recordings of subnucleus reticularis dorsalis neurones revealed novel electrophysiological properties and windup mechanisms

Aδ- and/or C-fibre nociceptive inputs drive subnucleus reticularis dorsalis (SRD) neurones projecting to a variety of regions including the spinal cord and the nucleus reticularis gigantocellularis (NRGc), but their electrophysiological properties are largely unknown. Here we intracellularly recorded the SRD neuronal responses to injection of polarising current pulses as well as to electrical stimulation of the cervical spinal posterior quadrant (PQ) and the NRGc. Three different classes of neurones with distinct electrophysiological properties were found: type I were characterised by the absence of a fast postspike hyperpolarisation, type II by the presence of a postspike hyperpolarisation followed by a depolarisation resembling low threshold calcium spikes (LTSs), and type III (lacking LTSs) had a fast postspike hyperpolarisation deinactivating A-like potassium channels leading to enlarged interspike intervals. All three classes generated depolarising sags to hyperpolarising current pulses and showed 3-4.5 Hz subthreshold oscillatory activity leading to windup when intracellularly injecting low-frequency repetitive depolarising pulses as well as in response to 0.5-2 Hz NRGc and PQ electrical stimulation. About half of the 132 sampled neurones responded antidromically to NRGc stimulation with more than 65% of the NRGc-antidromic cells, pertaining to all three types, also responding antidromically to PQ stimulation. NRGc stimulation induced exclusively excitatory first-synaptic-responses whilst PQ stimulation induced first-response excitation in most cases, but inhibitory postsynaptic potentials in a few type II and type III neurones not projecting to the spinal cord that also displayed cumulative inhibitory effects (inverse windup). The results show that SRD cells (i) can actively regulate different temporal firing patterns due to their intrinsic electrophysiological properties, (ii) generate windup upon gradual membrane depolarisation produced by low-frequency intracellular current injection and by C-fibre tonic input, both processes leading subthreshold oscillations to threshold, and (iii) collateralise to the NRGc and the spinal cord, potentially providing simultaneous regulation of ascending noxious information and motor reactions to pain.

Central modulation of pain

It has long been appreciated that the experience of pain is highly variable between individuals. Pain results from activation of sensory receptors specialized to detect actual or impending tissue damage (i.e., nociceptors). However, a direct correlation between activation of nociceptors and the sensory experience of pain is not always apparent. Even in cases in which the severity of injury appears similar, individual pain experiences may vary dramatically. Emotional state, degree of anxiety, attention and distraction, past experiences, memories, and many other factors can either enhance or diminish the pain experience. Here, we review evidence for "top-down" modulatory circuits that profoundly change the sensory experience of pain.

Participation of mu-opioid, GABA(B), and NK1 receptors of major pain control medullary areas in pathways targeting the rat spinal cord: implications for descending modulation of nociceptive transmission

Several brain areas modulate pain transmission through direct projections to the spinal cord. The descending modulation is exerted by neurotransmitters acting both at spinally projecting neurons and at interneurons that target the projection neurons. We analyzed the expression of mu-opioid (MOR), gamma-aminobutyric acid GABA(B), and NK1 receptors in spinally projecting neurons of major medullary pain control areas of the rat: rostroventromedial medulla (RVM), dorsal reticular nucleus (DRt), nucleus of the solitary tract, ventral reticular nucleus, and lateralmost part of the caudal ventrolateral medulla. The retrograde tracer cholera toxin subunit B (CTb) was injected into the spinal dorsal horn, and medullary sections were processed by double immunocytochemistry for CTb and each receptor. The RVM contained the majority of double-labeled neurons followed by the DRt. In general, high percentages of MOR- and NK1-expressing neurons were retrogradely labeled, whereas GABA(B) receptors were mainly expressed in neurons that were not labeled from the cord. The results suggest that MOR and NK1 receptors play an important role in direct and indirect control of descending modulation. The co-localization of MOR and GABA(B) in DRt neurons also demonstrated by the present study suggests that the pronociceptive effects of this nucleus may be controlled by local opoidergic and GABAergic inhibition of the pronociception increased during chronic pain.

Processing noxious information at the subnucleus reticularis dorsalis (SRD) of anesthetized cats: wind-up mechanisms

With the exception of one monkey's study, where wind-up was not reported, electrophysiological data from SRD neurons were obtained in rodents where they show wind-up. This work was designed to examine the response properties of SRD neurons in anesthetized cats to study how general the data from rats may be. Since cat's SRD cells showed wind-up, its underlying mechanisms were approached, an issue not previously addressed at supraspinal level. Electrical stimulation, extracellular (combined with microiontophoresis) and intracellular techniques revealed that A delta information reaches the SRD via the ventrolateral cord, whereas C information preferentially follows a dorsal route. Wind-up was usually generated by spinal and peripheral stimulation, but it was also evoked either by stimulating the nucleus reticularis gigantocellularis (NRGc), even after spinal cord section and bilateral full thickness removal of the cerebral cortex, or by applying microiontophoretic pulses of l-glutamate at 0.3-1 Hz. Wind-up relied on afferent repetitive activity gradually depolarizing the SRD neurons leading 3-4.5 Hz subthreshold membrane rhythmic activity to threshold. Riluzole retarded wind-up generation and decreased the number of spikes per stimulus during wind-up. GABA or glycine abolished spontaneous and sensory-evoked activity and bicuculline, but not strychnine, increased spontaneous and stimulus-evoked activity. These results demonstrate that wind-up at the SRD is not merely the reflection of spinal wind-up, but (i) can be locally generated, (ii) is partially dependent upon persistent sodium currents, and (iii) is under the modulation of a tonic GABAa-dependent inhibition decreasing SRD excitability. Injury and/or inflammation producing tonic C-fiber activation will surpass tonic inhibition generating wind-up.

From neuroanatomy to gene therapy: searching for new ways to manipulate the supraspinal endogenous pain modulatory system

The endogenous pain modulatory system is a complex network of brain areas that control nociceptive transmission at the spinal cord by inhibitory and facilitatory actions. The balance between these actions ensures effective modulation of acute pain, while during chronic pain the pronociceptive effects appear to prevail. The mechanisms underlying this imbalance were studied as to the role of two medullary components of the pain modulatory system: the dorsal reticular nucleus and the caudal ventrolateral medulla, which function primarily as pronociceptive and antinociceptive centres, respectively. Both areas are connected with the spinal dorsal horn by closed reciprocal loops. In the spino-dorsal reticular nucleus loop, the ascending branch is strongly inhibited by spinal GABAergic neurons, which may act as a buffering system of the dorsal reticular nucleus-centred amplifying effect. In the spino-caudal ventrolateral medulla loop, the ascending branch is under potent excitation of substance P (SP) released from primary afferents, which is likely to trigger the intense descending inhibition detected in acute pain. During chronic pain, the activity in the lateral reticular formation of the caudal ventrolateral medulla changes, so that the action of the caudal ventrolateral medulla upon SP-responsive spinal neurons shifts from inhibitory to excitatory. The mechanisms of this modulatory shift are unknown but probably relate to the decreased expression of micro-opioid, delta-opioid and GABAB receptors. Normalizing receptor expression in the caudal ventrolateral medulla or controlling noci-evoked activity at the dorsal reticular nucleus or caudal ventrolateral medulla by interfering with neurotransmitter release is now possible by the use of gene therapy, an approach that stands out as a unique tool to manipulate the supraspinal endogenous pain control system.

Differential expression of NK1 and GABAB receptors in spinal neurones projecting to antinociceptive or pronociceptive medullary centres

The balance between excitatory and inhibitory input exerted upon spinal cord neurones that belong to spinofugal pathways determines the ultimate type of information transmitted to the brain. We compared the relative expression of NK1 and GABAB receptors in two spinomedullary pathways targeting an antinociceptive area and a pronociceptive centre, respectively, the lateral part of the caudal ventrolateral medulla (VLMlat) and the dorsal reticular nucleus (DRt). Spinal cord sections of rats injected in the VLMlat or DRt with the retrograde tracer cholera toxin subunit B were triple-immunoreacted for the tracer, NK1 receptors and GABAB receptors. The dorsal horn neurones labelled from the VLMlat mainly co-localized the two receptors while those labelled from the DRt mainly expressed GABAB receptors, which was particularly evident in neurones of laminae IV-V. The morphological classification of lamina I neurones projecting to the VLMlat showed that fusiform, flattened and pyramidal cells mainly co-localized NK1 and GABAB receptors. As to lamina I neurones projecting to the DRt, multipolar neurones mainly expressed GABAB receptors while the majority of flattened and pyramidal neurones co-localized NK1 and GABAB receptors. The present results suggest that the expression of NK1 and GABAB receptors varies in neurones participating to different spinofugal pathways. The importance of the present findings in the knowledge of the endogenous supraspinal pain control system is discussed.

Brain projections from the medullary dorsal reticular nucleus: an anterograde and retrograde tracing study in the rat

In the last 15 years a role has been ascribed for the medullary dorsal reticular nucleus as a supraspinal pain modulating area. The medullary dorsal reticular nucleus is reciprocally connected with the spinal dorsal horn, is populated mainly by nociceptive neurons and regulates spinal nociceptive processing. Here we analyze the distribution of brain projections from the medullary dorsal reticular nucleus using the iontophoretic administration of the anterograde tracer biotinylated-dextran amine and the retrograde tracer cholera toxin subunit B. Fibers and terminal boutons labeled from the medullary dorsal reticular nucleus were located predominately in the brainstem, although extending also to the forebrain. In the medulla oblongata, anterograde labeling was observed in the orofacial motor nuclei, inferior olive, caudal ventrolateral medulla, rostral ventromedial medulla, nucleus tractus solitarius and most of the reticular formation. Labeling at the pons-cerebellum level was present in the locus coeruleus, A5 and A7 noradrenergic cell groups, parabrachial and deep cerebellar nuclei, whereas in the mesencephalon it was located in the periaqueductal gray matter, deep mesencephalic, oculomotor and anterior pretectal nuclei, and substantia nigra. In the diencephalon, fibers and terminal boutons were found mainly in the parafascicular, ventromedial, and posterior thalamic nuclei and in the arcuate, lateral, posterior, peri- and paraventricular hypothalamic areas. Telencephalic labeling was consistent but less intense and concentrated in the septal nuclei, globus pallidus and amygdala. The well-known role of the medullary dorsal reticular nucleus in nociception and its pattern of brain projections in rats suggests that the nucleus is possibly implicated in the modulation of: (i) the ascending nociceptive transmission involved in the motivational-affective dimension of pain; (ii) the endogenous supraspinal pain control system centered in the periaqueductal gray matter-rostral ventromedial medulla-spinal cord circuitry; (iii) the motor reactions associated with pain.

Anterior cingulate cortex contributes to the descending facilitatory modulation of pain via dorsal reticular nucleus

Supraspinal centres biphasically modulate spinal nociceptive transmission, including descending inhibition and facilitation. Recent studies have revealed that descending facilitatory modulation is a key mechanism underlying induction and maintenance of neuropathic and inflammatory pain. The anterior cingulate cortex (ACC) is not only involved in the transmission of pain sensation but also plays a role in processing pain-related emotion. The ACC also widely connects with relevant regions of the descending modulation system. Here we used electrophysiological and behavioural techniques to study the possible pathways behind the modulation of spinal nociceptive transmission from the ACC. C-fibre-evoked field potentials in the spinal dorsal horn were produced by electrical stimulation of the sciatic nerve at an intensity high enough to excite C fibres, and paw withdrawal latencies (PWLs) to noxious heating were recorded. The results showed that high-frequency tetanic electrical stimulation of the ACC both unilaterally enhanced the C-fibre-evoked field potentials in the spinal dorsal horn and bilaterally shortened PWLs, indicating a facilitation of spinal nociception. A similar effect was observed after microinjection of N-methyl-d-aspartic acid (NMDA; 10 nm, 1 microL) or homocysteic acid (HCA; 0.1 m, 1 microL) into the ACC. When the dorsal reticular nucleus (DRt) was electrolytically lesioned, ACC-induced facilitation of spinal nociception was blocked. These results imply that: (i) activation of the ACC may facilitate spinal nociception; (ii) NMDA receptors in the ACC may be involved in descending facilitation; and (iii) the DRt plays a crucial role in mediating ACC-induced facilitation of spinal nociception.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

The medullary dorsal reticular nucleus enhances the responsiveness of spinal nociceptive neurons to peripheral stimulation in the rat

Single-unit spinal recordings combined with application of glutamate into the medullary dorsal reticular nucleus were used to assess the action of this nucleus upon deep dorsal horn neurons in rats. Injection of high glutamate concentrations (10 and 100 mm) induced a dramatic and long-lasting increase of the responses of wide-dynamic range neurons to electrical stimulation of the sciatic nerve in the noxious range, without affecting ongoing discharges. Post-stimulus time histograms revealed that this increase concerned the post-discharge, but not A- or C-fibre-mediated responses, which remained unchanged independently of the stimulation frequency applied. The onset of the glutamate-induced response enhancement occurred with a concentration-dependent time delay and developed slowly until its maximum. These data indicate that the medullary dorsal reticular nucleus exerts a facilitating action upon deep dorsal horn wide-dynamic range neurons by enhancing their capacity to respond to peripheral stimulation through prolongation of their discharge. This action is accompanied by the strengthening of wind-up of deep dorsal horn wide-dynamic range neurons, hence providing a plausible substrate for chronic pain states. These results are in agreement with previous behavioural studies suggesting a pronociceptive role for the dorsal reticular nucleus [Almeida et al. (1996) Brain Res. Bull., 39, 7-15; Almeida et al. (1999) Eur. J. Neurosci., 11, 110-122], and support the involvement of a reverberating circuit, previously described in morphological studies [Almeida et al. (1993) Neuroscience, 55, 1093-1106; Almeida et al. (2000) Eur. J. Pain, 4, 373-387], which probably operates only at a certain threshold of activation.

Convergence of cutaneous, muscular and visceral noxious inputs onto ventromedial thalamic neurons in the rat

We have recently described a population of neurons in the lateral part of the ventromedial thalamus (VMl), that respond exclusively to noxious cutaneous stimuli, regardless of which part of the body is stimulated. The purpose of the present study was to investigate the convergence of cutaneous, muscular and visceral noxious inputs onto single, VMl neurons in anesthetized rats. VMl neurons were characterized by their responses to Adelta- and C-fiber activation as well as noxious heat applied to the hindpaw. We investigated whether they responded also to colorectal distensions. In an additional series of experiments, we tested the effects of colorectal, intraperitoneal, intramuscular and subcutaneous applications of the chemical irritant mustard oil (MO). The present study shows that a population of neurons located within the thalamic VMl nucleus, carries nociceptive somatosensory signals from the entire body. All these neurons responded to noxious cutaneous and intramuscular stimuli but not to levels of distension that could be considered innocuous or noxious, of the intact and inflammed colon and rectum. Although colorectal distension did not elicit VMl responses, convergence of visceral as well as muscle and cutaneous nociceptors was demonstrated by the increases in ongoing (background) discharges following intracolonic MO. A distinct effect is seen after MO injection into the lumen of the colon: an increase in ongoing activity for 15min but still a lack of effect of colorectal distension. Moreover, following inflammation induced by subcutaneous injections of MO VMl neurons developed responses to both thermal and mechanical innocuous skin stimulation, reminiscent of allodynia phenomena. It is suggested that the VMl contributes to attentional aspects of nociceptive processing and/or to the integration of widespread noxious events in terms of the appropriate potential motor responses.

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.

Brain afferents to the medullary dorsal reticular nucleus: a retrograde and anterograde tracing study in the rat

The medullary dorsal reticular nucleus (DRt) was recently shown to belong to the supraspinal pain control system; neurons within this nucleus give origin to a descending projection that increases spinal nociceptive transmission and facilitates pain perception [Almeida et al. (1999), Eur. J. Neurosci., 11, 110-122]. In the present study, the areas of the brain that may modulate the activity of DRt neurons were investigated by using of tract-tracing techniques. Injection of a retrograde tracer into the DRt resulted in labelling in multiple areas of the brain. In the contralateral orbital, prelimbic, infralimbic, insular, motor and somatosensory cortices labelling was prominent, but a smaller ipsilateral projection from these same areas was also detected. Strong labelling was also noted in the central amygdaloid nucleus, bed nucleus of stria terminalis and substantia innominata. Labelled diencephalic areas were mainly confined to the hypothalamus, namely its lateral and posterior areas as well as the paraventricular nucleus. In the mesencephalon, the periaqueductal grey, red nucleus and deep mesencephalic nucleus were strongly labelled, whereas, in the brainstem, the parabrachial nuclei, rostroventromedial medulla, nucleus tractus solitarius, spinal trigeminal nucleus, and the parvocellular, dorsal, lateral and ventral reticular nuclei were the most densely labelled regions. All deep cerebellar nuclei were labelled bilaterally. These data suggest that the DRt integrates information from the somatosensory, antinociceptive, autonomic, limbic, pyramidal and extrapyramidal systems while triggering its descending facilitating action upon the spinal nociceptive transmission.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Projection sites of superficial and deep spinal dorsal horn cells in the nucleus tractus solitarii of the rat

By using anterograde transport of biotin dextran amine injected into the cervical spinal dorsal horn, we have shown that fibres from superficial and deep dorsal horn project to the nucleus tractus solitarii via two distinct pathways. Afferent fibres from the superficial lamina (I-III) were found to course in the dorsal funiculus and terminate bilaterally in the caudal zone of the nucleus tractus solitarii (NTS), mainly within the commissural subnucleus. In contrast, afferents from the deeper dorsal horn laminae (IV-V) were found to course in the dorsolateral fasciculus and terminate ipsilaterally, mostly in the lateral areas of the caudal nucleus tractus solitarii. Similar, but more extensive patterns of labelled fibres were produced by injections into the white matter of the dorsal funiculus and dorsolateral fasciculus, respectively. These observations suggest that the caudal NTS not only serves as a location of visceral afferent convergence and integration, but may also be a receptive area for monosynaptic projections from dorsal horn neurons receiving sensory afferent inputs. Such projections may represent pathways through which NTS neurons are influenced by nociceptive and non-nociceptive information from the dorsal horn and thereby can co-ordinate the appropriate autonomic response, including adjustments in cardiorespiratory reflex output.

Reciprocal connections between the medullary dorsal reticular nucleus and the spinal dorsal horn in the rat

The synaptic architecture of spinal afferents of the dorsal portion (DRtd) of the medullary dorsal reticular nucleus (DRt) is studied. After iontophoretic injections of cholera toxin subunit B (CTb) into the superficial (laminae I-II), deep (laminae IV-V) or entire (laminae I-V) dorso-ventral extent of the spinal dorsal horn at the cervico-thoracic or lumbo-sacral levels, axonal boutons of two distinct types were labelled in the DRtd. Type A boutons (82% after cervico-thoracic injections and 92% after lumbo-sacral injections) were roundish, small and presented few mitochondria and small, round synaptic vesicles. Type B boutons (18% after cervico-thoracic injections and 8% after lumbo-sacral injections) were elongated, two to three times larger, and exhibited numerous mitochondria and larger round vesicles. Both types of bouton established asymmetrical synaptic contacts with small dendritic profiles and, less frequently, with dendritic trunks and perikarya. Retrograde labelling occurred in the postsynaptic profile of 15-18% type A boutons labelled from any injection site. Taken together with previous data on DRt-spinal synaptic contacts at the superficial dorsal horn, the present results point to the occurrence of a reciprocal excitatory loop connecting the dorsal DRt and lamina I, which may be at the basis of the DRt-mediated pain-facilitating effects described recently.

Supraspinal metabolic activity changes in the rat during adjuvant monoarthritis

Pain is a multi-dimensional experience including sensory-discriminative and affective-motivational components. The attribution of such components to a corresponding cerebral neuronal substrate in the brain refers to conclusions drawn from electrical brain stimulation, lesion studies, topographic mappings and metabolic imaging. Increases in neuronal metabolic activity in supraspinal brain regions, suggested to be involved in the central processing of pain, have previously been shown in various animal studies. The present investigation is the first to describe supraspinal structures which show increased metabolic activity during ongoing monoarthritic pain at multiple time-points. Experimental chronic monoarthritis of a hindlimb induced by complete Freund's adjuvant is one of the most used models in studies of neuronal plasticity associated with chronic pain. Such animals show typical symptoms of hyperalgesia and allodynia for a prolonged period. Metabolic activity changes in supraspinal brain regions during monoarthritis were assessed using the quantitative [14C]-2deoxyglucose technique at two, four, 14 days of the disease and, furthermore, in a group of 14-day monoarthritic rats which were mechanically stimulated by repeated extensions of the inflamed joint. Local glucose utilization was determined ipsi- and contralateral to the arthritic hindpaw in more than 50 brain regions at various supraspinal levels, and compared with saline-injected controls. At two and 14 days of monoarthritis significant bilateral increases in glucose utilization were seen in many brain structures, including brainstem, thalamic, limbic and cortical regions. Within the brainstem, animals with 14-day monoarthritis showed a higher number of regions with increased metabolic activity compared with two days. No differences between ipsi- and contralateral sides were detected in any of the experimental groups. Average increases ranged from 20 to 40% compared with controls and maximum values were detected in specific brain regions, such as the anterior pretectal nucleus, the anterior cingulate cortex and the nucleus accumbens. Interestingly, at four days of monoarthritis, the glucose utilization values were in the control range in almost all regions studied. Moreover, in monoarthritic rats receiving an additional noxious mechanical stimulation, the rates of glucose utilization were also comparable to controls in all brain areas investigated. Such patterns of brain metabolic activity agreed with concomitant changes in the lumbar spinal cord, described in the accompanying report. The present data show that a large array of supraspinal structures displays elevated metabolic activity during painful monoarthritis, with a non-linear profile for the time-points investigated. This observation most probably reflects mechanisms of transmission and modulation of nociceptive input arising from the monoarthritis and accompanying its development.

Natural noxious stimulation can induce long-term increase of spinal nociceptive responses

It is conceivable that plasticity in pain control systems and chronic pain may be due to mechanisms similar to learning. Long-term potentiation (LTP) in the hippocampus is often studied as a model of learning and memory. It has recently been shown that long-term excitation may be induced in single wide dynamic range (WDR) neurones in the spinal dorsal horn of rats after tetanic stimulation to the sciatic nerve. The present study shows that similar long-term changes can also be induced by a severe natural stimulus. Single unit extracellular recordings were made in urethane anaesthetized rats and the firing responses of WDR neurones evoked by a single electrical stimulus to the peripheral nerve were recorded every 4 min. After repeated crushing of tissue (including bone) corresponding to the receptive field of the WDR neurones (the conditioning stimulus) followed by a proximal total peripheral nerve block, the C-fibre evoked responses were increased (P < 0.001) for a 3 h observation period compared with baseline responses and control animals. In control animals the nerve block was applied before the conditioning stimulus. We suggest that a long-term increase of the excitability of WDR neurones may be important for the development of long lasting and chronic pain disorders after an acute but severe noxious stimulus.

Supraspinal contributions to hyperalgesia

Tissue injury is associated with sensitization of nociceptors and subsequent changes in the excitability of central (spinal) neurons, termed central sensitization. Nociceptor sensitization and central sensitization are considered to underlie, respectively, development of primary hyperalgesia and secondary hyperalgesia. Because central sensitization is considered to reflect plasticity at spinal synapses, the spinal cord has been the principal focus of studies of mechanisms of hyperalgesia. Not surprisingly, glutamate, acting at a spinal N-methyl-D-aspartate (NMDA) receptor, has been implicated in development of secondary hyperalgesia associated with somatic, neural, and visceral structures. Downstream of NMDA receptor activation, spinal nitric oxide (NO.), protein kinase C, and other mediators have been implicated in maintaining such hyperalgesia. Accumulating evidence, however, reveals a significant contribution of supraspinal influences to development and maintenance of hyperalgesia. Spinal cord transection prevents development of secondary, but not primary, mechanical and/or thermal hyperalgesia after topical mustard oil application, carrageenan inflammation, or nerve-root ligation. Similarly, inactivation of the rostral ventromedial medulla (RVM) attenuates hyperalgesia and central sensitization in several models of persistent pain. Inhibition of medullary NMDA receptors or NO. generation attenuates somatic and visceral hyperalgesia. In support, topical mustard oil application or colonic inflammation increases expression of NO. synthase in the RVM. These data suggest a prominent role for the RVM in mediating the sensitization of spinal neurons and development of secondary hyperalgesia. Results to date suggest that peripheral injury and persistent input engage spinobulbospinal mechanisms that may be the prepotent contributors to central sensitization and development of secondary hyperalgesia.

Long term potentiation of single WDR neurons in spinalized rats

We report long-term potentiation (LTP) in single spinal wide dynamic range (WDR) neurons in urethane-anaesthetized spinalized rats with a complete neuromuscular blockade. Peripheral influences were excluded by a complete lidocaine block distal to the stimulation site on the sciatic nerve. As previously shown A-beta fibre evoked responses were not increased by the tetanic stimulation when there was a neuromuscular blockade during the experiment. Spinalization, excluding influences from supraspinal structures, increased all firing responses, and the LTP of C-fibre evoked responses when calculated in number of action potentials compared to intact animals and to previous studies. Furthermore, an LTP of the post discharge was observed after spinalization. An LTP of the post discharge has previously not been reported. Therefore, we conclude that LTP in the dorsal horn normally seems to be inhibited by descending pathways.

The medullary dorsal reticular nucleus facilitates pain behaviour induced by formalin in the rat

The influence of the dorsal reticular nucleus (DRt) on pain behaviour during the formalin test was studied in the rat by lesioning the nucleus through local application of electrical current or quinolinic acid. Animals in which the DRt was lesioned ipsilaterally to the paw injected with formalin spent less time in focused (licking, biting or scratching the injected paw) and total (focused pain behaviour plus protection of the injected paw during movements) pain behaviour, and showed paw-jerks less frequently than non-lesioned animals in both phases 1 and 2 of the test. Animals in which the DRt was lesioned contralaterally to the injected paw presented a decrease in total pain behaviour and number of paw-jerks only during phase 2. The number of superficial (laminae I-II) and deep (laminae III-VI) spinal dorsal horn cells expressing the c-fos proto-oncogene 2 h after subcutaneous injection of formalin was reduced by 34% and 50%, respectively, in animals with an ipsilateral DRt lesion as compared to non-lesioned rats. No differences in c-fos expression were observed after lesioning the DRt contralateral to the formalin injection. The results indicate that the DRt is involved in the facilitation of nociception during the formalin test by enhancing the response capacity of dorsal horn neurons to noxious stimulation. It is suggested that the pronociceptive action of the DRt is mediated by the reciprocal connections it establishes with the spinal dorsal horn.

The pontine A5 noradrenergic cells which project to the spinal cord dorsal horn are reciprocally connected with the caudal ventrolateral medulla in the rat

A disynaptic pathway linking the caudal ventrolateral medulla (VLM) to the spinal cord via the A5 noradrenergic cell group of the pons has recently been described in the rat. In the present work, the projections of the A5 to the VLM and to the spinal dorsal horn were studied with double-tracing techniques combined with immunostaining of the noradrenaline-synthesizing enzyme dopamine-beta-hydroxylase. Cholera toxin subunit B (CTb) injected into the VLM and fluoro-gold injected into the spinal dorsal horn produced double retrograde labelling of A5 neurons immunoreactive for dopamine-beta-hydroxylase, which received appositions of fibre varicosities labelled anterogradely with CTb injected into the VLM. After injecting CTb into the A5, retrogradely labelled neurons occurred in the VLM. These neurons were contacted by anterogradely labelled fibres from the A5 group. These observations indicate that the VLM cells acting upon the A5 spinally projecting neurons, which are likely to exert an alpha2-adrenoreceptor-mediated inhibition on the spinal cord, are targeted by collaterals of the A5 spinal cord-bound axons. The A5-VLM pathway may be the anatomical substrate of a negative feedback circuit whereby the modulatory action of the VLM on the spinal cord is self-inhibited through activation of the A5.

Activation by cutaneous or visceral noxious stimulation of spinal neurons projecting to the medullary dorsal reticular nucleus in the rat: a c-fos study

The involvement of spinal neurons in the transmission of cutaneous and visceral nociceptive input to the medullary dorsal reticular nucleus was studied. Rats were injected with cholera toxin subunit B in the left dorsal reticular nucleus and subjected 4 days later to noxious mechanical, thermal or chemical stimulation of the proximal internal aspect of the left thigh, or to chemical stimulation of the urinary bladder. Sections of spinal segments T13-L3 were processed immunocytochemically for cholera toxin subunit B and Fos protein. The percentage of double-labelled cells in the population of Fos-positive cells was higher in lamina I (1-4%) than in deeper laminae (0-0.7%) following all stimuli. The percentage of double-labelled cells in the population of retrogradely labelled cells was 30-53% in lamina I and 0-5% in laminae III-X. Visceral stimulation activated more retrogradely labelled lamina I cells than any kind of cutaneous stimulation. Pyramidal cells were activated in higher numbers than multipolar and flattened cells after thermal cutaneous or visceral stimulation, and in lower numbers than multipolar cells after mechanical stimulation. These results suggest that, in the experimental conditions used, spinal cord cells conveying noxious input to the dorsal reticular nucleus are concentrated in lamina I. They further indicate that the spinal-dorsal reticular nucleus pathway plays a major role in the transmission of nociceptive visceral input, and point to the preferential involvement of pyramidal cells in cutaneous thermal and visceral processing.

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.

Organization of efferent projections from the spinal cervical enlargement to the medullary subnucleus reticularis dorsalis and the adjacent cuneate nucleus: a PHA-L study in the rat

The distribution and organization of projections from the spinal cervical enlargement to subnucleus reticularis dorsalis (SRD) and the neighbouring Cuneate nucleus (Cu) area was studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) into different laminae around the C7 level. The Cu received very dense projections from the dorsal horn, with the highest density being observed following injections into the medial part of laminae III-IV. The SRD received dense projections from laminae V-VII of the cervical enlargement, particularly from the reticular and medial aspects of lamina V, lamina VI, and the dorsal part of lamina VII. By contrast, the superficial part of the dorsal horn (laminae I to IV) and the dorsal part of lamina X provided only sparse projections to the SRD. Clusters of labelled terminals and boutons were observed mainly in the SRD areas subjacent to the Cu. In the caudorostral axis, labelled terminals were spread along the whole SRD from the cervicomedullary junction up to the caudal-most part of the area postrema. Contralateral projections to the SRD were scarce and were observed mainly after injections into the medial part of laminae VI-VII. These data give further support to the proposal that there are two parallel systems in neighbouring structures of the caudal medulla, viz. the Cu and the SRD, which, respectively, relay lemniscal and nociceptive information from the spinal cord to the thalamus.

The medullary dorsal reticular nucleus facilitates acute nociception in the rat

The influence on pain processing caused by destruction or stimulation of the dorsal reticular nucleus (DRt) was studied using the tail-flick and the increasing temperature hot-plate tests. Lesions of the DRt were obtained by injecting quinolinic acid (180 nmol/microliters) unilaterally or bilaterally, and nociceptive responses were evaluated by both tests. Following unilateral lesions, the tail-flick latencies and the hot-plate response temperatures were increased, values differing statistically from controls in the latter test. Bilateral lesions resulted in statistically significant increases of both tail-flick latency and hot-plate response temperature. Stimulation of the DRt was performed by injecting glutamate (100 nmol/microliters) unilaterally, which was followed 1 min later by a significant decrease in the tail-flick latency compared to saline injected controls. These results suggest that the DRt is involved in the facilitation of nociception after acute thermal noxious stimulation. This effect may be mediated through a spino-DRt-spinal loop causing a rebound of excitation in lamina I cells receiving noxious input from their own receptive field.

Lack of a bilateral projection of individual spinal neurons to the lateral reticular nucleus in the rat: a retrograde, non-fluorescent, double labeling study

The projection of spinal neurons to the lateral reticular nucleus of the rat was investigated with a non-fluorescent double retrograde tracing technique. Either a gold-lectin tracer or cholera toxin-b-subunit was injected into the lateral reticular nucleus on each side of the brain. Retrogradely labeled neurons were encountered bilaterally throughout the spinal cord. Double labeled neurons, however, were seldom seen (< 2% of the total number of labeled neurons) and those that were of passing fibers. It is concluded that most spinoreticular neurons project to either the ipsi- or contralateral lateral reticular nucleus, suggesting that each side receives a unique spinal input.

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.