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

Somatotopy of Mouse Spinothalamic Innervation and the Localization of a Noxious Stimulus Requires Deleted in Colorectal Carcinoma Expression by Phox2a Neurons

Anterolateral system (AS) neurons transmit pain signals from the spinal cord to the brain. Their morphology, anatomy, and physiological properties have been extensively characterized and suggest that specific AS neurons and their brain targets are concerned with the discriminatory aspects of noxious stimuli, such as their location or intensity, and their motivational/emotive dimension. Among the recently unraveled molecular markers of AS neurons is the developmentally expressed transcription factor Phox2a, providing us with the opportunity to selectively disrupt the embryonic wiring of AS neurons to gain insights into the logic of their adult function. As mice with a spinal-cord-specific loss of the netrin-1 receptor deleted in colorectal carcinoma (DCC) have increased AS neuron innervation of ipsilateral brain targets and defective noxious stimulus localization or topognosis, we generated mice of either sex carrying a deletion of Dcc in Phox2a neurons. Such DccPhox2a mice displayed impaired topognosis along the rostrocaudal axis but with little effect on left-right discrimination and normal aversive responses. Anatomical tracing experiments in DccPhox2a mice revealed defective targeting of cervical and lumbar AS axons within the thalamus. Furthermore, genetic labeling of AS axons revealed their expression of DCC on their arrival in the brain, at a time when many of their target neurons are being born and express Ntn1 Our experiments suggest a postcommissural crossing function for netrin-1:DCC signaling during the formation of somatotopically ordered maps and are consistent with a discriminatory function of some of the Phox2a AS neurons.SIGNIFICANCE STATEMENT How nociceptive (pain) signals are relayed from the body to the brain remains an important question relevant to our understanding of the basic physiology of pain perception. Previous studies have demonstrated that the AS is a main effector of this function. It is composed of AS neurons located in the spinal cord that receive signals from nociceptive sensory neurons that detect noxious stimuli. In this study, we generate a genetic miswiring of mouse AS neurons that results in a decreased ability to perceive the location of a painful stimulus. The precise nature of this defect sheds light on the function of different kinds of AS neurons and how pain information may be organized.

Lateralized kappa opioid receptor signaling from the amygdala central nucleus promotes stress-induced functional pain

The response of diffuse noxious inhibitory controls (DNIC) is often decreased, or lost, in stress-related functional pain syndromes. Because the dynorphin/kappa opioid receptor (KOR) pathway is activated by stress, we determined its role in DNIC using a model of stress-induced functional pain. Male, Sprague-Dawley rats were primed for 7 days with systemic morphine resulting in opioid-induced hyperalgesia. Fourteen days after priming, when hyperalgesia was resolved, rats were exposed to environmental stress and DNIC was evaluated by measuring hind paw response threshold to noxious pressure (test stimulus) after capsaicin injection in the forepaw (conditioning stimulus). Morphine priming without stress did not alter DNIC. However, stress produced a loss of DNIC in morphine-primed rats in both hind paws that was abolished by systemic administration of the KOR antagonist, nor-binaltorphimine (nor-BNI). Microinjection of nor-BNI into the right, but not left, central nucleus of the amygdala (CeA) prevented the loss of DNIC in morphine-primed rats. Diffuse noxious inhibitory controls were not modulated by bilateral nor-BNI in the rostral ventromedial medulla. Stress increased dynorphin content in both the left and right CeA of primed rats, reaching significance only in the right CeA; no change was observed in the rostral ventromedial medulla or hypothalamus. Although morphine priming alone is not sufficient to influence DNIC, it establishes a state of latent sensitization that amplifies the consequences of stress. After priming, stress-induced dynorphin/KOR signaling from the right CeA inhibits DNIC in both hind paws, likely reflecting enhanced descending facilitation that masks descending inhibition. Kappa opioid receptor antagonists may provide a new therapeutic strategy for stress-related functional pain disorders.

Changes in responses of neurons in spinal and medullary subnucleus reticularis dorsalis to acupoint stimulation in rats with visceral hyperalgesia

The purpose of this study was to explore the mechanism of acupoints sensitization phenomenon at the spinal and medulla levels. Experiments were performed on adult male Sprague-Dawley rats and visceral noxious stimuli was generated by colorectal distension (CRD). The activities of wide dynamic range (WDR) and subnucleus reticularis dorsalis (SRD) neurons were recorded. The changes of the reactions of WDR and SRD neurons to electroacupuncture (EA) on acupoints of “Zusanli-Shangjuxu” before and after CRD stimulation were observed. The results showed that visceral nociception could facilitate the response of neurons to acupoints stimulation. In spinal dorsal horn, EA-induced activation of WDR neurons further increased to 106.84 ± 17.33% (1.5 mA) (P < 0.001) and 42.27 ± 13.10% (6 mA) (P < 0.01) compared to the neuronal responses before CRD. In medulla oblongata, EA-induced activation of SRD neurons further increased to 63.28 ± 15.96% (1.5 mA) (P < 0.001) and 25.02 ± 7.47% (6 mA) (P < 0.01) compared to that before CRD. Taken together, these data suggest that the viscerosomatic convergence-facilitation effect of WDR and SRD neurons may underlie the mechanism of acupoints sensitization. But the sensitizing effect of visceral nociception on WDR neurons is stronger than that on SRD neurons.

The nucleus raphe magnus OFF-cells are involved in diffuse noxious inhibitory controls

Diffuse noxious inhibitory controls (DNIC) are very powerful long-lasting descending inhibitory controls which are pivotal in modulating the activity of spinal and trigeminal nociceptive neurons. DNIC are subserved by a loop involving supraspinal structures such as the lateral parabrachial nucleus and the subnucleus reticularis dorsalis. Surprisingly, though, whether the nucleus raphe magnus (NRM), another supraspinal area which is long known to be important in pain modulation, is involved in DNIC is still a matter of discussion. Here, we reassessed the role of the NRM neurons in DNIC by electrophysiologically recording from wide dynamic range (WDR) neurons in the trigeminal subnucleus oralis and pharmacologically manipulating the NRM OFF- and ON-cells. In control conditions, C-fiber-evoked responses in trigeminal WDR neurons are inhibited by a conditioning noxious heat stimulation applied to the hindpaw. We show that inactivating the NRM by microinjecting the GABAA receptor agonist, muscimol, both facilitates C-fiber-evoked responses of trigeminal WDR neurons and strongly attenuates their inhibition by heat applied to the hindpaw. Interestingly, selective blockade of ON-cells by microinjecting the broad-spectrum excitatory amino acid antagonist, kynurenate, into the NRM neither affects C-fiber-evoked responses nor attenuates DNIC of trigeminal WDR neurons. These results indicate that the NRM tonically inhibits trigeminal nociceptive inputs and is involved in the neuronal network underlying DNIC. Moreover, within NRM, OFF-cells might be more specifically involved in both the tonic and phasic descending inhibitory controls of trigeminal nociception.

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.

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.

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.

A quantitative study of brainstem projections from lamina I neurons in the cervical and lumbar enlargement of the rat

Lamina I of the rat spinal cord contains neurons that project to various brain areas including thalamus, periaqueductal grey matter (PAG), lateral parabrachial area (LPb), caudal ventrolateral medulla and a region in dorsal medulla that includes the nucleus tractus solitarius and dorsal reticular nucleus. We have shown that spinothalamic lamina I neurons are infrequent in rat lumbar enlargement, where they constitute approximately 5% of the estimated 400 projection neurons on each side of the L4 segment (Al-Khater and Todd, 2009). They are more numerous in cervical enlargement, but the total number of lamina I projection neurons in this region was not known. Here we have used paired injections of retrograde tracers into the brainstem to estimate the number of lamina I projection cells in the C7 segment. Our results suggest that there are approximately 215 lamina I projection cells per side, and that spinothalamic cells therefore make up approximately 42% of this population. The proportion of lamina I projection neurons labelled from PAG is higher in cervical than lumbar enlargement, while the proportion labelled from dorsal medulla is similar in the two regions. We also found that lamina I cells in L4 that project to the dorsal medulla are included in the population retrogradely labelled from LPb, thus confirming the estimate that there are around 400 lamina I projection cells in this segment.

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.

The calcium binding proteins calbindin, parvalbumin, and calretinin have specific patterns of expression in the gray matter of cat spinal cord

Calcium binding proteins (CBPs) regulate intracellular levels of calcium (Ca(2+)) ions. CBPs are particularly interesting from a morphological standpoint, because they are differentially expressed in certain sub-populations of cells in the nervous system of various species of vertebrate animals. However, knowledge on the cellular regulation governing such cell-specific CBP expression is still incomplete. In this work on the L7 segment of the cat spinal cord, we analyzed the localization and morphology of neurons expressing the CBPs calbindin-28 KD (CB), parvalbumin (PV), and calretinin (CR), and co-expressing CB and PV, CB and CR, and PV and CR. Single CBP-positive ((+)) neurons showed specific distributions: (1) CB was present in small neurons localized in laminae I, II, III and X, in small to medium size neurons in laminae III-VI, and in medium to large neurons in laminae VI-VIII; (2) PV was present in small size neurons in laminae III and IV and in medial portions of laminae V and VI, medium neurons and in lamina X at the border with lamina VII, in medium to large neurons in laminae VII and VIII; (3) CR labeling was detected in small size neurons in laminae I, II, III and VIII, in medium to large size neurons in laminae I and III-VII, and in small to medium size neurons in lamina X. Double labeled neurons were a small minority of the CBP(+) cells. Co-expression of CB and PV was seen in 1 to 2% of the CBP(+) cells, and they were detected in the ventral and intermediate portions of lamina VII and in lamina X. Co-localization of CB and CR was present in 0.3% of the cells and these cells were localized in lamina II. Double labeling for PV and CR occurred in 6% of the cells, and the cells were localized in ventral part of lamina VII and in lamina VIII. Overall, these results revealed distinct and reproducible patterns of localization of the neurons expressing single CBPs and co-expressing two of them. Distinct differences of CBP expression between cat and other species are discussed. Possible relations between the cat L7 neurons expressing different CBPs with the neurons previously analyzed in cat and other animals are suggested.

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.

Defining projections from the caudal pressor area of the caudal ventrolateral medulla

We previously defined a functional area in the caudal medulla oblongata that elicits an increase in arterial pressure when stimulated (Sun and Panneton [2002] Am. J. Physiol. 283:R768-R778). In the present study, anterograde and retrograde tracing techniques were used to investigate the projections of this caudal pressor area (CPA) to the medulla and pons. Injections of biotinylated dextran amine into the CPA resulted in numerous labeled fibers with varicosities in the ipsilateral subnucleus reticularis dorsalis, commissural subnucleus of the nucleus tractus solitarii, lateral medulla, medial facial nucleus, A5 area, lateral vestibular nucleus, and internal lateral subnucleus of the parabrachial complex. Sparser projections were found ipsilaterally in the pressor and depressor areas of the medulla and the spinal trigeminal nucleus and contralaterally in the CPA. Injections of the retrograde tracer Fluoro-Gold into these areas labeled neurons in the CPA as well as the nearby medullary dorsal horn and reticular formation. However, we conclude that the CPA projects preferentially to the subnucleus reticularis dorsalis, commissural nucleus tractus solitarii, lateral medulla, A5 area, and internal lateral parabrachial nucleus. Weaker projections were seen to the CVLM and RVLM and to the contralateral CPA. The projection to the facial nucleus arises from nearby reticular neurons, whereas projections to the vestibular nucleus arise from the lateral reticular nucleus. Labeled neurons in the CPA consisted mostly of small bipolar and some triangular neurons. The projection to the CVLM, or to A5 area, may provide for the increase in arterial pressure with CPA stimulation. However, most of the projections described herein are to nuclei implicated in the processing of noxious information. This implies a unique role for the CPA in somatoautonomic regulation.

Organization of parabrachial projections from the spinal trigeminal nucleus oralis: An anterograde tracing study in the rat

In recent years, we have accumulated data showing that the spinal trigeminal nucleus oralis (Sp5O) contributes to the processing of somatosensory inputs from the orofacial region. Although the parabrachial area (PB) represents the main brainstem relay for autonomic, nociceptive, and gustatory afferents, few data are available regarding the topographical distribution of the efferent projections from the Sp5O to the PB. We have addressed this question with the rat, by using the anterograde tracer Phaseolus vulgaris leucoagglutinin. A dense trigeminoparabrachial pathway from the Sp5O toward, predominantly, the ipsilateral PB was revealed. Projections come mainly from the dorsal part of the Sp5O that was found to innervate densely the medial, external medial, and ventral lateral subnuclei. In contrast, the ventral part of the Sp5O projected almost exclusively to an as yet not formally described region, located dorsally and laterally to the lateral tip of the brachium conjunctivum, close to the Kölliker-Fuse nucleus. These results suggest that distinct regions within the Sp5O may be involved in the processing of gustatory and nociceptive information.

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.

Separate, parallel sensory and hedonic pathways in the mammalian somatosensory system

We propose that separate sensory and hedonic representations exist in each of the primary structures of the somatosensory system, including brainstem, thalamic and cortical components. In the dorsal horn of the spinal cord, the hedonic representation, which consists primarily of nociceptive-specific, wide dynamic range, and thermoreceptive neurons, is located in laminae I and II, while the sensory representation, composed primarily by low-threshold and wide dynamic range neurons, is found in laminae III through V. A similar arrangement is found in the caudal spinal trigeminal nucleus. Based on the available anatomical and electrophysiological data, we then determine the corresponding hedonic and sensory representations in the area of the dorsal column nuclei, ventrobasal and posterior thalamic complex, and cortex. In rodent primary somatosensory cortex, a hedonic representation can be found in laminae Vb and VI. In carnivore and primate primary and secondary somatosensory cortical areas no hedonic representation exists, and the activities of neurons in both areas represent the sensory aspect exclusively. However, there is a hedonic representation in the posterior part of insular cortex, bordering on retroinsular cortex, that receives projections from two thalamic areas in which hedonics are represented. The functions of the segregated components of the system are discussed, especially in relation to the subjective awareness of pain.

Systemic morphine selectively depresses a thalamic link of widespread nociceptive inputs in the rat

The lateral part of the ventromedial thalamus (VM l) relays nociceptive inputs from the whole body surface to the dorsolateral frontal cortex. The aim of the present study was to investigate the effects of systemic morphine on nociceptive activity evoked in VM l neurones either by thermal (48 degrees C) or by supramaximal percutaneous electrical stimuli. The noxious thermal evoked responses were depressed by 10.8 +/- 10.1%, 48.3 +/- 23.0% and 67.3 +/- 10.1%, 5 min after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. Moreover, strong depressive effects on the Adelta- and C-fibre responses were already present 5 min after the injection. The responses were significantly reduced by 7.2 +/- 5.9%, 32.5 +/ 11.1% and 37.2 +/- 11.8% for Adelta fibres after i.v. injections of 1.0, 1.73 and 3.0 mg/kg of morphine, respectively. The corresponding values for C-fibre evoked responses were 16.3 +/- 16.2%, 57.0 +/- 12.0% and 69.0 +/- 8.2%. The dose of morphine that reduced VM l neuronal nociceptive responses by 50% (1.73 mg/kg) was around 3.5 times lower than that necessary to inhibit the responses of its spinal or medullary relays under similar experimental conditions. These results, added to the data of the literature, suggest that supraspinal effects of morphine are primarily mediated at the thalamic level. It is tempting to speculate that morphine-induced reductions of attentional or psychomotor responses related to pain may be mediated by its action on VM l.

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.

Dorsal column-thalamic pathway is involved in thalamic hyperexcitability following peripheral nerve injury: a lesion study in rats with experimental mononeuropathy

A total of 68 neurons were recorded from the ventro-postero-lateral nucleus of thalamus (VPL) in rats with a unilateral chronic constriction injury (CCI) of the sciatic nerve (n=20), sham operation (n=24) and naive rats (n=24), and effects of the lesion of dorsal column (DC) pathway [DC lesion or DC+gracile nucleus lesions] on VPL nucleus neuronal activities were studied. In the VPL nucleus contralateral to the CCI (receiving input from the injured nerve), response latencies of low threshold mechanoreceptive (LTM) and wide dynamic range (WDR) neurons to electrical stimulation of the sciatic nerve were significantly longer than that in the contralateral VPL nucleus receiving input from the sham-operated side (P<0.05). In contrast, response latencies of LTM and WDR neurons to DC stimulation were not different between the sham operated and CCI sides (0.05). Background activity of WDR neurons was significantly higher in the VPL nucleus contralateral to the CCI side when compared to neurons in the VPL nucleus contralateral to the sham operated side and in naive animals. Responses of LTM and WDR neurons to innocuous mechanical stimulation of the receptive fields were significantly decreased after DC and DC+gracile nucleus lesions in all animals. However, the responses of WDR neurons to noxious stimuli were selectively reduced only in rats with CCI by DC and DC+gracile nucleus lesions (P<0.05). The decrease in noxious stimulus-evoked responses of WDR neurons in the VPL nucleus contralateral to the CCI side after DC and DC+gracile nucleus lesions was greater than that in the VPL nucleus contralateral to the sham operated side and naive animals. These results indicated that DC and DC+gracile nucleus lesions produced selective and stronger effect on noxious responses of VPL nucleus WDR neurons receiving input from the site of nerve injury. The findings suggest that the gracile nucleus-thalamic pathway conveys, or modulates, nociceptive information to the VPL nucleus following peripheral nerve injury, resulting in an increase in VPL nucleus response to noxious stimuli that contributes to the development of mechanical hyperalgesia.

Ventromedial thalamic neurons convey nociceptive signals from the whole body surface to the dorsolateral neocortex

The somatosensory properties of ventromedial (VM) thalamic neurons were investigated in anesthetized rats by examining their responses to calibrated cutaneous stimuli. A population of neurons within the lateral part of the ventromedial thalamus (VMl) showed two peaks of activation after percutaneous electrical stimuli, regardless of which part of the body was stimulated. The early and late peaks were elicited by Adelta- and C-fiber activities with mean conduction velocities of 12.9 +/- 0.9 and 1 +/- 0.2 m/sec, respectively. These responses were strongly depressed or blocked after microinjections within the medullary subnucleus reticularis dorsalis of xylocaine or the NMDA antagonist MK-801. None of the VMl neurons responded to innocuous cutaneous or proprioceptive stimuli. In contrast, all these neurons responded to noxious mechanical and thermal stimulation of the limbs and showed monotonic increases in their discharges to increasingly strong noxious cutaneous stimuli. In addition, some VMl neurons were antidromically activated by stimulation in layer I of the dorsolateral frontal cortex. These findings suggest that the rat VMl conveys and encodes cutaneous nociceptive inputs from any part of the body surface to layer I of the dorsolateral neocortex. This reticulo-thalamo-cortical network may allow any signal of pain to gain access to widespread areas of the neocortex and thus help prime the cortex for attentional reactions and/or the coordination of motor responses.

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 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.