Morphine enhances the activity of thermoreceptive cold-specific lamina I spinothalamic neurons in the cat

Thermosensory thalamus: parallel processing across model organisms

The thalamus acts as an interface between the periphery and the cortex, with nearly every sensory modality processing information in the thalamocortical circuit. Despite well-established thalamic nuclei for visual, auditory, and tactile modalities, the key thalamic nuclei responsible for innocuous thermosensation remains under debate. Thermosensory information is first transduced by thermoreceptors located in the skin and then processed in the spinal cord. Temperature information is then transmitted to the brain through multiple spinal projection pathways including the spinothalamic tract and the spinoparabrachial tract. While there are fundamental studies of thermal transduction via thermosensitive channels in primary sensory afferents, thermal representation in the spinal projection neurons, and encoding of temperature in the primary cortical targets, comparatively little is known about the intermediate stage of processing in the thalamus. Multiple thalamic nuclei have been implicated in thermal encoding, each with a corresponding cortical target, but without a consensus on the role of each pathway. Here, we review a combination of anatomy, physiology, and behavioral studies across multiple animal models to characterize the thalamic representation of temperature in two proposed thermosensory information streams.

Encoding of cutaneous stimuli by lamina I projection neurons

ABSTRACT

Lamina I of the dorsal horn, together with its main output pathway, lamina I projection neurons, has long been implicated in the processing of nociceptive stimuli, as well as the development of chronic pain conditions. However, the study of lamina I projection neurons is hampered by technical challenges, including the low throughput and selection biases of traditional electrophysiological techniques. Here we report on a technique that uses anatomical labelling strategies and in vivo imaging to simultaneously study a network of lamina I projection neurons in response to electrical and natural stimuli. Although we were able to confirm the nociceptive involvement of this group of cells, we also describe an unexpected preference for innocuous cooling stimuli. We were able to characterize the thermal responsiveness of these cells in detail and found cooling responses decline when exposed to stable cold temperatures maintained for more than a few seconds, as well as to encode the intensity of the end temperature, while heating responses showed an unexpected reliance on adaptation temperatures.

Selective-cold output through a distinct subset of lamina I spinoparabrachial neurons

Spinal projection neurons are a major pathway through which somatic stimuli are conveyed to the brain. However, the manner in which this information is coded is poorly understood. Here, we report the identification of a modality-selective spinoparabrachial (SPB) neuron subtype with unique properties. Specifically, we find that cold-selective SPB neurons are differentiated by selective afferent input, reduced sensitivity to substance P, distinct physiological properties, small soma size, and low basal drive. In addition, optogenetic experiments reveal that cold-selective SPB neurons do not receive input from Nos1 inhibitory interneurons and, compared with other SPB neurons, show significantly smaller inhibitory postsynaptic currents upon activation of Pdyn inhibitory interneurons. Together, these data suggest that cold output from the spinal cord to the parabrachial nucleus is mediated by a specific cell type with distinct properties.

Thermal-Tactile Integration in Object Temperature Perception

The brain consistently faces a challenge of whether and how to combine the available information sources to estimate the properties of an object explored by hand. While object perception is an inference process involving multisensory inputs, thermal referral (TR) is an illusion demonstrating how the interaction between thermal and tactile systems can lead to deviations from physical reality-when observers touch three stimulators simultaneously with the middle three fingers of one hand but only the outer two stimulators are heated (or cooled), thermal uniformity is perceived across three fingers. Here, we used TR of warmth to examine the thermal-tactile interaction in object temperature perception. We show that TR is consistent with precision-weighted averaging of thermal sensation across tactile locations. Furthermore, we show that prolonged contact with TR stimulation results in adaptation to the local variations of veridical temperatures instead of the thermal uniformity perceived across three fingers. Our results illuminate the flexibility of processing that underlies thermal-tactile interactions and serve as a basis for thermal display design.

Central neural substrates involved in temperature discrimination, thermal pain, thermal comfort, and thermoregulatory behavior

A phylogenetically novel pathway that emerged with primate encephalization is described, which conveys high-fidelity cutaneous thermosensory activity in "labeled lines" to a somatotopic map in the dorsal posterior insular cortex. It originates in lamina I of the superficial dorsal horn and ascends by way of the lateral spinothalamic tract and a distinct region in posterolateral thalamus. It evolved from the homeostatic sensory activity that represents the physiologic (interoceptive) condition of the body and drives the central autonomic network, which underlies all affective feelings from the body. Accordingly, human discriminative thermal sensations are accompanied by thermally motivated behaviors and thermal feelings of comfort or discomfort (unless neutral), which evidence suggests are associated with activity in the insular, cingulate, and orbitofrontal cortices, respectively. Yet, the substrates for thermoregulatory behavior have not been established, and several strong candidates (including the hypothalamus and the bed nucleus of the stria terminalis) are discussed. Finally, the neural underpinnings for relationships between thermal affect and social feelings (warm-positive/cold-negative) are addressed, including the association of hyperthermia with clinical depression.

Corneal sensitivity following lacrimal gland excision in the rat

PURPOSE

Dry eye disease (DED) produces ocular pain and irritation, yet a detailed characterization of ocular sensitivity in a preclinical model of DED is lacking. The aim of the present study was to assess nociceptive behaviors in an aqueous tear deficiency model of DED in the rat.

METHODS

Spontaneous blinking, corneal mechanical thresholds, and eye wipe behaviors elicited by hypertonic saline (5.0 M) were examined over a period of 8 weeks following the unilateral excision of either the exorbital lacrimal gland or of the exorbital and infraorbital lacrimal glands, and in sham surgery controls. The effect of topical proparacaine on spontaneous blinking and of systemic morphine (0.5-3.0 mg/kg, subcutaneous [SC]) on spontaneous blinking and eye wipe responses were also examined.

RESULTS

Lacrimal gland excision resulted in mechanical hypersensitivity and an increase in spontaneous blinking in the ipsilateral eye over an 8-week period that was more pronounced after infra- and exorbital gland excision. The time spent eye wiping was also enhanced in response to hypertonic saline (5.0 M) at both 1- and 8-week time-points, but only in infra- and exorbital gland excised animals. Morphine attenuated spontaneous blinking, and the response to hypertonic saline in dry eye animals and topical proparacaine application reduced spontaneous blinking down to control levels.

CONCLUSIONS

These results indicate that aqueous tear deficiency produces hypersensitivity in the rat cornea. In addition, the increase in spontaneous blinks and their reduction by morphine and topical anesthesia indicate the presence of persistent irritation elicited by the activation of corneal nociceptors.

Topographically organized projection to posterior insular cortex from the posterior portion of the ventral medial nucleus in the long-tailed macaque monkey

Prior anterograde tracing work identified somatotopically organized lamina I trigemino- and spinothalamic terminations in a cytoarchitectonically distinct portion of posterolateral thalamus of the macaque monkey, named the posterior part of the ventral medial nucleus (VMpo; Craig [2004] J. Comp. Neurol. 477:119-148). Microelectrode recordings from clusters of selectively thermoreceptive or nociceptive neurons were used to guide precise microinjections of various tracers in VMpo. A prior report (Craig and Zhang [2006] J. Comp. Neurol. 499:953-964) described retrograde tracing results, which confirmed the selective lamina I input to VMpo and the anteroposterior (head to foot) topography. The present report describes the results of microinjections of anterograde tracers placed at different levels in VMpo, based on the anteroposterior topographic organization of selectively nociceptive units and clusters over nearly the entire extent of VMpo. Each injection produced dense, patchy terminal labeling in a single coherent field within a distinct granular cortical area centered in the fundus of the superior limiting sulcus. The terminations were distributed with a consistent anteroposterior topography over the posterior half of the superior limiting sulcus. These observations demonstrate a specific VMpo projection area in dorsal posterior insular cortex that provides the basis for a somatotopic representation of selectively nociceptive lamina I spinothalamic activity. These results also identify the VMpo terminal area as the posterior half of interoceptive cortex; the anterior half receives input from the vagal-responsive and gustatory neurons in the basal part of the ventral medial nucleus.

De novo expression of neurokinin-1 receptors by spinoparabrachial lamina I pyramidal neurons following a peripheral nerve lesion

Lamina I of the spinal dorsal horn is a major site of integration and transmission to higher centers of nociceptive information from the periphery. One important primary afferent population that transmits such information to the spinal cord expresses substance P (SP). These fibers terminate in contact with lamina I projection neurons that express the SP receptor, also known as the neurokinin-1 receptor (NK-1r). Three types of lamina I projection neurons have been described: multipolar, fusiform, and pyramidal. Most neurons of the first two types are thought to be nociceptive and express the NK-1r, whereas most pyramidal neurons are nonnociceptive and do not express the NK-1r. In this immunocytochemical and behavioral study, we induced a neuropathic pain-like condition in the rat by means of a polyethylene cuff placed around in the sciatic nerve. We document that this lesion led to a de novo expression of NK-1r on pyramidal neurons as well as a significant increase in SP-immunoreactive innervation onto these neurons. These phenotypic changes were evident at the time of onset of neuropathic pain-related behavior. Additionally, we show that, after a noxious stimulus (intradermal capsaicin injection), these NK-1r on pyramidal neurons were internalized, providing evidence that these neurons become responsive to peripheral noxious stimulation. We suggest that the changes following nerve lesion in the phenotype and innervation pattern of pyramidal neurons are of significance for neuropathic pain and/or limb temperature regulation.

Itch and analgesia resulting from intrathecal application of morphine: contrasting effects on different populations of trigeminothalamic tract neurons

Intrathecal application of morphine is among the most powerful methods used to treat severe chronic pain. However, this approach commonly produces itch sufficiently severe that patients are forced to choose between relief of pain or itch. The neuronal populations responsible for processing and transmitting information underlying itch caused by intrathecal application of morphine have not been identified and characterized. We describe two populations of antidromically identified trigeminothalamic tract (VTT) neurons in anesthetized rats that are differentially affected by morphine and explain several aspects of opioid-induced itch and analgesia. We found that intrathecal application of morphine increased ongoing activity of itch-responsive VTT neurons. In addition, intrathecal application of morphine increased responses to pruritogens injected into the skin and greatly heightened responses to innocuous mechanical stimuli. In contrast, the ongoing activity and responses to noxious pinches in nociceptive VTT neurons were frequently inhibited by the same dose of morphine. These results reveal that i.t. application of morphine affects specific subpopulations of VTT neurons in ways that may produce itch, hyperknesis, alloknesis, and analgesia.

Distribution of trigeminothalamic and spinothalamic lamina I terminations in the cat

The distribution in the thalamus of terminal projections from lamina I neurons of the trigeminal, cervical, and lumbosacral dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in the cat. Iontophoretic injections were guided by single- and multi-unit physiological recordings. The injections in particular cases were essentially restricted to lamina I, whereas in others they spread across laminae I-III or laminae I-V. The trigemino- and spinothalamic (TSTT) terminations were identified immunohistochemically. In all cases, regardless of the level of the injections, terminal fibers were consistently distributed in three main locations: the submedial nucleus; the ventral aspect of the basal ventral medial nucleus and ventral posterior nuclei; and, the dorsomedial aspect of the ventral posterior medial nucleus. The terminal fields in the submedial nucleus and the ventral aspect of the ventral posterior group were topographically organized. Terminations along the ventral aspect of the ventral posterior group extended posterolaterally into the caudal part of the posterior nucleus and anteromedially into the ventromedial part of the ventral lateral nucleus. In several cases with trigeminal lamina I injections, a terminal labeling patch was observed within the core of the ventral posterior medial nucleus. In cases with spinal lamina I injections, terminations were also consistently found in the lateral habenula, the parafascicular nucleus, and the nucleus reuniens. Isolated terminal fibers were occasionally seen in the zona incerta, the dorsomedial hypothalamus, and other locations. These anatomical observations extend prior studies of TSTT projections and identify lamina I projection targets that are important for nociceptive, thermoreceptive, and homeostatic processing in the cat. The findings are consistent with evidence from physiological (single-unit and antidromic mapping) and behavioral studies. The novel identification of spinal lamina I input to the lateral habenula could be significant for homeostatic behaviors.

The cannabinoid receptor agonist, WIN 55,212-2, inhibits cool-specific lamina I medullary dorsal horn neurons

Cannabinoid receptor agonists have been demonstrated to inhibit medullary and spinal cord dorsal horn nociceptive neurons. The effect of cannabinoids on thermoreceptive specific neurons in the spinal or medullary dorsal horn remains unknown. In the present study, single-unit recordings from the rat medullary dorsal horn were performed to examine the effect of a cannabinoid receptor agonists on cold-specific lamina I spinothalamic tract neurons. The cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (WIN-2), was locally applied to the medullary dorsal horn and the neuronal activity evoked by cooling the receptive field was recorded. WIN-2 (1 microg/microl and 2 microg/microl) significantly attenuated cold-evoked activity. Co-administration of the CB1 receptor antagonist SR 141716 with WIN-2 did not affect cold-evoked activity. These results demonstrate a potential mechanism by which cannabinoids produce hypothermia, and also suggest that cannabinoids may affect non-noxious thermal discrimination.

Increased nociceptive response in mice lacking the adenosine A1 receptor

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

Electrophysiological characterisations of rat lamina I dorsal horn neurones and the involvement of excitatory amino acid receptors

Lamina I of the spinal cord plays a key role in sensory transmission between afferent activity and the CNS. Studies have shown lamina I neurones to have distinct response properties compared to deep dorsal horn neurones, but little is known regarding excitatory amino acid mechanisms in their responses. Spinal electrophysiological recordings of lamina I neurones confirmed that the majority of these neurones (74%) are nociceptive specific (NS) in their responses, of which 18% can be termed polymodal nociceptive (HPC) (13% of the total population). The remainder (26%) were wide dynamic range. Lamina I neurones had smaller mechanical and heat-evoked responses compared to deeper dorsal horn neurones. The electrically evoked responses were also smaller, with a distinct lack of an NMDA-mediated 'wind-up' effect. NBQX (AMPA receptor antagonist, 0.5, 5, 50 microg/50 microl) produced dose-dependent inhibitions of the electrically evoked neuronal responses, but APV (NMDA receptor antagonist, 50, 100, 500 microg/50 microl) had minimal effects on their responses. These results implicate mainly AMPA receptors in the responses of lamina I neurones. Bicuculline (GABA(A) receptor antagonist, 0.5, 5, 50 microg/50 microl) demonstrated a role exerted by GABA(A) receptors in the control of A-delta fibre-mediated mechanical responses in lamina I. Overall, this study describes a high threshold, AMPA receptor possessing population of lamina I neurones, which seem to lack functional NMDA receptors, and are partially controlled by GABA(A) receptor activity.

Low dose systemic morphine attenuates operant escape but facilitates innate reflex responses to thermal stimulation

Effects of systemic morphine on operant escape responses of rats to thermal stimulation were compared directly with effects on innate licking and guarding responses. For these independent tests, thermal stimulation was delivered via the floor of testing chambers with or without platforms that provided an escape option. The principal findings were (1) administration of 0.5 to 1.5 mg/kg morphine attenuated escape from nociceptive heat and (2) in distinct contrast, licking and guarding responses to heat were enhanced by these doses. When escape responding was calculated as time on the heated plate without licking or guarding, sensitivity to morphine was greater for 44 degrees C than for 47 degrees C or 50 degrees C. Also, escape responses to cold (0 degrees C or 10 degrees C) were unaffected by 1.5 mg/kg morphine. The preferential reduction of heat nociception by morphine was demonstrated also by an operant preference task that gave the animals the option of standing on a cold (10 degrees C) or a hot (45 degrees C) surface. Administration of 0.5 mg/kg morphine increased occupancy of the hot surface. Platform time during operant tests was low and variable for warm stimulation (36 degrees C) and was significantly increased by each level of heat, showing that platform occupancy represented escape from nociception rather than avoidance responses. A lack of significant effects of 1.5 mg/kg morphine on operant performance during cold or warm stimulation controls for effects of systemic morphine other than antinociception.

Quantitative response characteristics of thermoreceptive and nociceptive lamina I spinothalamic neurons in the cat

The physiological characteristics of antidromically identified lamina I spinothalamic (STT) neurons in the lumbosacral spinal cord were examined using quantitative thermal and mechanical stimuli in barbiturate-anesthetized cats. Cells belonging to the three main recognized classes were included based on categorization with natural cutaneous stimulation of the hindpaw: nociceptive-specific (NS), polymodal nociceptive (HPC), or thermoreceptive-specific (COOL) cells. The mean central conduction latencies of these classes differed significantly; NS = 130.8 +/- 55.5 (SD) ms (n = 100), HPC = 72.1 +/- 28.0 ms (n = 128), and COOL = 58.6 +/- 25.3 ms (n = 136), which correspond to conduction velocities of 2.5, 4.6, and 5.6 m/s. Based on recordings made prior to any noxious stimulation, the mean spontaneous discharge rates of these classes also differed: NS = 0.5 +/- 0.7 imp/s (n = 47), HPC = 0.9 +/- 0.7 imp/s (n = 59), and COOL = 3.3 +/- 2.6 imp/s (n = 107). Standard, quantitative, thermal stimulus sequences applied with a Peltier thermode were used to characterize the stimulus-response functions of 76 COOL cells, 47 HPC cells, and 37 NS cells. The COOL cells showed a very linear output from 34 degrees C down to approximately 15 degrees C and a maintained plateau thereafter. The HPC cells showed a fairly linear but accelerating response to cold below a median threshold of approximately 24 degrees C and down to 9 degrees C (measured at the skin-thermode interface with a thermode temperature of 2 degrees C). The HPC cells and the NS cells both showed rapidly increasing, sigmoidal response functions to noxious heat with a fairly linear response between 45 and 53 degrees C, but they had significantly different thresholds; half of the HPC cells were activated at ~45.5 degrees C and half of the NS cells at approximately 43 degrees C. The 20 HPC lamina I STT cells and 10 NS cells tested with quantitative pinch stimuli showed fairly linear responses above a threshold of approximately 130 g/mm(2) for HPC cells and a threshold of approximately 100 g/mm(2) for NS cells. All of these response functions compare well (across species) with the available data on the characteristics of thermoreceptive and nociceptive primary afferent fibers and the appropriate psychophysics in humans. Together these results support the concept that these classes of lamina I STT cells provide discrete sensory channels for the sensations of temperature and pain.

Differential projections of thermoreceptive and nociceptive lamina I trigeminothalamic and spinothalamic neurons in the cat

The projections of 40 trigeminothalamic or spinothalamic (TSTT) lamina I neurons were mapped using antidromic activation from a mobile electrode array in barbiturate anesthetized cats. Single units were identified as projection cells from the initial array position and characterized with natural cutaneous stimuli as nociceptive-specific (NS, n = 9), polymodal nociceptive (HPC, n = 8), or thermoreceptive-specific (COOL, n = 22; WARM, n = 1) cells. Thresholds for antidromic activation were measured from each electrode in the mediolateral array at vertical steps of 250 microm over a 7-mm dorsoventral extent in two to eight (median = 6.0) anteroposterior planes. Histological reconstructions showed that the maps encompassed all three of the main lamina I projection targets observed in prior anatomical work, i.e., the ventral aspect of the ventroposterior complex (vVP), the dorsomedial aspect of the ventroposterior medial nucleus (dmVPM), and the submedial nucleus (Sm). The antidromic activation foci were localized to these sites (and occasional projections to other sites were also observed, such as the parafascicular nucleus and zona incerta). The projections of thermoreceptive and nociceptive cells differed. The projections of the thermoreceptive-specific cells were 20/23 to dmVPM, 21/23 to vVP, and 17/23 to Sm, whereas the projections of the NS cells were 1/9 to dmVPM, 9/9 to vVP, and 9/9 to Sm and the projections of the HPC cells were 0/8 to dmVPM, 7/8 to vVP, and 6/8 to Sm. Thus nearly all thermoreceptive cells projected to dmVPM, but almost no nociceptive cells did. Further, thermoreceptive cells projected medially within vVP (including the basal ventral medial nucleus), while nociceptive cells projected both medially and more laterally, and the ascending axons of thermoreceptive cells were concentrated in the medial mesencephalon, while the axons of nociceptive cells ascended in the lateral mesencephalon. These findings provide evidence for anatomical differences between these physiological classes of lamina I cells, and they corroborate prior anatomical localization of the lamina I TSTT projection targets in the cat. These results support evidence indicating that the ventral aspect of the basal ventral medial nucleus is important for thermosensory behavior in cats, consistent with the view that this region is a primordial homologue of the posterior ventral medial nucleus in primates.

Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch

We found a class of lamina I spinothalamic tract (STT) neurons selectively excited by iontophoretic histamine. The responses of this class of neurons parallel the pure itching sensation this stimulus elicits in humans, and match the responses of peripheral C-fibers that have similar selectivity. These neurons have distinct central conduction velocities and thalamic projections, indicating that they constitute a unique subset of STT neurons. These findings can explain why a lesion of the lateral STT disrupts itch along with pain and temperature sensations. Our findings provide strong evidence that itch is subserved by specific neural elements both peripherally and centrally.

Superficial dorsal horn neurons identified by intracutaneous histamine: chemonociceptive responses and modulation by morphine

We have investigated whether neurons in superficial laminae of the spinal dorsal horn respond to intracutaneous (ic) delivery of histamine and other irritant chemicals, and thus might be involved in signaling sensations of itch or chemogenic pain. Single-unit recordings were made from superficial lumbar dorsal horn neurons in pentobarbital sodium-anesthetized rats. Chemoresponsive units were identified using ic microinjection of histamine (3%, 1 microl) into the hindpaw as a search stimulus. All superficial units so identified [9 nociceptive-specific (NS), 26 wide-dynamic-range (WDR)] responded to subsequent ic histamine. A comparison group of histamine-responsive deep dorsal horn neurons (n = 16) was similarly identified. The mean histamine-evoked discharge decayed to 50% of the maximal rate significantly more slowly for the superficial (92.2 s +/- 65.5, mean +/- SD) compared with deep dorsal horn neurons (28. 2 s +/- 11.6). In addition to responding to histamine, most superficial dorsal horn neurons were also excited by ic nicotine (22/25 units), capsaicin (21/22), topical mustard oil (5/6), noxious heat (26/30), and noxious and/or innocuous mechanical stimuli (except for 1 unit that did not have a mechanosensitive receptive field). Application of a brief noxious heat stimulus during the response to ic histamine evoked an additive response in all but two cases, followed by transient depression of firing in 11/20 units. Intrathecal (IT) administration of morphine had mixed effects on superficial dorsal horn neuronal responses to ic histamine and noxious heat. Low morphine concentrations (100 nM to 1 microM) facilitated histamine-evoked responses (to >130% of control) in 9/24 units, depressed the responses (by >70%) in 11/24, and had no effect in 4. Naloxone reversed morphine-induced effects in some but not all cases. A higher morphine concentration (10 microM) had a largely depressant, naloxone-reversible effect on histamine responses. Responses of the same superficial neurons to noxious heat were facilitated (15/25), reduced (8/25), or unaffected (2/25) by low morphine concentrations and were depressed by the higher morphine concentration. In contrast, deep dorsal horn neuronal responses to both histamine and noxious heat were primarily depressed by low concentrations of morphine in a naloxone-reversible manner. These results indicate that superficial dorsal horn neurons respond to both pruritic and algesic chemical stimuli and thus might participate in transmitting sensations of itch and/or chemogenic pain. The facilitation of superficial neuronal responses to histamine by low concentrations of morphine, coupled with inhibition of deep dorsal horn neurons, might underlie the development of pruritus that is often observed after epidural morphine.

Physiological properties of the lamina I spinoparabrachial neurons in the rat

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (</=2 toes or pads). They were all activated by both peripheral A (mainly Adelta) and C fibers after intense transcutaneous electrical stimulation. Their discharge always increased in response to noxious natural stimuli of increasing intensities. The great majority (75%) of spino-PB neurons were nociceptive specific, i.e., they were excited only by noxious stimuli. The remaining (25%) still were excited primarily by noxious stimuli but also responded moderately to innocuous stimuli. Almost all spino-PB neurons (92%, 49/53) were activated by both mechanical and heat noxious stimuli. Among them, 35% were in addition moderately activated by noxious cold (thresholds between +20 and -10 degrees C). Only (8%, 4/53) responded exclusively to noxious heat. Spino-PB neurons clearly encoded the intensity of mechanical (n = 39) and thermal (n = 38) stimuli in the noxious range, and most of the individual stimulus-response functions were monotonic and positive up to 40/60 N. cm(-2) and 50 degrees C, respectively. For the mechanical modality, the mean threshold was 11.5 +/- 1.25 N. cm(-2) (mean +/- SE), the response increased almost linearly with the logarithm of the pressure between 10 and 60 N. cm(-2), the mean p(50) (pressure evoking 50% of the maximum response) and the maximum responsiveness were: 30 +/- 2.4 N. cm(-2) and 40.5 +/- 5 Hz, respectively. For the thermal modality, the mean threshold was 43.6 +/- 0.5 degrees C, the mean curve had a general sigmoid aspect, the steepest portion being in the 46-48 degrees C interval, the mean t(50) and the maximum responsiveness were: 47.4 +/- 0.3 degrees C and 40 +/- 4.4 Hz, respectively. Most of the spino-PB neurons tested (13/16) had their noxiously evoked responses clearly inhibited by heterotopic noxious stimuli. The mean response to noxious stimuli during heterotopic stimuli was 31.7 +/- 6.1% of the control response. We conclude that the nociceptive properties of the lamina I spino-PB neurons are reflected largely by those of PB neurons that were suggested to be involved in autonomic and emotional/aversive aspects of pain.

Spinal laminae I-II neurons in rat recorded in vivo in whole cell, tight seal configuration: properties and opioid responses

Using the in vivo whole cell recording procedure described previously, we recorded 73 neurons in laminae I and II in the lumbar spinal cord of the rat. Input impedances averaged 332 MOmega, which indicated that prior sharp electrode recordings contained a significant current shunt. Characterization of the adequate stimuli from the excitatory hindlimb receptive field indicated that 39 of 73 neurons were nociceptive, 6 were innocuous cooling cells, 20 responded maximally to brush, and 8 cells were not excited by stimulation of the skin of the hindlimb. The locations of 15 neurons were marked with biocytin. Nociceptive neurons were mostly found in lamina I and outer II, cooling cells in lamina I, and innocuous mechanoreceptive cells were mostly found in inner II or in the overlying white matter. The mu-opioid agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-Enkephalin (DAMGO) hyperpolarized 7 of 19 tested neurons with a conductance increase. This hyperpolarization was reversed by naloxone in the neurons in which it was applied. DAMGO also decreased the frequency of spontaneous PSPs in 13 neurons, 7 of which were also hyperpolarized by DAMGO. Five of the seven hyperpolarized neurons were nociceptive, responding to both heat and mechanically noxious stimuli, whereas two responded to slow, innocuous brush. These results indicate that whole cell, tight seal recordings sample a similar population of lamina I and II neurons in the rat as those found with sharp electrode recordings in cat and monkey. They further indicate that DAMGO hyperpolarizes a subset of the nociceptive neurons that have input from both heat and mechanical nociceptors and that presynaptic DAMGO effects can be observed in nociceptive neurons that are not hyperpolarized by DAMGO.

Responses of medullary dorsal horn neurons to corneal stimulation by CO(2) pulses in the rat

Corneal-responsive neurons were recorded extracellularly in two regions of the spinal trigeminal nucleus, subnucleus interpolaris/caudalis (Vi/Vc) and subnucleus caudalis/upper cervical cord (Vc/C1) transition regions, from methohexital-anesthetized male rats. Thirty-nine Vi/Vc and 26 Vc/C1 neurons that responded to mechanical and electrical stimulation of the cornea were examined for convergent cutaneous receptive fields, responses to natural stimulation of the corneal surface by CO(2) pulses (0, 30, 60, 80, and 95%), effects of morphine, and projections to the contralateral thalamus. Forty-six percent of mechanically sensitive Vi/Vc neurons and 58% of Vc/C1 neurons were excited by CO(2) stimulation. The evoked activity of most cells occurred at 60% CO(2) after a delay of 7-22 s. At the Vi/Vc transition three response patterns were seen. Type I cells (n = 11) displayed an increase in activity with increasing CO(2) concentration. Type II cells (n = 7) displayed a biphasic response, an initial inhibition followed by excitation in which the magnitude of the excitatory phase was dependent on CO(2) concentration. A third category of Vi/Vc cells (type III, n = 3) responded to CO(2) pulses only after morphine administration (>1.0 mg/kg). At the Vc/C1 transition, all CO(2)-responsive cells (n = 15) displayed an increase in firing rates with greater CO(2) concentration, similar to the pattern of type I Vi/Vc cells. Comparisons of the effects of CO(2) pulses on Vi/Vc type I units, Vi/Vc type II units, and Vc/C1 corneal units revealed no significant differences in threshold intensity, stimulus encoding, or latency to sustained firing. Morphine (0.5-3.5 mg/kg iv) enhanced the CO(2)-evoked activity of 50% of Vi/Vc neurons tested, whereas all Vc/C1 cells were inhibited in a dose-dependent, naloxone-reversible manner. Stimulation of the contralateral posterior thalamic nucleus antidromically activated 37% of Vc/C1 corneal units; however, no effective sites were found within the ventral posteromedial thalamic nucleus or nucleus submedius. None of the Vi/Vc corneal units tested were antidromically activated from sites within these thalamic regions. Corneal-responsive neurons in the Vi/Vc and Vc/C1 regions likely serve different functions in ocular nociception, a conclusion reflected more by the difference in sensitivity to analgesic drugs and efferent projection targets than by the CO(2) stimulus intensity encoding functions. Collectively, the properties of Vc/C1 corneal neurons were consistent with a role in the sensory-discriminative aspects of ocular pain due to chemical irritation. The unique and heterogeneous properties of Vi/Vc corneal neurons suggested involvement in more specialized ocular functions such as reflex control of tear formation or eye blinks or recruitment of antinociceptive control pathways.

A role for spinal lamina I neurokinin-1-positive neurons in cold thermoreception in the rat

Lamina I neurons of the spinal cord convey specific nociceptive activity to the brain. A subpopulation of lamina I cells bears substance P receptors (neurokinin-1) and recent studies have shown that these neurons encode for the intensity of noxious peripheral stimulation. Here, we report that cool thermal stimuli, applied to the hindpaw of anaesthetized rats, induce Fos expression in lamina I neurokinin-1 neurons that is graded with respect to the intensity of the thermal stimulus. Thus, as the temperature of the stimulus was reduced, both the total number of neurokinin-l-positive neurons expressing Fos and the proportion of Fos nuclei present within neurokinin-1 cells showed a significant increase. These data show that lamina I neurokinin-1 cells encode the intensity of noxious cooling of the skin. In laminae III and IV, although there was no correlation between neurokinin-1 cell activation and stimulus intensity, the total Fos count in these layers was inversely related to the depth of cooling. Thus, neurons in laminae III and IV may also play a role in thermoreception.

Thalamic relay site for cold perception in humans

The neural pathways subserving the sensation of temperature are virtually unknown. However, recent findings in the monkey suggest that the sensation of cold may be mediated by an ascending pathway relaying in the posterior part of the thalamic ventromedial nucleus (VMpo). To test this hypothesis we examined the responses of neurons to thermal stimulation of the skin and determined the perceptual effects of microstimulation in the VMpo region in awake patients undergoing functional stereotactic surgery. In 16 patients, microstimulation in the VMpo region evoked cold sensations in a circumscribed body part. Furthermore, at some of these sites thalamic neurons were found that responded to innocuous cooling of the skin area corresponding to the stimulation-evoked cold sensations. These data provide the first direct demonstration of a pathway mediating cold sensation and its location in the human thalamus.

Nociceptive and thermoreceptive lamina I neurons are anatomically distinct

Pain and temperature stimuli activate neurons of lamina I within the dorsal horn of the spinal cord, and although these neurons can be classified into three basic morphological types and three major physiological classes, earlier studies did not establish a structure/function correlation between their morphology and their physiological responses. We recorded and intracellularly labeled 38 cat lamina I neurons. All 12 fusiform cells were nociceptive-specific, responsive only to pinch and/or heat. All 11 pyramidal cells were thermoreceptive-specific, responsive only to innocuous cooling. Of ten multipolar cells, six were polymodal, responsive to heat, pinch and cold, and four were nociceptive-specific. Five unclassified cells had features consistent with this pattern. These results support the view that central pain and temperature pathways contain anatomically discrete sets of modality-selective neurons.

Anti-hyperalgesic and anti-allodynic effects of intrathecal nociceptin/orphanin FQ in rats after spinal cord injury, peripheral nerve injury and inflammation

We examined the effects of intrathecal nociceptin, the endogenous ligand for the orphan opioid receptor-like receptor, on abnormal pain-related behaviors in rats after carrageenan-induced inflammation and photochemically-induced peripheral nerve or spinal cord ischemic injury. Intrathecal nociceptin dose-dependently alleviated mechanical and cold allodynia-like behavior in the two models of neuropathic pain. The heat hyperalgesia associated with peripheral inflammation was also significantly reduced, although the efficacy of the antihyperalgesic effect of nociceptin in the inflammation model was decreased. Intrathecal nociceptin also induced significant antinociception on the tail-flick test in all three groups of rats. However, the antinociceptive effect of nociceptin was significantly reduced in rats with peripheral nerve injury. These results indicated that spinally administered nociceptin has anti-allodynic and anti-hyperalgesic effects in animal models of tonic or chronic pain of different origins. Peripheral inflammation and nerve injury may induce spinal plasticity which leads to altered potency and efficacy of nociceptin.

The cold plate as a test of nociceptive behaviors: description and application to the study of chronic neuropathic and inflammatory pain models

A cold plate apparatus was designed to test the responses of unrestrained rats to low temperature stimulation of the plantar aspect of the paw. At plate temperatures of 10 degrees C and 5 degrees C, rats with either chronic constriction injury (CCI) of the sciatic nerve or complete Freund's adjuvant (CFA) induced inflammation of the hindpaw displayed a stereotyped behavior. Brisk lifts of the treated hindpaw were recorded, while no evidence of other nociceptive behaviors could be discerned. The most consistent responses were obtained with a plate temperature of 5 degrees C in three 5-min testing periods, separated by 10-min intervals during which the animals were returned to a normal environment. Concomitantly to cold testing, the rats were evaluated for their response to heat (plantar test) and mechanical (von Frey hairs) stimuli. In both injury models, while responses to heat stimuli had normalized at 60 days post-injury, a clear lateralization of responses to cold was observed throughout the entire study period. Systemic lidocaine, clonidine, and morphine suppressed responses to cold in a dose-related fashion. At doses that did not affect motor or sensory behavior, both lidocaine and its quaternary derivative QX-314 similarly reduced paw lifts, suggesting that cold hyperalgesia is in part due to peripheral altered nociceptive processing. Clonidine was more potent in CCI then in CFA rats in reducing the response to cold. Paradoxically, clonidine increased the withdrawal latencies to heat in the CCI hindpaw at 40 days and thereafter, at a time when both hindpaws had the same withdrawal latencies in control animals. Morphine was also more potent on CCI than CFA cold responses, indicating that, chronically, CFA-induced hyperalgesia might be opiate resistant. Evidence for tonic endogenous inhibition of cold hyperalgesia was obtained for CFA rats, when systemic naltrexone significantly increased the number of paw lifts; this was not found in rats with CCI. At 60 days, neither morphine nor naltrexone affected cold-induced paw lifting in CFA rats, suggesting that the neuronal circuit mediating cold hyperalgesia in these animals had become opiate insensitive. In conclusion, the cold plate was found to be a reliable method for detecting abnormal nociceptive behavior even at long intervals after nerve or inflammatory injuries, when responses to other nociceptive stimuli have returned to near normal. The results of pharmacological studies suggest that cold hyperalgesia is in part a consequence of altered sensory processing in the periphery, and that it can be independently modulated by opiate and adrenergic systems.

Quantitative analysis of orofacial thermoreceptive neurons in the superficial medullary dorsal horn of the rat

Surprisingly little is known concerning the central processing of innocuous thermal somatosensory information. The aim of the present study was to obtain quantitative data on the characteristics of neurons in the rat superficial medullary dorsal horn (sMDH) that responded to innocuous thermal stimulation of the rat's face and tongue. Single-unit extracellular recordings were obtained in chloralose-urethane anesthetized rats. A total of 153 thermoreceptive neurons was studied. Of these, 146 were excited by cooling and inhibited by warming and were classified as COLD cells. The remaining seven cells were excited by innocuous warming of the skin or tongue. Of 123 COLD cells tested, 33% were excited by touch and 22% by pinch stimuli delivered to the thermoreceptive field. Of the 50 COLD cells tested, 46% were excited also by noxious heating (> or = 50 degrees C for 5 s). Most (82/121) of the receptive fields were located on the upper lip, 25 on the tongue, and most of the remaining on the lower lip. Receptive fields were generally small (1-5 mm2). In some experiments, electrical stimulation in the thalamus was performed, and nine COLD cells could be activated antidromically. The responses of 38 COLD cells to incremental 5 degrees C cooling steps were examined quantitatively. Thermal stimuli were applied to facial or lingual receptive fields of sMDH neurons with a computer-controlled Peltier thermode starting from 33 degrees C, decreasing to 8 or 3 degrees C, and returning to 33 degrees C. Most COLD cells (26/38) had both static and dynamic responses; 7 had mainly dynamic and 5 mainly static responses to step decreases in temperature. Rat sMDH COLD cells could be classified into three groups depending on their stimulus-response functions. The first group (Type 1, n = 19) had a bell-shaped static stimulus response function. The second group (Type 2) had a high maintained or increasing static firing rate as the temperature decreased < 18 degrees C (n = 10). Type 3 COLD cells had mainly dynamic properties (n = 7). Many of the cells in all groups were excited by noxious mechanical stimulation. Type 2 cells differed from the other two groups in that most did not respond to noxious thermal stimuli (hot) and many responded to innocuous tactile stimuli. Neurons from each of the three groups of COLD cells could be activated antidromically from contralateral thalamus. These data suggest that there is little central processing of thermal information at the first central synapse for Type 1 neurons, however, the responses of the other two types may be due to central processing and convergence. The demonstration of rat sMDH COLD cells with distinctive stimulus-response functions to thermal shifts suggests separate functional roles of these neurons in the ascending thermal sensory pathway.

Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

The present study evaluated the ability of humans to discriminate temperature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxillary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detecting temperature decreases were determined using an adaptive psychophysical paradigm. Visual analogue scale (VAS) ratings of cold and pain sensation were also recorded at 5-min intervals throughout each session. A second group of five subjects performed a similar detection task, but in this case using classical psychophysical techniques (method of constant stimuli) and adapting temperatures of 22 degrees, 16 degrees, 10 degrees and 6 degrees C. These subjects described the quality of the detected change in sensation, in addition to rating overall cold and pain sensation throughout the session. Detection thresholds were 0.27 degrees, 0.48 degrees, 4.8 degrees, 8.0 degrees and >10.0 degrees C for baselines of 22 degrees, 16 degrees, 10 degrees, 6 degrees and 0 degrees C, respectively, indicating that discrimination was better in the innocuous cool (22 degrees and 16 degrees C) than in the noxious and near-noxious cold (10-0 degrees C) range (P < 0.05). Tonic adapting temperatures of 22 degrees and 16 degrees C were always rated as cool but not painful, whereas adapting temperatures of 10 degrees and 6 degrees were sometimes and 0 degrees C usually rated as painful. Phasic temperature decreases from 22 degrees and 16 degrees C always produced cooling sensations, whereas decreases from baselines of 10 degrees and 6 degrees C produced primarily sensations of painful and non-painful prickle. These data suggest that different afferent channels mediate cool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.

Fine structural organization of spinothalamic and trigeminothalamic lamina I terminations in the nucleus submedius of the cat

We examined lamina I trigemino- and spinothalamic tract (TSTT) terminals labeled with Phaseolus vulgaris leucoagglutinin in the nucleus submedius (Sm), a nociceptive relay in the cat's thalamus. Volume-rendered (three-dimensional) reconstructions of ten lamina I TSTT terminals identified with light and electron microscopy were built from serial ultrathin sections by computer, which enabled the overall structures of the terminal complexes to be characterized in detail. Two fundamentally different terminations were observed: compact clusters of numerous boutons, which predominate in the dense focus of a lamina I terminal field in the Sm, and boutons-of-passage, which are present throughout the terminal field and predominate in its periphery. Reconstructions of cluster terminations reveal that all boutons of each cluster make synaptic contact with protrusions and branch points on a single dendrite and involve presynaptic dendrites (PSDs) in triadic arrangements, providing a basis for the secure relay of sensory information. In contrast, reconstructions show that boutons-of-passage are generally characterized by simple contacts with PSDs, indicating an ascending inhibitory lamina I influence. These different synaptic arrangements are consistent with physiological evidence indicating that the morphologically distinct nociceptive-specific and thermoreceptive-(cold)-specific lamina I TSTT neurons terminate differently within the Sm. Thus, a suitable structural substrate exists in the cat's Sm for the inhibitory effect of cold on nociception, a behavioral and physiological phenomenon of fundamental significance. We conclude that the Sm is more than a simple relay for nociception, and that it may be an integrative comparator of ascending modality-selective information that arrives from neurons in lamina I.

Morphological classes of spinothalamic lamina I neurons in the cat

We examined the morphology and distribution of retrogradely labeled spinothalamic tract (STT) neurons in lamina I (the marginal zone) of the spinal dorsal horn after large injections of cholera toxin subunit B (CTb) or Fast Blue (FB) into the contralateral thalamus of cats. Based on the shape and orientation of the somata and proximal dendrites in horizontal sections, three distinct cell types were identified: (1) fusiform cells with small, spindle-shaped somata and bipolar, longitudinal dendritic arbors; (2) pyramidal cells with triangular somata and three main dendritic origins with primarily longitudinal arborizations; and (3) multipolar cells with larger, multiangular somata and four or more radiating dendritic arbors directed both longitudinally and mediolaterally. These three morphological types differed significantly in the number of primary dendrites and the size of the somata. Subclasses of multipolar cells were noted. Nearly all cells could be categorized into these three classes consistently in horizontal sections. A small number of cells with transitional shapes or with small, round somata were unclassified. The proportional distributions of these cell types were found to vary over the length of the spinal cord (from the third cervical through the coccygeal segments) in three cats. The overall proportions of cell types were 34% fusiform, 36% pyramidal, 25% multipolar, and 5% unclassified. The proportions of pyramidal and multipolar cells were strikingly higher within the C7-8 and L6-7 segments and lowest in the thoracic segments. In contrast, fusiform cells formed about 20% of the labeled lamina I STT population in the C7-8 and L6-7 segments but more than 60% in thoracic segments. Across all nine cats, the proportions were similar within the cervical (C5-8) and lumbosacral (L5-S1) enlargements, although considerable interanimal variability was noted. These distinct morphological types of lamina I STT cells with differential longitudinal distributions probably have different functional roles. They may correspond to the three main physiological classes of lamina I STT cells.

Comparisons of dose-dependent effects of systemic morphine on flexion reflex components and operant avoidance responses of awake non-human primates

Electromyographic activity and the force of reflex and operant responses were recorded following administration of morphine. Low doses facilitated reflex responses to input from A-delta afferents but not from A-beta input. Higher doses inhibited A-delta responses but not A-beta responses. Operant avoidance responses to visual cues were unchanged. Thus, depending on the dose, nociceptive reflexes were facilitated or inhibited, without associated effects on non-nociceptive input or on motor output.

The thermal grill illusion: unmasking the burn of cold pain

In Thunberg's thermal grill illusion, first demonstrated in 1896, a sensation of strong, often painful heat is elicited by touching interlaced warm and cool bars to the skin. Neurophysiological recordings from two classes of ascending spinothalamic tract neurons that are sensitive to innocuous or noxious cold showed differential responses to the grill. On the basis of these results, a simple model of central disinhibition, or unmasking, predicted a quantitative correspondence between grill-evoked pain and cold-evoked pain, which was verified psychophysically. This integration of pain and temperature can explain the thermal grill illusion and the burning sensation of cold pain and may also provide a basis for the cold-evoked, burning pain of the classic thalamic pain syndrome.

Differential action of morphine and various opioid agonists on thermal allodynia and hyperalgesia in mononeuropathic rats

In a rat model of mononeuropathy produced by 4 loose ligatures around the common sciatic nerve, the effects of 1 mg/kg morphine and mu-, delta- and kappa-agonists, DAMGO (2 and 3 mg/kg), BUBUC (3 and 6 mg/kg) and U-69,593 (1.5 mg/kg), were evaluated by measuring the struggle latency (SL in sec) to immersion of the paw on the nerve-injured side in a cold (10 degrees C) or hot (42 degrees C, 44 degrees C, and 46 degrees C) water bath. Experiments were performed 2 weeks after surgery. The agonists were used at doses that produced potent antinociceptive effects on the vocalization test in this model. At 46 degrees C (clearly in the noxious range), only morphine and DAMGO had significant effects. The effect of morphine lasted for more than 2 h with a maximum at 40 min (SL = 13.8 +/- 1.6 sec, 252% of control values). For 2 and 3 mg/kg DAMGO, the dose-related effect lasted for 120 min at least, with a maximum at 20-40 min (SL = 6.0 +/- 0.5 and 8.8 +/- 0.7 sec, 148% and 170% of control values, respectively). These effects were more potent and prolonged than in normal rats and were reversed by 0.1 mg/kg naloxone i.v. By contrast, morphine and all selective agonists failed to relieve the abnormal reactions to 10 degrees C, 42 degrees C (in the non-noxious range) and 44 degrees C (at the noxious threshold) stimuli. Our data illustrate a differential effect of opioids on nociceptive tests based on different stimulus modalities and intensities in this model of mononeuropathic pain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of systemic morphine on lamina I spinothalamic tract neurons in the cat

Lamina I spinothalamic tract (STT) neurons are an integral component of the central representation of pain and temperature and thus their sensitivity to various analgesics needs to be examined. In the present study, the effects of successive, cumulative doses (0.125-2.0 mg/kg) of intravenous morphine sulfate on the quantitative stimulus-response properties of nociceptive lamina I STT cells have been tested in the intact, barbiturate-anesthetized cat. Both nociceptive-specific (n = 7) and multireceptive (heat, pinch and cold sensitive; n = 7) lamina I STT cells were inhibited in a dose-dependent manner. Parallel dose-dependent effects on responses to noxious heat and pinch were generally observed that reduced ongoing discharge levels and the slopes of the stimulus-response functions. However, non-STT lamina I cells (n = 5) differed significantly; the responses of one multireceptive (heat, pinch and cold-sensitive) cell and the responses to pinch of 3 of 4 wide dynamic range cells were not inhibited. In addition, two-thirds of the nociceptive lamina I STT cells showed enhanced responses at the lowest dose of morphine (0.125 mg/kg). These results contrast with the varied effects of morphine reported for superficial dorsal horn cells with uncharacterized projections and they support the role of lamina I STT cells in pain. Furthermore, these observations are consistent with previous findings indicating that lamina I STT neurons are a distinct subpopulation of lamina I cells. These results support previous evidence that opiatergic modulation of sensory activity in lamina I is functionally organized.

Morphine differentially modulates nociceptive input in the superficial versus the deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in the rat

Morphine enhanced the noxious thermal stimulus-evoked responses in 4/13 (31%) selectively nocireceptive neurons in the superficial dorsal horn, inhibited the responses in 4/13 (31%) neurons and produced a biphasic effect in 2/13 (17%) neurons. Naloxone antagonized these effects in 7/7 neurons. In contrast, morphine produced a naloxone reversible reduction in the nociceptive responses of 4/4 (100%) multireceptive neurons in the deeper dorsal horn of the medulla. The data are interpreted to indicate that opiates may differentially modulate nociceptive input in the superficial versus the deeper dorsal horn.

Propriospinal input to thoracolumbar sympathetic nuclei from cervical and lumbar lamina I neurons in the cat and the monkey

The possibility that specific thermoreceptive and nociceptive influences on sympathetic outflow are conveyed directly to spinal sympathetic regions by lamina I neurons was investigated anatomically with the immunofluorescent PHA-L technique in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-II in the cervical (C6-8) or lumbar (L6-7) enlargement. Bilateral (symmetric) terminal arborizations were observed (with an ipsilateral predominance) in the intermediolateral, intermediomedial, and intervening regions of the thoracolumbar intermediate zone. In serial horizontal sections, patches of terminal labeling appeared at regular longitudinal intervals in the intermediolateral region. Longitudinally coursing fibers that had multiple varicosities and gave off small terminal branches were observed in the intermediolateral and the intermediomedial regions. Mediolateral strips of labeling that extended from labeling in the intermediolateral region to labeling in the intermediomedial region occurred at fairly regular longitudinal intervals. Because the longitudinal distribution of these terminations corresponds very well with the characteristic (ladder-like) longitudinal pattern of organization of the neuropil of the thoracolumbar sympathetic nuclei, i.e., the principal part of the intermediolateral cell column, the central autonomic n., and the intervening n. intercalatus, it is inferred that these lamina I terminations occur within these nuclei. After cervical injections, the labeling was most dense in the upper thoracic T2-4 spinal cord segments in both the cat and the monkey; labeling was also present in the T10-12 segments. After lumbar injections, labeling in the cat was located in the L4 segment; labeling in the monkey was present in the T4-6 and T10-12 segments. The labeling obtained was much more dense in the monkey than in the cat. These observations reveal a spinal lamina I projection that could provide a direct pathway for the somatosympathetic reflex effects of thermal and noxious stimuli. Considered together with reports that lamina I and the sympathetic nuclei both receive descending input from certain key autonomic regions, this result emphasizes the importance of lamina I for homeostasis, in addition to its probable roles in behavioral arousal, affect, and sensation. These observations thus support the proposed concept that lamina I processes and distributes in a functionally specific manner the sensory input relevant to the physiological status of the tissues and organs of the entire organism.

Morphine alters the firing of cold-receptive neurons in the superficial dorsal horn of the medulla in the rat

Effects of morphine (1-3 mg/kg, i.v.) were tested on the innocuous cold-receptive input in the superficial dorsal horn of the medulla. The static activity of most cold-receptive (cold-specific) neurons (12/16) was reduced, whereas an enhancement (4/16) was observed in the remaining neurons. Naloxone (200 micrograms/kg, i.v.) reversed, partially or completely, the effects of morphine in 9/12 cold-receptive neurons, and enhanced the static activity of some cold-receptive neurons. Static activity, at different adapting temperatures, during a warming (10 degrees C-->40 degrees C) and a cooling (40 degrees C-->10 degrees C) sequence at steps of 5 degrees C was reduced by morphine. The effects of morphine were also tested on the static as well as the dynamic responses of 9 cold-receptive neurons. The effects of morphine on the dynamic responses were not dependent on the static firing frequency. Morphine produced similar effects, excitatory or inhibitory, on the static as well as the dynamic responses of 7/9 neurons whether the static firing frequency was high (17-33 Hz) or low (< 12 Hz). However, morphine effects on static and dynamic responses were different in the remaining 2 neurons (high static firing frequency). We suggest that the predominantly inhibitory effect of morphine on the innocuous cold receptive input in the medullary dorsal horn may explain the inhibitory effect on the perception of cooling stimuli by systemic morphine in behavioral studies.

Afferent modulation of warmth sensation and heat pain in the human hand

The hands of 14 normal humans were used to determine the somatotopic organization of the modulation of warmth sensation and heat pain by different forms of cutaneous stimuli. Test stimuli were 5-sec heat pulses ranging from 36 degrees to 51 degrees C, delivered to the fingerpads of digits 1, 2, 4, and 5 with a contact thermode. Conditioning stimuli (15 sec) bracketed the test stimuli and included vibration, noxious and innocuous heat, cold, and electrical pulses delivered to the fingerpads of digits that were adjacent or nonadjacent to the tested digits. Noxious (48 degrees +/- 1.3 degrees C), but not innocuous (43 degrees C), heat stimuli increased the perceived magnitude estimation of innocuous test stimuli (36-43 degrees C) by 20-37% when delivered to adjacent, but not to nonadjacent, digits. No other conditioning stimuli had any effect on the intensity of warmth perception. In contrast, both noxious and innocuous heat or electrical conditioning reduced the magnitude estimation of noxious (50-51 degrees C), but not innocuous, test pulses by 12-22% when delivered to adjacent digits. Conditioning of nonadjacent digits was significantly less effective. The analgesic effects of noxious and innocuous conditioning were approximately equal. Vibratory (120 Hz, 3.5 microns) and cold (15 degrees C) conditioning stimuli were ineffective. The results are consistent with a dermatomal somatotopic organization of tactile and heat modulatory influences on warmth sensation and heat pain. The results further suggest that the neural mechanism subserving warmth mediate a negative feedback influence on heat pain intensity.

Photochemically induced transient spinal ischemia induces behavioral hypersensitivity to mechanical and cold stimuli, but not to noxious-heat stimuli, in the rat

Pain-like sensory disorders lasting several days were observed in rats after transient ischemia in the lumbar region of the spinal cord. The ischemia was induced with a recently developed photochemical technique. Rats exhibited strong allodynia, a pain-like reaction to innocuous stimuli, to mechanical stimulation of the caudal trunk, hind limbs, and hind paws, which were areas innervated by the ischemic spinal segments. The rats also expressed hypersensitivity to cold stimuli. However, no changes in sensitivity to noxious heat could be detected with the hot-plate test. Furthermore, no morphological damage could be observed in the spinal cord at the light microscopic level in the majority of rats after transient spinal cord ischemia. The present results indicate that allodynia-like symptoms to mechanical stimuli after spinal ischemia may be mediated by myelinated afferents and could be associated with dysfunction of inhibitory transmission in the spinal cord. We suggest that this pain-related syndrome after spinal ischemia could be considered as an animal model of painful states of spinal origin.

Demonstration of dynorphin A 1–8 immunoreactive axons contacting spinal cord projection neurons in a rat model of peripheral inflammation and hyperalgesia

Using a double-labeling technique, we evaluated the input of afferents immunoreactive for dynorphin peptide onto a population of lumbar spinal neurons contributing to the spinoparabrachial tract in rats with 1 inflamed hind paw. We found that the frequency and distribution with which dynorphin immunoreactive varicosities were in apposition to projection neurons varied according to neuron location. In particular, neurons in the superficial dorsal horn and neck of the dorsal horn receive a high degree of dynorphin input. We also determine that unilateral peripheral inflammation is associated with both an increase in the number of projection neurons receiving detectable DYN input and in the frequency of this input onto a given neuron, with the largest increase seen in the superficial dorsal horn. Since almost all superficial dorsal horn neurons contributing to the spinoparabrachial tract respond either exclusively or maximally to noxious stimulation, our data supports dynorphin's involvement in nociception.