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

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.

Cold Stress Effects on Exposure Tolerance and Exercise Performance

Cold weather can have deleterious effects on health, tolerance, and performance. This paper will review the physiological responses and external factors that impact cold tolerance and physical performance. Tolerance is defined as the ability to withstand cold stress with minimal changes in physiological strain. Physiological and pathophysiological responses to short-term (cold shock) and long-term cold water and air exposure are presented. Factors (habituation, anthropometry, sex, race, and fitness) that influence cold tolerance are also reviewed. The impact of cold exposure on physical performance, especially aerobic performance, has not been thoroughly studied. The few studies that have been done suggest that aerobic performance is degraded in cold environments. Potential physiological mechanisms (decreases in deep body and muscle temperature, cardiovascular, and metabolism) are discussed. Likewise, strength and power are also degraded during cold exposure, primarily through a decline in muscle temperature. The review also discusses the concept of thermoregulatory fatigue, a reduction in the thermal effector responses of shivering and vasoconstriction, as a result of multistressor factors, including exhaustive exercise.

Genes, molecules and patients--emerging topics to guide clinical pain research

This review selectively explores some areas of pain research that, until recently, have been poorly understood. We have chosen four topics that relate to clinical pain and we discuss the underlying mechanisms and related pathophysiologies contributing to these pain states. A key issue in pain medicine involves crucial events and mediators that contribute to normal and abnormal pain signaling, but remain unseen without genetic, biomarker or imaging analysis. Here we consider how the altered genetic make-up of familial pains reveals the human importance of channels discovered by preclinical research, followed by the contribution of receptors as stimulus transducers in cold sensing and cold pain. Finally we review recent data on the neuro-immune interactions in chronic pain and the potential targets for treatment in cancer-induced bone pain.

Spinal processing of noxious and innocuous cold information: differential modulation by the periaqueductal gray

In addition to cold being an important behavioral drive, altered cold sensation frequently accompanies pathological pain states. However, in contrast to peripheral mechanisms, central processing of cold sensory input has received relatively little attention. The present study characterized spinal responses to noxious and innocuous intensities of cold stimulation in vivo and established the extent to which they are modulated by descending control originating from the periaqueductal gray (PAG), a major determinant of acute and chronic pain. In lightly anesthetized rats, hindpaw cooling with ethyl chloride, but not acetone, was sufficiently noxious to evoke withdrawal reflexes, which were powerfully inhibited by ventrolateral (VL)-PAG stimulation. In a second series of experiments, subsets of spinal dorsal horn neurons were found to respond to innocuous and/or noxious cold. Descending control from the VL-PAG distinguished between activity in nociceptive versus non-nociceptive spinal circuits in that innocuous cold information transmitted by non-nociceptive class 1 and wide-dynamic-range class 2 neurons remained unaltered. In contrast, noxious cold information transmitted by class 2 neurons and all cold-evoked activity in nociceptive-specific class 3 neurons was significantly depressed. We therefore demonstrate that spinal responses to cold can be powerfully modulated by descending control systems originating in the PAG, and that this control selectively modulates transmission of noxious versus innocuous information. This has important implications for central processing of cold somatosensation and, given that chronic pain states are dependent on dynamic alterations in descending control, will help elucidate mechanisms underlying aberrant cold sensations that accompany pathological pain states.

Behavioral and anatomical characterization of the bilateral sciatic nerve chronic constriction (bCCI) injury: correlation of anatomic changes and responses to cold stimuli

Background

Unilateral constrictive sciatic nerve injury (uCCI) is a common neuropathic pain model. However, the bilateral constrictive injury (bCCI) model is less well studied, and shows unique characteristics. In the present study, we sought to correlate effects of bCCI on nocifensive responses to cold and mechanical stimuli with selected dorsal horn anatomic markers. bCCI or sham ligation of both rat sciatic nerves were followed up to 90 days of behavioural testing. Additional rats sacrificed at 15, 30 and 90 days were used for anatomic analyses. Behavioural tests included hindpaw withdrawal responses to topical acetone, cold plate testing, an operant thermal preference task and hindpaw withdrawal thresholds to mechanical probing.

Results

All nocifensive responses to cold increased and remained enhanced for >45 days. Mechanical withdrawal thresholds decreased for 25 days only. Densitometric analyses of immunoperoxidase staining in the superficial dorsal horn at L4-5 revealed decreased cholecystokinin (CCK) staining at all times after bCCI, decreased mu opiate receptor (MOR) staining, maximal at 15 days, increased neuropeptide Y (NPY) staining only at days 15 and 30, and increased neurokinin-1 receptor (NK-1R) staining at all time points, maximal at 15 days. Correlation analyses at 45 days post-bCCI, were significant for individual rat nocifensive responses in each cold test and CCK and NK-1R, but not for MOR or NPY.

Conclusions

These results confirm the usefulness of cold testing in bCCI rats, a new approach using CCI to model neuropathic pain, and suggest a potential value of studying the roles of dorsal horn CCK and substance P in chronic neuropathic pain. Compared to human subjects with neuropathic pain, responses to cold stimuli in rats with bCCI may be a useful model of neuropathic pain.

Neurons in Superficial Trigeminal Subnucleus Caudalis Responsive to Oral Cooling, Menthol, and Other Irritant Stimuli

The recent discoveries of cold-sensitive transient receptor potential (TRP) channels prompted us to investigate the responses of neurons in trigeminal subnucleus caudalis (Vc) to intraoral cooling and agonists of TRPM8 and TRPA1. Single units responsive to lingual cooling were recorded in superficial laminae of Vc in thiopental-anesthetized rats. All units responded to noxious heat and 88% responded to menthol. Responses increased with menthol concentration from 0.1 to 1% (6.4–64 mM) and plateaued at 10% (640 mM). Noxious cold-evoked responses were significantly enhanced after menthol in a concentration-dependent manner. Constant-flow application of 1% menthol elicited a phasic discharge that adapted over 2–8 min and significantly enhanced subsequent cold-evoked but not heat-evoked responses; vehicle (10% ethanol) was ineffective. Reapplication of menthol 15 min later elicited a significantly reduced response (self-desensitization). Vc units were similarly excited phasically by 1% menthol dissolved in 40% ethanol. The 40% ethanol briefly excited Vc units during the first minute and reduced subsequent responses to noxious heat and cold while exhibiting neither self-desensitization nor cross-desensitization to menthol. Menthol cross-desensitized Vc responses to 40% ethanol. Most menthol-responsive units also responded to the TRPA1 agonists cinnamaldehyde and mustard oil, and the TRPV1 agonist capsaicin. Units in superficial Vc receive convergent input from primary afferents that express TRPM8, TRPA1, and/or TRPV1 channels, either directly or indirectly via intersubnuclear pathways. The convergent nature of these units suggests a general role in signaling noxious stimuli.

Respiratory responses to cold water immersion: neural pathways, interactions, and clinical consequences awake and asleep

The ventilatory responses to immersion and changes in temperature are reviewed. A fall in skin temperature elicits a powerful cardiorespiratory response, termed “cold shock,” comprising an initial gasp, hypertension, and hyperventilation despite a profound hypocapnia. The physiology and neural pathways of this are examined with data from original studies. The respiratory responses to skin cooling override both conscious and other autonomic respiratory controls and may act as a precursor to drowning. There is emerging evidence that the combination of the reestablishment of respiratory rhythm following apnea, hypoxemia, and coincident sympathetic nervous and cyclic vagal stimulation appears to be an arrhythmogenic trigger. The potential clinical implications of this during wakefulness and sleep are discussed in relation to sudden death during immersion, underwater birth, and sleep apnea. A drop in deep body temperature leads to a slowing of respiration, which is more profound than the reduced metabolic demand seen with hypothermia, leading to hypercapnia and hypoxia. The control of respiration is abnormal during hypothermia, and correction of the hypoxia by inhalation of oxygen may lead to a further depression of ventilation and even respiratory arrest. The immediate care of patients with hypothermia needs to take these factors into account to maximize the chances of a favorable outcome for the rescued casualty.

Fos induction in lamina I projection neurons in response to noxious thermal stimuli

Lamina I of the spinal cord contains many projection neurons: the majority of these are activated by noxious stimulation, although some respond to other stimuli, such as innocuous cooling. In the rat, approximately 80% of lamina I projection neurons express the neurokinin 1 (NK1) receptor, on which substance P acts. Lamina I neurons can be classified into three main morphological classes: pyramidal, fusiform and multipolar cells. It has been reported that in the cat, pyramidal cells respond to innocuous cooling, and whilst both fusiform and multipolar cells are activated by noxious mechanical and heat stimuli, only cells in the latter group respond to noxious cold [Nat Neurosci 1 (1998) 218]. However, we have previously shown that NK1 receptor-immunoreactive projection neurons belonging to each morphological class are equally likely to up-regulate the transcription factor Fos after noxious chemical stimulation, and that the density of innervation by substance P-containing (nociceptive) afferents is similar for cells of each type [J Neurosci 22 (2002) 4103]. This suggests that the morphological-physiological correlation that has been reported in the cat may not apply in the rat. We have tested this further by examining Fos expression in lamina I spinoparabrachial neurons in the rat after application of noxious heat or noxious cold stimuli under general anesthesia. Following noxious heat, 57-69% of NK1 receptor-immunoreactive spinoparabrachial neurons expressed Fos, and the proportion did not differ significantly between morphological groups. However, after noxious cold stimulation Fos was present in 63% of multipolar neurons, but only 19-26% of fusiform or pyramidal cells. These results suggest that although most NK1 receptor-expressing spinoparabrachial neurons are activated by noxious stimuli, responsiveness to noxious cold is significantly more common in those of the multipolar type. There therefore appears to be a correlation between morphology and function for lamina I projection neurons in the rat.

Effects of intrathecal BAM22 on noxious stimulus-evoked c-fos expression in the rat spinal dorsal horn

The effects of bovine adrenal medulla 22 (BAM22), a cleaved product of proenkephalin A, were investigated on the noxious stimulus-evoked expressions of spinal c-fos-like immunoreactivity (FLI). Heat (51 degrees C) applied to the tail evoked FLI predominantly in laminae I-II of the sacral spinal cord. Intrathecal (i.t.) BAM22 at a dose of 7 nmol decreased the expressions of the heat-evoked FLI by 68%, 64% and 56% in laminae I-II, III-IV and V-VI, respectively, and the decrease pattern was comparable to that induced by i.t. morphine (10 mug). Naloxone (1 mg/kg, i.p.) significantly enhanced the heat-evoked FLI in laminae III-VI, prevented the morphine-induced inhibition, and decreased the potencies of BAM22 in laminae I-II and V-VI by 23-40%. Higher dose of naloxone (10 mg/kg, i.p.) also partially reduced the BAM22-induced suppression. Following intraplantar injection of formalin (2.5%), FLI neurons were preferentially distributed not only in laminae I-II but also in laminae III-IV and V-VI of segments L4-L5. Pretreatment with BAM22 (7 nmol, i.t.) reduced the formalin-evoked FLI neurons by 72%, 61% and 58%, in laminae I-II, III-IV and V-VI, respectively. Naloxone (1 mg/kg. i.p.) enhanced the formalin-evoked expressions of FLI in laminae III-VI and decreased the potencies of BAM22 by 22-38% in laminae I-II and V-VI. The present study provided evidence at a cellular level showing that opioid and non-opioid effects of BAM22 on nociceptive processing in acute and persistent pain models were associated with modulation of noxious stimulus-evoked activity of the spinal dorsal horn neurons.

Substance P (NK1) and somatostatin (sst2A) receptor immunoreactivity in NTS-projecting rat dorsal horn neurones activated by nociceptive afferent input

Spinal neurones that receive inputs from primary afferent fibres and have axons projecting supraspinally to the medulla oblongata may represent a pathway through which nociceptive and non-nociceptive peripheral stimuli are able to modulate cardiorespiratory reflexes. Expression of the neurokinin-1 (NK1) receptor is believed to be an indicator of lamina I cells that receive nociceptive inputs from substance P releasing afferents, and similarly, sst2A receptor expression may be a marker for neurones receiving somatostatinergic inputs. In this study, immunoreactivity for these two receptors was investigated in rat spinal neurones retrogradely labelled by injections of cholera toxin B or Fluorogold into the nucleus of the solitary tract (NTS). In addition, nociceptive activation of these labelled cells was studied by immunodetection of Fos protein in response to cutaneous and visceral noxious chemical stimuli. NK1 and sst2A receptors in lamina I were localised to mainly separate populations of retrogradely labelled cells with fusiform, flattened and pyramidal morphologies. Examples of projection neurones expressing both receptors were, however observed. With visceral stimulation, many retrogradely labelled cells expressing c-fos were immunoreactive for the NK1 receptor, and a smaller population was sst2A positive. In contrast, with cutaneous stimulation, only NK1 positive retrogradely labelled cells showed c-fos expression. These data provide evidence that lamina I neurones receiving noxious cutaneous and visceral stimuli via NK1 receptor activation project to NTS and so may be involved in coordinating nociceptive and cardiorespiratory responses. Moreover, a subpopulation of projection neurones that respond to visceral stimuli may receive somatostatinergic inputs of peripheral, local or supraspinal origins.

Cooling of the urinary bladder activates neurons in the dorsal horn of the spinal cord

Although visceral innocuous cold receptors have been documented, the central termination of their afferents is unknown. We used menthol solution (0.6 mM) to obtain selective activation of cold receptors in the urinary bladder of rats. Innocuous cold stimulation induced Fos expression in a population of neurons in the superficial dorsal horn of L6-S1 segments of the spinal cord. Neurons in other regions of the spinal cord, e.g. the lumbar parasympathetic nucleus or the dorsal commissure region, were activated to a similar degree by menthol and control infusions, indicating a response to bladder filling. Our results are consistent with the proposal that subsets of modality-specific dorsal horn neurons convey specific information regarding the exteroceptive and interoceptive state of the animal.

Effects of the novel analgesic, cizolirtine, in a rat model of neuropathic pain

Cizolirtine (5-9[(N,N-dimethylaminoethoxy)phenyl]methyl0-1-methyl-1H-pyrazol citrate) is a centrally acting analgesic with a currently unknown mechanism of action, whose efficacy has been demonstrated in various models of acute and inflammatory pain in rodents. Further studies were performed in order to assess its potential antinociceptive action in a well-validated model of neuropathic pain, i.e. that produced by unilateral sciatic nerve constriction in rats. Animals were subjected to relevant behavioural tests based on mechanical (vocalization threshold to paw pressure) and thermal (struggle latency to paw immersion in a cold (10 degrees C) water bath) stimuli, 2 weeks after sciatic nerve constriction, when pain-related behaviour was fully developed. Acute pretreatment with 2.5-10 mg/kg p.o. of cizolirtine reversed both mechanical and thermal allodynia. These effects were antagonized by prior injection of the alpha(2)-adrenoceptor antagonist idazoxan (0.5 mg/kg i.v.), but not the opioid receptor antagonist naloxone (0.1 mg/kg i.v.). On the other hand, cizolirtine (10 mg/kg p.o.) produced no motor deficits in animals using the rotarod test. Our study showed that cizolirtine suppressed pain-related behavioural responses to mechanical and cold stimuli in neuropathic rats, probably via an alpha(2)-adrenoceptor-dependent mechanism. These results suggest that cizolirtine may be useful for alleviating some neuropathic somatosensory disorders, in particular cold allodynia, with a reduced risk of undesirable side effects.

Noxious-evoked c-fos expression in brainstem neurons immunoreactive for GABAB, mu-opioid and NK-1 receptors

Modulation of nociceptive transmission at the brainstem involves several neurochemical systems. The precise location and specific characteristics of nociceptive neurons activated in each system was never reported. In this study, the presence of GABA(B), mu-opioid, and neurokinin-1 (NK-1) receptors in brainstem nociceptive neurons was investigated by double-immunocytochemical detection of each receptor and noxious-evoked induction of the c-fos proto-oncogene. Noxious cutaneous mechanical stimulation significantly increased the proportions of neurons double-labelled for Fos and GABA(B) receptors in several brainstem regions, namely, the reticular formation of the caudal ventrolateral medulla (VLMlat and VLMrf), lateral reticular nucleus, spinal trigeminal nucleus, pars caudalis (Sp5C), nucleus of the solitary tract, dorsal reticular nucleus, ventral reticular nucleus, raphe obscurus nucleus and dorsal parabrachial nucleus (DPB). For mu-opioid receptors, the proportions of double-labelled neurons in noxious-stimulated animals were higher than in controls only in the VLMlat, VLMrf, Sp5C, DPB and A5 noradrenergic cell group. As for the NK-1 receptor, no significant differences were found between control and stimulated animals. According to these results, neurons expressing GABA(B), mu-opioid and NK-1 receptors at several pain control centres of the brainstem are differentially involved in processing nociceptive mechanical input. The data provide the definition of new supraspinal targets for selective modulation of nociceptive neurons in order to define better strategies of pain control.

Enhanced responses of spinal dorsal horn neurons to heat and cold stimuli following mild freeze injury to the skin

The effects of a mild freeze injury to the skin on responses of nociceptive dorsal horn neurons to cold and heat stimuli were examined in anesthetized rats. Electrophysiological recordings were obtained from 72 nociceptive spinal neurons located in the superficial and deep dorsal horn. All neurons had receptive fields (RFs) on the glabrous skin of the hindpaw, and neurons were functionally divided into wide dynamic range (WDR) and high-threshold (HT) neurons. Forty-four neurons (61%) were classified as WDR and responded to both innocuous and noxious mechanical stimuli (mean mechanical threshold of 12.8 +/- 1.6 mN). Twenty-eight neurons (39%) were classified as HT and were excited only by noxious mechanical stimuli (mean mechanical threshold of 154.2 +/- 18.3 mN). Neurons were characterized for their sensitivity heat (35 to 51 degrees C) and cold (28 to -12 degrees C) stimuli applied to their RF. Among WDR neurons, 86% were excited by both noxious heat and cold stimuli, while 14% responded only to heat. For HT neurons, 61% responded to heat and cold stimuli, 32% responded only to noxious heat, and 7% responded only to noxious cold. Effects of a mild freeze injury (-15 degrees C applied to the RF for 20 s) on responses to heat and cold stimuli were examined in 30 WDR and 22 HT neurons. Skin freezing was verified as an abrupt increase in skin temperature at the site of injury due to the exothermic reaction associated with crystallization. Freezing produced a decrease in response thresholds to heat and cold stimuli in most WDR and HT neurons. WDR and HT neurons exhibited a mean decrease in response threshold for cold of 9.0 +/- 1.3 degrees C and 10.0 +/- 1.6 degrees C, respectively. Mean response thresholds for heat decreased 4.0 +/- 0.4 degrees C and 4.3 +/- 1.3 degrees C in WDR and HT neurons, respectively. In addition, responses to suprathreshold cold and heat stimuli increased. WDR and HT neurons exhibited an 89% and a 192% increase in response across all cold stimuli, and a 93 and 92% increase in responses evoked across all heat stimuli, respectively. Our results demonstrate that many spinal neurons encode intensity of noxious cold as well as noxious heat over a broad range of stimulus temperatures. Enhanced responses of WDR and HT neurons to cold and heat stimuli after a mild freeze injury is likely to contribute to thermal hyperalgesia following a similar freeze injury in humans.

New members of the parathyroid hormone/parathyroid hormone receptor family: the parathyroid hormone 2 receptor and tuberoinfundibular peptide of 39 residues

The parathyroid hormone (PTH) family currently includes three peptides and three receptors. PTH regulates calcium homeostasis through bone and kidney PTH1 receptors. PTH-related peptide, probably also through PTH1 receptors, regulates skeletal, pancreatic, epidermal, and mammary gland differentiation and bladder and vascular smooth muscle relaxation and has a CNS role that is under investigation. Tuberoinfundibular peptide of 39 residues (TIP39) was recently purified from bovine hypothalamus based on selective PTH2 receptor activation. PTH2 receptor expression is greatest in the CNS, where it is concentrated in limbic, hypothalamic, and sensory areas, especially hypothalamic periventricular neurons, nerve terminals in the median eminence, superficial layers of the spinal cord dorsal horn, and the caudal part of the sensory trigeminal nucleus. It is also present in a number of endocrine cells. Thus TIP39 and PTH2 receptor-influenced functions may range from pituitary and pancreatic hormone release to pain perception. A third PTH-recognizing receptor has been found in zebrafish.

Morphologic features and electrical membrane properties of projection neurons in the marginal layer of the medullary dorsal horn of the rat

Possible correspondence between morphologic features and electrical membrane properties of projection neurons in lamina I [the marginal zone (MZ)] of the caudal subnucleus of the spinal trigeminal nucleus [the medullary dorsal horn (MDH)] was examined by using intracellular recordings and biocytin-injections combined with histochemical and immunohistochemical staining techniques. The experiments were done in horizontal slice preparations of the rat brain. Thirteen MZ neurons were recorded stably and stained successfully. These neurons were confirmed to send their axons to the brain regions outside the MDH by camera lucida reconstruction. They were divided into two types on the basis of branching patterns of their axons within the MDH: Type I projection (P-I) neurons (n = 7 neurons) had main axons that rarely emitted axon collaterals within the MDH, whereas type II projection (P-II) neurons (n = 6 neurons) had main axons that emitted many axon collaterals within laminae I, II (substantia gelatinosa), and III (magnocellular part) of the MDH and also to the spinal tract of the trigeminal nerve; these axon collaterals usually constituted a dense mesh of axonal processes within laminae I and II of the MDH, especially in lamina II. About half of the neurons of each type showed immunoreactivity for the neurokinin-1 receptor. Resting membrane potentials were significantly more positive in P-I neurons than in P-II neurons. The P-II neurons had higher input resistance, a longer membrane time constant, and a higher threshold for spike than P-I neurons. In response to weak, long depolarizing current pulses, P-II neurons often showed slow ramp depolarization; the same neurons exhibited delayed repolarization to the resting potential (slow after depolarization) after the offset of the long depolarizing current pulses. Neither the slow-ramp depolarization nor the slow after depolarization was observed in P-I neurons. Slow return to resting membrane potential after offset of hyperpolarizing current pulses also was observed frequently in P-II neurons but not in P-I neurons. The results indicate that P-II neurons differ in their membrane properties compared with P-I neurons, and P-II neurons may be involved in the local circuit mechanism within the MDH more deeply than P-I neurons.

Spinal lamina I neurons that express neurokinin 1 receptors: morphological analysis

The morphology of neurons in lamina I of the dorsal horn of the lumbar spinal cord which express neurokinin 1 receptors in the rat has been investigated. On the basis of soma and dendritic measurements, these neurons form two populations. One group consists of large neurons that stain intensely for the neurokinin 1 receptor with the immunochemical methods employed. They have a large soma, typically giving rise to between three and five thick principal dendrites. The dendritic tree, however, is relatively sparse, with the principal dendrites giving rise to small numbers of second- and third-order branches. All these dendrites are almost spine free. The dendritic tree spreads extensively in the rostrocaudal (approximately 550microm) and mediolateral (approximately 30microm) orientations, with few ventrally directed branches. These cells give rise to a single axon from their soma or a principal dendrite that generates a few local branches and also ramifies sparsely in deeper laminae (II-IV). The details of axonal morphology were established from intracellularly labelled material. Ultrastructural analysis of the synaptic input to these neurons reveals that they receive synapses with both clear round, flattened and dense-core vesicles; however, they do not form components of glomerular synapses. The second neuron type stains less intensely and typically has a small fusiform soma, giving rise to dendrites at its rostral and caudal poles. The dendritic tree is long in the rostrocaudal orientation (approximately 350microm), but restricted mediolaterally (approximately 40microm). The primary dendrites of these neurons bifurcate and soon give rise to third-order branches that are spiny. No pattern of organization could be detected for the distribution of either neuron type. These observations are discussed in the light of other recent studies indicating a central role for lamina I neurons expressing neurokinin 1 in the perception of severe pain.