Neonatal capsaicin treatment prevents the normal postnatal withdrawal of A fibres from lamina II without affecting fos responses to innocuous peripheral stimulation

Spinal anesthesia in children: A review

Even after a vast safety record, the role of spinal anesthesia (SA) as a primary anesthetic technique in children remains contentious and is mainly limited to specialized pediatric centers. It is usually practiced on moribund former preterm infants (<60 weeks post-conception) to reduce the incidence of post-operative apnea when compared to general anesthesia (GA). However, there is ample literature to suggest its safety and efficacy for suitable procedures in older children as well. SA in children has many advantages as in adults with an added advantage of minimal cardio-respiratory disturbance. Recently, several reports from animal studies have raised serious concerns regarding the harmful effects of GA on young developing brain. This may further increase the utility of SA in children as it provides all components of balanced anesthesia technique. Also, SA can be an economical option for countries with finite resources. Limited duration of surgical anesthesia in children is one of the major deterrents for its widespread use in them. To overcome this, several additives like epinephrine, clonidine, fentanyl, morphine, neostigmine etc. have been used and found to be effective even in neonates. But, the developing spinal cord may also be vulnerable to drug-related toxicity, though this has not been systematically evaluated in children. So, adjuvants and drugs with widest therapeutic index should be preferred in children. Despite its widespread use, incidence of side-effects is low and permanent neurological sequalae have not been reported with SA. Literature yields encouraging results regarding its safety and efficacy. Technical skills and constant vigilance of experienced anesthesia providers is indispensable to achieve good results with this technique.

Neuraxial analgesia in neonates and infants: a review of clinical and preclinical strategies for the development of safety and efficacy data

Neuraxial drugs provide robust pain control, have the potential to improve outcomes, and are an important component of the perioperative care of children. Opioids or clonidine improves analgesia when added to perioperative epidural infusions; analgesia is significantly prolonged by the addition of clonidine, ketamine, neostigmine, or tramadol to single-shot caudal injections of local anesthetic; and neonatal intrathecal anesthesia/analgesia is increasing in some centers. However, it is difficult to determine the relative risk-benefit of different techniques and drugs without detailed and sensitive data related to analgesia requirements, side effects, and follow-up. Current data related to benefits and complications in neonates and infants are summarized, but variability in current neuraxial drug use reflects the relative lack of high-quality evidence. Recent preclinical reports of adverse effects of general anesthetics on the developing brain have increased awareness of the potential benefit of neuraxial anesthesia/analgesia to avoid or reduce general anesthetic dose requirements. However, the developing spinal cord is also vulnerable to drug-related toxicity, and although there are well-established preclinical models and criteria for assessing spinal cord toxicity in adult animals, until recently there had been no systematic evaluation during early life. Therefore, in the second half of this review, we present preclinical data evaluating age-dependent changes in the pharmacodynamic response to different spinal analgesics, and recent studies evaluating spinal toxicity in specific developmental models. Finally, we advocate use of neuraxial drugs with the widest demonstrable safety margin and suggest minimum standards for preclinical evaluation before adoption of new analgesics or preparations into routine clinical practice.

Skin incision-induced receptive field responses of mechanosensitive peripheral neurons are developmentally regulated in the rat

Maturation of the nervous system results in changes in both central and peripheral processing. To better understand responses to injury in the young, developmental differences in the acute response to incision were investigated in both tactile and nociceptive myelinated peripheral mechanosensitive afferent neurons in vivo. Neuronal intrasomal recordings were performed in juvenile and infant rats in 34 L5 dorsal root ganglia, and each neuron was phenotypically defined. Neurons had a mechanosensitive receptive field in the glabrous skin on the plantar surface of the hind paw, which was characterized at baseline and for up to 45 min after incision. Fundamental maturational differences in the effect of incision were clear: in high-threshold nociceptive mechanoreceptors, the mechanical threshold decreased immediately and the receptive field size increased rapidly in juvenile rats but not in infant rats. Additionally, a divergence in changes in the instantaneous response frequency of tactile afferents occurred between the two ages. These differences may help explain maturational differences in responses to peripheral injury and suggest that differences in central nervous system responses may be partially mitigated by spatially confined and frequency-dependent differences resulting from tactile and nociceptive mechanosensitive input.

Acute pain and a motivational pathway in adult rats: influence of early life pain experience

Background

The importance of neonatal experience upon behaviour in later life is increasingly recognised. The overlap between pain and reward pathways led us to hypothesise that neonatal pain experience influences reward-related pathways and behaviours in adulthood.

Methodology/Principal Findings

Rat pups received repeat plantar skin incisions (neonatal IN) or control procedures (neonatal anesthesia only, AN) at postnatal days (P)3, 10 and 17. When adult, rats with neonatal ‘pain history’ showed greater sensory sensitivity than control rats following acute plantar skin incision. Motivational behaviour in the two groups of rats was tested in a novelty-induced hypophagia (NIH) paradigm. The sensitivity of this paradigm to pain-induced changes in motivational behaviour was shown by significant increases in the time spent in the central zone of the arena (43.7±5.9% vs. 22.5±6.7%, p<0.05), close to centrally placed food treats, and decreased number of rears (9.5±1.4 vs. 19.2±2.3, p<0.001) in rats with acute plantar skin incision compared to naive, uninjured animals. Rats with a neonatal ‘pain history’ showed the same pain-induced behaviour in the novelty-induced hypophagia paradigm as controls. However, differences were observed in reward-related neural activity between the two groups. Two hours after behavioural testing, brains were harvested and neuronal activity mapped using c-Fos expression in lateral hypothalamic orexin neurons, part of a specific reward seeking pathway. Pain-induced activity in orexin neurons of control rats (18.4±2.8%) was the same as in uninjured naive animals (15.5±2.6%), but in those rats with a ‘pain history’, orexinergic activity was significantly increased (27.2±4.1%, p<0.01). Furthermore the extent of orexin neuron activation in individual rats with a ‘pain history’ was highly correlated with their motivational behaviour (r = −0.86, p = 0.01).

Conclusions/Significance

These results show that acute pain alters motivational behaviour and that neonatal pain experience causes long-term changes in brain motivational orexinergic pathways, known to modulate mesolimbic dopaminergic reward circuitry.

Postnatal changes in the Rexed lamination and markers of nociceptive afferents in the superficial dorsal horn of the rat

In this study, we investigated postnatal changes in Rexed's laminae and distribution of nociceptive afferents in the dorsal horn of the rat lumbar spinal cord at postnatal days 0, 5, 10, 15, 20, and 60. Transverse sections of the L4-L5 segments were processed for triple labeling with isolectin B4 (IB4)-binding as a marker of nonpeptidergic C-fibers, calcitonin gene-related peptide (CGRP) immunoreactivity to label peptidergic nociceptive afferents, and a fluorescent Nissl stain to visualize cells and lamination at different stages of postnatal development. The Nissl staining revealed that the thickness of lamina I (LI) and outer lamina II remained mostly unchanged from birth until adulthood. CGRP afferents terminated mostly in LI and the outer two-thirds of lamina II, whereas the termination area of fibers binding IB4 was centered on the middle one-third of lamina II at all ages studied. In absolute values, the overall width of the bands of intense CGRP and IB4 labeling increased with age but decreased as a percentage of the overall thickness of the dorsal horn with maturation. The overlap of CGRP termination area with that of IB4 afferents increased with age. The consequences of these findings are twofold. First, the size of the different laminae does not grow evenly across the dorsal horn. Second, CGRP and IB4 labeling cannot be considered per se to be reliable markers of lamination during development. These findings have implications for comparing data obtained in immature and mature tissues with respect to localization of structures in the dorsal horn.

Action-based body maps in the spinal cord emerge from a transitory floating organization

During development primary afferents grow into and establish neuronal connections in the spinal cord, thereby forming the basis for how we perceive sensory information and control our movements. In the somatosensory system, myriads of primary afferents, conveying information from different body locations and sensory modalities, get organized in the dorsal horn of the spinal cord so that spinal multisensory circuits receive topographically ordered information. How this intricate pathfinding is brought about during development is, however, largely unknown. Here we show that a body representation closely related to motor patterns emerges from a transitory floating and plastic organization through profound activity-dependent rewiring, involving both sprouting and elimination of afferent connections, and provide evidence for cross-modality interactions in the alignment of the multisensory input. Thus, far from being inborn and stereotypic, the dorsal horn of the spinal cord now appears to be a highly adaptive brain-body interface.

Involvement of stem cell factor and its receptor tyrosine kinase c-kit in pain regulation

The c-kit receptor tyrosine kinase is expressed in a subpopulation of small- and medium-sized neurons of the dorsal root ganglia (DRG) and in the superficial layer of the spinal cord. Stem cell factor (SCF), a ligand of the c-kit receptor, induces neurite outgrowth from DRG and supports the survival of c-kit-expressing neurons. To clarify the possible function of the SCF/c-kit receptor system in the adult animal, we investigated the expression of c-kit receptor in the spinal cord and DRG in relation to pain by using H2C7, a newly developed anti-c-kit monoclonal antibody. S.c. and intrathecal injection of SCF markedly reduced the paw withdrawal threshold to mechanical stimuli and intrathecal SCF at 10 pg maximally induced mechanical allodynia in conscious mice. Intrathecal SCF also reduced the paw withdrawal latency to heat stimuli significantly but transiently. The c-kit receptor was co-expressed in 58.4% of calcitonin gene-related peptide (CGRP) -positive, but only 5.1% of isolectin B4-positive, DRG neurons. In the spinal cord, the c-kit receptor was detected in the superficial layer of the dorsal horn and co-localized there with CGRP in central terminals of DRG neurons. Selective elimination of unmyelinated C-fibers by neonatal capsaicin treatment resulted in marked reduction of the c-kit receptor and CGRP expression in the superficial layer of the spinal cord. Cell-size profiles showed that c-kit receptor expression was significantly up-regulated and down-regulated in medium-sized DRG neurons after neonatal capsaicin treatment and nerve injury, respectively. These results suggest that the c-kit receptor is mainly expressed in peptidergic small-sized DRG neurons and may be involved in pain regulation both peripherally and centrally.

Postnatal tuning of cutaneous inhibitory receptive fields in the rat

Spinal nociceptive processing undergoes extensive maturation in the postnatal period. The large excitatory cutaneous receptive fields and sensitivity to mechanical stimulation in the first weeks of life suggest a lack of inhibitory control in developing spinal sensory pathways, which cannot be easily explained at the synaptic level. We hypothesized that developmental changes in dorsal horn inhibition occur at the network level, and tested this by mapping the spatial and modality organization of dorsal horn cell inhibitory receptive fields (RFs) in decerebrate spinal adult and neonatal rats. We report two novel results. First, although contralateral inhibition of dorsal horn cells was well established by postnatal day 3 (P3), inhibitory RFs were significantly less spatially restricted at P3 than in the adult and the intensity of inhibition across the RF was more evenly distributed in the neonate. Second, contralateral inhibitory RFs could be activated by both low- and high-intensity stimulation in the neonate, in contrast to the situation in adult where high-intensity pinch is normally required. These results demonstrate substantial postnatal changes in the organization or 'tuning' of inhibition in the developing dorsal horn, which are likely to contribute to the maturation of tactile and nociceptive spinal processing and coordinated sensorimotor and pain behaviour.

Bee venom injection significantly reduces nociceptive behavior in the mouse formalin test via capsaicin-insensitive afferents

Peripheral bee venom (BV) administration produces 2 contrasting effects, nociception and antinociception. This study was designed to evaluate whether the initial nociceptive effect induced by BV injection into the Zusanli acupoint is involved in producing the more prolonged antinociceptive effect observed in the mouse formalin test, and whether capsaicin-sensitive primary afferents are involved in these effects. BV injection into the Zusanli point increased spinal Fos expression but not spontaneous nociceptive behavior. BV pretreatment 10 minutes before intraplantar formalin injection dose-dependently attenuated nociceptive behavior associated with the second phase of the formalin test. The destruction of capsaicin-sensitive primary afferents by resiniferatoxin (RTX) pretreatment selectively decreased BV-induced spinal Fos expression but did not affect BV-induced antinociception. Furthermore, BV injection increased Fos expression in tyrosine hydroxylase immunoreactive neurons in the locus caeruleus, and this expression was unaltered by RTX pretreatment. Finally, BV's antinociception was blocked by intrathecal injection of 10 microg idazoxan, and this effect was not modified by RTX pretreatment. These findings suggest that subcutaneous BV stimulation of the Zusanli point activates central catecholaminergic neurons via capsaicin-insensitive afferent fibers without induction of nociceptive behavior. This in turn leads to the activation of spinal alpha2-adrenoceptors, which ultimately reduces formalin-evoked nociceptive behaviors.

PERSPECTIVE

This study demonstrates that BV acupuncture produces a significant antinociception without nociceptive behavior in rodents, which is mediated by capsaicin-insensitive afferents and involves activation of central adrenergic circuits. These results further suggest that BV stimulation into this acupuncture point might be a valuable alternative to traditional electrical or mechanical acupoint stimulation.

Postnatal development of substance P-immunoreaction in the trigeminal caudalis of neonatally capsaicin-treated mice

The trigeminal subnucleus caudalis (Vc) is a critical relay site for processing nociceptive afferent input from the orofacial area in addition to its modulation by neuroplastic change. Although an administration of capsaicin in neonates induces a selective destruction of substance P (SP)-immunoreactive nerve fibers, little information is available regarding its detailed effects on the Vc, particularly during postnatal development. The present study examined postnatal changes in the distribution of SP in the Vc and trigeminal ganglion (TG) by immunohistochemical techniques in naïve (NV) and neonatally capsaicin-treated (CP) mice, combined with a quantitative analysis. The neonatal mice received a single subcutaneous injection of capsaicin (50 mg/kg) at 48 hours after birth. The neural density of the SP-immunoreaction decreased to approximately a quarter of that in 1-week-old NV mice but increased to three-quarters of that in the NV in the superficial area after postnatal week 2. A double staining with SP and myelin basic protein confirmed the absence of any SP-immunoreaction in the myelinated nerve fibers in both NV and CP mice. The SP-immunoreaction never overlapped with non-peptidergic IB4-labeled neurons in the Vc and TG of either group. Neither the size distribution of SP-positive neurons nor their relative ratio in the TG differed between NV and CP mice at the ages of postnatal weeks 1 and 8. These findings indicate two putative origins for the emergent SP-immunoreaction in the superficial layer of the Vc of the CP mice: the surviving trigeminal neurons with SP against capsaicin treatment and/or intrinsic neurons/interneurons in the Vc without SP under normal conditions.

Ethanol withdrawal-associated allodynia and hyperalgesia: age-dependent regulation by protein kinase C epsilon and gamma isoenzymes

Ethanol (EtOH) withdrawal increases sensitivity to painful stimuli in adult rats. In this study, withdrawal from a single, acute administration of EtOH dose-dependently produced mechanical allodynia and thermal hyperalgesia in postnatal day 7 (P7) rats. In contrast, P21 rats exhibited earlier and more prolonged mechanical allodynia but not thermal hyperalgesia. For both P7 and P21 rats, blood and spinal cord EtOH levels peaked at 30 minutes after administration, with P7 rats achieving overall higher spinal cord concentrations. Protein kinase C (PKC) has been implicated in mediating pain responses. Inhibitory PKC- and gamma-specific peptides attenuated mechanical allodynia and thermal hyperalgesia in P7 rats, whereas only the PKCgamma inhibitor prevented mechanical allodynia in P21 rats. Immunoreactive PKC in dorsal root ganglion and PKCgamma in lumbar spinal cord increased at 6 hours after EtOH administration in P7 rats. In P21 rats, the density of PKC immunoreactivity remained unchanged, whereas the density of PKCgamma immunoreactivity increased and translocation occurred. These studies demonstrate developmental differences in neonatal nociceptive responses after withdrawal from acute EtOH and implicate a role for specific PKC isozymes in EtOH withdrawal-associated allodynia and hyperalgesia.

PERSPECTIVE

This study examines age-specific nociceptive responses after ethanol exposure by using 2 different ages of rats. The results suggest that ethanol age-dependently alters sensitivity to mechanical and thermal stimuli via specific protein kinase C isozymes. These results begin to ascertain the mechanisms that produce abnormal pain after alcohol exposure.

Ablation of primary afferent terminals reduces nicotinic receptor expression and the nociceptive responses to nicotinic agonists in the spinal cord

A variety of studies indicate that spinal nicotinic acetylcholine receptors modulate the behavioral and autonomic responses elicited by afferent stimuli. To examine the location of and role played by particular subtypes of nicotinic receptors in mediating cardiovascular and nociceptive responses, we treated neonatal and adult rats with capsaicin to destroy C-fibers in primary afferent terminals. Reduction of C-fiber terminals was ascertained by the loss of isolectin B4, CGRP and vanilloid receptors as monitored by immunofluorescence. Receptor autoradiography shows a reduction in number of epibatidine binding sites following capsaicin treatment. The reduction is particularly marked in the dorsal horn and primarily affects the class of high affinity epibatidine binding sites thought to modulate nociceptive responses. Accompanying the loss of terminals and nicotinic binding sites were significant reductions in the expression of alpha 3, alpha 4, alpha 5, beta 2 and beta 4 nicotinic receptor subunits in the superficial layers of the spinal cord as determined by antibody staining and confocal microscopy. The loss of nicotinic receptors that follows capsaicin treatment results in attenuation of the nociceptive responses to both spinal cytisine and epibatidine. Capsaicin treatment also diminishes the capacity of cytisine to desensitize nicotinic receptors mediating nociception, but it shows little effect on intrathecal nicotinic agonist elicited pressor and heart rate responses. Hence, our data suggest that alpha 3, alpha 4, alpha 5, beta 2 and beta 4 subunits of nicotinic receptors are localized in the spinal cord on primary afferent terminals that mediate nociceptive input. A variety of convergent data based on functional studies and subunit expression suggest that alpha 3 and alpha 4, in combination with beta 2 and alpha 5 subunits, form the majority of functional nicotinic receptors on C-fiber primary afferent terminals. Conversely, spinal nicotinic receptors not located on C-fibers play a primary role in the spinal pathways evoking spinally coordinated autonomic cardiovascular responses.

The development of nociceptive circuits

The study of pain development has come into its own. Reaping the rewards of years of developmental and molecular biology, it has now become possible to translate fundamental knowledge of signalling pathways and synaptic physiology into a better understanding of infant pain. Research has cast new light on the physiological and pharmacological processes that shape the newborn pain response, which will help us to understand early pain behaviour and to design better treatments. Furthermore, it has shown how developing pain circuitry depends on non-noxious sensory activity in the healthy newborn, and how early injury can permanently alter pain processing.

Alterations in primary afferent input to substantia gelatinosa of adult rat spinal cord after neonatal capsaicin treatment

Primary afferent fibers are divided into three main subgroups: Abeta-, Adelta-, and C-fibers. Morphological studies have demonstrated that neonatal capsaicin treatment (NCT) depletes C-fiber inputs in the spinal dorsal horn; the electrophysiological features of the NCT-induced changes, however, remain unclear. This issue was addressed by performing whole-cell voltage-clamp recordings from substantia gelatinosa (SG) neurons in dorsal root-attached spinal cord slices. When estimated from excitatory postsynaptic currents (EPSCs) evoked by stimulating primary afferent fibers, 24 (49%) of 49 neurons examined exhibited C-fiber EPSCs that were either monosynaptic (n = 15) or polysynaptic (n = 9) in origin; only two of the neurons had Abeta-fiber responses. In NCT rats, however, SG neurons exhibiting C-fiber-mediated EPSCs decreased to 7% (3 of 41 neurons tested), whereas Abeta-fiber EPSCs were observed in 21 (51%) of the neurons, and 14 (67%) of them exhibited monosynaptic ones. There was no change in the cell proportion having Adelta-fiber innervation after NCT. Our electrophysiological data suggest that NCT diminishes primary afferent C-fiber inputs while enhancing Abeta-fiber direct innervation in the SG in adulthood.

The postnatal reorganization of primary afferent input and dorsal horn cell receptive fields in the rat spinal cord is an activity-dependent process

The dorsal horn of the spinal cord in the newborn rat is characterized by large cutaneous mechanoreceptive fields, a predominance of A-fibre synaptic inputs and diffuse primary afferent A-fibre projections, all of which are gradually reduced and refined over the first postnatal weeks. This may be partly responsible for the reduction in cutaneous flexion reflex sensitivity of rats over the postnatal period. Here we show that chronic, local exposure of the dorsal horn of the lumbar spinal cord to the NMDA antagonist MK801 from birth prevents the normal functional and structural reorganization of A-fibre connections. Dorsal horn cells in spinal MK801-treated animals, investigated at eight weeks of age by in vivo electrophysiological recording, had significantly larger cutaneous mechanoreceptive fields and greater A-fibre evoked responses than vehicle controls. C-fibre evoked responses were unaffected. Chronic MK801 also prevented the normal structural reorganization of A-fibre terminals in the spinal cord. The postnatal withdrawal of superficially projecting A-fibre primary afferents to deeper laminae did not occur in treated animals although C-fibre afferent terminals and cell density in the dorsal horn were apparently unaffected. Spinal MK801-treated animals also had significantly reduced behavioural reflex thresholds to mechanical stimulation of the hindpaw compared to naïve and vehicle-treated animals, whereas noxious heat thresholds remained unaffected. The results indicate that the normal postnatal structural and functional development of A-fibre sensory connectivity within the spinal cord is an activity-dependent process requiring NMDA receptor activation.

The neurobiology of pain: developmental aspects

Invasive procedures that would be painful in children and adults are frequently performed on infants admitted to the neonatal intensive care unit. This article discusses sensory responses to these procedures in the immature nervous system and highlights the fact that, in addition to causing distress and delayed recovery, pain in infancy is also a developmental issue. First, the immaturity of sensory processing within the newborn spinal cord leads to lower thresholds for excitation and sensitization, therefore potentially maximizing the central effects of these tissue-damaging inputs. Second, the plasticity of both peripheral and central sensory connections in the neonatal period means that early damage in infancy can lead to prolonged structural and functional alterations in pain pathways that can last into adult life.

Origins of spontaneous and noxious stimuli-evoked miniature EPSCs in substantia gelatinosa

The origins of spontaneous and noxious stimuli-evoked glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in substantia gelatinosa (SG) neurons were investigated by using whole-cell voltage-clamp technique on adult rat spinal cord slice. The properties of mEPSCs of SG neurons from rats either neonatally capsaicin-treated or sciatic nerve ligated showed no difference from those of intact SG neurons, indicating independence of spontaneous mEPSCs on primary afferent fibers. In the presence of tetrodotoxin (TTX), capsaicin, which noxiously stimulated fine primary afferent fibers, caused increase of the mEPSCs frequency, but did not affect the amplitude profiles or mean amplitudes. TTX affected neither the spontaneous mEPSCs nor capsaicin-induced mEPSCs frequency increase. The results suggest that spontaneous mEPSCs in SG are mediated by presynaptic spontaneous glutamate release predominantly originating from interneuron terminals rather than from primary afferent terminals; under noxious stimulation, however, mEPSCs frequency increase is mediated by primary afferent excitation.