Immunohistochemical localization of GABA(B) receptors in the rat central nervous system

The recent cloning of two gamma-aminobutyric acid(B) (GABA(B)) receptor isoforms (GABA(B)R1a/b), which are probably splice variants of the same gene transcript, allowed us to develop an antiserum that recognized the receptors in fixed tissue and to map their distribution in the rat central nervous system (CNS). We also investigated whether GABA(B)R1 colocalizes with glutamic acid decarboxylase (GAD), a marker of GABAergic cell bodies and terminals. Although GABA(B)R1-like immunoreactivity (GABA(B)R1-LI) was distributed throughout the CNS, several distinct distribution patterns emerged: (1) all monoaminergic brainstem cell groups appeared to contain very high levels of GABA(B)R1, (2) a very high intensity of GABA(B)R1-LI was observed in the majority of the cholinergic regions in the CNS, with exception of motoneurons of the third through sixth cranial nerve nuclei, and (3) a low density of the receptor was observed in most of the nuclei that contain cell bodies of GABAergic projection neurons. The highest GABA(B)R1 labeling was observed in the thalamus, interpeduncular nucleus and medial habenula. Cell bodies were labeled throughout the neuroaxis. We also observed dense neuropil labeling in many regions, suggesting that this receptor is localized in dendrites and/or axon terminals. However, in immunofluorescent double-labeling experiments for GABA(B)R1 and GAD, we never observed GABA(B)R1-LI in GAD-positive axon terminals; this result suggests that the GABA(B)R1 may not function as an autoreceptor. Double labeling was observed in the cell bodies of Purkinje neurons and in some interneurons. In general, the immunohistochemical localization of the GABA(B)R1 correlates well with physiologic and autoradiographic data on the distribution of GABA(B) receptors, but some critical differences were noted. Thus, it is likely that additional GABA(B) receptor subtypes remain to be identified.

Differential effects of neurotoxic destruction of descending noradrenergic pathways on acute and persistent nociceptive processing

Although many pharmacological studies indicate that bulbospinal noradrenergic projections contribute to antinociception, lesions of the major brainstem noradrenergic cell groups have provided conflicting evidence. Here we used a new immunotoxin, anti-dopamine beta-hydroxylase-saporin, to re-examine the contribution of noradrenergic pathways to nociception and to morphine analgesia. We treated rats intrathecally by lumbar puncture with the immunotoxin and examined dopamine beta-hydroxylase (DbetaH) immunoreactivity seven and 14 days after treatment. There was no change in DbetaH staining at 7 days; however, 14 days after treatment we demonstrated significant destruction of noradrenergic neurons in the locus coeruleus and in the A5 and A7 cell groups. There was a concomitant loss of noradrenergic axons in the dorsal and ventral horns of the lumbosacral and cervical cord. Consistent with the lack of anatomical changes, we found no difference in nociceptive responses in the hot-plate, tail-flick or formalin tests one week post-toxin. On day 14 we examined the behavioral response to injection of formalin into the hindpaw and found that responses during the second phase of pain behavior were significantly reduced. There was no change during the first phase. Formalin-evoked fos expression in the spinal cord was also reduced. We also evaluated morphine analgesia in the formalin test and found that toxin-treated animals exhibited enhanced morphine analgesia. These results establish the utility of using this immunotoxin to selectively destroy subpopulations of noradrenergic cell groups and provide evidence that acute and persistent nociception are differentially regulated by descending noradrenergic pathways.

Neurons in the dorsal column white matter of the spinal cord: complex neuropil in an unexpected location

It is common to think of gray matter as the site of integration in neural circuits and white matter as the wires that connect different groups of neurons. The dorsal column (DC) white matter, for example, is the spinal cord axonal pathway through which a topographic map of the body is conveyed to the somatosensory cortex. We now describe a network of neurons located along the midline of the DCs. The neurons are present in several mammals, including primates and birds, and have a profuse dendritic arbor that expresses both the neuron-specific marker, microtubule-associated protein-2, and the neurokinin-1 receptor, a target of the neuropeptide, substance P. Electron microscopy and double immunostaining for synaptophysin and a marker of gamma-aminobutyric acid-ergic terminals documented a rich synaptic input to these neurons. Finally, injection of a gamma-aminobutyric acid type A receptor antagonist or of substance P into the cerebrospinal fluid of the rat spinal cord induced Fos expression and internalization of the neurokinin-1 receptor in these neurons, respectively, indicating that the DC neurons are under tonic inhibitory control and can respond to neurotransmitters that circulate in the cerebrospinal fluid.

Spinal mechanisms of acute and persistent pain

Although there is considerable information about the mechanisms through which injury stimuli produce acute pain, recent studies indicate that there are significant long-term consequences of persistent injury. Pain is exacerbated, in part, because of a reorganization of spinal cord circuitry in the setting of persistent injury. This review describes our studies of the contribution of the primary afferent neurotransmitter, substance P (SP), to these changes. By following internalization of the SP receptor in spinal cord dorsal horn neurons, we have identified the stimuli that evoke SP release and the neurons that respond to these stimuli. Importantly, based on the intensities of stimuli required to evoke internalization, we conclude that SP is only released under conditions in which severe pain would be produced, that the release can be evoked by intense stimulation of somatic and visceral tissue, and that multiple stimulus modalities are effective. We also found that the numbers of neurons that are influenced increases dramatically in the setting of inflammation. Using a knockout strategy, we have also raised mice with a deletion of the preprotachykinin-A (PPT-A) gene, which encodes for SP and neurokinin A (NKA), and have identified a specific behavioral phenotype in which the animals do not detect a window of "pain" intensities; this window cuts across stimulus modalities. These results provide an important behavioral correlate of the receptor internalization studies. On the other hand, the allodynia (lowered pain threshold) that occurs in the setting of injury was not altered in these animals. Among the factors that could underlie injury-induced allodynia are the second messenger systems that are activated in dorsal horn neurons. Our studies have recently implicated the gamma isoform of protein kinase C (PKCgamma) in the development of nerve injury-induced neuropathic pain. Specifically, we found that although acute pain responses of mice with a deletion of PKCgamma are not altered, partial injury to the sciatic nerve (which induces a severe thermal and mechanical allodynia in the wild type mouse) is without effect in the knockout. Furthermore, the anatomical/neurochemical reorganization that typically follows sciatic nerve section does not occur in the PKCgamma mutant mice. Because the spinal cord distribution of interneurons that express PKCgamma is concentrated almost exclusively in the inner part of lamina II, we believe that changes in the properties of these neurons are key to the development of nerve injury-induced neuropathic pain conditions. Taken together, these studies emphasize that persistent pain should be considered a disease state of the nervous system, not merely a symptom of some other disease conditions. In the setting of persistent injury, the nervous system undergoes dramatic changes that exacerbate and prolong the pain condition. Our studies underscore the importance of preventing the long-term changes that result from persistent injury.

The contribution of capsaicin-sensitive afferents to the dorsal root ganglion sprouting of sympathetic axons after peripheral nerve injury in the rat

Transection of the sciatic nerve leads to sprouting of sympathetic efferent, noradrenergic axons and terminals around large cell bodies in the dorsal root ganglion. Here we examined whether injury to unmyelinated afferents contributes to the sprouting. Neonatal treatment with the C-fiber neurotoxin capsaicin increased sprouting after nerve injury. We conclude that injury to large, rather than small diameter fibers, triggers the sprouting of sympathetic efferents after nerve injury.

The cloned capsaicin receptor integrates multiple pain-producing stimuli

Capsaicin, the main pungent ingredient in "hot" chili peppers, elicits buming pain by activating specific (vanilloid) receptors on sensory nerve endings. The cloned vanilloid receptor (VR1) is a cation channel that is also activated by noxious heat. Here, analysis of heat-evoked single channel currents in excised membrane patches suggests that heat gates VR1 directly. We also show that protons decrease the temperature threshold for VR1 activation such that even moderately acidic conditions (pH < or = 5.9) activate VR1 at room temperature. VR1 can therefore be viewed as a molecular integrator of chemical and physical stimuli that elicit pain. Immunocytochemical analysis indicates that the receptor is located in a neurochemically heterogeneous population of small diameter primary afferent fibers. A role for VR1 in injury-induced hypersensitivity at the level of the sensory neuron is presented.

Pain: nocistatin spells relief

Tissue or nerve injury can dramatically alter the transmission of sensory stimuli by spinal cord neurons, so that a light touch produces pain. The discovery that peptide products of prepronociceptin processing either facilitate or inhibit these mechanisms suggests novel approaches to treating these conditions.

The effects of prior chronic stress on cardiovascular responses to acute restraint and formalin injection

Exposure to acute stressors activates both the hypothalamic-pituitary-adrenal (HPA) and cardiovascular systems. Prior chronic stress enhances HPA responses to novel, acute stressors, but whether it alters cardiovascular responsivity to novel, acute stress is unknown. In the present study, we examined mean arterial blood pressure (MAP) and heart rate (HR) to two distinct stimuli, restraint and formalin, following prior exposure to 7 days of intermittent cold. In two sets of control and chronically stressed animals, we measured MAP and HR for 60 min following onset of 30 min restraint and MAP, HR and behavioral responses to intraplantar injection of formalin. Chronic stress raised MAP and HR under resting conditions and elevated HR during, but not following termination of, restraint. These increases in HR during restraint were due to the differences in resting levels of HR, since both control and chronically stressed animals exhibited similar increases from resting levels in HR during restraint. Conversely, chronically stressed animals exhibited lower changes in MAP and HR from resting levels following termination of restraint. Formalin produced the characteristic biphasic pattern of cardiovascular and behavioral responses. Prior chronic stress did not alter behavior, but increased MAP and HR in Interphase and only MAP in Phase 2. The increases in MAP during Interphase and Phase 2 were a result of the elevations in resting levels of MAP, but even when differences in resting levels were taken into account, HR remained elevated in the Interphase in chronically stressed animals. Together, these data demonstrate that prior chronic intermittent cold stress modifies cardiovascular function both under resting conditions and, in very specific ways, under stimulated conditions produced by restraint and formalin. We propose that these modifications are produced by brain regions that are known to regulate cardiovascular function and which are activated by chronic stress.

Activation of coeruleospinal noradrenergic inhibitory controls during withdrawal from morphine in the rat

We previously reported that withdrawal from morphine induces the expression of Fos, a marker of neuronal activity, in spinal cord neurons, particularly in laminae I and II of the superficial dorsal horn, and that the magnitude of Fos expression is increased in rats with a midthoracic spinal transection. We suggested that loss of withdrawal-associated increases in descending inhibitory controls that arise in the brainstem underlie the increased Fos expression after spinal transection. Here, we addressed the origin of the supraspinal inhibition. We injected rats intracerebroventricularly with saline or anti-dopamine-beta-hydroxylase-saporin, a toxin that destroys noradrenergic neurons of the locus coeruleus. Eleven days later, we implanted rats with morphine or placebo pellets, and after 4 d, we precipitated withdrawal with naltrexone. One hour later, the rats were killed, their brains and spinal cords were removed, and transverse sections of the brains and spinal cords were immunoreacted with an antibody to Fos. In placebo-pelleted rats, the toxin injection did not alter behavior and did not induce expression of the Fos protein. However, compared with saline-injected withdrawing rats, the toxin-treated rats that underwent withdrawal demonstrated an intense withdrawal behavior rarely seen in the absence of toxin, namely forepaw fluttering. The rats also had significantly increased Fos-like immunoreactivity in all laminae of the cervical cord and in laminae I and II and the ventral horn of the lumbar cord. No differences were recorded in the sacral cord. We conclude that the effects of spinal transection in rats that withdraw from morphine in part reflect a loss of coeruleospinal noradrenergic inhibitory controls.

Partial sciatic nerve injury in the mouse as a model of neuropathic pain: behavioral and neuroanatomical correlates

The generation of knock-out and transgenic mice offers a promising approach to the identification of novel biochemical factors that contribute to persistent pain conditions. To take advantage of these mice, however, it is important to demonstrate that the traditional models of persistent pain, which were largely developed for studies in the rat, can be used in the mouse. Here, we combined behavioral and anatomical methods to characterize the pathophysiology of a partial nerve injury-evoked pain condition in the 'normal' mouse. In male C57BL6 mice we tied a tight ligature around 1/3 to 1/2 of the diameter of the sciatic nerve and evaluated the time-course and magnitude of the ensuing mechanical and thermal allodynia. We also used immunocytochemistry to analyze nerve injury-induced changes in substance P (SP) and NK-1 (SP) receptor expression in the spinal cord. As in the rat, partial nerve injury markedly decreased paw withdrawal thresholds to both mechanical and thermal stimuli on the injured side. We detected threshold changes one day after the injury. The thermal allodynia resolved by 49 days, but the mechanical allodynia persisted for the duration of the study (70 days). We found no changes contralateral to the nerve injury. Sympatholytic treatment with guanethidine significantly reduced both the thermal and mechanical allodynia. We observed a reduction of SP immunoreactivity in the superficial dorsal horn on the injured side at 7 and 14, but not at 3 or 70 days after the nerve injury, and we observed an increase of NK-1 receptor expression at 3, 7, 14 and 42, but not at 70 days after the injury. We conclude that partial injury to the sciatic nerve produces a comparable allodynia and neurochemical plasticity in the rat and mouse. These results establish a valuable model for future studies of the biochemical basis of neuropathic pain in mice with specific gene modifications.

Pituitary-adrenocortical responses to persistent noxious stimuli in the awake rat: endogenous corticosterone does not reduce nociception in the formalin test

Although glucocorticoids inhibit inflammation and are used to treat painful inflammatory rheumatic diseases, the contribution, if any, of endogenous pituitary-adrenocortical activity to the control of pain remains unclear. We report that injection of dilute formalin into the hindpaw not only evokes inflammation and pain-related behavior, but it also increases ACTH and corticosterone to a greater extent than restraint and saline injection alone. This difference was particularly robust during the final periods of pain-related behavior in the formalin test, when the ACTH and corticosterone (B) levels in the restraint/saline control group had returned to normal. These results indicate that formalin-evoked increases in ACTH and B reflect nociceptive input, rather than the stress associated with handling. To test the hypothesis that the formalin-induced increase in corticosterone reduces pain and inflammation, we next evaluated the effect of adrenalectomy (to prevent activation of glucocorticoid receptors) or high-dose dexamethasone (to saturate glucocorticoid receptors) on nociceptive processing in the formalin test. Neither adrenalectomy nor dexamethasone changed behavioral or cardiovascular nociceptive responses. Furthermore, the increases in blood pressure and heart rate produced by formalin may not be mediated by adrenomedullary catecholamine release. In addition, we conclude that the nociceptive component of the formalin stimulus is sufficient to activate the pituitary-adrenocortical system in the awake rat, but that the resulting release of corticosterone does not feed back and reduce nociceptive processing.

Primary afferent tachykinins are required to experience moderate to intense pain

The excitatory neurotransmitter glutamate coexists with the peptide known as substance P in primary afferents that respond to painful stimulation. Because blockers of glutamate receptors reliably reduce pain behaviour, it is assumed that 'pain' messages are mediated by glutamate action on dorsal horn neurons. The contribution of substance P, however, is still unclear. We have now disrupted the mouse preprotachykinin A gene (PPT-A), which encodes substance P and a related tachykinin, neurokinin A. We find that although the behavioural response to mildly painful stimuli is intact in these mice, the response to moderate to intense pain is significantly reduced. Neurogenic inflammation, which results from peripheral release of substance P and neurokinin A, is almost absent in the mutant mice. We conclude that the release of tachykinins from primary afferent pain-sensing receptors (nociceptors) is required to produce moderate to intense pain.

Differential effects of intrathecally administered delta and mu opioid receptor agonists on formalin-evoked nociception and on the expression of Fos-like immunoreactivity in the spinal cord of the rat

This study examined the effects of intrathecally (i.t.) administered mu and delta opioid receptor agonists on the flinching behavior and the expression of Fos-like immunoreactivity (Fos-LI) in the spinal cord elicited by s.c. injection of 5% formalin in one hindpaw of the rat. Intrathecal pretreatment with either the delta-1 opioid receptor agonist [D-Pen2,5]enkephalin (DPDPE) or the delta-2 opioid receptor agonist [D-Ala2,Glu4]deltorphin (DELT) produced a dose-dependent inhibition of flinching behavior in phase 1 and phase 2 that was antagonized by coadministration of the delta-1 opioid receptor antagonist 7-benzylidinenaltrexone or the delta-2 opioid receptor antagonist Naltriben, respectively. Although i.t. pretreatment with 60 micrograms of DPDPE produced a small decrease in the numbers of Fos-LI neurons in laminae I, IIi and IIo, as well as laminae V and VI and laminae VII-X, i.t. pretreatment with 30 micrograms of DELT did not decrease the number of Fos-LI neurons in any region of the spinal cord. In contrast, i.t. pretreatment with an equieffective dose of the mu opioid receptor agonist [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO) not only significantly decreased the number of flinches in phase 1 and phase 2, but also nearly completely prevented the expression of Fos-LI in all regions of the spinal cord. These effects were antagonized by pretreatment with the mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Phe-Thr-NH2. The efficacy of i.t. administered DAMGO suggests that a direct spinal action contributes to the inhibition of noxious stimulus-evoked Fos-LI in the spinal cord produced by systemically administered mu opioid receptor agonists such as morphine. The relative lack of effect of DPDPE or DELT suggests that delta opioid receptors do not modulate the early-immediate gene c-fos. Alternatively, because delta opioid receptor agonists inhibit synaptic transmission in the spinal cord by predominantly presynaptic mechanisms and do not hyperpolarize dorsal horn neurons, the excitatory inputs that persist in the presence of these agonists may be sufficient to activate the c-fos gene. Taken together, these results provide new evidence, at the level of a "third messenger," that the antinociception produced by i.t. administration of delta and mu opioid receptor agonists is mediated by different mechanisms.

Inflammation increases the distribution of dorsal horn neurons that internalize the neurokinin-1 receptor in response to noxious and non-noxious stimulation

Although the neurokinin-1 (NK-1)/substance P (SP) receptor is expressed by neurons throughout the spinal dorsal horn, noxious chemical stimulation in the normal rat only induces internalization of the receptor in cell bodies and dendrites of lamina I. Here we compared the effects of mechanical and thermal stimulation in normal rats and in rats with persistent hindpaw inflammation. Electron microscopic analysis confirmed the upregulation of receptor that occurs with inflammation and demonstrated that in the absence of superimposed stimulation, the increased receptor was, as in normal rats, concentrated on the plasma membrane. In general, noxious mechanical was more effective than noxious thermal stimulation in inducing NK-1 receptor internalization, and this was increased in the setting of inflammation. Although a 5 sec noxious mechanical stimulus only induced internalization in 22% of lamina I neurons in normal rats, after inflammation, it evoked near-maximal (98%) internalization in lamina I, produced significant changes in laminae III-VI, and expanded the rostrocaudal distribution of neurons with internalized receptor. Even non-noxious (brush) stimulation of the inflamed hindpaw induced internalization in large numbers of superficial and deep neurons. For thermal stimulation, the percentage of cells with internalized receptor increased linearly at >45 degrees C, but in normal rats, these were restricted to lamina I. After inflammation, however, the 52 degrees C stimulus also induced internalization in 25% of laminae III-IV cells. These studies provide a new perspective on the reorganization of dorsal horn circuits in the setting of persistent injury and demonstrate a critical contribution of SP.

Preserved acute pain and reduced neuropathic pain in mice lacking PKCgamma

In normal animals, peripheral nerve injury produces a persistent, neuropathic pain state in which pain is exaggerated and can be produced by nonpainful stimuli. Here, mice that lack protein kinase C gamma (PKCgamma) displayed normal responses to acute pain stimuli, but they almost completely failed to develop a neuropathic pain syndrome after partial sciatic nerve section, and the neurochemical changes that occurred in the spinal cord after nerve injury were blunted. Also, PKCgamma was shown to be restricted to a small subset of dorsal horn neurons, thus identifying a potential biochemical target for the prevention and therapy of persistent pain.

Diminished inflammation and nociceptive pain with preservation of neuropathic pain in mice with a targeted mutation of the type I regulatory subunit of cAMP-dependent protein kinase

To assess the contribution of PKA to injury-induced inflammation and pain, we evaluated nociceptive responses in mice that carry a null mutation in the gene that encodes the neuronal-specific isoform of the type I regulatory subunit (RIbeta) of PKA. Acute pain indices did not differ in the RIbeta PKA mutant mice compared with wild-type controls. However, tissue injury-evoked persistent pain behavior, inflammation of the hindpaw, and ipsilateral dorsal horn Fos immunoreactivity was significantly reduced in the mutant mice, as was plasma extravasation induced by intradermal injection of capsaicin into the paw. The enhanced thermal sensitivity observed in wild-type mice after intraplantar or intrathecal (spinal) administration of prostaglandin E2 was also reduced in mutant mice. In contrast, indices of pain behavior produced by nerve injury were not altered in the mutant mice. Thus, RIbeta PKA is necessary for the full expression of tissue injury-evoked (nociceptive) pain but is not required for nerve injury-evoked (neuropathic) pain. Because the RIbeta subunit is only present in the nervous system, including small diameter trkA receptor-positive dorsal root ganglion cells, we suggest that in inflammatory conditions, RIbeta PKA is specifically required for nociceptive processing in the terminals of small-diameter primary afferent fibers.

Cholecystokinin and enkephalin in brain stem pain modulating circuits

Neurons in rostral ventromedial medulla and the periaqueductal gray modulate dorsal horn nociceptive transmission. Endogenous peptides implicated in this modulation include enkephalin (ENK), which is antinociceptive, and cholecystokinin (CCK), which has anti-opioid effects. In this study double-label fluorescence immunocytochemistry demonstrated somata and terminals with ENK- or CCK-like immunoreactivity in these regions. Although the distribution of CCK- and ENK-immunoreactive terminal fields overlapped significantly, co-localization was rare. Furthermore, CCK- and ENK-immunoreactive somata had different morphologies and distinct distributions. The overlap of CCK- and ENK- immunoreactive terminals arbors provides a morphological substrate for an antagonistic interaction of CCK and ENK within brainstem pain modulating circuits, as has been demonstrated in the spinal cord.

The contribution of supraspinal, peripheral and intrinsic spinal circuits to the pattern and magnitude of Fos-like immunoreactivity in the lumbar spinal cord of the rat withdrawing from morphine

Withdrawal from morphine evokes increases in Fos-like immunoreactivity in the spinal cord, particularly in the superficial dorsal horn, laminae I/II. To determine the origin of the increased Fos-like immunoreactivity, we selectively targeted central or peripheral opioid receptors with naloxone-methiodide, an antagonist that does not cross the blood-brain barrier, or induced withdrawal after eliminating possible sources of input to the superficial dorsal horn. To induce tolerance, we implanted rats with morphine or placebo pellets (75 mg, six pellets over three days). On day 4, withdrawal was precipitated and after 1 h, the rats were killed, their spinal cords removed and 50 microm transverse sections of the spinal cord immunoreacted with a rabbit polyclonal antiserum directed against the Fos protein. In placebo-pelleted rats, none of the different procedures, viz. spinal transection, unilateral dorsal rhizotomy (L4-S2), neonatal capsaicin treatment or direct intrathecal opioid antagonist injection, induced expression of the Fos protein. However, both spinally transected and rhizotomized withdrawing animals showed significant increases in Fos-like immunoreactivity in laminae I/II, compared to intact withdrawing rats. Neonatal treatment with capsaicin, which eliminates C-fibres, did not alter Fos-like-immunoreactivity. Selective withdrawal of morphine from peripheral opioid receptors by naloxone-methiodide did not induce Fos-like immunoreactivity in the lumbar spinal cord greater than that recorded in nonwithdrawing rats. However, intrathecal injection of naloxone-methiodide increased Fos-like immunoreactivity in laminae I/II and the ventral horn to a greater extent than did subcutaneous injection of naloxone. We hypothesize that the increased Fos expression after systemic withdrawal in spinally-transected rats results from a loss of descending inhibitory control that is activated during withdrawal. The increase in withdrawal-induced Fos-like immunoreactivity after rhizotomy may be secondary to loss of inhibitory controls exerted by large diameter primary afferents or to deafferentation-induced reorganization in the dorsal horn. Since capsaicin did not alter the magnitude of Fos-like immunoreactivity in withdrawing rats, we conclude that hyperactivity of opioid receptor-laden C-fibres is not a necessary contributor to the withdrawal-induced increase in Fos-like immunoreactivity in laminae I and II. Taken together with the results recorded after intrathecal injection of naloxone-methiodide in tolerant rats, we conclude that the pattern of lumbar spinal cord Fos expression following systemic withdrawal is primarily a consequence of increased activity in opioid receptor-containing circuits intrinsic to the dorsal horn and that the magnitude of Fos expression is normally dampened by supraspinal and primary afferent-derived inhibitory inputs.

Formalin-evoked Fos expression in spinal cord is enhanced in morphine-tolerant rats

It has been hypothesized that tolerance to the analgesic effects of morphine results from the development of a compensatory response in neurons that express the opioid receptor or in neural circuits in which those neurons participate. The compensatory response establishes a sensitized state in these neurons. To determine if administration of a noxious stimulus can unmask a sensitization of dorsal horn neurons in morphine-pelleted rats, we injected morphine-tolerant and control rats with formalin into the plantar surface of the hindpaw, counted the number of flinches for 2 h and then processed the lumbar cord for Fos immunocytochemistry. Although there was no significant difference in flinching behavior between the morphine-tolerant and control groups, we recorded significantly increased total Fos-like immunoreactivity at the L4/5 and L2 segments both ipsilateral and contralateral to the site of formalin injection in the morphine-tolerant rats compared to the control rats. These results suggest that lumbar spinal cord neurons are sensitized during the development of tolerance, that the sensitization can be unmasked by the administration of a noxious stimulus and that it is manifested as increased expression of the Fos protein in the lumbar cord.

Noxious cutaneous thermal stimuli induce a graded release of endogenous substance P in the spinal cord: imaging peptide action in vivo

Dorsal root ganglia (DRG) neurons synthesize and transport substance P (SP) to the spinal cord where it is released in response to intense noxious somatosensory stimuli. We have shown previously that SP release in vivo causes a rapid and reversible internalization of SP receptors (SPRs) in dorsal horn neurons, which may provide a pharmacologically specific image of neurons activated by SP. Here, we report that noxious heat (43 degrees, 48 degrees, and 55 degrees C) and cold (10 degrees, 0 degrees, -10 degrees, and -20 degrees C) stimuli, but not innocuous warm (38 degrees C) and cold (20 degrees C) stimuli, applied to the hindpaw of anesthetized rats induce SPR internalization in spinal cord neurons that is graded with respect to the intensity of the thermal stimulus. Thus, with increasing stimulus intensities, both the total number of SPR+ lamina I neurons showing SPR internalization and the number of internalized SPR+ endosomes within each SPR immunoreactive neuron showed a significant increase. These data suggest that thermal stimuli induce a graded release of SP from primary afferent terminals and that agonist dependent receptor endocytosis provides evidence of a spatially and pharmacologically unique "neurochemical signature" after specific somatosensory stimuli.

Transneuronal labeling of a nociceptive pathway, the spino-(trigemino-)parabrachio-amygdaloid, in the rat

Transneuronal tracing of a nociceptive pathway, the spino-(trigemino)-parabrachio-amygdaloid pathway, was performed using an alpha-herpes virus, the Bartha strain of pseudorabies virus (PRV). Microinjection of PRV into the central nucleus of the amygdala (Ce) resulted in progressive retrograde and transneuronal infection of a multisynaptic circuit involving neurons in the brainstem and spinal cord as detected immunocytochemically. At short survival (26 hr), retrogradely labeled neurons were concentrated in the external lateral nucleus of the parabrachial complex (elPB) but were absent from both the trigeminal nucleus caudalis (TNC) and the spinal cord. At longer survivals (52 hr), labeled cells were present in lamina I of both the TNC and spinal dorsal horn. Retrograde labeling from the Ce with Fluoro-gold demonstrated that elPB neurons have long dendrites extending laterally into the terminal field of spinal and trigeminal afferents, where transneuronal passage of PRV to these afferents could occur. Even longer survivals (76 hr) resulted in a columnar pattern of cell labeling in the TNC and spinal dorsal horn that extended from lamina I into lamina II. At this longest survival, primary sensory neurons became infected. Bilateral excitotoxic lesions of the elPB blocked almost all viral passage from the Ce to superficial laminae of the TNC and spinal dorsal horn. These results demonstrate that nociceptive input to the amygdala is relayed from neurons in lamina I through the elPB. We propose that this modular arrangement of lamina I and II neurons may provide the basis for spinal processing of peripheral input to the amygdala.

The differential contribution of capsaicin-sensitive afferents to behavioral and cardiovascular measures of brief and persistent nociception and to Fos expression in the formalin test

Intraplantar injection of dilute formalin evokes brief (Phase 1) and persistent (Phase 2) increases in primary afferent activity, pain behavior, and cardiovascular responses, and induces spinal cord Fos-like immunoreactivity (Fos-LI). Although previous studies demonstrated that the destruction of small diameter primary afferents with neonatal capsaicin treatment decrease formalin-evoked nociception, these studies only evaluated behavioral responses, and did not distinguish between Phase 1 and 2. To address these questions, we simultaneously evaluated formalin-evoked pain behavior (flinching of the afflicted paw), cardiovascular responses (heart rate and mean arterial pressure), and lumbar spinal cord Fos expression in control rats and in rats treated with capsaicin (100 mg/kg) one day postpartum. We found that neonatal capsaicin-treated rats, compared to controls, exhibited similar cardiovascular responses and slightly less flinching behavior during Phase 1. During Phase 2, however, capsaicin-treated rats exhibited 59% less flinching and 45% smaller heart rate responses. Also, in capsaicin-treated rats, we counted 59% fewer Fos-labeled neurons in the spinal cord. These results indicate that capsaicin-sensitive afferents contribute to formalin-evoked behavioral and cardiovascular responses and to spinal cord neuronal responses. The differential effect of neonatal capsaicin on nociception during Phase 1 and Phase 2 suggests that sensitization mechanisms during Phase 1 do not contribute to the magnitude of nociceptive responses during Phase 2.

NMDA-receptor regulation of substance P release from primary afferent nociceptors

Severe or prolonged tissue or nerve injury can induce hyperexcitability of dorsal horn neurons of the spinal cord, resulting in persistent pain, an exacerbated response to noxious stimuli (hyperalgesia), and a lowered pain threshold (allodynia). These changes are mediated by NMDA (N-methyl-D-aspartate)-type glutamate receptors in the spinal cord. Here we report that activation of the NMDA receptor causes release of substance P, a peptide neurotransmitter made by small-diameter, primary, sensory 'pain' fibres. Injection of NMDA in the cerebrospinal fluid of the rat spinal cord mimicked the changes that occur with persistent injury, and produced not only pain, but also a large-scale internalization of the substance P receptor into dorsal horn neurons, as well as structural changes in their dendrites. Both the pain and the morphological changes produced by NMDA were significantly reduced by substance P-receptor antagonists or by elimination of substance P-containing primary afferent fibres with the neurotoxin capsaicin. We suggest that presynaptic NMDA receptors located on the terminals of small-diameter pain fibres facilitate and prolong the transmission of nociceptive messages, through the release of substance P and glutamate. Therapies directed at the presynaptic NMDA receptor could therefore ameliorate injury-evoked persistent pain states.

Neurokinin-1 receptor-immunoreactive sympathetic preganglionic neurons: target specificity and ultrastructure

Substance P is involved in cardiovascular control at the spinal cord level, where it acts through neurokinin-1 receptors. In this study we used immunocytochemistry and retrograde tracing to investigate the presence of the neurokinin-1 receptor and its ultrastructural localization in rat sympathetic preganglionic neurons that project to the superior cervical ganglion or the adrenal medulla. Immunofluorescence for the neurokinin-1 receptor outlined the somatic and dendritic surfaces of neurons in autonomic subnuclei of spinal cord segments T1-T12, whereas immunofluorescence for the tracer, cholera toxin B subunit, filled retrogradely labelled cells. There was a significant difference in the proportion of neurokinin-1 receptor-immunoreactive sympathetic preganglionic neurons supplying the superior cervical ganglion and the adrenal medulla. Thirty-eight percent of the neurons that projected to the superior cervical ganglion were immunoreactive for the neurokinin-1 receptor compared to 70% of neurons innervating the adrenal medulla. Of neurons projecting to the superior cervical ganglion, significantly different proportions showed neurokinin-1 receptor immunoreactivity in spinal cord segment T1 (15%) versus segments T2 T6 (45%). At the ultrastructural level, neurokinin-1 receptor staining occurred predominantly on the inner leaflets of the plasma membranes of retrogradely labelled sympathetic preganglionic neurons. Deposits of intracellular label were often observed in dendrites and in the rough endoplasmic reticulum and Golgi apparatus of cell bodies. Neurokinin-1 receptor immunoreactivity was present at many, but not all, synapses as well as at non-synaptic sites, and occurred at synapses with substance P-positive as well as substance P-negative nerve fibres. Only 37% of the substance P synapses occurred on neurokinin-1-immunoreactive neurons in the intermediolateral cell column. These results show that presence of the neurokinin-1 receptor in sympathetic preganglionic neurons is related to their target. The ultrastructural localization of the receptor suggests that sympathetic preganglionic neurons may be affected (i) by substance P released at neurokinin-1 receptor-immunoreactive synapses, (ii) by other tachykinins (e.g., neurokinin A), which co-localize in substance P fibres in the intermediolateral cell column, acting through other neurokinin receptors, and (iii) by substance P that diffuses to neurokinin-1 receptors from distant sites.

GABA-immunoreactive boutons contact identified OFF and ON cells in the nucleus raphe magnus

The pontomedullary raphe magnus (RM) contains two physiologically defined types of neurons that participate in the opioid-induced modulation of dorsal horn nociceptive messages: OFF cells, which decrease, and ON cells, which increase their discharge rates when reflex behavior is evoked by noxious pinch or heat. Because both types of neuron have inhibitory inputs and because there is evidence that gamma-aminobutyric acid (GABA) inhibitory mechanisms within RM contribute to the antinociceptive action of opioids, we have sought anatomical evidence for a direct GABAergic input to OFF and ON cells. In this study, cells of each type located in the RM were electrophysiologically defined and intracellularly filled with horseradish peroxidase or Neurobiotin. One cell of each type was labeled in the cat, and 2-3 cells of each type were labeled in the rat. Thin sections were labeled by a postembedding immunogold procedure by using an antibody directed against glutaraldehyde-conjugated GABA. GABA-immunoreactive (GABA-ir) boutons contained small, round, clear vesicles and made symmetrical synapses with identified dendrites. GABA-ir boutons were apposed to soma and to proximal and distal dendrites of both cell types in both species. These findings demonstrate direct GABAergic input to identified OFF and ON cells in the RM. J. Comp. Neurol. 378:196-204, 1997.