There is growing evidence supporting a role for histamine H3 receptors in the modulation of pathological pain. To further our understanding of this modulation, we examined the effects of a selective H3 receptor antagonist, 6-((3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy)-N-methyl-3-pyridinecarboxamide (GSK189254), on spinal neuronal activity in neuropathic (L5 and L6 ligations) and sham rats. Systemic administration of GSK189254 (0.03–1 mg/kg i.v.) dose-dependently decreased both evoked (10-g von Frey hair for 15 s) and spontaneous firing of wide dynamic range (WDR) neurons in neuropathic, but not sham-operated, animals. The effects on spontaneous firing suggest that H3 receptors may have a role in central sensitization and/or modulating non-evoked pain. Transection of the spinal cord (T9-T10) completely eliminated the effects (both evoked and spontaneous) of systemic GSK189254 (1 mg/kg, i.v.) on WDR neuronal firing in neuropathic rats, indicating that the descending modulatory system has an important role in the H3-related dampening of spinal neuronal activity. Subsequently, lesions of the locus coeruleus, or direct GSK189254 (3 and 10 nmol/0.5 μl) injections into this site, demonstrate that the locus coeruleus is a key component of the H3 descending modulatory pathway. In summary, blockade of H3 receptors reduces spontaneous firing as well as the responses of spinal nociceptive neurons to mechanical stimulation. This effect is in large part mediated via supraspinal sites, including the locus coeruleus, that send descending projections to modulate spinal neuronal activity.
The histaminergic H3 receptor is one of four G-coupled histamine receptors and is mainly thought to be a presynaptic autoreceptor regulating the release of histamine, but it also exists as a heteroreceptor located on nonhistaminergic neurons influencing the release of other transmitters such as norepinephrine and serotonin (Arrang et al., 1983; Gemkow et al., 2009; Giannoni et al., 2010). The receptor has been associated with cognition and sleep (Onodera et al., 1998; Giovannini et al., 1999; Passani et al., 2004; Fox et al., 2005; Komater et al., 2005; Esbenshade et al., 2008), but more recently there has been emerging data pointing to a role for H3 in pain modulation (Cannon et al., 2003, 2007; Cannon and Hough, 2005; Medhurst et al., 2007, 2008; Hsieh et al., 2010). The H3 receptor is localized to key sites in the nociceptive system such as the dorsal horn of the spinal cord and several supraspinal regions including the locus coeruleus (LC) (Pillot et al., 2002; Korotkova et al., 2005; Cannon et al., 2007; Medhurst et al., 2008). Although there has been some conflicting pharmacological data with H3 agonists and a nonselective antagonist (Owen et al., 1994; Farzin et al., 2002; Cannon and Hough, 2005), more recent data with selective H3 receptor antagonists demonstrate a consistent level of efficacy in reducing allodynia and hyperalgesia in models of neuropathic, osteoarthritic, and inflammatory pain (Medhurst et al., 2007, 2008; Hsieh et al., 2010).
H3-related modulation of behavioral nociception may be mediated by multiple sites in the central nervous system, but the data gathered to date are limited and to some extent are contrasting. Intracerebroventricular injection of histamine enhanced allodynia in a model of neuropathic pain, and this may have been at least partially caused by activity at H3 receptors (Huang et al., 2007). However, intrathecal injection of H3 receptor agonists attenuated responses to tail pinch in naive rats (Cannon et al., 2003). Likewise, systemic injection of an agonist reduced the occurrence of formalin-induced nocifensive behaviors, and this effect was reversed by intrathecal injection of a nonselective H3 antagonist (Cannon et al., 2007). This suggests that H3 receptor agonists may have either a pronociceptive or antinociceptive effect depending on the site of action. Nonetheless, systemic, intrathecal, and intracerebroventricular injection of a selective H3 receptor antagonist improved grip force deficits in osteoarthritic rats (Hsieh et al., 2010).
To enhance our understanding of the role of H3 receptors in pain modulation, we have taken a mechanistic approach by studying the effects of an H3 receptor antagonist on a key class of nociceptive neuron in the spinal cord. The activity of spinal wide dynamic range (WDR) neurons is influenced by primary afferent input from A and C fibers, segmental and extra-segmental spinal connections, as well as output from the descending modulatory system (D'Mello and Dickenson, 2008). Animal behavior can be difficult to interpret, and responses to evoked stimulation can be delayed or facilitated by many factors (e.g., motoric, shifts in attention or anxiety, sleep state), even though the desired readout is focused on pain modulation. Therefore, examining the activity of an important group of nociceptive neurons after systemic and site-specific injections of an H3 receptor antagonist will demonstrate if the “pain system” is engaged by this mechanism, and where this modulation originates. To this end, the H3 receptor antagonist 6-((3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy)-N-methyl-3-pyridinecarboxamide (GSK189254) was injected systemically in intact, spinal-transected, and LC-lesioned neuropathic rats. In behavioral assays, systemic injection of GSK189254 has been shown to be anti-allodynic in neuropathic rats (Medhurst et al., 2008; Hsieh et al., 2010). In the current study, GSK189254 was also injected directly into LC and spinal tissue to evaluate its modulation of WDR activity. The LC is an important site in descending noradrenergic modulation of spinal nociceptive signaling (Millan, 2002; Pertovaara, 2006) and receives histaminergic projections from the tuberomammillary nucleus (Panula et al., 1989). Histamine has been shown to excite LC noradrenergic activity (Korotkova et al., 2005).
Materials and Methods
All animal handling and experimental protocols were approved by Abbott's Institutional Animal Care and Use Committee and conducted in accordance with the ethical principles for pain-related animal research of the American Pain Society. Male Sprague-Dawley rats (Charles River Laboratories, Inc., Wilmington, MA), weighing 250 to 400 g, were used for all experiments and housed in a temperature-controlled room with a 12-h day/night cycle. Food and water were available ad libitum.
The surgical procedure to produce the spinal nerve ligation (SNL) model of neuropathic pain in rats has been described previously (Kim and Chung, 1992). Either a unilateral tight ligation (5-0 black braided silk) of the L5 and L6 spinal nerves or sham surgery was performed on isoflurane-anesthetized rats 14 days before electrophysiological experiments.
On the day of neuronal recording, all SNL animals were tested for the development of mechanical allodynia. Animals that did not respond to a 6-g von Frey hair stimulation of the ipsilateral hind paw (8-s cutoff) were excluded from further study in electrophysiological experiments. All rats were then anesthetized with pentobarbital (50 mg/kg i.p.). In animals receiving systemic administration of compounds, a catheter was placed into the left and right external jugular veins. In animals receiving intraspinal or intra-LC injection of compound, a catheter was placed only into the left external jugular vein. A laminectomy was then performed to remove vertebral segments T12-L4 to gain access to spinal regions that receive input from the hind paw (spinal segments L4-L6). In a subgroup of rats, an additional small laminectomy was performed at the T9-T10 (vertebral segment) level to permit transection of the spinal cord with a curved scalpel and eliminate descending modulation of the recorded WDR neuron. Animals were secured in a stereotaxic apparatus (David Kopf Instruments, Tujunga, CA) by clamps attached to the vertebral processes on either side of the exposure site. The exposed area of the spinal cord was first enveloped by agar and then filled with mineral oil. In animals receiving bilateral injection of compound into the LC, or bilateral lesions of the LC, two small craniectomies were performed to allow stereotaxic placement of a cannula or lesioning electrode into the LC. The cannulas and electrodes entered the brain at a 15° angle (rostrally), and coordinates for LC placement were 3.6 mm caudal to lambda, ± 1.3 mm lateral from the saggital suture, and 7.2 mm from the skull surface (Paxinos and Watson, 2007). In some rats, bilateral electrolytic lesions of the locus coeruleus were made by passing 1.0 mA d.c. current for 10 s. A stable plane of anesthesia was maintained throughout the experiment by a continuous infusion of propofol at a rate of 8 to 12 mg/kg/h i.v. Body temperature was kept at approximately 37°C by placing the animals on a circulating water blanket.
Platinum-iridium microelectrodes (Frederick Haer, Brunswick, ME) were used to record extracellular activity of dorsal horn WDR neurons. Spike waveforms were monitored on an oscilloscope throughout the experiment, digitized (32 points), and then stored for off-line analysis (SciWorks; Datawave Technologies, Longmont, CO) to ensure that the unit under study was unambiguously discriminated throughout the experiment. Only one neuron was recorded per animal, except in two experiments in which waveforms from two separate neurons were clearly distinguishable.
At the onset of each experiment, spontaneous neuronal firing was recorded for 5 min to determine baseline levels. Each neuron was then characterized to confirm WDR response patterns. In order, the neuronal receptive field was gently tapped, brushed, given a noxious pinch with forceps, and stimulated with a 10-g von Frey hair for 2 to 3 s each. A neuron was classified as WDR when it responded to noxious and innocuous stimuli with a graded response. After neuronal characterization, baseline evoked firing was measured. A low-intensity mechanical stimulus (10-g von Frey hair) was used as the evoked stimulus to relate this electrophysiological study to the antiallodynic action of GSK189254 in behavioral studies (Medhurst et al., 2008; Hsieh et al., 2010). The 10-g von Frey hair was applied to the neuronal receptive field on the ipsilateral hind paw for 15 s, and this was repeated three times with a 5-min interstimulus interval. After baseline measurements, GSK189254 or vehicle was administered systemically or in site-specific injections. Spontaneous and evoked WDR neuronal activity was then remeasured 5, 15, 25, and 35 min after systemic and site injections.
Delivery and Preparation of Compounds for Electrophysiology.
GSK189254 (Medhurst et al., 2007) is a histaminergic H3 receptor antagonist that has been shown to have anti-allodynic effects in neuropathic animals (Medhurst et al., 2008; Hsieh et al., 2010) and is very potent at human (pKi = 9.59 −9.90) and rat (pKi = 8.51–9.17) H3 receptors. GSK189254 is reported to be very selective for the H3 receptor, including over other histamine receptors, produced less than 50% inhibition of 50 other receptors, ion channels, and enzymes at 1 μM, and does not alter serotonin release (Medhurst et al., 2007). Comparable data were generated for 10 μM GSK189254 including only 18% inhibition of α2 adrenergic receptors (Cerep, Celle L'Evescault, France). Thus, this compound is an excellent tool to investigate H3 mechanism of action. Systemic injection of GSK189254 (0.03–1 mg/kg i.v.) or vehicle (10% dimethyl sulfoxide in polyethylene glycol) was administered intravenously over a 7-min period (1 ml/kg). For intra-LC and intraspinal injection, GSK189254 was prepared in 40% N-methyl-2-pyrrolidone and sterile water. GSK189254 (3 and 10 nmol) or vehicle was injected directly into the LC (in 0.5 μl) through a 28-gauge stainless-steel cannula (Plastics One, Roanoke, VA) attached to a 2-μl Hamilton syringe (Hamilton Co., Reno, NV) via a length of PE-50 tubing. Infusions into the LC were completed on one side over a 1-min period, then repeated on the opposite side (right and left LC were each injected first in alternating experiments). For intraspinal delivery (McGaraughty et al., 2006) of 10 nmol GSK189254 or vehicle, a glass infusion pipette (o.d. 75–80 μm) with an angled beveled tip was attached to the recording electrode in such a way that the tips were separated by approximately 300 μm laterally and 30 to 100 μm dorsoventrally. The electrode and pipette were simultaneously lowered into the spinal tissue. The infusion pipette was attached to a 2-μl Hamilton syringe with a length of PE-50 tubing, and 0.5 μl of solution was delivered over a 2-min period.
LC injection sites were infused with Chicago Sky Blue dye through the cannula at the end of the experiment. The rat was then killed with an overdose of propofol and perfused intracardially with physiological saline followed by 10% formalin. Lesion and infusion sites were histologically verified. Lesions typically encompassed the LC and some surrounding tissue. Data from eight rats were not used for the study because the injection cannula missed the LC or the tract could not be found.
Postdrug spontaneous firing was measured in the 5 min leading up to each stimulus at 5, 15, 25, and 35 min after injection. Evoked activity was measured by counting the spikes during the time of stimulus presentation. Thus, the total number of spikes was counted for 15 s during the 10-g von Frey stimulation. A mean of the three predrug-evoked stimuli was calculated to represent baseline-evoked activity. For each neuron, the firing rate was converted to spikes/s, and the postdrug spontaneous and evoked activity was calculated as a percentage of the respective baseline levels.
Baseline levels (spikes/s) of evoked and spontaneous firing were compared for sham and SNL groups by using a t test. For comparisons to baseline firing levels, statistical significance of postdrug activity was established by using a repeated-measures ANOVA followed by a Fisher's least significant difference test. A two-way ANOVA followed by a Fisher's least significant difference test was used for comparison of drug and vehicle groups. Post hoc tests were performed only if the relevant parameter was significant in the ANOVA. All data are presented as mean ± S.E.M., and a difference was considered significant if it reached a p value < 0.05.
Baseline Neuronal Activity.
Discharge activity was recorded from 91 WDR neurons with a mean depth of 744.1 ± 17.5 μm from the surface of the spinal cord (approximately lamina V in the deep dorsal horn). Except in two experiments in which waveforms from two separate neurons were clearly distinguishable, only one neuron was recorded per animal (n = 89 rats).
The mean baseline (predrug) spontaneous and evoked firing for WDR neurons across groups is shown in Table 1. Baseline spontaneous firing of SNL rats was significantly elevated (2-fold) compared with the sham animals (p < 0.01) and is comparable with previous reports from our group and others (Chapman et al., 1998a; Chu et al., 2004; Suzuki and Dickenson, 2006; McGaraughty et al., 2007, 2008a,b, 2009). Application of the 10-g von Frey hair stimulus for 15 s to the neuronal receptive field evoked similar levels of baseline activity in the SNL and sham rats and is also consistent with previous reports (Elmes et al., 2004; Sagar et al., 2005; McGaraughty et al., 2007, 2008a,b, 2009).
Systemic GSK189254 in SNL and Sham Rats.
Systemic administration of GSK189254 (0.03–1 mg/kg i.v.) dose dependently attenuated both spontaneous and von Frey-evoked firing of WDR neurons in SNL rats (Fig. 1). The 0.03 and 0.1 doses of GSK189254 significantly (p < 0.05) reduced WDR responses to the 10-g von Frey hair stimulus, but the evoked response was nearly eliminated by the 1 mg/kg dose (decreasing firing by 82.6 ± 7.5% from baseline levels 35 min after injection) (Fig. 1A). Spontaneous firing of WDR neurons was decreased (p < 0.05) by the 0.1 and 1 mg/kg doses of GSK189254, but not by the 0.03 dose (Fig. 1B). The greatest magnitude of effect was observed 35 min after injection of 1 mg/kg of GSK189254, when spontaneous firing was decreased by 83.3 ± 4.4% from baseline levels. The effects on spontaneous and evoked firing at the two highest doses of GSK189254 typically occurred within the first 15 min after injection.
Sham animals received only the highest dose of GSK189254 (1 mg/kg i.v.). Neither evoked (10-g von Frey hair) nor the spontaneous firing of WDR neurons was altered by the systemic injection of GSK189254 (Fig. 2), and this was significantly different (p < 0.05) than the effects observed in the SNL rats.
Systemic GSK189254 in SNL Spinally Transected and LC-Lesioned Animals.
Transection of the spinal cord at the T9-T10 vertebral level completely abolished (p < 0.05) the systemic effects of 1 mg/kg i.v. GSK189254 on both evoked (10-g von Frey hair) and spontaneous firing of WDR neurons in SNL rats (Fig. 3). Descending influences on the recorded WDR neurons from supraspinal regions should have been eliminated because the recording sites were posterior to the site of transection. Similar to the transection data, bilateral lesions of the LC also completed eradicated (p < 0.05) the systemic effects of 1 mg/kg i.v. GSK189254 on WDR neuronal activity in SNL rats at every time point examined (Fig. 4).
Intra-LC Injections of GSK189254 in SNL Rats.
Intra-LC injection of GSK189254 (3 and 10 nmol) significantly (p < 0.05) reduced both evoked (10-g von Frey hair) and spontaneous firing of WDR neurons by 5 min postinjection in SNL rats (Fig. 5). For both doses, evoked responses of WDR neurons was significantly (p < 0.05) depressed compared with baseline levels for the entire 35 min of postdrug recording (Fig. 5A), and the largest reduction in evoked firing (46.8 ± 9.8%) was observed at the last time point after the 10-nmol dose. After the initial inhibition of spontaneous firing with the 3-nmol dose of GSK189254 (5-min time point), this activity subsequently returned to baseline firing levels (Fig. 5B). The 10-nmol dose significantly attenuated spontaneous (p < 0.01) WDR firing at all time points examined with a maximally observed decrease of 55.5 ± 9.2% from baseline levels 25 min after injection.
Intraspinal Injections of GSK189254 in SNL Rats.
The 10-nmol dose of GSK189254 did not alter the spontaneous or evoked (10-g von Frey hair) firing of WDR neurons when injected directly into spinal tissue close to the recording microelectrode (Fig. 6). The concentration of GSK189254 injected intraspinally was the same as that injected into the LC (10 nmol in 0.5 μl).
Histaminergic H3 receptors have recognized roles in diverse functions such as sleep and cognition (Onodera et al., 1998; Giovannini et al., 1999; Passani et al., 2004; Fox et al., 2005; Esbenshade et al., 2008). A contribution to pain modulation has also emerged. Systemic injection of selective H3 receptor antagonists have been shown efficacious in different behavioral models of pathological pain, including several models of neuropathic pain (Medhurst et al., 2007, 2008; Hsieh et al., 2010). Consistent with the behavioral results, systemic injection of an H3 receptor antagonist dose-dependently decreased the firing of spinal WDR neurons in SNL neuropathic rats. Thus, blocking H3 receptors reduced the output of an important class of neurons in the pain pathway, and this action probably contributed to the observed behavioral effects.
The effect of GSK189254 on WDR neurons was not observed in sham-neuropathic rats, suggesting that H3 receptors have an enhanced role in states of pathological pain. Previous studies have demonstrated that selective H3 receptor antagonists were not effective in altering the behavioral responses of naive animals to low- and high-intensity mechanical stimulation (Medhurst et al., 2007, 2008). These compounds were subsequently found to reduce tactile allodynia in the SNL, chronic constriction, and varicell-zoster models of neuropathic pain (Medhurst et al., 2007, 2008; Hsieh et al., 2010). Thus, data gathered with selective H3 receptor antagonists have been consistent in that the compounds are efficacious in reducing indications of hypersensitivity in animals with chronic pain, but are not effective in naive or sham animals. Mechanical nociception was also unaltered in H3 knockout animals (Cannon et al., 2003). Although injection of the H3 antagonist thioperamide is reported to be antinociceptive in naive animals (Malmberg-Aiello et al., 1994; Farzin et al., 2002), this compound has been shown to be nonselective for the H3 receptor (Leurs et al., 2005; Gbahou et al., 2006). Nonetheless, studies with H3 receptor agonists suggest a role for this mechanism in uninjured animals, but these data can be contradictory (Malmberg-Aiello et al., 1994; Farzin et al., 2002; Cannon et al., 2003; Cannon and Hough, 2005) and may be related to the intensity of the test stimulus and modalities examined (Hough and Rice, 2011).
WDR neurons are intensity-driven cells that respond to stimuli in both the innocuous and noxious ranges and thus are likely to factor into signaling for both allodynia and hyperalgesia after an injury. Injection of GSK189254 to neuropathic rats reduced the responses of WDR neurons to low-intensity mechanical stimulation, which is consistent with the anti-allodynic effect of H3 receptor antagonists in behavioral assays (Medhurst et al., 2007, 2008; Hsieh et al., 2010). In addition to effects on evoked firing, the H3 receptor antagonist reduced the spontaneous firing of WDR neurons in SNL but not sham rats. WDR spontaneous firing was significantly higher in SNL rats compared with the sham controls and probably reflects injury-related sensitization of this class of spinal neuron (Chapman et al., 1998b; Suzuki and Dickenson, 2006; McGaraughty et al., 2009). This heightened level of spontaneous firing may also indicate that there was ongoing or non-evoked discomfort in the animal because higher levels of WDR activity code for increased somatosensory intensity. Non-evoked pain is observed in the majority of patients with chronic pain and is a primary reason for seeking medical care (Birklein et al., 2000; Backonja and Stacey, 2004). There have been recent advances in measuring non-evoked pain in animals, but this is still a difficult endpoint to accurately assess (King et al., 2009; Langford et al., 2010; Leys et al., 2011; Miyagi et al., 2011). Suzuki and Dickenson (2006) have reported that heightened spontaneous discharges of WDR neurons are either reduced or unaltered by drugs that are effective or ineffective, respectively, in treating non-evoked pain in humans. Thus, blockade of H3 receptors reduces neuronal sensitization and possibly dampens non-evoked discomfort in nerve-injured rats.
The observed actions of systemic GSK189254 on WDR neuronal firing probably were mediated through supraspinal sites, which triggered descending inhibition of WDR neurons. To first assess this, it was demonstrated that transection of the spinal cord, at a level between the brain and the recorded WDR neuron, eliminated the effects of systemic GSK189254 on both evoked and spontaneous WDR firing in SNL animals. This clearly indicated that the brain was needed for the inhibitory effects of the H3 receptor antagonist. Subsequently, given the well acknowledged role of the LC in descending pain modulation (Millan 2002; Pertovaara, 2006) and localization of H3 receptors to this region (Panula et al., 1989; Pillot et al., 2002), we then performed bilateral lesions of the LC region. Similar to the transection data, these lesions abolished the systemic effects of the H3 antagonist on WDR neuronal firing. Direct local injection of GSK189254 confirmed LC involvement as the compound delivered to this site dose-dependently decreased both the evoked and spontaneous firing of WDR neurons in injured rats. GSK189254 readily crosses the blood-brain barrier when given systemically (Hsieh et al., 2010) and thus would have access to the LC after systemic administration. This is the first demonstration that selective blockade of supraspinal H3 receptors attenuates nociceptive signaling in neuropathic rats. Intracerebroventricular injection of GSK189254 improved grip force deficits in a model of osteoarthritis, and the nonselective antagonist, thioperamide (intracerebroventricularly), increased mechanical thresholds in rats with a partial ligation of the sciatic nerve (Huang et al., 2007). Taken together, supraspinal H3 receptors clearly contribute to the modulation of pathological nociception affecting both spinal neuronal activity and behavioral readouts of nociception.
Although it is likely that GSK189254 interacted with more than one supraspinal site, the LC seems to be a critical component to the attenuation of WDR activity. Electrical stimulation of the LC modulates nociception through both ascending and descending pathways, and these effects are mediated primarily through the activation of noradrenergic fibers (Jones, 1991; Zhang et al., 1997; Pertovaara, 2006). Stimulation of the LC region inhibits the firing of spinal nociceptive neurons, including WDR neurons, through descending projections (Hodge et al., 1983; Jones and Gebhart, 1986; Tsuruoka et al., 2004). H3 receptors in the LC are thought to be presynaptic autoreceptors as opposed to the somatodendritic heteroreceptors (Pillot et al., 2002). Using a slice preparation, it was shown that histamine excites noradrenergic neurons in the LC through H1 and H2 receptors (Korotkova et al., 2005). Thus, blockade of H3 autoreceptors in the LC after GSK189254 administration could lead to accumulation of histamine in the synapse and trigger excitation of noradrenergic fibers through H1 and H2 receptors. Future studies need to confirm GSK189254-related release of histamine in the LC and how this may affect noradrenergic neurons.
Activation of the noradrenergic fibers may be one mechanism through which intracerebroventricular injection of histamine decreases mechanical nociception in nerve-ligated animals (Huang et al., 2007). It is also worth noting that nociceptive thresholds in naive animals are largely unaffected by bilateral lesions of the LC or disruption of the noradrenergic system (Tsuruoka and Willis, 1996; Jasmin et al., 2002). However, noradrenergic neurons in the LC contribute to both the induction and maintenance of neuropathic pain (Brightwell and Taylor, 2009). In addition, the responses of LC neurons to nociceptive stimulation are significantly enhanced in neuropathic rats compared with sham or uninjured animals (Viisanen and Pertovaara, 2007). Thus, the GSK189254-induced inhibition of WDR neuronal firing in SNL, but not sham, rats is consistent with an enhanced role for the LC after injury.
Surprisingly, intraspinal delivery of GSK189254 did not alter the firing of WDR neurons. Immunohistochemical and binding data have shown that H3 receptors are localized to spinal tissue (Cannon et al., 2007; Medhurst et al., 2008). The injected concentration of antagonist was the same in both the spinal and LC sites, and because these were directly into tissue, it should result in relatively quick effects on the WDR readout. The intra-LC effect was seen within 5 min of injection, but WDR activity did not change up to 35 min after intraspinal delivery. The intraspinal technique has been used to investigate several different pharamacologies, and each has been effective in reducing WDR activity (McGaraughty et al., 2006, 2008a,b, 2009). It may be considered that a higher dose of GSK189254 should have been injected into the spinal cord, but the data with spinal transected rats indicate that this added dose would still be ineffective in attenuating WDR firing. Intrathecal injection of GSK189254 improved grip force deficits in a rat model of osteoarthritis (Hsieh et al., 2010), and agonists attenuated acute nociception as well as the second phase of the formalin assay in naive rats (Cannon et al., 2003, 2007). It is possible that the disparity in spinal efficacy could be related to differences in disease state or modality tested. Other possibilities include that behavior was influenced by some ancillary action of the mechanism (e.g., motoric) after intrathecal delivery, or a class of spinal neurons other than WDRs contributed to the observed behavioral effects.
In summary, systemic injection of a selective H3 receptor antagonist reduces signaling in an important class of spinal nociceptive neurons involved with transmitting this information to the brain. This effect was observed in neuropathic rats but not in sham rats, and the inhibition was mediated, at least partially, through descending pathways originating in the locus coeruleus. These physiological data demonstrate that blockade of the H3 receptor is an effective means to dampen activity in the nociceptive pathway after a nerve injury and provides a mechanistic explanation for previous behavioral data. The effects on both mechanically evoked and spontaneous firing of WDR neurons in SNL rats suggest that administration of an H3 receptor antagonist may be an effective means to alleviate both tactile allodynia and non-evoked discomfort after nerve injury.
Participated in research design: McGaraughty and Chu.
Conducted experiments: McGaraughty and Chu.
Performed data analysis: McGaraughty.
Wrote or contributed to the writing of the manuscript: McGaraughty, Cowart, and Brioni.
All research was funded by Abbott Laboratories.
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
- locus coeruleus
- wide dynamic range
- spinal nerve ligation
- analysis of variance
- Received March 25, 2012.
- Accepted June 21, 2012.
- Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics