Neural Mechanisms That Underlie Angina-Induced Referred Pain in the Trigeminal Nerve Territory: A c-Fos Study in Rats.

The present study was designed to determine whether the trigeminal sensory nuclear complex (TSNC) is involved in angina-induced referred pain in the trigeminal nerve territory and to identify the peripheral nerve conducting nociceptive signals that are input into the TSNC. Following application of the pain producing substance (PPS) infusion, the number of Fos-labeled cells increased significantly in the subnucleus caudalis (Sp5C) compared with other nuclei in the TSNC. The Fos-labeled cells in the Sp5C disappeared when the left and right cervical vagus nerves were sectioned. Lesion of the C1-C2 spinal segments did not reduce the number of Fos-labeled cells. These results suggest that the nociceptive signals that conduct vagal afferent fibers from the cardiac region are input into the Sp5C and then projected to the thalamus.

Biological implications of coeruleospinal inhibition of nociceptive processing in the spinal cord.

The coeruleospinal inhibitory pathway (CSIP), the descending pathway from the nucleus locus coeruleus (LC) and the nucleus subcoeruleus (SC), is one of the centrifugal pain control systems. This review answers two questions regarding the role coeruleospinal inhibition plays in the mammalian brain. First is related to an abnormal pain state, such as inflammation. Peripheral inflammation activated the CSIP, and activation of this pathway resulted in a decrease in the extent of the development of inflammatory hyperalgesia. During inflammation, the responses of the dorsal horn neurons to graded heat stimuli in the LC/SC-lesioned rats did not produce a further increase with the increase of stimulus intensity in the higher range temperatures. These results suggest that the function of CSIP is to maintain the accuracy of intensity coding in the dorsal horn because the plateauing of the heat-evoked response in the LC/SC-lesioned rats during inflammation is due to a response saturation that results from the lack of coeruleospinal inhibition. The second concerns attention and vigilance. During freezing behavior induced by air-puff stimulation, nociceptive signals were inhibited by the CSIP. The result implies that the CSIP suppresses pain system to extract other sensory information that is essential for circumstantial judgment.

A possible synaptic configuration underlying coeruleospinal inhibition of visceral nociceptive transmission in the rat.

A synaptic arrangement underlying descending inhibition from the locus coeruleus/subcoeruleus (LC/SC) on visceral nociceptive transmission in the spinal cord was investigated in the anesthetized rat. Extracellular recordings were made from the L(6)-S(2) segmental level using a carbon filament glass microelectrode (4-6 MΩ). Colorectal distention (CRD) was produced by inflating a balloon inside the descending colon and rectum. All neurons tested responded to both CRD and to cutaneous pinch (a force of 613 g/mm(2)), indicating that nociceptive signals from visceral organs and nociceptive signals from the cutaneous receptive field converge on a single neuron. These neurons were divided into two groups based on their response to CRD: short latency-abrupt and short latency-sustained neurons. Electrical stimulation of the LC/SC (30 or 50 µA, 100 Hz, 0.1 ms pulses) inhibited both CRD-evoked and cutaneous pinch-evoked responses in short latency-abrupt and short latency-sustained neurons. When graded CRD (20, 40, 60, and 80 mmHg) was delivered, LC/SC stimulation produced a reduction in slope of the linear CRD intensity-response magnitude curve without a change in the response threshold in both short latency-abrupt (n = 42) and short latency-sustained neurons (n = 11). This result suggests that coeruleospinal inhibition of visceral nociceptive transmission is due to a synaptic configuration in which inhibitory and excitatory terminals are in close spatial proximity, including presynaptic inhibition.

The nucleus locus coeruleus/subcoeruleus contributes to antinociception during freezing behavior following the air-puff startle in rats.

An air puff elicits a startle response in mammals. Following the startle response, rats react with a defensive-like, immobile posture (DIP) of approximately 2-5s in length. We have previously reported that air-puff stimulation (APS) activates the nucleus locus coeruleus/subcoeruleus (LC/SC) so that the DIP is induced. The LC/SC is one of the structures that plays an important role in endogenous pain control. Our particular interest is whether APS induces nociceptive modulation. Rats were tested for behavioral nociception with heating of the tail. Rats whisked their tail following heating and then bit the heat source when the tail could not escape heating by tail flick. The tail flick latency (TFL) and the bite latency (BL) were measured as an indicator of nociception. Compressed house air (14.4 psi in strength, 0.1s in duration) was presented for APS. Two weeks before the experiment, the rats received bilateral injections of 6 µg of the neurotoxin 6-hydroxydopamine to specifically lesion noradrenaline-containing neurons of the LC/SC. APS produced prolongation of the TFL and the BL. In both the TFL and the BL, APS-induced prolongation was not observed in rats with the LC/SC lesions. When BLs were plotted against DIP periods, the BL was almost constant regardless of the change in the DIP period. These results suggest that (1) APS produces nociceptive modulation, (2) the LC/SC is involved in APS-induced nociceptive modulation, and (3) two APS-induced events, the DIP and nociceptive modulation, are a parallel phenomenon.

Descending pathways from activated locus coeruleus/subcoeruleus following unilateral hindpaw inflammation in the rat.

We have previously shown that the descending pathways from the locus coeruleus (LC)/subcoeruleus (SC) to the spinal cord are activated during peripheral inflammation, and that activation of this coeruleospinal system decreases development of hyperalgesia. Anatomical evidence suggests that the descending modulation system from the LC/SC should be active bilaterally during inflammation when the LC/SC either ipsilateral or contralateral to the site of inflammation is activated. In the present study, the development of hyperalgesia following the induction of unilateral hindpaw inflammation was compared between rats with either bilateral or unilateral lesions of the LC/SC and rats with a sham operation. Four hours after carrageenan injection, in the inflamed paw, paw withdrawal latencies (PWLs) to thermal stimuli of the bilateral LC/SC-lesioned rats were significantly shorter than those of the unilateral LC/SC-lesioned and the sham-operated rats, whereas the decreased PWLs of the unilateral LC/SC-lesioned rats were equivalent to those of the sham-operated rats. A difference in PWL between the bilateral and the unilateral LC/SC-lesioned rats was not observed in the contralateral non-inflamed paw. The result suggests that in the LC/SC both ipsilateral and contralateral to the inflamed paw, only neurons which project to the dorsal horn ipsilateral to the inflamed paw were activated following peripheral inflammation.

Descending modulation of visceral nociceptive transmission from the locus coeruleus/subcoeruleus in the rat.

The purpose of the present investigation was to examine whether electrical stimulation in the locus coeruleus/subcoeruleus (LC/SC) could modulate visceral pain evoked by noxious colorectal distention (CRD). Experiments were performed on 40 pentobarbital anesthetized male Sprague-Dawley rats. Extracellular potentials of single L(6)-S(2) spinal neuron were recorded with a carbon filament electrode. CRD (80 mmHg) was produced by inflating a balloon inside the descending colon and rectum. Electrical stimulation of the LC/SC (30, 50 and 70 microA, 100 Hz, 0.1 ms pulses) was delivered either ipsilaterally or contralaterally. Results showed that for 42/62 (68%) short-latency abrupt (SL-A) neurons, all of the short-latency sustained (SL-S) and long-latency (LL) neurons, LC/SC stimulation produced intensity-dependent attenuation of the CRD-evoked discharge. For 10/62 (16%) SL-A neurons, 6/8 (75%) inhibited (INHIB) neurons LC/SC stimulation increased the evoked discharge, for 10/62 (16%) SL-A neurons and 2/8 (25%) INHIB neurons, the evoked discharges were unaffected by the LC/SC stimulation. LC/SC stimulation also had different effects on the spontaneous activities of these neurons. The effects of LC/SC stimulation were the same both ipsilaterally and contralaterally either for the evoked discharges or for spontaneous activities. Following LC/SC lesions, LC/SC stimulation did not inhibit nociceptive responses, whereas inhibitory effects were observed by stimulation of the intact LC/SC contralateral to the recording site. These data suggest that the transmission of visceral pain was under the control of the centrifugal pathways from the LC/SC.

NSAID loxoprofen inhibits high threshold or wide dynamic range neuronal responses in the rat at different time-courses.

The onset of the antinociceptive effect with loxoprofen sodium (LOX), a non-steroidal anti-inflammatory drug, was examined electrophysiologically during carrageenan-induced hindpaw inflammation in the rat. Extracellular recordings were made from either wide dynamic range (WDR) or high threshold (HT) neurons in the dorsal horn. Recordings from the same neuron were continued for at least 3 h after the injection of carrageenan. Three hours after the induction of inflammation, either a fresh solution of LOX (1 mg/kg) or distilled water was directly administered into the stomach through PE 50 tubing. LOX significantly reduced inflammation-increased background activity and noxious heat-evoked responses in both HT and WDR neurons, whereas distilled water did not produce any change. Asignificant difference in the onset of the inhibitory effect of LOX was observed between HT and WDR neurons. The results show that WDR neurons precede HT neurons regarding inhibition of nociceptive processing in the dorsal horn after administration of LOX.

Coeruleospinal inhibition of visceral nociceptive processing in the rat spinal cord.

Visceral nociceptive information is transmitted in two different areas of the spinal cord gray matter, the dorsal horn and the area near the central canal. The present study was designed to examine whether visceral nociceptive transmission in the two different areas is under the control of the centrifugal pathways from the locus coeruleus/subcoeruleus (LC/SC). Extracellular recordings were made from the L(6)-S(2) segmental level using a carbon filament glass microelectrode (4-6 MOmega). Colorectal distentions (80 mmHg) were produced by inflating a balloon inside the descending colon and rectum. In both dorsal horn and deep area neurons, responses to colorectal distention were inhibited during electrical stimulation (30, 50 and 70 microA, 100 Hz, 0.1 ms pulses) of the LC/SC. It is well known that spinothalamic tract (STT) neurons excited by visceral nociceptive stimuli are located in the dorsal horn and that postsynaptic dorsal column (PSDC) neurons which conduct visceral nociceptive signals in the dorsal column (DC) are located near the central canal of the spinal cord. The present study, therefore, suggests that the descending LC/SC system can inhibit visceral nociceptive signals ascending through the STT and the DC pathways.