Herbert K. Proudfit

Nociceptive responses to high and low rates of noxious cutaneous heating are mediated by different nociceptors in the rat: electrophysiological evidence

&NA; Several lines of evidence suggest that different classes of nociceptive afferents mediate the responses produced by different rates of noxious skin heating. More specifically, low skin heating rates evoke nociceptive responses that appear to be mediated by the activation of capsaicin‐sensitive C‐fiber nociceptors, whereas high skin heating rates appear to produce responses mediated by the activation of other nociceptors. This hypothesis was examined by both electrophysiological and behavioral experiments. This report describes the results of experiments designed to determine whether pharmacologic treatments that selectively alter the activity of C‐fiber nociceptive afferents also produce selective effects on foot withdrawal responses to either high or low rates of noxious foot heating. The results these experiments demonstrate that: (1) topical application of a low concentration of capsaicin, which sensitizes C‐fiber nociceptors, selectively decreased the latency of responses to low heating rates; (2) topical application of a high concentration of capsaicin, that desensitizes C‐fiber nociceptors, selectively increased the latency of responses to low heating rates; (3) low doses of systemic morphine, which selectively attenuate nociception produced by the activation of C‐fiber nociceptors, selectively increased response latencies for low skin heating rates. These results support the conclusion that foot withdrawal responses evoked by low skin heating rates are mediated by the activation of capsaicin‐sensitive C‐fiber nociceptors and foot withdrawal responses evoked by high skin heating rates are mediated by the activation of other nociceptors. This conclusion is supported by the results of the accompanying electrophysiological study which provides direct evidence that low rates of skin heating preferentially activate C‐fiber nociceptors while high rates of skin heating preferentially activate A&dgr; nociceptors.

The function of noradrenergic neurons in mediating antinociception induced by electrical stimulation of the locus coeruleus in two different sources of Sprague-Dawley rats

Although noradrenergic neurons in the nucleus locus coeruleus are known to project to the spinal cord, these neurons appear to innervate different regions of the spinal cord in Sprague-Dawley rats obtained from two different vendors. Recent anatomical studies demonstrated that the noradrenergic neurons in the locus coeruleus in Sasco Sprague-Dawley rats primarily innervate the ventral horn, whereas Harlan Sprague-Dawley rats have coeruleospinal projections that terminate in the dorsal horn of the spinal cord. This report describes the results of behavioral experiments that were designed to determine the functional significance of these anatomical differences. Electrical stimulation of neurons in the locus coeruleus produced antinociception in both Harlan and Sasco rats. The antinociception in Harlan rats was readily reversed by intrathecal injection of yohimbine, a selective alpha 2-adrenoceptor antagonist, or by phentolamine, a non-selective alpha 2-adrenoceptor antagonist. In contrast, these antagonists did not alter the antinociception produced by locus coeruleus stimulation in Sasco rats. Finally, the alpha 2-antagonist, idazoxan, did not alter the antinociceptive effect of locus coeruleus stimulation in either group of rats. These observations indicate that coeruleospinal noradrenergic neurons in Harlan and Sasco Sprague-Dawley rats have different physiological functions. Thus, electrical stimulation of noradrenergic neurons in the locus coeruleus that innervate the spinal cord dorsal horn (Harlan rats) produces antinociception, but stimulation of coeruleospinal noradrenergic neurons that project to the ventral horn (Sasco rats) does not produce antinociception. It is likely that genetic differences between these outbred stocks of rats account for the fundamental differences in the projections of coeruleospinal neurons and their function in controlling nociception.

The projections of noradrenergic neurons in the A5 catecholamine cell group to the spinal cord in the rat : anatomical evidence that A5 neurons modulate nociception

Brainstem noradrenergic neurons located in the A5, A6, and A7 catecholamine cell groups provide the entire noradrenergic innervation of the spinal cord. We have previously demonstrated that noradrenergic neurons in the A6 and A7 cell groups innervate the ventral and dorsal horns, respectively. Since the specific spinal cord terminations of the A5 cell group have not been clearly delineated, the present experiments were designed to trace the projections from this noradrenergic cell group to the spinal cord, using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) in combination with dopamine-beta-hydroxylase immunocytochemistry. The results of these experiments indicate that A5 noradrenergic neurons project ipsilaterally through the dorsolateral funiculus in cervical, thoracic, and lumbar segments. In cervical segments, these axons terminate primarily in the ipsilateral deep dorsal horn (laminae IV-VI) and the intermediate zone (lamina VII). In thoracic segments, the intermediolateral cell column is heavily innervated by A5 axons. In lumbar segments, the concentration of A5 axons is more diffuse and more widely distributed than that in cervical and thoracic segments. Although there is a higher density of axons in the deep dorsal horn and the intermediate zone, there are also scattered axons in the dorsal and ventral horns. The innervation of these regions of the spinal cord by A5 neurons provides anatomical support for the conclusion that these noradrenergic neurons are involved in modulating cardiovascular reflexes and nociceptive transmission in the spinal cord.

Chapter 27 Pharmacologic evidence for the modulation of nociception by noradrenergic neurons

Publisher Summary This chapter discusses the pharmacologic evidence for the modulation of nociception by noradrenergic (NA) neurons. Four major systems of NA neurons that modulate nociception have been described. Three of these systems are involved in producing antinociception, while the fourth enhances nociception. System of bulbospinal NA neurons that modulates nociception is located in the locus coeruleus (LC) and subcoeruleus regions of the dorsolateral medulla. Stimulation of these neurons produces antinociception and inhibition of spinal cord dorsal horn neurons activated by noxious cutaneous stimulation. The NA neurons located in the A1 nucleus, part of the lateral reticular nucleus of the ventrolateral medulla, constitute a second NA pain modulation system. Electrical or chemical stimulation of the neurons in the A1 region produces antinociception and inhibition of spinal cord dorsal horn neurons. A third bulbospinal NA system originates in the rostral part of the A5 nucleus and includes the A7 nucleus. Electrical stimulation of neurons in these regions produces antinociception that can be blocked by intrathecal injection of NA antagonists. The fourth system of NA neurons, located in the caudal A5 nucleus, projects to, and tonically inhibits spinally projecting neurons located in the ventromedial medulla (VMM).

The projection of locus coeruleus neurons to the spinal cord in the rat determined by anterograde tracing combined with immunocytochemistry

Pontospinal noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) nuclei are the major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of spinally-projecting noradrenergic neurons located in these nuclei have not been clearly defined. The purpose of the experiments described in this report was to more precisely define the spinal terminations of neurons located in the LC/SC using the anterograde tracer phaseolus vulgaris-leucoagglutinin in combination with dopamine-beta-hydroxylase (D beta H) immunocytochemistry. In addition, the spinal cord regions in which LC/SC neurons terminate was assessed by measuring the reduction in the density of D beta H-immunoreactive axon terminals in specific spinal cord regions after a unilateral electrolytic lesion that included LC/SC neurons. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord.

The projection of noradrenergic neurons in the A7 catecholamine cell group to the spinal cord in the rat demonstrated by anterograde tracing combined with immunocytochemistry

Noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) catecholamine cell groups innervate all levels of the spinal cord. However, the specific spinal cord terminations of these neurons have not been clearly delineated. This study determined the spinal cord terminations of the A7 catecholamine cell group using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in combination with dopamine-beta-hydroxylase (DBH) immunocytochemistry. In addition, the spinal cord projections of A7 neurons were examined by measuring the reduction in the density of DBH-immunoreactive axons in specific regions of the spinal cord after a unilateral electrolytic lesion of the A7 cell group. The results of these experiments indicate that noradrenergic neurons in the A7 cell group project primarily in the ipsilateral dorsolateral funiculus and terminate most heavily in the dorsal horn (laminae I-IV).

Alterations in nociceptirve threshold and morphine-induced analgesia produced by intrathecally administered amine antagonists ☆

Intrathecal administration of either methysergide or phentolamine hyperalgesia. This suggests that tonically active serotonergic and noradrenergic neuronal systems modulate sensitivity to nociceptive stimuli at the level of the spinal cord. Methysergide did not attenuate the analgesia induced by either 2.0 or 7.5 mg/kg morphine (s.c.), while phentolamine attenuated the analgesia induced by 2.0, but not 7.5 mg/kg morphine. These findings suggest that bulbospinal serotonergic neurons are not integral components of the neuronal circuitry which mediates opiate-induced analgesia. Noradrenergic neurons, however, appear to mediate a portion of such analgesia.

The projections of locus coeruleus neurons to the spinal cord

Spinally projecting noradrenergic neurons located in the locus coeruleus/subcoeruleus (LC/SC) are a major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of these neurons have not been clearly defined. The purpose of this chapter is to describe the results of experiments that used the anterograde tracer Phaseolus vulgaris leucoagglutinin in combination with dopamine-beta-hydroxylase immunocytochemistry to more precisely determine the spinal cord terminations of neurons located in the LC/SC. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord. Finally, the results of preliminary experiments indicate that different rat substrains may have LC neurons that exhibit qualitatively different termination patterns in the spinal cord. More specifically, LC neurons in some rat substrains innervate the dorsal horn, while those in other substrains primarily innervate the ventral horn and intermediate zone.

Effect of intrathecally administered noradrenergic antagonists on nociception in the rat

Recent evidence suggests that some groups of noradrenergic neurons found in the brainstem have axonal connections in the spinal cord dorsal horn and may be involved in the control of pain sensitivity. Such evidence includes the demonstration that intrathecal injection of noradrenergic agonists increases nociceptive threshold (hypoalgesia). The present studies examined whether the descending noradrenergic system is tonically active and, if so, what noradrenergic receptor subtypes mediate the actions of endogenously-released norepinephrine. These studies involved the measurement of nociceptive threshold before and after the intrathecal injection of noradrenergic antagonists having different relative affinities for alpha-noradrenergic receptor subtypes. The intrathecal administration of alpha-noradrenergic antagonists produced a dose-dependent decrease in nociceptive threshold (hyperalgesia). This finding is consistent with the proposal that tonically-active bulbospinal noradrenergic neurons modulate the processing of nociceptive information in the spinal cord. The potency and duration of the hyperalgesia was correlated with the relative potency of the antagonists for the alpha-2 noradrenergic receptor. The relative potencies were as follows: yohimbine greater than phentolamine greater than WB 4101 greater than prazosin. Thus, endogenous norepinephrine which is tonically released from bulbospinal axon terminals may interact preferentially with noradrenergic receptors of the alpha-2 type.

Analgesia produced by microinjection of baclofen and morphine at brain stem sites

Microinjection of either baclofen (1.5 microgram) or morphine (2.5 microgram), in equimolar doses (14 mM), at sites located in the caudal periaqueductal gray (PAG) resulted in a delay in tail flick latency (analgesia). The relative analgesic potency of baclofen among caudal PAG sites, however, did not correlate with that of morphine. Application of either drug into the caudal aspect of the cerebral aqueduct also produced analgesia, but neither agent caused analgesia when applied at PAG sites rostral to the interaural line. Baclofen also produced analgesia when microinjected in the lower brain stem at sites lateral to the midline in or near the nucleus gigantocellularis, but did not produce analgesia when applied on the midline at sites located within or near the raphe magnus. Conversely, morphine produced analgesia when applied locally at midline sites but not at sites located lateral to the midline. These data suggest that the analgesia produced by systemic administration of baclofen and morphine involves activation of different neuronal substrates.