A. Iggo

An α2 receptor mediates the selective inhibition by noradrenaline of nociceptive responses of identified dorsal horn neurones

Extracellular recordings were made of 59 neurones with long, ascending projections (spinocervical tract (SCT) and dorsal column postsynaptic (DCPS) neurones) in the lumbar dorsal horn of anaesthetized and paralyzed cats. All showed prominent excitatory responses to innocuous stimuli, applied to their cutaneous receptive fields on the ipsilateral hindlimb. The majority of the population investigated (83%) was multireceptive, being activated by noxious as well as innocuous cutaneous stimuli. Drug effects were examined on a regular cycle of responses to these cutaneous stimuli and also to DL-homocysteic acid (DLH). In 49 multireceptive SCT and DCPS neurones, ionophoretically-applied L-noradrenaline (NA) produced a potent selective inhibition of the nociceptive responses (to heat or pinch) in 40 out of 44 SCT and 3 out of 5 DCPS neurones, with no statistically significant change in the responses to innocuous brush or DLH, or in spontaneous activity. NA had no effect on the majority of cells (8 out of 11) that responded only to innocuous stimuli. In 19 SCT neurones that showed NA-selectivity, the alpha 2-selective agonists clonidine (in 12 out of 15) and metaraminol (in 2 out of 3) mimicked this selective effect, whereas, the alpha 1 agonist, phenylephrine and the beta agonist, isoprenaline did not. Furthermore, the alpha 2 antagonists, yohimbine and idazoxan (RX781094), either reversed or reduced the potency of the NA-elicited inhibition of nociceptive responses in all 7 SCT neurones tested. These results are discussed in relation to other evidence for spinal antinociceptive effects of noradrenergic systems acting at a spinal level and the possible involvement of an alpha 2 receptor in such effects.

Nociceptor-driven dorsal horn neurones in the lumbar spinal cord of the cat.

&NA; Single dorsal horn neurones have been recorded extracellularly in the lumbar spinal cord of cats anaesthetized with chloralose. Cold block at L1 was used to provide reversible spinalization. The location of the units in the dorsal horn was marked by the electrophoretic deposition of pontamine sky blue from the recording microelectrode. There was a clear somatotopic representation of the ventrolateral surface of the foot in the L6 segment. Thirty‐five of the 46 units recorded in the marginal zone of the L6 dorsal horn (lamina I) could only be excited by volleys in small afferent fibres and by noxious stimulation of the skin in the foot regions and were termed class 3 cells. The remaining 11 units could, in addition, be excited by sensitive cutaneous mechanosensitive afferent units — they were class 2 units. The ‘specific’ nociceptor‐driven neurones could be divided into 2 subclasses on the basis of their excitability by afferent fibres. Class 3 (a) were excited only by A&dgr; cutaneous afferents and class 3 (b) by both A&dgr; and C cutaneous afferents. Some of the latter were also excited by A&dgr; and C afferent fibres in the lateral gastrocnemius nerve. When tested by natural stimulation all class 3 cells were excited by noxious mechanical stimuli, but only the 3 (b) units were effectively excited by heating the skin. This discharge in 3 (b) units could be suppressed by electrical stimulation of large (group II) cutaneous myelinated afferent fibres and a similar effect could be produced in responses evoked by A&dgr; and C afferent volleys. Additional inhibition was accomplished by electrical stimulation of the slower myelinated cutaneous (A&dgr; or group III) afferent fibres. The excitability of the class 3 cells was greater in spinal preparations but the tonic descending inhibition was weaker than the apparently similar descending tonic inhibition of class 2 cells. The results are discussed with reference to pain mechanisms.

Functional characteristics of mechanoreceptors in sinus hair follicles of the cat

1. The discharge of impulses in afferent fibres dissected from the infraorbital and ulnar nerves of anaesthetized cats was recorded during controlled movements of the maxillary and carpal sinus hairs.

Responses of spinocervical tract neurones to noxious stimulation of the skin.

1. Activity of single spinocervical tract neurones has been recorded in the lumbar spinal cord of chloralose anaesthetized or decerebrated cats. Reversible spinalization was produced by cold block at L3. Sensitivity of these neurones to noxious stimulation was studied by heating their cutaneous receptive fields above 40‐45 degrees C. 2. Most of the units were located in lamina IV of the dorsal horn and had their receptive fields in the ipsilateral foot. All but one of the studied neurones were excited by moving hairs or by gentle mechanical stimulation of the skin. 3. Eighty‐four % of the units were affected by noxious stimuli and three kinds of response were obtained: (i) 61% were excited (E‐cells) by noxious heat; (ii) 19% were inhibited (I‐cells); and (iii) 19% gave a mixed response reversing from excitatory to inhibitory (EI‐cells). 4. E‐cells had axons with a wider range of conduction velocities than the rest and also received the strongest descending inhibition from supraspinal structures. 5. The recording sites of EI‐cells were located in the medial third of the dorsal horn whereas E‐ and I‐cells were distributed over the full width of the dorsal horn. 6. The possible role of the spinocervical tract in nociception is discussed.

Sensory receptors in ankle joint capsules of normal and arthritic rats

SummaryThe responses of afferent fibers innervating the capsule of the ankle joint have been investigated in 34 normal and 19 arthritic anaesthetised rats. Afferent fiber diameter and conduction velocity were in the same range in normal and arthritic rats. All receptors examined were excited by mechanical stimulation of the joint capsule and the majority adapted slowly. A resting discharge was absent in normal rats in contrast to the arthritic animals where it was present in about 25% of the sample. The mechanical thresholds of the responses, measured using either von Frey hairs or the force transducer, ranged from 4.6 mN to 65 mN for 11 units recorded in normal and exceeded 80 mN for 11 others. For 30 units recorded in arthritic animals the thresholds ranged from 0.4 to 46 mN. When tested, pressure on the ankle or small degrees of flexion or extension produced a high rate of discharge in receptors of arthritics while similar stimuli were ineffective in normals. Repeated indentation at short intervals caused a progressive reduction in response to 10 to 15% of control values in normal and almost to extinction in arthritic rats. Recovery occurred within minutes in normals but was delayed in arthritics. These results suggest that the changes in responsiveness of somatosensory neurons and in behaviour, previously described in arthritic rats, can be partly accounted for in terms of the altered properties of the joint capsule receptors.

The role of PGE2 in the sensitization of mechanoreceptors in normal and inflamed ankle joints of the rat.

SummaryThe role of PGE2 in the sensitization of highthreshold tarsal joint mechanoreceptors (putative nociceptors) has been investigated in 11 arthritic and 16 normal rats. Injections of a low dose of Freunds complete adjuvant at multiple sites into the tissues surrounding the ankle joint induced a chronic unilateral monoarthritis in the injected limb. Measurements of both spontaneous activity and responses of tarsal joint mechanoreceptors to repeated graded mechanical stimuli were made. All of the mechanoreceptors examined had afferent fibres with conduction velocities in the C or A-δ range. Using this new model of joint inflammation we have shown that lysine acetylsalicylate reduces the mechanical sensitivity of these joint mechanoreceptors and reduces the spontaneous activity in afferent nerve fibres. Prostaglandin E2 is unable to restore either the spontaneous activity in the afferent axon or the mechanical sensitivity of tarsal joint mechanoreceptors after lysine acetylsalicylate in the arthritic rat. Similarly, PGE2 does not sensitize or excite tarsal joint mechanoreceptors in the normal rat. In the normal rat, however, PGE2 potentiates the excitatory action of bradykinin and enhances the sensitizing effect of bradykinin on the responses of joint mechanoreceptors to mechanical stimulation when both substances are injected simultaneously. These results indicate that PGE2 is not important in the sensitization of these joint mechanoreceptors in this model of chronic joint inflammation but that in other circumstances PGE2 may be able to contribute to a sensitization of joint mechanoreceptors by enhancing the action of bradykinin.

Descending control of spinal nociceptive transmission. Actions produced on spinal multireceptive neurones from the nuclei locus coeruleus (LC) and raphe magnus (NRM)

SummaryThe effects of electrical stimulation in the nuclei locus coeruleus (LC) and raphe magnus (NRM) were examined on the background and/or evoked discharge of neurones in the spinal dorsal horn of anaesthetized cats. These were qualitatively, and in most cases quantitatively similar, in their action on multireceptive neurones. In these neurones an inhibitory action on the discharge evoked by noxious cutaneous stimuli or by activation of Aδ and C fibres was most prominent although in some neurones (22%) an initial excitation lasting up to 100 ms preceded the inhibition which could last up to 1 s. Excitation alone was observed in only 3% of multireceptive neurones. Electrical stimulation also produced an inhibitory action on the discharge of low threshold mechanoreceptive neurones (80%). In four of ten multireceptive neurones examined in detail, LC stimulation produced a selective inhibitory action on the discharge evoked by noxious cutaneous stimuli. In the remaining six multireceptive neurones it was partially selective against noxious as compared with non-noxious inputs. The inhibitory action was also more pronounced on the discharge evoked by activity in Aδ and C fibres than fast conducting afferents. The inhibitory action evoked by electrical stimulation in LC on nociceptive transmission in the spinal cord is suggested to play a part in mediating analgesia from LC.

PGI2-induced activation and sensitization of articular mechanonociceptors

The effects of PGE2 PGI2 and the stable PGI2 analogue cicaprost on the afferent discharge of ankle joint mechanonociceptors were studied in the anaesthetized rat. Close-arterial injection of PGI2 (0.01-0.1 micrograms) or cicaprost (0.05-5 micrograms) caused both sensitization to mechanical stimulation and excitation of the majority of mechanonociceptors, whereas PGE2 (0.03-3 micrograms) had only weak effects on a small number of nociceptive units. These results suggest the existence of specific PGI2 sensitive receptors (IP receptors) on rat sensory afferent nerves, and support the hypothesis that in the rat endogenous PGI2 plays an important role in the lowering of nociceptive thresholds in inflamed joints.

A study of 5-HT-receptors associated with afferent nerves located in normal and inflamed rat ankle joints.

Neural recordings were made from sensory fibres in a nerve supplying the ankle joint in normal rats and in rats with a novel monoarticular arthritis. The responses of mechanically and chemically sensitive units to intra-arterial injections of 5-HT were measured. In most cases the mechanosensitivity of sensory receptors in the ankle joint was not altered by 5-HT. However, 5-HT produced an increase in afferent activity in units which were identified as C-fibres on the basis of action potential amplitude and duration. The receptive fields of these chemosensitive units were not located. The responses of these units to 5-HT were dose dependent and were abolished by the 5-HT2-antogonist, ketanserin, but not by the 5-HT3-receptor antagonist, MDL 72222. The responses of chemosensitive units to injections of 5-HT were similar in normal and arthritic rats although the response was slightly prolonged in arthritic animals.

Pathways mediating descending control of spinal nociceptive transmission from the nuclei locus coeruleus (LC) and raphe magnus (NRM) in the cat

SummaryWe have previously reported that electrical stimulation in LC or NRM when tested on the activity of a multireceptive neurone in the spinal cord produced similar inhibitory actions. The present study aimed to define the pathways that mediate this descending inhibitory action in the spinal cord by pharmacological means and by making surgical lesions in the spinal cord or NRM. Attempts to differentiate pathways pharmacologically did not succeed since the i.v. administration of the 5-HT antagonists, methysergide and cinnanserin failed to antagonise descending inhibition evoked from either NRM or LC. Lesions involving a part or whole of the ipsilateral ventral quadrant reduced the inhibition produced from LC to a greater extent than that from NRM in 24 multireceptive neurones. In seven of these neurones stimulation in LC was without any effect after the lesion. In 23 multireceptive neurones recorded after making lesions that spared the ipsilateral ventral quadrant the effects of LC stimulation were unchanged. NRM effectiveness was reduced by an ipsilateral dorsolateral funiculus (DLF) lesion but required a bilateral DLF lesion for an almost complete abolition. Similar results were obtained when the effect of the various lesions were studied on the dorsal root potentials (DRPs) generated from LC or NRM. Lesions in the midline raphe complex, that included NRM, did not block the inhibitory action of LC stimulation. The inhibition produced from both these nuclei was additive whereas excitation was not. We conclude that LC actions in the spinal cord are mediated primarily through a pathway in the ipsilateral ventral quadrant whereas those from NRM are mediated through bilateral projections in DLF. Furthermore, although NRM plays no part in mediating LC actions and separate and independent pathways mediate their spinal action yet these apparently independent pathways have plenty of scope for interaction in the dorsal horn of the spinal cord itself.