James W. Hu

Convergence of cutaneous, tooth pulp, visceral, neck and muscle afferents onto nociceptive and non-nociceptive neurones in trigeminal subnucleus caudalis (medullary dorsal horn) and its implications for referred pain

&NA; Because of the likely involvement of central convergence of afferent inputs in mechanisms underlying referred pain, the activity of single neurones was recorded in the cats trigeminal (V) subnucleus caudalis (medullary dorsal horn) to test for the presence and extent of convergent inputs to the neurones. In chloralose‐anaesthetized or decerebrate unanaesthetized cats, electrical stimuli were applied to afferents supplying facial skin, oral mucosa, canine and premolar tooth pulp, laryngeal mucosa, cervical skin and muscle, and jaw and tongue muscles, and tactile and noxious mechanical and thermal stimuli were applied to skin and mucosa. Considerable proportions of caudalis neurones which could be functionally classified on the basis of their cutaneous receptive field properties as low‐threshold mechanoreceptive (LTM), wide‐dynamic‐range (WDR), or nociceptive‐specific (NS) neurones, could be excited by electrical stimulation of several of these afferent inputs. Extensive convergence of afferent inputs, including inputs from skin or mucosal areas outside the neuronal oral‐facial receptive field delineated by natural stimuli, was a particular feature of the units classified as cutaneous nociceptive neurones (i.e., WDR and NS). On the basis of antidromic activation, 15% of these WDR and NS neurones were shown to have a direct projection to the contralateral thalamus. The findings question the use of terminology and classifications of somatosensory neurones based only on the cutaneous receptive field properties of the neurones since distinctions between the different neuronal populations become less obvious when properties other than those related to cutaneous afferent inputs are taken into account. Moreover, the observations of extensive convergence of different types of afferents, which was especially apparent in cutaneous nociceptive neurones, also suggest a role for these neurones in mediating deep pain and in spread and referral of pain.

Stimulation of craniofacial muscle afferents induces prolonged facilitatory effects in trigeminal nociceptive brain-stem neurones

&NA; Stimulation of small‐diameter afferents supplying deep tissues has been shown to increase the excitability of spinal cord neurones responding to cutaneous afferent inputs. This facilitation has been implicated as integral central mechanisms of deep pain that may contribute to the tenderness and spread and/or referral of pain following injury of deep tissues. In view of the recent documentation of deep craniofacial afferent inputs, as well as cutaneous afferent inputs to the trigeminal (V) spinal tract nucleus, we wished to determine the effects of deep inputs excited by the small‐fibre irritant mustard oil on trigeminal nociceptive neurones. The extracellular activity of single brain‐stem neurones was recorded in subnuclei caudalis and oralis of the V spinal tract nucleus of anaesthetized rats. The neurones were classified as low‐threshold mechanosensitive (LTM), wide dynamic range (WDR) and nociceptive specific (NS) on the basis of their cutaneous mechanoreceptive field properties and their responses evoked by electrical stimulation of their cutaneous afferent inputs. Injection of 5% mustard oil (2–5, &mgr;l) into the deep masseter muscle produced a facilitatory effect in 12 of 27 nociceptive neurones tested in caudalis and in 5 of 12 nociceptive neurones in oralis. This effect was reflected in an expansion of the cutaneous mechanoreceptive field, an increase in spontaneous activity or an increase in responsivity to electrical stimulation of cutaneous afferent inputs to the neurones. The facilitation was reversible and typically became apparent within 3–5 min of the injection, reached its peak at 5–10 min, and lasted for 20–30 min. These findings document for the first time the sensitivity of the cutaneous receptive field properties of V brain‐stem nociceptive neurones to facilitatory influences from deep craniofacial afferents, and point to possible mechanisms that may contribute to the tenderness, spread and referral of pain that may result from trauma to deep craniofacial tissues.

Response properties of nociceptive and non-nociceptive neurons in the rat's trigeminal subnucleus caudalis (medullary dorsal horn) related to cutaneous and deep craniofacial afferent stimulation and modulation by diffuse noxious inhibitory controls

&NA; An electrophysiological study was carried out in anesthetized rats to characterize the properties of single neurons in trigeminal (V) subnucleus caudalis. Each neuron was functionally classfied in terms of its cutaneous mechanoreceptive field properties as low‐threshold mechanoreceptive (LTM), wide dynamic range (WDR) or nociceptive‐specific (NS), and its responsiveness was also tested to electrical stimulation of hypoglossal (XII) nerve muscle afferents. Some neurons were also tested with noxious stimulation of the tail or forepaw for the presence of diffuse noxious inhibitory controls (DNIC) of evoked responses. A I–VI; the LTM neurons predominated in laminae II/IV whereas the nociceptive (WDR, NS) were located in the superficial and especially deeper laminae of caudalis. The majority of the WDR and NS neurons were also activated by noxious heating as well as by noxious mechanical and electrical stimulation of their orofacial mechanoreceptive field, and in contrast to our previous studies in cats, most of these caudalis nociceptive neurons received C fiber as well as A fiber cutaneous afferent inputs. In contrast to the LTM neurons, but consistent with our previous data in cats, many of the nociceptive neurons also received convergent excitatory inputs from XII muscle afferents and the stimulus‐response functions of the WDR neurons indicated that they were capable of coding the intensity of A and C fiber craniofacial muscle afferent inputs as well as those from cutaneous afferents. The study has also documented for the first time that muscle afferent‐evoked responses as well as those evoked by cutaneous afferent inputs to nociceptive neurons are subject to DNIC. These findings indicate that subnucleus caudalis plays an important role in the transmission of superficial and deep nociceptive information from the craniofacial region of the rat, and also reveal that response of the nociceptive neurons evoked by deep as well as superficial afferent inputs can be powerfully modulated by other nociceptive influences originating from widespread parts of the body.

Glutamate-evoked pain and mechanical allodynia in the human masseter muscle

&NA; The present study examined the effect of peripheral administration of the excitatory amino acid (EAA) glutamate on the intensity of perceived pain and pressure pain thresholds (PPTs) in healthy young women (n=17) and men (n=18). Two injections separated by 25 min of 0.2 ml, 1.0 M glutamate into the masseter muscle produced significantly higher scores of pain on 0–10 cm visual analogue scales (VAS) in women than in men (analysis of variance, ANOVA: P<0.001). There was no significant difference between the VAS scores for the first and the second injections in either men or women. The PPTs determined in the masseter muscle were significantly reduced following the first injection and further significantly reduced after the second injection (ANOVA: P<0.001). Furthermore, the PPTs were reduced to a similar extent in both women and men (maximum 44–56%), suggesting that gender did not influence the process of sensitization. There were no significant difference in VAS scores or PPTs between women taking oral contraceptives (n=9) and those who did not (n=8) (ANOVAs: P=0.709, P=0.153). It is concluded that the VAS scores produced by intramuscular administration of 1.0 M glutamate may reflect a gender‐dependent activation of nociceptive pathways which, in part, may be mediated through peripheral EAA receptors. The reduction of PPTs in the masseter muscle following administration of glutamate in a concentration of 1.0 M may reflect allodynia to mechanical stimuli. This process of sensitization was not gender‐dependent. The present results suggest that injection of 1.0 M glutamate into the masseter muscle may provide a useful experimental method to test sensitization and efficacy of peripheral EAA receptor antagonists in human subjects.

Trigeminal subnucleus caudalis : beyond homologies with the spinal dorsal horn

Trigeminal sensory nerves relay mechanical, thermal, chemical and proprioceptive information from craniofacial regions. Trigeminal sensory nerves act at the local level to maintain the integrity of craniofacial tissues through trophic in ̄uences and serve the whole animal through re ̄exes that protect other external sensory (e.g. retina, olfactory epithelium, taste receptors) and internal homeostatic (e.g. respiratory and digestive tracts) systems from damaging environmental changes. Despite awareness of the functional complexity of trigeminal sensory systems compared to spinal systems (see Kruger and Young, 1981) our understanding of central mechanisms of trigeminal pain has often relied on analogies with the spinal cord. It has been 50 years since Olszewski (1950) divided the trigeminal spinal nucleus (Vsp) into oralis (Vo), interpolaris (Vi) and caudalis (Vc) subdivisions and nearly 25 years since Gobel et al. (1977) detailed the structural homologies between Vc and the spinal dorsal horn. While these landmark studies have shaped discussions of the special relationship between Vc and craniofacial pain (Renehan and Jacquin, 1993; Sessle, 2000), this useful homology may need revision since recent evidence suggests that select portions of Vc are organized differently from spinal systems. The Vc, the largest subdivision of the Vsp, consists of an elongated laminated portion that merges without clear boundaries with the cervical dorsal horn, while the rostral Vc is displaced medially by the caudal tip of Vi to form a distinctive transition region. The ventral Vi/Vc transition region, which is well described in rodents, consists of rostral Vc with its fragmented laminar appearance, interstitial islands of neurons embedded in the trigeminal spinal tract, and a ventral crescent-shaped region of caudal Vi (Phelan and Falls, 1989). The dorsomedial aspect of the Vi/Vc transition region, an important integrative region for peri-oral and intra-oral sensory input, has its own unique complex features (see Sessle, 2000) and is not discussed further. This review proposes, based mainly on studies using corneal stimuli, that neurons in the ventral Vi/Vc transition region and caudal Vc are interconnected to serve different aspects of craniofacial pain associated with ophthalmic structures and display properties different from dorsal horn neurons in lower portions of the spinal cord.

Effects of inflammatory irritant application to the rat temporomandibular joint on jaw and neck muscle activity

&NA; An electromyographic (EMG) study was carried out in 40 anaesthetized rats to determine if the activity of jaw and neck muscles could be influenced by injection of the small‐fibre excitant and inflammatory irritant mustard oil into the region of the temporomandibular joint (TMJ). Injection of a vehicle (mineral oil, 20 &mgr;l) did not produce any significant change in EMG activity. In contrast, injection of mustard oil (20 &mgr;l, 20%) evoked increases in EMG activity in the jaw muscles but not in the neck muscles. The increased EMG activity evoked by mustard oil was reflected in 1 or 2 phases of increased activity. The early EMG increase occurred soon after the mustard oil injection (mean latency ± SD: 3.5 ± 2.3 sec), peaked within 1 min, and then subsided (mean duration: 7.5 ± 5.2 min). The later EMG increase occurred at 14.6 ± 10.0 min after the mustard oil injection and lasted 14.3 ± 12.3 min. These excitatory effects of mustard oil on the EMG activity of jaw muscles appear to have a reflex basis since they could be abolished by pre‐administration of local anaesthetic into the TMJ region. These results document that TMJ injection of mustard oil results in a sustained and reversible activation of jaw muscles that may be related to the reported clinical occurrence of increased muscle activity associated with trauma to the TMJ.

Raphe-induced suppression of the jaw-opening reflex and single neurons in trigeminal subnucleus oralis, and influence of naloxone and subnucleus caudalis.

AbstractThe effects of stimulation of the nucleus raphe magnus (NRM) and the periaqueductal gray (PAG) were tested on the digastric (jaw‐opening) reflex and on the activity of functionally identified single neurons recorded in trigeminal (V) subnucleus oralis in the brain stem. Reflex and neuronal responses evoked by tooth pulp stimulation could be readily suppressed for 250–1000 msec by PAG and NRM conditioning stimuli. The effects were not specific for tooth pulp afferent inputs, however, since suppression was also apparent in jaw‐opening reflex responses evoked by low‐intensity electrical or tactile stimulation of oral‐facial sites, and in the mechanically or electrically evoked responses of oralis neurons with localized low‐threshold mechanoreceptive fields.The modulatory effects on the jaw‐opening reflex and oralis neuron activity were not altered by reversible cold block of synaptic transmission in V subnucleus caudalis. Thus it appears that the PAG‐ and NRM‐induced effects on the reflex and oralis neurons are not dependent on relays via caudalis.Some of the suppressive influences on responses to oral‐facial stimuli could be reversed by the administration of the opiate antagonist naloxone. This suggests that some of the modulatory influences involve endogenous opiate‐related mechanisms.Many of the oralis neurons were identified as trigeminothalamic relay neurons on the basis of their antidromic response to ventrobasal thalamic stimulation; PAG and NRM conditioning produced not only a suppression of their orthodromic responses to oral‐facial stimuli but also caused a decrease in the antidromic excitability of the relay neurons. This decrease may be indicative of raphe‐induced postsynaptic inhibition of oralis neurons, and/or presynaptic facilitation of their thalamic endings.

Involvement of NMDA receptor mechanisms in jaw electromyographic activity and plasma extravasation induced by inflammatory irritant application to temporomandibular joint region of rats

&NA; The aim of this study was to examine the possible role of N‐methyl‐ d‐aspartate (NMDA) receptor mechanisms in responses induced by the small‐fibre excitant and inflammatory irritant mustard oil injected into the temporomandibular joint (TMJ) region of rats. The effects of the non‐competitive NMDA antagonist MK‐801 were tested on the mustard oil‐evoked increases in electromyographic (EMG) activity of the masseter and digastric muscles and Evans Blue plasma extravasation. Five minutes before the mustard oil injection, MK‐801 or its vehicle was administered systemically (i.v.), into the third ventricle (i.c.v.), or locally into the TMJ region. Compared with control animals receiving vehicle, the rats receiving MK‐801 at an i.v. dose of 0.5 mg/kg (n = 5) showed a significant reduction in the incidence and magnitude of EMG responses as well as in the plasma extravasation evoked by mustard oil; MK‐801 at an i.v. dose of 0.1 mg/kg (n = 5) had no significant effect on plasma extravasation or on the incidence and magnitude of EMG responses but did significantly increase the latency of EMG responses. An i.c.v. dose of 0.1 mg/kg (n = 5) or 0.01 mg/kg (n = 5) had no significant effect on plasma extravasation or incidence of EMG responses but did significantly reduce the magnitudes of the masseter EMG response; the 0.01 mg/kg dose also significantly increased the latency of the digastric EMG response. The magnitudes of both the masseter and digastric EMG responses were also significantly reduced by MK‐801 administered into the TMJ region at a dose of 0.1 mg/kg (n = 5) but not by 0.01 mg/kg (n = 5); neither dose significantly affected the incidence of EMG responses or the plasma extravasation. These data suggest that both central and peripheral NMDA receptor mechanisms may play an important role in EMG responses evoked by the small‐fibre excitant and inflammatory irritant mustard oil, but that different neurochemical mechanisms may be involved in the plasma extravasation induced by mustard oil.

Stimulation sites in periaqueductal gray, nucleus raphe magnus and adjacent regions effective in suppressing oral-facial reflexes

Electrode penetrations were made in the mesencephalon and caudal brainstem at the levels of the periaqueductal gray matter (PAG) and nucleus raphe magnus (NRM) in chloralose-anaesthetized or decerebrate cats. In a systematic fashion, mesencephalic and brainstem loci at every 1 mm of vertical depth were electrically stimulated in a series of mediolateral or anteroposterior electrode penetrations to determine the lowest stimulation threshold at each locus for suppressing the digastric jaw-opening reflex evoked by tooth pulp or infraorbital nerve stimulation; at some loci, the threshold current required for suppressing infraorbital nerve-evoked neck reflexes was also determined. Stimulation at sites within large regions of the mesencephalon and caudal brainstem was effective in suppressing these reflexes at less than 4 X the lowest threshold for reflex suppression in each animal. However, in these regions the areas of lowest threshold in the mesencephalon generally corresponded to the ventrolateral PAG and adjacent nucleus cuneiformis and part of the lateral reticular formation (LRF) and in the caudal brainstem they corresponded to NRM and the adjacent nuclei reticularis magnocellularis (RMC) and gigantocellularis (RGC). These findings suggest that there may be mesencephalic and caudal brainstem areas in addition to PAG and NRM that are equally effective in modulating reflex activity.

Involvement of glia in central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization is a crucial mechanism underlying the increased excitability of nociceptive pathways following peripheral tissue injury and inflammation. We have previously demonstrated that the small-fiber excitant and inflammatory irritant mustard oil (MO) applied to the tooth pulp produces glutamatergic- and purinergic-dependent central sensitization in brainstem nociceptive neurons of trigeminal subnucleus caudalis (Vc). Recent studies have implicated both astrocytes and microglia in spinal nociceptive mechanisms, showing, for example, that inhibition of spinal astroglial metabolism or spinal microglial p38MAPK activation can attenuate hyperalgesia in inflammatory pain models but have not tested effects of glial inhibitors on central sensitization in functionally identified spinal nociceptive neurons. The aim of the present study was to determine whether glial cells are involved in the MO-induced central sensitization in Vc nociceptive neurons, by examining the effects of intrathecally applied SB203580 (SB), an inhibitor of p38MAPK, and fluoroacetate (FA), an inhibitor of the astroglial metabolic enzyme aconitase. During continuous superfusion of phosphate-buffered saline over Vc, MO application to the pulp-induced central sensitization in Vc nociceptive neurons reflected in significant increases in cutaneous mechanoreceptive field (RF) size and responses to noxious mechanical stimuli and a decrease in mechanical activation threshold. The i.t. application of SB or FA markedly attenuated the MO-induced increases in pinch RF size and responses to noxious stimuli and the decrease in activation threshold. Neither SB nor FA application significantly affected the baseline (i.e., pre-MO application) RF and response properties. These results suggest that glial metabolic processes are important in the development of Vc central sensitization.