Harumitsu Hirata

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.

A Novel Class of Neurons at the Trigeminal Subnucleus Interpolaris/Caudalis Transition Region Monitors Ocular Surface Fluid Status and Modulates Tear Production

Reflex tears are produced by many conditions, one of which is drying of the ocular surface. Although peripheral neural control of the lacrimal gland is well established, the afferent pathways and properties of central premotor neurons necessary for this reflex are not known. Male rats under barbiturate anesthesia were used to determine whether neurons at the ventral trigeminal subnucleus interpolaris- caudalis (Vi/Vc) transition or the trigeminal subnucleus caudalis-cervical cord (Vc/C1) junction region in the lower brainstem were necessary for tears evoked by noxious chemical stimulation (CO2 pulses) or drying of the ocular surface. Both the Vi/Vc transition and Vc/C1 junction regions receive a dense direct projection from corneal nociceptors. Synaptic blockade of the Vi/Vc transition, but not the Vc/C1 junction, by the GABAA receptor agonist muscimol inhibited CO2-evoked tears. Glutamate excitation of the Vi/Vc transition, but not the Vc/C1 junction, increased tear volume. Single units recorded at the Vi/Vc transition, but not at the Vc/C1 junction, were inhibited by wetting and excited by drying the ocular surface. Nearly all moisture-sensitive Vi/Vc units displayed an initial inhibitory phase to noxious concentrations of CO2 followed by delayed excitation and displayed an inhibitory surround receptive field from periorbital facial skin. Drying of the ocular surface produced many Fos-positive neurons at the Vi/Vc transition, but not at the Vc/C1 junction. This is the first report of a unique class of moisture-sensitive neurons that exist only at the ventral Vi/Vc transition, and not at more caudal portions of Vc, that may underlie fluid homeostasis of the ocular surface.

Topical cannabinoid agonist, WIN55,212-2, reduces cornea-evoked trigeminal brainstem activity in the rat

&NA; Cannabinoids act at receptors on peripheral and central neurons to modulate diverse physiological functions and produce analgesia. Corneal sensory nerves express the CB1 cannabinoid receptor and project to two spatially discrete regions of the lower brainstem, the trigeminal interpolaris/caudalis (Vi/Vc) transition and subnucleus caudalis/upper cervical cord (Vc/C1) junction region. The function of CB1 expression on corneal nerves is not known. To determine if cannabinoid receptors in the anterior eye affect the activity of trigeminal brainstem neurons at the Vi/Vc and Vc/C1 the CB1 agonist, WIN55,212‐2 (WIN‐2), was applied topically prior to chemical excitation of corneal afferent fibers. In the first series of experiments WIN‐2 was applied topically prior to excitation of corneal nociceptors by mustard oil (MO). WIN‐2 reduced significantly the number of Fos‐like immunoreactive neuronal nuclei (Fos‐LI) at the Vi/Vc transition (−46.7±8.2%, P<0.05), while smaller non‐significant reductions occurred at the Vc/C1 junction region (−20.3±7.6%). The selective CB1 antagonist, SR141716A (1 mg/kg, i.v.), prevented WIN‐2‐evoked reduction in Fos‐LI after MO. Systemic administration of WIN‐2 (1 or 10 mg/kg, i.p.) or SR141716A (1 mg/kg, i.v.) or topical corneal application of morphine sulfate did not affect Fos‐LI produced by MO. In parallel experiments, topical WIN‐2 reduced the magnitude of single unit activity recorded at the Vi/Vc transition (−80±7%, P<0.025), but not at the Vc/C1 junction region (−34±30%) evoked by CO2 pulses applied to the cornea. Topical morphine did not alter CO2‐evoked unit activity at either recording location. These results indicated that cannabinoid receptor agonists acted, at least in part, at CB1 receptors in the anterior eye to reduce corneal stimulation‐evoked trigeminal brainstem neural activity. Corneal nociceptor‐evoked activity at the Vi/Vc transition was reduced significantly by topical WIN‐2, while activity at the Vc/C1 junction region displayed only minor decreases. These findings were consistent with the hypothesis that CB1 receptors affect the activity of corneal‐responsive neurons that preferentially contribute to homeostasis of the anterior eye and/or reflexive aspects of nociception rather than the sensory‐discriminative aspects of corneal nociception.

Differential modulation of TMJ neurons in superficial laminae of trigeminal subnucleus caudalis/upper cervical cord junction region of male and cycling female rats by morphine.

&NA; Sex differences in the cellular responses to morphine were examined in an animal model of temporomandibular joint (TMJ) pain. TMJ‐responsive neurons were recorded in the superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C2) junction region, the initial site of synaptic integration for TMJ afferents, in male and cycling female rats under barbiturate anesthesia. Unit activity was evoked by local injection of bradykinin into the TMJ capsule at 30 min intervals and the effects of morphine sulfate (0.03–3 mg/kg, i.v.) were assessed by a cumulative dose regimen. Morphine caused a dose‐related inhibition of bradykinin‐evoked unit activity in males and diestrous females in a naloxone‐reversible manner, while evoked unit activity in proestrous females was not reduced. The apparent sex hormone‐related aspect of morphine analgesia was selective for evoked unit activity, since the spontaneous activity of TMJ units was reduced similarly in all groups, while the convergent cutaneous receptive field area of TMJ units did not change in any group. These results were consistent with the hypothesis that sex hormone status interacts with pain control systems to modify neural activity at the level of the Vc/C2 junction region relevant for TMD pain.

Quantitative characterization reveals three types of dry-sensitive corneal afferents: pattern of discharge, receptive field, and thermal and chemical sensitivity.

This study reveals that the cold-sensitive (CS) + dry-sensitive (DS) corneal afferents reported in a previous study consist of two types: 1) low threshold (LT)-CS + DS neurons with <1°C cooling sensitivity, and 2) high threshold (HT)-CS + DS neurons with a wide range of cooling sensitivities (~1-10°C cooling). We also found DS neurons with no cooling sensitivity down to 19°C [cold-insensitive (CI) + DS neurons]. LT-CS + DS neurons showed highly irregular discharge patterns during the dry cornea characterized by numerous spiking bursts, reflecting small temperature changes in the cornea. Their receptive fields (RFs) were mainly located in the corneas center, the first place for tears to ebb from the surface and be susceptible to external temperature fluctuations. HT-CS and CI + DS neurons showed a gradual rise in firing rate to a stable level over ~60 s after the dry stimulus onset. Their RFs were located mostly in the corneas periphery, the last place for tears to evaporate. The exquisite sensitivity to cooling in LT-CS + DS neurons was highly correlated with heat sensitivity (~45°C). There was a perfect correlation between noxious heat sensitivity and capsaicin responsiveness in each neuron type. The high sensitivity to noxious osmotic stress was a defining property of the HT-CS and CI + DS neurons, while high sensitivity to menthol was a major characteristic of the LT-CS + DS neurons. These observations suggest that three types of DS neurons serve different innocuous and nociceptive functions related to corneal dryness.

Acute corneal epithelial debridement unmasks the corneal stromal nerve responses to ocular stimulation in rats: Implications for abnormal sensations of the eye.

It is widely accepted that the mechanisms for transducing sensory information reside in the nerve terminals. Occasionally, however, studies have appeared demonstrating that similar mechanisms may exist in the axon to which these terminals are connected. We examined this issue in the cornea, where nerve terminals in the epithelial cell layers are easily accessible for debridement, leaving the underlying stromal (axonal) nerves undisturbed. In isoflurane-anesthetized rats, we recorded extracellularly from single trigeminal ganglion neurons innervating the cornea that are excited by ocular dryness and cooling: low-threshold (<2°C cooling) and high-threshold (>2°C) cold-sensitive plus dry-sensitive neurons playing possible roles in tearing and ocular pain. We found that the responses in both types of neurons to dryness, wetness, and menthol stimuli were effectively abolished by the debridement, indicating that their transduction mechanisms lie in the nerve terminals. However, some responses to the cold, heat, and hyperosmolar stimuli in low-threshold cold-sensitive plus dry-sensitive neurons still remained. Surprisingly, the responses to heat in approximately half of the neurons were augmented after the debridement. We were also able to evoke these residual responses and follow the trajectory of the stromal nerves, which we subsequently confirmed histologically. The residual responses always disappeared when the stromal nerves were cut at the limbus, suggesting that the additional transduction mechanisms for these sensory modalities originated most likely in stromal nerves. The functional significance of these residual and enhanced responses from stromal nerves may be related to the abnormal sensations observed in ocular disease.NEW & NOTEWORTHY In addition to the traditional view that the sensory transduction mechanisms exist in the nerve terminals, we report here that the proximal axons (stromal nerves in the cornea from which these nerve terminals originate) may also be capable of transducing sensory information. We arrived at this conclusion by removing the epithelial cell layers of the cornea in which the nerve terminals reside but leaving the underlying stromal nerves undisturbed.

Ambient Air Currents Activate Corneal Nerves During Ocular Desiccation in Rats: Simultaneous Recordings of Neural Activity and Corneal Temperature

Purpose Previously we found two types of corneal neurons that we hypothesized to play an important role in tearing. One type is called low threshold–cold sensitive plus dry sensitive (LT-CS + DS), and the other is termed high threshold–cold sensitive plus dry sensitive (HT-CS + DS). The present study examined critical stimuli influencing the activity of these neurons to elucidate environmental factors that may trigger this ocular reflex. Methods Single corneal neurons were extracellularly recorded from the trigeminal ganglia in response to ocular stimuli that mimic environmental conditions one encounters in daily life. They included an ocular desiccation and slight air currents and were presented while simultaneously monitoring the ocular surface temperatures (OST) in rats. Results The results showed that the changes in steady state (SS) activity of the neurons closely followed the changes in SS OST: during the sustained ocular desiccation, neural firing displayed numerous small sudden increases in activities (“spiking”); these “spiking” activities of LT-CS + DS neurons were replicated by a minute air current that induced slight ocular surface cooling of approximately 0.2–0.1°C; and the responses of HT-CS + DS neurons showed an inconsistent relationship to the changes in SS OST or exhibited little evidence for “spiking” activities. Conclusions These results suggest that LT-CS + DS neurons play a role in the afferent trigger of tearing as we face the environment, exposing the cornea to prevailing air currents that produce a slight cooling of the ocular surface. By contrast, HT-CS + DS neurons may serve to protect the eyes from extreme dryness by eliciting nociception-evoked tearing when the OST or osmolarity of tears becomes injurious.