Changes in corneal innervation during postnatal development in normal rats and in rats treated at birth with capsaicin

This study was undertaken to learn more about the keratitis that follows the neonatal administration of capsaicin to rats. In the first part of the study the density and pattern of corneal innervation were followed throughout postnatal development in litters of control rats and in rats that received capsaicin as neonates. In rats treated with capsaicin, the density of corneal innervation was lower than that of control rats until 28 days of age. Thereafter nerve fiber sprouting was observed within the corneas of capsaicin-treated rats. By 56 days of age the capsaicin-treated rats had a denser corneal innervation than did control rats. In the second part of the study the incidence and severity of the corneal trophic changes induced by capsaicin were studied in litters of rats treated with chronic tarsorrhaphy and in litters raised in the dark. Neither of these manipulations altered the incidence or severity of keratitis. These observations demonstrate that sprouting of the remaining corneal axons is related temporally to the previously observed decrease in keratitis. Furthermore the more severe keratitis observed in early postnatal life does not appear to be related to physical injury of the cornea.     

Impulse activity in corneal sensory nerve fibers after photorefractive keratectomy

PURPOSE: To evaluate the changes in spontaneous and stimulus-evoked nerve impulse activity of corneal polymodal and mechanonociceptor sensory fibers of the cornea after photorefractive keratectomy (PRK). METHODS: A central corneal ablation 6 mm in diameter and 70 microm in depth was performed with an excimer laser in both eyes of three anesthetized cats, after removal of the corneal epithelium. Single nerve fiber activity was recorded in these animals 12 to 48 hours after surgery. Activity in corneal nerve fibers with receptive fields (RFs) within and/or close to the wound, as well as with RFs far from the lesioned area, was studied. Incidence and frequency of spontaneous discharges and nerve impulse firing responses to mechanical (Cochet-Bonet esthesiometer) and chemical (CO(2) gas pulses) stimuli were studied. RESULTS: The incidence of nociceptor fibers exhibiting ongoing activity (15/35 vs. 1/9) and the frequency of their spontaneous firing (0.25 +/- 0.09 impulses [imp]/s versus 0.08 +/- 0.08 imp/s) was higher in fibers with RFs within and/or bordering the wounded area than in those with RFs far away from the wound. Mechanical responsiveness of fibers with RFs within or nearby the ablated area was often reduced. In these fibers, CO(2) pulses evoked a lower-frequency impulse discharge (0.9 +/- 0.2 imp/s inside, 2.3 +/- 0.7 imp/s outside the wound). CO(2)-evoked discharges recorded from fibers innervating the intact wound border were similar to those recorded in corneal fibers of intact cats. CONCLUSIONS: The spontaneous impulse activity and the abnormal responsiveness shown by a part of the corneal nerve fibers innervating the injured cornea are presumably the neurophysiological substrate of the pain sensations experienced by human patients hours after PRK surgery.     

Tear secretion induced by selective stimulation of corneal and conjunctival sensory nerve fibers

PURPOSE: To measure the increase in tear secretion evoked by selective stimulation of the different populations of sensory receptors of the cornea and conjunctiva by using moderate and intense mechanical, chemical, and cold stimuli. METHODS: Six healthy subjects participated in the study. Tear secretion was measured in both eyes by the Schirmer's test conducted under control conditions and after stimulation of the center of the cornea and the temporal conjunctiva with a gas esthesiometer. Mechanical stimulation consisted in three pulses of 3 seconds' duration of warmed air (at 34 degrees C on the eye surface) applied at moderate (170 mL/min) and high (260 mL/min) flow rates. Cold thermal stimulation was made with cooled air that produced a corneal temperature drop of -1 degrees C or -4.5 degrees C. Chemical (acidic) stimulation was performed with a jet of gas containing a mixture of 80% CO(2) in air. RESULTS: The basal volume of tear secretion increased significantly (P < 0.05, paired t-test) after stimulation of the cornea with high-flow mechanical stimuli (260 mL/min), intense cooling pulses (-4.5 degrees C), and chemical stimulation (80% CO(2)). The same stimuli were ineffective when applied to the conjunctiva. Moderate mechanical (170 mL/min) and cold (-1 degrees C) stimulation of the cornea or the conjunctiva did not change significantly the volume of tear secretion. CONCLUSIONS: Reflex tear secretion caused by corneal stimulation seems to be chiefly due to activation of corneal polymodal nociceptors, whereas selective excitation of corneal mechanonociceptors or cold receptors appears to be less effective in evoking an augmented lacrimal secretion. Conjunctival receptors stimulated at equivalent levels do not evoke an increased tear secretion.     

大鼠三叉神经痛觉传入纤维感觉根通路的研究

目的研究大鼠三叉神经痛觉传入纤维感觉根通路分布特点,为指导三叉神经痛颅内段高度选择性手术提供实验依据。方法将逆行荧光示踪剂荧光金分别注入大鼠三叉神经脊束核各亚核。大鼠存活7d后,取三叉神经感觉根做连续冰冻切片,在荧光显微镜下观察荧光阳性纤维在感觉根横断面的分布情况。结果在出脑桥处,荧光金阳性纤维分布于感觉根周边薄层区域内,中央有少量散在分布;在感觉根中段,荧光亦分布于周边,中间可见少量散在分布,但逐渐向外侧集中;在靠近半月节处,荧光明显集中于感觉根外侧,其他部位则无分布;进入三叉神经半月节后,荧光金阳性纤维主要分布于半月节中部。结论大鼠三叉神经痛觉传人纤维的走行呈明显的规律性,在近脑桥端,主要分布于感觉根周边,在近半月节处,较集中地分布于感觉根外侧。（中国眼耳鼻喉科杂志,2009,9：77—79）     

Effects of capsaicin on corneal wound healing.

This study examined whether the depletion of neuropeptides from sensory nerve terminals induced by capsaicin modifies the healing rate of experimental corneal wounds in adult rabbits. Capsaicin (33 or 3.3 mM solutions) was administered topically and/or by a single retrobulbar injection to one eye while the fellow eye, treated with the vehicle, served as a control. After 1-3 weeks of treatment, an epithelial wound was made in the center of the cornea of both eyes with n-heptanol. Migration rates of epithelial cells surrounding the wound and estimated wound closure times were calculated by measuring the reduction in wound size. Combined treatment with 33 mM retrobulbar and 3.3 mM topical capsaicin for 3 weeks induced a significant delay in epithelial migration rates and in wound closure times (P less than 0.05). Topical or retrobulbar capsaicin alone for 3 weeks and combined treatment lasting only 1 week were not sufficient to modify wound healing times. The substance P antagonist, spantide (3 mM), applied topically for 1-3 weeks before or immediately after corneal wounding was also ineffective in changing wound closure rates. These findings suggest that the delayed wound healing observed after prolonged treatment with capsaicin could be due to a sustained depletion of neuropeptides from corneal sensory endings, supporting the hypothesis that trophic effects of sensory nerves on corneal epithelium are, at least in part, mediated by neuropeptides contained in peripheral nerve terminals.     

Capsaicin-induced corneal changes associated with sensory denervation in neonatal rat

A single subcutaneous injection of capsaicin (50 mg/kg) to neonatal Wistar rats induced prominent corneal changes consisting of initial edematous lesions and late degenerative opacities in the epithelium and stroma, with disintegrated epithelial cells on histologic sections. Qualitative and quantitative analyses of the corneal nerves by means of gold chloride impregnation revealed a marked degeneration of the intraepithelial terminal fibers, resulting in a significant decrease in neural density by over 70% of the normal value at 4 and 6 weeks after neonatal capsaicin treatment. Thereafter, regeneration of nerve fibers occurred, but the neural density did not return to the normal level at 6 months after treatment.     

Comparative effects of the nonsteroidal anti-inflammatory drug nepafenac on corneal sensory nerve fibers responding to chemical irritation

PURPOSE: To compare the corneal analgesic efficacy of the nonsteroidal anti-inflammatory drugs (NSAIDs) nepafenac, diclofenac, and ketorolac, and to evaluate the possibility that their inhibitory effects on corneal polymodal nociceptor fiber activity are partly mediated by a decrease in sodium currents. METHODS: Corneal sensory afferent units were recorded in the anesthetized cat. The response of thin myelinated polymodal nociceptor fibers to mechanical and acidic stimulation (98.5% CO(2)) was recorded before and at various times after topical application of the vehicle or of nepafenac 0.1% (Nevanac; Alcon Laboratories, Ltd., Fort Worth, TX), diclofenac 0.1% (Voltaren; Novartis, Basel, Switzerland), and ketorolac 0.4% (Acular LS; Allergan, Irvine, CA). Voltage-clamp recordings were performed in cultured trigeminal ganglion neurons. RESULTS: Nepafenac, diclofenac, and ketorolac reduced the mean frequency of the impulse response evoked by repeated CO(2) stimuli in polymodal nociceptor fibers. The progressive increase in ongoing activity, observed in vehicle-treated eyes after repeated acidic stimulation was also prevented. Nepafenac exhibited a more rapid and a slightly more pronounced effect on spontaneous and CO(2)-evoked activity than did diclofenac and ketorolac and did not affect the responsiveness of corneal mechanonociceptor or cold receptor fibers. In cultured mice trigeminal ganglion neurons, diclofenac significantly suppressed sodium currents, whereas nepafenac or its metabolite, amfenac, exhibited only minimal inhibitory effects. CONCLUSIONS: The inhibition of polymodal nociceptor activity by nepafenac, a weak inhibitor of cyclooxygenase, is most likely due to its greater lipophilicity compared with diclofenac and ketorolac, leading to a rapid saturation of the corneal epithelium where nociceptor terminals are located. In contrast to diclofenac, nepafenac does not exhibit local anesthetic effects.     

Irreversible corneal decompensation in patients treated with topical dorzolamide

PURPOSE: To describe irreversible corneal decompensation after topical dorzolamide hydrochloride (Trusopt; Merck and Co, Inc, West Point, Pennsylvania) therapy in nine patients who had histories consistent with corneal endothelial compromise. METHOD: Multicenter review of patients' charts. RESULTS: Nine eyes of nine patients developed overt corneal decompensation after starting topical dorzolamide, a condition that did not resolve with drug cessation. This occurred after 3 to 20 weeks (mean, 7.8) of therapy. All nine patients had undergone intraocular surgery. Eight patients had undergone cataract surgery; three were aphakic and three had posterior chamber intraocular lenses. Two patients had anterior chamber intraocular lenses and also had undergone trabeculectomies. Four patients had undergone penetrating keratoplasties, each case complicated by episodes of corneal allograft rejection that were successfully treated. Two patients had asymptomatic Fuchs endothelial dystrophy. Seven patients have since undergone successful penetrating keratoplasties. CONCLUSION: The reports suggest that dorzolamide can cause irreversible corneal edema in a subset of glaucoma patients with endothelial compromise. The findings suggest a rationale for research into the long-term effects of dorzolamide on the corneal endothelium.