RESUMEN
Mild traumatic brain injury (mTBI) is often accompanied by neurological and ocular symptoms that involve trigeminal nerve pathways. Laser-induced shock wave (LISW) was applied to the skull of male rats as a model for mTBI, while behavioral and neural recording methods were used to assess trigeminal function. The LISW caused greater eye wiping behavior to ocular instillation of hypertonic saline (Sham = 4.83 ± 0.65 wipes/5 min, LISW = 12.71 ± 1.89 wipes/5 min, p < 0.01) and a marked reduction in the time spent in bright light consistent with enhanced periocular and intraocular hypersensitivity, respectively (Sham = 16.3 ± 5.6 s, LISW = 115.5 ± 27.3 s, p < 0.01). To address the early neural mechanisms of mTBI, single trigeminal brainstem neurons, identified by activation to corneal or dural mechanical stimulation, were recorded in trigeminal subnucleus interpolaris/caudalis (Vi/Vc) and trigeminal subnucleus caudalis/upper cervical cord (Vc/C1) regions. The LISW caused marked sensitization to hypertonic saline and to exposure to bright light in neurons of both regions (p < 0.05). Laser speckle imaging revealed an increase in meningeal arterial blood flow to bright light after LISW (Sham = 4.7 ± 2.0 s, LISW = 469.0 ± 37.9 s, p < 0.001). Local inhibition of synaptic activity at Vi/Vc, but not at Vc/C1, by microinjection of CoCl2, prevented light-evoked increases in meningeal blood flow in LISW-treated rats. By contrast, topical meningeal application of phenylephrine significantly reduced light-evoked responses of Vi/Vc and Vc/C1 neurons. These data suggested that neurons in both regions became sensitized after LISW and were responsive to changes in meningeal blood flow. Neurons at the Vi/Vc transition and at Vc/C1, however, likely serve different roles in mediating the neurovascular and sensory aspects of mTBI.
Asunto(s)
Conmoción Encefálica , Ratas , Masculino , Animales , Ratas Sprague-Dawley , Conmoción Encefálica/metabolismo , Neuronas/metabolismo , Córnea/inervación , Córnea/fisiología , Tronco EncefálicoRESUMEN
The P2Y2 receptor agonist, diquafosol sodium, is commonly used to treat the signs and symptoms of dry eye disease (DE) patients. Although diquafosol improves tear film stability, the neural mechanisms underlying the reduction in ocular pain are not well defined. This study determined if repeated application of diquafosol reduces the sensitization of nociceptive neurons in the lower trigeminal brainstem nuclear complex (TBNC) via peripheral P2Y2 mechanisms in a rat model for DE. Diquafosol was applied to the ocular surface daily for 28 days, starting at day 0 or day 14, after exorbital gland removal. The number of eyeblinks, P2Y2-immunoreactive neurons in the trigeminal ganglion (TG), and correlates of TBNC neural excitability (i.e., cFos protein and phosphorylated extracellular signal-regulated kinase (pERK) expression) were assessed in male rats. Diquafosol increased spontaneous tear volume and reduced the number of ocular surface-evoked eyeblinks in DE rats. Fluorogold-labeled TG neurons that supply the cornea expressed P2Y2. The number of P2Y2-immunoreactive neurons was increased in DE rats and suppressed by diquafosol. Diquafosol also reduced the number of cFos- and pERK-immunoreactive neurons in the TBNC in DE rats. These findings suggest that diquafosol, regardless of late-phase treatment, relieves ocular nociception in DE by reducing peripheral P2Y2 expression.
Asunto(s)
Síndromes de Ojo Seco , Ganglio del Trigémino , Ratas , Masculino , Animales , Ganglio del Trigémino/metabolismo , Síndromes de Ojo Seco/tratamiento farmacológico , Síndromes de Ojo Seco/diagnóstico , Síndromes de Ojo Seco/metabolismo , Lágrimas/metabolismo , Tronco Encefálico , Neuronas/metabolismoRESUMEN
BACKGROUND: The rodent vibrissal (whisker) systcnsorimotor integration and active tactile sensing. Experiments on the vibrissal system often require highly repeatable stimulation of multiple whiskers and the ability to vary stimulation parameters across a wide range. The stimulator must also be easy to position and adjust. Developing a multi-whisker stimulation system that meets these criteria remains challenging. NEW METHOD: We describe a novel multi-whisker stimulator to assess neural selectivity for the direction of global motion. The device can generate repeatable, linear sweeps of tactile stimulation across the whisker array in any direction and with a range of speeds. A fiber optic beam break detects the interval of whisker contact as the stimulator passes through the array. RESULTS: We demonstrate the device's function and utility by recording from a small number of multi-whisker-responsive neurons in the trigeminal brainstem. Neurons had higher firing rates in response to faster stimulation speeds; some also exhibited strong direction-of-motion tuning. COMPARISON WITH EXISTING METHODS: The stimulator complements more standard piezo-electric stimulators, which offer precise control but typically stimulate only single whiskers, require whisker trimming, and travel through small angles. It also complements non-contact methods of stimulation such as air-puffs and electromagnetic-induced stimulation. Tradeoffs include stimulation speed and frequency, and the inability to stimulate whiskers individually. CONCLUSIONS: The stimulator could be used - in either anesthetized or awake, head-fixed preparations - as an approach to studying global motion selectivity of multi-whisker sensitive neurons at multiple levels of the vibrissal-trigeminal system.
Asunto(s)
Percepción del Tacto , Vibrisas , Animales , Neuronas/fisiología , Estimulación Física/métodos , Corteza Somatosensorial/fisiología , Tacto/fisiología , Percepción del Tacto/fisiología , Vibrisas/fisiologíaRESUMEN
The trigeminal brainstem sensory nuclear complex is the first central relay structure mediating orofacial somatosensory and nociceptive perception. Animal studies suggest a substantial involvement of neurochemical alterations at such basal CNS levels in acute and chronic pain processing. Translating this animal based knowledge to humans is challenging. Human related examining of brainstem functions are challenged by MR related peculiarities as well as applicability aspects of experimentally standardized paradigms. Based on our experience with an MR compatible human orofacial pain model, the aims of the present study were twofold: 1) from a technical perspective, the evaluation of proton magnetic resonance spectroscopy at 3 T regarding measurement accuracy of neurochemical profiles in this small brainstem nuclear complex and 2) the examination of possible neurochemical alterations induced by an experimental orofacial pain model. Data from 13 healthy volunteers aged 19-46 years were analyzed and revealed high quality spectra with significant reductions in total N-acetylaspartate (N-acetylaspartate + N-acetylaspartylglutamate) (-3.7%, p = 0.009) and GABA (-10.88%, p = 0.041) during the pain condition. These results might reflect contributions of N-acetylaspartate and N-acetylaspartylglutamate in neuronal activity-dependent physiologic processes and/or excitatory neurotransmission, whereas changes in GABA might indicate towards a reduction in tonic GABAergic functioning during nociceptive signaling. Summarized, the present study indicates the applicability of 1H-MRS to obtain neurochemical dynamics within the human trigeminal brainstem sensory nuclear complex. Further developments are needed to pave the way towards bridging important animal based knowledge with human research to understand the neurochemistry of orofacial nociception and pain.
Asunto(s)
Dolor Facial/metabolismo , Espectroscopía de Protones por Resonancia Magnética/métodos , Núcleos del Trigémino/metabolismo , Adulto , Ácido Aspártico/análogos & derivados , Ácido Aspártico/análisis , Ácido Aspártico/metabolismo , Dipéptidos/análisis , Dipéptidos/metabolismo , Femenino , Humanos , Masculino , Persona de Mediana Edad , Adulto Joven , Ácido gamma-Aminobutírico/análisis , Ácido gamma-Aminobutírico/metabolismoRESUMEN
Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.
Asunto(s)
Dolor , Animales , Córnea , Síndromes de Ojo Seco , Nociceptores , Sensación , TermorreceptoresRESUMEN
Cornea-evoked eyeblinks maintain tear film integrity on the ocular surface in response to dryness and protect the eye from real or potential damage. Eyelid movement following electrical stimulation has been well studied in humans and animals; however, the central neural pathways that mediate protective eyeblinks following natural nociceptive signals are less certain. The aim of this study was to assess the role of the trigeminal subnucleus interpolaris/caudalis (Vi/Vc) transition and subnucleus caudalis/upper cervical cord (Vc/C1) junction regions on orbicularis oculi electromyographic (OOemg) activity evoked by ocular surface application of hypertonic saline or exposure to bright light in urethane anesthetized male rats. The Vi/Vc and Vc/C1 regions are the main sites of termination for trigeminal afferent nerves that supply the ocular surface, while hypertonic saline (saline=0.15-5M) and bright light (light=5k-20klux) selectively activate ocular surface and intraocular trigeminal nerves, respectively, and excite second-order neurons at the Vi/Vc and Vc/C1 regions. Integrated OOemg activity, ipsilateral to the applied stimulus, increased with greater stimulus intensities for both modalities. Lidocaine applied to the ocular surface inhibited OOemg responses to hypertonic saline, but did not alter the response to light. Lidocaine injected into the trigeminal ganglion blocked completely the OOemg responses to hypertonic saline and light indicating a trigeminal afferent origin. Synaptic blockade by cobalt chloride of the Vi/Vc or Vc/C1 region greatly reduced OOemg responses to hypertonic saline and bright light. These data indicate that OOemg activity evoked by natural stimuli known to cause irritation or discomfort in humans depends on a relay in both the Vi/Vc transition and Vc/C1 junction regions.
Asunto(s)
Parpadeo/fisiología , Córnea/fisiología , Nervio Trigémino/fisiología , Anestésicos Intravenosos/farmacología , Animales , Parpadeo/efectos de los fármacos , Médula Cervical/efectos de los fármacos , Médula Cervical/fisiología , Cobalto/farmacología , Córnea/efectos de los fármacos , Electromiografía , Lidocaína/farmacología , Luz , Masculino , Músculo Esquelético/fisiología , Vías Nerviosas/efectos de los fármacos , Vías Nerviosas/fisiología , Neuronas Aferentes/efectos de los fármacos , Neuronas Aferentes/fisiología , Fármacos del Sistema Nervioso Periférico/farmacología , Estimulación Luminosa , Ratas , Solución Salina Hipertónica/administración & dosificación , Nervio Trigémino/efectos de los fármacos , Núcleos del Trigémino/efectos de los fármacos , Núcleos del Trigémino/fisiología , Uretano/farmacología , Bloqueadores del Canal de Sodio Activado por Voltaje/farmacologíaRESUMEN
Orexin-A (OxA) is synthesized in posterior and lateral regions of the hypothalamus and contributes to homeostatic regulation of body functions including pain modulation. To determine if orexinergic mechanisms contribute to posterior hypothalamus (PH)-induced modulation of ocular input to subnucleus caudalis/upper cervical (Vc/C1) neurons, the orexin-1 receptor antagonist SB334867 was applied to the dorsal brainstem surface prior to PH disinhibition, by bicuculline methiodide, in male rats under isoflurane anesthesia. Ocular input to Vc/C1 units by bright light or hypertonic saline was markedly reduced by PH disinhibition and reversed completely by local Vc/C1 application of SB334867. OxA applied to the Vc/C1 surface mimicked the effects of PH disinhibition in a dose-dependent manner. OxA-induced inhibition was prevented by co-application of SB334867, but not by the orexin-2 receptor antagonist TCS Ox2 29. PH disinhibition and local OxA application also reduced the high threshold convergent cutaneous receptive field area of ocular units, suggesting widespread effects on somatic input to Vc/C1 ocular units. Vc/C1 application of OxA or SB334867 alone did not affect the background discharge of ocular units and suggested that the PH-OxA influence on ocular unit activity was not tonically active. Vc/C1 application of OxA or SB334867 alone also did not alter mean arterial pressure, whereas PH disinhibition evoked prompt and sustained increases. These results suggest that stimulus-evoked increases in PH outflow acts through OxA and orexin-1 receptors to alter the encoding properties of trigeminal brainstem neurons responsive to input from the ocular surface and deep tissues of the eye.
Asunto(s)
Hipotálamo Posterior/fisiología , Neuronas/fisiología , Fenómenos Fisiológicos Oculares , Receptores de Orexina/metabolismo , Núcleos del Trigémino/fisiología , Vías Aferentes/fisiología , Animales , Masculino , Estimulación Luminosa , Estimulación Física , Ratas , Ratas Sprague-DawleyRESUMEN
Sensory input from the temporomandibular joint (TMJ) to neurons in superficial laminae at the spinomedullary (Vc/C1-2) region is strongly influenced by estrogen status. This study determined if GABAergic mechanisms play a role in estrogen modulation of TMJ nociceptive processing in ovariectomized female rats treated with high- (HE) or low-dose (LE) estradiol (E2) for 2days. Superficial laminae neurons were activated by ATP (1mM) injections into the joint space. The selective GABAA receptor antagonist, bicuculline methiodide (BMI, 5 or 50µM, 30µl), applied at the site of recording greatly enhanced the magnitude and duration of ATP-evoked responses in LE rats, but not in units from HE rats. The convergent cutaneous receptive field (RF) area of TMJ neurons was enlarged after BMI in LE but not HE rats, while resting discharge rates were increased after BMI independent of estrogen status. By contrast, the selective GABAA receptor agonist, muscimol (50µM, 30µl), significantly reduced the magnitude and duration of ATP-evoked activity, resting discharge rate, and cutaneous RF area of TMJ neurons in LE and HE rats, whereas lower doses (5µM) affected only units from LE rats. Protein levels of GABAA receptor ß3 isoform at the Vc/C1-2 region were similar for HE and LE rats. These results suggest that GABAergic mechanisms contribute significantly to background discharge rates and TMJ-evoked input to superficial laminae neurons at the Vc/C1-2 region. Estrogen status may gate the magnitude of GABAergic influence on TMJ neurons at the earliest stages of nociceptive processing at the spinomedullary region.