Intracellular response properties of neurons in the spinal trigeminal nucleus to peripheral and cortical stimulation in the cat

Overexpressed Na V 1.7 Channels Confer Hyperexcitability to in vitro Trigeminal Sensory Neurons of Ca V 2.1 Mutant Hemiplegic Migraine Mice

Trigeminal sensory neurons of transgenic knock-in (KI) mice expressing the R192Q missense mutation in the α1A subunit of neuronal voltage-gated Ca_V2.1 Ca²⁺ channels, which leads to familial hemiplegic migraine type 1 (FHM1) in patients, exhibit a hyperexcitability phenotype. Here, we show that the expression of Na_V1.7 channels, linked to pain states, is upregulated in KI primary cultures of trigeminal ganglia (TG), as shown by increased expression of its α1 subunit. In the majority of TG neurons, Na_V1.7 channels are co-expressed with ATP-gated P2X3 receptors (P2X3R), which are important nociceptive sensors. Reversing the trigeminal phenotype with selective Ca_V2.1 channel inhibitor ω-agatoxin IVA inhibited Na_V1.7 overexpression. Functionally, KI neurons revealed a TTX-sensitive inward current of larger amplitude that was partially inhibited by selective Na_V1.7 blocker Tp1a. Under current-clamp condition, Tp1a raised the spike threshold of both wild-type (WT) and KI neurons with decreased firing rate in KI cells. Na_V1.7 activator OD1 accelerated firing in WT and KI neurons, a phenomenon blocked by Tp1a. Enhanced expression and function of Na_V1.7 channels in KI TG neurons resulted in higher excitability and facilitated nociceptive signaling. Co-expression of Na_V1.7 channels and P2X3Rs in TGs may explain how hypersensitivity to local stimuli can be relevant to migraine.

Loss of inhibition by brain natriuretic peptide over P2X3 receptors contributes to enhanced spike firing of trigeminal ganglion neurons in a mouse model of familial hemiplegic migraine type-1

Purinergic P2X3 receptors (P2X3Rs) play an important role in pain pathologies, including migraine. In trigeminal neurons, P2X3Rs are constitutively downregulated by endogenous brain natriuretic peptide (BNP). In a mouse knock-in (KI) model of familial hemiplegic migraine type-1 with upregulated calcium CaV2.1 channel function, trigeminal neurons exhibit hyperexcitability with gain-of-function of P2X3Rs and their deficient BNP-mediated inhibition. We studied whether the absent BNP-induced control over P2X3Rs activity in KI cultures may be functionally expressed in altered firing activity of KI trigeminal neurons. Patch-clamp experiments investigated the excitability of wild-type and KI trigeminal neurons induced by either current or agonists for P2X3Rs or transient receptor potential vanilloid-1 (TRPV1) receptors. Consistent with the constitutive inhibition of P2X3Rs by BNP, sustained pharmacological block of BNP receptors selectively enhanced P2X3R-mediated excitability of wild-type neurons without affecting firing evoked by the other protocols. This effect included increased number of action potentials, lower spike threshold and shift of the firing pattern distribution toward higher spiking activity. Thus, inactivation of BNP signaling transformed the wild-type excitability phenotype into the one typical for KI. BNP receptor block did not influence excitability of KI neurons in accordance with the lack of BNP-induced P2X3R modulation. Our study suggests that, in wild-type trigeminal neurons, negative control over P2X3Rs by the BNP pathway is translated into tonic suppression of P2X3Rs-mediated excitability. Lack of this inhibition in KI cultures results in a hyperexcitability phenotype and might contribute to facilitated trigeminal pain transduction relevant for migraine.

Response properties of temporomandibular joint mechanosensitive neurons in the trigeminal sensory complex of the rabbit

The neurophysiological properties of neurons sensitive to TMJ movement (TMJ neurons) in the trigeminal sensory complex (Vcomp) during passive movement of the isolated condyle were examined in 46 rabbits. Discharges of TMJ neurons from the rostral part of the Vcomp were recorded with a microelectrode when the isolated condyle was moved manually and with a computer-regulated mechanostimulator. A total of 443 neurons responding to mechanical stimulation of the face and oral cavity were recorded from the brainstem. Twenty-one TMJ neurons were detected rostrocaudally from the dorsal part of the trigeminal principal sensory nucleus (NVsnpr), subnucleus oralis of the trigeminal spinal nucleus, and reticular formation surrounding the trigeminal motor nucleus. Most of the TMJ neurons were located in the dorso-rostral part of the NVsnpr. Of the TMJ units recorded, 90 % were slowly adapting and 26 % had an accompanying resting discharge. The majority (86 %) of the TMJ units responded to the movement of the isolated condyle in the anterior and/or ventral directions, and half were sensitive to the condyle movement in a single direction. The discharge frequencies of TMJ units increased as the condyle displacement and constant velocity (5 mm/s) increased within a 5-mm anterior displacement of the isolated condyle. Based on these results, we conclude that sensory information is processed by TMJ neurons encoding at least joint position and displacement in the physiological range of mandibular displacement.

Identification of c-Fos immunoreactive brainstem neurons activated during fictive mastication in the rabbit

In the present study we used the expression of the c-Fos-like protein as a "functional marker" to map populations of brainstem neurons involved in the generation of mastication. Experiments were conducted on urethane-anesthetized and paralyzed rabbits. In five animals (experimental group), rhythmical bouts of fictive masticatory-like motoneuron activity (cumulative duration 60-130 min) were induced by electrical stimulation of the left cortical "masticatory area" and recorded from the right digastric motoneuron pool. A control group of five animals (non-masticatory) were treated in the same way as the experimental animals with regard to surgical procedures, anesthesia, paralysis, and survival time. To detect the c-Fos-like protein, the animals were perfused, and the brainstems were cryosectioned and processed immunocytochemically. In the experimental group, the number of c-Fos-like immunoreactive neurons increased significantly in several brainstem areas. In rostral and lateral areas, increments occurred bilaterally in the borderzones surrounding the trigeminal motor nucleus (Regio h); the rostrodorsomedial half of the trigeminal main sensory nucleus; subnucleus oralis-gamma of the spinal trigeminal tract; nuclei reticularis parvocellularis pars alpha and nucleus reticularis pontis caudalis (RPc) pars alpha. Further caudally-enhanced labeling occurred bilaterally in nucleus reticularis parvocellularis and nucleus reticularis gigantocellularis (Rgc) including its pars-alpha. Our results provide a detailed anatomical record of neuronal populations that are correlated with the generation of the masticatory motor behavior.

Eye movement-related modulation of trigeminal neuron activity during active sleep and wakefulness

The amplitude of electrically-evoked mass action potentials recorded in the spinal cord and brainstem has been reported to decrease only during eye movement events of active sleep. In contrast, we have reported that the response of trigeminal sensory neurons to peripheral stimuli is modulated throughout the behavioral state of active sleep. It is unclear whether eye movement events contribute to the modulation of trigeminal sensory neuron activity during active sleep. In the present study, eye movement events were demarcated in order to investigate how these events affect peripheral input to trigeminal sensory neurons in chronic, intact, behaving cats. When compared with wakefulness, the mean response of 45 trigeminal sensory neurons to low-intensity electrical stimulation of the canine tooth pulp was significantly suppressed by 28% during periods of active sleep where no eye movement activity was present and by 41% during periods of active sleep with eye movement events. Hence, during active sleep, tooth pulp-evoked responses were significantly decreased by 16% during eye movement events when compared with non-eye movement active sleep. To investigate whether presynaptic inhibition played a role in this phenomenon, the excitability of eight individual tooth pulp afferent terminals during eye movement periods was compared with non-eye movement periods of active sleep. No evidence of eye movement-related depolarization of tooth pulp terminals was detected. When compared to wakefulness, the responses of six trigeminal sensory neurons to air puff stimulation of facial hair mechanoreceptors were significantly increased by 96% during periods of active sleep where no eye movement activity was present but were significantly decreased by 15% during eye movement events when compared with non-eye movement active sleep. The results of the present study indicate that neuronal responses to both tooth pulp and facial hair mechanoreceptor stimulation are significantly attenuated during eye movement events of active sleep.

Spontaneous discharge and peripherally evoked orofacial responses of trigemino-thalamic tract neurons during wakefulness and sleep

In the present study, ongoing and evoked activity of antidromically identified trigemino-thalamic tract (TGT) neurons was examined over the sleep-wake cycle in cats. There was no difference in the mean spike discharge rate of TGT neurons when quiet sleep (QS) and active sleep (AS) were compared with wakefulness (W). However, tooth pulp-evoked responses of TGT neurons were decreased during AS when compared to W. Conversely, the responses of TGT neurons to air puff activation of facial hair mechanoreceptors reciprocally increased during AS when compared to W. The present data demonstrate that ascending sensory information emanating from distinct orofacial areas is differentially modified during the behavioral state of AS. Specifically, the results obtained suggest that during AS, sensory information arising from hair mechanoreceptors is enhanced, whereas information arising from tooth pulp afferents is suppressed. These data may provide functional evidence for an AS-related gate control mechanism of sensory outflow to higher brain centers.

Morphology and synaptic connections of slowly adapting periodontal afferent terminals in the trigeminal subnuclei principalis and oralis of the cat

Previous studies suggest that sensory information from primary afferent fibers is processed in a distinct manner in the individual subnuclei of trigeminal sensory nuclear complex. The present study has addressed this issue by using intra-axonal labeling with horseradish peroxidase to examine the ultrastructure and synaptic organization of axon terminals from slowly adapting (SA) periodontal afferents in the ventral subdivision (Vpv) of principalis and the rostro-dorsomedial part (Vo.r) of oralis. Our observations are based on complete or near-complete reconstructions of 139 synaptic boutons in Vpv and 105 in Vo.r. All the labeled boutons contained clear, spherical, synaptic vesicles and were presynaptic to unlabeled dendrites, and they were frequently postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles (P-endings). The P-endings frequently formed axodendritic synapses on dendrites which received axodendritic synapses from labeled boutons (synaptic triads). On the basis of the number of contacts, synaptic arrangements associated with the labeled boutons could be subgrouped into simple (one or two contacts), intermediate (three or four contacts), and complex (five or more contacts) types. The labeled boutons varied from round to elongated forms with smooth to more irregular or scalloped contours. The boutons with scalloped contour were much more frequent in the complex type. The boutons of the intermediate type were significantly smaller than the complex type and larger than the simple type. The SA periodontal afferent terminals participated in each type of synaptic arrangements in Vpv, but were mostly of the simple type in Vo.r. The size of labeled boutons was significantly larger in Vpv than in Vo.r. The total number of axodendritic and axoaxonic contacts per labeled bouton was significantly higher in Vpv than in Vo.r. Another difference was the more frequent occurrence of synaptic triads in Vpv than in Vo.r. These observations provide evidence that sensory information from primary afferent fibers is processed in a different manner in the two subnuclei.