Optical recording of the neuronal activity in the brainstem-spinal cord: application of a voltage-sensitive dye

Effect of carbamazepine and gabapentin on excitability in the trigeminal subnucleus caudalis of neonatal rats using a voltage-sensitive dye imaging technique

Background

The antiepileptic drugs carbamazepine and gabapentin are effective in treating neuropathic pain and trigeminal neuralgia. In the present study, to analyze the effects of carbamazepine and gabapentin on neuronal excitation in the spinal trigeminal subnucleus caudalis (Sp5c) in the medulla oblongata, we recorded temporal changes in nociceptive afferent activity in the Sp5c of trigeminal nerve-attached brainstem slices of neonatal rats using a voltage-sensitive dye imaging technique.

Results

Electrical stimulation of the trigeminal nerve rootlet evoked changes in the fluorescence intensity of dye in the Sp5c. The optical signals were composed of two phases, a fast component with a sharp peak followed by a long-lasting component with a period of more than 500 ms. This evoked excitation was not influenced by administration of carbamazepine (10, 100 and 1,000 μM) or gabapentin (1 and 10 μM), but was increased by administration of 100 μM gabapentin. This evoked excitation was increased further in low Mg²⁺ (0.8 mM) conditions, and this effect of low Mg²⁺ concentration was antagonized by 30 μM DL-2-amino-5-phosphonopentanoic acid (AP5), a N-methyl-d-aspartate (NMDA) receptor blocker. The increased excitation in low Mg²⁺ conditions was also antagonized by carbamazepine (1,000 μM) and gabapentin (100 μM).

Conclusion

Carbamazepine and gabapentin did not decrease electrically evoked excitation in the Sp5c in control conditions. Further excitation in low Mg²⁺ conditions was antagonized by the NMDA receptor blocker AP5. Carbamazepine and gabapentin had similar effects to AP5 on evoked excitation in the Sp5c in low Mg²⁺ conditions. Thus, we concluded that carbamazepine and gabapentin may act by blocking NMDA receptors in the Sp5c, which contributes to its anti-hypersensitivity in neuropathic pain.

The central respiratory chemoreceptor: where is it located?-Invited article

We review previous reports on the localization of the central chemoreceptor focusing on our studies that used various experimental techniques including lesioning (brainstem transection and removal of pia mater), analyses of neuronal responses to CO(2) by electrophysiological and optical recording, mapping of CO(2)-excitable neurons by c-fos immunohistochemistry and local acidic stimulation. Among these experimental techniques, voltage imaging with calculation of cross correlation coefficients between the respiratory output activity and each pixel, i.e., correlation coefficient imaging technique, enabled us to effectively analyze imaging data without empirical signal processing. The reviewed studies have indicated that the most superficial layer of the rostral ventral medulla, i.e., the surface portions of the nucleus retrotrapezoideus/parafacial respiratory group, nucleus parapyramidal superficialis and nucleus raphe pallidus, is important in central chemoreception. We suggest that one of the major respiratory rhythm generators, i.e., the preBötzinger complex, is not chemosensitive in itself or rather inhibited by CO(2). Based on our detailed analysis of c-fos immunohistochemistry, we propose a cell-vessel architecture model for the central respiratory chemoreceptor. Primary chemoreceptor cells are mainly located beneath large surface vessels within the marginal glial layer of the ventral medulla, and surround fine penetrating vessels that branch from a large surface vessel. Respiratory neurons in the rostral portion of the ventral respiratory group could be intrinsically chemosensitive, but their role in chemoreception might be secondary. Definitive identification of chemosensitive sites and chemoreceptor cells needs further studies.

Iteration of high-frequency stimulation enhances long-lasting excitatory responses in the spinal dorsal horn of rats: characterization by optical imaging of signal propagation

To investigate plastic changes in nociceptive sensitivity of the dorsal horn, slow excitatory responses elicited by iteration of high-frequency stimulation were spatiotemporally observed in spinal cord slices of young-adult rats using membrane excitation imaging techniques. Single-pulse stimulation to the dorsal root elicited membrane excitation in lamina II, and high-frequency pulse-train stimulation evoked long-lasting excitation that expanded widely in the dorsal horn. Iteration of high-frequency stimulation enhanced the strength and extent of the excitatory responses, but such augmentation of the excitatory responses disappeared in the presence of an NMDA receptor antagonist (CPP) and was hindered by an NK1 receptor antagonist (L-703.606). The results suggest that activation of both NMDA and NK1 receptors is involved in the enhancement of slow excitatory responses evoked by iteration of high-frequency stimulation.

Ischemia-induced disturbance of neuronal network function in the rat spinal cord analyzed by voltage-imaging

Using a voltage-imaging technique, we analyzed the acute effect of ischemia, hypoxia and hypoglycemia on the neuronal network function of the rat spinal cord. Ischemic, hypoxic, or hypoglycemic stress was loaded to spinal cord slices with an oxygen- and glucose-free, oxygen-free, or glucose-free mock cerebrospinal fluid, respectively. Depolarizing signals in the dorsal horn, induced by dorsal root stimulation, consisted of fast (pre-synaptic) and slow (post-synaptic) components. The slow component was attenuated much more than the fast component under an ischemic condition (P<0.0002). Post-synaptic neuronal activities in lamina III-IV were suppressed earlier than those in lamina I-II. The nerve fiber was relatively resistant to ischemia. As long as the fast component was preserved in the dorsal horn, the suppression of the fast and slow components was reversible. There was a significant difference (P<0.05) in the recovered slow component sizes between the group in which the fast component was suppressed by more than 20% by ischemia and the group in which the suppression was less than 20%. Further prolonged stress irreversibly eliminated most of the slow component, and attenuated the fast component (to 59+/-8%) accompanied by cellular damage in histology. Suppression of neural activity by hypoxic or hypoglycemic stress was less prominent than that by ischemia. Prolonged ischemic stress suddenly and irreversibly eliminated depolarizing signals in the ventral horn accompanied by morphological damage of motoneurons. Immunohistochemical staining was negative for apoptosis. We have, for the first time, analyzed the processes of spinal cord disturbance induced by ischemia, hypoxia and hypoglycemia at the neuronal network level by directly observing the regional neuronal network activities within the spinal cord. We conclude that synaptic transmission in the dorsal horn, especially in deep regions, is vulnerable and first affected by these stresses. Severe ischemic stress induces irreversible dysfunction of neurons accompanied by eventual cell death in both dorsal and ventral horns.

Spatiotemporal patterns of dorsal root-evoked network activity in the neonatal rat spinal cord: optical and intracellular recordings

Spatiotemporal patterns of dorsal root-evoked potentials were studied in transverse slices of the rat spinal cord by monitoring optical signals from a voltage-sensitive dye with multiple-photodiode optic camera. Typically, dorsal root stimulation generated two basic waveforms of voltage images: dual-component images consisting of fast, spike-like signal followed by a slow signal in the dorsal horn, and small, slow signals in the ventral horn. To qualitatively relate the optical signals to membrane potentials, whole cell recordings were combined with measurements of light absorption in the area around the soma. The slow optical signals correlated closely with subthreshold postsynaptic potentials in all regions of the cord. The spike-like component was not associated with postsynaptic action potentials, suggesting that the fast signal was generated by presynaptic action potentials. Firing in a single neuron could not be detected optically, implying that local voltage images originated from synchronously activated neuronal ensembles. Blocking glutamatergic synaptic transmission inhibited excitatory postsynaptic potentials (EPSPs) and significantly reduced the slow optical signals, indicating that they were mediated by glutamatergic synapses. Suppressing glycine-mediated inhibition increased the amplitude of both optical signals and EPSPs, while blocking GABA(A) receptor-mediated synapses, increased the amplitude and time course of EPSPs and prolonged the duration of voltage images in larger areas of the slice. The close correlation between evoked EPSPs and their respective local voltage images shows the advantage of the high temporal resolution optical system in measuring both the spatiotemporal dynamics of segmental network excitation and integrated potentials of neuronal ensembles at identified sites.

Optical mapping of pontine chemosensitive regions of neonatal rat

We analyzed the neuronal response to hypercapnic acidosis, using an optical recording technique with a fluorescent voltage-sensitive dye (di-4-ANEPPS), in pontine slice preparations of neonatal rats, containing the locus coeruleus (LC), which has been electrophysiologically demonstrated to be chemosensitive. The dye-stained preparation was continuously superfused with artificial cerebrospinal fluid. Epifluorescence of the slice was detected using a high-sensitivity optical recording system. Changes in the intensity of fluorescence were serially analyzed while switching artificial cerebrospinal fluid from control to hypercapnic acidosis, or vice versa. The optical recording method revealed that the LC, as reported in previous studies, reversibly showed a depolarizing response to hypercapnic acidosis in 56% of the examined preparations. The A5 area (56%) also exhibited a reversible, depolarizing response to hypercapnic acidosis. The response was preserved under conditions in which chemical synaptic transmission was blocked by low Ca(2+)-high Mg(2+) solution. These results suggest that the optical recording method is applicable to identification of potentially chemosensitive areas, which deserve further electrophysiological analysis, and that the A5 area could be chemosensitive.

Synaptic Blockade Plays a Major Role in the Neural Disturbance of Experimental Spinal Cord Compression

We analyzed dynamic processes of neural excitation propagation in the experimentally compressed spinal cord using a high-speed optical recording system. Transverse slices of the juvenile rat cervical spinal cord were stained with a voltage-sensitive dye (di-4-ANEPPS). Two components were identified in the depolarizing optical responses to dorsal root electrical stimulation: a fast component of short duration corresponding to pre-synaptic excitation and a slow component of long duration corresponding to post-synaptic excitation. In the directly compressed dorsal horn, the slow component was attenuated more (attenuated to 37.4 +/- 9.1% of the control) than the fast component (to 70.5 +/- 14.9%) (p < 0.01) at 400 msec after stimulation. Depolarizing optical responses to compression and to chemical synaptic blockade were similar. There was a regional difference between white matter (attenuated to 86.2 +/- 10.5%) and gray matter (to 72.6 +/- 10.4%) (p < 0.03) in compression-induced changes of the fast components; neural activity in the white matter was resistant to compression, especially in the dorsal root entry zone. Depolarizing optical signals in the region adjacent to the directly compressed site were also attenuated; the fast component was attenuated to 77.6 +/- 10.4% and the slow component to 31.8 +/- 11.3% of the control signals (p < 0.01). Spinal cord dysfunction induced by purely mechanical compression without tissue destruction was virtually restored with early decompression. We suggest that a disturbance of synaptic transmission plays an important role in the pathophysiological mechanisms of spinal cord compression, at least under in vitro experimental conditions of juvenile rats.

Acetazolamide and respiratory chemosensitivity to CO(2) in the neonatal rat transverse medullary slice

Hypoglossal nerve rootlets in the transverse medullary slice prepared from neonatal rats exhibit a bursting 'respiratory' rhythm that increases in frequency with CO(2), presumably due to activation of chemosensitive cells such as the central chemoreceptors. Carbonic anhydrase is associated with areas of central chemoreception and we propose a hypothesis for its involvement in the chemoreception process. We tested this hypothesis by blocking its activity with acetazolamide in six slice preparations. However, the addition of 1 mM acetazolamide dissolved in dimethyl sulphoxide to the superfusing bathing solution produced no alteration in the bursting frequency response of the slice to CO(2). We concluded that the chemoreception process producing the CO(2) response of the superfused, transverse medullary slice does not involve carbonic anhydrase.