Slow intrinsic optical signals in the rat spinal dorsal horn in slice

Water diffusion closely reveals neural activity status in rat brain loci affected by anesthesia

Diffusion functional MRI (DfMRI) reveals neuronal activation even when neurovascular coupling is abolished, contrary to blood oxygenation level—dependent (BOLD) functional MRI (fMRI). Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity observed with local field potentials (LFPs), especially in regions involved in wakefulness. In contrast, BOLD signals showed nonspecific changes, reflecting systemic effects of the anesthesia on overall brain hemodynamics status. Electrical stimulation of the central medial thalamus nucleus (CM) exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion in the CM of furosemide, a specific neuronal swelling blocker, led the ADC to increase further locally, although LFP activity remained unchanged, and increased the current threshold awakening the animals under CM electrical stimulation. Oppositely, induction of cell swelling in the CM through infusion of a hypotonic solution (−80 milliosmole [mOsm] artificial cerebrospinal fluid [aCSF]) led to a local ADC decrease and a lower current threshold to wake up the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Together, those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI.

It has been reported that neuronal activation results in a decrease of water diffusion in activated neural tissue. This new approach, known as diffusion functional MRI (DfMRI), has high potential for functional imaging of the brain, as the currently widespread blood oxygenation level—dependent (BOLD)-functional MRI (fMRI) method, which is based on neurovascular coupling, remains an indirect marker of neuronal activation. Here, we show that the water apparent diffusion coefficient (ADC) derived from DfMRI increased in specific rat brain regions under anesthetic conditions, reflecting the decreased neuronal activity, especially in regions involved in wakefulness. Electrical stimulation of the central medial (CM) thalamic nucleus exhibiting this anesthesia-induced ADC increase led the animals to transiently wake up. Infusion of the CM with furosemide—a specific blocker of neuronal swelling—led the ADC to increase further locally and increased the current threshold for waking the animals. Conversely, induction of cell swelling in the CM through infusion of a hypotonic solution led to a local ADC decrease and a lower current threshold to wake the animals. Strikingly, the local ADC changes produced by blocking or enhancing cell swelling in the CM were also mirrored remotely in areas functionally connected to the CM, such as the cingulate and somatosensory cortex. Those results strongly suggest that neuronal swelling is a significant mechanism underlying DfMRI.

Attenuated Glial K(+) Clearance Contributes to Long-Term Synaptic Potentiation Via Depolarizing GABA in Dorsal Horn Neurons of Rat Spinal Cord

It has been reported that long-term enhancement of superficial dorsal horn (DHs) excitatory synaptic transmission underlies central sensitization, secondary hyperalgesia, and persistent pain. We tested whether impaired clearance of K(+) and glutamate by glia in DHs may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities. Transient exposure of the spinal cord slice to fluorocitrate (FC) is shown to be accompanied by a protracted decrease of the DHs optical response to repetitive electrical stimulation of the ipsilateral dorsal root, and by a similarly protracted increase in the postsynaptic response of the DHs like LTP. It also is shown that LTPFC does not occur in the presence of APV, and becomes progressively smaller as [K(+)]o in the perfusion solution decreased from 3.0 mM to 0.0 mM. Interestingly LTPFC is reduced by bath application of Bic. Whole-cell patch recordings were carried out to evaluate the effects of FC on the response of DHs neurons to puffer-applied GABA. The observations reveal that transient exposure to FC is reliably accompanied by a prolonged (>1 hr) depolarizing shift of the equilibrium potential for the DHs neuron transmembrane ionic currents evoked by GABA. Considered collectively, the findings demonstrate that LTPFC involves (1) elevation of [K(+)]o in the DHs, (2) NMDAR activation, and (3) conversion of the effect of GABA on DHs neurons from inhibition to excitation. It is proposed that a transient impairment of astrocyte energy production can trigger the cascade of dorsal horn mechanisms that underlies hyperalgesia and persistent pain.

Effect of excitatory and inhibitory agents and a glial inhibitor on optically-recorded primary-afferent excitation

The effects of GABA, excitatory amino-acid receptors antagonists and a glial metabolism inhibitor on primary-afferent excitation in the spinal dorsal horn were studied by imaging the presynaptic excitation of high-threshold afferents in cord slices from young rats with a voltage-sensitive dye. Primary afferent fibers and terminals were anterogradely labeled with a voltage-sensitive dye from the dorsal root attached to the spinal cord slice. Single-pulse stimulation of C fiber-activating strength to the dorsal root elicited compound action potential-like optical responses in the superficial dorsal horn. The evoked presynaptic excitation was increased by the GABA_A receptor antagonists picrotoxin and bicuculline, by glutamate receptor antagonists D-AP5 and CNQX, and by the glial metabolism inhibitor mono-fluoroacetic acid (MFA). The increase in presynaptic excitation by picrotoxin was inhibited in the presence of D-AP5, CNQX and MFA. Presynaptic modulation in the central terminal of fine primary afferents by excitatory and inhibitory amino acids may represent a mechanism that regulates the transmission of pain.

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.

Optically Recorded Response of the Superficial Dorsal Horn: Dissociation From Neuronal Activity, Sensitivity to Formalin-Evoked Skin Nociceptor Activation

In rat spinal cord, slice repetitive electrical stimulation of the dorsal root at an intensity that activates C-fibers evokes a slow-to-develop and prolonged (30–50 s) change in light transmittance (OISDR) in the superficial part of the ipsilateral dorsal horn (DHs). Inhibition of astrocyte metabolism [by bath-applied 400 μM fluoroacetate and 200 μM glutamine (FAc + Gln)] or interference with glial and neuronal K+ transport [by 100 μM 4-aminopyridine (4-AP)] leads to dissociation of the OISDR and the postsynaptic DHs response to a single-pulse, constant-current dorsal root stimulus (P-PSPDR). The OISDR decreases under FAc+Gln, whereas the P-PSPDR remains unaltered; under 4-AP, the P-PSPDR increases, but the OISDR decreases. In contrast, both the OISDR and P-PSPDR increase when K+o is elevated to 8 mM. These observations from slices from normal subjects are interpreted to indicate that the OISDR mainly reflects cell volume and light scattering changes associated with DHs astrocyte uptake of K+ and glutamate (GLU). In slices from subjects that received an intracutaneous injection of formalin 3–5 days earlier, both the OISDR and the response of the DHs ipsilateral to the injection site to 100-ms local application (via puffer pipette) of 15 mM K+ or 100 μM GLU were profoundly reduced, and the normally exquisite sensitivity of the DHs to elevated K+o is decreased. Considered collectively, the observations raise the possibility that impaired regulation of DHs K+o and GLUo may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities that develop following intracutaneous formalin injection.

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.

The relationship between changes in intrinsic optical signals and cell swelling in rat spinal cord slices

Changes in intrinsic optical signals could be related to cell swelling; however, the evidence is not compelling. We measured light transmittance, ECS volume fraction (alpha), and extracellular K+ in rat spinal cord slices during electrical stimulation and the application of elevated potassium, NMDA, or anisoosmotic solutions. Dorsal root stimulation (10 Hz/1 min) induced an elevation in extracellular K+ to 6-8 mM, a light transmittance increase of 6-8%, and a relative ECS volume decrease of less than 5%; all of these changes had different time courses. The application of 6 or 10 mM K+ or NMDA (10(-5) M) had no measurable effect on alpha, but light transmittance increased by 20-25%. The application of 50 or 80 mM K+ evoked a 72% decrease in alpha while the light transmittance increase remained as large as that in 6 or 10 mM K+. While the change in alpha persisted throughout the 45-min application, light transmittance, after peaking in 6-8 min, quickly returned to control levels and decreased below them. Astrocytic hypertrophy was observed in 6, 10, and 50 mM K+. The same results followed the application of 10(-4) M NMDA or hypotonic solution (160 mmol/kg). The elevation of extracellular K+ after NMDA application, corresponding to increased neuronal activity, had a similar time course as the light transmittance changes. Furosemide, Cl(-)-free, or Ca(2+)-free solution blocked or slowed down the decreases in alpha, while the light transmittance increases were unaffected. In hypertonic solution (400 mmol/kg), alpha increased by 30-40%, while light transmittance decreased by 15-20%. Thus, light transmittance changes do not correlate with changes in ECS volume but are associated with neuronal activity and morphological changes in astrocytes.

Optical imaging of intrinsic signals induced by peripheral nerve stimulation in the in vivo rat spinal cord

We examined neural response patterns evoked by peripheral nerve stimulation in in vivo rat spinal cords using an intrinsic optical imaging technique to monitor neural activity. Adult rats were anesthetized by urethane, and laminectomy was performed between C5 and Th1 to expose the dorsal surface of the cervical spinal cord. The median, ulnar, and radial nerves were dissected, and bipolar electrodes were implanted in the forelimb. Changes in optical reflectance were recorded from the dorsal cervical spinal cord in response to simultaneous stimulation of the median and ulnar nerves using a differential video acquisition system. In the region of the cervical spinal cord, intrinsic optical signals were detected between C5 and Th1 at wavelengths of 605, 630, 730, 750, and 850 nm: the image with the largest signal intensity and highest contrast was obtained at 605 nm. The signal intensity and response area expanded with an increase in the stimulation intensity and varied with the depth of the focal plane of the macroscope. The intrinsic optical response was mostly eliminated by Cd(2+), suggesting that the detected signals were mainly mediated by postsynaptic mechanisms activated by sensory nerve fibers. Furthermore, we succeeded in imaging neural activity evoked by individual peripheral nerve stimulation. We found that the response areas related to each peripheral nerve exhibited different spatial distribution patterns and that there were animal-to-animal variations in the evoked neural responses in the spinal cord. The results obtained in this study confirmed that intrinsic optical imaging is a very useful technique for acquiring fine functional maps of the in vivo spinal cord.

GABAergic input contributes to activity-dependent change in cell volume in the hippocampal CA1 region

Swelling of brain cells is one of the physiological responses associated with neuronal activation. To investigate underlying mechanisms, we analyzed interactions between changes in cell volume and synaptic responses in the hippocampal slices from rodents. Swelling within the CA1 area was detected as increases in transmittance of near-infrared light (IR), and field excitatory postsynaptic potentials (fEPSPs) were recorded simultaneously. High frequency stimulation (HFS) of afferent fibers induced a transient increase in IR transmittance in both somatic and dendritic regions, which was temporally associated with fEPSPs. Stimulus-induced increases in transmittance were strongly reduced in the presence of DL-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, indicating involvement of glutamate receptors. Application of a GABA-A receptor antagonist, bicuculline, increased the amplitude and time course of the fEPSPs but rather decreased HFS-induced optical signals. When the extracellular Cl(-) was reduced to 10.5 mM, HFS induced a decrease in transmittance, which was also blocked by bicuculline. In hippocampal slices obtained from mice deficient in the 65 kDa isoform of glutamic acid decarboxylase, HFS-induced signals were significantly smaller than in the wild-type mice, although fEPSP profiles did not differ. These results suggest that Cl(-) influx through GABA-A receptors contributes to synaptically evoked swelling in the hippocampal CA1 region.

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

Although there are several limitations, optical recording techniques are superior to multi-electrode mapping methods, as it is possible to record at large number of points in a small area without destroying the tissue and possible to know relative changes of membrane potentials. Optical recording techniques using voltage-sensitive dyes will be more importantly applied in the study of central respiratory control (e.g., mechanisms of respiratory rhythm generation) in the near future.

Intrinsic optical signals in the dorsal horn of rat spinal cord slices elicited by brief repetitive stimulation

With repetitive electrical stimulation of the dorsal root (20 Hz for 1 s at C-fibre strength), intrinsic optical signals (IOSs), measured as changes in light transmittance, were recorded in the superficial dorsal horn of rat spinal cord slices using a photodiode array imaging device. The mechanism underlying the induction of IOSs was investigated. IOSs elicited by brief repetitive stimulation persisted for 1-2 min and were decreased by reducing external Cl- concentration or by cation-chloride cotransport inhibitors. Furosemide was most effective whilst bumetanide was least effective among the inhibitors tested. A 1-min elevation of external K+ concentration evoked IOSs in the dorsal horn in the absence of stimulation, and K+-induced IOSs were inhibited by furosemide. These results suggest that the uptake of excess K+ via the furosemide-sensitive, cation-chloride cotransporters underlies the induction of the IOSs. One-minute exposure to hypotonic solutions, which would cause cell swelling, induced IOSs in the superficial dorsal horn. Whilst osmotic-induced IOSs were not affected by furosemide, they were inhibited by HgCl2 in a 2-mercaptoethanol-sensitive manner. The stimulation-induced IOSs were similarly depressed by HgCl2. In contrast, voltage-sensitive dye signals and field potentials, evoked by single electrical stimuli, were significantly less affected by HgCl2. These results suggest that there is a specialized water transport pathway in the superficial dorsal horn, and that IOSs elicited by brief repetitive activation of C-fibres are attributable to cell swelling caused by water influx through this pathway, as an osmotic gradient is established by the uptake of K+ via the furosemide-sensitive cotransporters.

Effect of halothane on neuronal excitation in the superficial dorsal horn of rat spinal cord slices: evidence for a presynaptic action

The action of the volatile anaesthetic halothane on optically recorded neuronal excitation in juvenile rat spinal cord slices was investigated. Prolonged neuronal excitation lasting approximately 100 ms was evoked in the superficial dorsal horn after single-pulse dorsal root stimulation that activated both A- and C-fibres. Halothane depressed the neuronal excitation in a concentration-dependent manner (IC(50) 0.21 mm, I(max) 28%). In Ca(2+)-free solution, dorsal root stimulation induced excitation with a short duration of several tens of milliseconds, in which the excitation of the postsynaptic component was largely eliminated. Under these conditions, halothane also depressed the excitation concentration-dependently (IC(50) 0.46 mm, I(max) 60%). Most of the suppression occurred within 5 min of halothane application, and the effect of halothane was fully reversible upon washout of the anaesthetic. Application of bicuculline and strychnine or picrotoxin, or reduction of extracellular Cl(-) concentration ([Cl(-)](o)), had no effect on halothane inhibition. Applications of K(+) channel blockers tetraethyl ammonium, 4-aminopyridine, Cs(+) or Ba(2+) either had no effect or augmented the inhibitory effect of halothane. On the other hand, the degree of inhibition by halothane was found to be dependent on [K(+)](o); the higher [K(+)](o), the larger the depression. In addition, decreases in [Na+]o and [Mg(2+)](o) reduced the excitation similar to that of halothane treatment, and the degree of halothane inhibition became larger with lower [Mg(2+)](o). These results lead to a hypothesis that halothane suppresses the excitation of presynaptic elements by inhibiting presynaptic Na(+) channels by shifting the steady-state inactivation curve in the hyperpolarizing direction.

Optical measurement of swelling and water transport in spinal cord slices from aquaporin null mice

Water movement between cells and interstitium in spinal cord and brain occurs during neural signal transduction and in response to injuries such as ischemia and blunt trauma. At least two aquaporin-type water channels are expressed in spinal cord: AQP1 in afferent sensory nerve fibers in the superficial layers of the dorsal horn, and AQP4 in glial cells throughout gray matter. An imaging method was developed to map thickness changes in viable spinal cord and brain slices cut by a vibratome, and applied to measure osmotically induced water transport in spinal cord slices from wildtype and aquaporin knockout mice. Spinal cord slices (300 microm thickness) were mounted in a perfusion chamber with < 2 s exchange time, and transmitted light (565 nm) was imaged by a CCD camera. Changes in slice thickness were mapped from the amount of light passing through a thin ( approximately 100 microm) layer of perfusate bathing the slice, in which hemoglobin (6 mg/ml) was added to the perfusate as an inert absorbing chromophore. In response to osmotic challenges imposed by changing perfusate osmolality by 100 mOsm, transmitted light intensity changed reversibly with approximately mono-exponential kinetics whose initial rate depended upon position in the slice. In the superficial dorsal horn where AQP1 is strongly expressed, the rate of osmotic swelling was 7.0 +/- 1.3 microm/s in wildtype mice and 2.0 +/- 0.2 microm/s in AQP1 null mice; osmotic swelling was slower in deeper lamina of dorsal horn, and was decreased in AQP4 but not AQP1 null mice. These results establish a simple imaging method to map changes in water content of spinal cord slices, and provide evidence that aquaporins facilitate osmotic water transport in functionally relevant areas of the spinal cord.

Extracellular ATP reduces optically monitored electrical signals in hippocampal slices through metabolism to adenosine

Electrical signals in rat hippocampal slices were optically monitored using a voltage-sensitive dye to determine whether extracellular ATP exhibits direct effects through its own receptors or indirect effects after its hydrolysis to adenosine. The dentate gyrus was stimulated and electrical signals in the CA1 and the CA3 region were analyzed. The signals were divided into two components: a transient component peaking within 10 ms (fast component) and a subsequent sustained component (slow component). ATP (10 to 100 microM) inhibited both the fast and the slow components in the CA1 region by about 30% and 70%, respectively. ADP, AMP and adenosine also inhibited the fast and the slow components. The inhibition by ATP was antagonized by aminophylline and other adenosine receptor antagonists, and by alpha,beta-methylene ADP, an inhibitor of 5'-nucleotidases. These results suggest that extracellular ATP inhibits neuronal electrical signals in hippocampal slices after its metabolism to adenosine.

mu-Opioid receptors often colocalize with the substance P receptor (NK1) in the trigeminal dorsal horn

Substance P (SP) is a peptide that is present in unmyelinated primary afferents to the dorsal horn and is released in response to painful or noxious stimuli. Opiates active at the mu-opiate receptor (MOR) produce antinociception, in part, through modulation of responses to SP. MOR ligands may either inhibit the release of SP or reduce the excitatory responses of second-order neurons to SP. We examined potential functional sites for interactions between SP and MOR with dual electron microscopic immmunocytochemical localization of the SP receptor (NK1) and MOR in rat trigeminal dorsal horn. We also examined the relationship between SP-containing profiles and NK1-bearing profiles. We found that 56% of SP-immunoreactive terminals contact NK1 dendrites, whereas 34% of NK1-immunoreactive dendrites receive SP afferents. This result indicates that there is not a significant mismatch between sites of SP release and available NK1 receptors, although receptive neurons may contain receptors at sites distant from the peptide release site. With regard to opioid receptors, we found that many MOR-immunoreactive dendrites also contain NK1 (32%), whereas a smaller proportion of NK1-immunoreactive dendrites contain MOR (17%). Few NK1 dendrites (2%) were contacted by MOR-immunoreactive afferents. These results provide the first direct evidence that MORs are on the same neurons as NK1 receptors, suggesting that MOR ligands directly modulate SP-induced nociceptive responses primarily at postsynaptic sites, rather than through inhibition of SP release from primary afferents. This colocalization of NK1 and MORs has significant implications for the development of pain therapies targeted at these nociceptive neurons.

Functional circuitry for the induction of prolonged excitation in the rat spinal dorsal horn

The neuronal circuitry through which prolonged excitation is generated in the spinal dorsal horn was investigated using optical imaging of neuronal excitation in transverse slices of rat spinal cords. It is known that tetanic stimulation (20 Hz for 1 s) of the dorsal root that activates both A and C primary afferent fibres elicits slow intrinsic optical signals (IOS) in the dorsal horn, seen most intensely in the substantia gelatinosa (SG), lamina II, and that IOS expresses in part the slow synaptic response recorded intracellularly in dorsal horn neurons. We here report that the slow IOS within the SG were completely abolished after an incision was made at the border between the SG and the deeper laminae, but not after an incision within the deeper dorsal horn of the laminae III-V. The result demonstrates directly that, in order to generate prolonged excitation in the SG, the neuronal elements in the deeper dorsal horn must be intact. Thus, the afferent information might be received first by the deeper elements and then transmitted to the SG region, and/or collaboration between the SG and deeper elements is necessary to maintain prolonged excitation in the SG.

Optical responses evoked by single-pulse stimulation to the dorsal root in the rat spinal dorsal horn in slice

Neuronal excitation evoked after dorsal-root (DR) stimulation in the spinal dorsal horn (DH) of rats was visualized with a high-resolution optical-imaging method, and the propagation mechanism was studied. Transverse slices of the spinal cord were obtained from 2-4 week-old rats and stained with the voltage-sensitive dye RH-482. Single-pulse stimulation to the primary-afferent A fibers in the DR attached to the slice evoked a weak, brief (<10 ms) excitatory optical response in the laminae I and III-V. When the stimulus intensity and duration were increased to activate both A and C fibers, an additional, much greater, and longer-lasting (>100 ms) excitatory response was generated in the laminae I-III, most intensely in the lamina II. A treatment with excitatory amino acid (EAA) antagonists, dl-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2, 3-dione, significantly reduced the amplitude and duration of the response in the lamina II. The optical response in the antagonists-containing solution was quite similar to that recorded in a Ca2+-free solution that blocked afferent synaptic transmission. The late component (>10 ms) was, however, slightly greater than that in the Ca2+-free solution. Treatment with the ATP-receptor antagonist, suramin, had a minimal effect on the response in the presence of EAA antagonists. These results suggested that the propagation of the DR-stimulus-elicited excitation was contributed largely by EAA receptors, but also by other receptors to a much lesser extent.