An integrated spinal cord–hindlimbs preparation for studying the role of intrinsic properties in somatosensory information processing

Neuronal intrinsic properties shape naturally evoked sensory inputs in the dorsal horn of the spinal cord

Intrinsic electrophysiological properties arising from specific combinations of voltage-gated channels are fundamental for the performance of small neural networks in invertebrates, but their role in large-scale vertebrate circuits remains controversial. Although spinal neurons have complex intrinsic properties, some tasks produce high-conductance states that override intrinsic conductances, minimizing their contribution to network function. Because the detection and coding of somato-sensory information at early stages probably involves a relatively small number of neurons, we speculated that intrinsic electrophysiological properties are likely involved in the processing of sensory inputs by dorsal horn neurons (DHN). To test this idea, we took advantage of an integrated spinal cord–hindlimbs preparation from turtles allowing the combination of patch-clamp recordings of DHN embedded in an intact network, with accurate control of the extracellular milieu. We found that plateau potentials and low threshold spikes (LTS) -mediated by L- and T-type Ca²⁺channels, respectively- generated complex dynamics by interacting with naturally evoked synaptic potentials. Inhibitory receptive fields could be changed in sign by activation of the LTS. On the other hand, the plateau potential transformed sensory signals in the time domain by generating persistent activity triggered on and off by brief sensory inputs and windup of the response to repetitive sensory stimulation. Our findings suggest that intrinsic properties dynamically shape sensory inputs and thus represent a major building block for sensory processing by DHN. Intrinsic conductances in DHN appear to provide a mechanism for plastic phenomena such as dynamic receptive fields and sensitization to pain.

GABAergic signalling in a neurogenic niche of the turtle spinal cord

The region that surrounds the central canal (CC) in the turtle spinal cord is a neurogenic niche immersed within already functional circuits, where radial glia expressing brain lipid binding protein (BLBP) behave as progenitors. The behaviour of both progenitors and neuroblasts within adult neurogenic niches must be regulated to maintain the functional stability of the host circuit. In the brain, GABA plays a major role in this kind of regulation but little is known about GABAergic signalling in neurogenic niches of the postnatal spinal cord. Here we explored the action of GABA around the CC of the turtle spinal cord by combining patch-clamp recordings of CC-contacting cells, immunohistochemistry for key components of GABAergic signalling and Ca(2+) imaging. Two potential sources of GABA appeared around the CC: GABAergic terminals and CC-contacting neurones. GABA depolarized BLBP(+) progenitors via GABA transporter-3 (GAT3) and/or GABA(A) receptors. In CC-contacting neurones, GABA(A) receptor activation generated responses ranging from excitation to inhibition. This functional heterogeneity appeared to originate from different ratios of activity of the Na(+)-K(+)-2Cl(-) co-transporter (NKCC1) and the K(+)-Cl(-) co-transporter (KCC2). In both progenitors and immature neurones, GABA induced an increase in intracellular Ca(2+) that required extracellular Ca(2+) and was blocked by the selective GABA(A) receptor antagonist gabazine. Our study shows that GABAergic signalling around the CC shares fundamental properties with those in the embryo and adult neurogenic niches, suggesting that GABA may be part of the mechanisms regulating the production and integration of neurones within operational spinal circuits in the turtle.

Intrinsic membrane properties of spinal dorsal horn neurones modulate nociceptive information processing in vivo

The dorsal horn of the spinal cord is the first central relay where nociceptive inputs are processed. Based on the expression and modulation of intrinsic electrophysiological properties in in vitro slice preparations, dorsal horn neurones (DHNs) display different discharge patterns (tonic, plateau or rhythmic), which shape the neurone's response to sensory inputs. However, it is unclear whether intrinsic properties play any role in sensory processing in vivo. Using in vivo patch clamp recordings in the adult rat, we here examine whether these intrinsic properties are present, and to what extent they determine the DHN response to natural stimulation. We focused primarily on wide dynamic range neurones in deep laminae. These cells displayed a multicomponent peripheral receptive field, comprising an excitatory firing zone, a low-probability firing fringe, and adjacent inhibitory zones. Deep DHNs presented similar intrinsic properties to those observed in vitro, including plateau potentials. These plateaus, underlying high frequency accelerating discharges and after-discharges, were triggered by mechanical stimulation of the excitatory receptive field. Persistent activities induced by activation of plateau potentials were interrupted by stimulation of peripheral inhibitory zones. Moreover, we show that plateau activation is necessary for the expression of windup in response to repetitive, nociceptive stimulation. Finally, using the spinal nerve ligation model of neuropathy, we demonstrate a significant increase in the proportion of plateau neurones in deep dorsal laminae. Our data, therefore, establish that intrinsic amplification properties are expressed within intact spinal circuits and suggest their involvement in neuropathy-induced hyperexcitability of deep DHNs.

Analysis of quantal size of voltage responses to retinal stimulation in the accessory optic system

In the intact vertebrate central nervous system, the quantal nature of synaptic transmission is difficult to measure because the postsynaptic sites may be distributed along a tortuous dendritic tree that cannot be readily clamped spatially to a uniform potential. Titrating the intact brain's extracellular concentration of calcium ions is also challenging because of its strong buffering mechanisms. In this study, using a whole brain with eye attached preparation, quantal neurotransmission was examined in the turtle brainstem in vitro, by recording from accessory optic system neurons that receive direct input from visually responsive retinal ganglion cells. Unitary EPSPs, evoked by microstimulation of a single ganglion cell, were measured during whole cell current-clamp recordings. In this preparation, the neurons exhibit direction-selectivity, despite the hypoxic conditions. Bath application of cadmium to reduce calcium influx also reduced evoked EPSP amplitudes to that of the spontaneous synaptic events. Statistical analyses indicated that these evoked response amplitudes could be well fitted to a Poisson distribution for most brainstem neurons. Therefore, the spontaneous miniature excitatory synaptic events of approximately 1 mV, as also observed during spike blockade of the retina [Kogo, N., Ariel, M., 1997. Membrane properties and monosynaptic retinal excitation of neurons in the turtle accessory optic system. Journal of Neurophysiology 78, 614-627], are likely responses to the neurotransmitter of single vesicles release by retinal axon terminals.

Immunohistochemical and hodological characterization of calbindin-D28k-containing neurons in the spinal cord of the turtle,Pseudemys scripta elegans

Neurons and fibers containing the calcium-binding protein calbindin-D28k (CB) were studied by immunohistochemical techniques in the spinal cord of adult and juvenile turtles, Pseudemys scripta elegans. Abundant cell bodies and fibers immunoreactive for CB were widely and distinctly distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. In the dorsal horn, most CB-containing cells were located in close relation to the synaptic fields formed by primary afferents, which were not labeled for CB. Double immunohistofluorescence demonstrated distinct cell populations in the dorsal horn labeled only for CB or nitric oxide synthase, whereas in the dorsal part of the ventral horn colocalization of nitric oxide synthase was found in about 6% of the CB-immunoreactive cells in this region. Choline acetyltransferase immunohistochemistry revealed that only about 2% of the neurons in the dorsal part of the ventral horn colocalized CB, whereas motoneurons were not CB-immunoreactive. The involvement of CB-containing neurons in ascending spinal projections to the thalamus, tegmentum, and reticular formation was demonstrated combining the retrograde transport of dextran amines and immunohistochemistry. Similar experiments demonstrated supraspinal projections from CB-containing cells mainly located in the reticular formation but also in the thalamus and the vestibular nucleus. The revealed organization of the neurons and fibers containing CB in the spinal cord of the turtle shares distribution and developmental features, colocalization with other neuronal markers, and connectivity with other tetrapods and, in particular with mammals.

Cytological organization of the central gelatinosa in the turtle spinal cord

This paper deals with the cytological organization of the central gelatinosa (CG) in the spinal cord of juvenile (2-12 months) turtles. We found two main cell classes in the CG: one with characteristics of immature neurons, the other identified as radial glia (RG). The cells surrounding the central canal formed radial conglomerates in such a way that the RG lamellae covered the immature neurons. We found three major subpopulations of RG that expressed S-100, glial fibrillary acidic protein, or both proteins. Electron microscopic images showed gap junctions interconnecting RG. As with the mammalian neuroepithelial cells, most CG cells displayed intrinsic polarity expressed by structural and molecular differences between the most apical and basal cell compartments. The apical zone was characterized by the occurrence of a single cilium associated with a conspicuous centrosomal complex. We found a prominent expression of the PCM-1 centrosomal protein concentrated close to the central canal lumen. In the particular case of RG, the peripheral end feet contacted the subpial basement membrane. We also found "transitional cell forms" difficult to classify by the usual imaging approaches. Functional clues obtained by patch-clamp recordings of CG cells defined some of them as already committed to follow the neuronal lineage, whereas others had properties of less mature or migrating cells. The CG appeared as a richly innervated region receiving terminal branches from nerve plexuses expressing gamma-aminobutyric acid, serotonin, and glutamate. The results presented here support our previous studies indicating that the CG is an extended neurogenic niche along the spinal cord of turtles.

Switching-on and -off of bistable spontaneous discharges in rat spinal deep dorsal horn neurons

Somatosensory deep dorsal horn spinal neurons were previously shown to present in vitro a bistable state of activity in which a fixed firing rate is maintained over prolonged periods in the absence of stimulation. Those periods of enhanced spinal spontaneous discharge may play a role in the genesis or maintenance of hyperalgesic states, where episodes of durable spontaneous pain are commonly reported. Here we show in vivo that a small percentage of deep spinal neurons (4% of the recorded population) are capable of rapidly shifting between low-frequency and high-frequency levels of spontaneous activity. At least one of the transitions between the two states was induced by stimulation of the receptive field, making this an interesting and unique case in which stable firing rates are switched-on or -off by somatosensory stimuli.

Modulation of visual inputs to accessory optic system by theophylline during hypoxia

Neural tissues from fresh water turtles have been electrophysiologically studied in vitro due to their significant resistance to hypoxia. Such neurons have resting membrane potentials that are similar to intact animals and receive similar synaptic inputs evoked by sensory stimuli. One mechanism to reduce the brain's metabolic requirement in the absence of oxygenated blood flow was investigated by blocking adenosine receptors before and during hypoxia. Extracellular and whole-cell patch recordings were made from the basal optic nucleus, whose neurons respond to visual stimuli in vitro. While the addition of the adenosine antagonist theophylline to oxygenated superfusate had minimal effect on the neural activity, theophylline in superfusate bubbled with nitrogen strongly increased activity compared to either oxygenated theophylline or control superfusate bubbled with nitrogen. The increase in spontaneous activity was due to increases to both amplitude and frequency of excitatory synaptic events. Even during these increases, the neurons continued to exhibit their direction-sensitive responses. These results indicate that adenosine may play a role in protecting the viability of the brainstem during hypoxia without reducing visually mediated brainstem reflex control.