Dynamics of synaptic transmission in the CNS: Contribution of neuron-glia interactions

The last 30 years have seen a growing appreciation of the importance for CNS functioning of the internal state of neural tissues, which is exquisitely reflective of the immediate-to-long-term history of the preceding neural activity experienced by those tissues. This dissertation comprises three research projects that together address different, but related aspects of dynamics of the state of neural tissues, with a focus on the roles played by astroglia and GABAergic synaptic transmission.;Project 1: Origins of Optical Intrinsic Signal and its significance. 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 fluoroacetate and glutamine (FAc+Gln), or interference with glial and neuronal K+ transport by 4-aminopyridine (4-AP) lead to dissociation of the OISDR and the postsynaptic DHs response to a single-pulse 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. These observations indicate that the OISDR mainly reflects cell volume and light scattering changes associated with DHs astrocyte uptake of K+ and glutamate (GLU).;Project 2: Effects of alteration of glia on neuronal plasticity. Transient (20min) exposure of the spinal cord slice to fluorocitrate (FC; a reversible inhibitor of glial energy production via the TCA cycle) is shown to be accompanied by a protracted decrease of the superficial dorsal horn (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 to single-pulse stimulation of the attached dorsal root (LTPFC). The observations reveal that transient exposure of the slice 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 (average EGABApreFC : -75 mV; EGABApostFC : -50 mV). Considered collectively, the findings demonstrate that LTPFC involves (1) elevation of [K+]o in the DHs, (2) NMDA receptor 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 via the TCA cycle can trigger the cascade of dorsal horn mechanisms that underlies hyperalgesia and persistent pain.;Project 3: Contribution of GABA to cerebral cortical dynamics. Imaging of the optical intrinsic signal (OIS), evoked in the rat sensorimotor cortical slice by 1s-long 20Hz electrical stimulation applied to locus at the layer VI/white matter junction, was used to delineate a column-shaped cortical region responding to a local thalamocortical input drive, and whole-cell patch clamp recordings were obtained from layer II-III pyramidal neurons residing in that region. Puffs of pressure-ejected GABA were released from a micropipette in a close vicinity of the recorded neuron's soma before and also immediately after "conditioning" electrical stimulation. Prior to conditioning stimulation, GABA puffs hyperpolarized the recorded neurons, whereas for ∼15s subsequent to conditioning stimulation GABA puffs depolarized the same neurons. Finally, OIS imaging in the presence of GABA antagonist bicuculline suggests that the depolarizing action of GABA is confined to the center of the stimulated cortical region, while at its margins GABA remains hyperpolarizing. Taken together, these findings suggest that synaptically released GABA can be either inhibitory or excitatory, depending on the activity state of the local network. (Abstract shortened by UMI.)...