Upregulation of Barrel GABAergic Neurons Is Associated with Cross-Modal Plasticity in Olfactory Deficit

Background: Loss of a sensory function is often followed by the hypersensitivity of other modalities in mammals, which secures them well-awareness to environmental changes. Cellular and molecular mechanisms underlying cross-modal sensory plasticity remain to be documented. Methodology/Principal Findings: Multidisciplinary approaches, such as electrophysiology, behavioral task and immunohistochemistry, were used to examine the involvement of specific types of neurons in cross-modal plasticity. We have established a mouse model that olfactory deficit leads to a whisking upregulation, and studied how GABAergic neurons are involved in this cross-modal plasticity. In the meantime of inducing whisker tactile hypersensitivity, the olfactory injury recruits more GABAergic neurons and their fine processes in the barrel cortex, as well as upregulates their capacity of encoding action potentials. The hyperpolarization driven by inhibitory inputs strengthens the encoding ability of their target cells. Conclusion/Significance: The upregulation of GABAergic neurons and the functional enhancement of neuronal networks may play an important role in cross-modal sensory plasticity. This finding provides the clues for developing therapeuti

Upregulation of excitatory neurons and downregulation of inhibitory neurons in barrel cortex are associated with loss of whisker inputs

Abstract Loss of a sensory input causes the hypersensitivity in other modalities. In addition to cross-modal plasticity, the sensory cortices without receiving inputs undergo the plastic changes. It is not clear how the different types of neurons and synapses in the sensory cortex coordinately change after input deficits in order to prevent loss of their functions and to be used for other modalities. We studied this subject in the barrel cortices from whiskers-trimmed mice vs. controls. After whisker trimming for a week, the intrinsic properties of pyramidal neurons and the transmission of excitatory synapses were upregulated in the barrel cortex, but inhibitory neurons and GABAergic synapses were downregulated. The morphological analyses indicated that the number of processes and spines in pyramidal neurons increased, whereas the processes of GABAergic neurons decreased in the barrel cortex. The upregulation of excitatory neurons and the downregulation of inhibitory neurons boost the activity of network neurons in the barrel cortex to be high levels, which prevent the loss of their functions and enhances their sensitivity to sensory inputs. These changes may prepare for attracting the innervations from sensory cortices and/or peripheral nerves for other modalities during cross-modal plasticity.</p

Extracellular and intracellular acidosis impairs the production of action potentials at the cortical GABAergic neurons.

<p>Sequential spikes of GABAergic neurons in cortical slices were evoked by injecting depolarization pulse (200 ms) and recorded by using whole-cell current-clamp. Extracellular acidosis was made by perfusing the cortical slices with the acidic ACSF (pH 6.75) after the control ACSF (pH 7.35). Intracellular acidosis was made by using the recording pipettes whose tips were filled with control pipette solution (pH 7.35) and back-filled with acidification pipette solution (pH 6.75). <b>A)</b> shows the evoked spikes under the control (red trace) and subsequent intracellular acidification (blue trace), respectively. <b>B)</b> shows the values of spike frequencies under the conditions of control (pH 7.35, red bar) and intracellular acidification (pH 6.75; blue bar). Two asterisks show p<0.01 (n = 15, paired t-test). <b>C)</b> shows the spikes under the control (red trace) and extracellular acidification (dark blue), respectively. <b>D)</b> shows the values of spike frequencies under the conditions of control (pH 7.35, red bar) and extracellular acidification (pH 6.75; dark-blue). Two asterisks show p<0.01 (n = 15, paired t-test). Dash-lines in <b>A)</b> & <b>C)</b> show the levels of threshold potentials.</p

Acidosis-Induced Dysfunction of Cortical GABAergic Neurons through Astrocyte-Related Excitotoxicity

<div><p>Background</p><p>Acidosis impairs cognitions and behaviors presumably by acidification-induced changes in neuronal metabolism. Cortical GABAergic neurons are vulnerable to pathological factors and their injury leads to brain dysfunction. How acidosis induces GABAergic neuron injury remains elusive. As the glia cells and neurons interact each other, we intend to examine the role of the astrocytes in acidosis-induced GABAergic neuron injury.</p><p>Results</p><p>Experiments were done at GABAergic cells and astrocytes in mouse cortical slices. To identify astrocytic involvement in acidosis-induced impairment, we induced the acidification in single GABAergic neuron by infusing proton intracellularly or in both neurons and astrocytes by using proton extracellularly. Compared the effects of intracellular acidification and extracellular acidification on GABAergic neurons, we found that their active intrinsic properties and synaptic outputs appeared more severely impaired in extracellular acidosis than intracellular acidosis. Meanwhile, extracellular acidosis deteriorated glutamate transporter currents on the astrocytes and upregulated excitatory synaptic transmission on the GABAergic neurons. Moreover, the antagonists of glutamate NMDA-/AMPA-receptors partially reverse extracellular acidosis-induced injury in the GABAergic neurons.</p><p>Conclusion</p><p>Our studies suggest that acidosis leads to the dysfunction of cortical GABAergic neurons by astrocyte-mediated excitotoxicity, in addition to their metabolic changes as indicated previously.</p></div

Extracellular and intracellular acidosis prolongs the refractory periods of action potentials at the cortical GABAergic neurons.

<p>Refractory periods were measured by injecting paired-depolarization pulses and recorded under whole-cell current-clamp. <b>A)</b> shows the measurements of refractory periods under the control (red trace) and subsequent intracellular acidification (blue), respectively. <b>B)</b> shows the averaged values of spike refractory periods under the conditions of control (pH 7.35, red bar) and intracellular acidification (pH 6.75; blue). Two asterisks show p<0.01 (n = 15, paired t-test). <b>C)</b> shows the measurement refractory periods under the control (red trace) and extracellular acidification (dark blue), respectively. <b>D)</b> shows the values of refractory periods under the conditions of control (pH 7.35, red bar) and extracellular acidification (pH 6.75; dark-blue). Two asterisks show p<0.01 (n = 15, paired t-test).</p

The dysfunction of glutamate transporter in the astrocyte leads to the impairment of GABAergic neuron during acidosis.

<p>Extracellular acidification impairs the function of astrocytic glutamate transporter (Glu-T), and the subsequent glutamate accumulation deteriorates GABAergic neurons through activating ionotropic glutamate receptors, such as NMDAR and AMPAR.</p

Extracellular and intracellular acidosis impairs the production of action potentials at the cortical GABAergic neurons.

<p>Sequential spikes of GABAergic neurons in cortical slices were evoked by injecting depolarization pulse (200 ms) and recorded by using whole-cell current-clamp. Extracellular acidosis was made by perfusing the cortical slices with the acidic ACSF (pH 6.75) after the control ACSF (pH 7.35). Intracellular acidosis was made by using the recording pipettes whose tips were filled with control pipette solution (pH 7.35) and back-filled with acidification pipette solution (pH 6.75). <b>A)</b> shows the evoked spikes under the control (red trace) and subsequent intracellular acidification (blue trace), respectively. <b>B)</b> shows the values of spike frequencies under the conditions of control (pH 7.35, red bar) and intracellular acidification (pH 6.75; blue bar). Two asterisks show p<0.01 (n = 15, paired t-test). <b>C)</b> shows the spikes under the control (red trace) and extracellular acidification (dark blue), respectively. <b>D)</b> shows the values of spike frequencies under the conditions of control (pH 7.35, red bar) and extracellular acidification (pH 6.75; dark-blue). Two asterisks show p<0.01 (n = 15, paired t-test). Dash-lines in <b>A)</b> & <b>C)</b> show the levels of threshold potentials.</p

The inhibition of glutamate receptors partially reverses the impairment of intrinsic properties at cortical GABAergic neurons induced by extracellular acidosis.

<p>Sequential spikes, threshold potentials and refractory period at GABAergic neurons were recorded by whole-cell voltage-clamp under the conditions of sequential manipulations, i.e., control, extracellular acidosis and extracellular acidosis plus 40 μM D-AP5 and 10 μM CNQX. <b>A)</b> shows the averaged values of spike frequency under the conditions of control (red bar), extracellular acidosis (blue bar) and extracellular acidosis plus glutamate receptor blockers (green bar). <b>B)</b> shows the averaged values of spike refractory periods under the conditions of control (red bar), extracellular acidosis (blue bar) and extracellular acidosis plus glutamate receptor blockers (green bar). <b>C)</b> shows the averaged values of spike threshold potentials under the conditions of control (red bar), extracellular acidosis (blue bar) and extracellular acidosis plus glutamate receptor blockers (green bar). Two asterisks present p<0.01, such as blue and green bars versus red bar. An asterisk presents p<0.05, such as green bar versus red bar. # presents p<0.05, such as green bar versus blue bar (n = 15; one-way ANOVA).</p

Extracellular acidosis upregulates glutamatergic synaptic transmission at cortical GABAergic neurons dominantly.

<p>Spontaneous EPSCs were recorded on GABAergic neurons by whole-cell voltage-clamp without stimulating presynaptic axons. <b>A)</b> shows the recorded sEPSCs under the control (top trace), intracellular acidification (middle trace) and extracellular acidification (bottom trace). <b>B)</b> illustrates the differences of sEPSC amplitudes between intracellular acidosis and control (∆EPSC amplitudes, red bar) as well as the differences between extracellular acidosis and control (∆EPSC amplitudes, blue bar; p<0.01, n = 15; one-way ANOVA). <b>C)</b> shows the differences of inter-sEPSC interval between intracellular acidosis and control (∆inter-IPSC interval, red bar) as well as the differences between extracellular acidosis and control (blue bar; p<0.01, n = 15; one-way ANOVA).</p