Abstract
Widely used for mapping afferent activated brain areas in vivo, the label-free intrinsic optical signal (IOS) is mainly ascribed to blood volume changes subsequent to glial glutamate uptake. By contrast, IOS imaged in vitro is generally attributed to neuronal and glial cell swelling, however the relative contribution of different cell types and molecular players remained largely unknown. We characterized IOS to Schaffer collateral stimulation in the rat hippocampal slice using a 464-element photodiode-array device that enables IOS monitoring at 0.6 ms time-resolution in combination with simultaneous field potential recordings. We used brief half-maximal stimuli by applying a medium intensity 50 Volt-stimulus train within 50 ms (20 Hz). IOS was primarily observed in the str. pyramidale and proximal region of the str. radiatum of the hippocampus. It was eliminated by tetrodotoxin blockade of voltage-gated Na+ channels and was significantly enhanced by suppressing inhibitory signaling with gamma-aminobutyric acid(A) receptor antagonist picrotoxin. We found that IOS was predominantly initiated by postsynaptic Glu receptor activation and progressed by the activation of astroglial Glu transporters and Mg2+-independent astroglial N-methyl-D-aspartate receptors. Under control conditions, role for neuronal K+/Cl− cotransporter KCC2, but not for glial Na+/K+/Cl− cotransporter NKCC1 was observed. Slight enhancement and inhibition of IOS through non-specific Cl− and volume-regulated anion channels, respectively, were also depicted. High-frequency IOS imaging, evoked by brief afferent stimulation in brain slices provide a new paradigm for studying mechanisms underlying IOS genesis. Major players disclosed this way imply that spatiotemporal IOS reflects glutamatergic neuronal activation and astroglial response, as observed within the hippocampus. Our model may help to better interpret in vivo IOS and support diagnosis in the future.
Highlights
Reflecting genuine excitability of neural cells, the label-free intrinsic optical signal (IOS) [1,2] can essentially be detected at cellular resolution both in vivo and in vitro [3,4,5,6,7]
Amplitude of the IOS signal in the close vicinity of the electrophysiological recording site as well as summa amplitude measured on all diodes covering the entire hippocampus were compared to electrophysiological signal parameters, namely the amplitude of the population spike (PS) and the slope of the field excitatory postsynaptic potential
The amplitude of the population spike measures the synchronous firing of the neighboring neurons [33], while the slope of the field excitatory postsynaptic potential (fEPSP) measures the currents generated by the synaptic activation of pyramidal cells [34]
Summary
Reflecting genuine excitability of neural cells, the label-free intrinsic optical signal (IOS) [1,2] can essentially be detected at cellular resolution both in vivo and in vitro [3,4,5,6,7]. Despite the diagnostic power of IOS imaging technique, detailed understanding of molecular and cellular processes underlying the generation of afferent evoked spatiotemporal IOS is still lacking [11], inviting to study the relationship between synaptic activity and the IOS signal. Local changes in light scattering due to activity-dependent cell swelling [1,16] and alterations of the extracellular volume [17,18] are regarded as the principal component of the osmotic pressure induced and afferent stimulation evoked IOS [1,19]
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