In vivo patch-clamp analysis of synaptic responses in rat primary somatosensory barrel cortex (SI)

Rat primary somatosensory (barrel) cortex (SI) contains a detailed representation of the whiskers on the face. The somatosensory information is transferred to the SI from whiskers via the ventroposteromedial nucleus (VPM) of the thalamus. To investigate detailed synaptic activities of cortical neurons receiving input from the thalamus, we have applied the in vivo whole-cell patch clamp recording technique to rat SI. Male Sprague–Dawley rats were anesthetized with urethane (1.2 g/kg, i.p.). Recording pipettes were filled with a standard solution containing 0.2% neurobiotin. Rats were then perfused for the morphological studies. Most recorded neurons exhibited spontaneous excitatory postsynaptic currents (EPSCs) at a holding potential (Vh) of −70 mV and spontaneous IPSCs (Vh=0 mV). Deflection of contralateral whiskers elicited increased EPSCs in both frequency and amplitude. Some neurons exhibited membrane current fluctuation (oscillation). Application of CNQX (100 μM) to the surface of the cortex abolished the oscillatory current, indicating that the oscillation consisted of the summated EPSCs. Whisker stimulation distorted the regularity of the oscillation indicating that oscillation could be related to neural processing in a silent period of sensory input. A single pulse delivered to the VPM via bipolar electrode induced an EPSC with short latency (2 ms), as well as EPSCs with long latency (120 ms) in the cortical neurons. This stimulation also suppressed the oscillations suggesting that thalamic activities could regulate oscillation.