Direct inhibition evoked by whisker stimulation in somatic sensory (SI) barrel field cortex of the awake rat

Whisker deflection typically evokes a transient volley of action potentials in rat somatic sensory (SI) barrel cortex. Postexcitatory inhibition is thought to quickly terminate the cortical cell response to whisker deflection. Using dual electrode extracellular recording in awake rats, we describe an infrequent type of cell response in which stimulation of single hairs consistently blocks the ongoing discharge of neurons without prior excitation (I-only inhibition). Reconstruction of the recording sites indicates that I-only inhibition occurs most frequently when the recording site is clearly in the septum or at the barrel-septum junction. The same cells that respond with I-only inhibition to one whisker can show an excitatory discharge to other whiskers, usually followed by inhibition. Stimulation of either nose hairs or the large mystacial vibrissa can evoke I-only inhibition in SI cortex. I-only inhibition is most commonly observed at low stimulus frequencies (similar to 1 Hz). At stimulus frequencies of >6 Hz, I-only inhibition typically converts to excitation. We conclude that single whisker low-frequency stimulation can selectively block the spontaneous discharge of neurons in SI barrel field septa. The observation that this cell response is found most often in or at the edge of septa and at relatively long latencies supports the idea that I-only inhibition is mediated through cortical circuits. We propose that in these cells inhibition alone or a combination of inhibition and disfacilitation play a role in suppressing neuronal discharge occasioned by low frequency contact of the whiskers with the environment.