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 th ought to quickly terminate the cortical cell response to whisker deflection . Using dual electrode extracellular recording in awake rats, we describe a n infrequent type of cell response in which stimulation of single hairs con sistently 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 cor tex. I-only inhibition is most commonly observed at low stimulus frequencie s (similar to 1 Hz). At stimulus frequencies of >6 Hz, I-only inhibition ty pically converts to excitation. We conclude that single whisker low-frequen cy stimulation can selectively block the spontaneous discharge of neurons i n SI barrel field septa. The observation that this cell response is found m ost often in or at the edge of septa and at relatively long latencies suppo rts the idea that I-only inhibition is mediated through cortical circuits. We propose that in these cells inhibition alone or a combination of inhibit ion and disfacilitation play a role in suppressing neuronal discharge occas ioned by low frequency contact of the whiskers with the environment.