Fast-spiking, parvalbumin + GABAergic interneurons: From cellular design to microcircuit function

Background Neuronal networks in the brain include glutamatergic principal neurons and GABAergic interneurons (GABA, γ-aminobutyric acid). The latter may be a minority cell type, but they are vital for normal brain function because they regulate the activity of principal neurons. If interneuron function is impaired, higher brain function can be damaged and seizures may result. The fast-spiking, parvalbumin-positive interneurons (PV + interneurons) are readily characterized and, consequently, have been adopted as a research model for systematic and quantitative investigations. These cells contribute to feedback and feedforward inhibition and are critically involved in the generation of network oscillations. They can convert an excitatory input signal into an inhibitory output signal within a millisecond, but it is unclear how these signaling properties are implemented at the molecular and cellular levels, nor how PV + interneurons shape complex network functions. Advances Recent work sheds light on the subcellular signaling properties of PV + interneurons. PV + cells show a high degree of polarity. The weakly excitable dendrites allow PV + interneurons to sample activity in the surrounding network, whereas the highly excitable axons enable analog-to-digital conversion and fast propagation of the digital signal to a large number of target cells. Additionally, tight coupling of Ca 2+ channels and release sensors at GABAergic output synapses increases the efficacy and speed of the inhibitory output. Recent results also provide a better understanding of how PV + interneurons operate in neuronal networks. Not only are PV + interneurons involved in basic microcircuit functions, such as feedforward and feedback inhibition or gamma-frequency oscillations, but they also play a role in complex network operations, including expansion of dynamic activity range, pattern separation, modulation of place and grid field shapes, phase precession, and gain modulation of sensory responses. Thus, PV + interneurons are critically involved in advanced computations in microcircuits and neuronal networks. Outlook Parvalbumin-expressing interneurons may also play a key role in numerous brain diseases. These include epilepsy, but also complex psychiatric diseases such as schizophrenia. Thus, PV + interneurons may become important therapeutic targets in the future. However, much needs to be learned about the basic function of these interneurons before clinical neuroscientists will have a chance to successfully use PV + interneurons for therapeutic purposes.