Parvalbumin-expressing interneurons in cortical networks are coupled by gap-junctions, forming a syncytium that supports propagating epileptiform discharges, induced by 4-aminopyridine. It remains unclear, however, whether these propagating events occur under more natural states, without pharmacological blockade. In particular, we investigated whether propagation also happens when extracellular K+ rises, as is known to occur following intense network activity, such as during seizures. We examined how increasing [K+]o affects the likelihood of propagating activity away from a site of focal (200-400µm) optogenetic activation of PV-interneurons. Activity was recorded using a linear 16-electrode array placed along layer V of primary visual cortex. At baseline levels of [K+]o (3.5mM), induced activity was recorded only within the illuminated area. However, when [K+]o was increased above a threshold level (50th percentile= 8.0mM; interquartile range= 7.5-9.5mM), time-locked, fast-spiking unit activity, indicative of parvalbumin-expressing interneuron firing, was also recorded outside the illuminated area, propagating at 59.1 mm/s. Blockade of glutamatergic synaptic transmission reduced the efficacy of propagation, but could be restored by further increasing [K+]o. Propagation was further reduced, and in most cases prevented altogether, by pharmacological blockade of gap-junctions, achieved by any of three different drugs, quinine, mefloquine or carbenoxolone. Wash-out of quinine rapidly re-established the pattern of propagating activity. Computer simulations show qualitative differences between propagating discharges in high [K+]o and 4-aminopyridine, arising from differences in the electrotonic effects of these two manipulations. These interneuronal syncytial interactions are likely to affect the complex electrographic dynamics of seizures, once [K+]o is raised above this threshold level.Significance statementWe demonstrate the spatially extended propagation of activity through a gap-junction mediated syncytium of parvalbumin-expressing interneurons, in conditions that are known to exist at times within the brain. Previous work has only shown gap-junction coordination very locally, through directly connected cells, or induced at a distance by pharmacological means. We show that cell-class specific spread is facilitated by raised extracellular K+. This is highly pertinent to what happens at the onset of, and during, seizures, when extracellular K+ can rise rapidly to levels well in excess of the measured threshold for propagation. Our data suggests that interneuronal coupling will be enhanced at this time, and this has clear implications for the behaviour of these cells as seizures progress.
