A clinical hallmark of human alphaherpesvirus
infections is peripheral
pain or
itching. Pseudorabies virus (PRV), a broad host range alphaherpesvirus, causes violent
pruritus in many different animals, but the mechanism is unknown. Previous in vitro studies have shown that infected, cultured peripheral nervous system (PNS) neurons exhibited aberrant electrical activity after PRV
infection due to the action of viral membrane fusion
proteins, yet it is unclear if such activity occurs in infected PNS ganglia in living animals and if it correlates with disease symptoms. Using two-photon microscopy, we imaged autonomic ganglia in living mice infected with PRV strains expressing GCaMP3, a genetically encoded
calcium indicator, and used the changes in
calcium flux to monitor the activity of many neurons simultaneously with single-cell resolution.
Infection with virulent PRV caused these PNS neurons to fire synchronously and cyclically in highly correlated patterns among infected neurons. This activity persisted even when we severed the presynaptic axons, showing that
infection-induced firing is independent of input from presynaptic brainstem neurons. This activity was not observed after
infections with an attenuated PRV recombinant used for circuit tracing or with PRV mutants lacking either viral
glycoprotein B, required for membrane fusion, or viral
membrane protein Us9, required for sorting virions and viral
glycoproteins into axons. We propose that the
viral fusion proteins produced by virulent PRV
infection induce electrical coupling in unmyelinated axons in vivo. This action would then give rise to the synchronous and cyclical activity in the ganglia and contribute to the characteristic
peripheral neuropathy.