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Pseudorabies virus (PRV), a neurovirulent α-herpesvirus, spreads between neurons at synaptic connections. PRV-infected neurons have been shown to exhibit functional deficits with the attenuated PRV152 Bartha strain negatively influencing neuronal functioning in in vitro model systems. However, the impact of this attenuated PRV152 Bartha strain on the native central nervous system has not been fully explored. Using a combination of in vivo stereotactic injections and post-hoc in vitro whole-cell recordings, we investigated the functional impact of PRV152 Bartha in the auditory system of juvenile Mongolian gerbils. The specificity of this virus strain to spread exclusively trans-synaptically in a retrograde fashion and the well-defined structure of the ascending auditory brainstem pathways allowed us to determine the physiological alterations in primary and secondary infected neurons. We find at primary and secondary infections sites, the inferior colliculus (IC) and dorsal nucleus of the lateral lemniscus respectively, a reduced excitability of infected cells. The loss of excitability is manifested by an increase in current threshold and a loss of action potential generation. The minor changes in the approximated passive membrane parameters induced by the infection cannot explain the full loss in excitability, indicating that channel densities and properties have changed. This impact on neuronal functioning might contribute to the lethal neurovirulent effects of PRV viruses as vital neuronal circuits might cease activity. Since the detrimental effects of the attenuated PRV152 Bartha strain are reduced compared to wild-type strains, it comprises an excellent tool to study the neuropathological mechanisms of viral infections.

Motor behavior is generated by specific neural circuits. Those producing locomotion are located in the spinal cord, and their activation depends on descending inputs from the brain or on sensory inputs. In this study, we have used an in vitro brainstem-spinal cord preparation from adult zebrafish to localize a region where stimulation of descending inputs can induce sustained locomotor activity. We show that a brief stimulation of descending inputs at the junction between the brainstem and spinal cord induces long-lasting swimming activity. The swimming frequencies induced are remarkably similar to those observed in freely moving adult fish, arguing that the induced locomotor episode is highly physiological. The motor pattern is mediated by activation of ionotropic glutamate and glycine receptors in the spinal cord and is not the result of synaptic interactions between neurons at the site of the stimulation in the brainstem. We also compared the activity of motoneurons during locomotor activity induced by electrical stimulation of descending inputs and by exogenously applied NMDA. Prolonged NMDA application changes the shape of the synaptic drive and action potentials in motoneurons. When escape activity occurs, the swimming activity in the intact zebrafish was interrupted and some of the motoneurons involved became inhibited in vitro. Thus, the descending inputs seem to act as a switch to turn on the activity of the spinal locomotor network in the caudal spinal cord. We propose that recurrent synaptic activity within the spinal locomotor circuits can transform a brief input into a well coordinated and long-lasting swimming pattern.

Neurons in the dorsal nucleus of the lateral lemniscus (DNLL) receive excitatory and inhibitory inputs from the superior olivary complex (SOC) and convey GABAergic inhibition to the contralateral DNLL and the inferior colliculi. Unlike the fast glycinergic inhibition in the SOC, this GABAergic inhibition outlasts auditory stimulation by tens of milliseconds. Two mechanisms have been postulated to explain this persistent inhibition. One, an “integration-based” mechanism, suggests that postsynaptic excitatory integration in DNLL neurons generates prolonged activity, and the other favors the synaptic time course of the DNLL output itself. The feasibility of the integration-based mechanism was testedin vitroin DNLL neurons of Mongolian gerbils by quantifying the cellular excitability and synaptic input–output functions (IO-Fs). All neurons were sustained firing and generated a near monotonic IO-F on current injections. From synaptic stimulations, we estimate that activation of approximately five fibers, each on average liberating ∼18 vesicles, is sufficient to trigger a single postsynaptic action potential. A strong single pulse of afferent fiber stimulation triggered multiple postsynaptic action potentials. The steepness of the synaptic IO-F was dependent on the synaptic NMDA component. The synaptic NMDA receptor current defines the slope of the synaptic IO-F by enhancing the temporal and spatial EPSP summation. Blocking this NMDA-dependent amplification during postsynaptic integration of train stimulations resulted into a ∼20% reduction of the decay time course of the GABAergic inhibition. Thus, our data show that the NMDA-dependent amplification of the postsynaptic activity contributes to the GABAergic persistent inhibition generated by DNLL neurons.