AAV-hChR2(H134R) was used for optogenetic manipulation to manipulate the neuronal activity of RtTg. AAV-taCasp3 was used for ablating RtTg neurons. AAV-Gcamp6m was used to monitor the neuronal activity of RtTg glutamatergic neurons during startle. RV and AAV helper were used for retrograde monosynaptic tracing of spinal MNs-projecting premotor neurons. (From BrainVTA)

The viruses used in this article from BrainVTA are in the table below

The reticulotegmental nucleus (RtTg) has long been recognized as a crucial component of brainstem reticular formation (RF). However, the function of RtTg and its related circuits remain elusive. Here, we report a role of the RtTg in startle reflex, a highly conserved innate defensive behaviour. Optogenetic activation of RtTg neurons evokes robust startle responses in mice. The glutamatergic neurons in the RtTg are significantly activated during acoustic startle reflexes (ASR). Chemogenetic inhibition of the RtTg glutamatergic neurons decreases the ASR amplitudes. Viral tracing reveals an ASR neural circuit that the cochlear nucleus carrying auditory information sends direct excitatory innervations to the RtTg glutamatergic neurons, which in turn project to spinal motor neurons. Together, our findings describe a functional role of RtTg and its related neural circuit in startle reflexes, and demonstrate how the RF connects auditory system with motor functions.

Figure 1. The CN sends monosynaptic glutamatergic projections to RtTg glutamatergic neurons.

In this study, the authors investigated the function of RtTg and its related circuits utilizing cell-type-specific viral tracing approaches as well as optogenetic/chemogenetic tools combined with in vitro/in vivo electrophysiological recordings. The result shows that the RtTg controls a conservative startle response, and identified a neural circuit from the cochlear nucleus (CN) to spinal motor neurons that underlies this innate defensive behaviour.