Fluoxetine treatment rewires vestibular circuitry and corrects deficits in egocentric navigation of adult mice

Abstract

As egocentric navigation relies on vestibular input, neonatal exposure of the medial vestibular nucleus (MVN) to GABAA receptor antagonist bicuculline (BIC) produced navigation impairment in adult mice. Fluoxetine (FLX), a selective serotonin (5-HT) reuptake inhibitor, is known to reinstate exclusive plasticity in mature neural circuitry. Co-treatment of FLX and wobble vestibular stimulation (WOB) in these mice at postnatal day (P) 21-28 was shown to correct such navigation deficits in adulthood. This study hypothesizes that FLX+WOB treatment resets the deranged vestibular circuitry for egocentric navigation.

Since the transplantation of immature neuron can reinstate plasticity in mature circuitry, it was examined whether FLX treatment induces neurogenesis in the MVN, thereby abolishing navigation deficits. Prominin1 (PROM1)-expressing stem cells in the (sub-)ependymal layer of the fourth ventricle were hypothesized to provide the neighboring MVN neuron progenitors which proliferate with FLX induction. Both in vivo genetic lineage tracing and in vitro neurosphere assay found no neurons differentiated from PROM1 progeny in the MVN and (sub-)ependymal layer. FLX, or 5-HT mimicking its effect in vitro, did not promote proliferation of cells in the MVN. These results illustrate that the mechanism underlying FLX+WOB treatment might be independent of cell proliferation.

To ascertain whether FLX+WOB treatment alters the connectivity of GABAergic parvalbumin (PV)-expressing MVN neurons, thereby abolishing navigation deficits, various virus tracing tools were used to elucidate the central vestibular circuitry. Through anterograde monosynaptic tracing, postsynaptic targets of these PV neurons were found in relays of the circuitry for egocentric navigation, including the nucleus prepositus hypoglossi (NPH), dorsal paragigantocellular reticular nucleus (PGRNd), supragenual nucleus (SGN), and dorsal tegmental nucleus (DTN). In this ascending vestibular circuitry, SGN processes direct input from the MVN for generating head direction signals. Projection to the SGN from the MVN was mainly ipsilateral, with over 30% of SGN-projecting MVN neurons expressing PV. Retrograde monosynaptic tracing further demonstrated that neonatal BIC exposure and subsequent FLX+WOB treatment had no effect on input from the aforementioned relays onto SGN-projecting MVN neurons.

Viral retrograde tracing revealed that neonatal BIC exposure increased the projection of PV-expressing MVN neurons to the contralateral SGN but reduced such projection to the bilateral DTN. The deranged projection of PV neurons in this ascending circuitry may be responsible for the BIC-induced deficits in egocentric navigation.

Compared with mice with BIC treatment only, anterograde monosynaptic tracing revealed that BIC and subsequent FLX+WOB treatments reduced the number of ipsilateral NPH neurons postsynaptic to PV-expressing MVN neurons. This provided evidence that FLX+WOB treatment rewired the ascending vestibular circuitry, thereby contributing to the correction of BIC-induced deficits in egocentric navigation.

To conclude, neonatal BIC produced derangement in vestibular circuitry and caused deficits in egocentric navigation of adult mice. FLX+WOB treatment induced re-establishment of the circuitry and corrected the navigation deficits. These results shed light on how miswiring of vestibular circuitry for navigation may occur and offer a therapeutic approach for restoring egocentric navigation in adults through harnessing the plasticity of vestibular circuitry.