Synaptic Refinement of an Inhibitory Topographic Map in the Auditory Brainstem Requires Functional Ca_V1.3 Calcium Channels

Abstract

Synaptic refinement via the elimination of inappropriate synapses and strengthening of appropriate ones is crucially important for the establishment of specific, topographic neural circuits. The mechanisms driving these processes are poorly understood, particularly concerning inhibitory projections. Here, we address the refinement of an inhibitory topographic projection in the auditory brainstem in functional and anatomical mapping studies involving patch-clamp recordings in combination with minimal and maximal stimulation, caged glutamate photolysis, and single axon tracing. We demonstrate a crucial dependency of the refinement on Ca V 1.3 calcium channels: Ca V 1.3 Ϫ/Ϫ mice displayed virtually no elimination of projections up to hearing onset. Furthermore, strengthening was strongly impaired, in line with a reduced number of axonal boutons. The mediolateral topography was less precise and the shift from a mixed GABA/ glycinergic to a purely glycinergic transmission before hearing onset did not occur. Together, our findings provide evidence for a Ca V 1.3-dependent mechanism through which both inhibitory circuit formation and determination of the neurotransmitter phenotype are achieved.

Time-dependent Gene Expression Analysis of the Developing Superior Olivary Complex

“…Notably, all developmentally regulated voltage-gated Ca 2ϩ channels were downregulated. This indicates an important role of these channels in the perinatal period, which is in accord with the recent observation of the requirement of Ca v 1.3 for proper development and refinement of auditory circuits (12,46,49).…”

“…At P4, SOC neurons still show many signs of immaturity, such as depolarizing GABAergic and glycinergic action (7,8), and axonal (9) and dendritic pruning (10). Furthermore, there is ongoing neurite outgrowth and functional refinement (9,11,12). The stage of P16, about 4 -6 days after hearing onset, is often used for in vitro electrophysiological analyses of the mature-like auditory system (7,(13)(14)(15).…”

L-type calcium channels in the auditory system

“…To verify this hypothesis, and at the same time test if Ca v 1.3 plays a critical role in the process, we studied the process of refinement in the MNTB-LSO connection in Cacna1d −/− mice. As expected, refinement did not occur [13]. At present, we are still unable to rule out that the absence of refinement may, at least in part, be due to a lack of spontaneous activity in the auditory pathway.…”

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

“…On D2 receptor-expressing striatopallidal medium spiny neurons, they are required for synaptic pruning induced by dopamine depletion (Olson et al, 2005;Fieblinger et al, 2014). A role for Ca V 1.3 in synaptic refinement has been described in the auditory pathway during development (Hirtz et al, 2012). Together these data point to an important role of Ca V 1.3 for the generation and maintenance of neuronal connectivity.…”