Excitatory action of an immature glycinergic/GABAergic sound localization pathway.

Most mammals determine the azimuthal direction of incoming sound using auditory cues arising from differences in interaural sound intensity. The first station in the ascending auditory pathway, which processes interaural intensity differences, is the lateral superior olive (LSO), a binaural nucleus in the auditory brainstem. LSO neurons encode interaural intensity differences by integrating excitatory input from the ipsilateral cochlea and inhibitory input from the contralateral cochlea. Both inputs converge on single neurons in a highly organized, frequency-specific manner. The correct development of the precise arrangement of these inputs and their physiological properties depends on neuronal activity. Previous studies have shown that inhibitory, glycinergic/GABAergic inputs to the LSO are transiently depolarizing, and it has been hypothesized that this depolarizing action enables developing inhibitory inputs to act as excitatory inputs. In support of this hypothesis, we recently demonstrated that depolarizing glycinergic/GABAergic inputs can increase the intracellular calcium concentration in immature LSO neurons and elicit action potentials. These results provide support for the notion that the influence of glycinergic/GABAergic synaptic activity on development of the LSO involves calcium-dependent signaling mechanisms.

Metabotropic glutamate receptors in the lateral superior olive activate TRP-like channels: age- and experience-dependent regulation.

The lateral superior olive (LSO) is the primary auditory nucleus for processing of interaural sound level differences, which is one of the major cues for sound localization. During development, survival and maturation of LSO neurons critically depend on synaptic activity and intracellular calcium signaling. Before hearing onset, glutamatergic synaptic inputs from the cochlear nucleus (CN) to the LSO activate group I metabotropic glutamate receptors (mGluRs), which leads to calcium release from intracellular stores and large calcium influx from the extracellular milieu. Here, we investigated the nature of the mGluR-activated membrane channel that mediates the influx of extracellular calcium. Using Fura-2 calcium imaging in brain stem slices of neonatal and juvenile mice, we found that this calcium channel is blocked by Ni(2+), La(3+), and 2-aminoethoxydiphenylborane (2-APB), known antagonists of transient receptor potential (TRP) channels. During postnatal development, the contribution of extracellular calcium influx to mGluR-mediated Ca(2+) responses gradually decreased and was almost abolished by the end of the third postnatal week. Over this period, the contribution of Ca(2+) release from internal stores remained unchanged. The developmental decrease of TRP-like channel-mediated calcium influx was significantly less in congenitally deaf waltzer mice, suggesting that early auditory experience is necessary for the normal age-dependent downregulation of functional TRP channels.

Glycinergic and GABAergic calcium responses in the developing lateral superior olive.

The lateral superior olive (LSO), a binaural nucleus involved in sound localization, receives tonotopically organized inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). During development, the tonotopic organization of this glycinergic/GABAergic MNTB-LSO pathway is established by activity-dependent axonal reorganization. However, the underlying mechanisms by which this reorganization takes place have remained largely unknown. As cytosolic calcium is one of the most important second messengers responsible for inducing synaptic plasticity and reorganization, we examined whether and how activity in the MNTB-LSO pathway changes the intracellular calcium concentration ([Ca2+]i) in developing LSO neurons. By applying calcium imaging techniques to Fura-2-labelled slices from neonatal rats and mice, we found that glycine and GABA (gamma-aminobutyric acid) affect [Ca2+]i in LSO neurons in an age-dependent manner; during the first postnatal week, the period at which glycine and GABA are depolarizing in the LSO, glycine and GABA always increased [Ca2+]i. However, in 2-week-old animals, the time around hearing onset when glycine and GABA are hyperpolarizing, glycine and GABA slightly decreased [Ca2+]i. Calcium responses could also be elicited by stimulation of afferent fibres from the MNTB, and these synaptic responses were mediated by glycine and GABA(A) receptors. Furthermore, GABA, which is a neurotransmitter only in the immature MNTB-LSO pathway, played a major role in generating MNTB-elicited Ca2+ responses. The direct link of glycinergic/GABAergic synaptic activity to intracellular calcium signalling during the period of inhibitory synaptic plasticity could be one of the mechanisms by which tonotopic MNTB-LSO connections become established.

Glutamatergic calcium responses in the developing lateral superior olive: receptor types and their specific activation by synaptic activity patterns.

The lateral superior olive (LSO) is a binaural auditory brain stem nucleus that plays a central role in sound localization. Survival and maturation of developing LSO neurons critically depend on intracellular calcium signaling. Here we investigated the mechanisms by which glutamatergic afferents from the cochlear nucleus increase intracellular calcium concentration in LSO neurons. Using fura-2 calcium imaging in slices prepared from neonatal mice, we found that cochlear nucleus afferents can activate all major classes of ionotropic and metabotropic glutamate receptors, each of which contributes to an increase in intracellular calcium. The specific activation of different glutamate receptor classes was dependent on response amplitudes and afferent stimulus patterns. Low-amplitude responses elicited by single stimuli were entirely mediated by calcium-impermeable AMPA/kainate receptors that activated voltage-gated calcium channels. Larger-amplitude responses elicited by either single stimuli or stimulus trains resulted in additional calcium influx through N-methyl-d-aspartate receptors. Finally, high-frequency stimulation also recruited group I and group II metabotropic glutamate receptors, both of which mobilized intracellular calcium. This calcium release in turn activated a strong influx of extracellular calcium through a membrane calcium channel that is distinct from voltage-gated calcium channels. Together, these results indicate that before hearing onset, distinct patterns of afferent activity generate qualitatively distinct types of calcium responses, which likely serve in guiding different aspects of LSO development.