Expression of the postsynaptic scaffold PSD-95 and development of synaptic physiology during giant terminal formation in the auditory brainstem of the chicken

Tonotopic differentiation of presynaptic neurotransmitter-releasing machinery in the auditory brainstem during the prehearing period and its selective deficits in Fmr1 knockout mice

Tonotopic organization is a fundamental feature of the auditory system. In the developing auditory brainstem, the ontogeny and maturation of neurotransmission progress from high to low frequencies along the tonotopic axis. To explore the underlying mechanism of this tonotopic development, we aim to determine whether the presynaptic machinery responsible for neurotransmitter release is tonotopically differentiated during development. In the current study, we examined vesicular neurotransmitter transporters and calcium sensors, two central players responsible for loading neurotransmitter into synaptic vesicles and for triggering neurotransmitter release in a calcium-dependent manner, respectively. Using immunocytochemistry, we characterized the distribution patterns of vesicular glutamate transporters (VGLUTs) 1 and 2, vesicular gamma-aminobutyric acid transporter (VGAT), and calcium sensor synaptotagmin (Syt) 1 and 2 in the developing mouse medial nucleus of the trapezoid body (MNTB). We identified tonotopic gradients of VGLUT1, VGAT, Syt1, and Syt2 in the first postnatal week, with higher protein densities in the more medial (high-frequency) portion of the MNTB. These gradients gradually flattened before the onset of hearing. In contrast, VGLUT2 was distributed relatively uniformly along the tonotopic axis during this prehearing period. In mice lacking Fragile X mental retardation protein, an mRNA-binding protein that regulates synaptic development and plasticity, progress to achieve the mature-like organization was altered for VGLUT1, Syt1, and Syt2, but not for VGAT. Together, our results identified novel organization patterns of selective presynaptic proteins in immature auditory synapses, providing a potential mechanism that may contribute to tonotopic differentiation of neurotransmission during normal and abnormal development.

Expression patterns of chloride transporters in the auditory brainstem of developing chicken

GABAergic transmission changes from depolarization to hyperpolarization in most vertebrate brain regions during development. By contrast, in the auditory brainstem of chicken a depolarizing effect of GABA persists after hatching. Since auditory brainstem neurons that receive GABAergic input have a Cl^- reversal potential above resting membrane potential, a specifically tuned activity of Cl^- transporters is likely. We here present a developmental study of the expression patterns of several members of the SLC12 family (NKCC1, NKCC2, KCC1, KCC2, KCC4, CCC6, CCC9) and of AE3 at developmental ages E7, E10, E12, E15, E17, and P1 with quantitative RT-PCR. NKCC2 and CCC9 were not detected in auditory brainstem (positive control: kidney). KCC1, CCC6 and AE3 were expressed, but not regulated, while NKCC1, KCC2 and KCC4 were regulated. The expression of the latter transporters increased, with KCC2 exhibiting the strongest expression at all time points. Biochemical analysis of the protein expression of NKCC1, KCC2 and KCC4 corroborated the findings on the mRNA level. All three transporters showed a localization at the outer rim of the cells, with NKCC1 and KCC2 expressed in neurons, and KCC4 predominantly in glia. The comparison of the published chloride reversal potential and expression of transporter proteins suggest strong differences in the efficiency of the three transporters. Further, the strong KCC2 expression could reflect a role in the structural development of auditory brainstem synapses that might lead to changes in the physiological properties.

Intrinsic and Synaptic Dynamics Contribute to Adaptation in the Core of the Avian Central Nucleus of the Inferior Colliculus

The reduction of neuronal responses to repeated stimulus presentation occurs in many sensory neurons, also in the inferior colliculus of birds. The cellular mechanisms that cause response adaptation are not well described. Adaptation must be explicable by changes in the activity of input neurons, short-term synaptic plasticity of the incoming connections, excitability changes of the neuron under consideration or influences of inhibitory or modulatory network connections. Using whole-cell recordings in acute brain slices of the embryonic chicken brain we wanted to understand the intrinsic and synaptic contributions to adaptation in the core of the central nucleus of the inferior colliculus (ICCc). We described two neuron types in the chicken ICCc based on their action potential firing patterns: Phasic/onset neurons showed strong intrinsic adaptation but recovered more rapidly. Tonic/sustained firing neurons had weaker adaptation but often had additional slow components of recovery from adaptation. Morphological analysis suggested two neuron classes, but no physiological parameter aligned with this classification. Chicken ICCc neurons received mostly mixed AMPA- and NMDA-type glutamatergic synaptic inputs. In the majority of ICCc neurons the input synapses underwent short-term depression. With a simulation of the putative population output activity of the chicken ICCc we showed that the different adaptation profiles of the neuron classes could shift the emphasize of stimulus encoding from transients at long intervals to ongoing parts at short intervals. Thus, we report here that description of biophysical and synaptic properties can help to explain adaptive phenomena in central auditory neurons.

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.