No abstract

Activity-dependent BDNF (brain-derived neurotrophic factor) expression is hypothesized to be a cue for the context-specificity of memory formation. So far, activity-dependent BDNF cannot be explicitly monitored independently of basal BDNF levels. We used the BLEV (BDNF-live-exon-visualization) reporter mouse to specifically detect activity-dependent usage of Bdnf exon-IV and -VI promoters through bi-cistronic co-expression of CFP and YFP, respectively. Enriching acoustic stimuli led to improved peripheral and central auditory brainstem responses, increased Schaffer collateral LTP, and enhanced performance in the Morris water maze. Within the brainstem, neuronal activity was increased and accompanied by a trend for higher expression levels of Bdnf exon-IV-CFP and exon-VI-YFP transcripts. In the hippocampus BDNF transcripts were clearly increased parallel to changes in parvalbumin expression and were localized to specific neurons and capillaries. Severe acoustic trauma, in contrast, elevated neither Bdnf transcript levels, nor auditory responses, parvalbumin or LTP. Together, this suggests that critical sensory input is essential for recruitment of activity-dependent auditory-specific BDNF expression that may shape network adaptation.

Background/Aims: From invertebrates to mammals, Gα_i proteins act together with their common binding partner Gpsm2 to govern cell polarization and planar organization in virtually any polarized cell. Recently, we demonstrated that Gα_i3-deficiency in pre-hearing murine cochleae pointed to a role of Gα_i3 for asymmetric migration of the kinocilium as well as the orientation and shape of the stereociliary (“hair”) bundle, a requirement for the progression of mature hearing. We found that the lack of Gα_i3 impairs stereociliary elongation and hair bundle shape in high-frequency cochlear regions, linked to elevated hearing thresholds for high-frequency sound. How these morphological defects translate into hearing phenotypes is not clear. Methods: Here, we studied global and conditional Gnai3 and Gnai2 mouse mutants deficient for either one or both Gα_i proteins. Comparative analyses of global versus Foxg1-driven conditional mutants that mainly delete in the inner ear and telencephalon in combination with functional tests were applied to dissect essential and redundant functions of different Gα_i isoforms and to assign specific defects to outer or inner hair cells, the auditory nerve, satellite cells or central auditory neurons. Results: Here we report that lack of Gα_i3 but not of the ubiquitously expressed Gα_i2 elevates hearing threshold, accompanied by impaired hair bundle elongation and shape in high-frequency cochlear regions. During the crucial reprogramming of the immature inner hair cell (IHC) synapse into a functional sensory synapse of the mature IHC deficiency for Gα_i2 or Gα_i3 had no impact. In contrast, double-deficiency for Gα_i2 and Gα_i3 isoforms results in abnormalities along the entire tonotopic axis including profound deafness associated with stereocilia defects. In these mice, postnatal IHC synapse maturation is also impaired. In addition, the analysis of conditional versus global Gα_i3-deficient mice revealed that the amplitude of ABR wave IV was disproportionally elevated in comparison to ABR wave I indicating that Gα_i3 is selectively involved in generation of neural gain during auditory processing. Conclusion: We propose a so far unrecognized complexity of isoform-specific and overlapping Gα_i protein functions particular during final differentiation processes.

Enzymatically isolated type I spiral ganglion neurons of the guinea pig have been investigated in the present study. The identity of the cells was confirmed by using anti-neuron-specific enolase immunostaining. The presence and shredding of the myelin sheath was also documented by employing anti-S100 immunoreaction. The membrane characteristics of the cells were studied by using the whole-cell patch-clamp technique. The whole-cell capacitance of the cells was 9 +/- 2 pF (n = 51), while the resting membrane potential of the cells was -62 +/- 9 mV (n = 19). When suprathreshold depolarizing stimuli were applied, the neurons fired a single action potential at the beginning of the stimulation. It was confirmed in this study that type I spiral ganglion cells possess a hyperpolarization-activated nonspecific cationic current (Ih). The major characteristics of this current component were unaffected by the enzyme treatment. Type I spiral ganglion cells also expressed various depolarization-activated K+ current components. A high-threshold outward current was sensitive to 1-10 mm TEA+ application. The ganglion cells also expressed a relatively small, but nevertheless present, transient outward current component which was less sensitive to TEA+ but could be inhibited by 100 micro m 4-aminopyridine. A DTX-I-sensitive current was responsible for some 30% of the total outward current (at 0 mV), showed rapid activation at membrane potentials positive to -50 mV and demonstrated very little inactivation. However, inhibition of the highly 4-AP- or DTX-I-sensitive component did not alter the rapidly inactivating nature of the firing pattern of the cells.

Whole‐cell patch clamp recordings were made from bushy cells of the anterioventral cochlear nucleus (aVCN) and their synaptic terminals (calyx of Held) in the medial nucleus of the trapezoid body (MNTB). Both high voltage‐activated (HVA) and low voltage‐activated (LVA) calcium currents were present in acutely dissociated aVCN neurones and in identified bushy neurones from a cochlear nucleus slice. The transient LVA calcium current activated rapidly on depolarization (half‐activation, −59 mV) and inactivated during maintained depolarization (half‐inactivation, −89 mV). This T‐type current was observed in somatic recordings but was absent from presynaptic terminals. On the basis of their pharmacological sensitivity, P/Q‐type Ca2+ channels accounted for only 6 % of the somatic HVA, while L‐, N‐ and R‐type Ca2+ channels each accounted for around one‐third of the somatic calcium current. The divalent permeabilities of these native calcium channels were compared. The Ba2+/Ca2+ conductance ratios of the somatic HVA and LVA channels were 1.4 and 0.7, respectively. The conductance ratio of the presynaptic HVA current was 0.9, significantly lower that that of the somatic HVA current. We conclude that LVA currents are expressed in the bushy cell body, but are not localized to the excitatory synaptic terminal. All of the HVA current subtypes are expressed in bushy cells, but there is a strong polarity to their localization; P‐type contribute little to somatic currents but predominate at the synaptic terminal; L‐, N‐ and R‐types dominate at the soma, but contribute negligibly to calcium currents in the terminal.