The elaboration of dendrites is fundamental to the establishment of neuronal polarity and connectivity, but the mechanisms that underlie dendritic morphogenesis are poorly understood. We found that the genetic knockdown of the transcription factor NeuroD in primary granule neurons including in organotypic cerebellar slices profoundly impaired the generation and maintenance of dendrites while sparing the development of axons. We also found that NeuroD mediated neuronal activity-dependent dendritogenesis. The activity-induced protein kinase CaMKII catalyzed the phosphorylation of NeuroD at distinct sites, including endogenous NeuroD at Ser336 in primary neurons, and thereby stimulated dendritic growth. These findings uncover an essential function for NeuroD in granule neuron dendritic morphogenesis. Our study also defines the CaMKII-NeuroD signaling pathway as a novel mechanism underlying activity-regulated dendritic growth that may play important roles in the developing and mature brain.

We studied acutely dissociated neurons from the dorsomedial (shell) region of the rat suprachiasmatic nucleus (SCN) with the aim of determining the ionic conductances that underlie spontaneous firing. Most isolated neurons were spontaneously active, firing rhythmically at an average frequency of 8 Ϯ 4 Hz. After application of TTX, oscillatory activity generally continued, but more slowly and at more depolarized voltages; these oscillations were usually blocked by 2 M nimodipine. To quantify the ionic currents underlying normal spontaneous activity, we voltage clamped cells using a segment of the spontaneous activity of each cell as voltage command and then used ionic substitution and selective blockers to isolate individual currents. TTX-sensitive sodium current flowed throughout the interspike interval, averaging Ϫ3 pA at Ϫ60 mV and Ϫ11 pA at Ϫ55 mV. Calcium current during the interspike interval was, on average, fourfold smaller. Except immediately before spikes, calcium current was outweighed by calcium-activated potassium current, and in current clamp, nimodipine usually depolarized cells and slowed firing only slightly (average, ϳ8%). Thus, calcium current plays only a minor role in pacemaking of dissociated SCN neurons, although it can drive oscillatory activity with TTX present. During normal pacemaking, the early phase of spontaneous depolarization (Ϫ85 to Ϫ60 mV) is attributable mainly to background conductance; cells have relatively depolarized resting potentials (with firing stopped by TTX and nimodipine) of Ϫ55 to Ϫ50 mV, although input resistance is high (9.5 Ϯ 4.1 G⍀). During the later phase of pacemaking (positive to Ϫ60 mV), TTX-sensitive sodium current is dominant.

Summary The transient receptor potential channel 5 (TRPC5) is predominantly expressed in the brain where it can form heterotetrameric complexes with TRPC1 and TRPC4 channel subunits. These excitatory, non-selective cationic channels are regulated by G protein, phospholipase C-coupled receptors. Here, we show that TRPC5−/− mice exhibit diminished innate fear levels in response to innately aversive stimuli. Moreover, mutant mice exhibited significant reductions in responses mediated by synaptic activation of Group I metabotropic glutamate and cholecystokinin 2 receptors in neurons of the amygdala. Synaptic strength at afferent inputs to the amygdala was diminished in P10–P13 null mice. In contrast, baseline synaptic transmission, membrane excitability, and spike timing-dependent long-term potentiation at cortical and thalamic inputs to the amygdala were largely normal in older null mice. These experiments provide genetic evidence that TRPC5, activated via G protein-coupled neuronal receptors, has an essential function in innate fear.

We characterized the properties and functional roles of voltage-dependent potassium channels in the dendrites of Purkinje neurons studied in rat cerebellar slices. Using outside-out patches formed

Transient receptor potential (TRP) channels are abundant in the brain where they regulate transmission of sensory signals. The expression patterns of different TRPC subunits (TRPC1, 4, and 5) are consistent with their potential role in fear-related behaviors. Accordingly, we found recently that mutant mice lacking a specific TRP channel subunit, TRPC5, exhibited decreased innate fear responses. Both TRPC5 and another member of the same subfamily, TRPC4, form heteromeric complexes with the TRPC1 subunit (TRPC1/5 and TRPC1/4, respectively). As TRP channels with specific subunit compositions may have different functional properties, we hypothesized that fear-related behaviors could be differentially controlled by TRPCs with distinct subunit arrangements. In this study, we focused on the analysis of mutant mice lacking the TRPC4 subunit, which, as we confirmed in experiments on control mice, is expressed in brain areas implicated in the control of fear and anxiety. In behavioral experiments, we found that constitutive ablation of TRPC4 was associated with diminished anxiety levels (innate fear). Furthermore, knockdown of TRPC4 protein in the lateral amygdala via lentiviralmediated gene delivery of RNAi mimicked the behavioral phenotype of constitutive TRPC4-null (TRPC4 ؊/؊ ) mouse. Recordings in brain slices demonstrated that these behavioral modifications could stem from the lack of TRPC4 potentiation in neurons in the lateral nucleus of the amygdala through two G ␣q/11 protein-coupled signaling pathways, activated via Group I metabotropic glutamate receptors and cholecystokinin 2 receptors, respectively. Thus, TRPC4 and the structurally and functionally related subunit, TRPC5, may both contribute to the mechanisms underlying regulation of innate fear responses.