Neurons from varied regions of the central nervous system can show widely divergent responses to electrical stimuli that are determined by cell-specific differences in ion channel composition. The well-ordered and highly characterized peripheral auditory system allows one to explore the significance of this diversity during the final stages of postnatal development. We examined the electrophysiological features of murine spiral ganglion neurons in vitro at a time when recordings could be made from the cell bodies before myelination. These cells carry information about sound stimuli from hair cell receptors in the basilar membrane and are arranged tonotopically. Spiral ganglion neuron responses to depolarizing current injection were assessed with whole-cell current clamp recordings from cells that were isolated separately from the apical and basal thirds of the mouse cochlea. These cells displayed systematic variation in their firing. Apex neurons (low frequency coding) showed longer latency, slowly adapting responses, whereas base neurons (high frequency coding) showed short latency, rapidly adapting responses to the same stimuli. This physiological diversity was mirrored by regional differences in ion channel content assessed immunohistochemically. Apex neurons had a preponderance of Kv4.2 subunits, whereas base neurons possessed greater levels of K(Ca), Kv1.1, and Kv3.1 subunits. Taken together, these results indicate that the distribution of a set of voltage-gated potassium channels may relate specifically to a particular range of coding frequencies. These studies also suggest that intrinsic properties of spiral ganglion neurons can contribute to the characteristic responses of the peripheral auditory system. Their potential role in development and adult function is discussed.

It is now well established that sensory neurons and receptors display characteristic morphological and electrophysiological properties tailored to their functions. This is especially evident in the auditory system, where cells are arranged tonotopically and are highly specialized for precise coding of frequency-and timing-dependent auditory information. Less well understood, however, are the mechanisms that give rise to these biophysical properties. We have provided insight into this issue by using whole-cell current-clamp recordings and immunocytochemistry to show that BDNF and NT-3, neurotrophins found normally in the cochlea, have profound effects on the firing properties and ion channel distribution of spiral ganglion neurons in the murine cochlea. Exposure of neurons to BDNF caused all neurons, regardless of their original cochlear position, to display characteristics of the basal neurons. Conversely, NT-3 caused cells to show the properties of apical neurons. These results are consistent with oppositely oriented gradients of these two neurotrophins and/or their high-affinity receptors along the tonotopic map, and they suggest that a combination of neurotrophins are necessary to establish the characteristic firing features of postnatal spiral ganglion neurons.

We have previously identified two broad electrophysiological classes of spiral ganglion neuron that differ in their rate of accommodation (Mo & Davis, 1997a). In order to understand the underlying ionic basis of these characteristic firing patterns, we used α‐dendrotoxin (α‐DTX) to eliminate the contribution of a class of voltage‐gated K+ channels and assessed its effects on a variety of electrophysiological properties by using the whole‐cell configuration of the patch‐clamp technique. Exposure to α‐DTX caused neurons that initially displayed rapid accommodation to fire continuously during 240 ms depolarizing test pulses within a restricted voltage range. We found a non‐monotonic relationship between number of action potentials fired and membrane potential in the presence of α‐DTX that peaked at voltages between –40 to –10 mV and declined at more depolarized and hyperpolarized test potentials. The α‐DTX‐sensitive current had two components that activated in different voltage ranges. Analysis of recordings made from acutely isolated neurons gave estimated half‐maximal activation voltages of –63 and 12 mV for the two components. Because α‐DTX blocks the Kv1.1, Kv1.2 and Kv1.6 subunits, we examined the action of the Kv1.1‐selective blocker dendrotoxin K (DTX‐K). We found that this antagonist reproduced the effects of α‐DTX on neuronal firing, and that the DTX‐K‐sensitive current also had two separate components. These data suggest that the transformation from a rapidly adapting to a slowly adapting firing pattern was mediated by the low voltage‐activated component of DTX‐sensitive current with a potential contribution from the high voltage‐activated component at more depolarized potentials. In addition, the effects of DTX‐K indicate that Kv1.1 subunits are important constituents of the underlying voltage‐gated potassium channels.

Aim-This review posits that fatty acid amide hydrolase (FAAH) inhibition has therapeutic potential against neuropathological states including traumatic brain injury, Alzheimer's, Huntington's, and Parkinson's diseases, and stroke.Main Methods-This proposition is supported by data from numerous in vitro and in vivo experiments establishing metabolic and pharmacological contexts for the neuroprotective role of the endogenous cannabinoid ("endocannbinoid") system and selective FAAH inhibitors.Key Findings-The systems biology of endocannabinoid signaling involves two main cannabinoid receptors, the principal endocannabinoid lipid mediators N-arachidonoylethanolamine ("anandamide") (AEA) and 2-arachidonoyl glycerol (2-AG), related metabolites, and the proteins involved in endocannabinoid biosynthesis, biotransformation, and transit. The endocannabinoid system is capable of activating distinct signaling pathways on-demand in response to pathogenic events or stimuli, thereby enhancing cell survival and promoting tissue repair. Accumulating data suggest that endocannabinoid system modulation at discrete targets is a promising pharmacotherapeutic strategy for treating various medical conditions. In particular, neuronal injury activates cannabinoid signaling in the central nervous system as an intrinsic neuroprotective response. Indirect potentiation of this salutary response through pharmacological inhibition of FAAH, an endocannabinoid-deactivating enzyme, and consequent activation of signaling pathways downstream from cannabinoid receptors, have been shown to promote neuronal maintenance and function.Significance-This therapeutic modality has the potential to offer site-and event-specific therapeutic relief in those tissues where endocannabinoids are being produced as part of a physiological protective mechanism. In contrast, direct application of cannabinoid receptor agonists to the central nervous system may activate CB receptors indiscriminately and invite unwanted psychotrophic effects.

The exocyst is a 734-kDa complex essential for development. Perturbation of its function results in early embryonic lethality. Extensive investigation has revealed that this complex participates in multiple biological processes, including protein synthesis and vesicle/ protein targeting to the plasma membrane. In this article we report that the exocyst may also play a role in modulating microtubule dynamics. Using monoclonal antibodies, we observed that endogenous exocyst subunits co-localized with microtubules and mitotic spindles in normal rat kidney cells. To test for a functional relationship between the exocyst complex and microtubules, we established an in vitro exocyst reconstitution assay and studied exocyst effect on microtubule dynamics. We found that the exocyst complex reconstituted from eight recombinant exocyst subunits inhibited tubulin polymerization in vitro. Deletion of exocyst subunit sec5, sec6, sec15, or exo70 diminished its tubulin polymerization inhibition activity. Surprisingly, exocyst subunit exo70 itself was also capable of inhibiting tubulin polymerization, although exocyst complex with exo70 deletion did not lose its activity completely. Overexpression of exo70 in NRK cells resulted in microtubule network disruption and the formation of filopodia-like plasma membrane protrusions. The formation of these membrane protrusions was greatly hampered by stabilizing microtubules with taxol. Overexpression of exo84, an exocyst subunit that did not show tubulin polymerization inhibition activity, did not cause this phenotype. Results shown in this article, along with a previous report that localized microtubule instability induces plasma membrane addition, implicates a novel role for the exocyst in modulating microtubule dynamics underlying exocytosis.