Adenylyl cyclase isoforms in the vestibular periphery of the rat

M2 muscarinic ACh receptors sensitive BK channels mediate cholinergic inhibition of type II vestibular hair cells

There are two types of hair cells in the sensory epithelium of vestibular end organ. Type II vestibular hair cell (VHC II) is innervated by the efferent nerve endings, which employ a cholinergic inhibition mediated by SK channels through the activation of α9-containing nAChR. Our previous studies demonstrated that a BK-type cholinergic inhibition was present in guinea pig VHCs II, which may be mediated by an unknown mAChR. In this study, BK channel activities triggered by ACh were studied to determine the mAChR subtype and function. We found the BK channel was insensitive to α9-containing nAChR antagonists and m1, m3, m4 muscarinic antagonists, but potently inhibited by the m2 muscarinic antagonist. Muscarinic agonists could mimic the effect of ACh and be blocked by m2 antagonist. cAMP analog activated the BK current and adenyl cyclase (AC) inhibitor inhibited the ACh response. Inhibitor of Giα subunit failed to affect the BK current, but inhibitor of Giα and Giβγ subunits showed a potent inhibition to these currents. Our findings provide the physiological evidence that mAChRs may locate in guinea pig VHCs II, and m2 mAChRs may play a dominant role in BK-type cholinergic inhibition. The activation of m2 mAChRs may stimulate Giβγ-mediated excitation of AC/cAMP activities and lead to the phosphorylation of Ca(2+) channels, resulting in the influx of Ca(2+) and opening of the BK channel.

Capturing adenylyl cyclases as potential drug targets

Cyclic AMP (cAMP) is an important intracellular signalling mediator. It is generated in mammals by nine membrane-bound and one soluble adenylyl cyclases (ACs), each with distinct regulation and expression patterns. Although many drugs inhibit or stimulate AC activity through the respective upstream G-protein coupled receptors (for example, opioid or beta-adrenergic receptors), ACs themselves have not been major drug targets. Over the past decade studies on the physiological functions of the different mammalian AC isoforms as well as advances in the development of isoform-selective AC inhibitors and activators suggest that ACs could be useful drug targets. Here we discuss the therapeutic potential of isoform-selective compounds in various clinical settings, including neuropathic pain, neurodegenerative disorders, congestive heart failure, asthma and male contraception.

Urocortin expression in mouse cochlear nucleus and Scarpa's ganglion

OBJECTIVES/HYPOTHESIS

Clinical and basic studies have correlated tinnitus with stress. Although the etiology of tinnitus is unknown, the cochlear nucleus (CN) appears to play a role. To better understand the potential impact of stress on tinnitus and modulation of the central auditory system in general, the goal of the current study was to examine the presence and distribution of axon terminals containing urocortin in the CN of the mouse.

STUDY DESIGN

Prospective description of histological findings.

METHODS

Three different forms of urocortin were labeled in brainstem sections collected from 10 wild-type mice by immunohistochemistry. Immunoreactive terminal fibers in the CN were digitally photographed, as well as reconstructed in the CN under a drawing tube attached to a light microscope.

RESULTS

Specific staining was found in en passant type fibers scattered throughout the CN but situated mostly within the granule cell domains. Clusters of labeled fibers were observed in the nerve root. Labeled axons were observed in the three tracts known to carry olivocochlear fibers to the CN, as well as in the olivocochlear bundle itself. As the axons within the olivocochlear bundle departed the brainstem in the vestibular nerve, numerous labeled en passant fibers were observed among somata of the vestibular ganglion (Scarpa's). Centrally, labeled axons were followed from the CN to the lateral superior olive, an established source of urocortin-positive efferents to the cochlea. These findings indicate that lateral olivocochlear efferents innervate the CN and Scarpa's ganglion, and also that urocortin is likely a neuromodulator in particular CN circuits.

CONCLUSIONS

The current study supports innervation of specific regions of the mouse CN and Scarpa's ganglion by neurons expressing urocortin. The innervation may be one substrate by which stress modulates particular CN processes. Further studies are necessary to establish the role of urocortin in CN models of tinnitus.

In silico analysis of 2085 clones from a normalized rat vestibular periphery 3' cDNA library

The inserts from 2400 cDNA clones isolated from a normalized Rattus norvegicus vestibular periphery cDNA library were sequenced and characterized. The Wackym-Soares vestibular 3' cDNA library was constructed from the saccular and utricular maculae, the ampullae of all three semicircular canals and Scarpa's ganglia containing the somata of the primary afferent neurons, microdissected from 104 male and female rats. The inserts from 2400 randomly selected clones were sequenced from the 5' end. Each sequence was analyzed using the BLAST algorithm compared to the Genbank nonredundant, rat genome, mouse genome and human genome databases to search for high homology alignments. Of the initial 2400 clones, 315 (13%) were found to be of poor quality and did not yield useful information, and therefore were eliminated from the analysis. Of the remaining 2085 sequences, 918 (44%) were found to represent 758 unique genes having useful annotations that were identified in databases within the public domain or in the published literature; these sequences were designated as known characterized sequences. 1141 sequences (55%) aligned with 1011 unique sequences had no useful annotations and were designated as known but uncharacterized sequences. Of the remaining 26 sequences (1%), 24 aligned with rat genomic sequences, but none matched previously described rat expressed sequence tags or mRNAs. No significant alignment to the rat or human genomic sequences could be found for the remaining 2 sequences. Of the 2085 sequences analyzed, 86% were singletons. The known, characterized sequences were analyzed with the FatiGO online data-mining tool (http://fatigo.bioinfo.cnio.es/) to identify level 5 biological process gene ontology (GO) terms for each alignment and to group alignments with similar or identical GO terms. Numerous genes were identified that have not been previously shown to be expressed in the vestibular system. Further characterization of the novel cDNA sequences may lead to the identification of genes with vestibular-specific functions. Continued analysis of the rat vestibular periphery transcriptome should provide new insights into vestibular function and generate new hypotheses. Physiological studies are necessary to further elucidate the roles of the identified genes and novel sequences in vestibular function.

Expression of adenylyl cyclase isoforms in neutrophils

In the present study, we have identified the expression of adenylyl cyclase (AC) isoforms in rat neutrophils according to the mRNA analysis and the distinct mode of regulation of isoform activity. Agarose gel electrophoresis of reverse transcription-polymerase chain reaction (RT-PCR)-amplified products resulted in a single band of the expected size for each product with nucleotide sequences corresponding to AC1 to AC9. AC1 was abundant, while AC2, 6 and 9 were of moderate expression among the AC isoforms in neutrophils based on the quantitative real-time RT-PCR analysis. Exposure of neutrophils to Ca(2+) ionophore A23187, isoproterenol and forskolin stimulated cellular cyclic AMP accumulation. EDTA and the calmodulin (CaM) antagonist, trifluoperazine, prevented the A23187-induced response. Pretreatment with pertussis toxin (PTX) inhibited the alpha(2)-adrenergic agonist, UK14304-induced cellular cyclic AMP elevation. In addition, UK14304 augmented the cyclic AMP elevation when cells were stimulated by isoproterenol. Phorbol 12-myristate 13-acetate (PMA) attenuated the augmentation response of UK14304 and isoproterenol. Treatment of the membrane preparations from rat neutrophils with Ca(2+)/CaM, forskolin, isoproterenol, GTPgammaS or Gbetagamma all increased cyclic AMP production. The addition of protein kinase C (PKC) catalytic fragment and Gbetagamma augmented the Ca(2+)/CaM- and isoproterenol-stimulated AC activity, respectively. However, forskolin and the activated protein kinase A (PKA) attenuated the GTPgammaS- and isoproterenol-stimulated AC activity, respectively. KT5720, a PKA inhibitor, reversed the inhibition by PKA. Taken together, these data suggest the presence of four groups of AC isoforms in rat neutrophils.