Effects Of Noradrenaline On The GABA Response In Rat Isolated Spiral Ganglion Neurons In Culture

Aim:Spiral ganglion neurons (SGNs) in the cochlea receiveγ-aminobutyric acid (GABA)-ergic modulation through the lateral olivocochlear system from the superior olivary complex, and noradrenaline (NA)-ergic innervation from the superior cervical ganglion. But the modulatory effects of NA on GABA-activated responses in the SGNs remain unknown. In the present study, the modulatory effects of NA on the GABA responses were investigated in the isolated cultured SGNs of rat.Methods:The culture of SGN was carried out with postnatal day 0～5 SD rats. The process of cellular growth and differentiation of SGN were observed by fluorescent inverted/phase contrast microscope. Immunocytochemical identification was performed on the cultured SGN of SD rats by methods of fluorescence and the antibody of neuron-specific nuclear protein (NeuN). The effect of NA on GABA responses in the SGN was studied by using nystatin perforated patch recording configuration under voltage-clamp conditions. Results:The trypsin dissociated and cultured SGN of SD rats could survive well and had a normal phenotypic differentiation in vitro. The stable neuronal plasticity of SGN existed in the postnatal SD rats under the present experimental conditions. NA reversibly depressed GABA responses in a concentration-dependent manner and neither changed the reversal potential of the GABA response nor affected the apparent affinity of GABA to its receptor.α2-adrenoceptor agonist and antagonist, clonidine and yohimbine mimicked and blocked the NA action on GABA response, respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), a protein kinase A inhibitor, mimicked the effect of NA on GABA response. NA failed to affect the GABA response in the presence of both cyclic adenosine 5'-monophosphoric acid (cAMP) and protein kinase A modulators. However, NA still depressed the GABA response even in the presence of both phorbol-12-myristate-13-acetate (PMA), a protein kinase C activator and chelerythrine, a protein kinase C inhibitor.Conclusion:The cell amount and surviving period of cultured SGN were able to meet the requirements of cytological experiments. NA suppresses GABA response viaα2-adrenoceptor which reduces intracellular cAMP formation through the inhibition of adenyl cyclase. Therefore, NA input to the SGNs may modulate the auditory transmission by affecting the GABA responses.