ATP- and ACh-induced responses in isolated cat petrosal ganglion neurons.

Chemoreceptor (glomus) cells of the carotid body are synaptically connected to the sensory nerve endings of petrosal ganglion (PG) neurons. In response to natural stimuli, the glomus cells release transmitters, which acting on the nerve terminals of petrosal neurons increases the chemosensory afferent discharge. Among several transmitter molecules present in glomus cells, acetylcholine (ACh) and adenosine 5'-triphosphate (ATP) are considered to act as excitatory transmitter in this synapse. To test if ACh and ATP play a role as excitatory transmitters in the cat CB, we recorded the electrophysiological responses from PG neurons cultured in vitro. Under voltage clamp, ATP induces a concentration-dependent inward current that partially desensitizes during 20-30 s application pulses. The ATP-induced current has a threshold near 100 nM and saturates between 20-50 muM. ACh induces a fast, inactivating inward current, with a threshold between 10-50 muM, and saturates around 1 mM. A large part of the population of PG neurons (60%) respond to both ATP and ACh. Present results support the hypothesis that ACh and ATP act as excitatory transmitters between cat glomus cells and PG neurons.

A "minimal" sodium channel construct consisting of ligated S5-P-S6 segments forms a toxin-activatable ionophore.

The large size (six membrane-spanning repeats in each of four domains) and asymmetric architecture of the voltage-dependent Na+ channel has hindered determination of its structure. With the goal of determining the minimum structure of the Na+ channel permeation pathway, we created two stable cell lines expressing the voltage-dependent rat skeletal muscle Na+ channel (micro1) with a polyhistidine tag on the C terminus (muHis) and pore-only micro1 (muPore) channels with S1-S4 in all domains removed. Both constructs were recognized by a Na+ channel-specific antibody on a Western blot. muHis channels exhibited the same functional properties as wild-type micro1. In contrast, muPore channels did not conduct Na+ currents nor did they bind [3H]saxitoxin. Veratridine caused 40 and 54% cell death in muHis- and muPore-expressing cells, respectively. However, veratridine-induced cell death could only be blocked by tetrodotoxin in cells expressing muHis, but not muPore. Furthermore, using a fluorescent Na+ indicator, we measured changes in intracellular Na+ induced by veratridine and a brevotoxin analogue, pumiliotoxin. When calibrated to the maximum signal after addition of gramicidin, the maximal percent increases in fluorescence (deltaF) were 35 and 31% in cells expressing muHis and muPore, respectively. Moreover, in the presence of 1 microm tetrodotoxin, deltaF decreased significantly to 10% in muHis- but not in muPore-expressing cells (43%). In conclusion, S5-P-S6 segments of micro1 channels form a toxin-activable ionophore but do not reconstitute the Na+ channel permeation pathway with full fidelity.

Emisiones otoacústicas en productos de distorsión cocleares en chinchillas / Otoacoustic emissions of cochlear distortion products in chinchillas

Al estimular el oído con dos tonos puros simultáneos (primarios), se obtienen respuestas cocleares no sólo a los tonos primarios sino también a productos de distorsión (PDs) con frecuencia que son combinaciones de las frecuencias de los estímulos. Estos PDs han sido estudiados en experimentos psicofísicos, en registros de fibras aisladas del nervio coclear y mas recientemente en emisiones otoacústicas medidas en el meato externo. Hemos montado la técnica de medición de PDs en emisiones otoacústicas en la chinchilla usando una sonda acústica construida en nuestro laboratorio. Se presentaron simultáneamente dos tonos primarios en el conducto auditivo externo por medio de fonos miniatura ubicados en la sonda. Esta presión se amplificó, se muestreó y se analizó para obtener sus componentes espectorales. Resultados obtenidos en 8 chinchillas muestran que, para tonos primarios (Ÿ1 y Ÿ2) con frecuencias entre 2 kHz y 8 kHz, se registran PDs a frecuencias tanto menores (p.ej., 2Ÿ1-Ÿ2), como mayores (p.ej., 2Ÿ2-Ÿ1), que las de los tonos primarios. Los PDs a 2Ÿ1-Ÿ2 alcanzan niveles de hasta 24 dB bajo el nivel de los tonos primarios, mientras que los de frecuencia 2Ÿ2-Ÿ1 son de menor intensidad. Se discuten algunas aplicaciones clínicas de las emisiones de PDs en humanos