Rationale Acid-Sensing Ion Channels (ASICs) are Na+ channels that are activated by acidic pH. Their expression in cardiac afferents and remarkable sensitivity to small pH changes has made them leading candidates to sense cardiac ischemia. Objective Four genes encode six different ASIC subunits, however it is not yet clear which of the ASIC subunits contribute to the composition of ASICs in cardiac afferents. Methods and Results Here we labeled cardiac afferents using a retrograde tracer dye in mice, which allowed for patch-clamp studies of murine cardiac afferents. We found that a higher percentage of cardiac sensory neurons from the dorsal root ganglia (DRG) respond to acidic pH and generated larger currents compared to those from the nodose ganglia (NG). The ASIC-like current properties of the cardiac DRG neurons from wild-type mice most closely matched the properties of ASIC2a/3 heteromeric channels. This was supported by studies in ASIC null mice: acid-evoked currents from ASIC3 −/− cardiac afferents matched the properties of ASIC2a channels, and currents from ASIC2 −/− cardiac afferents matched the properties of ASIC3 channels. Conclusions We conclude that ASIC2a and -3 are the major ASIC subunits in cardiac DRG neurons, and provide potential molecular targets to attenuate chest pain and deleterious reflexes associated with cardiac disease.

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These experiments were designed to examine a paradox present in the literature with regard to the fine structure of nigrostriatal dopamine terminals within the rat striatum. Previous studies have shown that anterograde transport of tritiated labeled proteins from the substantia nigra to the striatum over short survival times primarily labels asymmetric synapses (and that these asymmetric synapses are preferentially vulnerable to selective dopaminergic neurotoxins such as 6-hydroxydopamine). In contrast, fine structural immunohistochemical studies with antibodies to tyrosine hydroxylase and dopamine have consistently labeled primarily symmetric synapses en passant within the striatum. We have now confirmed that these two seemingly contradictory types of labeled synapses (radio- and immuno-labeled) can both be present, but most often separate from one another, in single ultrathin sections. However, we also found that radiolabeled unmyelinated axons were usually double-labeled by tyrosine hydroxylase immunohistochemistry. Employing longer survival times (10 days after the nigral isotope injections) in order to enhance the ratio of "en passant" to terminal labeling produced a large increase in the occurrence of radiolabeled striatal axonal varicosities with the result that many symmetric synapses en passant were double-labeled with both the autoradiographic and the immunohistochemical markers. Given that more than 95% of the nigrostriatal projection arises from dopamine fluorescent neurons, it would appear that both the asymmetric and symmetric terminals belong to the same type of neuron. Thus, we suggest that single dopaminergic neurons in the substantia nigra make two types of synaptic contact with striatal cells: 1) symmetric synapses en passant, which can be stained with tyrosine hydroxylase and dopamine and which contact dendritic spine necks, and 2) asymmetric terminal boutons of unknown chemical nature which end on dendritic spine heads. We conclude that both the asymmetric terminal and symmetric en passant synapses take origin from a single nigrostriatal dopaminergic neuronal population and that dopaminergic transmitter markers occur only in one of these synaptic types in the rat striatum.

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