Chemically-mediated cross-excitation has been described between neurons within sensory ganglia. However, the identity and source of the chemical mediators is not known. Ca2+-dependent release of neurotransmitters from cultured sensory neurons in vitro has been observed, although neurite outgrowth may confound the ability to extrapolate findings from culture systems to in vivo conditions. Thus, the present studies evaluate the hypothesis of capsaicin-sensitive intraganglionic neuropeptide release from freshly prepared slices of rat sensory ganglia. The ganglionic slice preparation provides an advantage over neuronal cultures, because release may be assessed within minutes after tissue collection (minimizing phenotypic changes) and while maintaining gross anatomical relationships. Trigeminal ganglia (TGG) were quickly removed from male, Sprague-Dawley rats (175-200 g), chopped into 200 μm slices and placed into chambers within 3 min of collection. Chambers were perfused with buffer, and superfusates were collected and assayed for immunoreactive calcitonin gene-related peptide (iCGRP) release via radioimmunoassay. After about 90 min of baseline collection, tissue was treated with capsaicin followed by a washout period. Capsaicin (1-100 μM) evoked concentration-dependent increases in iCGRP release. A competitive capsaicin receptor antagonist, capsazepine, significantly inhibited capsaicin-evoked release of iCGRP. In addition, capsaicin-evoked release of iCGRP was dependent on the presence of extracellular calcium. Furthermore, capsaicin-evoked release from TGG slices was significantly greater than that from slices of equivalent weights of adjacent trigeminal nerve shown histologically to be free of neuronal somata. These data support the hypothesis that Ca2+-dependent exocytosis of neuropeptides may occur within the TGG in vivo and that the majority of this release derives from neuronal somata.
Bibliographical noteFunding Information:
The authors thank A. Leong for technical assistance. This work was supported by a Howard Hughes Medical Institute Predoctoral Fellowship (Y.M.U.), DE9860, DE7288, DA10510, DE11277, and a Lasby Visiting Professorship (H.E.G).
- Neuronal somata
- Trigeminal ganglion