SUMMARY1. Intracellular recordings were made in dorsal root ganglia in vitro at 37 'C. The L4, L5 and L6 ganglia from 46-to 51-day-old female Wistar rats were used. In each neuron conduction velocity (CV) was measured and fluorescent dye was injected. Later the intensity of the immunoreactivity to RT97 (a monoclonal antibody to the phosphorylated 200 kDa neurofilament subunit) as well as the cell size (crosssectional area at the nuclear level) were measured in the dye-injected neurons. RT97 was used to distinguish between the L (light, neurofilament-rich) and the SD (small dark, neurofilament-poor) neuronal somata.2. Neurons were classified as C neurons (CV < 1-3 m/s), C/Ad neurons (1-3-2 m/s),A neurons (2-12 m/s) or Aa//? neurons (> 12 m/s).3. All A-fibre somata were RT97 positive (L) and all C-fibre somata were RT97 negative (SD), although in the C/M group both positive and negative neurons were seen. Thus, RT97-negative somata had C (unmyelinated) or C/Ad fibres, while RT97-positive somata had A (myelinated) or C/Ad fibres. 4. The size distributions of A neurons and C neurons were consistent with their classification as L-and SD-cell neurons respectively. The size distribution of Ad cells was skewed with a peak of small cells and a tail of medium-sized cells.5. There was a loose positive correlation between cell size and fibre CV. 6. RT97 intensity was positively correlated with CV if all neurons were considered together, but no correlation was seen within the C, Ad or Aoc/, CV groups.7. RT97 intensity was positively correlated with cell size when all neurons were considered together. Although no correlation was seen within the C or the Ad CV groups, a clear positive correlation was seen for Atz// neurons. 8. The relationship of RT97 intensity to cell size was not demonstrably altered by axotomy, time in vitro or the presence of intracellular dye in control experiments.9. RT97-negative and -positive neurons could be seen in neonatal rat ganglia. Their size distributions resembled, respectively, the SD-and L-neuron populations at this age. RT97 immunoreactivity may therefore be a useful predictor of the cell type and myelinated state which a sensory cell is destined to reach in the adult rat.
Capsaicin (0.01-50 microM) was applied to adult rat vagus nerve in vitro to examine the C-fibre conduction block and to compare its time course with that of the depolarisation caused by this drug. The conduction block was assessed by the reduction in the C-wave of the compound action potential. Capsaicin caused a dose-dependent decrease in the height and area under the C-wave; the threshold dose was between 0.01 and 0.3 microM and maximum C-wave reduction of about 85% occurred with doses of 5 microM or above. The C-wave reduction was divided into reversible and irreversible components which differed in dose dependency. The threshold for the reversible block was below 0.3 microM and for the irreversible block it was about 1 microM. The onset of the block took about 5 min. regardless of dose. Where there was more than 50% recovery after the block the reversible component was measured. This was dose dependent and its duration was 10-90 min. Removal of external calcium did not affect the magnitude of the C-wave block, although it did seem to prevent recovery of the C-wave.
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