STRUCTURAL BASIS OF NEURON-TO-NEURON CROSS-EXCITATION IN DORSAL-ROOT GANGLIA

Lanthanum was used as a tracer substance to determine whether small mo lecules in the bulk extracellular space in dorsal root ganglia have ac cess to the narrow cleft that separates sensory neurons from their sur rounding satellite cell sheath. Results showed that lanthanum is able to diffuse into this cleft, especially when the tissue is incubated wi th the tracer before fixation. Lanthanum gained access to the cleft at the seam where adjacent satellite cell processes meet. There appears to be preferential access in the axon hillock-initial segment region. Large diameter light neurons, which generally support fast conducting myelinated axons and carry information about non-nociceptive sensory e vents, proved more likely to admit lanthanum than small diameter dark neurons, which tend to have thin myelinated and unmyelinated axons and typically carry nociceptive information. Peripheral axotomy triggered a reduction in the access of lanthanum to the neuron-satellite cell c left. These data bear on the mechanism of non-synaptic cell-to-cell cr oss-excitation within dorsal root ganglia, and in particular, lend sup port to the hypothesis that this interaction is mediated chemically ra ther than electrically.