Postsynaptic effects of long-range afferents in distant segments caudal to their entry point in rat spinal cord under the influence of picrotoxin or strychnine.

1. Previous work has shown that substantial numbers of arriving myelinated afferent nerve fibers travel for many segments caudal to their entry point and terminate in the gray matter of distant segments. This fact is surprising because no monosynaptic post-synaptic responses attributable to these long-range afferents are observable in the distant segments. Evidence has been produced to explain this paradox by showing that impulse transmission is normally blocked in these long-range afferents by the tonic operation of a primary afferent depolarization (PAD) mechanism. Impulse transmission is restored if the PAD mechanism is disabled with gamma-aminobutyric acid antagonists. In this paper we examine the postsynaptic consequence of restoring conduction in the long-range afferents with picrotoxin. Because picrotoxin blocks PAD and increases the excitability of cells, we here contrast its action with that of the glycine antagonist strychnine, which increases the excitability of dorsal horn cells but does not affect PAD. 2. The preparation used throughout these experiments was decerebrate and spinal at T11. On one side dorsal roots T12, T13, and L1 were intact but all more caudal roots on that side were cut. Recordings of single units were made in the L6 segment with a rigid search pattern. The innervated area of skin on the flank was repeatedly stimulated with pressure in a repeated pattern. In the control state, few cells were detected responding in the L6 segment, which was five to seven segments caudal to the intact dorsal roots. After picrotoxin, the number of cells with excitatory receptive fields rose by a factor of 8, whereas there was no change in the number of inhibited cells. By contrast, after strychnine there was no change in the number of excited cells but the number of inhibited cells rose by a factor of 10. Details of the size of receptive fields, adequate stimuli, and location of the responding cells are provided. 3. To determine the number of synapses involved in producing the excitatory responses of L6 cells, we applied electrical stimuli to the peripheral receptive fields and to the L1 dorsal root. In the presence of picrotoxin, many cells were recorded with a latency consistent with monosynaptic connection. Some of these cells responded twice to two stimuli separated by 2 ms, but there was a latency variation between successive responses. Some cells responded with a short fixed latency and twice in 2 ms. No such cells were detected in the control state or in the presence of strychnine.(ABSTRACT TRUNCATED AT 400 WORDS)