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 myelin ated afferent nerve fibers travel for many segments caudal to their en try point and terminate in the gray matter of distant segments. This f act is surprising because no monosynaptic postsynaptic responses attri butable to these long-range afferents are observable in the distant se gments. Evidence has been produced to explain this paradox by showing that impulse transmission is normally blocked in these long-range affe rents 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 ex amine the postsynaptic consequence of restoring conduction in the long -range afferents with picrotoxin. Because picrotoxin blocks PAD and in creases the excitability of cells, we here contrast its action with th at of the glycine antagonist strychnine, which increases the excitabil ity of dorsal horn cells but does not affect PAD. 2. The preparation u sed throughout these experiments was decerebrate and spinal at T-11. O n one side dorsal roots T-12, T-13, and L(1) were intact but all more caudal roots on that side were cut. Recordings of single units were ma de in the L(6) segment with a rigid search pattern. The innervated are a of skin on the flank was repeatedly stimulated with pressure in a re peated pattern. In the control state, few cells were detected respondi ng in the L(6) segment, which was five to seven segments caudal to the intact dorsal roots. After picrotoxin, the number of cells with excit atory receptive fields rose by a factor of 8, whereas there was no cha nge in the number of inhibited cells. By contrast, after strychnine th ere was no change in the number of excited cells but the number of inh ibited cells rose by a factor of 10. Details of the size of receptive fields, adequate stimuli, and location of the responding cells are pro vided.3. To determine the number of synapses involved in producing the excitatory responses of L(6) cells, we applied electrical stimuli to the peripheral receptive fields and to the L(1) dorsal root. In the pr esence of picrotoxin, many cells were recorded with a latency consiste nt with monosynaptic connection. Some of these cells responded twice t o two stimuli separated by 2 ms, but there was a latency variation bet ween successive responses. Some cells responded with a short fixed lat ency and twice in 2 ms. No such cells were detected in the control sta te or in the presence of strychnine. 4. The results are consistent wit h the proposal that in the presence of picrotoxin a tonic PAD mechanis m is disabled and that this permits the unblocking of impulse transmis sion in long-range afferents that are capable of producing monosynapti c responses in dorsal horn cells at least five segments caudal to the root carrying the afferent impulses.