GABA-Receptor–Independent Dorsal Root Afferents Depolarization in the Neonatal Rat Spinal Cord

Kremer, E. and A. Lev-Tov. GABA-receptor–independent dorsal root afferents depolarization in the neonatal rat spinal cord. J. Neurophysiol. 79: 2581–2592, 1998. Dorsal root afferent depolarization and antidromic firing were studied in isolated spinal cords of neonatal rats. Spontaneous firing accompanied by occasional bursts could be recorded from most dorsal roots in the majority of the cords. The afferent bursts were enhanced after elevation of the extracellular potassium concentration ([K+]e) by 1–2 mM. More substantial afferent bursts were produced when the cords were isolated with intact brain stems. Rhythmic afferent bursts could be recorded from dorsal roots in some of the cords during motor rhythm induced by bath-applied serotonin and N-methyl-d-aspartate (NMDA). Bilaterally synchronous afferent bursts were produced in pairs of dorsal roots after replacing the NaCl in the perfusate with sodium-2-hydroxyethansulfonate or after application of the γ-aminobutyric acid-A (GABAA) receptor antagonist bicuculline with or without serotonin (5-HT) and NMDA. Antidromic afferent bursts also could be elicited under these conditions by stimulation of adjacent dorsal roots, ventrolateral funiculus axons, or ventral white commissural (VWC) fibers. The antidromic bursts were superimposed on prolonged dorsal root potentials (DRPs) and accompanied by a prolonged increase in intraspinal afferent excitability. Surgical manipulations of the cord revealed that afferent firing in the presence of bicuculline persisted in the hemicords after hemisection and still was observed after removal of their ventral horns. Cutting the VWC throughout its length did not perturb the bilateral synchronicity of the discharge. These findings suggest that the activity of dorsal horn neurons is sufficient to produce the discharge and that the bilateral synchronicity can be maintained by cross connectivity that is relayed from side to side dorsal to the VWC. Antagonists of GABAB, 5-HT2/5-HT1C, or glutamate metabotropic group II and III receptors could not abolish afferent depolarization in the presence of bicuculline. Depolarization comparable in amplitude to DRPs, could be produced in tetrodotoxin-treated cords by elevation of [K+]e to the levels reported to develop in the neonatal rat spinal cord in response to dorsal root stimulation. A mechanism involving potassium transients produced by neuronal activity therefore is suggested to be the major cause of the GABA-independent afferent depolarization reported in our study. Possible implications of potassium transients in the developing and the adult mammalian spinal cord are discussed.

The activity of interneurons during locomotion in the in vitro necturus spinal cord

1. Less than two segments of the cervical spinal cord of the mudpuppy (Necturus maculatus) is sufficient to generate a locomotor rhythm with application of N-methyl-D-aspartic acid (NMDA). We have recorded intracellularly from rhythmically active interneurons in these segments and classified them according to their phase of firing within the step cycle and their afferent input. 2. Four classes of interneurons were found: flexor, flexor-->extensor, extensor, and extensor-->flexor. Interneurons that burst during the transition from flexion to extension or vice versa are referred to as “transitional” interneurons and represent the majority (68%) of rhythmically active interneurons studied in the mudpuppy spinal cord. 3. All flexor interneurons received only inhibitory input from cutaneous and dorsal root afferents, whereas the flexor-->extensor interneurons that responded received only excitatory input from dorsal root and cutaneous afferents. All extensor interneurons and all but one extensor-->flexor interneuron received no afferent input from the cutaneous or dorsal root afferents we stimulated. 4. Other interneurons have been classified as “tonic” cells. They fire continuously when the mudpuppy is walking and are silent when the mudpuppy is not walking. These interneurons receive no afferent input from the sources tested and may be responsible for turning locomotion on and off. 5. In conclusion, the presence of many transitional interneurons with specific patterns of afferent input may be required for the phasing of legged locomotion. We believe the in vitro preparation of the mudpuppy spinal cord and forelimb is an excellent model for studying the firing properties of interneurons during legged locomotion.

Long range afferents in rat spinal cord. III. Failure of impulse transmission in axons and relief of the failure after rhizotomy of dorsal roots

Dorsal root afferents entering the spinal cord form a T-junction with a caudal branch descending m any segments and giving off side branches terminating in the dorsal horn. This anatomical finding contrasts with the physiological observation that the postsynaptic effects of arriving afferents in the dorsal horn are limited to a few segments on either side of the root carrying the input. This paper explores the possibility that one explanation for this paradox is that orthodromic impulse conduction fails to penetrate the long range parts of the caudal branch. The experiments show that when all roots are intact, very few fibres can be detected carrying orthodromic impulses as far as 20 mm caudal to the entry point. After section of neighbouring dorsal roots, however, large numbers of conducting fibres can be recorded at that point. Signs of orthodromic conduction begin 7 days after root section. These results were confirmed by another method which com pared the relative refractory period of the membrane of the descending branch produced either after a local stimulus had evoked an action potential or after a rostral distant stimulus had produced an orthodromic action potential. Again, in the intact cord, the results indicate that impulses fail to penetrate long distances into the descending branches but that, as soon as 2 days after rhizotomy in the area of suspected conduction failure, orthodromic conduction is restored. It is proposed that the failure and release of conduction may depend on the control of membrane potential in the primary afferents, which would form a pre-presynaptic control mechanism.

Modulation of monosynaptic excitation in the neonatal rat spinal cord

1. The effects of high-frequency (5-50 Hz) stimulation of dorsal root afferents on monosynaptic excitation of alpha motoneurons was studied in the in vitro spinal cord preparation of the neonatal rat, using sharp-electrode intracellular recordings. 2. Double pulse stimulation of dorsal root afferents induced severe depression of testing excitatory postsynaptic potentials (EPSPs) at each of the tested interstimulus intervals (15 ms-5 s). After perfusion of the preparation with low-calcium, high-magnesium Krebs saline, the amplitude of the conditioning EPSPs was markedly decreased and the testing EPSPs exhibited substantial facilitation that was maximal at the 20-ms interval and that was accompanied by depression at intervals > or = 60-100 ms. 3. Short-duration stimulus trains applied to dorsal root afferents normally induced tetanic depression of the intracellularly recorded monosynaptic EPSPs. Switching the bathing solution to low-calcium, high-magnesium saline decreased the control EPSP and induced facilitation and then tetanic potentiation (TP) of the EPSPs within the applied train. The magnitude of potentiation (% potentiation) of these EPSPs depended on the interpulse interval of the short stimulus train and on the degree of attenuation of the unpotentiated control EPSP after the solution was changed from normal- to low-calcium Krebs solution. 4. Long-duration stimulus trains applied to dorsal root afferents at 5-10 Hz induced marked depression of monosynaptic EPSPs during the train. The depression was alleviated after cessation of the tetanic stimulation and was followed in some cases by slight posttetanic potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)