The contribution of increases in extracellular potassium to primary afferent depolarization in the bullfrog spinal cord

Dorsal root potentials in the isolated frog spinal cord: amino acid neurotransmitters and magnesium ions

Sucrose gap techniques recorded dorsal root potentials evoked by supramaximal dorsal root stimulation in in vitro, hemisected frog spinal cords. In 0 mM Mg2+ large (mean 13.0 mV), long lasting (mean 8.1 s) dorsal root potentials were recorded which consisted of two components: (1) an early component sensitive to picrotoxin, bicuculline, and low [Cl-]o and presumably produced by activation of GABAA receptors; and (2) a long-duration second component enhanced and lengthened by picrotoxin, bicuculline and low [Cl-]o and thought to result from increased interneuron discharges resulting from depression of GABA-mediated pre- and postsynaptic inhibition. Both the early and late components were reduced by over 90% in amplitude and duration by 20 mM Mg2+ or by kynurenate and bicuculline. The early component of the dorsal root potential may depend mainly upon activation of non-N-methyl-D-aspartate receptors. Thus, the N-methyl-D-aspartate antagonist D-(-)-2-amino-5- phosphonovalerate caused only a modest reduction in the amplitude of the early dorsal root potential component while the non N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione caused a much more substantial reduction. Exposure of the spinal cord to a "physiological" concentration of Mg2+ (1.0 mM) greatly reduced the duration and somewhat reduced the amplitude of the dorsal root potential. The reduction of dorsal root potentials by 1.0 mM Mg2+ appears to be caused by both pre- and postsynaptic factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical transmission in the dorsal column nuclei

The transmitter chemistry of the dorsal column nuclei is reviewed, with special emphasis on the monosynaptic component of the dorsal column-medial lemniscal pathway. It is maintained that in this anatomically addressed system concerned mainly with fast, secure sensory transmission, amino acids represent the predominant mechanism used for chemical relay of primary afferent impulses. The major excitatory primary afferent transmitter is most likely glutamic acid, whereas gamma-aminobutyric acid (GABA) fulfills adequately the role of transmitter of recurrent, postsynaptic and presynaptic inhibition. Recent immunohistochemical and physiological evidence indicates that 5-hydroxytryptamine, originating mainly from neurons of the raphé nuclei, plays a modulatory role in dorsal column transmission of innocuous sensory information. The basic synaptic elements involved in transmission across this relay, along with their corresponding chemical identities, are presented in the form of a speculative model.

Spontaneous dorsal root potentials arise from interneuronal activity in the isolated frog spinal cord

Spontaneous dorsal root potentials (sDRPs) were recorded from the dorsal roots of the isolated frog spinal cord using sucrose gap techniques. sDRPs were always negative (depolarizing) in sign and ranged in size from about 100 microV to 6.0 mV. The largest sDRPs were 25-40% of the amplitude of DRPs evoked by stimulation of adjacent dorsal roots. Hypoxia or accumulation of extracellular K+ ions did not appear responsible for the generation of this spontaneous activity since exposing the cord to unoxygenated Ringer's solution decreased sDRPs and K+-sensitive microelectrodes indicated that only small changes in extracellular K+ (approximately 0.15 mM) were produced coincidently with the largest sDRPs. Chemically-mediated synaptic transmission was found to be necessary for the production of sDRPs because the addition of Mn2+ or Mg2+ ions or tetrodotoxin to the Ringer's solution or reduction of its Na+ concentration blocked sDRPs, whereas application of 4-aminopyridine enhanced them. It did not seem that a direct action of GABA on afferent fiber terminals was responsible for the generation of spontaneous potentials since an increase in sDRPs was seen after: application of the GABA antagonists, bicuculline and picrotoxin; exposure to the glutamic acid decarboxylase inhibitor, semicarbazide (which significantly reduced the concentration of GABA in the cord); and lowering of the external Cl- concentration. Similarly taurine is probably not significant since the taurine antagonist, TAG, increased the amount of spontaneous activity. On the other hand, (--)-baclofen, which is thought to reduce excitatory amino acid release, D,L-alpha-aminoadipic acid, alpha, epsilon-diaminopimelic acid, and 2-amino-4-phosphonobutyric acid, which are believed to be selective postsynaptic excitatory amino acid antagonists, and [D-Pro2-D-Phe7-D-Trp9]-substance P, a postsynaptic blocker of the action of substance P, markedly and reversibly reduced sDRPs. Experiments were performed on isolated cords without supraspinal or afferent input; therefore sDRPs must be generated by intraspinal structures. It would seem that interneurons are responsible because addition of mephenesin or pentobarbital--compounds which inhibit polysynaptic reflex transmission involving interneurons--reduced the production of sDRPs. sDRPs may result from the action of excitatory transmitters such as L-glutamate, L-aspartate, or substance P released by interneuronal firing in the spinal cord. Moreover, because sDRPs were increased by application of yohimbine, corynanthine and propanolol and reduced by haloperidol, such interneurons may be under descending control of adrenergic and dopaminergic fibers.

Depolarization of feline primary afferent fibres by acidic amino acids

When administered micro-electrophoretically into the spinal grey matter of cats anaesthetized with pentobarbitone, acidic amino acids known to be neuronal excitants lower the threshold of electrically stimulated muscle and cutaneous primary afferent fibres and terminations. This depolarizing effect was not observed with fibres stimulated in the white matter. Depolarization by micro-electrophoretic potassium and excitant amino acids appeared not be be associated with an alteration in terminal membrane conductance since there was no change in synaptically evoked primary afferent depolarization. Excitant amino acid depolarization was not blocked by the gamma-aminobutyric acid antagonist bicuculline methochloride, but was reduced by selective excitant amino acid antagonists. The results are discussed in relation to the probable absence of specific excitant amino acid receptors on afferent terminals, the depolarizing effect of the amino acids on myelinated fibres and non-myelinated terminals being more likely a consequence of changes in the extracellular medium associated with the depolarization and firing of neurones.

Excitatory amino acids and potassium release in the frog spinal cord

Synaptic release of excitatory amino acids such as L-glutamate and/or L-aspartate and subsequent activation of specific receptors by these putative transmitters appears necessary for the release of K+ by afferent stimulation in the isolated frog spinal cord. This conclusion is based on the findings that (-)baclofen, which is thought to reduce the presynaptic release of putative excitatory amino acid transmitters, and some amino dicarboxylic amino acids (D, L-alpha-aminoadipic acid, 2-amino-4-phosphonobutyric acid, and D, L-alpha, epsilon-diaminopimelic acid), which are believed to interfere with the activation of receptors by these same excitatory amino acids, significantly attenuate the increment in extracellular K+ evoked by tetanic dorsal root stimulation.

Selective action of piretanide on primary afferent GABA responses in the frog spinal cord

The effects of piretanide, a drug which blocks active chloride transport in other systems, was examined on amino acid responses and synaptic potentials in the frog spinal cord. It was found that piretanide had no effect on GABA ventral root hyperpolarizing responses, but abolished dorsal root depolarizing responses. Dorsal root glutamate responses were little affected by piretanide. The depression of GABA responses on dorsal root ganglion cells was accompanied by a large decrease in the conductance change elicited by GABA. In addition, there was a small shift of the GABA equilibrium potential toward the resting membrane potential, which is consistent with a reduced inward pumping of chloride ions. Piretanide abolished the dorsal root potential elicited by ventral root stimulation. In addition it reduced the early part of the dorsal root potential evoked by stimulating an adjacent dorsal root but had remarkably little effect on the late phase.