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In the rat visual cortex in vitro, single-shock stimulations applied to the border between layer VI and the white matter evoke synchronized burst-firing by units in layer III. We have examined the effects of glutamate receptor antagonists on this activity, with antagonists applied via the bath to allow correlation of effects with concentrations. All synaptically driven components (recorded extracellularly as field potential 'S2' spikes, dipoles 'W1' and 'W2', and coinciding single-unit spikes) were inhibited by greater than 90% in 1.0 mM kynurenic acid and in 3 or 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, which selectively blocks AMPA/kainate receptors). S2 spike amplitudes were reduced by half in 0.7 microM CNQX. 2-Amino-5-phosphonovalerate (APV), a specific blocker of NMDA receptors, did not prevent S2 spike burst or horizontal spread of bursting within layer III. However, APV reduced the duration of synchronized bursts and the slower potentials which followed. In Mg(2+)-free medium, new components appeared which were APV-sensitive: (1) low amplitude spikes, distributed spatially like S2 spike, but recurring more slowly, and (2) slow potentials, distributed spatially like W1 and W2 potentials, but lasting for hundreds of milliseconds. The amplitudes of these spikes were reduced by half in 3 microM D-APV. Our data imply that: (1) glutamate receptors play a major role in mediating local, excitatory neurotransmission in the supragranular layers of neocortex, with NMDA and AMPA/kainate subtypes each subserving somewhat different functions; (2) AMPA/kainate receptors mediate rapid excitatory transmission between layer III neurons, responsible for driving the first 15 ms of synchronized bursts; (3) currents gated by NMDA receptors determine the duration of coherent firing bursts, and drive asynchronous neuronal firing following bursts; and (4) under conditions which circumvent block by extracellular Mg2+, activation of NMDA receptors greatly enhances and prolongs the response to single-shock stimulations. In vivo, activation of layer III neurons is likely to depend significantly upon currents gated by NMDA receptors whenever repetitively firing excitatory inputs summed over several tens of milliseconds provide enough depolarization to lift block by extracellular Mg2+.

The marine natural product lophotoxin has produced a non-reversible antagonism of parasympathetic and sympathetic functions that are known to be mediated by C6 sub-type nicotinic receptors. Transmission through anuran paravertebral ganglia was eliminated in 20-40 min by 10-30-min treatments with 16-32 microM lophotoxin, in a time course resembling the onset of block of C10 sub-type nicotinic receptors at the neuromuscular junction and in cultured BC3H-1 cells. The action persisted through 16 h of washout. Nerve conduction was unaffected. Somewhat longer treatments (80 min) of in vitro ileal sections resulted in loss of sensitivity to nicotine, but not to acetylcholine, for at least 5 h. These data indicate that lophotoxin can serve as a more universal nicotinic receptor probe than the alpha-neurotoxins, which may bind to both C6 and C10 sub-types, but block only the C10.

The hypothesis that excitatory retinotectal transmission is mediated primarily by a glutamate or glutamate-related transmitter-receptor system was examined by recording extracellular field potentials in isolated sections of goldfish tectum while stimulating the optic tract and applying antagonists of excitatory amino acid (EAA) neurotransmission via the tissue bath. Three antagonists of EAA receptors produced greater than 90% reduction in the postsynaptic components of these evoked potentials. In order of potency, these were (with the concentrations that produced 50% block): kynurenic acid (0.15 mM), gamma-D-glutamylglycine (0.33 mM), and cis-2,3-piperidine dicarboxylic acid (0.47 mM). All 3 log concentration-effect curves were parallel, symmetrically sigmoidal, and somewhat steeper than non-cooperative single-site binding isotherms. All antagonist actions stabilized within 15 min and were completely reversible. An EAA antagonist potent and selective for the N-methyl-D-aspartate (NMDA) subtype of receptor, 2-amino-5-phosphonovalerate, had little or no effect in either normal, low [Ca2+]/high [Mg2+], or Mg2+-free media. These data indicate that an excitatory amino acid receptor not of the NMDA subtype plays an essential role in fast excitatory retinotectal transmission, and would be most consistent with the mediation of most or all excitatory retinotectal transmission by a single class and subtype of glutamate receptor.

A software system is described which has been used to identify, average, and otherwise analyse different kinds of electrophysiological events. Spontaneous miniature end-plate potentials were identified by assessing the fit of subintervals within suspected event-containing intervals to flexibly defined model subevents. It was thus practical to apply complex, explicit, and stable criteria in the estimation of the frequency and mean time course of these spontaneous events. This software also mediated the objective rejection of noise-contaminated elements from samples of either evoked or spontaneous potentials. Average event time courses were comprised of a series of estimates of the standard deviations as well as the means. Each analysis generated thousands of individual event amplitudes which were used to improve estimates of quantal release and response parameters. This software allowed more complete analysis of tape-recorded experiments which conserved test compounds and animals, and enabled the observation of simultaneous and equal degrees of action upon spontaneous and evoked potentials, which suggested that a common mechanism was involved.

Field potentials and radial current source-densities (CSDs) evoked by optic tract stimulation were observed in goldfish tectum in vitro. The effects of different concentrations of calcium and magnesium were studied to improve understanding of component events in retinotectal transmission and interpretations of effects of pharmacological agents. Responses were of 3 forms: 'non-synaptic', 'subthreshold', and 'complex.' Subthreshold responses occurred when amplitudes were less than 40% of their maximal levels. They consisted of a sink-source pair with the sink in the superficial optic neuropil and the source 100-150 micron deeper. They were monophasic, rising in 1-2 ms and decaying as simple exponentials with time constants between 3.3 and 4.5 ms. Several other conditions which reduce amplitudes (besides low [Ca2+] and/or high [Mg2+]) produce responses of the subthreshold form. Complex responses, observed when amplitudes were 40-100% of maximal, were characterized by more rapid rise and decay and included, in the most extreme cases, a late, long, low-amplitude sink-source pair of opposite polarity. We propose that the time course of decay of subthreshold responses is determined by the passive cable properties of bi- or multistratified neurons with one dendritic arbor in the optic neuropil and a second arbor 100-250 micron deeper. Complex responses probably include recurrent inhibition to depolarized dendrites of the optic neuropil, latent by 4 ms to the monosynaptic excitation. Pharmacological assessments of retinotectal transmission may be made more precise by using low [Ca2+]/high [Mg2+] media to isolate monosynaptic activity.