Calcium, magnesium and the electrical activity of guinea-pig olfactory coex in vitro

1. Evoked electrical activity was observed in slices from guinea-pig olfactory cortex maintained in vitro. This evoked activity was studied in saline solutions containing various concentrations of Ca(2+) and Mg(2+).2. The evoked potentials recorded from the surface of the prepiriform cortex comprised a negative wave (N-wave) of about 10-15 msec duration upon which a variable number of short duration positive notches were superimposed. The N-wave was identified as a population excitatory post-synaptic potential (EPSP) of the olfactory cortex neurones and the positive notches were identified with synchronous discharge of cortical neurones.3. The relation between the EPSP amplitude and saline Ca(2+) concentration was sigmoid. The N-wave amplitude (EPSP) was proportional to [Ca(2+)](o) (n). When saline Ca(2+) was between 0.4 and 1 mMn averaged 2.5 (range 1.5-4.3).4. The EPSP amplitude was reduced by increasing saline [Mg(2+)]. High Mg(2+) salines (about 10 mM) abolished the EPSP.5. Both Ca(2+) and Mg(2+) in high concentrations depressed the positive notches.6. Salines containing little or no Ca(2+) and Mg(2+) caused the preparation to show signs of hyperexcitability. Under these conditions the evoked potentials were very variable and unstable. ;Ca(2+)-free' salines caused a reversible loss of excitability.7. The results indicated that Ca(2+) and Mg(2+) had antagonistic effects on the release of the transmitter substance(s). Increased Ca(2+) concentrations increased the output of transmitter, increased Mg(2+) concentrations reduced the output of transmitter. Both Ca(2+) and Mg(2+) in increasing concentration increased the threshold for action potential generation.