The effects of Ca2+, Mg2+ and kynurenate on primary afferent synaptic potentials evoked in cat spinal cord neurones in vivo.

1. A technique was developed for perfusing the central canal of the cat spinal cord over a defined region to alter the extracellular environment and examine the effects of various ions and pharmacological agents on synaptic transmission in vivo. 2. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by hindlimb muscle nerve stimulation were recorded intracellularly from dorsal spinocerebellar tract (DSCT) neurones in Clarke's column, in close proximity to the central canal. 3. The effects of central canal perfusion of solutions containing Ca2+, Mg2+ and kynurenate on the monosynaptic afferent EPSP were examined. 4. Perfusion of the central canal with solutions containing a high Mg2+ concentration completely and reversibly blocked the monosynaptic EPSP, while perfusion with solutions containing a high Ca2+ concentration produced up to a fourfold increase in the peak amplitude of the EPSP. This large increase in the EPSP indicates that the pool of quanta available for release is considerably greater than estimated from previous quantal analysis studies at this synaptic connection. 5. Perfusion of the central canal with kynurenate, an antagonist at excitatory amino acid receptors, resulted in a complete block of the monosynaptic EPSP in DSCT neurones. This provides direct evidence that an excitatory amino acid, such as glutamate, is released from primary muscle afferent terminals in Clarke's column of the cat spinal cord in vivo.