Cat spinal motoneurons exhibit topographic sensitivity to glutamate and glycine

Abstract

Spinal neurones from the 6th and 7th lumbar segments of cat were recorded intracellularly. l-Glutamate (GLU) and glycine (GLY) were preferentially applied to ‘somatic’ or ‘dendritic’ regions. To accomplish this, single micropipette electrodes for intracellular recording were assembled alongside drug-delivering micropipettes at varying distances from the recording tip. Microiontophoretic application of GLU to predominantly ‘denritic’ sites led to a depolarizing response after a significantly shorter delay than applications to more ‘somatic’ sites. ‘Dendritic’ applications did not cause measurable increases in membrane conductance at the recording site. ‘Somatic’ application of high amounts of GLU caused a depolarization associated with a progressively increasing membrane conductance. Thus, GLU may increase membrane conductance at ‘dendritic’ sites, but this effect is hardly detectable by conventional resistance measurements at the soma membrane. Microiontophoretic application of GLY at ‘dendritic’ or ‘somatic’ sites produced hyperpolarizing responses with marked increases of membrane conductance. The onset latency of the response was significantly longer for ‘dendritic’ applications than for ‘somatic’ applications. Action potentials, EPSPs and IPSPs were all attenuated by shunting during the GLY (or GLU) increases of membrane conductance. The equilibrium potentials measured for the hyperpolarizing action of GLY (range: −75 to −82 mV) and for the orthodromically evoked IPSPs (range: −74 to −78 mV) were similar and were closer to the resting potential than antidromically evoked IPSPs (range: −80 to −84 mV). The specific distribution of excitatory glutaminergic and inhibitory glycinergic receptive sites on cat spinal motoneurons further support a physiologic role for these amino acids as synaptic transmitters.