A comparison of extracellular and intracellular recording during extracellular microiontophoresis

A technique is described in which a central recording microelectrode can be moved independently of a concentrically arranged multibarrelled electrode prepared for microiontophoresis. Recordings were made from cat spinal motoneurones during microiontophoretic applications of excitatory amino acids and biogenic amines with the central electrode placed first extracellularly and then intracellularly. Recording were also made from one of the iontophoretic barrels. Both intra- and extracellular electrodes were used to record action potential firing, the ventral root field (VRF) evoked by antidromic ventral root stimulation and the membrane potential (EM). They were also used to record 'focal potentials' evoked by the extracellular application of drugs to nearby neurones. The firing pattern evoked by extracellular iontophoretic applications of DL-homocysteate and glutamate was not altered significantly following impalement of the cell by the recording microelectrode. Excitatory amino acids usually caused a reduction of the VRF negative wave and evoked an additional late positive wave. These VRF changes recovered at the same rate as the extracellularly recorded, negative 'focal potentials' (Flatman and Lambert, 1979). Iontophoretic applications of biogenic amines caused small increases, small decreases, or no change of the VRF negative wave. Variable responses were also seen during intracellular recording: hyperpolarization, no response and, occasionally, depolarizations were recorded. It is concluded that, during the drug action, VRF changes are difficult to interpret and are a poor index of drug-evoked changes in neuronal excitability or EM.