A multi-electrode system that permits simultaneous recordings from multiple neurons and iontophoretic applications at two or three
different brain sites during acute experiments is described. This system consists of two or three microdrive terminals, each of which
includes four electrodes that can be moved independently and used for both extracellular recordings and microiontophoretic drug
administration. Drug applications were performed during standard extracellular recordings of multiple single-units via specialized
combined electrodes (CEs), which enable ejection of neuroactive substances and recording of neuronal activity from the same
electrode. With this system, we were able to successfully record simultaneously from different levels (brainstem, thalamus, and cortex)
of the vibrissal ascending pathway of the anesthetized rat. Herein, examples of simultaneous recordings from the brainstem and
thalamus and from the thalamus and cortex are presented. An effect of iontophoretic applications of agonists and antagonists of
metabotropic glutamate receptors (mGluRs) in the thalamus is demonstrated, and the extent of drug diffusion in the barrel cortex is
demonstrated with biocytin. This new multi-electrode system will facilitate the study of transformations of sensory information acquired
by the whiskers into cortical representations.