Limitations of the technique of pressure microinjection of excitatory amino acids for evoking responses from localized regions of the CNS

The aim of this study, performed on anaesthetized cats and rabbits, was to test the assumption that pressure microinjections of excitatory amino acids cause long-lasting excitation of neurones located close to the injection site. Unitary action potentials on antidromic field potentials were recorded from respiratory or 'reticular' neurones in the medulla oblongata and from phrenic motoneurones at different distances from the injection site. Injection of 10-150 nl (5-150 nmol) of l-glutamate or dl-homocysteic acid into these areas resulted in complex and widespread neuronal events. Generally, more distant neurones (500-1300 μm) were excited for variable periods of time (3-15 min), while neurones in the vicinity of the injection site (0-500 μm) showed, after a brief period of excitation time, a long-lasting (up to 30 min) decrease in excitability or silencing of discharge, probably due to a depolarizing block and disturbances in the ionic composition of the extracellular space. These findings show that interpretation of physiological responses following such injections should not be based on an assumption of local neuronal excitation. Some recommendations regarding the use of this technique are made.