Effects of ionophoresed noradrenaline on the spontaneous activity of neurones in rat primary somatosensory cortex.

Changes in spontaneous activity of rat S1 cortical neurones with identified receptive fields were investigated in reply to ionophoresed noradrenaline (NA). Extracellular levels of NA were maintained constant by continuous electrochemical analysis at the carbon fibre recording tip of the multibarrel micro‐electrode. In the absence of NA there were clear differences in spike amplitude, firing rate and pattern of firing of deep (800‐1400 micron) and superficial (0‐800 micron) cells. Superficial cells responded to low (5 X 10(‐8) to 5 X 10(‐7) M) NA concentrations with simple inhibition. Recovery occurred within a minute or so of extracellular NA concentrations falling below detectable (10(‐8) M) levels. Increases in local concentration merely stopped cells firing. In contrast, cells located in the deep zone could often be excited by very low NA concentrations (less than 10(‐8) M), with inhibition occurring at levels 10‐100 times greater. Most cells, however, were inhibited, with threshold doses for a 50% change in firing rate much higher than for superficial cells. Some cells in the deep zone showed sustained increases in firing rate following an ionophoretic trial. This could occur for periods of up to 1 h after ceasing a trial. Such effects could be produced by levels as low as 10(‐7) M‐NA. Interspike interval analysis for deep cells suggested that their spontaneous activity resembled that established for slow‐wave sleep. During and after excitation by NA the pattern of firing of small groups of these cells changed to that established for the waking state. The effect could persist for up to 1 h following a short (2‐5 min) ionophoretic trial.