Receptive fields and responses to ionophoretically applied noradrenaline and 5-hydroxytryptamine of units recorded in laminae I-III of cat dorsal horn.

Recordings were made with carbon fibre microelectrodes from 87 units in laminae I, II and III of the spinal cord in anaesthetized cats, and responses of the units to ionophoretically applied noradrenaline (NA) and 5-hydroxytryptamine (5-HT) were observed. Units with low threshold receptive fields were situated in laminae II and III, while those with high threshold or wide dynamic range fields were mainly restricted to laminae I and II. NA excited nearly half of the units in laminae I and II but had no effect on most units in lamina III. 5-HT excited 68% of the units and these were distributed throughout all 3 laminae. Excitations, particularly by 5-HT, could be very prolonged. NA and 5-HT excited units in all 3 receptive field classes. A few units in lamina I were inhibited. It is suggested that the cells recorded from in the present experiments may be inhibitory interneurones which act on large dorsal horn cells or on their primary afferent inputs.