Response characteristics of thalamic neurons to microiontophoretically applied morphine

The extracellular unit activity of parafasciculus thalamic neurons was studied before and after incremental microiontophoretic applications of morphine and naloxone. The parafasciculus neurons could be divided into three groups according to their initial spontaneous discharges. Only two groups responded to morphine as a function of dose and, within these groups, there were 12 different patterns. These findings suggest that the parafasciculus neurons responded in a heterogenous manner to morphine and they also provide physiological support for the biochemical evidence indicating the existence of multiple opiate receptors.