STEREOSPECIFIC AUGMENTATION OF HIPPOCAMPAL PYRAMIDAL CELL RESPONSES BY MORPHINE IS DEPENDENT ON THEIR POSITION ON THE INPUT-OUTPUT CURVE

The effect of morphine (0.5-50 μM) was examined on CA1 field potentials in the transverse hippocampal slice. Morphine consistently produced an augmentation of orthodromic evoked activity in stratum pyramidale (population spike) manifest as (i) a shift to lower threshold of the entire input-output curve for the neurons which was naloxone reversible near threshold only, and (ii) generation of an additional population spike whose amplitude was proportional to the position of the sampled response on its intput-output curve, and which was fully naloxone reversible. Both effects were stereospecific. The phenomena were not accompanied by simultaneous changes in potentials in the dendritic regions (population excitatory postsynaptic potential) of CA1, and only the second effect was observed during antidromic activation of the pyramidal cells. The results indicate the importance of assessing full input-output characteristics of drug effects.