Figure 1.

Morphine alters the firing rate and pattern of the VTA DA neurons in naïve rats. The firing activities were recorded before and after morphine injection (5 mg/kg, i.v.). A: firing rate of a representative DA neuron. B: distribution of firing rate in the same neuron. C: representative segment of firing spike train recorded from a DA neuron before and after morphine treatment. D: spectral chart before (upper panel) and after (lower panel) morphine treatment. SO: slow oscillation.