Morphine and naloxone: effects on conditioned fear as measured with the potentiated startle paradigm.

Morphine (0.6 to 10 mg/kg) produced a dose-dependent reduction of the potentiated startle effect where acoustic startle amplitude is normally increased in the presence of a light previously paired with a shock. The effect was selective since the same doses did not appreciably depress baseline levels of startle. Naloxone (2 mg/kg) did not significantly affect potentiated startle, but antagonized the ability of morphine (10 mg/kg) to block potentiated startle. Morphine did not block potentiated startle by accelerating extinction. The advantages of this paradigm for studying fear or anxiety were discussed.