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Fear behaviors are regulated by adaptive mechanisms that dampen their expression in the absence of danger. By studying circuits and the molecular mechanisms underlying this adaptive response, we show that cholinergic neurons of the medial habenula reduce fear memory expression through GABA_B presynaptic excitation. Ablating these neurons or inactivating their GABA_B receptors impairs fear extinction in mice, whereas activating the neurons or their axonal GABA_B receptors reduces conditioned fear. Although considered exclusively inhibitory, here, GABA_B mediates excitation by amplifying presynaptic Ca²⁺ entry through Ca_v2.3 channels and potentiating co-release of glutamate, acetylcholine, and neurokinin B to excite interpeduncular neurons. Activating the receptors for these neurotransmitters or enhancing neurotransmission with a phosphodiesterase inhibitor reduces fear responses of both wild-type …