Inhibitory synapses play key roles in the modulatory circuitry that regulates pain signaling and generation of migraine headache. A rare, dominant form of this common disease, familial hemiplegic migraine type 1 (FHM1), arises from missense mutations in the pore-forming α_1A subunit of P/Q-type Ca²⁺ channels. These channels are normally vital for presynaptic Ca²⁺ entry and neurotransmitter release at many central synapses, raising questions about effects of FHM1 mutations on neuronal Ca²⁺ influx and inhibitory and excitatory neurotransmission. We have expressed the four original FHM1 mutant channels in hippocampal neurons from α_1A knockout mice. Whole-cell recordings indicated that FHM1 mutant channels were less effective than wild-type channels in their ability to conduct P/Q-type current, but not generally different from wild type in voltage-dependent channel gating. Ca²⁺ influx triggered by action potential waveforms was also diminished. In keeping with decreased channel activity, FHM1 mutant channels were correspondingly impaired in supporting the P/Q-type component of inhibitory neurotransmission. When expressed in wild-type inhibitory neurons, FHM1 mutant channels reduced the contribution of P/Q-type channels to GABAergic synaptic currents, consistent with a competition of mutant and endogenous channels for P/Q-specific slots. In all cases, N-type channels took up the burden of supporting transmission and homeostatic mechanisms maintained overall synaptic strength. The shift to reliance on N-type channels greatly increased the susceptibility to G protein-coupled modulation of neurotransmission, studied with the GABA_B agonist baclofen. Thus, mutant-expressing synapses might be weakened in a heightened state of neuromodulation like that provoked by triggers of migraine such as stress.