GABAB receptor‐mediated presynaptic inhibition in guinea‐pig hippocampus is caused by reduction of presynaptic Ca2+ influx.

1. The hypothesis that activation of GABAB receptors inhibits evoked synaptic transmission by reducing the presynaptic Ca2+ influx was tested using a recently developed technique for simultaneously recording the presynaptic Ca2+ transient ([Ca2+]t) and the field excitatory postsynaptic potential (fEPSP) evoked by a single electrical stimulus at CA3 to CA1 synapses of guinea‐pig hippocampus. 2. The GABAB receptor agonist baclofen reversibly blocked, in a dose‐dependant manner, both the fEPSP and the presynaptic [Ca2+]t with similar time courses. During application of baclofen, the fEPSP was proportional to about the fourth power of the presynaptic [Ca2+]t, and the presynaptic fibre volley and the resting Ca2+ level did not change. These results are similar to those we previously observed following application of several voltage‐dependent Ca2+ channel blockers, suggesting that baclofen inhibits the fEPSP by blocking the presynaptic Ca2+ influx. 3. The inhibition by baclofen of both the fEPSP and the presynaptic [Ca2+]t was blocked by the GABAB receptor antagonist CGP 35348, consistent with the causal relationship between the GABAB receptor‐mediated presynaptic inhibition of the [Ca2+]t and the fEPSP. 4. The inhibition by baclofen of the [Ca2+]t was partially occluded by application of the voltage‐dependent Ca2+ channel blocker omega‐conotoxin‐GVIA (omega‐CgTX‐GVIA), but not omega‐agatoxin‐IVA (omega‐AgaTX‐IVA), suggesting that baclofen reduces the presynaptic [Ca2+]t by blocking Ca2+ channels including the omega‐CgTX‐GVIA‐sensitive type. 5. We conclude that baclofen inhibits evoked transmitter release by reducing presynaptic Ca2+ influx.(ABSTRACT TRUNCATED AT 250 WORDS)