Midbrain slices containing the dorsal and medial raphe nuclei were prepared from rat brain in order to study serotonergic-GABAergic interaction. The slices were loaded with either [3H] serotonin or [3H]GABA, superfused and the electrically induced efflux of radioactivity was determined. The GABA(A) receptor agonist muscimol (3 to 30 microM) and the GABA(B) receptor agonist baclofen (30 and 100 microM) inhibited [3H]serotonin and [3H]GABA release. These effects of muscimol were reversed by the GABA(A) antagonists bicuculline (100 microM). The GABA(B) antagonist phaclofen (100 microM) also antagonized the baclofen-induced inhibition of [3H]serotonin and [3H]GABA release. Phaclofen by itself increased [3H]serotonin release but it did not alter [3H]GABA overflow. Muscimol (10 microM) and baclofen (100 microM) also inhibited [3H]serotonin release after depletion of GABAergic neurons by isoniazid pretreatment. These findings indicate the presence of postsynaptic GABA(A) and GABA(B) receptors located on serotonergic neurons. The 5-HT1A receptor agonist 8-OH-DPAT (0.01 to 1 microM) and the 5-HT1B receptor agonist CGS-12066A (0.01 to 1 microM) inhibited the electrically stimulated [3H]serotonin and [3H]GABA release. The 5-HT1A antagonist WAY-100135 (1 microM) was without effect on [3H]serotonin and [3H]GABA efflux by itself but it reversed the 8-OH-DPAT-induced transmitter release inhibition. During KCl (22 mM)-induced depolarization, tetrodotoxin (1 microM) did not alter the inhibitory effect of CGS-12066A (1 microM) on [3H]GABA release, it did blocked, however, the ability of 8-OH-DPAT (1 microM) to reduce [3H]GABA efflux. After depletion of raphe serotonin neurons by p-chlorophenylalanine pretreatment, CGS-12066A (1 microM) still inhibited [3H]GABA release whereas in serotonin-depleted slices, 8-OH-DPAT (1 microM) was without effect on the release. We conclude that reciprocal influence exists between serotonergic projection neurons and the GABAergic interneurons or afferents in the raphe nuclei and these interactions may be mediated by 5-HT1A/B and GABA(A/B) receptors. Both synaptic and non-synaptic neurotransmission may be operative in the 5-HTergic-GABAergic reciprocal interaction which may serve as a local tuning in the neural connection between cerebral cortex and midbrain raphe nuclei.