Dopamine D1-like receptor-mediated presynaptic inhibition of excitatory transmission onto rat magnocellular basal forebrain neurones.

1. Excitatory postsynaptic currents (EPSCs) following focal afferent stimulation were recorded from patch-clamped magnocellular neurones in a thin-slice preparation of the rat basal forebrain. Evoked EPSCs had a mean decay time constant of 3.81 +/- 0.09 ms and were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM). 2. Bath-applied dopamine (DA) reduced evoked EPSC amplitude by up to 54.2 +/- 2.3% with an IC50 of 19.9 microM in normal Krebs solution (2.5 mM Ca2+, 1.2 mM Mg2+) without effect on postsynaptic holding current. 3. DA (30 microM) reduced the mean frequency of spontaneous miniature EPSCs recorded in 0.5 microM tetrodotoxin without affecting their mean amplitude, rise time or decay time constant. This effect was diminished by 100 microM Cd2+. 4. The effect of DA on evoked EPSCs was mimicked by the D1-like receptor agonist, SKF 81297 (IC50 25.6 microM), but not by the D2-like receptor agonist R(-)-TNPA (30 microM) or (-)-quinpirole (30 microM), and was antagonized by the D1-like receptor antagonist R(+)-SCH 23390 (estimated dissociation constant KB = 1.7 microM) but not by the D2-like receptor antagonist S(-)-eticlopride (10 microM). 5. Forskolin (10 microM) reduced evoked EPSCs to approximately 60% of the control amplitude, and occluded the effect of subsequent application of DA. 6. These results suggest that glutamatergic afferents to magnocellular basal forebrain neurones possess presynaptic D1-like DA receptors, and that activation of these receptors reduces excitatory glutamatergic transmission, probably via an adenylyl cyclase-dependent pathway.