Estimation of Agonist Dissociation Constants at Central Presynaptic Alpha-2 Autoreceptors in the Absence of

The agonist dissociation constants (KA) and relative efficacies of UK-i 4304, norepinephnne (NE) and clonidine at presynaptic release-modulating alpha-2 adrenoceptors were determined in rat cerebral cortex slices using the irreversible alpha-2 adreno-captor antagonist N-ethoxycarbonyl-2-ethoxy-i,2-dihydroquino-line (EEDQ)(0.8 mg/kg i.p.) to reduce the receptor pool. Eighteen hours after treatment, the slices were incubated with [3H]NE, superfused in the presence of reuptake inhibitor and stimulated electrically with short bursts of 4 pulses delivered at 100 Hz (pseudo-one-pulse; POP) or trains of 36 or 72 pulses delivered at 3 Hz. EEDQ treatment did not affect the overflow of radioac-tivity evoked by POP, but greatly facilitated overflow at 36 or 72 pulses/3 Hz indicating that the autoinhibition seen under the latter conditions is totally lacking with POP stimulation. KA value