Spatiotemporal Interaction of α2 Autoreceptors and Noradrenaline Transporters in the Rat Locus Coeruleus

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Abstract
We investigated the roles of α2 autoreceptors and noradrenaline (NA) transporters on NA efflux and uptake in the rat locus coeruleus after electrical stimulation. NA efflux was evoked by various trains (50 pulses, 10-500 Hz) and measured by fast cyclic voltammetry. NA efflux and uptake half-time (t1/2) were stimulus-dependent, ranging from 43 ± 3 nM and 2.45 ± 0.21 s, respectively, with 500-Hz stimuli to 127 ± 11 nM and 4.41 ± 0.34 s, respectively, with 100-Hz trains. Based on these data, we calculate that each transporter removes 0.19 NA molecules from the extracellular space every second, a velocity compatible more with transporter-than channel-mode conduction. Dexmedetomidine (10 nM) decreased NA efflux by ∼30% on stimulations of ≤1 s in duration. BRL 44408 (1 μM) increased NA efflux on stimuli of ≥2 s (by up to 92 ± 16%). Desipramine (50 nM) increased NA efflux on stimuli of ≥1 s (by 113 ± 24%) but slowed NA uptake on all stimuli. When given together, the effects of desipramine and BRL 44408 were additive at stimuli of ≥1 s but showed potentiation on shorter trains. There was a significant time delay for the elevation of NA efflux by blockade of uptake (0.79 s) or autoreceptors (1.14 s), suggesting that both are located extrasynaptically and that NA must diffuse through the extracellular space to these structures. We suggest that released NA may interact with α2 autoreceptors and NA transporters as far as 10 μm from the release sites, an action compatible with a volume transmission role of NA in the locus coeruleus.

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