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The binding of [³H]desipramine to human brain tissue was characterized. Competition studies in the frontal cortex and hypothalamus revealed a single-site binding model for noradrenaline (K_i 120–190 μM). The noradrenaline uptake inhibitors nisoxetine, nortriptyline and desipramine fitted two-site binding models and these compounds exhibited 10–80 times lower K_i values than the serotonin uptake inhibitor citalopram. The high-affinity component of the nisoxetine-sensitive [³H]desipramine binding (K_i 50–110 nM) approximated the binding sensitive to noradrenaline. This binding fraction was defined as that sensitive to 1 μM nisoxetine and showed a maximum binding capacity (B_max) of 380 ± 80 fmol/mg protein and an apparent K_d of 5.1 (4.5–5.7) nM in the hypothalamus. The binding was also investigated in 25 additional brain regions without finding detectable amounts of binding. However, when the specific binding was defined as that sensitive to 100 μM nisoxetine, low-affinity binding where B_max and K_d were not possible to determine was obtained in all brain regions investigated. It is concluded that [³H]desipramine binding to human brain tissue represents multiple binding sites. Only when regarding binding sensitive to noradrenaline and to the high-affinity component of noradrenaline uptake inhibitors is the binding saturable and of high affinity. It is possible that this site represents the uptake site for noradrenaline.

The pharmacological and biochemical characteristics of [3H]desipramine binding to rat brain tissue were investigated. Competition studies with noradrenaline, nisoxetine, nortriptyline, and desipramine suggested the presence of more than one [3H]desipramine binding site. Most of the noradrenaline-sensitive binding represented a high-affinity site, and this site appeared to be the same as the high-affinity site of nisoxetine-sensitive binding. The [3H]desipramine binding sites were abolished by protease treatment, a result suggesting that the binding sites are protein in nature. When specific binding was defined by 0.1 microM nisoxetine, the binding was saturable and fitted a single-site binding model with a binding affinity of approximately 1 nM. This binding fraction was abolished by lesioning of the noradrenaline neurons with the noradrenaline neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4). In contrast, when 10 microM nisoxetine was used to define the specific binding, the binding was not saturable over the nanomolar range, but the binding fitted a two-site binding model with KD values of 0.5 and greater than 100 nM for the high- and low-affinity components, respectively. The high-affinity site was abolished after DSP4 lesioning, whereas the low-affinity site remained. The binding capacity (Bmax) for binding defined by 0.1 microM nisoxetine varied between brain regions, with very low density in the striatum (Bmax not possible to determine), 60-90 fmol/mg of protein in cortical areas and cerebellum, and 120 fmol/mg of protein in the hypothalamus. The binding capacities of these high-affinity sites correlated significantly with the regional distribution of [3H]noradrenaline uptake but not with 5-[3H]hydroxytryptamine uptake.(ABSTRACT TRUNCATED AT 250 WORDS)