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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 (Ki 120-190 microM). The noradrenaline uptake inhibitors nisoxetine, nortriptyline and desipramine fitted two-site binding models and these compounds exhibited 10-80 times lower Ki values than the serotonin uptake inhibitor citalopram. The high-affinity component of the nisoxetine-sensitive [3H]desipramine binding (Ki 50-110 nM) approximated the binding sensitive to noradrenaline. This binding fraction was defined as that sensitive to 1 microM nisoxetine and showed a maximum binding capacity (Bmax) of 380 +/- 80 fmol/mg protein and an apparent Kd 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 microM nisoxetine, low-affinity binding where Bmax and Kd 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.

Antidepressant agents with properties to inhibit 5-hydroxytryptamine (5-HT, serotonin) uptake in brain tissue and platelets bind with high affinities to neuronal and platelet membranes. [3H]Imipramine, [3H]paroxetine and [3H]citalopram label specific binding sites related to the 5-HT transporter. [3H]Paroxetine and [3H]citalopram appear to be better ligands than [3H]imipramine. The former label a homogenous population of binding sites, whereas the displaceable binding of [3H]imipramine is heterogenous. Recent observations in several laboratories, which have taken the heterogeneity of [3H]imipramine binding into account, indicate that the binding of antidepressants to the 5-HT transporter probably occurs to the same site that binds 5-HT for transport and not to a separate site as previously suggested. Additional bonds to subsites in close vicinity to the 5-HT recognition site may contribute to the binding. No convincing evidence has been presented of the existence of an endogenous ligand other than 5-HT itself that binds to the [3H]imipramine binding site. Recent studies also suggest that repeated treatment of rats with antidepressant agents does not produce any alterations of the binding of [3H]imipramine or [3H]paroxetine to membranes of cerebral cortex. It is also doubtful whether the density of the 5-HT uptake site in platelets measured with these ligands is decreased in affective disorders as first reported.

Aging was associated with an increase in the density of specific binding sites for [3H]imipramine in postmortem specimens of human hypothalamus, frontal cortex, and parietal cortex. In general, [3H]imipramine binding was not affected by factors considered difficult to control in postmortem studies, i.e., time from death to autopsy and cause of death. The in vitro regulation of [3H]imipramine binding by sodium was impaired with age in hypothalamic homogenates. In vitro regulation of [3H]imipramine binding by chloride was intact. Determination of the concentrations of 5-hydroxytryptamine (serotonin) and 5-hydroxyindoleacetic acid in hypothalamus and frontal cortex indicated no apparent age-related changes in indole metabolism. The age-related increase in brain [3H]imipramine binding and impairment in the in vitro regulation of binding by ions are similar to changes observed previously in aged mouse brain. The increase in brain antidepressant binding sites is discussed in relationship to other indices of brain serotonergic function in aging and to the relationship of [3H]imipramine binding and depression.

Cerebral microvessels were prepared from fresh and frozen human brain samples obtained from autopsy cases. Structural integrity and purity of the microvessels were confirmed by light and electron microscopy, and by measurement of the enzymatic marker gamma-glutamyltranspeptidase. Similar morphological and enzymatic characteristics were found for the microvessels prepared from fresh and frozen brain samples. Radioligand binding experiments indicated the presence both in the "fresh" and "frozen" microvessel preparations of specific alpha 1-, alpha 2-, and beta-adrenergic, histamine H1, serotonin S1 and imipramine binding sites, although the density of beta-adrenergic and histamine H1 specific binding sites were lower in the frozen samples than in the fresh samples. Low levels of specific binding to muscarinic, GABAergic and serotonin S2 sites (with respect to the specific binding densities in the crude homogenates) were found in the microvessel preparations.