Modulation of neuronal serotonin uptake by a putative endogenous ligand of imipramine recognition sites.

Abstract

Imipramine inhibits the serotonin uptake by binding with high affinity to regulatory sites of this uptake located on axons that release serotonin. The number of imipramine recognition sites located on crude synaptic membrane preparations is reduced by two daily injections of imipramine or desmethylimipramine for 3 weeks. When the binding sites for [3H]imipramine are down-regulated the Vma of the neuronal uptake of serotonin is increased. Moreover, in minces prepared from the brain hippocampus of rats receiving imipramine in a dose regimen that reduces the number of [3H]imipramine recognition sites, the efficiency of imipramine as a blocker of the serotonin uptake is diminished. Hence the high-affinity binding sites for [3H]imipramine may have a physiological role in modulation of serotonin reuptake. Probably this is mediated by an endogenous effector of these regulatory sites. A nonpeptidic constituent of rat brain capable of displacing [3H]imipramine from its high-affinity binding site and of inhibiting the serotonin uptake in a dose-related manner has been extracted and its partial purification is described.Imipramine and desmethylimipramine (DMI) have been used for longer than two decades to relieve the symptoms of depression (1, 2). Their capacity to block norepinephrine (NE) and serotonin (5-hydroxytryptamine; 5HT) uptake has been considered to be important for their therapeutic action, but when it was noted that imipramine and DMI block amine uptake almost instantaneously, whereas their antidepressant action occurs with a latency time of 1-2 weeks (1), the importance of the uptake inhibition in explaining their therapeutic action was deemphasized. Upon repeated daily administration to rats imipramine and a number of its congeners down-regulate the function of NE recognition sites in several brain structures (3-6); because the onset of this down-regulation and that of the antidepressant action are delayed with respect to the beginning of the treatment, it has been suggested that the NE response downregulation and the antidepressant action may be related (6). An additional tool to study the mechanism of the down-regulation of /3-adrenergic receptors became available when Langer and his colleagues reported that specific Na+-dependent and highaffinity binding sites for [3H]imipramine are located in crude synaptic membranes prepared from various structures of mammalian brains (7,8). The maximal number, Bmax, of [3H]imipramine binding sites is down-regulated by repeated injections of imipramine or DMI (9-11). These recognition sites are located on 5HT axon terminals (12)(13)(14) and appear to be anatomically and functionally associated with the 5HT uptake mechanism (15)(16)(17).In 1981 at a symposium on the mode of action of antidepressants we reported that the selective destruction of 5HT axons by 5,7-dihydroxytryptamine (5,7-DHT), which eliminates[3H]imipramine recognition sites, prevented the down-regulation of f3-adrenergic recognition sites elicited by two daily injections of DMI repeated for 3 wee...

Selective effects of thiol reagents on the binding sites for imipramine and neurotransmitter amines in the rat brain

“…Taken together, the present results suggest that imipramine binding and 5-HT transport systems share a similarly reacting protein. This is offurther relevance for the purported functional and anatomical association of imipramine binding sites with the 5-HT presynaptic uptake system Paul et al, 1981;Barbaccia et al, 1983). Preliminary experiments with the noradrenaline uptake system seem to indicate that changes in [3H]-5-HT uptake were specific (unpublished results).…”

Interaction of Putative Endogenous Tryptolines with the Hypothalamic Serotonergic System and Prolactin Secretion in Adult Male Rats

“…Re cently tryptolines have also been shown to bind to the JHimipramine ('H-IMI) recognition site [23,25] which is lo cated presynaptically on 5HT neurons and is associated with the 5HT uptake molecular complex [24]. Even if the precise functional significance and the endogenous ligand of 3H-IMI binding sites are still unknown [4], recent reports indicate that these sites may ailosterically modulate the 5HT transport mechanism [34,37]. 5-MeOT has been shown to interact with 3H-IM1 recognition and 5HT uptake sites through the mechanisms of competitive and noncom petitive inhibition, respectively [44].…”

Biochemical evidence for the involvement of central serotonergic neurotransmission in the action of the antidepressant drug Medifoxamine

“…[3H]-5-HT uptake was studied as described by Barbaccia et al [1983] after minor modifications. Microprisms were incubated for 5 min at 37°C under gentle agitation with [3H]-5-HT.…”