Down-regulation of tryptamine binding sites following chronic molindone administration. A comparison with responses of dopamine and 5-hydroxytryptamine receptors

Trace Amine-Associated Receptor 1 Contributes to Diverse Functional Actions of O-Phenyl-Iodotyramine in Mice but Not to the Effects of Monoamine-Based Antidepressants

Trace Amine-Associated Receptor 1 (TAAR1) is a potential target for the treatment of depression and other CNS disorders. However, the precise functional roles of TAAR1 to the actions of clinically used antidepressants remains unclear. Herein, we addressed these issues employing the TAAR1 agonist, o-phenyl-iodotyramine (o-PIT), together with TAAR1-knockout (KO) mice. Irrespective of genotype, systemic administration of o-PIT led to a similar increase in mouse brain concentrations. Consistent with the observation of a high density of TAAR1 in the medial preoptic area, o-PIT-induced hypothermia was significantly reduced in TAAR1-KO mice. Furthermore, the inhibition of a prepulse inhibition response by o-PIT, as well as its induction of striatal tyrosine hydroxylase phosphorylation and elevation of extracellular DA in prefrontal cortex, were all reduced in TAAR1-KO compared to wildtype mice. O-PIT was active in both forced-swim and marble-burying tests, and its effects were significantly blunted in TAAR1-KO mice. Conversely, the actions on behaviour and prefrontal cortex dialysis of a broad suite of clinically used antidepressants were unaffected in TAAR1-KO mice. In conclusion, o-PIT is a useful tool for exploring the hypothermic and other functional antidepressant roles of TAAR1. By contrast, clinically used antidepressants do not require TAAR1 for expression of their antidepressant properties.

A high-affinity [3H]tryptamine binding site in human brain

In vitro filtration binding revealed high-affinity specific [3H]tryptamine binding sites in human brain. These binding sites are heterogeneously distributed throughout brain, ranging from 280 fmol/mg protein in hippocampus and thalamus to approximately 90 fmol/mg protein in medulla oblongata and cerebellum. Preliminary autoradiographic studies indicate a heterogeneous distribution within layers of the frontal cortex. The observed stereoselectivity of the site, the interaction of the site with a G protein and the observed region-selective downregulation of the site in a human pathological condition, i.e. hepatic encephalopathy (Mousseau et al., 1994), suggests that this binding site is a functional [3H]tryptamine receptor. A similarity in kinetics and distribution of the [3H]tryptamine receptor in human and rat brain indicates that these two entities represent homologous structures, although the difference in pharmacological profiles suggests species variants. One cannot exclude the possibility that the rat and human [3H]tryptamine receptors do represent distinct subtypes. Finally, the suggested role for tryptamine in neuropsychiatric disorders as originally suggested by Dewhurst (1968) is supported by the present series of experiments.

Effects of low- and high-dose tranylcypromine on [3H]tryptamine binding sites in the rat hippocampus and striatum

Chronic studies were initiated in rats to determine the effects of high- and low-dose tranylcypromine (TCP) on [3H]tryptamine (3H-T) binding sites. Male Sprague-Dawley rats were administered TCP (0.5 or 2.5 mg/kg/day) or vehicle (distilled water) for 4, 10 or 28 days via Alzet minipumps. After decapitation, the hippocampus and striatum were used to prepare membrane fragments for single point 3H-T binding. Hippocampal 3H-T binding was reduced after 10 and 28 days with the low dose and after 4, 10 and 28 days with the high dose. Striatal 3H-T binding was reduced by both doses at all time intervals. The high dose resulted in a significantly greater reduction in striatal 3H-T binding than did the low-dose after 4, 10, and 28 days. These results suggest that a more rapid reduction of 3H-T binding in the hippocampus and/or a greater reduction of 3H-T binding in the striatum by high-dose TCP than by low-dose TCP may be contributing factors in the reported efficacy of the former in refractory depression.

Tryptamine: a metabolite of tryptophan implicated in various neuropsychiatric disorders

Although early interest in the biomedical relevance of tryptamine has waned in recent years, it is clear from the above discussion that the study of tryptamine is worthy of serious consideration as a factor in neuropsychiatric disorders. The study of [3H]-tryptamine binding sites indicates an adaptive responsiveness characteristic of functional receptors. The question raised by Jones (1982d) on whether tryptamine is acting centrally as a neurotransmitter or a neuromodulator still remains mostly unanswered, although the evidence cited within this review strongly suggests a modulatory role for this neuroactive amine (see also Juorio and Paterson, 1990). The synthesis and degradative pathways of tryptamine, as well as the intricate neurochemical and behavioral consequences of altering these pathways, are now more fully understood. It is not yet clear what the role of tryptamine is under normal physiological [homeostatic] conditions, however, its role during pathological conditions such as mental and physical stress, hepatic dysfunction and other disorders of metabolism (i.e. electrolyte imbalance, increased precursor availability, enzyme induction or alterations in enzyme co-factor availability) may be quite subtle, perhaps accounting for various sequelae hitherto considered idiopathic. The evidence for a primary role for tryptamine in the etiology of mental or neurological diseases is still relatively poor, although the observations that endogenous concentrations of tryptamine are particularly susceptible to pharmacological as well as physiological manipulations serve to reinforce the proposition that this indoleamine is not simply a metabolic accident but rather a neuroactive compound in its own right. Finally, one might wonder what proportion of the data attributed to modifications of 5-HT metabolism might, in fact, involve unrecognized changes in the concentrations of other neuroactive metabolites of tryptophan such as tryptamine.

Effects of age and of chronic antidepressant treatment on [3H]tryptamine and [3H]dihydroalprenolol binding to rat cortical membranes

1. The effects of age and of chronic antidepressant treatment on [3H]tryptamine and [3H]dihydroalprenolol binding site density were measured in brain cortical membranes from male Sprague-Dawley rats. 2. The density but not the affinity of [3H]tryptamine binding sites was increased in 18-month-old rats relative to 3-month-old rats. Neither the density nor the affinity of [3H]dihydroalprenolol binding sites was affected by age. 3. Chronic administration (28 days s.c. via Alzet osmotic minipumps) of tricyclic antidepressant drugs (daily doses: imipramine.HCl, 30 mg kg-1; desipramine.HCl, 10 mg kg-1; clomipramine.HCl, 10 mg kg-1) resulted in decreases in [3H]dihydroalprenolol binding site density but no changes in [3H]tryptamine binding site density; no changes in affinity of either site were observed. 4. Chronic administration (s.c. via Alzet osmotic minipumps) of monoamine oxidase inhibitor antidepressant drugs (daily doses: tranylcypromine.HCl, 0.5 and 1.0 mg kg-1; phenelzine sulfate, 5 and 10 mg kg-1, each for 28 days; clorgyline.HCl, 1.0 mg kg-1; (-)-deprenyl.HCl, 1.0 mg kg-1, each for 14 days) resulted in decreases in [3H]tryptamine binding site density, without any effects on the affinity of this site. In addition, each of these monoamine oxidase inhibitors except (-)-deprenyl resulted in a decrease in [3H]dihydroalprenolol binding site density. No affinity changes were observed. 5. These data indicate that the [3H]tryptamine binding site exhibits physiological changes with aging and is differentially sensitive to the actions of tricyclic antidepressants and monoamine oxidase inhibitor antidepressants, respectively.

The effects of chronic trimipramine treatment on biogenic amine metabolism and on dopamine D2, 5-HT2 and tryptamine binding sites in rat brain

1. Two week chronic administration of trimipramine increased the brain concentration and metabolism of dopamine and 5-hydroxytryptamine. 2. The treatment also produced a reduction in dopamine D2 and 5-hydroxytryptamine 5-HT2 receptors but no change in tryptamine binding was observed. 3. These findings suggest that trimipramine induces adaptive changes in dopamine and 5-hydroxytryptamine neurotransmission.