Effects of single and repeated administration of 1,2,3,4-tetrahydroisoquinoline analogs on the binding of [11C]raclopride to dopamine D2 receptors in the mouse brain

Effects of 1-methyl-1, 2, 3, 4-tetrahydroisoquinoline on a diabetic neuropathic pain model

Background: Neuropathy is a prevalent and debilitating complication of poorly managed diabetes, contributing towards poor quality of life, amputation risk, and increased mortality. The available therapies for diabetic neuropathic pain (DPN) have limitations in terms of efficacy, tolerability and patient compliance. Dysfunction in the peripheral and central monoaminergic system has been evidenced in various types of neuropathic and acute pain. The objective of the present study was to investigate 1-methyl 1, 2, 3, 4-tetrahydroisoquinoline (1MeTIQ), an endogenous amine found in human brain with a known neuroprotective profile, in a model of streptozotocin (STZ) induced neuropathic pain. Methods: Diabetic neuropathy in male BALB/c mice was induced by intraperitoneal injection of a single dose of STZ (200 mg/kg). Upon development of DPN after 4 weeks, mice were investigated for mechanical allodynia (von Frey filament pressure test) and thermal hyperalgesia (tail immersion test). Ondansetron (1.0 mg/kg i.p.), naloxone (3.0 mg/kg i.p.) and yohimbine (2.0 mg/kg i.p.) were used to elucidate the possible mechanism involved. Postmortem frontal cortical, striatal and hippocampal tissues were dissected and evaluated for changes in levels of dopamine, noradrenaline and serotonin using High-Performance Liquid Chromatography (HPLC) with UV detection. Results: Acute administration of 1MeTIQ (15-45 mg/kg i.p.) reversed streptozotocin-induced diabetic neuropathic static mechanical allodynia (von Frey filament pressure test) and thermal hyperalgesia (tail immersion test), these outcomes being comparable to standard gabapentin. Furthermore, HPLC analysis revealed that STZ-diabetic mice expressed lower concentrations of serotonin in all three brain regions examined, while dopamine was diminished in the striatum and 1MeTIQ reversed all these neurotransmitter modifications. These findings suggest that the antihyperalgesic/antiallodynic activity of 1MeTIQ may be mediated in part via supraspinal opioidergic and monoaminergic modulation since they were naloxone, yohimbine and ondansetron reversible. Conclusion: It was also concluded that acute treatment with 1MeTIQ ameliorated STZ-induced mechanical allodynia and thermal hyperalgesia and restored brain regionally altered serotonin and dopamine concentrations which signify a potential for 1MeTIQ in the management of DPN.

1-Methyl-1,2,3,4-tetrahydroisoquinoline, an endogenous amine with unexpected mechanism of action: new vistas of therapeutic application

This review outlines the effects of 1,2,3,4-tetrahydroisoquinoline (TIQ) and its derivative, 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), endogenous substances imbued with high pharmacological potential and broad spectrum of action in brain. 1MeTIQ has gained special interest as a neuroprotectant, and its ability to antagonize the behavioral syndrome produced by well-known neurotoxins (e.g., MPTP; rotenone). This review is thus focused on mechanisms of action of 1MeTIQ in behavioral, neurochemical, and molecular studies in rodents; also, effects of TIQ and 1MeTIQ on dopamine metabolism; and neuroprotective properties of TIQ and 1MeTIQ in vitro and in vivo. Finally, antiaddictive properties of 1MeTIQ will be described in cocaine self-administered rats. Findings implicate TIQ and especially its methyl derivative 1MeTIQ in unique and complex mechanisms of neuroprotection in various neurodegenerative illnesses of the central nervous system. We believe that MAO inhibition, free radicals scavenging properties, and antagonism to the glutamatergic system may play an essential role in neuroprotection. In addition, the results strongly support the view that 1MeTIQ has a considerable potential as a drug for combating substance abuse, through the attenuation of craving.

Synthesis and neurotoxicity of tetrahydroisoquinoline derivatives for studying Parkinson's disease

Parkinson's disease involves the progressive degeneration of dopaminergic neurons in the substantia nigra. However, the etiology of the disease remains to be elucidated. Endogenous amines, such as 1,2,3,4-tetrahydroisoquinoline (TIQ) derivatives present in the mammalian brain, are known to participate in the pathogenesis of Parkinson's disease. These endogenous neurotoxins have been extensively studied because of their structural resemblance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), an agent widely used for generating animal models of Parkinson's disease-like symptoms. Investigations of the synthesis and pharmacological properties of TIQ derivatives are expected to contribute to the development of new therapeutic agents for treating Parkinson's disease. In the present study, we describe more efficient synthesis methods for TIQ derivatives via Pummerer-type cyclization of the substrate N-acyl sulfoxide. Furthermore, the modified Pummerer reaction provided a convenient and efficient method for synthesizing various TIQs. TIQ and its derivative, 1-benzyl-TIQ, can induce parkinsonism in primates and rodents. On the other hand, one TIQ derivative, 1-methyl-TIQ, has been shown to prevent MPTP, TIQ, and 1-benzyl-TIQ induced behavioral abnormalities. Therefore, TIQ derivatives are considered to play an important role in both the onset and prevention of Parkinson's disease. In this article, we focus on the synthesis and pharmacological aspects of 1,2,3,4-tetrahydroisoquinoline derivatives in Parkinson's disease.

Investigation of the use of positron emission tomography for neuroreceptor imaging in rabbit eyes

To determine whether positron emission tomography (PET) can be used for imaging of neuroreceptors in eyes of rabbits. PET imaging of dopamine D(2) receptor, dopamine transporter, serotonin(1A) receptor and sigma(1) receptor in the eyes and brain was performed using corresponding positron-emitting ligands in baseline, pretreatment and displacement conditions. The 4 radioligands outlined the eyes and brain in the baseline. Pretreatment resulted in a slight reduction (26-28%) in the uptake in the anterior segments of eyes. The binding of each radioligand in the iris-ciliary body and retina was confirmed by ex vivo autoradiography. However, the PET signal in the eyes was unexpectedly higher than the autoradiography signal. The identification of radioligand-neuroreceptor binding by PET in the rabbit eyes is not specific enough.

Evaluation of O-[11C]methyl-L-tyrosine and O-[18F]fluoromethyl-L-tyrosine as tumor imaging tracers by PET

We investigated the potential of O-[(11)C]methyl-L-tyrosine and O-[(18) F]fluoromethyl-L-tyrosine as positron-emitting tracers for tumor imaging. The two tracers had similar distribution patterns in rats bearing AH109A hepatoma, with pancreas and, on a lesser extent, AH109A showing the highest uptake. Uptake of both tracers in the AH109A and uptake ratios of AH109A-to-tissues (with the exception of AH109A-to-bone) gradually increased for 60 min. O-[(11)C]methyl-L-tyrosine was metabolically stable, whereas a negligible low amount of metabolites was observed for O-[(18)F]fluoromethyl-L-tyrosine. Both tracers showed the potential for tumor imaging.

Quantitative measurement of histamine H1 receptors in human brains by PET and [11C]doxepin

The aim of this study is to establish a method for quantitative measurement of histamine H(1) receptor (H1R) in human brain by PET and [(11)C]doxepin ([(11)C]DOX). The estimated parameters with a two-compartment model were stable for the initial values for parameter estimation but those with a three-compartment model were not. This finding suggests that the H1R measured by the [(11)C]DOX and PET can be evaluated with a two-compartment model.

Age-related changes of the [11C]CFT binding to the striatal dopamine transporters in the Fischer 344 rats: a PET study

We investigated the age-related changes of the binding of [11C]CFT to striatal dopamine transporters (DATs) in vivo in Fischer 344 rats by positron emission tomography (PET). The tissue dissection method represented an age-related decrease in the uptake ratio of the striatum to the cerebellum and in the specific binding-to-nonspecific binding ratio of [11C]CFT. PET demonstrated an age-dependent decrease in the striatal uptake of [11C]CFT, however, the kinetic analysis represented the age-related decrease in both the association rate constant (k3) and dissociation rate constant (k4), but not the binding potential (k3/k4) that was a parameter including both of density and affinity of the binding sites. The PET finding was not necessarily coincident with the result investigated in vitro previously. Therefore, careful interpretation is necessary for PET studies using [11C]CFT and small animals such as rats.

Sigma1 and dopamine D2 receptor occupancy in the mouse brain after a single administration of haloperidol and two dopamine D2-like receptor ligands

We investigated sigma(1) and dopamine D(2) receptor occupancy in mouse brain after a single injection of haloperidol, nemonapride, or spiperone using [(11)C]SA4503 and [(11)C]raclopride, respectively. Co-injection of the three compounds significantly blocked the uptake of each radioligand. Six hours later, only haloperidol blocked [(11)C]SA4503 uptake, while all three reduced [(11)C]raclopride uptake. Sigma(1) receptor occupancy by haloperidol was reduced to 19% at day 2 when D(2) receptor occupancy disappeared. [(11)C]SA4503 would be applicable to the investigation of sigma(1) receptor occupancy of antispychotic drugs using PET.

No reduction of dopamine transporter binding sites in mice following treatment with the TIQ analogue 1-benzyl-1,2,3,4-tetrahydroisoquinoline

1-Benzyl-1,2,3,4-tetrahydroisoquinoline (1-BnTIQ) and TIQ are endogenous substances inducing bradykinesia, one of the symptoms of parkinsonism, in rodents and primates, and 2-methyl-TIQ is postulated to be an active form of TIQ. We investigated the effect of 1-BnTIQ-, TIQ- or 2-methyl-TIQ-treatment on the binding of 2-beta-carbomethoxy-3-beta-(4-fluorophenyl)-[N-methyl-11C]tropane to striatal dopamine transporters (DATs) in mice. Neither 1-BnTIQ (80 mg/kg, i.p., twice per day for 10 days) nor 2-methyl-TIQ (40 mg/kg, i.p., twice per day for 10 days) affected the radioligand-DAT binding, while TIQ (80 mg/kg, i.p., twice per day for 10 days) induced a 14% decrease. These results indicate that 1-BnTIQ does not affect the density of DATs on dopaminergic neurons, and that it is not clear whether or not 2-methyl-TIQ is an active form of TIQ.