5-HT2A receptor SPECT imaging with [¹²³I]R91150 under P-gp inhibition with tariquidar: More is better?

Alterations in dopamine system and in its connectivity with serotonin in a rat model of Alzheimer's disease

Dopamine pathways alterations are reported in Alzheimer’s disease. However, it is difficult in humans to establish when these deficits appear and their impact in the course of Alzheimer’s disease. In the TgF344-Alzheimer’s disease rat model at the age of 6 months, we showed a reduction in in vivo release of striatal dopamine due to serotonin 5HT_2A-receptor blockade, in the absence of alterations in 5HT_2A-receptor binding, suggesting a reduction in 5HT_2A-receptor-dopamine system connectivity. In addition, a functional hypersensitivity of postsynaptic dopamine D₂-receptors and D₂-autoreceptors was also reported without any change in D₂-receptor density and in the absence of amyloid plaques or overexpression of the 18 kDa translocator protein (an inflammatory marker) in areas of the dopamine system. Citalopram, a selective serotonin reuptake inhibitor, induced functional 5HT_2A-receptor−D₂-receptor connectivity changes but had no effect on D₂-autoreceptor hypersensitivity. In older rats, dopamine cell bodies overexpressed translocator protein and dopamine projection sites accumulated amyloid. Interestingly, the 5HT_2A-receptor density is decreased in the accumbens subdivisions and the substantia nigra pars compacta. This reduction in the striatum is related to the astrocytic expression of 5HT_2A-receptor. Our results indicate that both serotonin/dopamine connectivity and dopamine signalling pathways are dysregulated and potentially represent novel early diagnostic and therapeutic avenues.

Effect of 5-HT2A receptor antagonism on levels of D2/3 receptor occupancy and adverse behavioral side-effects induced by haloperidol: a SPECT imaging study in the rat

Several studies suggested that 5-HT_2A receptor (5-HT_2AR) blockade may provide a more favorable efficacy and side-effect profile to antipsychotic treatment. We hypothesized that a combined haloperidol (a D_2/3 receptor (D_2/3R) antagonist) and MDL-100,907 (a 5-HT_2AR antagonist) treatment would reverse the side effects and the neurochemical alterations induced by haloperidol alone and would potentialize its efficacy. We thus chronically treated male Mdr1a knock-out rats with several doses of haloperidol alone or in combination with a saturating dose of a MDL-100,907. Receptor occupancy at clinically relevant levels was validated with a dual-radiotracer in-vivo SPECT imaging of D_2/3R and 5-HT_2AR occupancy. Experimental tests of efficacy (dizocilpine-disrupted prepulse inhibition (PPI) of the startle reflex) and side effects (catalepsy, vacuous chewing movements) were performed. Finally, a second dual-radiotracer in-vivo SPECT scan assessed the neurochemical changes induced by the chronic treatments. Chronic haloperidol failed to reverse PPI disruption induced by dizocilpine, whilst administration of MDL-100,907 along with haloperidol was associated with a reversal of the effect of dizocilpine. Haloperidol at 0.5 mg/kg/day and at 1 mg/kg/day induced catalepsy that was significantly alleviated (by ~50%) by co-treatment with MDL-100,907 but only at 0.5 mg/kg/day dose of haloperidol. Chronic haloperidol treatment, event at doses as low as 0.1 mg/kg/day induced a significant upregulation of the D_2/3R in the striatum (by over 40% in the nucleus accumbens and over 20% in the caudate-putamen nuclei), that was not reversed by MDL-100,907. Finally, an upregulation of 5-HT_2AR after chronic haloperidol treatment at a moderate dose only (0.25 mg/kg/day) was demonstrated in frontal cortical regions and the ventral tegmental area. Overall, a partial contribution of a 5-HT_2AR antagonism to the efficacy and side-effect profile of antipsychotic agents is suggested.

In vivo absolute quantification of striatal and extrastriatal D2/3 receptors with [123I]epidepride SPECT

Background

[¹²³I]epidepride is a high-affinity radiotracer used in single-photon emission computed tomography (SPECT) imaging of the D_2/3 receptors. It binds with high affinity to striatal and extrastriatal receptors. Nevertheless, its slow kinetics in the striatum impedes quantification in this region. Thus, an approach that would allow a simultaneous quantification of both striatal and extrastriatal D_2/3 receptors would be of interest for preclinical and clinical SPECT neuroimaging. We describe a partial saturation protocol that allows us to produce an in vivo Scatchard plot and thus estimate B_avail and appK_d separately in both striatal and extrastriatal regions, through a single dynamic SPECT session. To validate this approach, a multi-injection protocol is used for the full kinetic modeling of [¹²³I]epidepride using a two-tissue compartment, 5-parameter model (2T-5k).

Methods

Eighteen male rats were used. Binding parameters were estimated using the multi-injection protocol. Various simulations were performed to estimate the optimal conditions for the partial saturation protocol, which was applied at the region and voxel level. The results of the partial saturation study were compared to those obtained with the 2T-5k model. To illustrate the interest of the partial saturation approach, we performed a preliminary study of the effect of a chronic, subcutaneous administration of haloperidol (1 mg/kg/day), a D₂ receptor antagonist, on the B_avail of [¹²³I]epidepride in the rat striatum.

Results

A series of simulations demonstrated that a mass of 3 ug/kg of unlabeled epidepride allows the formation of an in vivo Scatchard plot. The partial saturation study led to robust estimations of B_avail in all brain regions that highly correlated (r = 0.99) with the corresponding values from the multi-injection study. A chronic haloperidol treatment resulted in a 17.9% increase in the B_avail values in the left Caudate Putamen nucleus (CP) (p = 0.07) and a 13.8% increase in the right CP (p = 0.12).

Conclusion

A partial saturation method allowed the robust quantification of D_2/3 receptors in striatal and extrastriatal D_2/3 receptors with a single-scan approach. This approach may be applied in the mapping of the D_2/3 receptor in translational biological studies and potentially, in clinical SPECT imaging.

New Trends and Current Status of Positron-Emission Tomography and Single-Photon-Emission Computerized Tomography Radioligands for Neuronal Serotonin Receptors and Serotonin Transporter

The critical role of serotonin (5-hydroxytryptamine; 5-HT) and its receptors (5-HTRs) in the pathophysiology of diverse neuropsychiatric and neurodegenerative disorders render them attractive diagnostic and therapeutic targets for brain disorders. Therefore, the in vivo assessment of binding of 5-HT receptor ligands under a multitude of physiologic and pathologic scenarios may support more-accurate identification of disease and its progression and the patient's response to therapy as well as the screening of novel therapeutic strategies. The present Review aims to focus on the current status of radioligands used for positron-emission tomography (PET) and single-photon-emission computerized tomography (SPECT) imaging of human brain serotonin receptors. We further elaborate upon and emphasize the attributes that qualify a radioligand for theranostics on the basis of its frequency of use in clinics, its benefit to risk assessment in humans, and its continuous evolution, along with the major limitations.