Kinetic modeling of [99mTc]TRODAT-1: a dopamine transporter imaging agent

[99mTc]Technetium[2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo [3.2.1] oct-2-yl]-methyl] (2-mercaptoethyl) amino] ethyl] amino] ethane-thiolato(3-)-N2,N2',S2,S2']oxo-[1R-(exo-exo)] ([99mTc] TRODAT-1) is a useful imaging agent for central nervous system dopamine transporters. The purpose of this study was to characterize the in vivo binding potential and kinetic rate constants of this agent in nonhuman primates.

METHODS

A series of four SPECT scans were performed on each of two female baboons with a bolus injection of [99mTc]TRODAT-1 (717+/-78 MBq; 19.38+/-2.12 mCi). Dynamic images of the brain were acquired over 4 h using a triple-head camera equipped with fan-beam collimators. Arterial and venous blood were sampled frequently using a peristaltic pump throughout the duration of the study. Regions of interest were drawn on the corresponding MRI scan to which each functional image was coregistered. Using analytical solutions to the three-compartment model with the Levenberg-Marquardt minimization technique, each study was individually fitted to a kinetic parameter vector (method I). Additionally, within each subject, three corresponding intrasubject studies were fitted simultaneously to a single parameter vector by constraining the binding potential, distribution volume and dissociation rate constant to improve the identifiability of the parameter estimates (method II).

RESULTS

The results clearly indicated that [99mTc] TRODAT-1 localized in the striatum with slower washout rate than other brain regions. A maximal target/nontarget ratio of 3.5 between striatum and cerebellum was obtained. SPECT image analysis of the striatum yielded unconstrained k3/k4 values of 3.4+/-1.4, 2.4+/-0.7, 3.0+/-1.5, and 4.0+/-10.3, with respective constrained (fixed k4) values of 2.9 +/- 0.4, 2.4 +/- 0.4, 1.7+/-0.4 and 1.8+/-0.4 in one baboon using method I. With method II, the corresponding simultaneously fitted values were 2.1+/-0.3 using no constraints and 2.2+/-0.2 using a fixed k4. The second baboon had similar results.

CONCLUSION

These findings suggest that the binding potential and corresponding kinetic rate constants can be reliably estimated in nonhuman primates with dynamic SPECT imaging of the dopamine transporter using a technetium-based tropane analogue. Furthermore, method II parameter vectors compare favorably to those produced using method I based on SEEs.

Thyrotropin-releasing hormone phase shifts circadian rhythms in hamsters

The role of thyrotropin-releasing hormone (TRH) in regulating circadian rhythms was investigated by assessing the ability of TRH microinjections into the suprachiasmatic nucleus (SCN) to induce phase shifts in hamster wheel-running behavior. TRH injected into the SCN at 10 and 100 nM doses produced phase advances in wheel-running activity of 18.3 +/- 1.9 and 34.8 +/- 2.9 minutes, respectively, when administered at circadian time (CT) 6. Injections at CT 18 produced no effects. The temporal sensitivity of the SCN to TRH administration was examined by administering TRH at specific circadian times. TRH produced significant phase advances at CT 4, 6, and 8, while no significant changes in wheel-running onset were observed at other CT times. These studies represent the first evidence of TRH's ability to affect circadian function.

Alterations in TRH receptors in temporal lobe of schizophrenics: a quantitative autoradiographic study

We utilized quantitative autoradiography to determine the distribution of receptors for thyrotropin-releasing hormone (TRH) throughout the human temporal lobe and to examine the distribution of these receptors in discrete subregions of the temporal lobe from patients diagnosed premortem with schizophrenia. When compared to non-neurologic controls, schizophrenic patients demonstrated an increase of 51% in the concentration of TRH receptors in the molecular layer of the dentate gyrus. Within nuclei of the schizophrenic amygdala, marked decreases were found in the central (44%), medial (38%), cortical (36%), accessory cortical (52%), lateral (54%), and medial basal (22%) nuclei. We also examined postmortem brain samples from patients with Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease for alterations in the distribution of TRH receptors. No significant differences from non-neuropsychiatric controls were noted within the hippocampus in any of these disease states; however, slight alterations were noted in the central and medial basal amygdala in Huntington's disease and in the cortical amygdala in Alzheimer's disease. These disease-specific findings suggest that TRH may play a role in the neurochemical dysfunction of schizophrenia.

Serotonin uptake sites and serotonin receptors are altered in the limbic system of schizophrenics

Serotonin (5-HT) uptake sites were mapped by autoradiographic means with [3H]cyano-imipramine ([3H]CN-IMI), the 5-HT1A receptor with [3H]8-hydroxy-2-[di-n-propyl-amino]tetralin ([3H]8-OH-DPAT), and the 5-HT2 receptor with both [3H]ketanserin and [125I]lysergic acid diethylamide ([125I]LSD) in eight nonneurologic controls and 10 cases with a diagnosis of schizophrenia. In the striatum, there was a marked heterogeneous patterning of 5-HT uptake sites that corresponded to the striosomal/matrix compartmentalization of the striatum. This organization was not matched with an equally heterogeneous pattern of either 5-HT2 or 5-HT1A receptors. For the isocortex, a general organizational scheme was observed with the 5-HT1A receptor expression high in the external laminae and deep laminae, but 5-HT2 receptor expression was higher in the internal laminae. There was a laminar distribution of 5-HT uptake sites that approximated the combined distributions of the 5-HT1A receptor and the 5-HT2 receptor. In the parahippocampal gyrus and hippocampus, the distribution of 5-HT uptake sites was complementary to the distribution of 5-HT1A and 5-HT2 receptors. In schizophrenic cases, there was a large increase in the number and altered striosomal/matrix organization of 5-HT uptake sites in the striatum. There was also an increase in the numbers of 5-HT2 receptors in the nucleus accumbens and ventral putamen of the schizophrenics. The number of 5-HT1A receptors was not modified. There was a marked reduction in 5-HT uptake sites in the external and middle laminae of the anterior cingulate, frontal cortex, and posterior cingulate, and no changes were observed in the motor cortex, temporal cortex, or hippocampus. Increased numbers of 5-HT1A receptors were found in the posterior cingulate, motor cortex, and hippocampus. Serotonin2 receptors were substantially elevated in the posterior cingulate, temporal cortex, and hippocampus, but not in the frontal, anterior cingulate, or motor cortices. Examination of the temporal lobe and hippocampus of a group of nonschizophrenic suicides (n = 8) indicated the alterations in 5-HT system in the limbic regions of the striatum, the limbic cortex, and hippocampus of the schizophrenic cases may be disease specific.

Distribution of beta-adrenergic receptor subtypes in human post-mortem brain: alterations in limbic regions of schizophrenics

The distribution of the beta 1 (beta 1) and beta 2 (beta 2) subtypes of the beta-adrenergic receptor was examined in rat and nondiseased control human tissue. The distribution of the beta 1 and beta 2 receptors was also examined in schizophrenic cases, with additional studies in schizophrenic suicide and nonschizophrenic suicide cases. Scatchard analysis of the binding of [125I]iodopindolol (IPIN) to cortical membranes showed a similar Kd in human (177 pM) and rat (161 pM), but a lower maximum binding site (Bmax) in the human tissue (18.7 fmol/mg protein and 55.6 fmol/mg protein). For the autoradiographic studies [125I]IPIN was used to visualize both subtypes (total) or was displaced with the selective beta 1-receptor antagonist ICI-89,406 to visualize beta 2 sites, or with the selective beta 2-receptor antagonist ICI-118,551 to visualize beta 1 sites. Important differences in the regional distribution of the two subtypes of the beta-adrenergic receptors were noted between rat and human. In the nucleus accumbens and ventral putamen (ventral striatum), a patchy distribution of beta 1 receptors was observed that was not evident in the rat. These patches were aligned with markers of the matrix compartment of the striatum. The schizophrenic cases showed significant increases in the labeling of the beta 1-receptor patches with [125I]IPIN. In contrast to the frontal cortex of the nondisease controls, the parietal and temporal cortex showed a high ratio of beta 1 to beta 2 receptors and a highly laminar organization of the subtypes. [125I]IPIN binding to beta 1 receptors was highest in the external laminae with the reverse gradient for the beta 2 subtype. The medial temporal cortex displayed an alteration in the ratio of the 2 subtypes of the beta-adrenergic receptor, with the parahippocampus and hippocampus of the human, in contrast to the rat brain, predominantly expressing the beta 2 receptor. Moreover, there were consistently higher densities of beta 2 receptors in the hippocampus of the right hemisphere than the left hemisphere of the nondisease controls. There was not a left and right hemispheric asymmetry of beta 2 receptors in the hippocampus of elderly schizophrenics or in young schizophrenics who committed suicide. The asymmetry was evident in nonschizophrenic suicides, suggesting that the lack of asymmetry in the hippocampus of schizophrenics is evident early in the disease process. Thus limbic structures show alterations in the patterning of beta 1 and beta 2 receptors in the schizophrenic cases.

Organization of dopamine D1 and D2 receptors in human striatum: receptor autoradiographic studies in Huntington's disease and schizophrenia

The technique of quantitative autoradiography was used to examine the effects of Huntington's disease (HD) and schizophrenia on the organization of striatal dopamine (DA) D1 and D2 receptors. Whereas the striatum of HD cases showed a reduction in the density of D1 ([3H]SCH 23390) and D2 ([3H]spiroperidol) receptors, the patterning of D2 receptor loss did not match that of the D1 receptor loss. The HD loss of D1 D1 receptors (65%) is far greater than the loss of D2 receptors (28%). Whereas there was a dorsal-ventral gradient of effect on both receptor subtypes, the effects of HD on D2 receptors in the ventral putamen (PUT) and nucleus accumben septi (NAS) were minimal. Similarly, muscarinic M1 and M2 receptors demonstrate different patterns of alteration in HD. The M2 subtype, labeled with [3H]N-methylscopolamine (in the presence of excess pirenzepine to occlude M1 sites), was depleted far more than the M1 receptor subtype, labeled with [3H]pirenzepine. Although the effects of HD on [3H]mazindol labeling of DA terminals were more heterogeneous, there appeared to be a relative preservation of this afferent input to the striatum of the HD cases. In the schizophrenic cases, our autoradiographic studies confirm previous reports of an elevation of D2 receptor density in the striata of many schizophrenics. This increase was evident even though two of the three cases were known to have not been treated with neuroleptics, and the third case may also have been drug naive. However, the increase was far greater in the NAS (164%) and ventral PUT (173%) than more dorsally in the striatum (68%). The density of D1 receptors and DA terminals labeled with [3H]mazindol in the striatum of schizophrenics was not significantly different from that of control cases. Thus in both HD and schizophrenia, the ratio of D2/D1 receptors is altered in favor of the D2 population, particularly in the NAS.