Richard L. Ehrenkaufer

Social dominance in monkeys: Dopamine D2 receptors and cocaine self-administration

Disruption of the dopaminergic system has been implicated in the etiology of many pathological conditions, including drug addiction. Here we used positron emission tomography (PET) imaging to study brain dopaminergic function in individually housed and in socially housed cynomolgus macaques (n = 20). Whereas the monkeys did not differ during individual housing, social housing increased the amount or availability of dopamine D2 receptors in dominant monkeys and produced no change in subordinate monkeys. These neurobiological changes had an important behavioral influence as demonstrated by the finding that cocaine functioned as a reinforcer in subordinate but not dominant monkeys. These data demonstrate that alterations in an organisms environment can produce profound biological changes that have important behavioral associations, including vulnerability to cocaine addiction.

PET imaging of dopamine D2 receptors during chronic cocaine self-administration in monkeys

Dopamine neurotransmission is associated with high susceptibility to cocaine abuse. Positron emission tomography was used in 12 rhesus macaques to determine if dopamine D2 receptor availability was associated with the rate of cocaine reinforcement, and to study changes in brain dopaminergic function during maintenance of and abstinence from cocaine. Baseline D2 receptor availability was negatively correlated with rates of cocaine self-administration. D2 receptor availability decreased by 15–20% within 1 week of initiating self-administration and remained reduced by ∼20% during 1 year of exposure. Long-term reductions in D2 receptor availability were observed, with decreases persisting for up to 1 year of abstinence in some monkeys. These data provide evidence for a predisposition to self-administer cocaine based on D2 receptor availability, and demonstrate that the brain dopamine system responds rapidly following cocaine exposure. Individual differences in the rate of recovery of D2 receptor function during abstinence were noted.

Social Stress, Depression, and Brain Dopamine in Female Cynomolgus Monkeys

Socially subordinate adult female cynomolgus monkeys are hypercortisolemic, the targets of aggression, fearful, vigilant, receive little positive affiliative contact, exhibit pathological behaviors indicating anxiety, and are disengaged in the social events around them. Subordinates also have altered dopaminergic activity that may be due to decreased D2 receptor binding. Dopaminergic activity indices were more closely associated with affiliative than agonistic behaviors.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: In vitro binding studies and positron emission tomography studies in nonhuman primates

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)‐4‐fluorobenzyltrozamicol ([18F](+)‐FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)‐ and (−)‐ stereoisomers of FBT for the VAcChT as well as sigma (σ2 and σ2) receptors. (+)‐FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for σ1 (21.6 nM) and σ2 (35.9 nM) receptors, whereas (−)‐FBT had similar affinities for the VAcChT and σ1 receptors (∼20 nM) and a lower affinity for σ2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)‐FBT. [18F](+)‐FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)‐FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood‐brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)‐FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT. Synapse 25:368–380, 1997.

[18F]N-4'-fluorobenzyl-4-(3-bromophenyl) acetamide for imaging the sigma receptor status of tumors: comparison with [18F]FDG and [125I]IUDR

Abstract A series of biodistribution studies were conducted with the radiotracer, [ 18 F]N-(4′-fluorobenzyl)-4-(3-bromophenyl)acetamide, [ 18 F] 1 in nude mice bearing tumor xenografts of the mouse mammary adenocarcinoma, line 66. This radiotracer has a high affinity for both σ 1 and σ 2 receptors. In vivo studies were also conducted in order to assess the effect of blocking σ 1 receptors on tumor uptake and the tumor:background ratio of this radiotracer. The results of these studies revealed that blocking the σ 1 receptor so that only the σ 2 receptors are labeled in vivo, results in a higher tumor:background ratio with only a small reduction in the tumor uptake of the radiotracer relative to the no-carrier-added (i.e., nonselective) conditions. Comparative in vivo studies were also conducted with the anatomic and metabolic imaging agent, [ 18 F]FDG, and a radiolabeled DNA precursor, [ 125 I]IUdR. Both of these radiolabeled compounds represent classes of agents that have been proposed for imaging the proliferative status of solid tumors. The results of these studies indicated that a σ 2 -selective imaging agent may be, 1) a better anatomic imaging agent for breast cancer than [ 18 F]FDG, and 2) a better functional imaging agent than the radiolabeled DNA precursors, [ 123/124 I]IUdR and [ 11 C]thymidine, for measuring the proliferative status of breast tumors with PET and SPECT. However, additional studies will be needed to compare σ 2 -selective imaging agents with [ 18 F]FLT in order to determine which is the more appropriate imaging agent for measuring the proliferative status of breast tumors with PET.

PET imaging of dopamine D2 receptors with [18F]fluoroclebopride in monkeys: effects of isoflurane- and ketamine-induced anesthesia.

The purpose of the present study was to determine whether positron emission tomography (PET) studies in monkeys with the dopamine (DA) D2 receptor ligand [18F]fluoroclebopride (FCP) would be significantly influenced by anesthetic induction with isoflurane (∼5.0%) compared to induction with 10 mg/kg ketamine. Five experimentally-naive adult male cynomolgus monkeys (Macaca fascicularis) were trained to sit calmly in a primate restraint chair. Before the first PET scan, each monkey was anesthetized, by mask, with isoflurane. After complete sedation, the monkey was intubated and anesthesia was maintained throughout the PET study by isoflurane (∼1.5%). At least 1 month later, a second PET study was conducted in which anesthesia was induced with ketamine and maintained by isoflurane (∼1.5%). Irrespective of induction anesthetic, there was a high uptake of [18F]FCP and a linear rate of washout from the basal ganglia for all monkeys. There were also no differences in time to peak uptake (∼25 min), in clearance half-life (t1/2 = 140–164 min) or in D2 binding (distribution volume ratios of 2.48 vs. 2.50). These results indicate that induction anesthetic did not differentially affect D2 binding of [18F]FCP in monkeys. Furthermore, the low variability between studies indicates that [18F]FCP is an excellent ligand for longitudinal studies of D2 receptors in nonhuman primates.

Comparison of two fluorine-18 labeled benzamide derivatives that bind reversibly to dopamine D2 receptors: in vitro binding studies and positron emission tomography.

The purpose of the present set of studies was to characterize, in vitro and in vivo, two benzamide analogues, 2,3‐dimethoxy‐N‐[1‐(4‐fluorobenzyl)piperidin‐4yl]benzamide (MBP) and 4′‐fluoroclebopride (FCP), for studying dopamine D2 receptors with Positron Emission Tomography (PET). In vitro binding studies were conducted to determine the affinities of MBP and FCP to the three subtypes of dopamine D2 receptors: D2(long), D3, and D4 receptors. MBP was found to have a high affinity (Ki = 1–8 nM) for all three subtypes of the D2 receptor, whereas FCP had nanomolar affinity (Ki ∼ 5.5 nM) for D2(long) and D3 receptors, and a lower affinity for D4 receptors (Ki = 144 nM). In vitro binding studies also revealed that MBP had a relatively high affinity for σ1 receptors (Ki = 11 nM) compared to FCP (Ki = 340 nM). PET imaging studies were conducted in rhesus monkeys with the fluorine‐18 labeled analogues of each compound. Both [18F]MBP and [18]FCP displayed reversible binding kinetics during the 3 h time course of PET. [18]FCP was found to have a higher basal ganglia: cerebellum ratio and lower variability in the rate of washout from D2 receptors in vivo relative to [18F]MBP. Neither radiotracer was found to produce radiolabeled metabolites capable of crossing the blood‐brain barrier. The high σ1 binding affinity and low basal ganglia: cerebellum ratio of [18F]MBP indicate that this ligand may not be suitable for quantitative studies of D2 receptors. The results from the in vitro and in vivo studies indicate that [18F]FCP is a promising ligand for studying D2 receptors with PET.

Cholinergic activity of aged rhesus monkeys revealed by positron emission tomography.

In the present study, the radiotracer [18F] (+)‐4‐fluorobenzyltrozamicol ((+)‐[18F]FBT) and positron emission tomography (PET) were used to examine the vesicular acetylcholine transporter and determine if presynaptic cholinergic activity was altered with age in 23 rhesus monkeys that varied in age from 10 to 37 years. Binding of (+)‐[18F]FBT in the basal ganglia was reduced significantly with increasing age of the monkeys. However, there were individual differences noted in that some middle‐aged and aged monkeys demonstrated levels of (+)‐[18F]FBT binding that were comparable to the binding measured in adult monkeys. These data indicate that presynaptic cholinergic function may decrease with age, but that there may be a differential susceptibility of the cholinergic system to the aging process in different individuals. Synapse 39:95–100, 2001.

Fluorine‐18‐labeled tropane analogs for PET imaging studies of the dopamine transporter

A series of PET imaging studies were conducted with two fluorine‐18‐labeled tropane analoges, [18F](+)‐FTT and [18F](+)‐FCT. Both compounds possessed a high affinity and selectivity for the dopamine transporter and had a higher accumulation in the basal ganglia, a brain region having a high density of the dopamine transporter (DAT) than the cerebellum, a reference region devoid of dopaminergic terminals. [18F](+)‐FCT had a higher brain uptake and more suitable basal ganglia:cerebellum (BG:Cb) ratio than [18F](+)‐FTT. [18F](+)‐FCT also displayed reversible binding kinetics in vivo, indicating that the measurement of DAT density in vivo with PET will be relatively insensitive to changes in cerebral blood flow that can occur as a consequence of disease or prolonged cocaine abuse. The uptake of [18F](+)‐FCT was also displaced by an intravenous injection of cocaine (1.0 mg/kg), which is consistent with the labeling of the DAT in vivo by this radiotracer. These data suggest that [18F](+)‐FCT may be a suitable radiotracer for studying DAT function in vivo with PET. Synapse 37:109–117, 2000.

Morphine-induced spinal cholinergic activation: in vivo imaging with positron emission tomography.

&NA; Positron emission tomography (PET) imaging of spinal cord in monkeys with a cholinergic tracer demonstrates increased spinal cholinergic activity in response to an analgesic dose of morphine, and this PET result correlates with measurement of acetylcholine spillover into spinal cord extracellular space induced by morphine, as measured by microdialysis. Previous studies in rats, mice, and sheep demonstrate activation of spinal cholinergic neurons by systemic opioid administration, and participation of this cholinergic activity in opioid‐induced analgesia. Testing the relevance of this observation in humans has been limited to measurement of acetylcholine spillover into lumbar cerebrospinal fluid. The purpose of this study was to apply a recently developed method to image spinal cholinergic terminals non‐invasively via PET and to test the hypothesis that the tracer utilized would reflect changes in local cholinergic activity. Following Animal Care and Use Committee approval, seven adult male rhesus monkeys were anesthetized on three separate occasions. On two of the occasions PET scans were performed using [18F] (+)‐4‐fluorobenzyltrozamicol ([18F]FBT), which selectively binds to the vesicular acetylcholine (ACh) transporter in the presynaptic cholinergic terminals. PET scans were preceded by injection of either saline or an analgesic dose of IV morphine (10 mg/kg). On the third occasion, microdialysis catheters were inserted in the spinal cord dorsal horn and acetylcholine concentrations in dialysates determined before and after IV morphine injection. Morphine increased cholinergic activity in the spinal cord, as determined by blood flow corrected distribution volume of [18F]FBT in the cervical cord compared to the cerebellum. Morphine also increased acetylcholine concentrations in microdialysates from the cervical cord dorsal horn. The one animal which did not show increased spinal cholinergic activity by PET from this dose of morphine also did not show increased acetylcholine from this morphine dose in the microdialysis experiment. These data confirm the ability to use PET to image spinal cholinergic terminals in the monkey spinal cord and suggest that acute changes in cholinergic activity can be imaged with this non‐invasive technique. Following preclinical screening, PET scanning with [18F]FBT may be useful to investigate mechanisms of analgesic action in normal humans and in those with pain.