In vivo imaging of adenovirus-mediated over-expression of dopamine D2 receptors in rat striatum by positron emission tomography

Quantitative, noninvasive, in vivo longitudinal monitoring of gene expression in the brain by co-AAV transduction with a PET reporter gene

In vivo imaging of vector transgene expression would be particularly valuable for repetitive monitoring of therapy in the brain, where invasive tissue sampling is contraindicated. We evaluated adeno-associated virus vector expression of a dopamine-2 receptor (D2R) mutant (D2R80A) by positron emission tomography in the brains of mice and cats. D2R80A is inactivated for intracellular signaling and binds subphysiologic amounts of the radioactive [¹⁸F]-fallypride analog of dopamine. The [¹⁸F]-fallypride signal bound to D2R80A in the injection site was normalized to the signal from endogenous D2R in the striatum and showed stable levels of expression within individual animals. A separate adeno-associated virus type 1 vector with identical gene expression control elements, expressing green fluorescent protein or a therapeutic gene, was coinjected with the D2R80A vector at equal doses into specific sites. Both transgenes had similar levels of gene expression by immunohistochemistry, in situ hybridization, and quantitative PCR assays, demonstrating that D2R80A is a faithful surrogate measure for expression of a gene of interest. This dual vector approach allows the D2R80A gene to be used with any therapeutic gene and to be injected into a single site for monitoring while the therapeutic gene can be distributed more widely as needed in each disease.

The sodium iodide symporter (NIS) as an imaging reporter for gene, viral, and cell-based therapies

Preclinical and clinical tomographic imaging systems increasingly are being utilized for non-invasive imaging of reporter gene products to reveal the distribution of molecular therapeutics within living subjects. Reporter gene and probe combinations can be employed to monitor vectors for gene, viral, and cell-based therapies. There are several reporter systems available; however, those employing radionuclides for positron emission tomography (PET) or singlephoton emission computed tomography (SPECT) offer the highest sensitivity and the greatest promise for deep tissue imaging in humans. Within the category of radionuclide reporters, the thyroidal sodium iodide symporter (NIS) has emerged as one of the most promising for preclinical and translational research. NIS has been incorporated into a remarkable variety of viral and non-viral vectors in which its functionality is conveniently determined by in vitro iodide uptake assays prior to live animal imaging. This review on the NIS reporter will focus on 1) differences between endogenous NIS and heterologously-expressed NIS, 2) qualitative or comparative use of NIS as an imaging reporter in preclinical and translational gene therapy, oncolytic viral therapy, and cell trafficking research, and 3) use of NIS as an absolute quantitative reporter.

Methods to monitor gene therapy with molecular imaging

Recent progress in scientific and clinical research has made gene therapy a promising option for efficient and targeted treatment of several inherited and acquired disorders. One of the most critical issues for ensuring success of gene-based therapies is the development of technologies for non-invasive monitoring of the distribution and kinetics of vector-mediated gene expression. In recent years many molecular imaging techniques for safe, repeated and high-resolution in vivo imaging of gene expression have been developed and successfully used in animals and humans. In this review molecular imaging techniques for monitoring of gene therapy are described and specific use of these methods in the different steps of a gene therapy protocol from gene delivery to assessment of therapy response is illustrated. Linking molecular imaging (MI) to gene therapy will eventually help to improve the efficacy and safety of current gene therapy protocols for human application and support future individualized patient treatment.

Small animal imaging facility: new perspectives for the radiologist

In recent years, new technologies have become available for imaging small animals. The use of animal models in basic and preclinical sciences, for example, offers the possibility of testing diagnostic markers and drugs, which is becoming crucial in the success and timeliness of research and is allowing a more efficient approach in defining study objectives and providing many advantages for both clinical research and the pharmaceutical industry. The use of these instruments offers data that are more predictive of the distribution and efficacy of a compound. The mouse, in particular, has become a key animal model system for studying human disease. It offers the possibility of manipulating its genome and producing accurate models for many human disorders, thus resulting in significant progress in understanding pathologenic mechanisms. In neurobiology, the possibility of simulating neurodegenerative diseases has enabled the development and validation of new treatment strategies based on gene therapy or cell grafting. Noninvasive imaging in small living animal models has gained increasing importance in preclinical research, itself becoming an independent specialty. The aim of this article is to review the characteristics of these systems and illustrate their main applications.

Aging of the striatum: mechanisms and interventions

Motor function declines with increasing adult age. Proper regulation of the balance between dopamine (DA) and acetylcholine (ACh) in the striatum has been shown to be fundamentally important for motor control. Although other factors can also contribute to this age-associated decline, a decrease in the concentration and binding potential of the DA D(2) receptor subtype in the striatum, especially in the cholinergic interneurons, are involved in the mechanism. Our studies have shown that gene transfer of the DA D(2) receptor subtype with adenoviral vectors is effective in ameliorating age-associated functional decline of the striatal cholinergic interneurons. These achievements confirm that an age-associated decrease of D(2)R contributes functional alteration of the interaction of DA and ACh in the striatum and demonstrate that these age-associated changes indeed are modifiable.

D2R DNA transfer into the nucleus accumbens attenuates cocaine self-administration in rats

Dopamine (DA) D2 receptor (D2R) agonists and antagonists can modulate self-administration behavior, conditioned place preference, and locomotor responses to cocaine. Low levels of D2R have also been observed in cocaine addicted subjects and in non human primates after chronic cocaine exposures. Prior studies had shown that D2R upregulation in the nucleus accumbens (NAc) in rodents trained to self-administer alcohol markedly attenuated alcohol preference and intake. Here we assess the effects of D2R upregulation in the NAc on cocaine intake in rats trained to self-administer cocaine. Following 2 weeks of i.v. cocaine self-administration (CSA), rats were stereotaxically treated with an adenovirus that carried the D2R gene to upregulate D2R in the NAc. D2R vector treatment resulted in a significant decrease (75%) in cocaine infusions and lever presses (70%) for cocaine. This effect lasted 6 days before cocaine consumption returned to baseline levels, which corresponds roughly to the time it takes D2R to return to baseline levels. These findings show that CSA and D2R in the NAc are negatively correlated and suggest that cocaine intake is modulated in part by D2R levels in NAc. Thus strategies aimed at increasing D2R expression in NAc may be beneficial in treating cocaine abuse and addiction.

Functional recovery of the striatal cholinergic system in aged rats by adenoviral vector-mediated gene transfer of dopamine D2 receptor

We previously reported that the striatal dopamine-acetylcholine (ACh) interaction was affected by the aging process, possibly via a decrease of the striatal D(2)R expression. In the current study, the ACh responses to the infusion of 0.1microM of the D(2)R agonist quinpirole were measured with microdialysis techniques after adenoviral vector-mediated gene transfer of D(2)R into the striatum of 25-month-old rats. The D(2)R gene-transferred rats showed significantly stronger responses of the striatal cholinergic neurons to the infusion of the D(2)R agonist than did control vector-transferred rats. The current study suggests that age-associated functional decline with decreased gene expression of the receptor may be restored by intervention.

DRD2 Gene Transfer Into the Nucleus Accumbens Core of the Alcohol Preferring and Nonpreferring Rats Attenuates Alcohol Drinking

BACKGROUND

Transient overexpression of the dopamine D2 receptor (DRD2) gene in the nucleus accumbens (NAc) using an adenoviral vector has been associated with a significant decrease in alcohol intake in Sprague Dawley rats. This overexpression of DRD2 reduced alcohol consumption in a two-bottle-choice paradigm and supported the view that high levels of DRD2 may be protective against alcohol abuse.

METHODS

Using a limited access (1 hr) two-bottle-choice (water versus 10% ethanol) drinking paradigm, we examined the effects of the DRD2 vector in alcohol intake in the genetically inbred alcohol-preferring (P) and -nonpreferring (NP) rats. In addition, micro-positron emission tomography imaging was used at the completion of the study to assess in vivo the chronic (7 weeks) effects of ethanol exposure on DRD2 levels between the two groups.

RESULTS

P rats that were treated with the DRD2 vector (in the NAc) significantly attenuated their alcohol preference (37% decrease) and intake (48% decrease), and these measures returned to pretreatment levels by day 20. A similar pattern of behavior (attenuation of ethanol drinking) was observed in NP rats. Analysis of the [C]raclopride micro-positron emission tomography data after chronic (7 weeks) exposure to ethanol revealed clear DRD2 binding differences between the P and NP rats. P rats showed 16% lower [C]raclopride specific binding in striatum than the NP rats.

CONCLUSIONS

These findings further support our hypothesis that high levels of DRD2 are causally associated with a reduction in alcohol consumption and may serve as a protective factor against alcoholism. That this effect was seen in P rats, which are predisposed to alcohol intake, suggests that they are protective even in those who are genetically predisposed to high alcohol intake. It is noteworthy that increasing DRD2 significantly decreased alcohol intake but did not abolish it, suggesting that high DRD2 levels may specifically interfere with the administration of large quantities of alcohol. The significantly higher DRD2 concentration in NP than P rats after 7 weeks of ethanol therefore could account for low alcohol intake.

Dopamine D2R DNA transfer in dopamine D2 receptor-deficient mice: Effects on ethanol drinking

Dopamine (DA) signals are transmitted via specific receptors including the D2 receptors (D2R). Previous studies have shown that D2R upregulation in the nucleus accumbens (NAc) attenuated alcohol consumption. We hypothesized that upregulation of D2R in the NAc would significantly influence alcohol drinking. We tested this hypothesis by determining the effect that D2R upregulation has on alcohol intake in genetically altered mice lacking D2Rs. After a steady baseline of drinking behavior was established for all mice, a null vector or a genetically modified adenoviral vector containing the rat D2R cDNA was infused into the NAc of wild-type (Drd2+/+), heterozygous (Drd2+/-), and receptor-deficient mice (Drd2-/-). Ethanol intake and preference were then determined using the two-bottle choice paradigm. Our results indicated that Drd2+/+ mice treated with the D2R vector significantly attenuated (58 %) their ethanol intake as well as reduced preference. Drd2+/- and mutant mice showed a similar attenuation, although the change was not as marked (12 %) and did not last as long. In contrast, Drd2-/- mice treated with the D2R vector displayed a temporary but significant increase (46 %) in ethanol intake and preference (consumption). These results supported the notion that the D2R plays an important role in alcohol consumption in mice and suggest that a key threshold range of D2R levels is associated with elevated alcohol consumption. Significant deviations in D2R levels from this range could impact alcohol consumption, and could help to explain possible individual variations in alcohol response, metabolism, sensitivity and consumption.

Functional neuroimaging: a new generation of human brain studies in obesity research

Obesity is predominantly caused by overeating, an abnormal behaviour for which there is no unequivocal neurophysiological explanation. Functional neuroimaging techniques, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have recently emerged as new tools to search for regions of the brain that are involved in the regulation of eating behaviours and those that are involved in the pathophysiology of obesity. Using these techniques, a limited number of studies have provided the first in vivo images of the human hypothalamic response to nutritional stimuli and revealed the complexity of the human brain response to hunger, taste, and satiation. Selective differences have been reported in the functional architecture of the brain of obese and lean individuals. We discuss current use and possible future developments of functional neuroimaging applied to obesity research. We conclude that functional neuroimaging provides an increasingly important tool for investigating how different regions of the brain work in concert to orchestrate normal eating behaviours and how they conspire to produce obesity and other eating disorders.

Neuroimaging and mechanisms of drug abuse: interface of molecular imaging and molecular genetics

Whereas ligand studies can inform the end-products of dysregulation of genetic expression, reporter gene imaging can provide the means to understand the genetic origin of these end-products. As with radioligand studies, in vivo direct measurement of gene expression will allow genetic processes to be monitored over time in the same subject, use of a subject as his/her own control in intervention studies (i.e., measurement before and after an intervention), and monitoring the spatial distribution of molecular events in the whole brain. Furthermore, reporter gene imaging, by advancing knowledge of the biologic mechanisms of disease states, has important clinical implications, particularly in the development and monitoring of treatments. We expect PET to play a prominent role in the elucidation of substance abuse mechanisms and contribute significantly to the development of innovative treatment strategies.

Prospects of molecular imaging in neurology

Molecular imaging aims towards the non-invasive kinetic and quantitative assessment and localization of biological processes of normal and diseased cells in vivo in animal models and humans. Due to technological advances during the past years, imaging of molecular processes is a rapidly growing field, which has the potential of broad applications in the study of cell biology, biochemistry, gene/protein function and regulation, signal transduction, characterization of transgenic animals, development of new treatment strategies (gene or cell-based) and their successful implementation into clinical application. Most importantly, the possibility to study these parameters in the same subject repeatedly over time makes molecular imaging an attractive technology to obtain reliable data and to safe recourse; for example, molecular imaging enables the assessment of an exogenously introduced therapeutic gene and the related alterations of endogenously regulated gene functions directly in the same subject. Therefore, molecular imaging will have great implications especially when molecular diagnostic and treatment modalities have to be translated from experimental into clinical application. Here, we review the three main imaging technologies, which have been developed for studying molecular processes in vivo, the disease models, which have been studied so far, and the potential future applications.

Longitudinal follow-up study of adenoviral vector-mediated gene transfer of dopamine d2 receptors in the striatum in young, middle-aged, and aged rats: a positron emission tomography study

Overexpression of dopamine D(2) receptors by adenoviral vector-mediated gene transfer in the rat striatum was evaluated by positron emission tomography in vivo and by ex vivo autoradiography in 5-, 13-, and 24-month-old Fischer 344 rats. Each rat had hemilateral gene transfer of D(2) receptors mediated by adenoviral vectors (AdCMV.DopD(2)R) in the striatum with contralateral striatal injection of control vectors (AdCMV.LacZ). At day 2 or 3 after vector injection positron emission tomography or ex vivo autoradiography was performed. The binding potential of a radiolabeled D(2) receptors ligand, [11C]raclopride, was significantly higher in the D(2) receptors gene-transferred striatum than the control side in each age group at a similar degree. The binding potential in the AdCMV.DopD(2)R-injected striatum of 24-month-old rats was similar to that in the AdCMV.LacZ-injected striatum of 5-month-old rats (0.99+/-0.14 versus 0.91+/-0.08). A significant age-associated decrease of the binding potential of [11C]raclopride was found in the control vector-injected side, and a significant increase of the binding potential in the adenoviral vector-injected side in all three age groups, suggesting no aging effect on the overexpression of D(2) receptors. A group of rats underwent follow-up assessment by positron emission tomography. The overexpression of D(2) receptors decreased with time in all three groups; however, the decrease rate of the D(2) receptors expression was significantly smaller in the 24-month-old group than in the 5-month-old group. We confirmed that the adenoviral vector-mediated gene transfer of D(2) receptors compensated the decreased density of striatal D(2) receptors in the 24-month-old rats up to the level in the control striatum of 5-month-old rats, and the decrease rate of the overexpression was significantly smaller in aged rats.

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.

Molecular Imaging of Gliomas

Gliomas are the most common types of brain tumors. Although sophisticated regimens of conventional therapies are being carried out to treat patients with gliomas, the disease invariably leads to death over months or years. Before new and potentially more effective treatment strategies, such as gene- and cell-based therapies, can be effectively implemented in the clinical application, certain prerequisites have to be established. First of all, the exact localization, extent, and metabolic activity of the glioma must be determined to identify the biologically active target tissue for a biological treatment regimen; this is usually performed by imaging the expression of up-regulated endogenous genes coding for glucose or amino acid transporters and cellular hexokinase and thymidine kinase genes, respectively. Second, neuronal function and functional changes within the surrounding brain tissue have to be assessed in order to save this tissue from therapy-induced damage. Third, pathognomonic genetic changes leading to disease have to be explored on the molecular level to serve as specific targets for patient-tailored therapies. Last, a concerted noninvasive analysis of both endogenous and exogenous gene expression in animal models as well as the clinical setting is desirable to effectively translate new treatment strategies from experimental into clinical application. All of these issues can be addressed by multimodal radionuclide and magnetic resonance imaging techniques and fall into the exciting and fast growing field of molecular and functional imaging. Noninvasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging may reveal the assessment of the “location,” “magnitude,” and “duration” of therapeutic gene expression and its relation to the therapeutic effect. Detailed reviews on molecular imaging have been published from the perspective of radionuclide imaging (Gambhir et al., 2000; Blasberg and Tjuvajev, 2002) as well as magnetic resonance and optical imaging (Weissleder, 2002). The present review focuses on molecular imaging of gliomas with special reference on the status and perspectives of imaging of endogenous and exogenously introduced gene expression in order to develop improved diagnostics and more effective treatment strategies of gliomas and, in that, to eventually improve the grim prognosis of this devastating disease.

Molecular and functional imaging technology for the development of efficient treatment strategies for gliomas

Gliomas are the most common types of brain tumors, which invariably lead to death over months or years. Before new and potentially more effective treatment strategies, such as gene therapy, can be effectively introduced into clinical application the following goals must be reached: (1) the determination of localization, extent and metabolic activity of the glioma; (2) the assessment of functional changes within the surrounding brain tissue; (3) the identification of genetic changes on the molecular level leading to disease; and in addition (4) a detailed non-invasive analysis of both endogenous and exogenous gene expression in animal models and in the clinical setting. Non-invasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging techniques may reveal the assessment of the location, magnitude and duration of therapeutic gene expression and its relation to the therapeutic effect. Here, we review the main principles of PET imaging and its key roles in neurooncology research.

Gamma camera dual imaging with a somatostatin receptor and thymidine kinase after gene transfer with a bicistronic adenovirus in mice

PURPOSE

To compare two systems for assessing gene transfer to cancer cells and xenograft tumors with noninvasive gamma camera imaging.

MATERIALS AND METHODS

A replication-incompetent adenovirus encoding the human type 2 somatostatin receptor (hSSTr2) and the herpes simplex virus thymidine kinase (TK) enzyme (Ad-hSSTr2-TK) was constructed. A-427 human lung cancer cells were infected in vitro and mixed with uninfected cells at different ratios. A-427 tumors in nude mice (n = 23) were injected with 1 x 10(6) to 5 x 10(8) plaque-forming units (pfu) of Ad-hSSTr2-TK. The expressed hSSTr2 and TK proteins were imaged owing to internally bound, or trapped, technetium 99m ((99m)Tc)-labeled hSSTr2-binding peptide (P2045) and radioiodinated 2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl-5-iodouracil (FIAU), respectively. Iodine 125 ((125)I)-labeled FIAU was used in vitro and iodine 131 ((131)I)-labeled FIAU, in vivo. The (99m)Tc-labeled P2045 and (125)I- or (131)I-labeled FIAU were imaged simultaneously with different window settings with an Anger gamma camera. Treatment effects were tested with analysis of variance.

RESULTS

Infected cells in culture trapped (125)I-labeled FIAU and (99m)Tc-labeled P2045; uptake correlated with the percentage of Ad-hSSTr2-TK-positive cells. For 100% of infected cells, 24% +/- 0.4 (mean +/- SD) of the added (99m)Tc-labeled P2045 was trapped, which is significantly lower (P <.05) than the 40% +/- 2 of (125)I-labeled FIAU that was trapped. For the highest Ad-hSSTr2-TK tumor dose (5 x 10(8) pfu), the uptake of (99m)Tc-labeled P2045 was 11.1% +/- 2.9 of injected dose per gram of tumor (thereafter, dose per gram), significantly higher (P <.05) than the uptake of (131)I-labeled FIAU at 1.6% +/- 0.4 dose per gram. (99m)Tc-labeled P2045 imaging consistently depicted hSSTr2 gene transfer in tumors at all adenovirus doses. Tumor uptake of (99m)Tc-labeled P2045 positively correlated with Ad-hSSTr2-TK dose; (131)I-labeled FIAU tumor uptake did not correlate with vector dose.

CONCLUSION

The hSSTr2 and TK proteins were simultaneously imaged following dual gene transfer with an adenovirus vector.

Positron emission tomography and ex vivo and in vitro autoradiography studies on dopamine D2-like receptor degeneration in the quinolinic acid-lesioned rat striatum: comparison of [11C]raclopride, [11C]nemonapride and [11C]N-methylspiperone

With [11C]raclopride,[11C]nemonapride and [11C]N-methylspiperone, degeneration of dopamine D2-like receptors in the unilaterally quinolinic acid-lesioned rats was evaluated by positron emission tomography (PET) and ex vivo and in vitro autoradiography. PET showed a decreased uptake of [11C]raclopride in the lesioned striatum, but an increased uptake of [11C]nemonapride and [11C]N-methylspiperone despite a decreased binding in vitro. Ex vivo autoradiography showed an increased accumulation of the three ligands in the cortical region overlying the injured striatum, probably enlarging PET signals. PET has the limited potential for evaluating the receptor degeneration in the present animal model.

In vivo assessment of adenoviral vector-mediated gene expression of dopamine D(2) receptors in the rat striatum by positron emission tomography

For functional assessment of gene therapy in experimental animals, in vivo assessment of transferred genes will provide a major advance over an in vitro analysis which must be done post-hoc. In the current study we conducted positron emission tomography (PET) analysis in rats following injection of the adenoviral vector encoding the cDNA for the rat dopamine D(2) receptors (D(2)R) (AdCMV.DopD(2)R) into rat brain to provide a quantitative evaluation of D(2)R overexpression. Quantitative measurements as well as images by PET and ex vivo autoradiography demonstrated the significant increase of D(2)R binding of [(11)C]raclopride, a specific D(2)R radioligand, in the AdCMV.DopD(2)R-injected rat striatum 2 or 3 days after vector injection. Longitudinal in vivo assessment of the gene expression by PET demonstrated decreased binding of [(11)C]raclopride with time, which was in agreement with the observation in a cross-sectional autoradiographic study. The results of the current study demonstrate that PET can be used for longitudinal in vivo assessment of D(2)R expression mediated by adenoviral vector in rat brain.

Overexpression of dopamine D2 receptors reduces alcohol self-administration

The mechanism(s) underlying predisposition to alcohol abuse are poorly understood but may involve brain dopamine system(s). Here we used an adenoviral vector to deliver the dopamine D2 receptor (DRD2) gene into the nucleus accumbens of rats, previously trained to self-administer alcohol, and to assess if DRD2 levels regulated alcohol preference and intake. We show that increases in DRD2 (52%) were associated with marked reductions in alcohol preference (43%), and alcohol intake (64%) of ethanol preferring rats, which recovered as the DRD2, returned to baseline levels. In addition, this DRD2 overexpression similarly produced significant reductions in ethanol non-preferring rats, in both alcohol preference (16%) and alcohol intake (75%). This is the first evidence that overexpression of DRD2 reduces alcohol intake and suggests that high levels of DRD2 may be protective against alcohol abuse.

Development of an adenoviral vector for intracerebral delivery of the dopamine D(2) receptor

The age-related loss of striatal dopamine D(2) receptors (D(2)R) has been observed in numerous species, including rodents, monkeys, and man, and is partly responsible for impaired motor function in aged mammals. We have developed an adenoviral vector designed for intracerebral transfer of cDNA for D(2)R. Results of in vitro studies demonstrated that the vector produced abundant message for D(2)R and that the vector was membrane bound and capable of binding appropriate ligand. Results of in vivo studies provided clear evidence of D(2)R production when injected into the striatum of rats. The D(2)R produced were capable of binding appropriate ligand. In addition, evidence of functional receptors was produced by demonstrating apomorphine-induced rotational behavior in rats receiving a unilateral injection of the vector. Despite these successes, we have been unable to demonstrate improvement in the motor behavior of aged rats receiving bilateral injections of the vector. A major problem with this vector as with similar adenoviral vectors is the loss of expression beginning 3-5 days after injection to undetectable levels at 21 days. Because of the lack of motor functional effects in aged rats and the loss of expression of the vector, other strategies for development of the vector are being pursued. Regarding functional effects, we have examined the feasibility of manipulating hippocampal acetylcholine (ACh) release through D(2)R manipulation to improve memory performance. Using microdialysis, we have demonstrated in vivo in rats that treatment with a D(2)R agonist increases hippocampal ACh release while treatment with a D(2)R antagonist attentuates this effect as well as impairs performance in a complex maze task. In addition, a D(2)R null mutant mouse is being used to examine possible therapeutic effects of the vector. These mice show specific motor deficits. Recent studies using positron emission tomography have also demonstrated the feasibility of in vivo imaging of the vector. Thus, use of adenoviral vectors specific for neurotransmitter receptors can provide a highly useful research tool for examining age-related alterations in behavioral function and a possible strategy for therapeutic intervention.