Changes in rat cerebral blood volume due to modulation of the 5-HT(1A) receptor measured with susceptibility enhanced contrast MRI

Pharmacological MRI to investigate the functional selectivity of 5-HT1A receptor biased agonists

The emerging concept of "biased agonism" denotes the phenomenon whereby agonists can preferentially direct receptor signalling to specific intracellular responses among the different transduction pathways, thus potentially avoiding side effects and improving therapeutic effects. The aim of this study was to investigate biased agonism by using pharmacological magnetic resonance imaging (phMRI). The cerebral blood oxygen level dependent (BOLD) signal changes induced by increasing doses of two serotonin 5-HT_1A receptor biased agonists, NLX-112 and NLX-101, were mapped in anaesthetized rats. Although both compounds display high affinity, selectivity and agonist efficacy for 5-HT_1A receptors, NLX-101 is known to preferentially activate post-synaptic receptors, whereas NLX-112 targets both pre- and post-synaptic receptors. We used several doses of agonists in order to determine if the regional selectivity of NLX-101 was dose-dependent. NLX-112 and NLX-101 induced different positive and negative hemodynamic changes patterns at equal doses. Importantly, NLX-101 had no significant effect in regions expressing pre-synaptic receptors contrary to NLX-112. NLX-112 also produced higher BOLD changes than NLX-101 in the orbital cortex, the somatosensory cortex, and the magnocellular preoptic nuclei. In other regions such as the retrosplenial cortex and the dorsal thalamus, the drugs had similar effects. In terms of functional connectivity, NLX-112 induced more widespread changes than NLX-101. The present phMRI study demonstrates that two closely-related agonists display notable differences in their hemodynamic "fingerprints". These data support the concept of biased agonism at 5-HT_1A receptors and raise the prospect of identifying novel therapeutics which exhibit improved targeting of brain regions implicated in neuropsychiatric disorders. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Pharmacological MRI response to a selective dopamine transporter inhibitor, GBR12909, in awake and anesthetized rats

Pharmacological magnetic resonance imaging (phMRI) is a powerful tool for imaging the effects of drugs on brain activity. In preclinical phMRI studies, general anesthesia used for minimizing head movements is thought to influence the phMRI responses to drugs. In this study we investigated the phMRI responses to a selective dopamine transporter (DAT) inhibitor, GBR12909, and a dopamine (DA) releaser, d-amphetamine (AMPH), in the isoflurane anesthetized and awake rats using a relative cerebral blood volume (rCBV) method. AMPH (1 mg/kg i.p.) caused an increase in rCBV in the dopaminergic circuitry in the both anesthetized and awake rats. The striatal rCBV change was correlated with the change of the striatal DA concentration induced by AMPH in the both anesthetized and awake rats. GBR12909 (10 mg/kg i.p.) caused a positive rCBV response and showed a similar regional pattern of rCBV response to AMPH in the awake rats, and the correlation between the change of the striatal rCBV and the striatal DA concentration was observed. However, in the anesthetized rats, GBR12909 induced a widespread negative rCBV response, whereas an increase in striatal DA concentration was observed. These findings indicate that phMRI responses to activation of DA neurotransmission by GBR12909 or AMPH are overall identical in the awake state, while the phMRI response to a DAT inhibitor, GBR12909 but not to AMPH was changed by isoflurane anesthesia. For the evaluation of neuroactive drugs using phMRI, isoflurane anesthesia might be complicated the interpretation of pharmacodynamic effects of drugs in preclinical studies.

Pharmacologic magnetic resonance imaging (phMRI): imaging drug action in the brain

The technique of functional magnetic resonance (fMRI), using various cognitive, motor and sensory stimuli has led to a revolution in the ability to map brain function. Drugs can also be used as stimuli to elicit an hemodynamic change. Stimulation with a pharmaceutical has a number of very different consequences compared to user controllable stimuli, most importantly in the time course of stimulus and response that is not, in general, controllable by the experimenter. Therefore, this type of experiment has been termed pharmacologic MRI (phMRI). The use of a drug stimulus leads to a number of interesting possibilities compared to conventional fMRI. Using receptor specific ligands one can characterize brain circuitry specific to neurotransmitter systems. The possibility exists to measure parameters reflecting neurotransmitter release and binding associated with the pharmacokinetics and/or the pharmacodynamics of drugs. There is also the ability to measure up- and down-regulation of receptors in specific disease states. phMRI can be characterized as a molecular imaging technique using the natural hemodynamic transduction related to neuro-receptor stimulus. This provides a coupling mechanism with very high sensitivity that can rival positron emission tomography (PET) in some circumstances. The large numbers of molecules available, that do not require a radio-label, means that phMRI becomes a very useful tool for performing drug discovery. Data and arguments will be presented to show that phMRI can provide information on neuro-receptor signaling and function that complements the static picture generated by PET studies of receptor numbers and occupancies.

Lasting effects of chronic fluoxetine treatment on the late developing rat brain: age-dependent changes in the serotonergic neurotransmitter system assessed by pharmacological MRI

RATIONALE

With the growing prevalence of psychotropic drug prescriptions among children and adolescents, the need for studies on lasting effects of drug exposure on the developing brain rises. Fluoxetine is the only selective serotonin reuptake inhibitor (SSRI) officially registered to treat major depressive disorder in children. Although various (pre)clinical studies have assessed the (long-term) effects of fluoxetine exposure in the perinatal period and in adulthood, limited data is available on its effects on the developing brain later in life, i.e. during adolescence.

OBJECTIVE

The present study aimed at investigating the effects of age following chronic SSRI treatment on the central serotonin (5-HT) system. To this end, pharmacological MRI (phMRI) was performed in chronic fluoxetine-treated (5 mg/kg, oral gavage for 3 weeks) juvenile (PND25) and adult rats (PND65) after a 1-week washout period, using an acute fluoxetine challenge (5 mg/kg, i.v.) to trigger the 5-HT system.

RESULTS

We observed a diminished brain response to the acute challenge in adult treated animals when compared to control animals, whereas this response was increased in juvenile treated rats. As a result, a significant age by treatment interaction effect was seen in several (subcortical) 5-HT related brain regions.

CONCLUSION

An opposite effect of chronic fluoxetine treatment was seen in the developing brain compared to that in matured brain, as assessed non-invasively using phMRI. These findings most likely reflect neuronal imprinting effects of juvenile SSRI treatment and may underlie emotional disturbances seen in animals and children treated with this drug. Also, our findings suggest that phMRI might be ideally suited to study this important issue in the pediatric population.

Functional and pharmacological MRI in understanding brain function at a systems level

Functional magnetic resonance imaging (fMRI) methods have been extensively applied to study the human brain and its functional organization in healthy and disease states. A strong rationale exists for the extension of this approach to animal models as a translational tool to bridge clinical and preclinical research. Specifically, the development of pharmacological MRI (phMRI), i.e., the use of fMRI to map spatiotemporal patterns of brain activity induced by pharmacological agents, has provided a robust and flexible tool to resolve brain circuits and mechanism-specific functional changes produced by selective intervention in different neurotransmitter systems in vivo. This chapter describes the methodological aspects of fMRI and phMRI in preclinical species, and some of the key findings, with a special emphasis on the translational potential of these methods in neuropharmacological research.

Evidence that increased 5-HT release evokes region-specific effects on blood-oxygenation level-dependent functional magnetic resonance imaging responses in the rat brain

This study aimed to determine the potential of in vivo functional magnetic resonance imaging (fMRI) methods as a non-invasive means of detecting effects of increased 5-HT release in brain. Changes in blood-oxygenation level-dependent (BOLD) contrast induced by administration of the 5-HT-releasing agent, fenfluramine, were measured in selected brain regions of halothane-anesthetized rats. Initial immunohistochemical measurements of the marker of neural activation, Fos, confirmed that in halothane-anesthetized rats fenfluramine (10 mg/kg i.v.) evoked cellular responses in cortical regions which were attenuated by pre-treatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (300 mg/kg i.p. once daily for 2 days). Fenfluramine-induced Fos was demonstrated in numerous glutamatergic pyramidal neurons (Fos/excitatory amino acid carrier 1 (EAAC1) co-labeled), but also a small number of GABA interneurons (Fos/glutamic acid decarboxylase (GAD)(67) colabeled). Fenfluramine (10 mg/kg i.v.) evoked changes in BOLD signal intensity in a number of cortical and sub-cortical regions with the greatest effects being observed in the nucleus accumbens (-13.0%+/-2.7%), prefrontal cortex (-10.1%+/-3.2%) and motor cortex (+2.3%+/-1.0%). Pre-treatment with p-chlorophenylalanine, significantly attenuated the response to fenfluramine (10 mg/kg i.v.) in all regions with the exception of the motor cortex which showed a trend. These experiments demonstrate that increased 5-HT release evokes region-specific changes in the BOLD signal in rats, and that this effect is attenuated in almost all regions by 5-HT depletion. These findings support the use of fMRI imaging methods as a non-invasive tool to study 5-HT function in animal models, with the potential for extension to clinical studies.

5-Hydroxytryptamine2C Receptor Contribution to m-Chlorophenylpiperazine and N-Methyl-β-carboline-3-carboxamide-Induced Anxiety-Like Behavior and Limbic Brain Activation

Activation of 5-hydroxytryptamine2C (5-HT(2C)) receptors by the 5-HT(2) receptor agonist m-chlorophenylpiperazine (m-CPP) elicits anxiety in humans and anxiety-like behavior in animals. We compared the effects of m-CPP with the anxiogenic GABA(A) receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG-7142) on both anxiety-like behavior and regional brain activation using functional magnetic resonance imaging (fMRI) in the rat. We also determined whether the selective 5-HT(2C) receptor antagonist SB 242084 [6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride] would blunt m-CPP or FG-7142-induced neuronal activation. Both m-CPP (3 mg/kg i.p.) and FG-7142 (10 mg/kg i.p.) elicited anxiety-like behavior when measured in the social interaction test, and pretreatment with SB 242084 (1 mg/kg i.p.) completely blocked the behavioral effects of both anxiogenic drugs. Regional brain activation in vivo in response to anxiogenic drug challenge was determined by blood oxygen level-dependent (BOLD) fMRI using a powerful 9.4T magnet. Region of interest analyses revealed that m-CPP and FG-7142 significantly increased BOLD signals in brain regions that have been linked to anxiety, including the amygdala, dorsal hippocampus, and medial hypothalamus. These BOLD signal increases were blocked by pretreatment with SB 242084. In contrast, injection of m-CPP and FG-7142 resulted in BOLD signal decreases in the medial prefrontal cortex that were not blocked by SB 242084. In conclusion, the brain activation signals produced by anxiogenic doses of both m-CPP and FG-7142 are mediated at least partially by the 5-HT(2C) receptor, indicating that this receptor is a key component in anxiogenic neural circuitry.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Functional magnetic resonance mapping of intracerebroventricular infusion of a neuroactive peptide in the anaesthetised rat

Pharmacological magnetic resonance imaging (phMRI) methods map the cerebral haemodynamic response to challenge with psychotropic agents as a surrogate for drug-induced changes in brain activity. However, many neuroactive compounds present low blood-brain barrier penetration and thus systemic administration may result in insufficient brain concentration. Intracerebroventricular (ICV) administration has been long used as an effective way of bypassing the blood-brain barrier in studies with non-brain-penetrant compounds, such as neuropeptides. In order to extend the range of pharmacological substances accessible to phMRI, we have developed methods to map relative cerebral blood volume (rCBV) changes induced by in situ ICV administration of neuroactive agents in the anaesthetised rat. We have applied this method to study for the first time the phMRI response to central administration of a neuropeptide, the metabolically stable and potent NK1 receptor agonist GR-73632. ICV administration of 4.2 pmol of GR-73632 produced a rapid onset and sustained rCBV increase in several brain structures, such as the amygdala, the caudate putamen and the cortex. These results demonstrate the feasibility of phMRI as a tool to study the functional correlates of brain activity induced by central administration of neuroactive agents.

Toward brain imaging of serotonin 5-HT1A autoreceptor internalization

Enhancing cerebral serotonin (5-hydroxytryptamine, 5-HT) neurotransmission is a common property of antidepressant treatments and the basis for their efficacy. 5-HT1A receptors located on the cell body and dendrites of 5-HT neurons (autoreceptors) play a key role in this regard. Because they normally mediate an inhibition of neuronal firing, their desensitization is a prerequisite to the delayed enhancement of 5-HT neurotransmission upon treatment with monoamine oxidase (MAOI) inhibitors or specific serotonin reuptake inhibitors (SSRI). Using beta-sensitive microprobes in vivo, we measured a significant decrease (-30%) in binding sites for the 5-HT1A PET radioligand [18F]MPPF associated with an equivalent reduction (-34%) in the cell surface density of 5-HT1A receptor immunoreactivity (internalization), in the nucleus raphe dorsalis (autoreceptors), but not hippocampus (heteroreceptors), of rats given a single dose of the specific 5-HT1A receptor agonist, 8-OH-DPAT (0.5 mg/kg, iv). This effect was completely blocked by pretreatment with the selective 5-HT1A antagonist WAY 100635. Having ruled out that this decreased density of [18F]MPPF binding in the nucleus raphe dorsalis of 8-OH-DPAT-treated rats resulted from a local blood flow effect, we obtained autoradiographic evidence indicating that the total amount of specific binding of [18F]MPPF in tissue sections was unaffected by the 8-OH-DPAT treatment in either NRD or hippocampus. It was therefore concluded that the internalization of 5-HT1A autoreceptors accounted for the decreased binding in vivo of [18F]MPPF in the nucleus raphe dorsalis of rats treated with 8-OH-DPAT. Thus, PET imaging might provide a mean to measure 5-HT1A receptor internalization in human brain and thus assess responsiveness to antidepressant treatment.

Cerebral blood volume and clinical changes on the third day of placebo substitution for SSRI treatment

BACKGROUND

Interruptions in SSRI treatment have been associated with adverse effects that can resemble depressive illness. We hypothesized that brain regions implicated in depression, with extensive serotonergic innervation, would exhibit changes in activity associated with emergence of symptoms following drug discontinuation.

METHODS

Subjects meeting DSM-IV criteria for remitted major depression on 20 mg/day of either fluoxetine or paroxetine were recruited into this 6-week study. During weeks 2 and 6, subjects underwent a 3-day period in which either active drug or placebo was substituted for their medication under double-blind conditions. Cerebral blood volume (CBV) maps were obtained via dynamic susceptibility magnetic resonance imaging at the end of each double-blind period.

RESULTS

In the paroxetine group, change in CBV in left medial superior frontal region and left caudate nucleus correlated significantly with change in Discontinuation Emergent Symptom Scale and Hamilton Depression Rating Scale (HDRS; R2 = 0.66, p =.0007; R2 = 0.51, p =.006; and R2 = 0.43, p =.015; R2 = 0.32, p =.043, respectively).

CONCLUSIONS

These data demonstrate that changes in regional CBV of left prefrontal cortex and left caudate nucleus correlate with the emergence of discontinuation symptoms and increased HDRS after interruption of paroxetine treatment.