William Rea

Synchronized delta oscillations correlate with the resting-state functional MRI signal

Synchronized low-frequency spontaneous fluctuations of the functional MRI (fMRI) signal have recently been applied to investigate large-scale neuronal networks of the brain in the absence of specific task instructions. However, the underlying neural mechanisms of these fluctuations remain largely unknown. To this end, electrophysiological recordings and resting-state fMRI measurements were conducted in α-chloralose-anesthetized rats. Using a seed-voxel analysis strategy, region-specific, anesthetic dose-dependent fMRI resting-state functional connectivity was detected in bilateral primary somatosensory cortex (S1FL) of the resting brain. Cortical electroencephalographic signals were also recorded from bilateral S1FL; a visual cortex locus served as a control site. Results demonstrate that, unlike the evoked fMRI response that correlates with power changes in the γ bands, the resting-state fMRI signal correlates with the power coherence in low-frequency bands, particularly the δ band. These data indicate that hemodynamic fMRI signal differentially registers specific electrical oscillatory frequency band activity, suggesting that fMRI may be able to distinguish the ongoing from the evoked activity of the brain.

Sensitization to the Behavioral Effects of Cocaine: Modulation by Dynorphin and κ-Opioid Receptor Agonists

Several lines of evidence suggest an involvement of the mesolimbic dopamine (DA) system in the mediation of psychostimulant-induced sensitization. It is also apparent that endogenous opioid peptide systems can modulate the activity of this same DA system. Psychostimulant-induced alterations in opioid peptide gene expression have also been reported. In this review, evidence will be presented that demonstrates that the administration of kappa-opioid agonists can prevent the initiation of behavioral sensitization to cocaine and that such treatment is also effective in preventing alterations in mesolimbic DA neurotransmission that occur as a consequence of repeated cocaine administration. The putative role of opioid-DA interactions in the modulation of psychostimulant-induced sensitization will also be discussed.

Cocaine-induced brain activation detected by dynamic manganese-enhanced magnetic resonance imaging (MEMRI)

Dynamic manganese-enhanced magnetic resonance imaging (MEMRI) detects neuronal activity based on the passage of Mn2+ into active neurons. Because this mechanism is independent of any hemodynamic response, it is potentially ideal for pharmacological studies and was applied to investigate the acute CNS effects of cocaine in the rat. Dose-dependent, region-specific MEMRI signals were seen mostly in cortical and subcortical mesocorticolimbic structures. To verify the spatial accuracy and physiological mechanisms of MEMRI, neuronal activation following electrical forepaw stimulation revealed somatotopic signal enhancement in the primary and secondary somatosensory cortices, which was blocked by diltiazem, a Ca2+ channel antagonist. These data suggest that MEMRI may serve as a tool for investigating the effects of pharmacological agents and opens an application of MRI to study CNS drug effects at a systems level.

Allosteric interactions between agonists and antagonists within the adenosine A2A receptor-dopamine D2 receptor heterotetramer

Significance G protein-coupled receptors (GPCRs) constitute the largest plasma membrane protein family involved in cell signaling. GPCR homodimers are predominant species, and GPCR heteromers likely are constituted by heteromers of homodimers. The adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromer is a target for the nonselective adenosine receptor antagonist caffeine. This study uncovers allosteric modulations of A2AR antagonists that mimic those of A2AR agonists, challenging the traditional view of antagonists as inactive ligands. These allosteric modulations disappear when agonist and antagonist are coadministered, however. A model is proposed that considers A2AR-D2R heteromers as heterotetramers, constituted by A2AR and D2R homodimers. The model predicted that high concentrations of A2AR antagonists would behave as A2AR agonists and decrease D2R function in the brain. Adenosine A2A receptor (A2AR)-dopamine D2 receptor (D2R) heteromers are key modulators of striatal neuronal function. It has been suggested that the psychostimulant effects of caffeine depend on its ability to block an allosteric modulation within the A2AR-D2R heteromer, by which adenosine decreases the affinity and intrinsic efficacy of dopamine at the D2R. We describe novel unsuspected allosteric mechanisms within the heteromer by which not only A2AR agonists, but also A2AR antagonists, decrease the affinity and intrinsic efficacy of D2R agonists and the affinity of D2R antagonists. Strikingly, these allosteric modulations disappear on agonist and antagonist coadministration. This can be explained by a model that considers A2AR-D2R heteromers as heterotetramers, constituted by A2AR and D2R homodimers, as demonstrated by experiments with bioluminescence resonance energy transfer and bimolecular fluorescence and bioluminescence complementation. As predicted by the model, high concentrations of A2AR antagonists behaved as A2AR agonists and decreased D2R function in the brain.

Phentermine and Fenfluramine: Preclinical Studies in Animal Models of Cocaine Addiction

ABSTRACT: Combined dopamine (DA) and 5‐hydroxytryptamine (5‐HT) releasers such as phentermine (PHEN) and fenfluramine (FEN) are reported, in open label studies, to reduce craving for alcohol and cocaine and to prevent relapse. The objective of the studies reported here was to assess the actions of these agents alone and in combination in various animal models of drug addiction. Study 1. in vivo microdialysis experiments demonstrate that these agents preferentially release mesolimbic DA (PHEN) and 5‐HT (FEN). Patients who relapse and use cocaine while taking these medications report diminished cocaine‐like subjective effects. Microdialysis experiments were performed in awake rats, and dialysate samples were analyzed for DA and 5‐HT. PHEN (1 mg/kg, intravenously (i.v.)) elevated DA (2–3‐fold) for over 1.5 hr. Administration of cocaine (3 mg/kg, i.v.) increased DA 6‐fold in saline‐treated rats, but only 3‐fold in PHEN‐treated rats. PHEN did not reduce cocaine‐induced increases in 5‐HT. Study 2. These agents were assessed in a mouse model of cocaine‐conditioned motoric activity (CCMA). Pretreatment with non‐activating doses of PHEN (4.6 mg/kg, intraperitoneally (i.p.)) enhanced CCMA, whereas non‐depressing doses of FEN (0.1 mg/kg, i.p.) did not alter CCMA or the PHEN‐induced increase in CCMA. In contrast, sub‐effective doses of FEN reduced CCMA stereotypy‐like locomotion, whereas sub‐effective doses of PHEN were without effect. PHEN reversed the FEN‐induced increase in CCMA stereotypy‐like locomotion. Study 3. PHEN and FEN were assessed in the conditioned place preference model. FEN produced marked aversions for an environment previously associated with its administration and the minimum dose producing this effect was 3.0 mg/kg. In contrast, administration of PHEN, amphetamine (1.0–3.0 mg/kg) or morphine (3.0–5.0 mg/kg) produced dose‐related preferences for the drug‐paired place. However, the magnitude of the response to PHEN was less than that produced by the other prototypic drugs of abuse. In rats that received FEN (0.3 or 3.0 mg/kg) in combination with PHEN (3.0 mg/kg), the conditioned rewarding effects of PHEN were abolished. These data demonstrate that the rewarding effects of PHEN can be conditioned to stimuli previously associated with its administration. However, the conditioned response to this agent is less then that produced by prototypic drugs of abuse. The finding that PHEN‐induced place preferences were attenuated by doses of FEN demonstrates that the combination of FEN/PHEN is devoid of motivational effects. The preclinical data obtained with PHEN/FEN in various models of drug provide a strong rationale for pursuing controlled clinical trials in humans with agents that act via a similar mechanism of action.

Modulation of behavioral sensitization to cocaine by NAALADase inhibition.

Sensitization to cocaine has been attributed to alterations in excitatory amino acid and dopamine neurotransmission in the mesolimbic system. The present study sought to determine whether inhibition of NAALADase, an enzyme that cleaves glutamate from the endogenous neuropeptide, N‐acetyl‐aspartyl‐glutamate (NAAG), attenuates sensitization to the psychomotor stimulant effects of cocaine. Rats received daily injections of cocaine (20.0 mg/kg/day; i.p.) or saline for 5 days. Fifteen minutes prior to these injections they received an i.p. injection of the NAALADase inhibitor, 2‐PMPA (50.0–100 mg/kg), or vehicle. Locomotor activity and stereotypy produced by a challenge dose of cocaine (15.0 mg/kg) were assessed 3 days later. Acute cocaine administration increased locomotor activity in control animals. In animals with a prior history of cocaine administration, the behavioral response to cocaine was significantly enhanced. In animals that had received 2‐PMPA in combination with cocaine, the enhancement of cocaine‐induced locomotor activity was attenuated. No alteration in cocaine‐evoked activity was observed in animals that had received once daily injections of 2‐PMPA, alone. Acute administration of 2‐PMPA also did not modify saline‐induced locomotor activity or activity produced by an acute cocaine challenge. These data demonstrate that NAALADase inhibition attenuates the development of sensitization to the locomotor‐activating effects of cocaine. Furthermore, this action cannot be attributed to an antagonism of the acute effects of cocaine. Synapse 38:161–166, 2000.

Orexin–Corticotropin-Releasing Factor Receptor Heteromers in the Ventral Tegmental Area as Targets for Cocaine

Release of the neuropeptides corticotropin-releasing factor (CRF) and orexin-A in the ventral tegmental area (VTA) play an important role in stress-induced cocaine-seeking behavior. We provide evidence for pharmacologically significant interactions between CRF and orexin-A that depend on oligomerization of CRF1 receptor (CRF1R) and orexin OX1 receptors (OX1R). CRF1R–OX1R heteromers are the conduits of a negative crosstalk between orexin-A and CRF as demonstrated in transfected cells and rat VTA, in which they significantly modulate dendritic dopamine release. The cocaine target σ1 receptor (σ1R) also associates with the CRF1R–OX1R heteromer. Cocaine binding to the σ1R–CRF1R–OX1R complex promotes a long-term disruption of the orexin-A–CRF negative crosstalk. Through this mechanism, cocaine sensitizes VTA cells to the excitatory effects of both CRF and orexin-A, thus providing a mechanism by which stress induces cocaine seeking.

Registering and Analyzing Rat fMRI Data in the Stereotaxic Framework by Exploiting Intrinsic Anatomical Features

The value of analyzing neuroimaging data on a group level has been well established in human studies. However, there is no standard procedure for registering and analyzing functional magnetic resonance imaging (fMRI) data into common space in rodent fMRI studies. An approach for performing rat imaging data analysis in the stereotaxic framework is presented. This method is rooted in the biological observation that the skull shape and size of rat brain are essentially the same as long as their weights are within certain range. Registration is performed using rigid-body transformations without scaling or shearing, preserving the unique properties of the stable shape and size inherent in rat brain structure. Also, it does not require brain tissue masking and is not biased towards surface coil sensitivity profile. A standard rat brain atlas is used to facilitate the identification of activated areas in common space, allowing accurate region of interest analysis. This technique is evaluated from a group of rats (n=11) undergoing routine MRI scans; the registration accuracy is estimated to be within 400 microm. The analysis of fMRI data acquired with an electrical forepaw stimulation model demonstrates the utility of this technique. The method is implemented within the Analysis of Functional NeuroImages (AFNI) framework and can be readily extended to other studies.

Local Control of Extracellular Dopamine Levels in the Medial Nucleus Accumbens by a Glutamatergic Projection from the Infralimbic Cortex

It is generally assumed that infralimbic cortex (ILC) and prelimbic cortex, two adjacent areas of the medial prefrontal cortex (mPFC) in rodents, provide selective excitatory glutamatergic inputs to the nucleus accumbens (NAc) shell and core, respectively. It is also generally believed that mPFC influences the extracellular levels of dopamine in the NAc primarily by an excitatory collateral to the ventral tegmental area (VTA). In the present study, we first established the existence of a selective functional connection between ILC and the posteromedial portions of the VTA (pmVTA) and the mNAc shell (pmNAc shell), by measuring striatal neuronal activation (immunohistochemical analysis of ERK1/2 phosphorylation) and glutamate release (in vivo microdialysis) upon ILC electrical stimulation. A novel optogenetic-microdialysis approach allowed the measurement of extracellular concentrations of glutamate and dopamine in the pmNAc shell upon local light-induced stimulation of glutamatergic terminals from ILC. Cortical electrical and local optogenetic stimulation produced significant increases in the extracellular concentrations of glutamate and dopamine in the pmNAc shell. Local blockade of glutamate release by perfusion of an adenosine A2A receptor antagonist in the pmNAc shell blocked the dopamine release induced by local optogenetic stimulation but only partially antagonized dopamine release induced by cortical electrical stimulation. The results demonstrate that ILC excitatory afferents directly modulate the extracellular concentration of dopamine in the pmNAc shell, but also support the involvement of an indirect mechanism of dopamine control, through a concomitant ILC-mediated activation of the pmVTA. SIGNIFICANCE STATEMENT We established the existence of a functional connection between the infralimbic cortex (ILC) and the posteromedial portions of the ventral tegmental area (pmVTA) and the medial nucleus acumbens shell (pmNAc shell). A novel optogenetic-microdialysis approach allowed us to demonstrate that local glutamate release from glutamatergic terminals from the ILC exert a significant modulation of extracellular concentration of dopamine in the pmNAc shell. This mechanism provides the frame for a selective cortical-mediated tonic dopaminergic modulation of specific striatal compartments.

Low- but Not High-Frequency LFP Correlates with Spontaneous BOLD Fluctuations in Rat Whisker Barrel Cortex

Resting-state magnetic resonance imaging (rsMRI) is thought to reflect ongoing spontaneous brain activity. However, the precise neurophysiological basis of rsMRI signal remains elusive. Converging evidence supports the notion that local field potential (LFP) signal in the high-frequency range correlates with fMRI response evoked by a task (e.g., visual stimulation). It remains uncertain whether this relationship extends to rsMRI. In this study, we systematically modulated LFP signal in the whisker barrel cortex (WBC) by unilateral deflection of rat whiskers. Results show that functional connectivity between bilateral WBC was significantly modulated at the 2 Hz, but not at the 4 or 6 Hz, stimulus condition. Electrophysiologically, only in the low-frequency range (<5 Hz) was the LFP power synchrony in bilateral WBC significantly modulated at 2 Hz, but not at 4- or 6-Hz whisker stimulation, thus distinguishing these 2 experimental conditions, and paralleling the findings in rsMRI. LFP power synchrony in other frequency ranges was modulated in a way that was neither unique to the specific stimulus conditions nor parallel to the fMRI results. Our results support the hypothesis that emphasizes the role of low-frequency LFP signal underlying rsMRI.