Glucagon-like peptide 1 receptor activation regulates cocaine actions and dopamine homeostasis in the lateral septum by decreasing arachidonic acid levels.

Agonism of the glucagon-like peptide 1 (GLP-1) receptor (GLP-1R) has been effective at treating aspects of addictive behavior for a number of abused substances, including cocaine. However, the molecular mechanisms and brain circuits underlying the therapeutic effects of GLP-1R signaling on cocaine actions remain elusive. Recent evidence has revealed that endogenous signaling at the GLP-1R within the forebrain lateral septum (LS) acts to reduce cocaine-induced locomotion and cocaine conditioned place preference, both considered dopamine (DA)-associated behaviors. DA terminals project from the ventral tegmental area to the LS and express the DA transporter (DAT). Cocaine acts by altering DA bioavailability by targeting the DAT. Therefore, GLP-1R signaling might exert effects on DAT to account for its regulation of cocaine-induced behaviors. We show that the GLP-1R is highly expressed within the LS. GLP-1, in LS slices, significantly enhances DAT surface expression and DAT function. Exenatide (Ex-4), a long-lasting synthetic analog of GLP-1 abolished cocaine-induced elevation of DA. Interestingly, acute administration of Ex-4 reduces septal expression of the retrograde messenger 2-arachidonylglycerol (2-AG), as well as a product of its presynaptic degradation, arachidonic acid (AA). Notably, AA reduces septal DAT function pointing to AA as a novel regulator of central DA homeostasis. We further show that AA oxidation product γ-ketoaldehyde (γ-KA) forms adducts with the DAT and reduces DAT plasma membrane expression and function. These results support a mechanism in which postsynaptic septal GLP-1R activation regulates 2-AG levels to alter presynaptic DA homeostasis and cocaine actions through AA.

Evidence against dopamine D1/D2 receptor heteromers.

Hetero-oligomers of G-protein-coupled receptors have become the subject of intense investigation, because their purported potential to manifest signaling and pharmacological properties that differ from the component receptors makes them highly attractive for the development of more selective pharmacological treatments. In particular, dopamine D1 and D2 receptors have been proposed to form hetero-oligomers that couple to Gαq proteins, and SKF83959 has been proposed to act as a biased agonist that selectively engages these receptor complexes to activate Gαq and thus phospholipase C. D1/D2 heteromers have been proposed as relevant to the pathophysiology and treatment of depression and schizophrenia. We used in vitro bioluminescence resonance energy transfer, ex vivo analyses of receptor localization and proximity in brain slices, and behavioral assays in mice to characterize signaling from these putative dimers/oligomers. We were unable to detect Gαq or Gα11 protein coupling to homomers or heteromers of D1 or D2 receptors using a variety of biosensors. SKF83959-induced locomotor and grooming behaviors were eliminated in D1 receptor knockout (KO) mice, verifying a key role for D1-like receptor activation. In contrast, SKF83959-induced motor responses were intact in D2 receptor and Gαq KO mice, as well as in knock-in mice expressing a mutant Ala(286)-CaMKIIα that cannot autophosphorylate to become active. Moreover, we found that, in the shell of the nucleus accumbens, even in neurons in which D1 and D2 receptor promoters are both active, the receptor proteins are segregated and do not form complexes. These data are not compatible with SKF83959 signaling through Gαq or through a D1/D2 heteromer and challenge the existence of such a signaling complex in the adult animals that we used for our studies.

Repeated i.v. cocaine exposure produces long-lasting behavioral sensitization in pregnant adults, but behavioral tolerance in their offspring.

Repeated exposure to cocaine during sensitive periods of forebrain development produces specific, long-lasting changes in the structure and function of maturing neural circuits. Similar regimens of drug exposure in adult animals with mature, homeostatically regulated nervous systems produce neuroadaptations that appear to be quite different in nature and magnitude. We studied the ability of cocaine to induce behavioral sensitization and/or tolerance following repeated administration of i.v. cocaine (3 mg/kg, twice daily) to pregnant rabbits during the period of peak differentiation within the rabbit cerebral cortex (embryonic day [E] 16-E25). Offspring and the adult mothers were behaviorally tested following acute administration of amphetamine 2 months after the litters were born. The offspring, having received cocaine during the prenatal sensitive period, showed profound behavioral tolerance to the amphetamine challenge. In contrast, the mothers of these offspring, who received cocaine at the same dose and duration, and experienced the same period of withdrawal, exhibited robust behavioral sensitization. These data indicate that specific adaptive changes in neural signaling and/or circuitry that occur in response to repeated exposure to psychostimulants are highly dependent upon the maturational state of the brain during which the exposure occurs.

Prenatal cocaine exposure produces consistent developmental alterations in dopamine-rich regions of the cerebral cortex.

Administration of cocaine to pregnant rabbits produces robust and long-lasting anatomical alterations in the dopamine-rich anterior cingulate cortex of offspring. These effects include increased length and decreased bundling of layer III and V pyramidal neuron dendrites, increases in parvalbumin expression in the dendrites of interneurons, and increases in detectable GABAergic neurons. We have now examined multiple cortical regions with varying degrees of catecholaminergic innervation to investigate regional variations in the ability of prenatal cocaine exposure to elicit these permanent changes. All regions containing a high density of tyrosine hydroxylase-immunoreactive fibers, indicative of prominent dopaminergic input, exhibited alterations in GABA and parvalbumin expression by interneurons and microtubule-associated protein-2 labeling of apical dendrites of pyramidal neurons. These regions included the medial prefrontal, entorhinal, and piriform cortices. In contrast, primary somatosensory, auditory and motor cortices exhibited little tyrosine hydroxylase staining and no measurable cocaine-induced changes in cortical structure. From these data we suggest that the presence of dopaminergic afferents contributes to the marked specificity of the altered development of excitatory pyramidal neurons and inhibitory interneurons induced by low dose i.v. administration of cocaine in utero.

Disease-specific changes in regulator of G-protein signaling 4 (RGS4) expression in schizophrenia.

Complex defects in neuronal signaling may underlie the dysfunctions that characterize schizophrenia. Using cDNA microarrays, we discovered that the transcript encoding regulator of G-protein signaling 4 (RGS4) was the most consistently and significantly decreased in the prefrontal cortex of all schizophrenic subjects examined. The expression levels of ten other RGS family members represented on the microarrays were unchanged and hierarchical data analysis revealed that as a group, 274 genes associated with G-protein signaling were unchanged. Quantitative in situ hybridization verified the microarray RGS4 data, and demonstrated highly correlated decreases in RGS4 expression across three cortical areas of ten subjects with schizophrenia. RGS4 expression was not altered in the prefrontal cortex of subjects with major depressive disorder or in monkeys treated chronically with haloperidol. Interestingly, targets for 70 genes mapped to the major schizophrenia susceptibility locus 1q21--22 were present on the microarrays, of which only RGS4 gene expression was consistently altered. The combined data indicate that a decrease in RGS4 expression may be a common and specific feature of schizophrenia, which could be due either to genetic factors or a disease- specific adaptation, both of which could affect neuronal signaling.

Identification of a sensitive period of prenatal cocaine exposure that alters the development of the anterior cingulate cortex.

Administration of cocaine to pregnant rabbits produces robust and long-lasting anatomical, neurochemical and behavioral alterations in their offspring. For example, exposure to cocaine following implantation [embryonic day (E) 8] through gestation (E29) produces increased length and decreased bundling of layer III and V pyramidal neuron dendrites, increased parvalbumin expression in the dendrites of interneurons, and increased GABA levels in the anterior cingulate cortex (ACC) and other dopamine-rich cortical areas. We have investigated the presence of a sensitive period of in utero exposure during specific developmental epochs prior to and during the onset of cortical development that might be capable of producing such effects. Cocaine (3 mg/kg i.v., twice daily) or saline was administered during embryonic days E16-E25 (onset and peak of corticogenesis), E8-E15 (prior to cortical plate formation), E8-E25 or E8-E29 as in earlier studies. Examination of the ACC in offspring exposed from E8 to E25 and from E16 to E25 were found to induce alterations in the development of pyramidal neurons and interneurons that are nearly identical to those induced by the complete exposure (E8-E29) paradigm. No alterations were observed following the E8-E15 cocaine exposure. These data suggest that exposure to cocaine during E16-E25, the time of peak corticogenesis, appearance of cortical monoamines and onset of D(1) dopamine receptor expression, is necessary and sufficient to produce long-term effects on the organization of excitatory pyramidal neurons and inhibitory interneurons in the ACC.

Quantitative autoradiographic mapping of rat brain dopamine D3 binding with [(125)I]7-OH-PIPAT: evidence for the presence of D3 receptors on dopaminergic and nondopaminergic cell bodies and terminals.

The regional distribution and cellular localization of dopamine D3 receptors in the rat brain was examined using quantitative autoradiography. [(125)I]7-OH-PIPAT bound in a saturable and reversible manner and exhibited subnanomolar affinity for a single population of GTP-insensitive sites. The pharmacological profile was characteristic of cloned D3 receptors and nonspecific binding was uniformly low. The highest levels of D3 receptors were measured in the islands of Calleja, nucleus accumbens, ventral pallidum, substantia nigra, and lobules 9 and 10 of the cerebellum. The high specific activity of this ligand also allowed detection of D3 receptors in other regions, including the serotonergic dorsal and median raphe nuclei, indicating that the distribution of this receptor is more widespread than previously appreciated. The cellular localization of D3 receptors in regions containing dopaminergic cells and terminals was examined by discrete injection of neurotoxins. Lesion of dopaminergic neurons with 6-hydroxydopamine produced 50% decreases in [(125)I]7-OH-PIPAT binding in the nucleus accumbens and substantia nigra. Quinolinic acid lesion of neurons originating in the nucleus accumbens also produced approximately 50% decreases in D3 receptors in the nucleus accumbens, substantia nigra, and ventral pallidum. 5, 7-Dihydroxytryptamine lesion of serotonergic cells and processes produced no changes in [(125)I]7-OH-PIPAT binding. These results demonstrate the presence of D3 receptors in several brain regions not previously identified and suggest that D3 receptors are expressed at somatodendritic and terminal levels of both dopaminergic and nondo-paminergic cells within the mesolimbic dopamine system.

Differential regulation of dopamine D2 and D3 receptors by chronic drug treatments.

Regulation of the expression of dopamine D2 and D3 receptors in the rat brain was examined using quantitative autoradiography after chronic (14 day) drug treatments designed to increase or decrease dopamine receptor stimulation. Reserpine treatment depleted endogenous dopamine by more than 90% and significantly increased the binding of [(125)I]NCQ 298 to D2 receptors in the nucleus accumbens, ventral pallidum, and substantia nigra. In contrast, this treatment significantly decreased the binding of [(125)I]7-OH-PIPAT to D3 receptors in each of these regions. Chronic stimulation of D2-like receptors with quinpirole (1 mg/kg/day) or 7-OH-DPAT (1 mg/kg/day) produced decreases in [(125)I]NCQ 298 binding in the nucleus accumbens, ventral pallidum, and substantia nigra as expected. As with depletion, chronic stimulation elicited an opposite response from D3 receptors with significant increases observed in the ventral pallidum and substantia nigra. D3 receptor expression in the nucleus accumbens was unchanged. Baclofen (30 mg/kg/day) or continuous administration of the psychomotor stimulant cocaine (20 mg/kg/day) produced no significant changes in D2 or D3 receptor binding in any region examined. Acute administration of the irreversible antagonist EEDQ (10 mg/kg) nearly eliminated D2 receptor binding in all regions, but inactivated D3 receptors only in the VP and SN, suggesting subtype-specific and region-specific differences in receptor occupancy. The existence of regional and subtype-specific heterogeneities in the regulation of these receptors supports the contention that despite their similar pharmacological profiles, D2 and D3 receptors may mediate different functional responses.

In utero cocaine-induced dysfunction of dopamine D1 receptor signaling and abnormal differentiation of cerebral cortical neurons.

Monoamines modulate neuronal differentiation, and alteration of monoamine neurotransmission during development produces specific changes in neuronal structure, function, and pattern formation. We have previously observed that prenatal exposure to cocaine in a clinically relevant animal model produces increased length of pyramidal neuron dendrites in the anterior cingulate cortex (ACC) postnatally. We now report that cocaine administered intravenously to pregnant rabbits at gestational stages preceding and during cortical histogenesis results in the early onset of hypertrophic dendritic outgrowth in the embryonic ACC. Confocal microscopy of DiI-labeled neurons revealed that the atypical, tortuous dendritic profiles seen postnatally in ACC-cocaine neurons already are apparent in utero. No defects in neuronal growth were observed in visual cortex (VC), a region lacking prominent dopamine innervation. In striking correlation with our in vivo results, in vitro experiments revealed a significant enhancement of spontaneous process outgrowth of ACC neurons isolated from cocaine-exposed fetuses but no changes in neurons derived from visual cortex. The onset of modified growth in vivo is paralleled by reduced D(1A) receptor coupling to its G-protein. These data suggest that the dynamic growth of neurons can be regulated by early neurotransmitter signaling in a selective fashion. Prenatal onset of defects in dopamine receptor signaling contributes to abnormal circuit formation and may underlie specific cognitive and behavioral dysfunction.

Effects of NADH on dopamine release in rat striatum.

Nicotinamide adenine dinucleotide (NADH) may be utilized for the synthesis and regeneration of tetrahydrobiopterin (BH(4)), which in turn is an essential cofactor for tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of dopamine (DA). NADH has been reported to relieve some of the symptoms of Parkinson's disease, presumably by altering dopaminergic function. The present study examines the efficacy of NADH in influencing DA activity in the rat striatum. In striatal slices, NADH (350 microM) significantly increased basal DA and DOPAC efflux and caused a 2-fold increase in the DA overflow evoked by high KCl (25 mM). Tissue levels of BH(4), basal BH(4) efflux, and KCl-evoked BH(4) overflow were unaffected by NADH, as was [(3)H]DA uptake into striatal synaptosomes. In contrast to the effects of NADH on DA function in vitro, no effects were observed when NADH was administered systemically. NADH (10 or 100 mg/kg, s.c.) did not influence the tissue content of DA, 5-HT, or their metabolites in the midbrain or striatum, nor did it alter DA extracellular concentrations. These results indicate that NADH can increase DA release from striatal slices, although we are as yet unable to detect this effect in vivo.

Ontogeny of dopamine D3 receptors in the nucleus accumbens of the rat.

The expression of dopamine D2 and D3 receptors in the developing rat nucleus accumbens and striatum was examined using quantitative receptor autoradiography. Male Sprague-Dawley rats were sacrificed on postnatal day 3, 7, 10, 14, 21, or 60. Sections were labeled with [125I]NCQ 298, which binds to both D2 and D3 receptor subtypes. Binding to D2/D3 receptors in the caudate-putamen appeared as early as P3 (approximately 20% of adult) and approached adult levels (75% of adult) by P21. D2/D3 receptors in the nucleus accumbens and olfactory tubercle developed with a similar time course. [125I]R(+)trans-7-hydroxy-2-(N-n-propyl-N-3'-iodo-2'-propenyl)aminotetra lin ([125I]7-OH-PIPAT) was used to selectively label D3 receptors in adjacent sections. [125I]7-OH-PIPAT binding was absent at P3 and just detectable at P7 and P10 (approximately 5% of adult). Appreciable D3 labeling appeared in the islands of Calleja at P14 and in the nucleus accumbens at P21. [125I]7-OH-PIPAT also detected a very low density of D3 receptors in the caudate-putamen which developed with a profile very similar to that of D3 receptors in the nucleus accumbens. Expression of the D2 receptor subtype therefore appears to precede expression of the D3 receptor subtype. Additionally, D2 receptors in different regions are expressed with a similar developmental profile, but there appears to be more heterogeneity in the ontogeny of forebrain D3 receptor expression.