Role of dopamine and GABA in the control of motor activity elicited from the rat nucleus accumbens

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Integrative opioid-GABAergic neuronal mechanisms regulating dopamine efflux in the nucleus accumbens of freely moving animals

The nucleus accumbens (NAc) is a terminal region of mesocorticolimbic dopamine (DA) neuronal projections from the ventral tegmental area. Accumbal DA release is integrated by afferents from other brain regions and by interneurons, which involve a diversity of neurotransmitters and neuropeptides. These integrative processes, implicated in the pathobiology of neuropsychiatric disorders, are mediated via receptor subtypes whose relative roles in the regulation of accumbal DA release are poorly understood. Such complex interactions are exemplified by how selective activation of opioid receptor subtypes enhances accumbal DA efflux in a manner that is modulated by changes in neural activity through GABA receptor subtypes. This review delineates the roles of GABA_A and GABA_B receptors in GABAergic neural mechanisms in NAc that participate in delta- and mu-opioid receptor-mediated increases in accumbal DA efflux in freely moving rats, focusing on studies using in vivo brain microdialysis. First, we consider how endogenous GABA exerts inhibition of accumbal DA efflux through GABA receptor subtypes. We also consider possible intra-neuronal source of the endogenous GABA that inhibits accumbal DA efflux. As NAc contains GABAergic neurons that express delta- or mu-opioid receptors, inhibition of accumbal GABAergic neurons is a candidate for mediating delta- or mu-opioid receptor-mediated increases in accumbal DA efflux. Therefore, we provide a detailed analysis of the effects of GABA receptor subtype ligands on delta- and mu-opioid receptor-mediated accumbal DA efflux. Finally, we present an integrative model to explain the mechanisms of interaction among delta- and mu-opioid receptors, GABAergic neurons and DAergic neurons in NAc.

Influence of intranasal exposure of MPTP in multiple doses on liver functions and transition from non-motor to motor symptoms in a rat PD model

Besides the effects on the striatum, the impairment of visceral organs including liver functions has been reported in Parkinson's disease (PD) patients. However, it is yet unclear if liver functions are affected in the early stage of the disease before the motor phase has appeared. The aim of our present study was thus to assess the effect of intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in different doses on striatum and liver functions. Deterioration of non-motor activities appeared on single exposure to MPTP along with rise in striatum oxidative stress and decline in antioxidant levels. Decreases in dopamine, noradrenaline, and GABA and increase in serotonin were detected in striatum. Motor coordination was impaired with a single dose of MPTP, and with repeated MPTP exposure, there was further significant impairment. Locomotor activity was affected from second exposure of MPTP, and the impairment increased with third MPTP exposure. Impairment of liver function through increase in serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels was observed after first MPTP insult, and it worsened with second and third administrations. First administration of MPTP triggered systemic inflammation showing significant increase in inflammatory markers in the liver. Our data shows for the first time that an intranasal route of entry of MPTP affects liver from the non-motor phase of PD itself, occurring concomitantly with the reduction of striatal dopamine. It also suggests that a single dose is not enough to bring about progression of the disease from non-motor to locomotor deficiency, and a repeated dose is needed to establish the motor severity phase in the rat intranasal MPTP model.

Disinhibition of the Nucleus Accumbens Leads to Macro-Scale Hyperactivity Consisting of Micro-Scale Behavioral Segments Encoded by Striatal Activity

The striatum comprises of multiple functional territories involved with multilevel control of behavior. Disinhibition of different functional territories leads to territory-specific hyperkinetic and hyperbehavioral symptoms. The ventromedial striatum, including the nucleus accumbens (NAc) core, is typically associated with limbic input but was historically linked to high-level motor control. In this study, performed in female Long-Evans rats, we show that the NAc core directly controls motor behavior on multiple timescales. On the macro-scale, following NAc disinhibition, the animals manifested prolonged hyperactivity, expressed as excessive normal behavior, whereas on the micro-scale multiple behavior transitions occurred, generating short movement segments. The underlying striatal network displayed population-based local field potential transient deflections (LFP spikes) whose rate determined the magnitude of the hyperactivity and whose timing corresponded to unitary behavioral transition events. Individual striatal neurons preserved normal baseline activity and network interactions following the disinhibition, maintaining the normal encoding of behavioral primitives and forming a sparse link between the LFP spikes and single neuron activity. Disinhibition of this classically limbic territory leads to profound motor changes resembling hyperactivity and attention deficit. These behavioral and neuronal results highlight the direct interplay on multiple timescales between different striatal territories during normal and pathological conditions.SIGNIFICANCE STATEMENT The nucleus accumbens (NAc) is a key part of the striatal limbic territory. In the current study we show that this classically limbic area directly controls motor behavior on multiple timescales. Focal disinhibition of the NAc core in freely behaving rats led to macro-scale hyperactivity and micro-scale behavioral transitions, symptoms typically associated with attention deficit hyperactivity disorder. The behavioral changes were encoded by the striatal LFP signal and single-unit spiking activity in line with the neuronal changes observed during tic expression following disinhibition of the striatal motor territory. These results point to the need to extend the existing parallel functional pathway concept of basal ganglia function to include the study of limbic-motor cross-territory interactions in both health and disease.

Prenatal cannabinoid exposure and altered neurotransmission

Marijuana is one of the most commonly used illicit drugs worldwide. In addition, use of synthetic cannabinoids is increasing, especially among adolescents and young adults. Although human studies have shown that the use of marijuana during pregnancy leads to adverse behavioral effects, such as deficiencies in attention and executive function in affected offspring, the rate of marijuana use among pregnant women is steadily increasing. Various aspects of human behavior including emotion, learning, and memory are dependent on complex interactions between multiple neurotransmitter systems that are especially vulnerable to alterations during the developmental period. Thus, exploration of neurotransmitter changes in response to prenatal cannabinoid exposure is crucial to develop an understanding of how homeostatic imbalance and various long-term neurobehavioral deficits manifest following the abuse of marijuana or other synthetic cannabinoids during pregnancy. Current literature confirms that vast alterations to neurotransmitter systems are present following prenatal cannabinoid exposure, and many of these alterations within the brain are region specific, time-dependent, and sexually dimorphic. In this review, we aim to provide a summary of observed changes to various neurotransmitter systems following cannabinoid exposure during pregnancy and to draw possible correlations to reported behavioral alterations in affected offspring.

Proteomic Analysis of the Spinophilin Interactome in Rodent Striatum Following Psychostimulant Sensitization

Glutamatergic projections from the cortex and dopaminergic projections from the substantia nigra or ventral tegmental area synapse on dendritic spines of specific GABAergic medium spiny neurons (MSNs) in the striatum. Direct pathway MSNs (dMSNs) are positively coupled to protein kinase A (PKA) signaling and activation of these neurons enhance specific motor programs whereas indirect pathway MSNs (iMSNs) are negatively coupled to PKA and inhibit competing motor programs. An imbalance in the activity of these two programs is observed following increased dopamine signaling associated with exposure to psychostimulant drugs of abuse. Alterations in MSN signaling are mediated by changes in MSN protein post-translational modifications, including phosphorylation. Whereas direct changes in specific kinases, such as PKA, regulate different effects observed in the two MSN populations, alterations in the specific activity of serine/threonine phosphatases, such as protein phosphatase 1 (PP1) are less well known. This lack of knowledge is due, in part, to unknown, cell-specific changes in PP1 targeting proteins. Spinophilin is the major PP1-targeting protein in striatal postsynaptic densities. Using proteomics and immunoblotting approaches along with a novel transgenic mouse expressing hemagglutainin (HA)-tagged spinophilin in dMSNs and iMSNs, we have uncovered cell-specific regulation of the spinophilin interactome following a sensitizing regimen of amphetamine. These data suggest regulation of spinophilin interactions in specific MSN cell types and may give novel insight into putative cell-specific, phosphatase-dependent signaling pathways associated with psychostimulants.

Stimulation of accumbal GABAB receptors inhibits delta1- and delta2-opioid receptor-mediated dopamine efflux in the nucleus accumbens of freely moving rats

The nucleus accumbens contains delta-opioid receptors that may decrease inhibitory neurotransmission. As GABA_B receptors inhibit dopamine release, decrease in activation of GABA_B receptors may be a mediator of delta-opioid receptor-induced accumbal dopamine efflux. If so, accumbal dopamine efflux induced by delta-opioid receptor activation should be suppressed by stimulating GABA_B receptors. As delta-opioid receptors are further subdivided into delta1- and delta2-opioid receptors, we analysed the effects of the GABA_B receptor agonist baclofen on delta1- and delta2-opioid receptor-mediated accumbal dopamine efflux in freely moving rats using in vivo microdialysis. Drugs were applied intracerebrally through the dialysis probe. Doses of compounds show total amount administered (mol) during 25-50 min infusions. Baclofen (2.5 and 5.0 nmol), which did not alter basal dopamine levels, inhibited the delta1-opioid receptor agonist DPDPE (5.0 nmol)-induced dopamine efflux. Baclofen (2.5 and 5.0 nmol) also inhibited the delta2-opioid receptor agonist deltorphin II (25.0 nmol)-induced dopamine efflux. A low dose of the GABA_B receptor antagonist 2-hydroxysaclofen (100.0 pmol), which failed to alter basal accumbal dopamine levels, counteracted the inhibitory effects of baclofen (5.0 nmol) on DPDPE (5.0 nmol)- and deltorphin II (25.0 nmol)-induced dopamine efflux. The present results show that reduction in accumbal GABA_B receptor-mediated inhibition of accumbal dopaminergic activity facilitates activation of delta1- and delta2-opioid receptor-induced increases in accumbal dopamine efflux. This study suggests that activation of delta1- and delta2-opioid receptors on the cell bodies and/or terminals of accumbal GABAergic interneurons inhibits GABA release and, accordingly, decreases GABA_B receptor-mediated inhibition of dopaminergic terminals, resulting in enhanced accumbal dopamine efflux.

Roles of Retinoic Acid Signaling in Shaping the Neuronal Architecture of the Developing Amphioxus Nervous System

The morphogen retinoic acid (RA) patterns vertebrate nervous systems and drives neurogenesis, but how these functions evolved remains elusive. Here, we show that RA signaling plays stage- and tissue-specific roles during the formation of neural cell populations with serotonin, dopamine, and GABA neurotransmitter phenotypes in amphioxus, a proxy for the ancestral chordate. Our data suggest that RA signaling restricts the specification of dopamine-containing cells in the ectoderm and of GABA neurons in the neural tube, probably by regulating Hox1 and Hox3 gene expression, respectively. The two Hox genes thus appear to serve distinct functions rather than to participate in a combinatorial Hox code. We were further able to correlate the RA signaling-dependent mispatterning of hindbrain GABA neurons with concomitant motor impairments. Taken together, these data provide new insights into how RA signaling and Hox genes contribute to nervous system as well as to motor control development in amphioxus and hence shed light on the evolution of these functions within vertebrates.

New Insights Into the Roles of Retinoic Acid Signaling in Nervous System Development and the Establishment of Neurotransmitter Systems

Secreted chiefly from the underlying mesoderm, the morphogen retinoic acid (RA) is well known to contribute to the specification, patterning, and differentiation of neural progenitors in the developing vertebrate nervous system. Furthermore, RA influences the subtype identity and neurotransmitter phenotype of subsets of maturing neurons, although relatively little is known about how these functions are mediated. This review provides a comprehensive overview of the roles played by RA signaling during the formation of the central and peripheral nervous systems of vertebrates and highlights its effects on the differentiation of several neurotransmitter systems. In addition, the evolutionary history of the RA signaling system is discussed, revealing both conserved properties and alternate modes of RA action. It is proposed that comparative approaches should be employed systematically to expand our knowledge of the context-dependent cellular mechanisms controlled by the multifunctional signaling molecule RA.

Monocrotophos induced oxidative stress and alterations in brain dopamine and serotonin receptors in young rats

Human exposure to monocrotophos, an organophosphate pesticide, could occur due to its high use in agriculture to protect crops. Recently, we found that postlactational exposure to monocrotophos impaired cholinergic mechanisms in young rats and such changes persisted even after withdrawal of monocrotophos exposure. In continuation to this, the effect of monocrotophos on noncholinergic targets and role of oxidative stress in its neurotoxicity has been studied. Exposure of rats from postnatal day (PD)22 to PD49 to monocrotophos (0.50 or 1.0 mg kg−1 body weight, perorally) significantly impaired motor activity and motor coordination on PD50 as compared to controls. A significant decrease in the binding of 3H-spiperone to striatal membrane (26%, p < 0.01; 30%, p < 0.05) in rats exposed to monocrotophos at both the doses and increase in the binding of 3H-ketanserin to frontocortical membrane (14%, p > 0.05; 37%, p < 0.05) in those exposed at a higher dose, respectively, was observed on PD50 compared with the controls. Alterations in the binding persisted even after withdrawal of monocrotophos exposure on PD65. Increased oxidative stress in brain regions following exposure of rats to monocrotophos was also observed on PD50 that persisted 15 days after withdrawal of exposure on PD65. The results suggest that monocrotophos exerts its neurobehavioral toxicity by affecting noncholinergic functions involving dopaminergic and serotonergic systems associated with enhanced oxidative stress. The results also exhibit vulnerability of developing brain to monocrotophos as most of the changes persisted even after withdrawal of its exposure.

Urinary delta-ALA: a potential biomarker of exposure and neurotoxic effect in rats co-treated with a mixture of lead, arsenic and manganese

Lead (Pb), arsenic (As) and manganese (Mn) are neurotoxic elements that often occur in mixtures for which practically no information is available on biomarkers (BMs) for the evaluation of exposure/effects. Exposures to these metals may increase delta-aminolevulinic acid (delta-ALA), which in itself may potentiate neurotoxicity. The objective of this study was to investigate the utility of urinary delta-ALA (delta-ALA-U) levels as BM of exposure and/or neurotoxic effects induced by this mixture. Five groups of Wistar rats were treated for 8 days with Pb (5mg/kg), As (60mg/L), Mn (10mg/kg), the 3-metal mixture (same doses of the single metals), and control group. Motor activity was evaluated and 24-h urine collected before and after the treatment. 24-hours (h) after the last dose, the rats were sacrificed and the brains removed for analyses. Delta-ALA and metal levels were determined in brain and urine. Co-treated rats showed a significant (p<0.05) correlation between increased Pb, As, Mn and delta-ALA levels in the brain and decreased motor activity. Delta-ALA-U concentrations were higher in the mixture-treated group than the sum of the delta-ALA-U levels in each single-treated groups and discriminated (p<0.05) between the mixture and untreated rats. Moreover, delta-ALA-U was correlated (p<0.05) with brain delta-ALA levels. These results establish that treatments with this metal mixture exacerbate behavioral dysfunction, increasing most prominently brain Pb levels. This study is the first to establish that delta-ALA-U levels represent a sensitive BM of exposure/neurotoxic effect to this metal mixture.

In vivo neurochemical evidence that newly synthesised GABA activates GABA(B), but not GABA(A), receptors on dopaminergic nerve endings in the nucleus accumbens of freely moving rats

GABA released from accumbal GABAergic interneurons plays an inhibitory role in the regulation of dopamine efflux through GABA(B) and GABA(A) receptors located on accumbal dopaminergic nerve endings. The cytosolic newly synthesised GABA alters vesicular GABA levels and, accordingly, the amount of GABA released from the neuron. Therefore, we hypothesised that glutamic acid decarboxylase (GAD) which generates GABA in accumbal GABAergic neurons, at least partly determines the GABA receptor subtype-mediated GABAergic tonus. To (in)validate this hypothesis, in vivo microdialysis was used to study the effects of an intra-accumbal infusion of the GAD inhibitor l-allylglycine (allylglycine) on the accumbal dopamine efflux of freely moving rats. The intra-accumbal infusion of allylglycine (50.0, 250.0 and 500.0 nmol) dose-dependently increased the accumbal dopamine levels. The co-administration of tetrodotoxin (720 pmol) suppressed the allylglycine (500.0 nmol)-induced dopamine efflux. The intra-accumbal infusion of GABA(B) receptor agonist baclofen (2.5 and 5.0 nmol) inhibited the allylglycine (500.0 nmol)-induced dopamine efflux. The baclofen's effects were counteracted by GABA(B) receptor antagonist saclofen (10.0 nmol). Neither GABA(A) receptor agonist (muscimol: 25.0 and 250.0 pmol) nor antagonist (bicuculline: 50.0 pmol) altered the allylglycine (250.0 and 500.0 nmol)-induced dopamine efflux. The present study provides in vivo neurochemical evidence that newly synthesised GABA that exerts an inhibitory tonus on the accumbal dopaminergic activity, acts at the level of GABA(B) receptors, but not GABA(A) receptors. The present study also shows that there is an allylglycine-insensitive GABA pool that release GABA exerting an inhibitory control of the accumbal dopaminergic activity, at the level of GABA(A) receptors. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Role of GABA B receptors in the endomorphin-1-, but not endomorphin-2-, induced dopamine efflux in the nucleus accumbens of freely moving rats

In vivo microdialysis was used to study the effects of the locally applied GABA B receptor antagonist 2-hydroxysaclofen and GABA B receptor agonist baclofen on the basal dopamine efflux as well as on the endomorphin-1- and endomorphin-2-induced dopamine efflux in the nucleus accumbens of freely moving rats. 2-Hydroxysaclofen (100 and 500 nmol) increased basal dopamine efflux. Baclofen (2.5 and 5 nmol) failed to affect basal dopamine efflux. 2-Hydroxysaclofen (1 and 10 nmol) which did not alter the basal dopamine efflux, enhanced the endomorphin-1 (25 nmol)-induced dopamine efflux. Baclofen (2.5 and 5 nmol) failed to affect endomorphin-1 (25 nmol)-induced dopamine efflux, but it counteracted the 2-hydroxysaclofen-induced increase of the endomorphin-1-elicited dopamine efflux. Neither 2-hydroxysaclofen (10 nmol) nor baclofen (5 nmol) affected the endomorphin-2 (25 nmol)-induced dopamine efflux. The doses mentioned are the total amount of drug over the infusion period that varied across the drugs (25 or 50 min). These results suggest that accumbal GABA B receptor plays an inhibitory role on the basal as well as the endomorphin-1-elicited accumbal dopamine efflux. The present results support our earlier reported notion that endomorphin-1 and endomorphin-2 increase accumbal dopamine efflux by different mechanisms. Finally, it is suggested that a decrease of endogenous accumbal GABA reduces the accumbal GABA B receptor-mediated GABA-ergic inhibition, enhancing thereby the accumbal dopamine efflux.

Role of GABAA receptors in the endomorphin-1-, but not endomorphin-2-, induced dopamine efflux in the nucleus accumbens of freely moving rats

In vivo microdialysis was used to study the effects of the locally applied GABA(A) receptor agonist muscimol and GABA(A) receptor antagonist bicuculline on the basal dopamine efflux as well as on the endomorphin-1- and endomorphin-2-induced dopamine efflux in the nucleus accumbens of freely moving rats. Muscimol (2500 pmol) and bicuculline (5 and 10 nmol) increased basal dopamine efflux. Bicuculline (50 pmol) inhibited the muscimol (2500 pmol)-induced dopamine efflux. Muscimol (250 pmol), but not bicuculline (50 and 500 pmol), enhanced the endomorphin-1 (25 nmol)-induced dopamine efflux. Bicuculline (50 pmol) counteracted the muscimol (250 pmol)-induced increase of the endomorphin-1-elicited dopamine efflux. Neither muscimol (25 and 250 pmol) nor bicuculline (50 and 500 pmol) affected the endomorphin-2 (25 nmol)-induced dopamine efflux. The doses mentioned are the total amount of drug over the infusion period (25 or 50 min) that varied across the drugs. The finding that muscimol and bicuculline increased basal dopamine efflux may imply that these drugs acted at different sites. It is suggested that (1) muscimol acts at GABA(A) receptors on GABA-ergic neurons that exert an inhibitory control of dopaminergic neurons and, accordingly, disinhibits these dopaminergic neurons, and that (2) bicuculline acts directly at GABA(A) receptors on dopaminergic neurons and, accordingly, removes the inhibitory control of these dopaminergic neurons. The finding that an agonist, but not antagonist, of GABA(A) receptors enhanced the endomorphin-1's effects might indicate that endomorphin-1 produced a floor effect at the level of GABA(A) receptors located on presynaptic, dopaminergic terminals. Finally, the present results support our earlier reported notion that endomorphin-1 and endomorphin-2 increase accumbal dopamine efflux by different mechanisms.

GABAA-receptor modification in taurine transporter knockout mice causes striatal disinhibition

The Striatum is involved in the regulation of movements and motor skills. We have shown previously, that the osmolyte and neuromodulator taurine plays a role in striatal plasticity. We demonstrate now that hereditary taurine deficiency in taurine-transporter knock-out (TAUT KO) mice results in disinhibition of striatal network activity, which can be corrected by taurine supplementation. Modification of GABAA but not glycine receptors (taurine is a ligand for both receptor types) underlies this disinhibition. Whole-cell recordings from acutely isolated as well as cultured striatal neurons revealed a decreased agonist sensitivity of the GABAA receptor in TAUT KO neurons in the absence of changes in the maximal GABA-evoked current amplitude. The striatal GABA level in TAUT KO mice was unchanged. The amplitude enhancement of spontaneous IPSCs by zolpidem was stronger in TAUT KO than in wild-type (WT) animals. Tonic inhibition was absent in striatal neurons under control conditions but was detected after incubation with the GABA-transaminase inhibitor vigabatrin: bicuculline induced a larger shift of baseline current in WT as compared to TAUT KO neurons. Lack of taurine leads to reduced sensitivity of synaptic and extrasynaptic GABAA receptors and consequently to disinhibition. These findings help in understanding neuropathologies accompanied by the loss of endogenous taurine, for instance in hepatic encephalopathy.

A possible mechanism for the effect of modifiable lateral inhibition in the striatum on the selection of conditioned reflex motor responses

A mechanism is proposed for the effects of striatal dopamine-modifiable lateral inhibition on the selection of conditioned reflex motor responses. According to this mechanism, activation of dopamine D1 (D2) receptors on strionigral (striopallidal) neurons facilitates long-term depression (potentiation) of the inhibitory inputs simultaneously with potentiation (depression) of the excitatory inputs, of sufficient strength to open NMDA channels. For " weak" excitation, insufficient to open NMDA channels, the modification rules were of the opposite sign. Activation of presynaptic D2 (D1) receptors leads to decreases (increases) in GABA release from strionigral (striopallidal) axon terminals innervating strionigral (striopallidal) cells. As a result, dopamine-modifiable lateral inhibition simultaneously increases both the potentiation (depression) of the excitatory inputs to "strongly" activated strionigral (striopallidal) neurons, increasing (decreasing) their activity, and increases the depression (potentiation) of the excitatory inputs to the "weakly" activated strionigral (striopallidal) neurons, decreasing (increasing) their activity. Subsequent reorganization of neuron activity in the cortex-basal ganglia-thalamus-cortex circuit facilitates selection of conditioned reflex motor responses by further increasing (decreasing) the activity of those motor cortex neurons which were "strongly" ("weakly") excited by the striatum in conditions of dopamine release in response to the conditioned stimulus.

Neurotensin antagonist acutely and robustly attenuates locomotion that accompanies stimulation of a neurotensin-containing pathway from rostrobasal forebrain to the ventral tegmental area

Neurotensin exerts complex effects on the mesolimbic dopamine system that alter motivation and contribute to neuroadaptations associated with psychostimulant drug administration. Activation of abundant neurotensin receptors in the ventral tegmental area (VTA) enhances dopamine neuron activity and associated release of dopamine in the nucleus accumbens (Acb) and cortex. In view of recent anatomical studies demonstrating that 70% of all neurotensin-containing neurons projecting to the VTA occupy the lateral preoptic area-rostral lateral hypothalamus (LPH) and lateral part of the medial preoptic area (MPOA), the present study examined functionality in the LPH-MPOA neurotensinergic pathway in the rat. Disinhibition (resulting ultimately in stimulation-like effects) of LPH-MPOA neurons with microinjected bicuculline (50 or 100 ng in 0.25 microL) produced locomotor activation that was considerably attenuated by systemic administration of the neurotensin antagonist SR 142948 A (0.03 and 0.1 mg/kg). In contrast, locomotion elicited in this manner was completely blocked by SR 142948 A infused directly into the VTA (5.0 and 15.0 ng in 0.25 microL). Baseline locomotion was unaffected by systemic or intra-VTA administration of SR 142948 A and LPH-MPOA-elicited locomotion was unaffected by infusion of SR 142948 A into the substantia nigra pars compacta and sites rostral and dorsal to the VTA. Locomotion was not elicited by infusions of bicuculline into the lateral hypothalamus at sites caudal to the LPH-MPOA, where neurotensin neurons projecting to the VTA are fewer. The results demonstrate the capacity of a neurotensin-containing pathway from LPH-MPOA to VTA to modulate locomotion. This pathway may be important in linking hippocampal and mesolimbic mechanisms in normal behaviour and drug addiction.

Kainate receptor activation potentiates GABAergic synaptic transmission in the nucleus accumbens core

Inhibitory synaptic transmission plays an important role in regulating the activity of medium spiny neurons (MSNs) in the nucleus accumbens (NAcc). The kainate (KA) subtype of ionotropic glutamate receptor has been shown to potently modulate GABAergic synaptic transmission in several brain regions. Although KA receptor subunits are expressed in the NAcc, KA receptor modulation of GABAergic synaptic transmission in this brain region has not been previously examined. In the current study, we sought to determine if KA receptor activation could alter inhibitory synaptic transmission in the NAcc as it has been shown to do in other brain regions. Using the whole cell patch-clamp technique, we demonstrate that KA receptor activation potentiates evoked GABAergic synaptic transmission and increases the frequency of spontaneous, but not miniature, GABA(A)-receptor-mediated IPSCs in the NAcc. In contrast, KA has no effect on currents evoked by exogenous application of GABA onto MSNs. Taken together, these data suggest that activation of KA receptors in the NAcc core potently facilitates action-potential-dependent GABAergic synaptic transmission, likely via an excitation of presynaptic GABAergic interneurons.

Modulation of the gait deficit in arthritic rats by infusions of muscimol and bicuculline

Gait analysis in the adjuvant-induced arthritic rat model of chronic pain was used to examine the role of GABA(A) receptors in the development of pain. Drug solutions were administered continuously at 5+/-0.75 microl/h for 14 days via Alzet osmotic pumps (2ML2) placed under the skin of the back. The GABA(A) receptor agonist, muscimol, produces a dose-dependent reversal of the gait deficits seen in arthritic rats without reducing the tibiotarsal joints inflammatory edema or the histological picture of joint erosion and inflammation. The higher infusion rate for muscimol, 20 microg/h, caused the gait for the arthritic rats to be indistinguishable from that of normal non-arthritic rats. In normal, non-arthritic rats, muscimol did not show any effect on gait. The GABA(A) receptor antagonist bicuculline showed small but significant exacerbation of stride length (P < 0.05) single and double stance time (P < 0.05) and swing time deficits (P < 0.05) in the arthritic rats, but no changes in measures of gait in the normal control rat. The results suggest that the development of arthritic pain is increased in the absence of GABA(A) receptor tone and that increasing GABA(A) receptor tone can reduce arthritic pain but does not affect the disease process.

Transient inactivation of the rat nucleus accumbens does not impair guidance of instrumental behaviour by stimuli predicting reward magnitude

The involvement of the nucleus accumbens (NAc) in the determination of reaction times (RTs) of instrumental responses by the expectancy of future reward was investigated. A simple RT task demanding conditioned lever release was used, in which the upcoming reward magnitude (5 versus 1 pellet) was signalled in advance by discriminative cues. In rats which acquired the task, RTs of instrumental responses were significantly shorter to the discriminative cue predictive of high reward magnitude. Inactivation of the NAc by lidocaine had no effect on RTs and their determination by cue-associated reward magnitudes, and did not affect the rate of correct responses. In keeping with an earlier study, intra-NAc infusion of amphetamine decreased RTs, impaired RT determination by cue-associated reward magnitudes and reduced the rate of correct responses. The unexpected finding that lidocaine inactivation of the NAc had no effect parallels previous data showing that lesions of NAc did not impair RT performance, while manipulation of intra-NAc glutamate or dopamine transmission impaired various aspects of RT performance in comparable tasks. It is suggested that experimental manipulations such as transient and permanent inactivation, which almost completely inhibit NAc neuronal output, allow alternative routes to be used to effectively control behaviour in the task employed here.

GABAA and GABAB receptors in the nucleus accumbens shell differentially modulate dopamine and acetylcholine receptor-mediated turning behaviour

The ability of GABAA and GABAB receptors in the shell of the nucleus accumbens to modulate distinct types of turning behaviour was investigated in freely moving rats, using the unilateral injection technique. The GABAA receptor agonist muscimol and the GABAA receptor antagonist bicuculline did not produce turning behaviour; the same holds for the GABAB agonist baclofen and the GABAB antagonist 2-hydroxysaclofen. A mixture of the dopamine D1 receptor agonist SKF 38393 and the dopamine D(2/3) receptor agonist quinpirole has been found to elicit contraversive pivoting, when injected into the shell. This pivoting was dose-dependently inhibited by muscimol, and the inhibitory effect of muscimol was antagonised by bicuculline. Pivoting was also dose-dependently inhibited by baclofen; however, 2-hydroxysaclofen did not antagonise the inhibitory effect. The acetylcholine receptor agonist carbachol has been found to elicit contraversive circling, when injected into the shell. This carbachol-induced circling was inhibited by baclofen, and 2-hydroxysaclofen antagonised the inhibitory effect. Carbachol-induced circling was also partially inhibited by muscimol; however, the inhibitory effect of muscimol was not antagonised by bicuculline. It is concluded that mesolimbic GABAA receptors exert an inhibitory control on dopamine-dependent pivoting that can be elicited from the shell of the nucleus accumbens, and that GABAB receptors exert an inhibitory control on acetylcholine-dependent circling that can be elicited from the shell of the nucleus accumbens. This data extends the earlier reported findings that the neurochemical substrate in the shell of the nucleus accumbens that mediates dopamine-dependent pivoting is fundamentally different from the shell substrate that mediates acetylcholine-dependent circling.

Baclofen attenuates conditioned locomotion to cues associated with cocaine administration and stabilizes extracellular glutamate levels in rat nucleus accumbens

We have used cocaine-conditioned locomotion in rats as an animal model for cocaine-conditioned responses that contribute to drug craving and relapse in human addicts. The purpose of the present study was to examine the ability of the GABA(B) agonist, baclofen, to attenuate such associative responses. First, experiments were conducted to identify a dose range of baclofen that did not impede exploratory or spontaneous behavior. This dose range was used during testing for conditioned locomotion specific to a flashing light and metronome, which were previously associated with administration of cocaine (PAIRED group) or saline (UNPAIRED group). At 2.0 mg/kg, baclofen attenuated conditioned locomotion in PAIRED subjects to the level of UNPAIRED subjects receiving saline or 2.0 mg/kg baclofen. Considering the importance of glutamate transmission in the nucleus accumbens (NAc) during associative responses to reward-related stimuli, the effect of baclofen on extracellular levels of glutamate in the NAc was tested with microdialysis. Baclofen (2.0 mg/kg) did not alter basal glutamate levels. However, baclofen pretreatment prevented the predatory odor, 2,5-dihydro-2,4,5-trimethylthiazoline, from increasing glutamate levels. This is the first report of baclofen modulating extracellular levels of glutamate in the NAc. Baclofen may prove to have general utility for suppressing stimulus-evoked increases in NAc glutamate levels. This could explain its ability to prevent cocaine-conditioned responses. In summary, our results suggest that enhancing GABA(B) transmission inhibits cocaine-conditioned responses, possibly by suppressing glutamate transmission in the NAc. A better understanding of interactions between GABA and glutamate transmission in the NAc may lead to the development of pharmacotherapies for cocaine craving.

Baclofen prevents hypoxia-induced consolidation impairment for passive avoidance in rats

We investigated the effects of baclofen, a selective GABA-B receptor agonist, on certain behaviours in rats after short-term hypoxia, as a model of experimentally induced amnesia. Baclofen given intraperitoneally (i.p.) in a dose of 0.25 mg kg(-1) increased the number of crossings and bar approaches in the open field, but was ineffective in the passive avoidance tests; it also shortened the time spent in open arms and reduced the number of open arms entries in an elevated 'plus' maze, being a measure of anxiety. Hypoxia (2% O2, 98% N2) within 4 min profoundly impaired locomotor activity, consolidation and retrieval of conditioned responses, and exhibited a proaxiogenic effect in the elevated 'plus' maze in rats--it reduced the time spent in open arms and the number of entries to closed and open arms. Baclofen's effect on locomotor and exploratory activity was substantially impaired after hypoxia, i.e. rats exhibited a significant reduction in those activities. This agonist of GABA-B receptor used before hypoxia significantly improved consolidation, but had no effect on retrieval. In the elevated 'plus' maze rats pre-treated with baclofen and then subjected to hypoxia prolonged the time spent in open arms, reduced the time spent in closed arms, and increased the number of entries to the arms, i.e. exhibited anxiolytic effect. We conclude, therefore, that baclofen improved consolidation of passive avoidance in rats undergoing hypoxia.

Facilitatory effect of unilateral lesion of the ventral tegmental area on locomotor response to stimulation of the contralateral ventral tegmental area: involvement of GABAergic transmission

It was found previously that in the rat, unilateral electrolytic lesion of the ventral tegmental area (VTA) facilitated feeding induced by electrical stimulation of the homologous VTA tissue in the contralateral hemisphere. In the present work, VTA stimulation-induced locomotor response was tested in male Wistar rats using a latency to move/stimulation frequency curve shift paradigm in order to check for functional generality of the "contralateral facilitation effect" and also with the aim of elaborating an easy and reliable behavioral model to study this phenomenon. In a further step, the hypothesis was tested that enhancement of function of the intact VTA results from elimination of tonic GABAergic influence derived normally from the lesioned VTA. GABA(A) (bicuculline, doses 0, 0.5 and 5.0 ng) and GABA(B) (phaclofen, doses 0, 500 and 1000 ng) receptors antagonists, and for comparison, a GABA(A) receptor agonist (muscimol, doses 0, 12.5, 25. 0 and 50.0 ng), were injected unilaterally to VTA and their effect on locomotor response elicited by electrical stimulation of the contralateral VTA was tested in a latency/frequency paradigm. It was found that similar to feeding, locomotor response evoked by unilateral electrical stimulation of the VTA was facilitated after contralateral VTA lesion which manifested as a decrease of the locomotion threshold and a leftward shift of the function relating latency to move to stimulation frequency. The effect was immediate, long-lasting and specific to the VTA destruction; lesions outside the VTA area caused gradual impairment of the locomotor response to stimulation. The facilitatory effect of the electrolytic lesion could be replicated by bicuculline, which significantly facilitated stimulation-induced behavior. Phaclofen exerted slight facilitating influence only at a low dose. No effect of muscimol on the locomotion threshold was found. We conclude that "the contralateral facilitation effect" at the level of VTA reflects the interhemispheric regulation of activity of the dopaminergic (DA) cells in which GABA(A)-mediated interhemispheric communication plays a significant role.

Diazepam-induced hyperphagia in mice is sensitive to quinpirole

The present trial examined the possibility that diazepam (DZP, 1 mg/kg) induces hyperphagia by acting on the dopaminergic system. Quinpirole (QP), dopamine D-2 receptor agonist, was used for this purpose. Mice fasted for 24 hr were treated with QP 1 (QP-1) or 2 (QP-2) mg/kg 30 min prior to termination of the starvation. DZP was given to untreated mice and half of the QP-1 and QP-2 treated mice 10 min before the termination of the starvation. Food consumed during six 30 min intervals (30 min-feeding), food consumed for 3 hr (total feeding), time required to enter the room containing food by passing through a maze with four multiple routes (time to banquet), latent period to commencement of eating food after entering the banquet room (latent period), and feeding frequency for the 30 min intervals as well as for 3 hr were measured. DZP stimulated feeding, shortened the latent period without affecting the time to banquet and increased the feeding frequency. The hyperphagic effect was restricted to the first 30 min interval only. Both QP-1 and QP-2 first reduced, then progressively stimulated, and finally reduced feeding without modifying total feeding, thus making a bell-shaped profile. They also prolonged both the time to banquet and the latent period, and reduced the feeding frequency of the first 30 min interval but not that for 3 hr. Both QP-1 and QP-2 canceled all the effects of DZP. These results imply that dopamine D2 receptor is involved in the induction of hyperphagia by DZP.

Extracellular perfusion of rat brain nuclei using microdialysis: a method for studying differential neurotransmitter release in response to neurotoxins

Microdialysis probes, stereotactically placed in rat brain nuclei, allow detailed kinetic comparisons of neurotransmitter release from in situ chemical lesioning over a continuum from acute early changes (minutes) to chronic late changes (days). This technique insures a localized mechanism of action independent of systemic effects apparent with other routes of administration and independent of mechanical damage patterns encountered in conventional chemical lesioning. The example provided compares changes in extracellular gamma-aminobutyric acid (GABA) concentrations in the striatum in response to quinolinic acid (QA, 0.24 M) and 3-nitropropionic acid (3-NPA, 0.25 M). These examples of chemical lesioning represent two theoretical mechanisms of neurodeterioration in Huntington's disease, QA representing the excitotoxic component, and 3-NPA representing the impaired mitochondrial energy component [M.F. Beal, N.W. Kowall, D.W. Ellison, M.F. Mazurek, K.J. Swartz, J.M. Martin, Replication of the neurochemical characteristics of Huntington's disease by quinolinic acid, Nature 321 (1986) 168-171; M.F. Beal, E. Brouillet, B.G. Jenkins, R.J. Ferrante, N.W. Kowall, J.M. Miller, E. Storey, R. Srivastava, B.R. Rosen, B.T. Hyman, Neurochemical and histologic characterization of striatal excitotoxic lesions produced by the mitochondrial toxin 3-nitropropionic acid. J. Neurosci. 13 (1993) 4181-4192; N.C. Reynolds, W. Lin, C.M. Cameron, D.L. Roerig, Differential responses of extracellular GABA to intrastriatal perfusions of 3-nitropropionic acid and quinolinic acid in the rat, Brain Res. 778 (1997) 140-149]. An auxillary microdialysis probe implanted in the ipsilateral nucleus accumbens is used to define the physiologic extent of the cytotoxic lesion. Pre-column derivatization of perfusate fractions with o-phthalaldehyde/t-butylthiol (OPA) provides electroactivity to the OPA-GABA conjugate and facilitates electrochemical detection following high performance liquid chromatography [J.M. Peinado, K.T. McManus, R.B. Myers, Rapid method for microanalysis of endogenous amino acid neurotransmitters in brain perfusates in the rat by isocratic HPLC-EC, J. Neurosci. Methods 18 (1986) 269-276].

Contribution of presynaptic dopaminergic receptors to the mechanism of the reactivating effects of blockade of the GABA-benzodiazepine-ionophore complex

Changes in the reactivating efficiency of blockade of components of the GABA-benzodiazepine-ionophore complex were analyzed in conditions of preliminary activation and inhibition of dopamine autoreceptors using (+)ZRRR and haloperidol respectively. A conditioned passive escape reflex was used, along with amnesia produced by detaining mice in the danger sector of a chamber immediately after imposition of a painful stimulus. Doses of bicuculline (1 mg/kg), picrotoxin (1 mg/kg), and flumazenil (10 mg/kg) given before testing restored performance of the conditioned response without altering the neurochemical background. Reductions and increases in dopaminergic activity during the training period prevented restoration of the conditioned passive escape response by blockade of GABAa and benzodiazepine receptors and chloride channels. It is suggested that the neurochemical mechanisms involved in restoring the damaged memory trace are based on the formation of te optimal balance in the activities of the various components of the GABA-benzodiazepine-ionophore complex and the dopaminergic system.

Differential responses of extracellular GABA to intrastriatal perfusions of 3-nitropropionic acid and quinolinic acid in the rat

Although both quinolinic acid and 3-nitropropionic acid destroy medium sized, GABAergic, spiny projection neurons after direct perfusion of neurotoxin into the rat striatum, changes in extracellular GABA concentration in the striatum within the first 90 min reflect different toxic mechanisms in these two animal models for Huntington's disease. Since quinolinic acid acts as a potent excitotoxin, the early depolarizing response in GABAergic neurons results in an early increase in extracellular GABA activity (peak at 40 min) whereas the more indirect action of 3-nitropropionic acid on mitochondrial energy metabolism results in a delayed increase in extracellular GABA activity (peak at 60 min) with a pattern of gradual increase and decline. The localized delivery of cytotoxin provides an opportunity for kinetic comparisons of direct and indirect cytotoxic mechanisms that can be useful in developing neuroprotective treatment strategies in Huntington's disease.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Increased waking after intra-accumbens injection of m-chlorophenylbiguanide: prevention with serotonin or dopamine receptor antagonists

Bilateral injection of the selective 5-HT3 receptor agonist m-chlorophenylbiguanide (5.0-40.0 micrograms) into the nucleus accumbens of the rat significantly increased waking and decreased slow wave sleep. Rapid eye movement (REM) sleep remained unchanged. Pretreatment with the 5-HT3 receptor antagonist MDL 72222 (1aH,3a,5a, H-tropan-3-yl-3,5-dichloro-benzoate) (0.5 mg/kg s.c.) reversed the effects of m-chlorophenylbiguanide (10.0-20.0 micrograms) on sleep and waking. Blockade of the dopamine D1 or D2 receptor with (+)-SCH 23390 (0.25 mg/kg s.c.) or YM-09151-2 (cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4- methylaminobenzamide) (0.5 mg/kg s.c.), respectively antagonized the increase of waking and reduction of slow wave sleep induced by m-chloro-phenylbiguanide (10.0 micrograms). Our results tend to indicate that the increase of wakefulness after injection of the selective 5-HT3 receptor agonist m-chlorophenylbiguanide into the nucleus accumbens is partly related to the release of endogenous dopamine. In addition, they suggest that concomitant stimulation of both accumbens dopamine D1 and D2 receptor-related mechanisms is a necessary prerequisite to increase wakefulness.

Simulation of spatial learning in the Morris water maze by a neural network model of the hippocampal formation and nucleus accumbens

Cells in the hippocampal formation show spatial firing correlates thought to be critical to the role played by this structure in spatial learning. Place cells in the hippocampus proper show location-specific activity, whereas cells in the postsubiculum fire as a function of momentary directional heading. One question which has received little attention is how these spatial signals are used by motor structures to actually guide spatial behavior. Here we present a model of how one kind of spatial behavior, instrumental learning in the Morris water maze, could be guided by the spatial information in the hippocampal formation. For this, we concentrate on the hippocampal projection to the nucleus accumbens, which is strongly implicated in instrumental learning. In the model, simulated firing patterns of place cells and head direction cells activate "motor" cells in the "accumbens." Each motor cell causes a particular locomotor movement in a simulated rat. In this way, the "rat" locomotes through the simulated environment. Each step places the animal in a slightly different location and directional orientation, which, in turn, activates a different set of place and head direction cells, thus causing the next locomotor response, and so on. Connection strengths between cells are initially set randomly. When the animal encounters the reward location, however, connections are altered, so that recently active synapses are strengthened. Thus, successful moves in a particular locational and directional context are "stamped in." Simulated rats show rapid learning, similar in many ways to that of actual rats. In particular, they generate efficient routes to the goal after minimal experience, and can do so from somewhat novel starting positions. Consideration of the model architecture shows that 1) combined use of directional and place information is an example of a linearly inseparable problem and that 2) some types of novel route generation, often thought to require a "cognitive mapping" strategy, can be generated from the S-R type model used here.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.