N-methyl-D-aspartate (NMDA) receptors in the ventral tegmental area: subcellular distribution and colocalization with 5-hydroxytryptamine(2A) receptors

TCB-2, a 5-hydroxytryptamine 2A receptor agonist, disrupts prepulse inhibition in the ventral pallidum and nucleus accumbens

The 5-hydroxytryptamine 2A (5-HT_2A) receptor plays an important role in schizophrenia. The 5-HT_2A receptor is also involved in the regulation of prepulse inhibition (PPI) in rodents. The aim of this study was to determine whether selective 5-HT_2A receptor agonizts or antagonists may alter PPI in rats and to identify the critical brain regions in which the activity of 5-HT_2A receptors regulates PPI. The results showed that infusion of the 5-HT_2A receptor agonist TCB-2 into the lateral ventricle disrupted PPI, but the 5-HT_2A receptor antagonist M100907 had no such effect. In addition, local infusion of TCB-2 into the nucleus accumbens and ventral pallidum disrupted PPI, whereas the same manipulation in the medial prefrontal cortex, ventral hippocampus, and ventral tegmental area did not disrupt PPI. In conclusion, agonism of 5-HT_2A receptors in the ventral pallidum and nucleus accumbens can disrupt PPI. The ventral pallidum and nucleus accumbens are critical brain regions responsible for the regulation of PPI by serotonin. These findings contribute to the extensive exploration of the molecular and neural mechanisms underlying the regulatory effect of 5-HT_2A receptor activity on PPI, especially the neural circuits modulated by 5-HT_2A receptor activity.

The Resilient Phenotype Induced by Prophylactic Ketamine Exposure During Adolescence Is Mediated by the Ventral Tegmental Area-Nucleus Accumbens Pathway

BACKGROUND

Major depressive disorder is prevalent in children and adolescents and is associated with a high degree of morbidity throughout life, with potentially devastating personal consequences and public health impact. The efficacy of ketamine (KET) as an antidepressant has been demonstrated in adolescent rodents; however, the neurobiological mechanisms underlying these effects are unknown. Recent evidence showed that KET reverses stress-induced (i.e., depressive-like) deficits within major mesocorticolimbic regions, such as the prefrontal cortex, nucleus accumbens (NAc), and hippocampus, in adult rodents. However, little is known about KET's effect in the ventral tegmental area (VTA), which provides the majority of dopaminergic input to these brain regions.

METHODS

We characterized behavioral, biochemical, and electrophysiological effects produced by KET treatment in C57BL/6J male mice during adolescence (n = 7-10 per condition) within the VTA and its major projection regions, namely, the NAc and prefrontal cortex. Subsequently, molecular targets within the VTA-NAc projection were identified for viral gene transfer manipulations to recapitulate the effects of stress or KET treatment.

RESULTS

Repeated KET treatment produced a robust proresilient response to chronic social defeat stress. This effect was largely driven by Akt signaling activity within the VTA and NAc, and it could be blocked or recapitulated through direct Akt-viral-mediated manipulation. Additionally, we found that the KET-induced resilient phenotype is dependent on VTA-NAc, but not VTA-prefrontal cortex, pathway activity.

CONCLUSIONS

These findings indicate that KET exposure during adolescence produces a proresilient phenotype mediated by changes in Akt intracellular signaling and altered neuronal activity within the VTA-NAc pathway.

Low Doses of Psilocybin and Ketamine Enhance Motivation and Attention in Poor Performing Rats: Evidence for an Antidepressant Property

Long term benefits following short-term administration of high psychedelic doses of serotonergic and dissociative hallucinogens, typified by psilocybin and ketamine respectively, support their potential as treatments for psychiatric conditions such as major depressive disorder. The high psychedelic doses induce perceptual experiences which are associated with therapeutic benefit. There have also been anecdotal reports of these drugs being used at what are colloquially referred to as "micro" doses to improve mood and cognitive function, although currently there are recognized limitations to their clinical and preclinical investigation. In the present studies we have defined a low dose and plasma exposure range in rats for both ketamine (0.3-3 mg/kg [10-73 ng/ml]) and psilocybin/psilocin (0.05-0.1 mg/kg [7-12 ng/ml]), based on studies which identified these as sub-threshold for the induction of behavioral stereotypies. Tests of efficacy were focused on depression-related endophenotypes of anhedonia, amotivation and cognitive dysfunction using low performing male Long Evans rats trained in two food motivated tasks: a progressive ratio (PR) and serial 5-choice (5-CSRT) task. Both acute doses of ketamine (1-3 mg/kg IP) and psilocybin (0.05-0.1 mg/kg SC) pretreatment increased break point for food (PR task), and improved attentional accuracy and a measure of impulsive action (5-CSRT task). In each case, effect size was modest and largely restricted to test subjects characterized as "low performing". Furthermore, both drugs showed a similar pattern of effect across both tests. The present studies provide a framework for the future study of ketamine and psilocybin at low doses and plasma exposures, and help to establish the use of these lower concentrations of serotonergic and dissociative hallucinogens both as a valid scientific construct, and as having a therapeutic utility.

Targeting Serotonin 5-HT2A Receptors to Better Treat Schizophrenia: Rationale and Current Approaches

Schizophrenia is a major mental illness associated with profound disability. Current treatments for schizophrenia (antipsychotics) all have a similar mechanism of action and are primarily dopamine type 2 receptor (D₂R) antagonists. Antipsychotics are not fully effective for the majority of schizophrenia patients, suggesting the need for alternative approaches. The primary focus of this review is to assess the evidence for the role of the serotonin type 2A receptor (5-HT_2AR) in schizophrenia. Topics include an overview of 5-HT_2AR physiology and pathophysiology in schizophrenia, 5-HT_2AR interaction with other neurotransmitter systems, including the glutamatergic system, a review of the 5-HT_2AR/d-lysergic acid diethylamide (LSD) model of schizophrenia, a contrast of the 5-HT_2AR and glutamatergic models of schizophrenia, and finally, a review of Food and Drug Administration (FDA)-approved and investigational 5-HT_2AR-modulating compounds. Recent studies with lumeteperone, pimavanserin, and roluperidone are highlighted.

A systematic investigation of the differential roles for ventral tegmentum serotonin 1- and 2-type receptors on food intake in the rat

Central serotonin (5-HT) pathways are known to influence feeding and other ingestive behaviors. Although the ventral tegmentum is important for promoting the seeking and consumption of food and drugs of abuse, the roles of 5-HT receptor subtypes in this region on food intake have yet to be comprehensively examined. In these experiments, food restricted rats were given 2-h access to rat chow; separate groups of non-restricted animals had similar access to a sweetened fat diet. Feeding and locomotor activity were monitored following ventral tegmentum stimulation or blockade of 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, or 5-HT2C receptors. 5-HT1A receptor stimulation transiently inhibited rearing behavior and chow intake in food-restricted rats, and had a biphasic effect on non-restricted rats offered the palatable diet. 5-HT1B receptor agonism transiently inhibited feeding in restricted animals, but did not affect intake of non-restricted rats. In contrast, 5-HT1B receptor antagonism decreased palatable feeding. Although stimulation of ventral tegmental 5-HT2B receptors with BW723C86 did not affect hunger-driven food intake, it significantly affected palatable feeding, with a trend for an increasing intake at 2.0µg/side but not at 5.0µg/side. Antagonism of the same receptor modestly but significantly inhibited feeding of the palatable diet at 5.0µg/side ketanserin. Neither stimulation nor blockade of 5-HT2A or 5-HT2C receptors caused prolonged effects on intake or locomotion. These data suggest that serotonin's effects on feeding within the ventral tegmentum depend upon the specific receptor targeted, as well as whether intake is motivated by food restriction or the palatable nature of the offered diet.

Serotonin 2A receptors differentially contribute to abuse-related effects of cocaine and cocaine-induced nigrostriatal and mesolimbic dopamine overflow in nonhuman primates

Two of the most commonly used procedures to study the abuse-related effects of drugs in laboratory animals are intravenous drug self-administration and reinstatement of extinguished behavior previously maintained by drug delivery. Intravenous self-administration is widely accepted to model ongoing drug-taking behavior, whereas reinstatement procedures are accepted to model relapse to drug taking following abstinence. Previous studies indicate that 5-HT2A receptor antagonists attenuate the reinstatement of cocaine-maintained behavior but not cocaine self-administration in rodents. Although the abuse-related effects of cocaine have been closely linked to brain dopamine systems, no previous study has determined whether this dissociation is related to differential regulation of dopamine neurotransmission. To elucidate the neuropharmacological and neuroanatomical mechanisms underlying this phenomenon, we evaluated the effects of the selective 5-HT2A receptor antagonist M100907 on intravenous cocaine self-administration and drug- and cue-primed reinstatement in rhesus macaques (Macaca mulatta). In separate subjects, we evaluated the role of 5-HT2A receptors in cocaine-induced dopamine overflow in the nucleus accumbens (n = 4) and the caudate nucleus (n = 5) using in vivo microdialysis. Consistent with previous studies, M100907 (0.3 mg/kg, i.m.) significantly attenuated drug- and cue-induced reinstatement but had no significant effects on cocaine self-administration across a range of maintenance doses. Importantly, M100907 (0.3 mg/kg, i.m.) attenuated cocaine-induced (1.0 mg/kg, i.v.) dopamine overflow in the caudate nucleus but not in the nucleus accumbens. These data suggest that important abuse-related effects of cocaine are mediated by distinct striatal dopamine projection pathways.

Increased hippocampal CA1 density of serotonergic terminals in a triple transgenic mouse model of Alzheimer's disease: an ultrastructural study

Alzheimer's disease (AD) is a neurodegenerative pathology that deteriorates mnesic functions and associated brain regions including the hippocampus. Serotonin (5-HT) has an important role in cognition. We recently demonstrated an increase in 5-HT transporter (SERT) fibre density in the hippocampal CA1 in an AD triple transgenic mouse model (3xTg-AD). Here, we analyse the ultrastructural localisation, distribution and numerical density (N_v) of hippocampal SERT axons (SERT-Ax) and terminals (SERT-Te) and their relationship with SERT fibre sprouting and altered synaptic N_v in 3xTg-AD compared with non-transgenic control mice. 3xTg-AD animals showed a significant increase in SERT-Te N_v in CA1 at both, 3 (95%) and 18 months of age (144%), being restricted to the CA1 stratum moleculare (S. Mol; 227% at 3 and 180% at 18 months). 3xTg-AD animals also exhibit reduced N_v of perforated axospinous synapses (PS) in CA1 S. Mol (56% at 3 and 52% at 18 months). No changes were observed in the N_v of symmetric and asymmetrical synapses or SERT-Ax. Our results suggest that concomitant SERT-Te N_v increase and PS reduction in 3xTg-AD mice may act as a compensatory mechanism maintaining synaptic efficacy as a response to the AD cognitive impairment.

How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

The ability to learn about conditioned stimuli (CS) associated with rewards is a crucial adaptive mechanism. Activity in the mesocorticolimbic dopamine (DA) system, as well as in the ventral tegmental area (VTA), is correlated with responding to and learning about CSs. The mechanism by which VTA neurons become activated by signals associated with conditioned stimuli is not fully understood. Our model suggests that NMDA receptor stimulation in the VTA allows originally weak glutamate signals carrying information about environmental stimuli, coincident with strong excitation correlated with primary rewards, to be strengthened and thereby acquire the ability to activate VTA neurons in themselves, producing approach. Furthermore, once synaptic strengthening occurs, the model suggests that NMDA receptor stimulation in VTA is not necessary for the expression of reward-related learning. In this review we survey evidence that VTA cells respond to cues associated with primary rewards, that this responding is acquired, and that the VTA possesses the attributes to function as a site of integration of signals of primary and conditioned stimuli.

Colocalisation of serotonin2A receptors with the glutamate receptor subunits NR1 and GluR2 in the dentate gyrus: an ultrastructural study of a modulatory role

The serotonin(2A) receptor (5-HT(2A)R) is implicated in many neurological disorders and has a role in cognitive processes, reliant upon hippocampal glutamate receptors. Recent studies show that 5-HT(2A)R agonists and/or antagonists can influence cognitive function, suggesting a critical hippocampal role for these receptors, yet their cellular and subcellular distribution within this region has not been comprehensively analysed. Here, we have conducted an electron microscopic examination of 5-HT(2A)R distribution with the glutamate N-methyl-D-aspartate (NMDA) and amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor subunits NR1 and GluR2 in the hippocampal dentate gyrus (DG) in order to investigate whether 5-HT(2A)R location is compatible with a modulatory role over NMDA and/or AMPA receptor mediated neurotransmission. Of 5-HT(2A)R positive profiles, 56% were dendrites and 16% were dendritic spines. Labelling was both cytoplasmic and membranous. Spinous labelling was more frequently membranous at peri- and extra-synaptic sites, though was also associated with synaptic specialisations. Profiles displaying colocalisation of immunoreactivity for 5-HT(2A)Rs with NR1 or GluR2 were predominantly dendrites, representing 11% and 8% of 5-HT(2A)R positive profiles, respectively. Additionally, 12% of 5-HT(2A)R labelled profiles also displayed immunoreactivity for gamma-aminobutyric acid (GABA). These data indicate most 5-HT(2A)Rs are expressed on granule cell projections, with a smaller subpopulation expressed on GABAergic interneurons.

Molecular targets for treating cognitive dysfunction in schizophrenia

Cognitive impairment is a core feature of schizophrenia as deficits are present in the majority of patients, frequently precede the onset of other positive symptoms, persist even with successful treatment of positive symptoms, and account for a significant portion of functional impairment in schizophrenia. While the atypical antipsychotics have produced incremental improvements in the cognitive function of patients with schizophrenia, overall treatment remains inadequate. In recent years, there has been an increased interest in developing novel strategies for treating the cognitive deficits in schizophrenia, focusing on ameliorating impairments in working memory, attention, and social cognition. Here we review various molecular targets that are actively being explored for potential drug discovery efforts in schizophrenia and cognition. These molecular targets include dopamine receptors in the prefrontal cortex, nicotinic and muscarinic acetylcholine receptors, the glutamatergic excitatory synapse, various serotonin receptors, and the gamma-aminobutyric acid (GABA) system.

MK-801 infusions to the ventral tegmental area and ventromedial hypothalamus produce opposite effects on lordosis of hormone-primed rats

Progesterone initiates female sexual behavior of rodents (lordosis) through actions at intracellular progestin receptors in the ventromedial hypothalamus. Progesterone's metabolite, 5alpha-pregnan-3alpha-ol-20-one, mediates the intensity and duration of lordosis through its actions at GABA(A) receptors in the ventral tegmental area. Whether progestins can influence sexual behavior through actions that involve N-methyl-D-aspartate receptors (NMDARs) in the ventromedial hypothalamus and ventral tegmental area was investigated. The current study examines the effect of bilateral ventral tegmental area or ventromedial hypothalamus infusions of the non-competitive NMDAR antagonist (+)-MK-801 hydrogen maleate (MK-801; 0, 20, or 200 ng) on lordosis, motor activity, and NMDA R1 subtype (NMDAR1) immunoreactivity in estradiol benzoate (10 microg)+progesterone (50 microg)- and estradiol benzoate+vehicle primed rats. Compared to vehicle infusions, infusions of MK-801 to the ventral tegmental area facilitated lordosis of estradiol benzoate (10 microg)+progesterone (50 microg)- and estradiol benzoate+vehicle primed rats. Infusions of MK-801 to the ventromedial hypothalamus inhibited lordosis of estradiol benzoate (10 microg)+progesterone (50 microg)- and estradiol benzoate+vehicle primed rats, compared to vehicle. There was no effect of MK-801 infusions to the ventral tegmental area or the ventromedial hypothalamus on motor behavior. Immunocytochemistry for NMDAR1 revealed MK-801 (200 ng) infusions to the ventral tegmental area or ventromedial hypothalamus of estradiol benzoate (10 microg)+progesterone (50 microg)- or estradiol benzoate+vehicle primed rats significantly reduced the number of darkly stained NMDAR1-immunoreactive cells, compared to vehicle infusions. These data suggest NMDARs may be important in the mediation of hormonal actions in both the ventral tegmental area and the ventromedial hypothalamus for sexual receptivity of rodents, but in different ways.

Odour suppression in binary mixtures

It has been suggested that odours causing stronger trigeminal activation suppress weaker trigeminal stimuli and that mixed olfactory-trigeminal stimuli suppress odorants that only activate one of these systems. Volunteer normosmic participants (n=20) were exposed to six odorants with varying trigeminal impact to test the hypothesis that more intense "trigeminal" odorants would suppress weaker trigeminal stimuli in binary odour mixtures. It was also hypothesised that stronger trigeminal odorants would dominate six-odour mixtures. The predicted linear pattern of suppression was not seen, with a quadratic model emerging from the data. Stronger trigeminal stimuli failed to dominate six-odour mixtures. Despite the fact that the major hypothesis was not supported, it can be hypothesised from this experiment that the effect of suppression in binary mixtures is reliant upon two major effects: (1) the association formed between odours and the multiple memory systems that they interact with during the encoding and recognition processes, and (2) the balance between activation of the olfactory and trigeminal systems.

In the ventral tegmental area, G-proteins mediate progesterone's actions at dopamine type 1 receptors for lordosis of rats and hamsters

RATIONALE

Previous reports indicate that the ventral tegmental area (VTA) and/or progesterone (P) can modulate the reinforcing effects of drugs of abuse, food, and sexual behavior.

OBJECTIVES

We investigated if, in the VTA, P's membrane-mediated actions for lordosis involve dopamine type 1 receptors (D(1)). Also, whether P's actions at D(1) for lordosis are mediated by typical G-protein coupled mechanisms was examined.

METHODS

In Exps 1 and 2, rats received estradiol (E(2) 10 microg) at h 0 and infusions of the D(1) antagonist SCH23390 (100 ng), the D(1) agonist SKF38393 (100 ng), or vehicle, to the VTA, at h 44. Thirty minutes later, rats received systemic P (0, 50, 100, or 200 microg). Subjects were tested for lordosis and motor behavior 2.5 h later. In Exps 3 and 4, E(2)+P (rats 0 or 100 microg; hamsters 200 microg)-primed animals were pretested for lordosis and motor behavior at h 47.5 and infused with SKF38393 (100 ng) or vehicle to the VTA. Thirty minutes later, subjects were retested and infused with the G-protein inhibitor guanosine 5'-O-(2-Thiodiphosphate) (GDP-beta-S; 50 microM) or vehicle. Post-testing occurred 30 min later.

RESULTS

Pretreatment with SCH23390-reduced and SKF38393-enhanced P's actions, in the VTA, for lordosis of E(2)-primed rats and hamsters. As well, D(1)-mediated increases in P-facilitated lordosis of rats and hamsters were inhibited by GDP-beta-S. Changes in lordosis were independent of large alterations in motor behavior.

CONCLUSIONS

In the VTA, P has actions for modulating reinforcing behaviors, such as lordosis, at D(1) that are G-protein-mediated.

Neurochemical characterization of the release and uptake of dopamine in ventral tegmental area and serotonin in substantia nigra of the mouse

In the present report, fast-scan cyclic voltammetry was used to identify the monoamines that were released by electrical stimulation in mouse brain slices containing ventral tegmental area (VTA), substantia nigra (SN) -pars compacta (SNc) and -pars reticulata (SNr). We showed that voltammograms obtained in mouse VTA were consistent with detection of a catecholamine, while those in both subregions of the SN were consistent with detection of an indolamine, based on the reduction peak potentials. We used pharmacological blockade and genetic deletion of monoamine transporters to further confirm the identity of released monoamines in mouse midbrain and to assess the control of monoamines by their transporters in each brain region. Inhibition of dopamine and norepinephrine transporters by nomifensine (1 and 10 microm) decreased uptake rates in the VTA, but did not change uptake rates in either subregion of the SN. Serotonin transporter inhibition by fluoxetine (10 microm) decreased uptake rates in the SNc and SNr, but was without effect in the VTA. Selective inhibition of the norepinephrine transporter by desipramine (10 microm) had no effect in any brain region. Using dopamine transporter- and serotonin transporter-knockout mice, we found decreased uptake rates in VTA and SN subregions, respectively. Peak signals recorded in each midbrain region were pulse number dependent and exhibited limited frequency dependence. Thus, dopamine is predominately detected by voltammetry in mouse VTA, while serotonin is predominately detected in mouse SNc and SNr. Furthermore, active uptake occurs in these areas and can be altered only by specific uptake inhibitors, suggesting a lack of heterologous uptake. In addition, somatodendritic dopamine release in VTA was not mediated by monoamine transporters. This work offers an initial characterization of voltammetric signals in the midbrain of the mouse and provides insight into the regulation of monoamine neurotransmission in these areas.

Serotonin receptors represent highly favorable molecular targets for cognitive enhancement in schizophrenia and other disorders

RATIONALE

Current treatments for schizophrenia adequately treat the positive symptoms of schizophrenia but only modestly improve cognitive deficits. This review provides evidence for and against the use of selective 5-HT receptor drugs as cognition enhancing agents for schizophrenia and other disorders.

METHODS

Pre-clinical and clinical literature concerned with the role of the serotonergic system in cognition and memory as it relates to schizophrenia is reviewed. Individual 5-HT receptor subtypes for which selective drugs are available that are likely to improve cognition are reviewed. Recommendations for clinical testing are proposed.

RESULTS AND CONCLUSIONS

Four 5-HT receptor systems (5-HT(1A), 5-HT(2A), 5-HT(4), 5-HT(6)) are highlighted as suitable targets for enhancing cognition and memory. Because many clinically available antipsychotic drugs already target 5-HT(1A), 5-HT(2A) and 5-HT(6) receptors, design of clinical trials will need to take into account the serotonergic pharmacology of concurrently administered antipsychotic medications. 5-HT(1A) partial agonists and 5-HT(2A) antagonists have shown modest effectiveness in improving cognition in schizophrenia. 5-HT(6)-selective compounds for cognition enhancement are in late-stage clinical trials, while 5-HT(4) compounds have not yet been tested in humans for cognition enhancement.

RECOMMENDATIONS

For stand-alone therapy for enhancing cognition, 5-HT(1A) partial agonists, 5-HT(2A) antagonists, 5-HT(4) partial agonists and 5-HT(6) antagonists are all likely to induce at least modest improvement in cognition in schizophrenia. If "add-on therapy" is contemplated, antipsychotic drugs with weak affinities for serotonin receptors should be used to avoid confounds. It is likely that serotonergic drugs will soon be available as cognition enhancing medications for disorders other than schizophrenia (e.g. dementia).

Specific involvement of neurotensin type 1 receptor in the neurotensin-mediated in vivo dopamine efflux using knock-out mice

Abstract Neurotensin is a tridecapeptide neurotransmitter known to be involved in psychiatric disorders, various physiological processes and several different neurobiological mechanisms, including modulation of accumbal dopamine release. Two neurotensin extracellular binding sites, namely NT1- and NT2-receptor (NT1R and NT2R), have been cloned from the rat brain. These receptors are distinguishable by their different in vitro pharmacological properties but the available pharmacological tools have weak in vivo potency and specificity. The use of genetically engineered knock-out mice has provided a powerful alternative to the classical pharmacological approach to investigate their respective roles. In this study, using in vivo differential pulse amperometry, we show that, in wild-type mice, neurotensin application into the ventral tegmental area dose-dependently evokes dopamine efflux in the nucleus accumbens. This neurotensin-mediated efflux is dramatically decreased in mice lacking NT1R while it is unaffected in NT2R-deleted mice. This finding indicates that a large part of the dopamine efflux evoked by neurotensin in the nucleus accumbens of wild-type mice is mediated via NT1R present in the ventral tegmental area.

Localization of serotonin and its possible role in early embryos of Tritonia diomedea(Mollusca: Nudibranchia)

A classical neurotransmitter serotonin (5-HT) was detected immunochemically using laser scanning microscopy at the early stages of Tritonia diomedea development. At the one- to eight-cell stages, immunolabeling suggested the presence of 5-HT in the cytoplasm close to the animal pole. At the morula and blastula stages, a group of micromeres at the animal pole showed immunoreactivity. At the gastrula stage no immunoreactive cells were detected, but they arose again at the early veliger stage. Antagonists of 5-HT(2) receptors, ritanserin and cyproheptadine, as well as lipophilic derivatives of dopamine blocked cleavage divisions or distorted their normal pattern. These effects were prevented by 5-HT and its highly lipophilic derivates, serotoninamides of polyenoic fatty acids, but not by the hydrophilic (quaternary) analog of 5-HT, 5-HTQ. The results confirm our earlier suggestion that endogenous 5-HT in pre-nervous embryos acts as a regulator of cleavage divisions in nudibranch molluscs.

Similar ultrastructural distribution of the 5-HT(2A) serotonin receptor and microtubule-associated protein MAP1A in cortical dendrites of adult rat

As visualized by light and electron microscopic immunocytochemistry, the distribution of the neuronal serotonin-2A (5-HT(2A)) receptor is mainly intracellular throughout adult rat brain. This localization is particularly striking in the pyramidal cells of cerebral cortex, the dendrites of which are intensely immunoreactive, but without any labeling of their spines. In view of recent yeast two-hybrid and biochemical results suggesting an association of 5-HT(2A) receptors with the cytoskeletal microtubule-associated protein MAP1A, the respective subcellular distributions of the receptors and of MAP1A were compared by quantitative electron microscopic immunocytochemistry in dendrites of adult rat frontoparietal cortex. Counts of silver-intensified immunogold particles revealed a higher density of 5-HT(2A) receptors in smaller rather than larger dendrites, and an apportionment between pre-defined compartments representing the plasma membrane and the cytoplasm that was proportional to the relative surface area of these compartments. MAP1A immunoreactivity also predominated in smaller versus larger dendrites, but with a slightly lower proportion of labeling in the plasma membrane versus cytoplasmic compartment. The co-localization of 5-HT(2A) receptors and MAP1A protein in the same dendrites could be demonstrated in double immunolabeling experiments. These results confirmed the predominantly somato-dendritic, intracellular localization of 5-HT(2A) receptors in cerebral cortex, showed their higher concentration in distal as opposed to proximal dendrites, and suggested their potential association to the cytoskeleton in cortical neurons in vivo. Such a distribution of 5-HT(2A) receptors reinforces our earlier hypothesis that 5-HT(2A) receptors participate in intraneuronal signaling processes involving the cytoskeleton, and raises the possibility that their activation could be dependent upon that of another co-localized, plasma membrane-bound, 5-HT receptor.

Electron microscopic immunolabeling of transporters and receptors identifies transmitter-specific functional sites envisioned in Cajal's neuron

Neuronal arborizations that were so elegantly demonstrated in the early drawings of Santiago Ramón y Cajal can now be viewed by high resolution electron microscopic immunocytochemical localization of vesicular and plasmalemmal neurotransmitter transporters and receptors. The subcellular distribution of these proteins confers both chemical selectivity and functional specificity to the dendritic and axonal arborizations described by Cajal. This is illustrated by central dopaminergic and cholinergic neurons. Dopamine terminals in the striatum and ventral pallidum, as well as dendrites of midbrain dopaminergic neurons in the ventral tegmental area and substantia nigra express the plasmalemmal dopamine transporter (DAT) and the vesicular monoamine transporter (VMAT2). In forebrain regions, the dopamine D2 receptor (D2R) autoreceptor is localized to dopamine terminals, but also is targeted to pre- and postsynaptic neuronal profiles at a distance from the dopamine terminals. In somata and dendrites of the midbrain dopaminergic neurons, D2R labeling is expressed in most dendrites that contain VMAT2 storage vesicles, as well as in both excitatory and inhibitory afferents. Together, these observations indicate that dopamine is stored in and released from vesicles in both dendrities and axons, and may activate either local or more distant receptors through volume transmission. By analogy, the vesicular acetylcholine transporter (VachT) is similarly localized to the membranes of axon terminals and tubulovesicles in dendrities in the mesopontine tegmental cholinergic nuclei, suggesting that there also may be release of acetylcholine from both dendrities and axons. These results identify chemically selective functional sites for neuronal signaling envisioned by Cajal and redefined by modern technology.

Synaptic regulation of somatodendritic dopamine release by glutamate and GABA differs between substantia nigra and ventral tegmental area

Midbrain dopamine (DA) cells of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) exhibit somatodendritic release of DA. To address how somatodendritic release is regulated by synaptic glutamatergic and GABAergic input, we examined the effect of ionotropic-receptor antagonists on locally evoked extracellular DA concentration ([DA]o) in guinea pig midbrain slices. Evoked [DA]o was monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry. In SNc, evoked [DA]o was 160% of control in the presence of the AMPA-receptor antagonist, GYKI-52466, or the NMDA-receptor antagonist, AP5. Similar increases were seen with the GABAA-receptor antagonist, picrotoxin, or the GABA(B)-receptor antagonist, saclofen. The increase seen with GYKI-52466 was prevented when both picrotoxin and saclofen were present, consistent with normal, AMPA-receptor mediated activation of GABAergic inhibition. The increase with AP5 persisted, however, implicating NMDA-receptor mediated activation of another inhibitory circuit in SNc. In the VTA, by contrast, evoked [DA]o was unaffected by GYKI-52466 and fell slightly with AP5. Neither picrotoxin nor saclofen alone or in combination had a significant effect on evoked [DA]o. When GABA receptors were blocked in the VTA, evoked [DA]o was decreased by 20% with either GYKI-52466 or AP5. These data suggest that in SNc, glutamatergic input acts predominantly on GABAergic or other inhibitory circuits to inhibit somatodendritic DA release, whereas in VTA, the timing or strength of synaptic input will govern whether the net effect on DA release is excitatory or inhibitory.

Targeting of serotonin 1A receptors to dopaminergic neurons within the parabrachial subdivision of the ventral tegmental area in rat brain

Serotonin (5-hydroxytryptamine [5-HT]) modulates dopamine-related cognitive functions and motor activity through activation of selective receptor subtypes including 5-HT1A. Potential targets for these 5-HT1A-mediated actions of 5-HT include mesocortical and mesolimbic dopaminergic neurons having partially segregated distribution in the parabrachial and paranigral subdivisions of the ventral tegmental area (VTA), respectively. We therefore examined the ultrastructural immunocytochemical localization of the 5-HT1A receptor in the parabrachial (VTApb) and paranigral (VTApn) subdivisions of rat VTA, to determine 1) the functional sites for receptor activation, and 2) the cellular associations between this receptor and dopaminergic neurons identified by their tyrosine hydroxylase (TH) content. In each region, 5-HT1A immunoreactivity was mainly observed in somatodendritic profiles, but it was also present in small unmyelinated axons and in a few axon terminals and glia, suggesting a role for 5-HT1A receptors in presynaptic and glial functions, as well as postsynaptic neuronal activation, in VTA. In somatodendritic profiles, 5-HT1A gold particles were mainly localized to tubulovesicles presumed to be smooth endoplasmic reticulum. In addition, however, in distal dendrites receiving multiple inputs the receptor was targeted to selective postsynaptic junctions, or more randomly distributed on nonsynaptic portions of the plasma membrane. Of the 5-HT1A-labeled dendrites, 64% in VTApb and 44% in VTApn contained TH. These findings suggest a reserve of cytoplasmic 5-HT1A receptors that are mobilized to functional postsynaptic sites on the plasma membrane by afferent input to distal dendrites in the VTA. They also indicate that 5-HT1A activation may affect a larger population of dopaminergic neurons in VTApb compared with VTApn, thus having a potentially greater impact on cognitive functions modulated by mesocortical dopaminergic neurons.

Acute and repeated administration of the selective 5-HT(2A) receptor antagonist M100907 significantly alters the activity of midbrain dopamine neurons: an in vivo electrophysiological study

We examined the effect of the acute and repeated administration of M100907 (formerly MDL 100907), a selective 5-HT(2A) receptor antagonist, on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) of rats. This was accomplished using in vivo, extracellular single unit recording. The i.v. administration of M100907 (0.01-0.64 mg/kg) did not significantly alter the basal firing rate or pattern of spontaneously active SNC and VTA DA neurons. A single injection of either 0.01 or 0.03 mg/kg i.p. of M100907 did not significantly alter the number of spontaneously active DA neurons in either the SNC or VTA areas. However, 0.1 mg/kg i.p. of M100907 significantly increased the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. A single injection of all doses of M100907 significantly decreased the degree of bursting in VTA DA neurons, whereas the 0.1 mg/kg dose increased the degree of bursting in SNC DA neurons. The repeated administration (one injection per day for 21 days) of 0.03 and 0.1 mg/kg i.p. of M100907 produced a significant decrease in the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. The repeated administration of M100907 did not significantly alter the firing pattern of VTA DA neurons but significantly altered the firing pattern of SNC DA neurons. The results of this study indicate that M100907 administration alters the activity of midbrain DA neurons in anesthetized rats.

Postnatal inorganic lead exposure decreases the number of spontaneously active midbrain dopamine neurons in the rat

This study examined the effect of lead (Pb) exposure during postnatal development on the electrophysiological activity of midbrain dopamine (DA)-containing neurons. Single-cell electrophysiological recordings were made in the substantia nigra (SN) and ventral tegmental area (VTA) of chloral hydrate anesthetized rats. In this post-weaning exposure protocol 22-day-old male Sprague-Dawley rats were exposed to Pb- (100, 250, and 500 ppm) or Na-acetate in the drinking water for a period ranging from 3 to 6 weeks. Animals were exposed up to the day of electrophysiological recording. One Pb- and one Na-treated animal were recorded each experimental day. The post-weaning exposure protocol used in this study resulted in a significant Pb-dependent decrease in the number of spontaneously active DA neurons at the time of electrophysiological recording. Analysis of covariance, using duration of exposure as the covariate (i.e. 3, 4, 5, or 6 weeks), did not indicate that there was a consistent relationship between exposure duration and the number of spontaneously active DA neurons. However, the effect of Pb was dependent on the level of Pb exposure through the drinking water. At the 250 and 500 ppm level of exposure, Pb produced a significant decrease in the number of spontaneously active DA neurons in both anatomical regions. The number of active DA neurons was not significantly affected by the 100 ppm Pb treatment over the 3-6 weeks exposure period. The average discharge rate, and the percentage of spontaneously active DA neurons classified as having discharge patterns with bursts (i.e. 'bursting DA neurons'), was not changed at any of the three levels of Pb exposure. Based on results obtained from electrophysiological studies, the effect of selected Pb exposure levels, 250 and 500 ppm, were examined during the postnatal period using tyrosine hydroxylase (TH) immuno-histochemistry to determine if Pb affects the survival of dopamine neurons within SN and VTA. TH immuno-histochemical studies revealed that the reduction in the number of spontaneously active DA neurons in animals treated with 250 and 500 ppm Pb was probably not related to the physical loss of cells (e.g. necrosis or apoptosis).