Ultrastructural relationships of serotonin axon terminals in the cerebral cortex of the adult rat

Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission

Serotonin is a neurotransmitter involved in the modulation of a multitude of physiological and behavioral processes. In spite of the relatively reduced number of serotonin-producing neurons present in the mammalian CNS, a complex long-range projection system provides profuse innervation to the whole brain. Heterogeneity of serotonin receptors, grouped in seven families, and their spatiotemporal expression pattern account for its widespread impact. Although neuronal communication occurs primarily at tiny gaps called synapses, wiring transmission, another mechanism based on extrasynaptic diffusion of neuroactive molecules and referred to as volume transmission, has been described. While wiring transmission is a rapid and specific one-to-one modality of communication, volume transmission is a broader and slower mode in which a single element can simultaneously act on several different targets in a one-to-many mode. Some experimental evidence regarding ultrastructural features, extrasynaptic localization of receptors and transporters, and serotonin-glia interactions collected over the past four decades supports the existence of a serotonergic system of a dual modality of neurotransmission, in which wiring and volume transmission coexist. To date, in spite of the radical difference in the two modalities, limited information is available on the way they are coordinated to mediate the specific activities in which serotonin participates. Understanding how wiring and volume transmission modalities contribute to serotonergic neurotransmission is of utmost relevance for the comprehension of serotonin functions in both physiological and pathological conditions.

All the brain's a stage for serotonin: the forgotten story of serotonin diffusion across cell membranes

In the conventional model of serotonin neurotransmission, serotonin released by neurons in the midbrain raphe nuclei exerts its actions on forebrain neurons by interacting with a large family of post-synaptic receptors. The actions of serotonin are terminated by active transport of serotonin back into the releasing neuron, which is mediated by the serotonin reuptake transporter (SERT). Because SERT is expressed pre-synaptically and is widely thought to be the only serotonin transporter in the forebrain, the conventional model does not include serotonin transport into post-synaptic neurons. However, a large body of evidence accumulating since the 1970s has shown that serotonin, despite having a positive charge, can cross cell membranes through a diffusion-like process. Multiple low-affinity, high-capacity, sodium-independent transporters, widely expressed in the brain, allow the carrier-mediated diffusion of serotonin into forebrain neurons. The amount of serotonin crossing cell membranes through this mechanism under physiological conditions is considerable. Most prominent textbooks fail to include this alternative method of serotonin uptake in the brain, and even most neuroscientists are unaware of it. This failure has limited our understanding of a key regulator of serotonergic neurotransmission, impeded research on the potential intracellular actions of serotonin in post-synaptic neurons and glial cells, and may have impeded our understanding of the mechanism by which antidepressant medications reduce depressive symptoms.

Chondroitinase ABC Administration Facilitates Serotonergic Innervation of Motoneurons in Rats With Complete Spinal Cord Transection

Chondroitinase ABC (ChABC) is an enzyme that degrades glycosaminoglycan side-chains of chondroitin sulfate (CS-GAG) from the chondroitin sulfate proteoglycan (CSPG) core protein. Previous studies demonstrated that the administration of ChABC after spinal cord injury promotes nerve regeneration by removing CS-GAGs from the lesion site and promotes the plasticity of spinal neurons by removing CS-GAGs from the perineuronal nets (PNNs). These effects of ChABC might enhance the regeneration and sprouting of descending axons, leading to the recovery of motor function. Anatomical evidence, indicating that the regenerated axons innervate spinal motoneurons caudal to the lesion site, however, has been lacking. In the present study, we investigated whether descending axons pass through the lesion site and innervate the lumbar motoneurons after ChABC administration in rats with complete spinal cord transection (CST) at the thoracic level. At 3 weeks after CST, 5-hydroxytryptamine (5-HT) fibers were observed to enter the lesion in ChABC-treated rats, but not saline-treated rats. In addition, 92% of motoneurons in the ventral horn of the fifth lumbar segment (L5) in saline-treated rats, and 38% of those in ChABC-treated rats were surrounded by chondroitin sulfate-A (CS-A) positive structures. At 8 weeks after CST, many 5-HT fibers were observed in the ventral horn of the L5, where they terminated in the motoneurons in ChABC-treated rats, but not in saline-treated rats. In total, 54% of motoneurons in the L5 ventral horn in saline-treated rats and 39% of those in ChABC-treated rats were surrounded by CS-A-positive structures. ChABC-treated rats had a Basso, Beattie, and Bresnahan (BBB) motor score of 3.8 at 2 weeks, 7.1 at 3 weeks, and 10.3 at 8 weeks after CST. These observations suggest that ChABC administration to the lesion site immediately after CST may promote the regeneration of descending 5-HT axons through the lesion site and their termination on motoneurons at the level of caudal to the lesion site. ChABC administration might facilitate reinnervation by degrading CS-GAGs around motoneurons. Motor function of the lower limbs was significantly improved in ChABC-treated rats even before the 5-HT axons terminated on the motoneurons, suggesting that other mechanisms may also contribute to the motor function recovery.

Neuronal Redevelopment and the Regeneration of Neuromodulatory Axons in the Adult Mammalian Central Nervous System

One reason that many central nervous system injuries, including those arising from traumatic brain injury, spinal cord injury, and stroke, have limited recovery of function is that neurons within the adult mammalian CNS lack the ability to regenerate their axons following trauma. This stands in contrast to neurons of the adult mammalian peripheral nervous system (PNS). New evidence, provided by single-cell expression profiling, suggests that, following injury, both mammalian central and peripheral neurons can revert to an embryonic-like growth state which is permissive for axon regeneration. This “redevelopment” strategy could both facilitate a damage response necessary to isolate and repair the acute damage from injury and provide the intracellular machinery necessary for axon regrowth. Interestingly, serotonin neurons of the rostral group of raphe nuclei, which project their axons into the forebrain, display a robust ability to regenerate their axons unaided, counter to the widely held view that CNS axons cannot regenerate without experimental intervention after injury. Furthermore, initial evidence suggests that norepinephrine neurons within the locus coeruleus possess similar regenerative abilities. Several morphological characteristics of serotonin axon regeneration in adult mammals, observable using longitudinal in vivo imaging, are distinct from the known characteristics of unaided peripheral nerve regeneration, or of the regeneration seen in the spinal cord and optic nerve that occurs with experimental intervention. These results suggest that there is an alternative CNS program for axon regeneration that likely differs from that displayed by the PNS.

Axo-axonic synapses: Diversity in neural circuit function

The chemical synapse is the principal form of contact between neurons of the central nervous system. These synapses are typically configured as presynaptic axon terminations onto postsynaptic dendrites or somata, giving rise to axo-dendritic and axo-somatic synapses, respectively. Beyond these common synapse configurations are less-studied, non-canonical synapse types that are prevalent throughout the brain and significantly contribute to neural circuit function. Among these are the axo-axonic synapses, which consist of an axon terminating on another axon or axon terminal. Here, we review evidence for axo-axonic synapse contributions to neural signaling in the mammalian nervous system and survey functional neural circuit motifs enabled by these synapses. We also detail how recent advances in microscopy, transgenics, and biological sensors may be used to identify and functionally assay axo-axonic synapses.

Potential role of serotonin as a biological reductant associated with copper transportation

Serotonin (5-HT) is a neurotransmitter that is derived from tryptophan. Owing to a hydroxyl group attached to the indole nucleus, 5-HT exhibits a considerably higher redox activity than tryptophan. To gain insight into the biological relevance of the redox activity of 5-HT, the effect of Cu(I)-binding ligands on the 5-HT-mediated copper reduction was investigated. The d-d transition band of Cu(II) complexed with glycine [Cu(II)-Gly₂] was not affected by addition of 5-HT alone but was diminished when a thioether-containing compound coexists with 5-HT. Concomitant with disappearance of the d-d transition band of Cu(II)-Gly₂, the π-π* transition band of 5-hydroxyindole of 5-HT exhibits a red-shift which is consistently explained by oxidation of 5-HT and subsequent formation of a dimeric species. The redox reactions between 5-HT and copper are also accelerated by a peptide composed of a methionine (Met)-rich region in the extracellular domain of an integral membrane protein, copper transporter 1 (Ctr1). Since Ctr1 transports copper across the plasma membrane with specificity for Cu(I), reduction of extracellular Cu(II) to Cu(I) is required for copper uptake by Ctr1. Metalloreductases that can donate Cu(I) for Ctr1 have been identified in yeast but not yet been found in mammals. The results of this study indicate that the Met-rich region in the N-terminal extracellular domain of Ctr1 promotes the 5-HT-mediated Cu(II) reduction in order to acquire Cu(I) via a non-enzymatic process.

Serotonin Regulation of the Prefrontal Cortex: Cognitive Relevance and the Impact of Developmental Perturbation

The prefrontal cortex is essential for both executive function and emotional regulation. The interrelationships among these behavioral domains are increasingly recognized, as well as their sensitivity to serotonin (5-hydroxytryptamine, 5-HT). Prefrontal cortex receives serotonergic inputs from the dorsal and median raphe nuclei and is modulated by multiple subtypes of 5-HT receptor across its layers and cell types. Extremes of serotonergic modulation alter mood regulation in vulnerable individuals, yet the impact of serotonin under more typical physiological parameters remains unclear. In this regard, new tools are permitting a closer examination of the behavioral impact of the serotonin system. Optogenetic and chemogenetic manipulations of dorsal raphe 5-HT neurons reveal that serotonin has a greater impact on executive function than previously appreciated. Domains that appear sensitive to fluctuations in 5-HT neuronal excitability include patience and cognitive flexibility. This work is broadly consistent with ex vivo research investigating how 5-HT regulates prefrontal cortex and its output projections. A growing literature suggests 5-HT modulation of these prefrontal circuits is unexpectedly flexible to alteration during development by genetic, behavioral, environmental or pharmacological manipulations, with lasting repercussions for cognition and emotional regulation. Here, we review the cellular and circuit mechanisms of prefrontal serotonergic modulation, investigate recent research into the cognitive consequences of the serotonergic system, and probe the lasting consequences of developmental perturbations. Understanding both the complexity of the prefrontal serotonin system and its sensitivity during development are essential to learn more about the vulnerabilities of this system in mood and anxiety disorders and the underappreciated cognitive consequences of these disorders and their treatment.

Melanin-concentrating hormone peptidergic system: Comparative morphology between muroid species

Melanin-concentrating hormone (MCH) is a conserved neuropeptide, predominantly located in the diencephalon of vertebrates, and associated with a wide range of functions. While functional studies have focused on the use of the traditional mouse laboratory model, critical gaps exist in our understanding of the morphology of the MCH system in this species. Even less is known about the nontraditional animal model Neotomodon alstoni (Mexican volcano mouse). A comparative morphological study among these rodents may, therefore, contribute to a better understanding of the evolution of the MCH peptidergic system. To this end, we employed diverse immunohistochemical protocols to identify key aspects of the MCH system, including its spatial relationship to another neurochemical population of the tuberal hypothalamus, the orexins. Three-dimensional (3D) reconstructions were also employed to convey a better sense of spatial distribution to these neurons. Our results show that the distribution of MCH neurons in all rodents studied follows a basic plan, but individual characteristics are found for each species, such as the preeminence of a periventricular group only in the rat, the lack of posterior groups in the mouse, and the extensive presence of MCH neurons in the anterior hypothalamic area of Neotomodon. Taken together, these data suggest a strong anatomical substrate for previously described functions of the MCH system, and that particular neurochemical and morphological features may have been determinant to species-specific phenotypes in rodent evolution.

Serotonergic mechanisms in spinal cord injury

Spinal cord injury (SCI) is a tragic event causing irreversible losses of sensory, motor, and autonomic functions, that may also be associated with chronic neuropathic pain. Serotonin (5-HT) neurotransmission in the spinal cord is critical for modulating sensory, motor, and autonomic functions. Following SCI, 5-HT axons caudal to the lesion site degenerate, and the degree of axonal degeneration positively correlates with lesion severity. Rostral to the lesion, 5-HT axons sprout, irrespective of the severity of the injury. Unlike callosal fibers and cholinergic projections, 5-HT axons are more resistant to an inhibitory milieu and undergo active sprouting and regeneration after central nervous system (CNS) traumatism. Numerous studies suggest that a chronic increase in serotonergic neurotransmission promotes 5-HT axon sprouting in the intact CNS. Moreover, recent studies in invertebrates suggest that 5-HT has a pro-regenerative role in injured axons. Here we present a brief description of 5-HT discovery, 5-HT innervation of the CNS, and physiological functions of 5-HT in the spinal cord, including its role in controlling bladder function. We then present a comprehensive overview of changes in serotonergic axons after CNS damage, and discuss their plasticity upon altered 5-HT neurotransmitter levels. Subsequently, we provide an in-depth review of therapeutic approaches targeting 5-HT neurotransmission, as well as other pre-clinical strategies to promote an increase in re-growth of 5-HT axons, and their functional consequences in SCI animal models. Finally, we highlight recent findings signifying the direct role of 5-HT in axon regeneration and suggest strategies to further promote robust long-distance re-growth of 5-HT axons across the lesion site and eventually achieve functional recovery following SCI.

Selective Modulation of Axonal Sodium Channel Subtypes by 5-HT1A Receptor in Cortical Pyramidal Neuron

Serotonergic innervation of the prefrontal cortex (PFC) modulates neuronal activity and PFC functions. However, the cellular mechanism for serotonergic modulation of neuronal excitability remains unclear. We performed patch-clamp recording at the axon of layer-5 pyramidal neurons in rodent PFC slices. We found surprisingly that the activation of 5-HT1A receptors selectively inhibits Na+ currents obtained at the axon initial segment (AIS) but not those at the axon trunk. In addition, Na+ channel subtype NaV1.2 but not NaV1.6 at the AIS is selectively modulated by 5-HT1A receptors. Further experiments revealed that the inhibitory effect is attributable to a depolarizing shift of the activation curve and a facilitation of slow inactivation of AIS Na+ currents. Consistently, dual somatic and axonal recording and simulation results demonstrate that the activation of 5-HT1A receptors could decrease the success rate of action potential (AP) backpropagation toward the somatodendritic compartments, enhancing the segregation of axonal and dendritic activities. Together, our results reveal a selective modulation of NaV1.2 distributed at the proximal AIS region and AP backpropagation by 5-HT1A receptors, suggesting a potential mechanism for serotonergic regulation of functional polarization in the dendro-axonal axis, synaptic plasticity and PFC functions.

The lateral habenula and the serotonergic system

The habenula (Hb) is an epithalamic structure differentiated into two nuclear complexes, medial (MHb) and lateral habenula (LHb). After decades of relative neglect, interest in the Hb resurged when it was demonstrated that LHb neurons play a key role in encoding disappointments and expectation of punishments. Consistent with such a role, the LHb has been implicated in a broad array of functions and pathologic conditions, notably in mechanisms of stress and pain, as well as in the pathophysiology of mood disorders. So far, the vast majority of research involving the LHb has focused on its role in regulating midbrain dopamine release. However, the LHb is also robustly interconnected in a reciprocal manner with a set of rostral serotonin (5-HT) nuclei. Thus, there is increasing evidence that the LHb is amply linked to the dorsal (DR) and median raphe nucleus (MnR) by a complex network of parallel topographically organized direct and indirect pathways. Here, we summarize research about the interconnections of the LHb with different subregions of the DR and MnR, as well as findings about 5-HT-dependent modulation of LHb neurons. Finally, we discuss the contribution of distinct LHb-raphe loops to stress and stress-related psychiatric disorders including anxiety and depression.

Neural connection supporting endogenous 5-hydroxytryptamine influence on autonomic activity in medial prefrontal cortex

5-hydroxytryptamine (5-HT) transmission in the medial prefrontal cortex (mPFC) enhances or suppresses signal outflow to influence emotion-/cognition-based function performances and, putatively, the autonomic responses. The top-down cortical modulation of autonomic activities may be mediated in part through projections from mPFC to brain stem dorsal vagal complex (DVC). The abundant and heterogeneous densities of 5-HT fibers across laminae in mPFC suggest serotonergic innervation of mPFC-DVC projection neurons whereby endogenous 5-HT acts to regulate autonomic activities. The present study investigated the physical relationship between 5-HT fibers and the autonomic-related mPFC neurons by examining and quantitatively characterizing the 5-HT contacts upon retrogradely labeled mPFC-DVC projection neurons in pre- and infra-limbic cortices (PrL/IL) with light and electron microscopies combined with immunocytochemistry for 5-HT and presynaptic vesicle marker synaptophysin (Syn). 5-HT varicosities were observed, under confocal microscope, to form close appositions to or, at ultrastructural level, to form asymmetric axodendritic synapses and direct contacts upon the target neurons. About 16% of the entire 5-HTergic varicosities in lamina V of PrL/IL coexpressed Syn and about 24% of the peri-somatic 5-HTergic swellings demonstrated Syn-immunoreactivity (ir), suggesting a low frequency of putative synapses estimated at optical level. Ultrastructurally, examination of thirty-seven serially cut thin 5-HT boutons closely apposed to the labeled dendritic profiles demonstrated that only three contacts presented with identifiable asymmetric, synaptic membrane specializations. These data provide the first and direct morphological evidence supporting that endogenous 5-HT may be released mainly via direct contacts bearing no identifiable synaptic specializations as well as synapses, targeting autonomic-related mPFC neurons for autonomic regulation.

The Diversity of Spine Synapses in Animals

Here we examine the structure of the various types of spine synapses throughout the animal kingdom. Based on available evidence, we suggest that there are two major categories of spine synapses: invaginating and non-invaginating, with distributions that vary among different groups of animals. In the simplest living animals with definitive nerve cells and synapses, the cnidarians and ctenophores, most chemical synapses do not form spine synapses. But some cnidarians have invaginating spine synapses, especially in photoreceptor terminals of motile cnidarians with highly complex visual organs, and also in some mainly sessile cnidarians with rapid prey capture reflexes. This association of invaginating spine synapses with complex sensory inputs is retained in the evolution of higher animals in photoreceptor terminals and some mechanoreceptor synapses. In contrast to invaginating spine synapse, non-invaginating spine synapses have been described only in animals with bilateral symmetry, heads and brains, associated with greater complexity in neural connections. This is apparent already in the simplest bilaterians, the flatworms, which can have well-developed non-invaginating spine synapses in some cases. Non-invaginating spine synapses diversify in higher animal groups. We also discuss the functional advantages of having synapses on spines and more specifically, on invaginating spines. And finally we discuss pathologies associated with spine synapses, concentrating on those systems and diseases where invaginating spine synapses are involved.

Mapping the connectivity of serotonin transporter immunoreactive axons to excitatory and inhibitory neurochemical synapses in the mouse limbic brain

Serotonin neurons arise from the brainstem raphe nuclei and send their projections throughout the brain to release 5-HT which acts as a modulator of several neuronal populations. Previous electron microscopy studies in rats have morphologically determined the distribution of 5-HT release sites (boutons) in certain brain regions and have shown that 5-HT containing boutons form synaptic contacts that are either symmetric or asymmetric. In addition, 5-HT boutons can form synaptic triads with the pre- and postsynaptic specializations of either symmetrical or asymmetrical synapses. However, due to the labor intensive processing of serial sections required by electron microscopy, little is known about the neurochemical properties or the quantitative distribution of 5-HT triads within whole brain or discrete subregions. Therefore, we used a semi-automated approach that combines immunohistochemistry and high-resolution confocal microscopy to label serotonin transporter (SERT) immunoreactive axons and reconstruct in 3D their distribution within limbic brain regions. We also used antibodies against key pre- (synaptophysin) and postsynaptic components of excitatory (PSD95) or inhibitory (gephyrin) synapses to (1) identify putative 5-HTergic boutons within SERT immunoreactive axons and, (2) quantify their close apposition to neurochemical excitatory or inhibitory synapses. We provide a 5-HTergic axon density map and have determined the ratio of synaptic triads consisting of a 5-HT bouton in close proximity to either neurochemical excitatory or inhibitory synapses within different limbic brain areas. The ability to model and map changes in 5-HTergic axonal density and the formation of triadic connectivity within whole brain regions using this rapid and quantitative approach offers new possibilities for studying neuroplastic changes in the 5-HTergic pathway.

Regional distribution of serotonergic receptors: a systems neuroscience perspective on the downstream effects of the multimodal-acting antidepressant vortioxetine on excitatory and inhibitory neurotransmission

Previous work from this laboratory hypothesized that the multimodal antidepressant vortioxetine enhances cognitive function through a complex mechanism, using serotonergic (5-hydroxytryptamine, 5-HT) receptor actions to modulate gamma-butyric acid (GABA) and glutamate neurotransmission in key brain regions like the prefrontal cortex (PFC) and hippocampus. However, serotonergic receptors have circumscribed expression patterns, and therefore vortioxetine's effects on GABA and glutamate neurotransmission will probably be regionally selective. In this article, we attempt to develop a conceptual framework in which the effects of 5-HT, selective serotonin reuptake inhibitors (SSRIs), and vortioxetine on GABA and glutamate neurotransmission can be understood in the PFC and striatum-2 regions with roles in cognition and substantially different 5-HT receptor expression patterns. Thus, we review the anatomy of the neuronal microcircuitry in the PFC and striatum, anatomical data on 5-HT receptor expression within these microcircuits, and electrophysiological evidence on the effects of 5-HT on the behavior of each cell type. This analysis suggests that 5-HT and SSRIs will have markedly different effects within the PFC, where they will induce mixed effects on GABA and glutamate neurotransmission, compared to the striatum, where they will enhance GABAergic interneuron activity and drive down the activity of medium spiny neurons. Vortioxetine is expected to reduce GABAergic interneuron activity in the PFC and concomitantly increase cortical pyramidal neuron firing. However in the striatum, vortioxetine is expected to increase activity at GABAergic interneurons and have mixed excitatory and inhibitory effects in medium spiny neurons. Thus the conceptual framework developed here suggests that vortioxetine will have regionally selective effects on GABA and glutamate neurotransmission.

Psychedelics

Psychedelics (serotonergic hallucinogens) are powerful psychoactive substances that alter perception and mood and affect numerous cognitive processes. They are generally considered physiologically safe and do not lead to dependence or addiction. Their origin predates written history, and they were employed by early cultures in many sociocultural and ritual contexts. After the virtually contemporaneous discovery of (5R,8R)-(+)-lysergic acid-N,N-diethylamide (LSD)-25 and the identification of serotonin in the brain, early research focused intensively on the possibility that LSD and other psychedelics had a serotonergic basis for their action. Today there is a consensus that psychedelics are agonists or partial agonists at brain serotonin 5-hydroxytryptamine 2A receptors, with particular importance on those expressed on apical dendrites of neocortical pyramidal cells in layer V. Several useful rodent models have been developed over the years to help unravel the neurochemical correlates of serotonin 5-hydroxytryptamine 2A receptor activation in the brain, and a variety of imaging techniques have been employed to identify key brain areas that are directly affected by psychedelics. Recent and exciting developments in the field have occurred in clinical research, where several double-blind placebo-controlled phase 2 studies of psilocybin-assisted psychotherapy in patients with cancer-related psychosocial distress have demonstrated unprecedented positive relief of anxiety and depression. Two small pilot studies of psilocybin-assisted psychotherapy also have shown positive benefit in treating both alcohol and nicotine addiction. Recently, blood oxygen level-dependent functional magnetic resonance imaging and magnetoencephalography have been employed for in vivo brain imaging in humans after administration of a psychedelic, and results indicate that intravenously administered psilocybin and LSD produce decreases in oscillatory power in areas of the brain's default mode network.

Dopamine synapse is a neuroligin-2-mediated contact between dopaminergic presynaptic and GABAergic postsynaptic structures

Midbrain dopamine neurons project densely to the striatum and form so-called dopamine synapses on medium spiny neurons (MSNs), principal neurons in the striatum. Because dopamine receptors are widely expressed away from dopamine synapses, it remains unclear how dopamine synapses are involved in dopaminergic transmission. Here we demonstrate that dopamine synapses are contacts formed between dopaminergic presynaptic and GABAergic postsynaptic structures. The presynaptic structure expressed tyrosine hydroxylase, vesicular monoamine transporter-2, and plasmalemmal dopamine transporter, which are essential for dopamine synthesis, vesicular filling, and recycling, but was below the detection threshold for molecules involving GABA synthesis and vesicular filling or for GABA itself. In contrast, the postsynaptic structure of dopamine synapses expressed GABAergic molecules, including postsynaptic adhesion molecule neuroligin-2, postsynaptic scaffolding molecule gephyrin, and GABAA receptor α1, without any specific clustering of dopamine receptors. Of these, neuroligin-2 promoted presynaptic differentiation in axons of midbrain dopamine neurons and striatal GABAergic neurons in culture. After neuroligin-2 knockdown in the striatum, a significant decrease of dopamine synapses coupled with a reciprocal increase of GABAergic synapses was observed on MSN dendrites. This finding suggests that neuroligin-2 controls striatal synapse formation by giving competitive advantage to heterologous dopamine synapses over conventional GABAergic synapses. Considering that MSN dendrites are preferential targets of dopamine synapses and express high levels of dopamine receptors, dopamine synapse formation may serve to increase the specificity and potency of dopaminergic modulation of striatal outputs by anchoring dopamine release sites to dopamine-sensing targets.

Activation of 5-HT2A receptors by TCB-2 induces recurrent oscillatory burst discharge in layer 5 pyramidal neurons of the mPFC in vitro

The medial prefrontal cortex (mPFC) is a region of neocortex that plays an integral role in several cognitive processes which are abnormal in schizophrenic patients. As with other cortical regions, large‐bodied layer 5 pyramidal neurons serve as the principle subcortical output of microcircuits of the mPFC. The coexpression of both inhibitory serotonin 5‐HT_1A receptors on the axon initial segments, and excitatory 5‐HT_2A receptors throughout the somatodendritic compartments, by layer 5 pyramidal neurons allows serotonin to provide potent top–down regulation of input–output relationships within cortical microcircuits. Application of 5‐HT_2A agonists has previously been shown to enhance synaptic input to layer 5 pyramidal neurons, as well as increase the gain in neuronal firing rate in response to increasing depolarizing current steps. Using whole‐cell patch‐clamp recordings obtained from layer 5 pyramidal neurons of the mPFC of C57/bl6 mice, the aim of our present study was to investigate the modulation of long‐term spike trains by the selective 5‐HT_2A agonist TCB‐2. We found that in the presence of synaptic blockers, TCB‐2 induced recurrent oscillatory bursting (ROB) after 15–20 sec of tonic spiking in 7 of the 14 cells. In those seven cells, ROB discharge was accurately predicted by the presence of a voltage sag in response to a hyperpolarizing current injection. This effect was reversed by 5–10 min of drug washout and ROB discharge was inhibited by both synaptic activity and coapplication of the 5‐HT_2A/2C antagonist ketanserin. While the full implications of this work are not yet understood, it may provide important insight into serotonergic modulation of cortical networks.

Using whole‐cell patch‐clamp recordings obtained from layer 5 pyramidal neurons of the mouse mPFC, we investigated the modulation of long‐term spike trains by the selective 5‐HT_2A agonist TCB‐2. In the presence of synaptic blockers, TCB‐2 induced recurrent oscillatory bursting (ROB) after 15–20 sec of tonic spiking in 7 of the 14 cells; ROB discharge was accurately predicted by the presence of a voltage sag in response to a hyperpolarizing current injection. We have identified a novel modulation of pyramidal neuron excitability by a 5HT receptor known to contribute to the pathophysiology of schizophrenia.

Distribution of VGLUT3 in highly collateralized axons from the rat dorsal raphe nucleus as revealed by single-neuron reconstructions

This study aimed at providing the first detailed morphological description, at the single-cell level, of the rat dorsal raphe nucleus neurons, including the distribution of the VGLUT3 protein within their axons. Electrophysiological guidance procedures were used to label dorsal raphe nucleus neurons with biotinylated dextran amine. The somatodendritic and axonal arborization domains of labeled neurons were reconstructed entirely from serial sagittal sections using a computerized image analysis system. Under anaesthesia, dorsal raphe nucleus neurons display highly regular (1.72±0.50 Hz) spontaneous firing patterns. They have a medium size cell body (9.8±1.7 µm) with 2–4 primary dendrites mainly oriented anteroposteriorly. The ascending axons of dorsal raphe nucleus are all highly collateralized and widely distributed (total axonal length up to 18.7 cm), so that they can contact, in various combinations, forebrain structures as diverse as the striatum, the prefrontal cortex and the amygdala. Their morphological features and VGLUT3 content vary significantly according to their target sites. For example, high-resolution confocal analysis of the distribution of VGLUT3 within individually labeled-axons reveals that serotonin axon varicosities displaying VGLUT3 are larger (0.74±0.03 µm) than those devoid of this protein (0.55±0.03 µm). Furthermore, the percentage of axon varicosities that contain VGLUT3 is higher in the striatum (93%) than in the motor cortex (75%), suggesting that a complex trafficking mechanism of the VGLUT3 protein is at play within highly collateralized axons of the dorsal raphe nucleus neurons. Our results provide the first direct evidence that the dorsal raphe nucleus ascending projections are composed of widely distributed neuronal systems, whose capacity to co-release serotonin and glutamate varies from one forebrain locus to the other.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Vertebrate melanophores as potential model for drug discovery and development: a review

Drug discovery in skin pharmacotherapy is an enormous, continually expanding field. Researchers are developing novel and sensitive pharmaceutical products and drugs that target specific receptors to elicit concerted and appropriate responses. The pigment-bearing cells called melanophores have a significant contribution to make in this field. Melanophores, which contain the dark brown or black pigment melanin, constitute an important class of chromatophores. They are highly specialized in the bidirectional and coordinated translocation of pigment granules when given an appropriate stimulus. The pigment granules can be stimulated to undergo rapid dispersion throughout the melanophores, making the cell appear dark, or to aggregate at the center, making the cell appear light. The major signals involved in pigment transport within the melanophores are dependent on a special class of cell surface receptors called G-protein-coupled receptors (GPCRs). Many of these receptors of adrenaline, acetylcholine, histamine, serotonin, endothelin and melatonin have been found on melanophores. They are believed to have clinical relevance to skin-related ailments and therefore have become targets for high throughput screening projects. The selective screening of these receptors requires the recognition of particular ligands, agonists and antagonists and the characterization of their effects on pigment motility within the cells. The mechanism of skin pigmentation is incredibly intricate, but it would be a considerable step forward to unravel its underlying physiological mechanism. This would provide an experimental basis for new pharmacotherapies for dermatological anomalies. The discernible stimuli that can trigger a variety of intracellular signals affecting pigment granule movement primarily include neurotransmitters and hormones. This review focuses on the role of the hormone and neurotransmitter signals involved in pigment movement in terms of the pharmacology of the specific receptors.

Timing is not Everything: Neuromodulation Opens the STDP Gate

Spike timing dependent plasticity (STDP) is a temporally specific extension of Hebbian associative plasticity that has tied together the timing of presynaptic inputs relative to the postsynaptic single spike. However, it is difficult to translate this mechanism to in vivo conditions where there is an abundance of presynaptic activity constantly impinging upon the dendritic tree as well as ongoing postsynaptic spiking activity that backpropagates along the dendrite. Theoretical studies have proposed that, in addition to this pre- and postsynaptic activity, a “third factor” would enable the association of specific inputs to specific outputs. Experimentally, the picture that is beginning to emerge, is that in addition to the precise timing of pre- and postsynaptic spikes, this third factor involves neuromodulators that have a distinctive influence on STDP rules. Specifically, neuromodulatory systems can influence STDP rules by acting via dopaminergic, noradrenergic, muscarinic, and nicotinic receptors. Neuromodulator actions can enable STDP induction or – by increasing or decreasing the threshold – can change the conditions for plasticity induction. Because some of the neuromodulators are also involved in reward, a link between STDP and reward-mediated learning is emerging. However, many outstanding questions concerning the relationship between neuromodulatory systems and STDP rules remain, that once solved, will help make the crucial link from timing-based synaptic plasticity rules to behaviorally based learning.

Unilateral lesion of the nigrostriatal pathway decreases the response of interneurons in medial prefrontal cortex to 5-HT 2A/2C receptor stimulation in the rat

The aim of the present study was to investigate changes in the firing rate and pattern of interneurons in the medial prefrontal cortex (mPFC), and effects of 5-HT(2A/2C) receptor agonist DOI and antagonist ritanserin, and the selective 5-HT(2C) receptor antagonist SB 242084 on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) by extracellular recording in vivo. The lesion of the SNc decreased the firing rate of the interneurons compared to sham-lesioned rats, and firing pattern of these interneurons changed toward a more burst-firing. Administration of DOI (20-320 microg/kg, i.v.) dose-dependently increased the firing rate of all interneurons examined in sham-lesioned and the 6-OHDA-lesioned rats. The excitation was significant at doses higher than 40 microg/kg and 320 microg/kg in sham-lesioned and the 6-OHDA-lesioned rats, respectively. This dose, which produced marked effect in the 6-OHDA-lesioned rats, was much higher than that of sham-lesioned rats. The local application of DOI (5 microg) in mPFC increased the firing rate of the interneurons in sham-lesioned rats, while having no effect on the firing rate in the 6-OHDA-lesioned rats. The excitatory effect of DOI in sham-lesioned and the 6-OHDA-lesioned rats was completely or partially reversed by ritanserin or SB 242084. The results of our study show that lesion of the SNc leads to a decrease in the firing rate of interneurons in mPFC and fire with a more burst pattern, and decreased response of the interneurons to DOI in rat.

Serotonin-immunoreactive axon terminals innervate pyramidal cells and interneurons in the rat basolateral amygdala

The basolateral nuclear complex of the amygdala (BLC) receives a dense serotonergic innervation that appears to play a critical role in the regulation of mood and anxiety. However, little is known about how serotonergic inputs interface with different neuronal subpopulations in this region. To address this question, dual-labeling immunohistochemical techniques were used at the light and electron microscopic levels to examine inputs from serotonin-immunoreactive (5-HT+) terminals to different neuronal subpopulations in the rat BLC. Pyramidal cells were labeled by using antibodies to calcium/calmodulin-dependent protein kinase II, whereas different interneuronal subpopulations were labeled by using antibodies to a variety of interneuronal markers including parvalbumin (PV), vasoactive intestinal peptide (VIP), calretinin, calbindin, cholecystokinin, and somatostatin. The BLC exhibited a dense innervation by thin 5-HT+ axons. Electron microscopic examination of the anterior basolateral nucleus (BLa) revealed that 5-HT+ axon terminals contained clusters of small synaptic vesicles and a smaller number of larger dense-core vesicles. Serial section reconstruction of 5-HT+ terminals demonstrated that 76% of these terminals formed synaptic junctions. The great majority of these synapses were symmetrical. The main targets of 5-HT+ terminals were spines and distal dendrites of pyramidal cells. However, in light microscopic preparations it was common to observe apparent contacts between 5-HT+ terminals and all subpopulations of BLC interneurons. Electron microscopic analysis of the BLa in sections dual-labeled for 5-HT/PV and 5-HT/VIP revealed that many of these contacts were synapses. These findings suggest that serotonergic axon terminals differentially innervate several neuronal subpopulations in the BLC.

Serotonin and Dopamine Interactions in Rodents and Primates: Implications for Psychosis and Antipsychotic Drug Development

Since the late 1950s, appreciation of dopamine receptor blockade has played a primary role in understanding the mechanism underlying the therapeutic effects of antipsychotic drugs in schizophrenic patients in treating the positive symptoms of schizophrenia (e.g., delusions and hallucinations). Development of the second generation of antipsychotic drugs, otherwise known as atypical antipsychotic drugs, has resulted in treatments with improved subjective tolerability but relatively modest improvements in the negative symptoms of schizophrenia such as avolition, flat affect, and anhedonia. The major current challenge is to develop medications which can further improve negative symptoms treatment and also tackle the intractable clinical problems of cognitive impairment associated with schizophrenia. Further advances along these lines with respect to the dopaminergic and serotonergic neurostransmitter systems will be aided by an appreciation of the interaction between dopamine and serotonin receptor subtypes in a range of key brain structures, such as the prefrontal cortex, thalamus, striatum, amygdala, hippocampus, and the brain stem nuclei, from which the cell bodies of monoaminergic-containing neurons originate. Increasing emphasis on the use of animal models which are homologous to critical aspects of the pathophysiology in the brains of schizophrenic patients will also be required, especially as negative symptoms and cognitive impairment become an important focus for generating novel therapeutics.

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

Nicotinic modulation of neuronal networks: from receptors to cognition

RATIONALE

Nicotine affects many aspects of human cognition, including attention and memory. Activation of nicotinic acetylcholine receptors (nAChRs) in neuronal networks modulates activity and information processing during cognitive tasks, which can be observed in electroencephalograms (EEGs) and functional magnetic resonance imaging studies.

OBJECTIVES

In this review, we will address aspects of nAChR functioning as well as synaptic and cellular modulation important for nicotinic impact on neuronal networks that ultimately underlie its effects on cognition. Although we will focus on general mechanisms, an emphasis will be put on attention behavior and nicotinic modulation of prefrontal cortex. In addition, we will discuss how nicotinic effects at the neuronal level could be related to its effects on the cognitive level through the study of electrical oscillations as observed in EEGs and brain slices.

RESULTS/CONCLUSIONS

Very little is known about mechanisms of how nAChR activation leads to a modification of electrical oscillation frequencies in EEGs. The results of studies using pharmacological interventions and transgenic animals implicate some nAChR types in aspects of cognition, but neuronal mechanisms are only poorly understood. We are only beginning to understand how nAChR distribution in neuronal networks impacts network functioning. Unveiling receptor and neuronal mechanisms important for nicotinic modulation of cognition will be instrumental for treatments of human disorders in which cholinergic signaling have been implicated, such as schizophrenia, attention deficit/hyperactivity disorder, and addiction.

Serotonergic modulation of the limbic system

The limbic system is composed of cortical as well as subcortical structures, which are intimately interconnected. The resulting macrostructure is responsible for the generation and expression of motivational and affective states. Especially high levels of serotonin are found in limbic forebrain structures. Serotonin projections to these structures, which arise from serotonergic cell body groups in the midbrain, form a dense plexus of axonal processes. In many areas of the limbic system, serotonergic neurotransmission can best be described as paracrine or volume transmission, and thus serotonin is believed to play a neuromodulatory role in the brain. Serotonergic projections to limbic structures, arising primarily from the dorsal and median raphe nuclei, compose two distinct serotonergic systems differing in their topographic organization, electrophysiological characteristics, morphology, as well as sensitivity to neurotoxins and perhaps psychoactive or therapeutic agents. These differences may be extremely important in understanding the role of these two serotonergic systems in normal brain function and in mental illness. Central serotonergic neurons or receptors are targets for a variety of therapeutic agents used in the treatment of disorders of the limbic system.

In situ expression of brain-derived neurotrophic factor or neurotrophin-3 promotes sprouting of cortical serotonergic axons following a neurotoxic lesion

Neurotrophins promote sprouting and elongation of central nervous system (CNS) axons following injury. Consequently, it has been suggested that neurotrophins could be used to repair the CNS by inducing axonal sprouting from nearby intact axons, thereby compensating for the loss of recently injured axons. We tested whether long-term overexpression of neurotrophins in the rat cortex would induce sprouting of cortical serotonergic axons following a neurotoxic injury. After a single subcutaneous injection of para-chloroamphetamine (PCA; 9 mg/ml) that lesions the majority of serotonergic axons in the rat cortex, we injected adenoviral vectors containing cDNAs for brain-derived neurotrophic factor (Adv.BDNF), neurotrophin-3 (Adv.NT-3), or nerve growth factor (Adv.NGF) into the rat frontal cortex. Nine days later, we measured significant increases in the concentration of the respective neurotrophins surrounding the vector injection sites, as measured by ELISA. Immunohistochemical localization of serotonin revealed a fourfold increase in the density of serotonergic fibers surrounding the injection sites of Adv.BDNF and Adv.NT-3, corresponding to a 50% increase in cortical serotonin concentration, compared with a control vector containing the cDNA for enhanced green fluorescent protein (Adv.EGFP). In contrast, there was no difference in serotonergic fiber density or cortical serotonin concentration surrounding the injection of Adv.NGF compared with Adv.EGFP. These data demonstrate that localized overexpression of BDNF or NT-3, but not NGF, is sufficient to promote sprouting of serotonergic axons in the cortex following an experimental neurotoxic injury.

Modulation of neocortical interneurons: extrinsic influences and exercises in self-control

Neocortical GABAergic interneurons are a highly heterogeneous cell population that forms complex functional networks and has key roles in information processing within the cerebral cortex. Mechanisms that control the output of these cells are therefore crucial in regulating excitability within the neocortex during normal and pathophysiological activities. In addition to subtype-specific modulation of GABAergic cells by neurotransmitters released by afferents from subcortical nuclei, interneurons belonging to different classes are controlled by distinct self-modulatory mechanisms, each unique and powerful. In this article, we review the diverse responses of neocortical interneurons to extrinsic and intrinsic neuromodulators. We discuss how specificity of responses might differentially influence inhibition in somatodendritic compartments of pyramidal neurons and affect the balance of activities in neocortical circuits.

Membrane-associated guidance cues direct the innervation of forebrain and midbrain by dorsal raphe-derived serotonergic axons

Unlike many neurons that extend an axon precisely to a single target, individual dorsal raphe 5-HT neurons project to multiple brain regions and their axon terminals often lack classical synaptic specializations. It is not known how 5-HT axon collaterals select between multiple target fields, or even if 5-HT axons require specific guidance cues to innervate their targets. Nor is it known how these axon collaterals are restrained within specific innervation target regions. To investigate this, we challenged explants of dorsal raphe with co-explants, or cell membrane preparations of ventral midbrain, striatum or cerebral cortex. We provide evidence for membrane-associated cues that promote 5-HT axon growth into each of these three target regions. The axon growth-promoting activity was heat-, protease- and phosphatidylinositol-phospholipase-C (PI-PLC)-sensitive. Interestingly, 5-HT axons specifically lost the ability to grow in heterotypic explants, or membrane carpets, following contact with ventral midbrain or striatal, but not cortical, explants or membranes. This inductive activity associated with striatal and ventral midbrain membranes was sensitive to both high salt extraction and PI-PLC treatment. By contrast, the activity that inhibited 5-HT axon growth onto heterotypic membranes was sensitive only to high salt extraction. These results provide evidence that a glycosylphosphatidylinositol (GPI)-linked membrane protein promotes 5-HT axon growth, and that short-range membrane-bound, as well as GPI-linked, molecules contribute to the guidance of 5-HT axon collaterals. These findings suggest that 5-HT axon collaterals acquire a target-induced growth-inhibitory response to alternative targets, increasing their selectivity for the newly innervated field.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Hallucinogens

Hallucinogens (psychedelics) are psychoactive substances that powerfully alter perception, mood, and a host of cognitive processes. They are considered physiologically safe and do not produce dependence or addiction. Their origin predates written history, and they were employed by early cultures in a variety of sociocultural and ritual contexts. In the 1950s, after the virtually contemporaneous discovery of both serotonin (5-HT) and lysergic acid diethylamide (LSD-25), early brain research focused intensely on the possibility that LSD or other hallucinogens had a serotonergic basis of action and reinforced the idea that 5-HT was an important neurotransmitter in brain. These ideas were eventually proven, and today it is believed that hallucinogens stimulate 5-HT(2A) receptors, especially those expressed on neocortical pyramidal cells. Activation of 5-HT(2A) receptors also leads to increased cortical glutamate levels presumably by a presynaptic receptor-mediated release from thalamic afferents. These findings have led to comparisons of the effects of classical hallucinogens with certain aspects of acute psychosis and to a focus on thalamocortical interactions as key to understanding both the action of these substances and the neuroanatomical sites involved in altered states of consciousness (ASC). In vivo brain imaging in humans using [(18)F]fluorodeoxyglucose has shown that hallucinogens increase prefrontal cortical metabolism, and correlations have been developed between activity in specific brain areas and psychological elements of the ASC produced by hallucinogens. The 5-HT(2A) receptor clearly plays an essential role in cognitive processing, including working memory, and ligands for this receptor may be extremely useful tools for future cognitive neuroscience research. In addition, it appears entirely possible that utility may still emerge for the use of hallucinogens in treating alcoholism, substance abuse, and certain psychiatric disorders.

Pretectal and tectal afferents to the dorsal lateral geniculate nucleus of the turtle: An electron microscopic axon tracing and γ-aminobutyric acid immunocytochemical study

The pretectal and tectal projections to the dorsal lateral geniculate nucleus (GLd) of two species of turtle (Emys orbicularis and Testudo horsfieldi) were examined under the electron microscope by using axonal tracing techniques (horseradish peroxidase or biotinylated dextran amine) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. After injection of tracer into the pretectum, two types of axon terminals were identified as those of pretectogeniculate pathways. Both contained pleomorphic synaptic vesicles and were more numerous in the inner part of the nucleus. They could be distinguished on the bases of size and shape of their synaptic vesicles, type of synaptic contact, and level of GABA immunoreactivity. One type had a higher density of immunolabeling and established symmetric synaptic contacts, whereas the other, less densely immunolabeled, made asymmetric synaptic contacts. In both cases, synaptic contacts were mainly with relay cells and occasionally with interneurons. We suggest that these two types of pretectogeniculate terminals originate in two separate pretectal nuclei. After injection of tracer into the optic tectum, a single population of GABA-immunonegative tracer-labeled terminals was identified as belonging to the tectogeniculate pathway. These were small, had smooth contours, contained very small round synaptic vesicles, and established asymmetric synaptic contacts with long active zones, predominantly with relay cells and less frequently with interneurons, in the inner part of the nucleus. In addition, a population of GABA-negative and occasionally GABA-positive terminals, labeled by tracer injected into either the pretectum or the tectum, was identified as retinal terminals; these were presumably labeled by the retrograde transport of tracer in collateral branches of visual fibers innervating both the GLd and the pretectum or tectum. Comparison of the present ultrastructural findings in turtles with those previously reported in mammals shows that the cytological features, synaptic morphology, and immunochemical properties of the pretectogeniculate and tectogeniculate terminals of both groups share many similarities. Nevertheless, the postsynaptic targets of these two categories of terminals display some pronounced differences between the two groups, which are discussed in terms of their possible functional significance.

Prefrontal cortical projections to the rat dorsal raphe nucleus: ultrastructural features and associations with serotonin and gamma-aminobutyric acid neurons

Studies of human brain indicate that both the ventromedial prefrontal cortex (PFC) and the dorsal raphe nucleus (DRN) may be dysfunctional in major depressive illness, making it important to understand the functional interactions between these brain regions. Anatomical studies have shown that the PFC projects to the DRN, although the synaptic targets of this excitatory pathway have not yet been identified. Electrophysiological investigations in the rat DRN report that most serotonin neurons are inhibited by electrical stimulation of the PFC, suggesting that this pathway is more likely to synapse onto neighboring gamma-aminobutyric acid (GABA) neurons than onto serotonin cells. We tested this hypothesis by electron microscopic examination of DRN sections dually labeled for biotin dextran amine anterogradely transported from the PFC and immunogold-silver labeling for tryptophan hydroxylase (TrH) or for GABA. In the DRN, the majority of PFC axons either synapsed onto unlabeled dendrites or failed to form detectable synapses in single sections. Other PFC axons synapsed onto either TrH- or GABA-immunolabeled processes. Considerably more tissue sampling was necessary to detect PFC synapses onto TrH- than onto GABA-labeled dendrites, suggesting that the latter connections are more common. In other cases, PFC terminals and TrH- or GABA-immunoreactive dendrites either were closely apposed, without forming detectable synapses, or were separated by glial processes. These results provide potential anatomical substrates whereby the PFC can both directly and indirectly regulate the activity of serotonin neurons in the DRN and possibly contribute to the pathophysiology of depression.

Heterogeneous actions of serotonin on interneurons in rat visual cortex

The effects of serotonin (5-HT) on excitability of two cortical interneuronal subtypes, fast-spiking (FS) and low threshold spike (LTS) cells, and on spontaneous inhibitory postsynaptic currents (sIPSCs) in layer V pyramidal cells were studied in rat visual cortical slices using whole-cell recording techniques. Twenty-two of 28 FS and 26 of 35 LTS interneurons responded to local application of 5-HT. In the group of responsive neurons, 5-HT elicited an inward current in 50% of FS cells and 15% of LTS cells, an outward current was evoked in 41% of FS cells and 81% of LTS cells, and an inward current followed by an outward current in 9% of FS cells and 4% LTS cells. The inward and outward currents were blocked by a 5-HT(3) receptor antagonist, tropisetron, and a 5-HT(1A) receptor antagonist, NAN-190, respectively. The 5-HT-induced inward and outward currents were both associated with an increase in membrane conductance. The estimated reversal potential was more positive than -40 mV for the inward current and close to the calculated K(+) equilibrium potential for the outward current. The 5-HT application caused an increase, a decrease, or an increase followed by a decrease in the frequency of sIPSCs in pyramidal cells. The 5-HT(3) receptor agonist 1-(m-chlorophenyl) biguanide increased the frequency of larger and fast-rising sIPSCs, whereas the 5-HT(1A) receptor agonist (+/-)8-hydroxydipropylaminotetralin hydrobromide elicited opposite effects and decreased the frequency of large events. These data indicate that serotonergic activation imposes complex actions on cortical inhibitory networks, which may lead to changes in cortical information processing.

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.

5-HT3 receptors mediate serotonergic fast synaptic excitation of neocortical vasoactive intestinal peptide/cholecystokinin interneurons

Neocortical neurons expressing the serotonin 5-HT3 receptor (5-HT3R) were characterized in rat acute slices by using patch-clamp recordings combined with single-cell RT-PCR and histochemical labeling. The 5-HT3A receptor subunit was expressed selectively in a subset of GABAergic interneurons coexpressing cholecystokinin (CCK) and vasoactive intestinal peptide (VIP). The 5-HT3B subunit was never detected, indicating that 5-HT3Rs expressed by neocortical interneurons did not contain this subunit. In 5-HT3A-expressing VIP/CCK interneurons, serotonin induced fast membrane potential depolarizations by activating an inward current that was blocked by the selective 5-HT3R antagonist tropisetron. Furthermore, we observed close appositions between serotonergic fibers and the dendrites and somata of 5-HT3R-expressing neurons, suggestive of possible synaptic contacts. Indeed, in interneurons exhibiting rapid excitation by serotonin, local electrical stimulations evoked fast EPSCs of large amplitude that were blocked by tropisetron. Finally, 5-HT3R-expressing neurons were also excited by a nicotinic agonist, indicating that serotonergic and cholinergic fast synaptic transmission could converge onto VIP/CCK interneurons. Our results establish a clear correlation between the presence of the 5-HT3A receptor subunit in neocortical VIP/CCK GABAergic interneurons, its functional expression, and its synaptic activation by serotonergic afferent fibers from the brainstem raphe nuclei.

Relationships of 5-hydroxytryptamine immunoreactive terminal-like varicosities to 5-hydroxytryptamine-2A receptor-immunoreactive neuronal processes in the rat forebrain

The distributions of 5-hydroxytryptamine (5-HT)-immunoreactive (IR) varicosities and 5-hydroxytryptamine-2A receptor (5-HT2A)-IR neuronal structures in the rat brain have previously been described individually. Using double labeling immunocytochemistry, the relationships between 5-HT2A-IR and 5-HT-IR elements in the forebrain of male rats has been studied at the light microscopic level. In neocortical regions (frontal, parietal and retrosplenial cortex), the strongest 5-HT2A-IR was found in the apical dendrites of pyramidal cells in layers III-V, while 5-HT-IR terminal-like varicosities were present in all layers but most prominently in the outer layers. In other forebrain regions, the olfactory bulb, the hippocampal formation, and the islands of Calleja and Calleja magna, localized discrepancies were present between the 5-HT2A-IR neuronal profiles and the 5-HT-IR terminal-like varicosities. Hardly any additional juxtapositions between the 5-HT2A-IR neuronal profiles and 5-HT-IR terminal-like varicosities were revealed when the intraneuronal level of 5-HT was increased by monoamine oxidase inhibitor pretreatment (nialamide, 250 mg/kg, 3 h). Thus, in most forebrain regions, there were overall few juxtapositions between 5-HT terminal-like varicosities and 5-HT2A-IR neuronal structures. This observation suggests that 5-HT2A receptor mediated 5-HT transmission in the rat forebrain is mainly a volume transmission process mediated via short distance diffusion in the extra-cellular space.

Regional changes in density of serotonin transporter in the brain of 5-HT1A and 5-HT1B knockout mice, and of serotonin innervation in the 5-HT1B knockout

5-HT1A knockout (KO) mice display an anxious-like phenotype, whereas 5-HT1B KOs are over-aggressive. To identify serotoninergic correlates of these altered behaviors, autoradiographic measurements of 5-HT1A and 5-HT1B serotonin (5-HT) receptors and transporter (5-HTT) were obtained using the radioligands [3H]8-OH-DPAT, [125I]cyanopindolol and [3H]citalopram, respectively. By comparison to wild-type, density of 5-HT1B receptors was unchanged throughout brain in 5-HT1A KOs, and that of 5-HT1A receptors in 5-HT1B KOs. In contrast, decreases in density of 5-HTT binding were measured in several brain regions of both genotypes. Moreover, 5-HTT binding density was significantly increased in the amygdalo-hippocampal nucleus and ventral hippocampus of the 5-HT1B KOs. Measurements of 5-HT axon length and number of axon varicosities by quantitative 5-HT immunocytochemistry revealed proportional increases in the density of 5-HT innervation in these two regions of 5-HT1B KOs, whereas none of the decreases in 5-HTT binding sites were associated with any such changes. Several conclusions could be drawn from these results: (i) 5-HT1B receptors do not adapt in 5-HT1A KOs, nor do 5-HT1A receptors in 5-HT1B KOs. (ii) 5-HTT is down-regulated in several brain regions of 5-HT1A and 5-HT1B KO mice. (iii) This down-regulation could contribute to the anxious-like phenotype of the 5-HT1A KOs, by reducing 5-HT clearance in several territories of 5-HT innervation. (iv) The 5-HT hyperinnervation in the amygdalo-hippocampal nucleus and ventral hippocampus of 5-HT1B KOs could play a role in their increased aggressiveness, and might also explain their better performance in some cognitive tests. (v) These increases in density of 5-HT innervation provide the first evidence for a negative control of 5-HT neuron growth mediated by 5-HT1B receptors.

Serotoninergic afferents preferentially innervate distinct subclasses of peptidergic interneurons in the rat visual cortex

Although it is well documented that the non-pyramidal neurons of the cerebral cortex are under the influence of the vast serotoninergic input, the ultrastructural substrate for such functional interactions appears largely obscure. We sought to address this issue by dual immunoelectron microscopy, combining antibodies against serotonin (5-HT) and three neurochemical markers for peptidergic interneurons, namely somatostatin (SRIF), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP). The gold-substituted silver-peroxidase method was employed to intensify and differentiate the end-product of the peptide-immunoreaction from the non-intensified 5-HT fibers. Mainly the SRIF but also the NPY neurons were encountered among the postsynaptic targets of the 5-HT boutons. Recipients of synapses were perikarya and proximal dendrites of SRIF and NPY cells but also distal dendrites of the SRIF neurons. Neither synaptic relationships nor close appositions were ever identified between 5-HT boutons and VIP-immunoreactive elements. This remarkable synaptic preference/avoidance of 5-HT afferents for specific peptidergic subpopulations reveals a 'wired' component of cortical serotonin neurotransmission, which should be carefully interpreted within the frame of the available literature for extrasynaptic serotonin release.

Ultrastructural localization of the serotonin transporter in superficial and deep layers of the rat prelimbic prefrontal cortex and its spatial relationship to dopamine terminals

Dopamine levels within the prefrontal cortex (PFC) can be manipulated by selective inhibitors of the serotonin transporter (SERT). However, the cellular mechanisms underlying these effects are not clear. The present study sought to examine the distribution of immunogold-silver labeling for SERT (SERT-ir) in the rat prelimbic PFC and to describe its ultrastructural spatial relationship to dopamine axons labeled by immunoperoxidase staining for tyrosine hydroxylase (TH-ir). SERT was localized to axonal profiles that ranged in size from fine caliber fibers containing dense SERT-ir, primarily along the membrane, and rarely forming synapses to large, spherical varicosities exhibiting less dense staining, mainly within the cytoplasm, and more commonly forming synapses. Synaptic contacts of SERT profiles were typically asymmetric, axospinous, and more frequent in superficial (38%) than deep (19%) layers. For TH-ir profiles, only 24% were within the same 13.8 microm(2) microenvironment as SERT-ir profiles. Furthermore, TH-ir and SERT-ir profiles were rarely directly apposed to each other or convergent onto common dendritic structures. Instead, these two profiles were typically separated by an average distance of 1.30 microm in the coronal plane, a value that did not vary with the size of SERT-ir axons, the amount of SERT labeling, or the cortical layer examined. These results are consistent with two populations of SERT profiles within the rat prelimbic PFC that may arise from different raphe nuclei or that represent varicose and intervaricose portions of the same axons. Moreover, the functional interactions between cortical serotonin and dopamine systems that may contribute to the therapeutic efficacy of antidepressant drugs are likely to occur over distances greater than 1 microm.