Interaction of serotonin with somatosensory cortical neuronal responses to afferent synaptic inputs and putative neurotransmitters

Probing the structure and function of locus coeruleus projections to CNS motor centers

The brainstem nucleus locus coeruleus (LC) sends projections to the forebrain, brainstem, cerebellum and spinal cord and is a source of the neurotransmitter norepinephrine (NE) in these areas. For more than 50 years, LC was considered to be homogeneous in structure and function such that NE would be released uniformly and act simultaneously on the cells and circuits that receive LC projections. However, recent studies have provided evidence that LC is modular in design, with segregated output channels and the potential for differential release and action of NE in its projection fields. These new findings have prompted a radical shift in our thinking about LC operations and demand revision of theoretical constructs regarding impact of the LC-NE system on behavioral outcomes in health and disease. Within this context, a major gap in our knowledge is the relationship between the LC-NE system and CNS motor control centers. While we know much about the organization of the LC-NE system with respect to sensory and cognitive circuitries and the impact of LC output on sensory guided behaviors and executive function, much less is known about the role of the LC-NE pathway in motor network operations and movement control. As a starting point for closing this gap in understanding, we propose using an intersectional recombinase-based viral-genetic strategy TrAC (Tracing Axon Collaterals) as well as established ex vivo electrophysiological assays to characterize efferent connectivity and physiological attributes of mouse LC-motor network projection neurons. The novel hypothesis to be tested is that LC cells with projections to CNS motor centers are scattered throughout the rostral-caudal extent of the nucleus but collectively display a common set of electrophysiological properties. Additionally, we expect to find these LC projection neurons maintain an organized network of axon collaterals capable of supporting selective, synchronous release of NE in motor circuitries for the purpose of coordinately regulating operations across networks that are responsible for balance and movement dynamics. Investigation of this hypothesis will advance our knowledge of the role of the LC-NE system in motor control and provide a basis for treating movement disorders resulting from disease, injury, or normal aging.

Acute tryptophan depletion alters affective touch perception

RATIONALE

Affiliative tactile interactions help regulate physiological arousal and confer resilience to acute and chronic stress. C-tactile afferents (CTs) are a population of unmyelinated, low threshold mechanosensitive cutaneous nerve fibres which respond optimally to a low force stimulus, moving at between 1 and 10 cm/s. As CT firing frequencies correlate positively with subjective ratings of touch pleasantness, they are hypothesised to form the first stage of encoding affiliative tactile interactions. Serotonin is a key modulator of social responses with known effects on bonding.

OBJECTIVES

The aim of the present study was to determine the effect of acutely lowering central serotonin levels on perceptions of CT-targeted affective touch.

METHODS

In a double blind, placebo-controlled design, the effect of acute tryptophan depletion (ATD) on 25 female participants' ratings of directly and vicariously experienced touch was investigated. Psychophysical techniques were used to deliver dynamic tactile stimuli; some velocities were targeted to optimally activate CTs (1-10 cm/s), whereas other, faster and slower strokes fell outside the CT optimal range. Discriminative tactile function, cold pain threshold and tolerance were also measured.

RESULTS

ATD significantly increased pleasantness ratings of both directly and vicariously experienced affective touch, increasing discrimination of the specific hedonic value of CT targeted velocities. While ATD had no effect on either tactile or cold pain thresholds, there was a trend for reduced tolerance to cold pain.

CONCLUSIONS

These findings are consistent with previous reports that depletion of central serotonin levels modulates neural and behavioural responsiveness to appetitive sensory signals.

Serotonergic modulation across sensory modalities

The serotonergic system has been widely studied across animal taxa and different functional networks. This modulatory system is therefore well positioned to compare the consequences of neuromodulation for sensory processing across species and modalities at multiple levels of sensory organization. Serotonergic neurons that innervate sensory networks often bidirectionally exchange information with these networks but also receive input representative of motor events or motivational state. This convergence of information supports serotonin's capacity for contextualizing sensory information according to the animal's physiological state and external events. At the level of sensory circuitry, serotonin can have variable effects due to differential projections across specific sensory subregions, as well as differential serotonin receptor type expression within those subregions. Functionally, this infrastructure may gate or filter sensory inputs to emphasize specific stimulus features or select among different streams of information. The near-ubiquitous presence of serotonin and other neuromodulators within sensory regions, coupled with their strong effects on stimulus representation, suggests that these signaling pathways should be considered integral components of sensory systems.

Effects of lorazepam and baclofen on short- and long-latency afferent inhibition

KEY POINTS

Short-latency afferent inhibition (SAI) is modulated by GABA_A receptor activity, whereas the pharmacological origin of long-latency afferent inhibition remains unknown. This is the first study to report that long-latency afferent inhibition (LAI) is reduced by the GABA_A positive allosteric modulator lorazepam, and that both SAI and LAI are not modulated by the GABA_B agonist baclofen. These findings advance our understanding of the neural mechanisms underlying afferent inhibition.

ABSTRACT

The afferent volley evoked by peripheral nerve stimulation has an inhibitory influence on transcranial magnetic stimulation induced motor evoked potentials. This phenomenon, known as afferent inhibition, occurs in two phases: short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI). SAI exerts its inhibitory influence via cholinergic and GABAergic activity. The neurotransmitter receptors that mediate LAI remain unclear. The present study aimed to determine whether LAI is contributed by GABA_A and/or GABA_B receptor activity. In a double-blinded, placebo-controlled study, 2.5 mg of lorazepam (GABA_A agonist), 20 mg of baclofen (GABA_B agonist) and placebo were administered to 14 males (mean age 22.7 ± 1.9 years) in three separate sessions. SAI and LAI, evoked by stimulation of the median nerve and recorded from the first dorsal interosseous muscle, were quantified before and at the peak plasma concentration following drug ingestion. Results indicate that lorazepam reduced LAI by ∼40% and, in support of previous work, reduced SAI by ∼19%. However, neither SAI, nor LAI were altered by baclofen. In a follow-up double-blinded, placebo-controlled study, 10 returning participants received placebo or 40 mg of baclofen (double the dosage used in Experiment 1). The results obtained indicate that SAI and LAI were unchanged by baclofen. This is the first study to show that LAI is modulated by GABA_A receptor activity, similar to SAI, and that afferent inhibition does not appear to be a GABA_B mediated process.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

An autism-associated serotonin transporter variant disrupts multisensory processing

Altered sensory processing is observed in many children with autism spectrum disorder (ASD), with growing evidence that these impairments extend to the integration of information across the different senses (that is, multisensory function). The serotonin system has an important role in sensory development and function, and alterations of serotonergic signaling have been suggested to have a role in ASD. A gain-of-function coding variant in the serotonin transporter (SERT) associates with sensory aversion in humans, and when expressed in mice produces traits associated with ASD, including disruptions in social and communicative function and repetitive behaviors. The current study set out to test whether these mice also exhibit changes in multisensory function when compared with wild-type (WT) animals on the same genetic background. Mice were trained to respond to auditory and visual stimuli independently before being tested under visual, auditory and paired audiovisual (multisensory) conditions. WT mice exhibited significant gains in response accuracy under audiovisual conditions. In contrast, although the SERT mutant animals learned the auditory and visual tasks comparably to WT littermates, they failed to show behavioral gains under multisensory conditions. We believe these results provide the first behavioral evidence of multisensory deficits in a genetic mouse model related to ASD and implicate the serotonin system in multisensory processing and in the multisensory changes seen in ASD.

Serotonergic Modulation of Sensory and Multisensory Processing in Superior Colliculus

The ability to integrate information across the senses is vital for coherent perception of and interaction with the world. While much is known regarding the organization and function of multisensory neurons within the mammalian superior colliculus (SC), very little is understood at a mechanistic level. One open question in this regard is the role of neuromodulatory networks in shaping multisensory responses. While the SC receives substantial serotonergic projections from the raphe nuclei, and serotonergic receptors are distributed throughout the SC, the potential role of serotonin (5-HT) signaling in multisensory function is poorly understood. To begin to fill this knowledge void, the current study provides physiological evidence for the influences of 5-HT signaling on auditory, visual and audiovisual responses of individual neurons in the intermediate and deep layers of the SC, with a focus on the 5HT2a receptor. Using single-unit extracellular recordings in combination with pharmacological methods, we demonstrate that alterations in 5HT2a receptor signaling change receptive field (RF) architecture as well as responsivity and integrative abilities of SC neurons when assessed at the level of the single neuron. In contrast, little changes were seen in the local field potential (LFP). These results are the first to implicate the serotonergic system in multisensory processing, and are an important step to understanding how modulatory networks mediate multisensory integration in the SC.

Acute effects of the designer drugs benzylpiperazine (BZP) and trifluoromethylphenylpiperazine (TFMPP) using functional magnetic resonance imaging (fMRI) and the Stroop task--a pilot study

RATIONALE

A novel group of designer drugs containing benzylpiperazine (BZP) and/or trifluoromethylphenylpiperazine (TFMPP) have been available worldwide for more than a decade; however, their effects on human brain function have not been extensively described.

OBJECTIVES

In a double-blind, placebo-controlled crossover study, the acute effects of BZP and TFMPP (alone and in combination) on the neural networks involved in executive function were investigated using an event-related Stroop functional magnetic resonance imaging (fMRI) paradigm.

METHODS

Thirteen healthy participants aged 18-40 years undertook the Stroop task 90 min after taking an oral dose of either BZP (200 mg), TFMPP (either 50 or 60 mg), BZP + TFMPP (100 + 30 mg) or placebo. A change in activity in neural regions reflects an increase in local demand for oxygen, due to an increase in neuronal activity.

RESULTS

Relative to placebo, an increase in neural activation was observed in the dorsal striatum following BZP, and in the thalamus following TFMPP, when performing the Stroop task.

CONCLUSION

These data suggest that additional compensatory resources were recruited to maintain performance during the Stroop task. When BZP and TFMPP were administered together, both the dorsal striatum and thalamus were activated. However, the combination of BZP/TFMPP attenuated activation in the caudate, possibly due to TFMPP's indirect effects on dopamine release via 5HT2C receptors.

Cortical reorganization after spinal cord injury: always for good?

Plasticity constitutes the basis of behavioral changes as a result of experience. It refers to neural network shaping and re-shaping at the global level and to synaptic contacts remodeling at the local level, either during learning or memory encoding, or as a result of acute or chronic pathological conditions. 'Plastic' brain reorganization after central nervous system lesions has a pivotal role in the recovery and rehabilitation of sensory and motor dysfunction, but can also be "maladaptive". Moreover, it is clear that brain reorganization is not a "static" phenomenon but rather a very dynamic process. Spinal cord injury immediately initiates a change in brain state and starts cortical reorganization. In the long term, the impact of injury - with or without accompanying therapy - on the brain is a complex balance between supraspinal reorganization and spinal recovery. The degree of cortical reorganization after spinal cord injury is highly variable, and can range from no reorganization (i.e. "silencing") to massive cortical remapping. This variability critically depends on the species, the age of the animal when the injury occurs, the time after the injury has occurred, and the behavioral activity and possible therapy regimes after the injury. We will briefly discuss these dependencies, trying to highlight their translational value. Overall, it is not only necessary to better understand how the brain can reorganize after injury with or without therapy, it is also necessary to clarify when and why brain reorganization can be either "good" or "bad" in terms of its clinical consequences. This information is critical in order to develop and optimize cost-effective therapies to maximize functional recovery while minimizing maladaptive states after spinal cord injury.

High serotonin levels during brain development alter the structural input-output connectivity of neural networks in the rat somatosensory layer IV

Homeostatic regulation of serotonin (5-HT) concentration is critical for “normal” topographical organization and development of thalamocortical (TC) afferent circuits. Down-regulation of the serotonin transporter (SERT) and the consequent impaired reuptake of 5-HT at the synapse, results in a reduced terminal branching of developing TC afferents within the primary somatosensory cortex (S1). Despite the presence of multiple genetic models, the effect of high extracellular 5-HT levels on the structure and function of developing intracortical neural networks is far from being understood. Here, using juvenile SERT knockout (SERT^−/−) rats we investigated, in vitro, the effect of increased 5-HT levels on the structural organization of (i) the TC projections of the ventroposteromedial thalamic nucleus toward S1, (ii) the general barrel-field pattern, and (iii) the electrophysiological and morphological properties of the excitatory cell population in layer IV of S1 [spiny stellate (SpSt) and pyramidal cells]. Our results confirmed previous findings that high levels of 5-HT during development lead to a reduction of the topographical precision of TCA projections toward the barrel cortex. Also, the barrel pattern was altered but not abolished in SERT^−/− rats. In layer IV, both excitatory SpSt and pyramidal cells showed a significantly reduced intracolumnar organization of their axonal projections. In addition, the layer IV SpSt cells gave rise to a prominent projection toward the infragranular layer Vb. Our findings point to a structural and functional reorganization of TCAs, as well as early stage intracortical microcircuitry, following the disruption of 5-HT reuptake during critical developmental periods. The increased projection pattern of the layer IV neurons suggests that the intracortical network changes are not limited to the main entry layer IV but may also affect the subsequent stages of the canonical circuits of the barrel cortex.

Association with reward negatively modulates short latency phasic conditioned responses of dorsal raphe nucleus neurons in freely moving rats

The dorsal raphe nucleus (DRN) is implicated in mood regulation, control of impulsive behavior, and in processing aversive and reward-related signals. DRN neurons show phasic responses to sensory stimuli, but whether association with reward modulates these responses is unknown. We recorded DRN neurons from rats in a contextual conditioned approach paradigm in which an auditory cue was either followed or not followed by reward, depending on a global context signal. Conditioned approach (licking) occurred after cues in the reward context, but was suppressed in the no-reward context. Many DRN neurons showed short-latency phasic activations in response to the cues. There was striking contextual bias, with more and stronger excitations in the no-reward context than in the reward context. Therefore, DRN activity scaled inversely with cue salience and with the probability of subsequent conditioned approach. Tonic changes were similarly discriminatory, with increases being dominant after cues in the no-reward context, when licking was suppressed, and tonic decreases in rate dominant after reward-predictive cues during expression of conditioned licking. Phasic and tonic DRN responses thus provide signals of consistent valence but over different timescales. The tonic changes in activity are consistent with previous data and hypotheses relating DRN activity to response suppression and impulse control. Phasic responses could contribute to this via online modulation of attention allocation through projections to sensory-processing regions.

Serotonergic pharmacotherapy promotes cortical reorganization after spinal cord injury

Cortical reorganization plays a significant role in recovery of function after injury of the central nervous system. The neural mechanisms that underlie this reorganization may be the same as those normally responsible for skilled behaviors that accompany extended sensory experience and, if better understood, could provide a basis for further promoting recovery of function after injury. The work presented here extends studies of spontaneous cortical reorganization after spinal cord injury to the role of rehabilitative strategies on cortical reorganization. We use a complete spinal transection model to focus on cortical reorganization in response to serotonergic (5-HT) pharmacotherapy without any confounding effects from spared fibers left after partial lesions. 5-HT pharmacotherapy has previously been shown to improve behavioral outcome after SCI but the effect on cortical organization is unknown. After a complete spinal transection in the adult rat, 5-HT pharmacotherapy produced more reorganization in the sensorimotor cortex than would be expected by transection alone. This reorganization was dose dependent, extended into intact (forelimb) motor cortex, and, at least in the hindlimb sensorimotor cortex, followed a somatotopic arrangement. Animals with the greatest behavioral outcome showed the greatest extent of cortical reorganization suggesting that the reorganization is likely to be in response to both direct effects of 5-HT on cortical circuits and indirect effects in response to the behavioral improvement below the level of the lesion.

Effects of repeated 3,4-methylenedioxymethamphetamine administration on neurotransmitter efflux and sensory-evoked discharge in the ventral posterior medial thalamus

3,4-Methylenedioxymethamphetamine (MDMA) is known to enhance tactile sensory perception, an effect that contributes to its popularity as a recreational drug. The neurophysiological basis for the effects of MDMA on somatosensation are unknown. However, MDMA interactions with the serotonin transporter (SERT) and subsequent enhancement of serotonin neurotransmission are well known. The rat trigeminal somatosensory system receives serotonergic afferents from the dorsal raphe nucleus. Because these fibers express SERT, they should be vulnerable to MDMA-induced effects. We found that administration of a challenge injection of MDMA (3 mg/kg i.p.) after repeated MDMA treatment (3 mg/kg per day for 4 days) elicits both serotonin and norepinephrine efflux in the ventral posterior medial (VPM) thalamus of Long-Evans hooded rats, the main relay along the lemniscal portion of the rodent trigeminal somatosensory pathway. We evaluated the potential for repeated MDMA administration to modulate whisker-evoked discharge of individual neurons in this region. After surgically implanting stainless steel eight-wire multichannel electrode bundles, we recorded spike train activity of single cells while activating the whisker pathway using a piezoelectric mechanical stimulator. We found that repeated MDMA administration increased the spontaneous firing rate but reduced both the magnitude and duration of whisker-evoked discharge in individual VPM thalamic neurons. The time course of drug action on neuronal firing patterns was generally consistent with fluctuations in neurotransmitter efflux as shown from our microdialysis studies. On the basis of these results, we propose that single use and repeated administration of MDMA may "distort," rather than enhance, tactile experiences in humans, in part, by disrupting normal spike firing patterns through somatosensory thalamic relay circuits.

Projections and interconnections of genetically defined serotonin neurons in mice

Brain serotonin neurons are heterogeneous and can be distinguished by several anatomical and physiological characteristics. Toward resolving this heterogeneity into classes of functional relevance, subtypes of mature serotonin neurons were previously identified based on gene expression differences initiated during development in different rhombomeric (r) segments of the hindbrain. This redefinition of mature serotonin neuron subtypes based on the criteria of genetic lineage, along with the enabling genetic fate mapping tools, now allows various functional properties, such as axonal projections, to be allocated onto these identified subtypes. Furthermore, our approach uniquely enables interconnections between the different serotonin neuron subtypes to be determined; this is especially relevant because serotonin neuron activity is regulated by several feedback mechanisms. We used intersectional and subtractive genetic fate mapping tools to generate three independent lines of mice in which serotonin neurons arising in different rhombomeric segments, either r1, r2 or both r3 and r5, were uniquely distinguished from all other serotonin neurons by their expression of enhanced green fluorescent protein. Each of these subgroups of serotonergic neurons had a unique combination of forebrain projection targets. Typically more than one subgroup innervated an individual target area. Unique patterns of interconnections between the different groups of serotonin neurons were also observed and these pathways could subserve feedback regulatory circuits. Overall, the current findings suggest that activation of subsets of serotonin neurons could result in topographic serotonin release in the forebrain coupled with feedback inhibition of serotonin neurons with alternative projection targets.

Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.

Inhibition of SK and M channel-mediated currents by 5-HT enables parallel processing by bursts and isolated spikes

Although serotonergic innervation of sensory brain areas is ubiquitous, its effects on sensory information processing remain poorly understood. We investigated these effects in pyramidal neurons within the electrosensory lateral line lobe (ELL) of weakly electric fish. Surprisingly, we found that 5-HT is present at different levels across the different ELL maps; the presence of 5-HT fibers was highest in the map that processes intraspecies communication signals. Electrophysiological recordings revealed that 5-HT increased excitability and burst firing through a decreased medium afterhyperpolarization resulting from reduced small-conductance calcium-activated (SK) currents as well as currents mediated by an M-type potassium channel. We next investigated how 5-HT alters responses to sensory input. 5-HT application decreased the rheobase current, increased the gain, and decreased first spike latency. Moreover, it reduced discriminability between different stimuli, as quantified by the mutual information rate. We hypothesized that 5-HT shifts pyramidal neurons into a burst-firing mode where bursts, when considered as events, can detect the presence of particular stimulus features. We verified this hypothesis using signal detection theory. Our results indeed show that serotonin-induced bursts of action potentials, when considered as events, could detect specific stimulus features that were distinct from those detected by isolated spikes. Moreover, we show the novel result that isolated spikes transmit more information after 5-HT application. Our results suggest a novel function for 5-HT in that it enables differential processing by action potential patterns in response to current injection.

MDMA (ecstasy) modulates locomotor and prefrontal cortex sensory evoked activity

Ingestion of 3, 4-methylenedioxymethamphetamine (MDMA) leads to heightened response to sensory stimulation; thus, MDMA is referred to as "ecstasy" because it produces pleasurable enhancement of such sensation. There have been no electrophysiological studies that report the consequences of MDMA on sensory input. The present study was initiated to study the effects of acute and chronic MDMA on locomotor activity and sensory evoked field potential from freely behaving rats previously implanted with permanent electrodes in the prefrontal cortex (PFC). The main findings of this study are that: (1) acute MDMA augments locomotor behavior and attenuates the incoming sensory input, (2) chronic treatment of MDMA elicits behavioral sensitization, (3) chronic administration of MDMA results in attenuation of the baseline activity of the sensory evoked field potential, and (4) administration of rechallenge MDMA result in enhancement of the PFC sensory evoked field potential.

Drug therapies for attentional disorders alter the signal-to-noise ratio in the superior colliculus

Despite high levels of use, the mechanism of action of effective pharmacotherapies in attention deficit hyperactivity disorder (ADHD) is unknown. It has recently been hypothesized that one site of therapeutic action is the midbrain superior colliculus, a structure traditionally associated with visual processing, but also strongly implicated in distractibility, a core symptom of ADHD. We used male juvenile Wistar rats to examine the effects of therapeutically relevant doses of methylphenidate and d-amphetamine on collicular activity in vitro. Here we report a novel shared mechanism of the two drugs whereby they enhance the signal-to-noise ratio in the superior colliculus. The effects on the signal-to-noise ratio were mediated by serotonin (5-HT) via a pre-synaptic mechanism. This modulatory action would bias the system towards salient events and lead to an overall decrease in distractibility.

Regulatory processes and the development of cognitive representations

Although the construct of regulation is usually applied to ongoing behavior, it also has implications for ongoing cognition and the development of cognitive representations. We propose that subcortical motivational systems influence cortical representations in two general ways. First, they regulate response processes, promoting a general selection of information to which the child is exposed. Second, motivational systems regulate attention, promoting a more selective stabilization of representations for motivationally relevant sources of information. Together with the environment, these regulatory processes shape the child's developing representations. Individual differences in these processes result in cortical representational systems that enhance the motivational systems* ability to detect relevant inputs and to guide behavior in light of them. Examples are provided that focus on fearful children, discussing how their self-representation may contribute to anxious psychopathology.

MDMA (3,4-methylenedioxymethamphetamine)-mediated distortion of somatosensory signal transmission and neurotransmitter efflux in the ventral posterior medial thalamus

MDMA (3,4-methylenedioxymethamphetamine, Ecstasy) is reported to enhance tactile sensory perception, an effect that is believed to contribute to its popularity as a recreational drug. To date, no literature exists that addresses the neurophysiological mechanisms underlying the effects of MDMA on somatosensation. However, MDMA interactions with the serotonin transporter protein (SERT) are well known. The rat trigeminal somatosensory system has been studied extensively and receives serotonergic afferents from the dorsal raphe nucleus. Given that these fibers express SERT, they should be vulnerable to MDMA-induced effects. We found that short-term low-dose MDMA administration (3 mg/kg i.p.) led to a significant increase in 5-hydroxytryptamine (5-HT) efflux in the ventral posterior medial (VPM) thalamus, the main relay along the lemniscal portion of the rodent trigeminal somatosensory pathway. We further evaluated the potential for MDMA to modulate whisker-evoked discharge (WED) of individual neurons in this region. After surgically implanting stainless steel 8-wire multichannel electrode bundles, we recorded spike train activity from single cells of halothane-anesthetized rats while mechanically activating the whisker pathway. We found that short-term low-dose MDMA (3 mg/kg i.p.) increased the spontaneous firing rate but reduced the magnitude and duration of WED in individual VPM thalamic neurons. It is noteworthy that the time course of drug action on neuronal firing patterns was generally consistent with increased 5-HT efflux as shown from our microdialysis studies. Based on these results, we propose the working hypothesis that MDMA may "distort" rather than enhance tactile experiences in humans, in part, by disrupting normal spike firing patterns through somatosensory thalamic relay circuits.

Uncoupling between noradrenergic and serotonergic neurons as a molecular basis of stable changes in behavior induced by repeated drugs of abuse

A challenge in drug dependence is to delineate long-term behavioral and neurochemical modifications induced by drugs of abuse. In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists many months after the last administration, thus mimicking long-term sensitivity to drugs observed in human addicts. Although addictive properties of drugs of abuse are generally considered to be mediated by an increased release of dopamine in the ventral striatum, recent pharmacological and genetic experiments indicate an implication of alpha1b-adrenergic receptors in behavioral and rewarding responses to psychostimulants and opiates. Later on, it was shown that not only noradrenergic but also serotonergic systems, through 5-HT(2A) receptors, were controlling behavioral effects of drugs of abuse. More recently, experiments performed in animals knockout for alpha1b-adrenergic or 5-HT(2A) receptors indicated that noradrenergic and serotonergic neurons, besides their activating effects, inhibit each other by means of the stimulation of alpha1b-adrenergic and 5-HT(2A) receptors and that this mutual inhibition vanishes in wild type mice with repeated injections of psychostimulants, opiates or alcohol. Uncoupling induced by repeated treatments with drugs of abuse installs a stable sensitization of noradrenergic and serotonergic neurons, thus explaining an increased reactivity of dopaminergic neurons and behavioral sensitization. We propose that noradrenergic/serotonergic uncoupling is a common stable neurochemical consequence of repeated drugs of abuse which may also occur during chronic stressful situations and facilitate the onset of mental illness. Drug consumption would facilitate an artificial re-coupling of these neurons, thus bringing a temporary relief.

Quantitative analysis of glutamatergic and GABAergic neurons expressing 5-HT(2A) receptors in human and monkey prefrontal cortex

5-hydroxytryptamine (5-HT) or serotonin 2A receptors play an important role in modulation of prefrontal cortex (PFC) activity and have been implicated in the physiopathology of psychiatric disorders. There is no quantitative information on the percentage of glutamatergic and GABAergic cells that express 5-HT(2A) receptors in human and monkey PFC. We have used double in situ hybridization to quantify the mRNA co-localization of 5-HT(2A) receptor with the glutamatergic transporter vesicular glutamate transporter 1, and with the GABAergic marker glutamic acid decarboxylase 65/67 and in parvalbumin and calbindin GABAergic cell populations. Our results show that nearly every glutamatergic cell (86-100%) in layers II-V expressed 5-HT(2A) receptor mRNA in both species. This percentage was lower in layer VI (13-31%). In contrast, not all the GABAergic interneurons (13-46%) expressed 5-HT(2A) receptor mRNA. This receptor was expressed in 45-69% of parvalbumin and in 61-87% of calbindin positive cells. These results indicate that, while the majority of glutamatergic neurons can be sensitive to 5-HT action via 5-HT(2A) receptors, this modulation occurs only in a limited population of GABAergic interneurons and provides new neuroanatomical information about the role played by serotonin through 5-HT(2A) receptors in the PFC and on the sites of action for drugs such as antipsychotics and antidepressants used in treatment of psychiatric disorders.

Serotonergic modulation of plasticity of the auditory cortex elicited by fear conditioning

In the awake big brown bat, 30 min auditory fear conditioning elicits conditioned heart rate decrease and long-term best frequency (BF) shifts of cortical auditory neurons toward the frequency of the conditioned tone; 15 min conditioning elicits subthreshold cortical BF shifts that can be augmented by acetylcholine. The fear conditioning causes stress and an increase in the cortical serotonin (5-HT) level. Serotonergic neurons in the raphe nuclei associated with stress and fear project to the cerebral cortex and cholinergic basal forebrain. Recently, it has been shown that 5-HT(2A) receptors are mostly expressed on pyramidal neurons and their activation improves learning and memory. We applied 5-HT, an agonist (alpha-methyl-5-HT), or an antagonist (ritanserin) of 5-HT(2A) receptors to the primary auditory cortex and discovered the following drug effects: (1) 5-HT had no effect on the conditioned heart rate change, although it reduced the auditory responses; (2) 4 mm 5-HT augmented the subthreshold BF shifts, whereas 20 mm 5-HT did not; (3) 20 mm 5-HT reduced the long-term BF shifts and changed them into short-term; (4) alpha-methyl-5-HT increased the auditory responses and augmented the subthreshold BF shifts as well as the long-term BF shifts; (5) in contrast, ritanserin reduced the auditory responses and reversed the direction of the BF shifts. Our data indicate that the BF shift can be modulated by serotonergic neurons that augment or reduce the BF shift or even reverse the direction of the BF shift. Therefore, not only the cholinergic system, but also the serotonergic system, plays an important role in cortical plasticity according to behavioral demands.

Cocaine preferentially enhances sensory processing in the upper layers of the primary sensory cortex

Sensory systems are believed to play an important role in drug addiction, particularly in triggering craving and relapse, and it has been shown in previous studies that administration of cocaine can enhance evoked responses in the primary sensory cortex of experimental animals. Primary sensory cortex comprises a multi-layered structure to which a variety of roles have been assigned; an understanding of how cocaine affects evoked activity in these different layers may shed light on how drug-associated sensory cues gain control over behavior. The aim of the present study was to examine how cocaine affects whisker sensory responses in different layers of the primary sensory (barrel) cortex. Field potential and multi-unit activity were recorded from the cortex of anesthetized rats using 16 channel linear probes during repetitive (air puff) stimulation of the whiskers. In control conditions (under saline, i.v.), responses strongly adapted to the repeated sensory stimulation. Following an i.v. injection of cocaine (0.5 mg/kg, i.v.), this adaptation was strongly attenuated, giving each stimulus a more equal representation and weight. Attenuation of adaptation was more marked in the upper cortical layers in both field potential and multi-unit data. Indeed, in these layers, not only was adaptation attenuated but multi-unit response amplitudes under cocaine exceeded those under saline for stimuli occurring early in the train. The results extend our previous findings concerning the enhancement by cocaine of primary sensory responses. Insofar as enhanced neural responses equate to enhanced stimulus salience, the results indicate that cocaine may play a previously under-appreciated role in the formation of associations between drug and drug-related environmental cues by enhancing stimulus salience. The associative process itself may be assisted by a preferential action in the upper cortical layers, thought to be involved in learning and plasticity.

The serotonin releaser fenfluramine alters the auditory responses of inferior colliculus neurons

Local direct application of the neuromodulator serotonin strongly influences auditory response properties of neurons in the inferior colliculus (IC), but endogenous stores of serotonin may be released in a distinct spatial or temporal pattern. To explore this issue, the serotonin releaser fenfluramine was iontophoretically applied to extracellularly recorded neurons in the IC of the Mexican free-tailed bat (Tadarida brasiliensis). Fenfluramine mimicked the effects of serotonin on spike count and first spike latency in most neurons, and its effects could be blocked by co-application of serotonin receptor antagonists, consistent with fenfluramine-evoked serotonin release. Responses to fenfluramine did not vary during single applications or across multiple applications, suggesting that fenfluramine did not deplete serotonin stores. A predicted gradient in the effects of fenfluramine with serotonin fiber density was not observed, but neurons with fenfluramine-evoked increases in latency occurred at relatively greater recording depths compared to other neurons with similar characteristic frequencies. These findings support the conclusion that there may be spatial differences in the effects of exogenous and endogenous sources of serotonin, but that other factors such as the identities and locations of serotonin receptors are also likely to play a role in determining the dynamics of serotonergic effects.

Dorsal raphe nucleus stimulation modulates the response of layers IV and V barrel cortical neurons in rat

The effect of dorsal raphe nucleus (DRN) electrical stimulation on response properties of layers IV and V barrel cortical neurons was studied. To assess the receptive field characteristics of cortical neurons, responses of neurons were recorded following the displacement of principal and adjacent whiskers individually or in a condition test paradigm. Then neuronal responses to the displacement of whiskers were analyzed following DRN stimulation at 0, 50, 100, 200 and 400 ms inter-stimulation intervals. Considering On responses, DRN stimulation suppressed the response magnitude of layer V neurons to principal whisker deflection, while it slightly increased that of layer IV neurons (not statistically significant). The response latency of layer IV neurons increased when DRN was stimulated 200 or 400 ms before principal whisker deflection, while the response latency of layer V was not changed. DRN stimulation had no effect on either magnitude or latency of neuronal response to the adjacent whisker deflections. We observed a decrease in the inhibitory effect of the adjacent whisker deflection on the magnitude of neuronal response to the principal whisker deflection in layer IV when DRN was stimulated 200 ms before the principal whisker deflection. Off responses did not show any significant effect of DRN stimulation. Our results suggest a modulating role for DRN in processing of the incoming information into barrel cortex. This effect might be location dependent.

Serotonin Reuptake Inhibitors in Auditory Processing Disorders in Elderly Patients: Preliminary Results

OBJECTIVE/HYPOTHESIS

One mechanism associated with degeneration in the elderly is the decrease of neurotransmitters. In the central auditory pathway serotonin, can be found from cochlear nucleus to the auditory cortex, and it constitutes one of the most important neuromodulatory circuits in hearing processing. The present study analyzed the action of citalopram, a selective inhibitor of serotonin reuptake, in aged patients with normal to moderate sensorineural hearing loss (HL) and low performance on auditory processing.

STUDY DESIGN/METHOD

Prospective, double-blind, randomized, placebo-controlled study. Thirty-eight selected patients were randomly divided into two groups. Nineteen patients made up group A and received placebo for 60 days. Nineteen patients of Group B received 20 mg per day of citalopram for 60 days. Hearing evaluation was performed initially and after 60 days and included pure-tone audiometry, speech discrimination test (SDT), emittanciometry (acoustic impedance audiometry), identification of synthetic sentences with an ipsilateral competitive message (SSI/ICM), tests of pitch-pattern sequences (PPS), and the staggered spondaic words test (SSW).

RESULTS

Comparisons of tests of auditory processing pre- and posttreatment in each group showed a statistical improvement in performance on all tests in group B after 2 months of therapy. Comparisons pre- and posttreatment between groups showed that patients who received citalopram presented statistically significantly better results in the SSI/ICM test (P < .0001) after treatment. The same comparison in results for the PPS test and the SSW test revealed a tendency (P = .09 and 0.058, respectively) toward better performance in the group receiving citalopram.

CONCLUSION

These preliminary results suggest that the use of citalopram can have a positive impact on auditory processes in elderly patients with low performance in auditory process.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Cocaine-induced vs. behaviour-related alterations of spontaneous and evoked discharge of somatosensory cortical neurons

While the abuse potential of cocaine stems mainly from its ability to increase dopaminergic transmission in limbic regions, drug actions on other monoamine-innervated circuits may contribute to the development and maintenance of cocaine addiction. Previous extracellular recordings in anaesthetized rats revealed a facilitatory influence of cocaine on primary sensory pathways, which could influence the processing of drug-related stimuli during the development of cocaine addiction. We further analysed these sensory effects of cocaine in freely behaving rats (n = 9). Using an array of eight microelectrodes chronically implanted in infragranular layers of primary somatosensory cortex, we recorded the basal activity of 40 single- and 64 multiunits and their response to electrical stimulation of the whisker pad before and after incremental doses of cocaine (0.25-2 mg/kg i.v.). Both spontaneous and cocaine-induced explorations were associated with elevated basal firing of the cortical neurons and suppression of their short-latency excitation and postexcitatory inhibition in response to the whisker-pad stimulation. In addition to exploration-related alterations, the administration of cocaine enhanced the long-latency rebound excitation induced by the whisker-pad stimulation. This component of the sensory response, which is more labile and does not seem to convey information about the physical characteristics of the stimulus, may participate in the processing of drug-related sensory stimuli.

Acute tryptophan depletion does not change somatosensory evoked magnetic fields

RATIONALE

Serotonin is shown to regulate the activity of primary auditory cortex, but little is known about serotonin modulation of other sensory cortices.

METHODS

We investigated somatosensory evoked magnetic fields (SEF) to left median nerve stimulation in eight healthy subjects in a double-blind, controlled, cross-over design study after acute tryptophan depletion (ATD) and control mixture. SEFs were recorded with the whole-head magnetoencephalography 6 h after ingestion of mixtures. The SEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole.

RESULTS

ATD decreased the total and free TPR levels by 75 and 48% and control mixture increased them by 98% and 44%. ATD had no effect on the amplitudes or latencies of SEF components. The source locations of the responses were not significantly affected by ATD.

CONCLUSION

Serotonin does not affect stimuli processing in the primary somatosensory cortex.

The thalamo-cortical auditory receptive fields: regulation by the states of vigilance, learning and the neuromodulatory systems

The goal of this review is twofold. First, it aims to describe the dynamic regulation that constantly shapes the receptive fields (RFs) and maps in the thalamo-cortical sensory systems of undrugged animals. Second, it aims to discuss several important issues that remain unresolved at the intersection between behavioral neurosciences and sensory physiology. A first section presents the RF modulations observed when an undrugged animal spontaneously shifts from waking to slow-wave sleep or to paradoxical sleep (also called REM sleep). A second section shows that, in contrast with the general changes described in the first section, behavioral training can induce selective effects which favor the stimulus that has acquired significance during learning. A third section reviews the effects triggered by two major neuromodulators of the thalamo-cortical system--acetylcholine and noradrenaline--which are traditionally involved both in the switch of vigilance states and in learning experiences. The conclusion argues that because the receptive fields and maps of an awake animal are continuously modulated from minute to minute, learning-induced sensory plasticity can be viewed as a "crystallization" of the receptive fields and maps in one of the multiple possible states. Studying the interplays between neuromodulators can help understanding the neurobiological foundations of this dynamic regulation.

Differential changes in signal and background firing of accumbal neurons during cocaine self-administration

Learning theories of drug addiction propose that the disorder is, at least in part, attributable to drug effects on accumbal mechanisms that are normally involved in reward-related learning. The neurophysiological mechanisms that might transduce such a drug effect on accumbal mechanisms have yet to be identified. Previous studies showed that a population of accumbal neurons exhibit phasic excitatory responses time locked to cocaine-reinforced lever presses during intravenous cocaine self-administration sessions (neurons referred to as lever-press neurons). Most of the same neurons, like the majority of accumbal neurons, also show a decrease in average firing rate during the drug self-administration session. Evidence indicates that the phasic firing patterns transmit information related to drug-reward-related events. On the other hand, the decreases in average firing reflect a primary pharmacological effect of self-administered cocaine. In the present study, we tested the hypothesis that the phasic firing associated with drug seeking (i.e., signal) is less sensitive than other accumbal firing (i.e., background) to the inhibitory effect of cocaine. During intravenous cocaine self-administration sessions, 45 of 68 neurons showed a decrease in average firing during the self-administration session relative to a predrug baseline period. Fourteen neurons showed both an inhibition in average firing and an excitatory phasic response. For these 14 neurons, signal either remained equal to the average predrug firing rate or exceeded the predrug firing rate during the self-administration session. For the same neurons, background firing generally fell below average predrug firing. The differential changes in signal and background were associated with an increase in the ratio of signal-to-background for the individual neurons. Moreover, the relatively unique resistance of signal to inhibition was associated with an increase in the ratio of signal firing of all lever-press neurons relative to the background firing of all recorded neurons. This type of differential inhibition in signal and background firing might be expected to increase the relative influence of the drug-reward-related signals on accumbal-related neural circuits and differentially influence susceptibility of drug- and non-drug-reward-related synaptic and neural responses to neuroplasticity. It thus represents a mechanism by which inhibitory effects of self-administered drug might amplify the accumbal contribution to behavior and learning and potentially contribute to drug addiction.

Serotonergic modulation of supragranular neurons in rat sensorimotor cortex

Numerous observations suggest diverse and modulatory roles for serotonin (5-HT) in cortex. Because of the diversity of cell types and multiple receptor subtypes and actions of 5-HT, it has proven difficult to determine the overall role of 5-HT in cortical function. To provide a broader perspective of cellular actions, we studied the effects of 5-HT on morphologically and physiologically identified pyramidal and nonpyramidal neurons from layers I-III of primary somatosensory and motor cortex. We found cell type-specific differences in response to 5-HT. Four cell types were observed in layer I: Cajal Retzius, pia surface, vertical axon, and horizontal axon cells. The physiology of these cells ranged from fast spiking (FS) to regular spiking (RS). In layers II-III, we observed interneurons with FS, RS, and late spiking physiology. Morphologically, these cells varied from bipolar to multipolar and included basket-like and chandelier cells. 5-HT depolarized or hyperpolarized pyramidal neurons and reduced the slow afterhyperpolarization and spike frequency. Consistent with a role in facilitating tonic inhibition, 5-HT2 receptor activation increased the frequency of spontaneous IPSCs in pyramidal neurons. In layers II-III, 70% of interneurons were depolarized by 5-HT. In layer I, 57% of cells with axonal projections to layers II-III (vertical axon) were depolarized by 5-HT, whereas 63% of cells whose axons remain in layer I (horizontal axon) were hyperpolarized by 5-HT. We propose a functional segregation of 5-HT effects on cortical information processing, based on the pattern of axonal arborization.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Differences in somatosensory processing in S1 barrel cortex between normal and monoamine oxidase A knockout (Tg8) adult mice

Spatio-temporal processing of whisker information was analysed in vivo for single neurons in D2 barrel columns of S1 cortex in Tg8 mutant mice, which lack barrels. Findings were compared with normal C3H mice of the same genetic background. The topographical organization of functional columns was similar in Tg8 and normal mice. Response magnitudes (RMs) to D2 principal whisker deflections in D2 columns for Tg8 were similar to normals for layers I-III and layer IV cells but short latency responses (>10 ms post-stimulus) were twice the magnitude of normal mice. The surrounding whiskers D1 and D3 yielded smaller RMs in layer IV of mutants than normal mice whereas RMs in layers I-III were equipotent (P>0.5). Modal latencies were shorter in Tg8 mice in all layers. Latency distributions for whisker D2 responses in both laminae were bimodal in normal mice, peaking at 6-8 and 12 ms post-stimulus, but unimodal in Tg8 mice in both laminae, peaking at 6-8 ms. Hence, despite an absence of barrels, segregation of columns is enhanced in layer IV and sensory processing is faster in layers I-IV compared with normal mice. This contrasts with adenylyl cyclase knockout mice where both an absence of barrels and enhanced surrounding whisker responses have been observed. These findings suggest that factors other than barrels and clustering of thalamo-cortical terminals define receptive field geometry.

Single-unit responses of serotonergic medullary and pontine raphe neurons to environmental cooling in freely moving cats

Brain serotonin has long been implicated in the regulation of body temperature, although its precise role is not completely understood. The present study examined the effects of environmental cooling (4-8 degrees C for 2 or 4h) on the single-unit activity of serotonergic neurons recorded in the medullary raphe nuclei obscurus and pallidus and in the pontine dorsal raphe nucleus of freely moving cats. These neuronal groups have primarily descending projections to the spinal cord and ascending projections to the forebrain, respectively. Cold exposure induced shivering and piloerection, but no appreciable changes in core temperature. Of the medullary serotonergic cells studied (n=14), seven were activated and seven were unresponsive to cold exposure. For the responsive cells, the mean increase and peak effect in unit activity relative to baseline were 31% and 46%, respectively. Of the seven cold-responsive cells, the activity of four was monitored when the animals were transferred back to room temperature (23 degrees C). Within 15-30 min, the activity of these cells returned to baseline. In contrast, none of the dorsal raphe nucleus cells studied (n=14) displayed a significant change in neuronal activity during cold exposure, suggesting that these neurons do not receive afferent input from cold-sensitive cutaneous receptors or participate in thermoregulatory responses evoked by low ambient temperatures.Overall, these results suggest that a subset of medullary serotonergic neurons play a role in physiological mechanisms underlying cold defense (e.g. increases in motor output and/or autonomic outflow). On the other hand, the lack of responsiveness of serotonergic dorsal raphe nucleus neurons to cold exposure does not support a specific role for these cells in thermoregulation.

Regional cerebral cortical activation in monoamine oxidase A-deficient mice: differential effects of chronic versus acute elevations in serotonin and norepinephrine

Mice deficient in monoamine oxidase A have previously been shown to demonstrate a chronic elevation of serotonin and norepinephrine in the brain. Using the autoradiographic [14C]iodo-antipyrine method, we examined cerebral cortical blood flow in conscious, restrained four- to five-month-old knock-out and wild-type animals following the intraperitoneal administration of either saline or D-fenfluramine. Knock-out animals administered saline, compared to their wild-type counterparts, demonstrated a significantly higher regional cortical blood flow in somatosensory and barrel field neocortex, an area which previous histological studies have shown to be characterized by abnormal serotonergic projection fibers and absent barrel formation. Regional cortical blood flow was significantly lower in knock-out than in wild-type mice in the entorhinal and midline motor cortex, with non-significant decreases noted in the olfactory, piriform and retrosplenial cortices and the amygdala. We compared the above findings to those obtained in response to D-fenfluramine which, in conjunction with its metabolite D-norfenfluramine, results in acute elevations of brain levels of serotonin and norepinephrine. Administration of D-fenfluramine (21. 2mg/kg) resulted in changes in regional cortical perfusion in most brain regions of both knock-out and wild-type mice that were opposite to the genotypic differences seen in perfusion in response to saline. Fenfluramine significantly increased regional cortical blood flow in the allocortex (olfactory, piriform, entorhinal) and the amygdala, and significantly decreased regional cortical blood flow in the somatosensory, barrel field, midline motor and retrosplenial cortices. Changes in regional perfusion in response to fenfluramine were topographically equivalent in knock-out and wild-type mice, although in knock-out mice such changes were of greater magnitude. Our study suggests that the effects on regional cortical blood flow of a lifelong absence of monoamine oxidase A, and the consequent chronic increase in serotonin and norepinephrine, differ from those attributable to acute increases in these neurotransmitters following fenfluramine administration. Such a differential response may reflect neurodevelopmental abnormalities and/or effects of a chronic physiological adaptation on the regulation of cortical activation.

A functional model of some Parkinson's disease symptoms using a Guided Propagation Network

This paper presents a computational model of Parkinson's Disease (PD) symptoms. Based on psychophysiological data, the underlying system (Guided Propagation Network) implements coincidence detection between internal flows and stimuli, and can be dynamically controlled for representing the action of neuromodulators such as dopamine (DA). By modelling the DA deficit involved in PD through a decrease of response thresholds in the production modules of a GPN, four symptoms are observed in experiments carried out on a computer simulation, and then attributed to a lack of synchrony between 'proprioceptive stimuli' and internal flows: reduced intensity, increased rate, saccades and spontaneous repetitions.

Systemically administered cocaine alters stimulus-evoked responses of thalamic somatosensory neurons to perithreshold vibrissae stimulation

Previous studies have shown that systemically administered cocaine can transiently alter responses of primary somatosensory cortical neurons to threshold level stimulation of peripheral receptive fields. The goal of the present investigation was 2-fold: (1) characterize the effects of systemic cocaine on stimulus-evoked responses of the ventral posterior medial (VPM) thalamic neurons which relay somatosensory information to the cortex and (2) determine the time course and magnitude of changes in monoamine levels within the somatosensory thalamus following systemic administration of cocaine. Extracellularly recorded responses of single VPM thalamic neurons to whisker stimulation were monitored before and after cocaine administration in halothane anaesthetized rats. Each cell was first characterized by assessing its response profile to a range of perithreshold level deflections of the optimal whisker on the contralateral face. Drug effects on stimulus-response curves, response magnitude and latency were determined from quantitative analysis of spike train data. The results indicate that cocaine elicits a predictable augmentation or attenuation of the sensory response magnitude, with the direction of the change inversely related to the initial magnitude of the stimulus-evoked discharge. In addition, cocaine consistently reduced the response time of somatosensory thalamic neurons to peripheral receptive field stimulation. At the same dose and over the same time period, cocaine also produced marked elevation of norepinephrine and serotonin levels within the ventrobasal thalamus, as determined by in vivo microdialysis. These results suggest that cocaine-induced increases in norepinephrine and serotonin are responsible for drug-related modulation of the transfer of sensory signals through primary thalamocortical relay circuits.

Arousal systems and attentional processes

Unitary concepts of arousal have outlived their usefulness and their psychological fractionation corresponds to a similar chemical differentiation of the reticular formation of the brain. Neurobiological characteristics of the monoaminergic and cholinergic systems can be described in terms of their anatomical, electrophysiological and neurochemical properties. Functional studies suggest that the coeruleo-cortical noradrenergic system, under certain circumstances, is implicated in processes of selective attention, that the mesolimbic and mesostriatal dopaminergic systems contribute to different forms of behavioural activation, and that the cortical cholinergic projections have fundamental roles in the cortical processing of signals, affecting attentional and mnemonic processes. The ascending serotoninergic systems contribute to behavioural inhibition and appear to oppose the functions of the other systems in several ways.

Serotonergic axons in monkey prefrontal cerebral cortex synapse predominantly on interneurons as demonstrated by serial section electron microscopy

Anatomical approaches were used to describe the distribution, appearance, and synaptic interactions of serotonin (5-HT)-immunoreactive axons in monkey prefrontal cortex. A plexus of 5-HT axons was found throughout the gray matter, with an especially high density in layer I and a slight increase in layer IV. They were strikingly heterogeneous, with a gradient of morphologies ranging from fine and nonvaricose to highly varicose or thick and nonvaricose. Electron microscopy showed that both varicose and nonvaricose axons were typically filled with clear vesicles and less abundant dense core vesicles. A serial section analysis of 5-HT varicosities in layers I, III, and V showed consistent results across layers. Only about 23% of labeled varicosities formed identifiable synapses. These synapses were consistently asymmetric and were 2-5 serial sections (or 0.08-0.38 mu) in diameter. Targets of identified 5-HT synapses were dendritic shafts with the exception of one cell soma. Followed in serial sections, postsynaptic dendrites typically had morphological features of interneurons, i.e. they lacked spines, had a high density of synaptic inputs, and often had a varicose morphology. Only 8% of postsynaptic shafts were classified as pyramidal dendrites. This is in striking contrast to our previous study in this cortex of dopamine axons, which synapsed predominantly on pyramidal dendrites. These are the first results to indicate that interneurons are the major recipient of identifiable 5-HT synapses in the monkey prefrontal cortex.

The yohimbine-induced anticonflict effect in the rat, Part II. Neurochemical findings

In a companion paper the alpha 2-adrenoceptor antagonist yohimbine was found to produce a dose-dependent anticonflict effect in a modified Vogel's conflict test. The behavioral data further indicated that noradrenergic and serotonergic neurons as well as the benzodiazepine (BDZ) receptor may be involved in the anticonflict effect of yohimbine. In the present study the effects on rat brain monoamine neurochemistry and GABAA/BDZ receptor function (36Cl-uptake in corticohippocampal synaptoneurosomes) of a maximally anticonflict producing dose of yohimbine (4.0 mg/kg, i.p.) were studied. The levels of rat brain catecholamines and indoleamines were measured ex vivo using high performance liquid chromatography with electrochemical detection (HPLC-ED). Yohimbine decreased noradrenaline levels both in the hippocampus and the hemispheres but instead increased DOPAC levels in these brain regions as well as in the limbic forebrain. Yohimbine also markedly enhanced DOPA accumulation in the hippocampus and the hemispheres after inhibition of 1-aromatic amino acid decarboxylase by means of NSD 1015, whereas in the limbic system only a modest increase was obtained. The yohimbine-induced effects on the catecholamine synthesis rate were largely abolished in animals severely depleted of NA by means of 6-hydroxy-dopamine (6-OH-DA) pretreatment. Yohimbine decreased both the 5-HIAA/5-HT quotient (an indicator of 5-HT turnover) and 5-HTP accumulation after NSD 1015 in the hemispheres, whereas in the hippocampus and the limbic system only 5-HTP accumulation was decreased. The yohimbine-induced effect on the indoleamine synthesis rate was not influenced by 6-OH-DA pretreatment, whereas this effect and that on the catecholamine synthesis rate were both abolished by reserpine pretreatment. Neither in vivo nor in vitro administration of yohimbine significantly altered baseline or GABA-induced accumulation of 36Cl- in corticohippocampal synaptoneurosomes. In conclusion, the present study provides neurochemical support for the suggestion that yohimbine may exert its anticonflict effect in a modified Vogel's conflict test by increasing and decreasing NA and 5-HT neurotransmission, respectively, whereas no evidence was obtained for a direct interaction of yohimbine with GABAA/BDZ receptor function.

Organization of serotoninergic projections from the raphé nuclei to the anterior thalamic nuclei in the rat: a combined retrograde tracing and 5-HT immunohistochemical study

We combined retrograde transport of horseradish peroxidase (HRP) with 5-hydroxytryptamine (5-HT) immunohistochemistry to study serotoninergic projections to the anterior thalamic nuclei (ATN) of the rat. Small iontophoretic injections of HRP into the anterodorsal thalamic nucleus resulted in double-labelled neurons predominantly in the ventromedial and also in the ventrolateral part of the ipsilateral dorsal raphé (DR). A smaller number of double-labelled neurons was also found in the dorsomedial part of the nucleus, predominantly ipsilaterally, and in the median raphé nucleus (MnR), close to the midline. After injection into the medial subdivision of the anteroventral thalamic nucleus, the pattern of labelling in DR and MnR was similar to that detected following injections into the anterodorsal thalamic nucleus. However, injection into the posterior subdivision of the anteroventral thalamic nucleus resulted in bilateral retrograde labelling of a few 5-HT-containing neurons in the dorsolateral part of the DR. Labelling in the ventromedial, ventrolateral and dorsomedial regions of DR and MnR was similar to that detected after injections into the medial subdivision of the anteroventral thalamic nucleus. After all injections into the ATN, double-labelled cells were found throughout the rostrocaudal extent of MnR and throughout the rostral two-thirds of DR. The caudal extension of DR was devoid of double-labelled cells. Although double-labelled cells were observed bilaterally in the dorsolateral part of the DR, the projection from DR to ATN was predominantly ipsilateral. These results show that there is an internal organization within DR such that subnuclei of the DR can be defined on the basis of their efferent projections to specific subdivisions of the ATN.

Serotonergic suppression of interhemispheric cortical synaptic potentials

The inhibitory effects of 5-hydroxytryptamine (5-HT) on interhemispheric and intracortical synaptic potentials in layer V neurons of the rat medial prefrontal (MFC) cortex were examined. Low concentrations (1-3 microM) of 5-HT selectively attenuated polysynaptic potentials that were similarly evoked by callosal or white matter stimulation. Maximally effective concentrations of 5-HT blocked interhemispheric transmission by 50-90%, as evidenced by an attention of the short latency callosal depolarizing synaptic potential (e-DPSP). These effects of 5-HT were not associated with a change in membrane potential or input resistance. The e-DPSP was characterized as having an N-methyl-D-aspartate (NMDA) and a non-NMDA component; the non-NMDA component was attenuated by 5-HT. Attenuation of the synaptic potential was accompanied by an attenuation of a postsynaptic glutamate potential. Suppression of both the e-DPSP and the glutamate potential was concentration dependent with 10-100 microM being maximally effective. The 5-HT1A/2 antagonist, spiperone, antagonized the effects of 5-HT on synaptic and glutamate potentials. Spiperone (1 microM) shifted the concentration-effect curves for suppression of the e-DPSP and the glutamate potential to the right; however, the Kb for the glutamate potential concentration-effect curve was 10 times that for the e-DPSP curve. The differential antagonist sensitivity of synaptic and glutamate potentials was an indication that serotonin acted on more than one receptor subtype to reduce interhemispheric transmission.

GABAergic control of odour-induced activity in the frog olfactory bulb: possible GABAergic modulation of granule cell inhibitory action

In the olfactory bulb, the activity of the output neurons, the mitral cells, is under inhibitory control exerted by GABAergic interneurons, the granule cells. Although the mechanisms of this inhibition are well known from in vitro studies, its physiological role in controlling mitral cell activity in response to odours has never been investigated. This study planned to improve understanding of the involvement of granule cells. To do so, GABAA-synaptic mechanisms were altered using GABAA antagonists in order to observe the consequences on mitral cell electrophysiological responses to odours, delivered over a wide concentration range. Due to the laminar organization of bulbar cell populations, the antagonists picrotoxin or bicuculline were injected into the bulbar ventricle in order to block granule cell inhibitory action at first. Surprisingly, the early consequence of the antagonist injection was a decrease in cell responsivity: response spike frequencies were lowered while thresholds were occasionally shifted toward higher concentrations. This initial depressive effect was followed by a recovery of control excitability and, later, by an increase in excitability: spike bursts became more sustained in frequency and in duration. At the same time, in most of the cells studied, spontaneous activity became bursting. The early depressive effect of GABAA antagonists is discussed in terms of an enhancement of the inhibitory influence of granule cells on mitral cells. This might reflect a blocking action of the antagonists at the level of GABAergic synapses located on granule cells themselves. The late effect, an increase in excitability, is explained as the consequence of the alteration of the functioning of dendrodendritic synapses between granule and mitral cells leading to a disinhibition of the latter. The comparison of the present findings with others obtained when antagonists were applied on to glomerular layers led us to infer that granule cell inhibition would be devoted to limiting mitral cell responses in frequency and in duration rather than to adjusting their response threshold. The chronology of the effects observed strongly supports the fact that, following the intraventricular injection, the antagonists acted primarily in the deep layers of the bulb. Nevertheless, due to free diffusion starting from the injection site, the possibility that drugs act later in the glomerular layer can not be rejected. It can be concluded that, in addition to its extensive involvement through intrinsic interneurons, GABA might also control the strength of the inhibition exerted by granule cells on mitral cells via centrifugal fibres.

Serotonergic influence on olfactory learning in the neonate rat

The role of the serotonergic innervation of the olfactory bulb was examined in neonate rat pups (Sprague-Dawley) by utilizing an olfactory learning paradigm (Sullivan, McGaugh, & Leon, 1991; Sullivan, Wilson, & Leon, 1989). One hundred fifty nanoliters of the neurotoxin 5,7-dihydroxytryptamine (5,7-dHT, 10 micrograms/microliters) was injected into each anterior olfactory nucleus of Postnatal Day 2 (PND 2) pups in order to selectively deplete the serotonergic (5-HT) innervation of the olfactory bulb. On PND 8, control, sham-operated, or 5-HT-depleted pups were presented with stroke (unconditioned stimulus) paired with peppermint odor (conditioned stimulus) or were presented with peppermint alone. Other pups remained naive, i.e., they were not presented with either stroke or odor. The following day, the pups were placed on a mesh screen above two boxes containing either fresh bedding or bedding with peppermint odor for five 1-min trials. Control and sham-operated pups that were previously trained by the odor/stroke paradigm spent significantly more time over the peppermint odor than similarly trained 5-HT-depleted pups, pups trained using the odor only paradigm, or naive pups. Immunocytochemistry verified that the 5,7-dHT injections specifically depleted the 5-HT innervation of the olfactory bulb and left the 5-HT innervation of more caudal levels of the neuraxis (e.g., piriform cortex) intact. The locomotor behavior of the pups was not affected by the 5-HT depletions. This study is the initial investigation to suggest that 5-HT innervation to the olfactory bulb is important in the acquisition or expression of olfactory-based learned behavior in the neonate rat.

NE/DA interactions in prefrontal cortex and their possible roles as neuromodulators in schizophrenia

The monoaminergic innervation of the rat prefrontal cortex arises from well-defined mesencephalic nuclei, with noradrenergic (NE) neurons located in the locus coeruleus, dopaminergic (DA) neurons located in the ventral tegmental area, and serotonergic (5-HT) neurons originating in the raphe nuclei. Specific destruction of the NE bundle was found to induce morphological (i.e., sprouting) as well as metabolic (i.e., changes in rate of DA utilization) modifications of mesocortical DA neurons, suggesting that these two catecholaminergic systems have functional interactions within the prefrontal cortex. This was substantiated by experiments showing that DA afferents modulate the sensitivity of cortical post-synaptic beta-adrenergic receptors and that, reciprocally, NE neurons control the sensitivity of cortical D1 receptors. Behavioural and pharmacological data have further indicated that the stimulation of cortical alpha-1 adrenergic receptors inhibits cortical DA transmission at D1 receptors. Secondly, we have attempted to analyze how such interactions between neuromodulatory systems may be related to the development of mental diseases such as schizophrenia. On the basis of studies in the literature describing the effects produced by the ingestion of hallucinogenic drugs or data collected regarding REM sleep, it is postulated that two modes of brain functioning exist: analogical and cognitive. Each mode is characterized by differences in the relative activities of NE, DA and 5-HT neurons. At birth, during REM sleep, and following the ingestion of hallucinogens, the mode of brain functioning is essentially analogical; in contrast, both analogic and cognitive modes are postulated to coexist in the awake state. Oscillations between these two modes are under the control of monoaminergic systems on which an increase in cortical DA release favours the cognitive processing mode, whereas intermittent activations of NE neurons would switch the brain into the analogical mode of processing. It is proposed that schizophrenic patients with "positive" symptoms suffer from an abnormal preponderance of the analogical mode while awake, whereas "negative" symptoms are due to the excessive presence of the cognitive mode. Although pure biological deficits cannot be excluded, these dysfunctions could be related to the absence of particular environmental variables early in the development of these patients. This condition is probably required to establish normal regulatory control of monoaminergic neuronal activity.