The modulation of excitatory amino acid responses by serotonin in the cat neocortex in vitro

The history of Danish neuroscience

The history of Danish neuroscience starts with an account of impressive contributions made at the 17th century. Thomas Bartholin was the first Danish neuroscientist, and his disciple Nicolaus Steno became internationally one of the most prominent neuroscientists in this period. From the start, Danish neuroscience was linked to clinical disciplines. This continued in the 19th and first half of the 20th centuries with new initiatives linking basic neuroscience to clinical neurology and psychiatry in the same scientific environment. Subsequently, from the middle of the 20th century, basic neuroscience was developing rapidly within the preclinical university sector. Clinical neuroscience continued and was even reinforced during this period with important translational research and a close co-operation between basic and clinical neuroscience. To distinguish 'history' from 'present time' is not easy, as many historical events continue in present time. Therefore, we decided to consider 'History' as new major scientific developments in Denmark, which were launched before the end of the 20th century. With this aim, scientists mentioned will have been born, with a few exceptions, no later than the early 1960s. However, we often refer to more recent publications in documenting the developments of initiatives launched before the end of the last century. In addition, several scientists have moved to Denmark after the beginning of the present century, and they certainly are contributing to the present status of Danish neuroscience-but, again, this is not the History of Danish neuroscience.

Antidepressant modulation of isolation and restraint stress effects on brain chemistry and morphology

Stress elicits adaptive responses from the brain, but it can also lead to maladaptive consequences. For example, stress can precipitate mental illness, including depression. Prolonged stress also causes damage to neurons in the hippocampus. Antidepressant drugs must be evaluated, not only for their ability to potentiate adaptive responses, but also to inhibit maladaptive consequences of stress. Ongoing research in our laboratory has compared the atypical tricyclic antidepressant, tianeptine, with the typical tricyclics, desipramine and imipramine, with respect to the effects of isolation and repeated restraint stress. Tianeptine and desipramine similarly attenuated isolation stress-induced increases in locus coeruleus and midbrain tyrosine hydroxylase mRNA levels and isolation-stress induced decreases in preproenkephalin mRNA levels in striatum and nucleus accumbens. However, tianeptine and imipramine differed in their effects in the cerebral cortex and hippocampus on 5HT2, and 5HT1A receptor levels but, surprisingly, produced similar effects on levels of the serotonin transporter labelled with [3H] paroxetine. Tianeptine also prevented stress-induced reductions in the length and number of branchpoints of dendrites of CA3 pyramidal neurons in hippocampus; comparison with effects of typical tricyclics are ongoing. Tianeptine also blocked effects of corticosterone treatment to reduce branching and length of CA3 dendrites. These actions of tianeptine may be due to interactions between 5HT and excitatory amino acids in the mossy fiber terminals on CA3 pyramidal neurons. Taken together, these results indicate that tianeptine has unique properties compared to some other antidepressant drugs, but shares in common with those drugs the ability to attenuate stress effects on tyrosine hydroxylase gene expression and on the serotonin transporter. It remains to be seen whether these actions are the basis of a common antidepressant action.

Experience-dependent reactivation of ocular dominance plasticity in the adult visual cortex

A crucial issue in neurobiology is to understand the main mechanisms restricting neural plasticity to brief windows of early postnatal life. The visual system is one of the paradigmatic models for studying experience-dependent plasticity. The closure of one eye (monocular deprivation, MD) causes a marked ocular dominance (OD) shift of neurons in the primary visual cortex only during the critical period. Here, we report that environmental enrichment (EE), a condition of increased sensory-motor stimulation, reactivates OD plasticity in the adult visual cortex, as assessed with both visual evoked potentials and single-unit recordings. This effect is accompanied by a marked increase in cerebral serotonin (5-HT) levels. Blocking 5-HT enhancement in the visual cortex of EE rats completely prevents the OD shift induced by MD. We also found that EE leads to a reduced intracortical GABAergic inhibition and an increased BDNF expression and that the modulation of these molecular factors is neutralized by cortical infusion of the 5-HT synthesis inhibitor pCPA. Our results show that EE rejuvenates the adult visual cortex and that 5-HT is a crucial factor in this process, triggering a cascade of molecular events that allow the reinstatement of neural plasticity. The non-invasive nature of EE makes this paradigm particularly eligible for clinical application.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

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.

Olanzapine attenuates brain damage after focal cerebral ischemia in vivo

Atypical antipsychotic drugs are widely used in the treatment of schizophrenia. These agents are discovered to have some additional beneficial effects beyond their effectiveness as antipsychotic drugs. Among these initially unexpected effects are their potential effects as mood stabilizers in bipolar disorder and their efficacy in improving long-term outcome in schizophrenia. These effects recently raised the question whether these drugs may also have some neuroprotective effect in the brain. To examine this matter, in this study we evaluated the neuroprotective effect of olanzapine after permanent focal cerebral ischemia. Anaesthetized male C57BL/6j mice were submitted to permanent thread occlusion of the middle cerebral artery (MCA). Olanzapine (0.1 and 1 mg/kg) or vehicle was applied intraperitoneally just after permanent ischemia. Twenty-four hours after permanent ischemia, brain injury was evaluated by triphenyltetrazolium chloride staining (TTC). Olanzapine (0.1 and 1 mg/kg) showed significant neuroprotection after permanent focal cerebral ischemia.

Modeling early cortical serotonergic deficits in autism

Autism is a developmental brain disorder characterized by deficits in social interaction, language and behavior. Brain imaging studies demonstrate increased cerebral cortical volumes and micro- and macro-scopic neuroanatomic changes in children with this disorder. Alterations in forebrain serotonergic function may underlie the neuroanatomic and behavioral features of autism. Serotonin is involved in neuronal growth and plasticity and these actions are likely mediated via serotonergic and glutamatergic receptors. Few animal models of autism have been described that replicate both etiology and pathophysiology. We report here on a selective serotonin (5-HT) depletion model of this disorder in neonatal mice that mimics neurochemical and structural changes in cortex and, in addition, displays a behavioral phenotype consistent with autism. Newborn male and female mice were depleted of forebrain 5-HT with injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the bilateral medial forebrain bundle (mfb). Behavioral testing of these animals as adults revealed alterations in social, sensory and stereotypic behaviors. Lesioned mice showed significantly increased cortical width. Serotonin immunocytochemistry showed a dramatic long-lasting depletion of 5-HT containing fibers in cerebral cortex until postnatal day (PND) 60. Autoradiographic binding to high affinity 5-HT transporters was significantly but transiently reduced in cerebral cortex of 5,7-DHT-depleted mice. AMPA glutamate receptor binding was decreased at PND 15. We hypothesize that increased cerebral cortical volume and sensorimotor, cognitive and social deficits observed in both 5-HT-depleted animals and in individuals with autism, may be the result of deficiencies in timely axonal pruning to key cerebral cortical areas.

Risperidone attenuates brain damage after focal cerebral ischemia in vivo

Since their introduction, atypical neuroleptic agents have been discovered to have some beneficial effects beyond their effectiveness as neuroleptic drugs. Among these initially unexpected effects are their potential effects as mood stabilizers in bipolar disorder and their efficacy in improving long-term outcome in schizophrenia. These effects recently raised the question whether these drugs may also have some neuroprotective effect in the brain. To examine this matter, in this study we evaluated the neuroprotective effect of risperidone after permanent focal cerebral ischemia. Anaesthetized male C57BL/6j mice were submitted to permanent thread occlusion of the middle cerebral artery (MCA). Risperidone (0.1, 1 or 10 mg/kg) or vehicle was applied intraperitoneally just after permanent ischemia. Twenty-four hours after permanent ischemia, brain injury was evaluated by triphenyltetrazolium chloride staining (TTC). Risperidone (0.1, 1 and 10 mg/kg) showed significant neuroprotection after permanent focal cerebral ischemia.

Modulation of behavior and cortical motor activity in healthy subjects by a chronic administration of a serotonin enhancer

SSRIs are postulated to modulate motor behavior. A single dose of selective serotoninergic reuptake inhibitors (SSRIs) like fluoxetine, paroxetine, or fluvoxamine, has been shown to improve motor performance and efficiency of information processing for simple sensorimotor tasks in healthy subjects. At a cortical level, a single dose of SSRI was shown to induce a hyperactivation of the primary sensorimotor cortex (S1M1) involved in the movement (Loubinoux, I., Boulanouar, K., Ranjeva, J. P., Carel, C., Berry, I., Rascol, O., Celsis, P., and Chollet, F., 1999. Cerebral functional magnetic resonance imaging activation modulated by a single dose of the monoamine neurotransmission enhancers fluoxetine and fenozolone during hand sensorimotor tasks. J. Cereb. Blood Flow Metab. 19 1365--1375, Loubinoux, I., Pariente, J., Boulanouar, K., Carel, C., Manelfe, C., Rascol, O., Celsis, P., and Chollet, F., 2002. A Single Dose of Serotonin Neurotransmission Agonist Paroxetine Enhances Motor Output. A double-blind, placebo-controlled, fMRI study in healthy subjects. NeuroImage 15 26--36). Since SSRIs are usually given for several weeks, we assessed the behavioral and cerebral effects of a one-month chronic administration of paroxetine on a larger group. In a double-blind, placebo controlled and crossover study, 19 subjects received daily 20 mg paroxetine or placebo, respectively, over a period of 30 days separated by a wash-out period of 3 months. After each period, the subjects underwent an fMRI (active or passive movement, dexterity task, sensory discrimination task) and a behavioral evaluation. Concurrently, a TMS (transcranial magnetic stimulation) study was conducted (Gerdelat-Mas, A., Loubinoux, I., Tombari, D., Rascol, O., Chollet, F., Simonetta-Moreau, M., 2005. Chronic administration of selective serotonin re-uptake inhibitor (SSRI) paroxetine modulates human motor cortex excitability in healthy subjects. NeuroImage 27,314--322).

RESULTS

On the one hand, paroxetine improved motor performances at the finger tapping test (P=0.02) without affecting choice reaction time, strength and dexterity significantly. Subjects were also faster in processing the spatial incongruency between a stimulus and the motor response (P=0.04). In order to differentiate behavioral components, a principal component analysis was performed on all motor tests, and several characteristics were differentiated: strength, speed, skill, attention, and motor response coding. Paroxetine would improve the efficiency of motor response coding (MANOVA on the factors; factor 3, P=0.01). On the other hand, the chronic administration induced a significant hypoactivation of S1M1 whatever the task: motor or sensory, simple or complex (random effect analysis, P<0.05). The hypoactivation correlated with the improvement of performances at the finger tapping test (P<0.05) suggesting more efficiency in cerebral motor processing.

CONCLUSIONS

Our results showed a clear modulation of sensory and motor cerebral activation after a chronic paroxetine administration. An improvement in both behavior and cerebral efficiency was suggested. It could be hypothesized that monoamines, by an unspecific effect, may tune the response of pyramidal neurons to optimize performances.

Serotonin modulates the suppressive effects of corticosterone on proliferating progenitor cells in the dentate gyrus of the hippocampus in the adult rat

This series of experiments explores the interaction between corticosterone and serotonin (5-HT) in the regulation of cell proliferation in the dentate gyrus of the adult rat. Intracerebroventricular 5,7-DHT (5,7-dihydroxytryptamine) (either 200 or 300 microg) resulted in highly significant depletion of 5-HT as measured by high performance liquid chromatography in the frontal cortex but had no effect on the number of proliferating cells in the dentate gyrus by measuring 5-bromo-2'-deoxyuridine (BrdU) and Ki-67 cytochemistry. Treatment with PCPA (p-chlorophenylalanine: a tryptophan hydroxylase inhibitor: 300 mg/kg initially followed by 100 mg/kg/day) resulted in reduced proliferation as measured by Ki-67 after 3 days treatment, but not by BrdU uptake, and not after 14 days treatment by either method. In addition, injection of corticosterone (10-40 mg/kg/day) for 8 days significantly reduced proliferation in the dentate gyrus, as expected, measured by both BrdU uptake and Ki-67 immunostaining. Adrenalectomized (ADX) rats with a replacement subcutaneous pellet of corticosterone showed reduced proliferation when given additional corticosterone (10 mg/kg/day for 8 days), but this was prevented by 5-HT depletion (i.c.v. 5,7-DHT). Finally, a dose-response study showed that progressive doses of corticosterone (0-40 mg/kg/day) in ADX rats resulted in diminished suppression of proliferation in 5-HT-depleted compared with 5-HT-intact rats. These results strongly suggest that 5-HT regulates the sensitivity of proliferating cells in the dentate gyrus to corticosterone.

Protective effect of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus: putative neurotransmitter receptors involved in their action

Rhynchophylline and isorhynchophylline are major tetracyclic oxindole alkaloid components of Uncaira species, which have been long used as medicinal plants. In this study we examined the protective effects of rhynchophylline and isorhynchophylline on in vitro ischemia-induced neuronal damage in the hippocampus and interaction of these alkaloids with neurotransmitter receptors in a receptor expression model of Xenopus oocytes. In vitro ischemia was induced by exposing the hippocampal slices to oxygen- and D-glucose-deprived medium over 8 min. The resultant neuronal damage was elucidated as deterioration of population spike (PS) amplitudes evoked trans-synaptically by electrical stimulation of Schaffer collaterals and recorded in the CA1 area. Rhynchophylline and isorhynchophylline, as well as the N-methyl-D-aspartate (NMDA) antagonist (+/-)-2-amino-5-phosphono-valeric acid (APV), the muscarinic M1 receptor antagonist pirenzepine, and the 5-HT2 receptor antagonist ketanserin, attenuated the in vitro ischemia-induced neuronal damage in a concentration-dependent manner. There was no difference in the extent of protection against the neuronal damage between rhynchophylline and isorhynchophylline treatment. In Xenopus oocytes expressing the rat brain receptors encoded by total RNA, both rhynchophylline and isorhynchophylline reduced muscarinic receptor- and 5-HT2 receptor-mediated current responses in a competitive manner. Together with our previous findings that rhynchophylline and isorhynchophylline have a non-competitive antagonistic effect on the NMDA-type ionotropic glutamate receptors, the present results suggest that these alkaloids exert their protective action against ischemia-induced neuronal damage by preventing NMDA, muscarinic M1, and 5-HT2 receptors-mediated neurotoxicity during ischemia.

Effects of citalopram on the excitability of the human motor cortex: a paired magnetic stimulation study

Several recent reports suggest the possibility of monitoring pharmacological effects on brain excitability through transcranial magnetic stimulation (TMS). Different drugs have been studied using paired magnetic stimulation in normal subjects and patients. In particular, it has been suggested that antidepressant drugs may have an appreciable effect on motor excitability. The aim of the present study was to investigate motor area excitability in normal subjects after oral administration of a single dose of citalopram, a selective serotonin reuptake inhibitor (SSRI) antidepressant. Motor cortex excitability was studied by single and paired transcranial magnetic stimulation before and 2.5 and 36 (t1/2=36 h) h after oral administration of 30 mg of citalopram. Cortical excitability was measured using different transcranial magnetic stimulation parameters: motor threshold (MT), motor-evoked potential (MEP) amplitude and latency, motor recruitment, duration of cortical silent period (CSP), intracortical inhibition and intracortical facilitation. Spinal excitability and peripheral nerve conduction were measured by F response and M wave. Temporary but significant increases in motor threshold, motor-evoked potentials, silent period and intracortical inhibition were observed 2.5 h after drug administration, without any significant changes in motor-evoked potential amplitude and latency and spinal excitability parameters. Our findings suggest that a single oral dose of citalopram can induce significant but transitory suppression of motor cortex excitability in normal subjects.

Chronic social stress: effects on limbic brain structures

Different types of stressors are known to activate distinct neuronal circuits in the brain. Acute physiological stimuli that are life threatening and require immediate reactions lead to a rapid stimulation of brainstem and hypothalamus to activate efferent visceral pathways. In contrast, psychological stressors activate higher-order brain structures for further interpretations of the perceived endangerment. Common to the later multimodal stressors is that they need cortical processing and, depending on previous experience or ongoing activation, the information is assembled within limbic circuits connecting, e.g., the hippocampus, amygdala and prefrontal cortex to induce neuroendocrine and behavioral responses. In view of the fact that stressful life events often contribute to the etiology of psychopathologies such as depressive episodes, several animal models have been developed to study central nervous mechanisms that are induced by stress. The present review summarizes observations made in the tree shrew chronic psychosocial stress paradigm with particular focus on neurotransmitter systems and structural changes in limbic brain regions.

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Complex modulation of human motor cortex excitability by the specific serotonin re-uptake inhibitor sertraline

Monoamines are powerful modulators of cortical function. Serotonin has complex excitatory and inhibitory effects on animal cortex. Here, the effects of a single oral dose (100mg) of the selective serotonin re-uptake inhibitor sertraline on human motor cortex excitability were investigated in healthy subjects. Transcranial magnetic stimulation was used to test motor threshold, motor evoked potential intensity curve, cortical silent period, paired-pulse inhibition and facilitation and I-wave facilitation. Sertraline resulted in a steeper intensity curve and in depressed paired-pulse facilitation (PPF). All other measures and spinal and neuromuscular excitability remained unaffected. The steeper intensity curve points to an increased excitability of the cortico-spinal neurone, while the depressed PPF suggests an enhanced control of the cortico-spinal neurone by inhibitory interneurones. These features may improve the signal-to-noise ratio of output cells in human motor cortex.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Laminar distribution of cholinergic- and serotonergic-dependent plasticity within kitten visual cortex

Both cholinergic and serotonergic modulatory projections to mammalian striate cortex have been demonstrated to be involved in the regulation of postnatal plasticity, and a striking alteration in the number and intracortical distribution of cholinergic and serotonergic receptors takes place during the critical period for cortical plasticity. As well, agonists of cholinergic and serotonergic receptors have been demonstrated to facilitate induction of long-term synaptic plasticity in visual cortical slices supporting their involvement in the control of activity-dependent plasticity. We recorded field potentials from layers 4 and 2/3 in visual cortex slices of 60--80 day old kittens after white matter stimulation, before and after a period of high frequency stimulation (HFS), in the absence or presence of either cholinergic or serotonergic agonists. At these ages, the HFS protocol alone almost never induced long-term changes of synaptic plasticity in either layers 2/3 or 4. In layer 2/3, agonist stimulation of m1 receptors facilitated induction of long-term potentiation (LTP) with HFS stimulation, while the activation of serotonergic receptors had only a modest effect. By contrast, a strong serotonin-dependent LTP facilitation and insignificant muscarinic effects were observed after HFS within layer 4. The results show that receptor-dependent laminar stratification of synaptic modifiability occurs in the cortex at these ages. This plasticity may underly a control system gating the experience-dependent changes of synaptic organization within developing visual cortex.

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.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

Columnar distribution of serotonin-dependent plasticity within kitten striate cortex

Recent studies have identified the potential for an important role for serotonin (5-HT) receptors in the developmental plasticity of the kitten visual cortex. 5-HT(2C) receptors are transiently expressed in a patchy fashion in the visual cortex of kittens between 30-80 days of age complementary to patches demarcated by cytochrome oxidase staining. 5-HT, operating via 5-HT(2C) receptors, increases cortical synaptic plasticity as assessed both in brain slices and in vivo. Herein, we report that bath application of 5-HT substantially increases the probability of long-term potentiation within 5-HT(2C) receptor-rich zones of cortex, but this effect is not observed in the 5-HT(2C) receptor-poor zones. Instead, in these zones, 5-HT application increases the probability of long-term depression. These location-specific effects of 5-HT may promote the formation of compartment-specific cortical responses.

Fluoxetine-induced plasticity in the rodent visual system

We studied the effect of fluoxetine, a selective serotonin reuptake inhibitor, in the development and lesion-induced plasticity of retinotectal axons in pigmented rats. Neonatal rats received a daily injection of either fluoxetine or vehicle from postnatal day 1 (PND 1) to PND 10 or from PND 14 to PND 28 (fluoxetine, 7.5 and 10.0 mg/kg, respectively). In the latter group, some animals received a single lesion at the temporal periphery of the left retina at PND 21. Unoperated animals were use as the control. At the end of the treatment, the animals received an intraocular injection of horseradish peroxidase (HRP) in the right (intact) eye to trace the uncrossed retinotectal pathway. Chronic fluoxetine treatment, induced, in unoperated rats, an expansion of the retinal terminal fields along the rostro-caudal axis of the tectum both in the PND 10 and PND 28 groups. Following a retinal lesion in the left eye at PND 21, the vehicle-treated group showed a small reorganization of the intact uncrossed projection. In this group only a few terminals were labeled invading the denervated tectal surface one-week after the lesion. Fluoxetine-treated animals on the other hand, showed a great amplification of plasticity with a conspicuous sprouting of the uncrossed retinal axons into denervated areas. The data suggest that fluoxetine induces extensive axonal rearrangements in neonatal and juvenile central nervous system and amplifies neuroplasticity following retinal lesions late in development.

Stress, glucocorticoids and structural plasticity of the hippocampus

Sustained stress can have numerous pathophysiological effects. Adrenal glucocorticoid hormones are principal effectors in the stress response. They have profound effects on mood and behavior and affect neurochemical transmission and neuroendocrine control. We have used the experimental paradigm of chronic psychosocial stress in tree shrews to investigate the impact of aversive social encounters on brain structures. Chronic stress in male tree shrews which is accompanied by constantly elevated levels of glucocorticoids leads to structural changes in hippocampal neurons. Whereas dendritic atrophy of hippocampal pyramidal neurons and impairment of neurogenesis in the dentate gyrus could be demonstrated in chronically stressed tree shrews, a loss of hippocampal neurons was not observed in this animal model. The present review summarizes recent results on the question which structural changes occur during chronic stress in neurons of the brain and whether glucocorticoids might be responsible for such stress effects. The role of transmitter systems in stress-related neuronal plasticity is also discussed.

NMDA receptor-mediated oscillatory activity in the neonatal rat spinal cord is serotonin dependent

The effect of serotonin (5-HT) receptor blockade on rhythmic network activity and on N-methyl--aspartate (NMDA) receptor-induced membrane voltage oscillations was examined using an in vitro neonatal rat spinal cord preparation. Pharmacologically induced rhythmic hindlimb activity, monitored via flexor and extensor electroneurograms or ventral root recordings, was abolished by 5-HT receptor antagonists. Intrinsic motoneuronal voltage oscillations, induced by NMDA in the presence of tetrodotoxin (TTX), either were abolished completely or transformed to long-lasting voltage shifts by 5-HT receptor antagonists. Conversely, 5-HT application facilitated the expression of NMDA-receptor-mediated rhythmic voltage oscillations. The results suggest that an interplay between 5-HT and NMDA receptor actions may be critical for the production of rhythmic motor behavior in the mammalian spinal cord, both at the network and single cell level.

Serotonergic modulation of the responses to excitatory amino acids of rat dorsal horn neurons in vitro: implications for somatosensory transmission

Serotonin (5-HT) is one of the major transmitters involved in supraspinal control of somatic sensation and nociception. The aim of the present study was to investigate if the 5-HT-induced modulation of sensory transmission in the dorsal horn could be due to regulation of neuronal responses to excitatory amino acids. Experiments were performed in an in vitro preparation of the young rat spinal cord. Responses to dorsal root stimulation (DR-EPSP) and to droplet application of N-methyl-D-aspartic acid (NMDA) and alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) were obtained by means of intracellular recordings of dorsal horn neurons. Bath applications of 5-HT (50 microM) generally caused reductions in amplitude and integrated area of DR-EPSPs and of responses to NMDA but the responses to AMPA were unaltered. A linear correlation was found between the effects of 5-HT on the DR-EPSP and on the NMDA response measured as percentage change in amplitude (r2 = 0.45; P < or = 0.01) and integrated area (r2 = 0.77; P < or = 0.001). The NMDA receptor antagonist d-AP5 (50 microM) completely abolished NMDA responses and caused a depression of the DR-EPSP similar to that of 5-HT. The 5-HT1 receptor agonist 5-carboxamidotryptamine (5-CT; 1 microM) mimicked the depressant effects of 5-HT but had a stronger depressant action on the DR-EPSP than 5-HT. The depression of NMDA responses induced by 5-HT and 5-CT was tetrodotoxin (1 microM) resistant. It is concluded that 5-HT-induced depression of NMDA responses explains partially the depressant action of 5-HT on dorsal horn synaptic transmission activating a postsynaptic site sensitive to 5-CT. The possible activation of coadjuvant mechanisms is discussed.

NMDA receptor activation triggers voltage oscillations, plateau potentials and bursting in neonatal rat lumbar motoneurons in vitro

Whole-cell recordings of lumbar motoneurons in the intact neonatal rat spinal cord in vitro were undertaken to examine the effects of N-methyl-D-aspartate (NMDA) receptor activation on membrane behaviour. Bath application of NMDA induced rhythmic voltage oscillations of 5.9+/-2.1 mV (SD) at a frequency of 4.4+/-1.5 Hz. Amplitude, but not frequency, of the voltage oscillations was membrane potential-dependent. Voltage oscillations could recruit action potentials and/or plateau potentials with or without superimposed bursting. Blockade of synaptic transmission with tetrodotoxin (TTX) sometimes resulted in a loss of oscillatory activity which could then be restored by increasing the NMDA concentration. After application of TTX, the trajectory of NMDA-induced oscillations was similar to the trajectory induced in the presence of intact synaptic networks, although the mean oscillation duration was longer and the oscillation frequency was slower (1.8+/-1.1 Hz). Current ramps delivered after bath application of NMDA demonstrated bistable membrane properties which may underlie the plateau potentials. Injection of intracellular current pulses could initiate, entrain and terminate individual plateau potentials. The results suggest that membrane depolarization produced by oscillations may activate other intrinsic conductances which generate plateau potentials, thereby providing the neuron with enhanced voltage sensitivity, compared to that produced by NMDA receptor activation alone. These oscillatory events may have a role in the regulation of motor output in a variety of rhythmic behaviours including locomotion.

Serotonin facilitates synaptic plasticity in kitten visual cortex: an in vitro study

We have addressed the role of serotonin-2C (5-HT2C) receptors in the development and maintenance of synaptic plasticity in the kitten visual cortex. In visual cortical slices, taken from 40- to 80-day-old kittens, bath application of serotonin markedly facilitated the induction of both long-term depression (LTD) and long-term potentiation (LTP). Field potential responses to white matter stimulation were recorded from layer IV after a regime of low frequency stimulation (LFS; 1 Hz, 15 min), which reliably induced LTP or LTD in younger kittens (less than 30 days of age). At 40-80 days, this protocol almost never induced LTD or LTP in layer IV. However, in 50% of the visual cortical slices studied in 40-80-day-old kittens, LTD or LTP was induced, if serotonin (1 or 10 microM) was co-applied with LFS. No such serotonin facilitation of long-term plasticity was ever detected in > 120-day-old animals, indicating that serotonin facilitates synaptic plasticity within a defined period of visual cortical development. Serotonergic 5-HT2C receptors are likely to contribute to the synaptic plasticity observed in layer IV, since mesulergine, an antagonist of the 5-HT2C receptor, completely blocked synaptic modifications induced by the combination of low frequency stimulation and serotonin application.

Auditory-evoked potentials as indicator of brain serotonergic activity--first evidence in behaving cats

Due to the increasing importance of the central serotonergic neurotransmission for pathogenetic concepts and as a target of pharmacotherapeutic interventions in psychiatry, reliable indicators of this system are needed. Several findings from basic and clinical research suggest that the stimulus intensity dependence of auditory evoked potentials (AEP) may be such an indicator of behaviorally relevant aspects of serotonergic activity (Hegerl and Juckel 1993, Biol Psychiatry 33:173-187). In order to study this relationship more directly, epidural recordings over the primary and secondary auditory cortex were conducted in chronically implanted cats under intravenous (i.v.) administration of drugs influencing the serotonergic and other modulatory systems (8-OH-DPAT, m-CPP, ketanserin, DOI, apomorphine, atropine, clonidine). The intensity dependence of the cat AEP component with the highest functional similarity to this of the N1/P2-component in humans was significantly changed by influencing 5-HT1a and 5-HT2 receptors, but not 5-HT1c receptors. This serotonergic modulation of the intensity dependence was only found for the primary auditory cortex which corresponds to the known different innervation of the primary and secondary auditory cortex by serotonergic fibers. Our study supports the idea that the intensity dependence of AEP could be a valuable indicator of brain serotonergic activity; however, this indicator seems to be of relative specificity because at least cholinergic effects on the intensity dependence were also observed.

Effects of hypomagnesia on histamine H1 receptor-mediated facilitation of NMDA responses

1. The ability of histamine to facilitate the N-methyl-D-aspartate (NMDA) induced depolarization of cortical projection neurones was examined by use of grease-gap recording. 2. Histamine (1 to 15 microM) reversibly facilitated the NMDA-induced depolarization yielding a bellshaped concentration-response relationship. The peak enhancement was 167% above the control at 10 microM histamine. Desensitization was present in 4 out of 5 slices on second exposure 40 min following the first exposure. 3. Histamine did not alter the depolarization induced by 10 microM kainate. 4. The histamine-induced facilitation persisted in the presence of tetrodotoxin, but was reduced in a concentration-dependent manner by diphenhydramine (IC50 = 7.6 nM). Cyproheptadine (10 nM) also reduced the facilitation, whereas ranitidine (200 nM) and thioperamide (10 nM) were ineffective in this regard. 5. Histamine (10 microM) facilitated the NMDA (25 microM)-induced depolarization in nominally Mg(2+)-free medium. The magnitude of the facilitation was smaller than that observed in Mg(2+)-containing medium (17% above the control) and desensitization was not observed. This facilitation was not reduced by cyproheptadine (10 nM) or diphenhydramine (1 microM). 6. We conclude that histamine facilitates the NMDA depolarization at cortical neurones via two distinct mechanisms. One mechanism involves activation of the histamine H1 receptor and is sensitive to Mg2+. The second mechanism is independent of histamine cell surface receptor activation and may reflect a direct action of histamine at the NMDA receptor.

Extracellular release of serotonin following fluid-percussion brain injury in rats

Serotonin has been implicated in the pathobiology of central nervous system trauma. Using microdialysis techniques, we performed measurements of extracellular serotonin release within the traumatized cerebral cortex of rats subjected to moderate fluid-percussion (F-P) brain injury. Twenty-four hours prior to TBI, a F-P interface was positioned parasagitally over the right cerebral cortex. On the second day, fasted rats were anesthetized with 70% nitrous oxide, 1% halothane and 30% oxygen. Under controlled physiological conditions and normothermic brain temperature (37-37.5 degrees C), rats were injured (n = 6) with a F-P pulse ranging from 1.8 to 2.0 atm. Following trauma, brain temperature was maintained for 4 h at 37 degrees C. Sham trauma animals (n = 7) were treated in an identical manner. Brain trauma induced acute elevations in the extracellular levels of serotonin (p < 0.01, ANOVA) compared to sham-operated controls. For example, serotonin levels increased from 18.85 +/- 7.12 pm/mL (mean +/- SD) in baseline samples to 65.78 +/- 11.36 in the first 10 min after trauma. The levels of serotonin remained significantly higher than control for the first 90-min sampling period. In parallel to the increase in serotonin levels after TBI, a significant 71.1% decrease (i.e., 182.29 +/- 30.08 vs 52.75 +/- 16.92) in extracellular 5-hydroxyindoleacetic acid (5-HIAA) levels was observed during the first 10 min after TBI. These data indicate that TBI is followed by a prompt increase in the extracellular levels of serotonin in cortical regions adjacent to the impact site. These neurochemical findings indicate that serotonin may play a significant role in the pathophysiology of TBI.

Action of 5-hydroxytryptamine in facilitating N-methyl-D-aspartate depolarization of cortical neurones mimicked by calcimycin, cyclopiazonic acid and thapsigargin

1. The ability of calcimycin, cyclopiazonic acid and thapsigargin to facilitate the N-methyl-D-aspartate (NMDA)-mediated depolarization of cortical projection neurones was investigated by use of grease-gap recording and the results compared with the facilitation that results from activation of 5-hydroxytryptamine2A receptors. 2. Calcimycin (0.25 to 3 microM), cyclopiazonic acid (5 to 30 microM), and thapsigargin (10 to 300 nM) reversibly facilitated the NMDA (50 microM)-induced depolarization in the presence of tetrodotoxin. The concentration-response relationships were bell-shaped with a mean enhancement of 550% for calcimycin (1 microM) and approximately 400% for cyclopiazonic acid (20 microM) and thapsigargin (100 nM). At the highest concentration of each agent tested, no facilitation was observed. 3. Chlorpromazine (1 microM) partially restored a facilitation at 3 microM calcimycin and 300 nM thapsigargin. Myo-inositol (10 mM) and 100 nM staurosporine were both ineffective in this regard. 4. The depolarization elicited by 10 microM quisqualate or 5 microM kainate was not facilitated by 10 microM cyclopiazonic acid. 5. Calcimycin (0.5 microM), cyclopiazonic acid (20 microM), and thapsigargin (100 nM) elicited a significant facilitation in the presence of an antagonist cocktail consisting of D,L-2-amino-3-phosphonopropionic acid, prazosin, ritanserin, and scopolamine, although the magnitude of the facilitation was reduced. 6. Facilitation of the NMDA depolarization elicited by both 30 microM 5-hydroxytryptamine and 10 microM phenylephrine was eliminated in nominally Mg(2+)-free medium. In contrast, the facilitation induced by 0.5 microM calcimycin remained intact. 7. Bis-(o-aminophenoxy)-ethane-N,N,N,N, tetraacetic acid aminoethoxy (50 microM) or perfusion with nominally Ca(2+)-free medium eliminated facilitation of the NMDA depolarization induced by 30 microM 5-hydroxytryptamine and 100 nM thapsigargin. 8. The facilitation induced by both 30 microM 5-hydroxytryptamine and 1 microM calcimycin was reduced in a concentration-dependent manner by nifedipine (1 to 10 microM). 9. Calcimycin, cyclopiazonic acid and thapsigargin facilitate the NMDA depolarization in a manner which closely mimics the facilitation induced by 5-hydroxytryptamine. It is concluded that enhancement of the NMDA depolarization at cortical projection neurones results from an elevation of Ca2+ in the cytosol and that several sources of Ca2+ contribute to the facilitation.

Chronic psychosocial stress causes apical dendritic atrophy of hippocampal CA3 pyramidal neurons in subordinate tree shrews

We have shown previously that repeated laboratory restraint stress or daily corticosterone administration affects the structure of CA3 hippocampal neurons in rats. In the present study, we investigated the effect of repeated daily psychosocial stress on the structure of hippocampal CA3 pyramidal neurons in male tree shrews (Tupaia belangeri). Male tree shrews develop social hierarchies in which subordinates show characteristic changes in physiological and behavioral parameters when confronted with a dominant. In the present experiments, subordinate animals lost body weight soon after starting the daily social conflict, and urinary excretion of cortisol was elevated throughout the experiment as compared with the control period. Golgi-impregnated brain tissue from subordinates exposed to 28 d (1 hr/d) of social confrontations was compared with that from control nonstressed animals. The apical dendrites of the CA3 pyramidal cells from subordinates had a decreased number of branch points and total dendritic length as compared with controls. No differences were observed in apical dendritic spine density or in the basal dendritic tree morphology. The stress-induced CA3 apical dendritic atrophy in subordinates was prevented by administering daily oral doses of the antiepileptic drug phenytoin (Dilantin, Sigma, St. Louis, MO) (200 mg/kg), which interferes with excitatory amino acid (EAA) action. These results suggest that the naturalistic stressor psychosocial stress induces specific structural changes in hippocampal neurons of subordinate male tree shrews. These changes, like those in the rat after glucocorticoid treatment or restraint stress, probably are mediated by activation of the hypothalamo-pituitary-adrenal-axis acting in concert with endogenous EAAs from mossy fiber input.

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.

Characterization of metabotropic glutamate receptor-mediated facilitation of N-methyl-D-aspartate depolarization of neocortical neurones

1. Facilitation of the N-methyl-D-aspartate (NMDA) receptor-mediated depolarization of cortical neurones induced by metabotropic glutamate receptor (mGluR) agonists in the presence of tetrodotoxin has been examined by use of grease-gap recording. 2. Quisqualate (1-2 microM) and 10 to 100 microM 1S,3R-I-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) facilitated the NMDA-, but not the kainate-induced depolarization with an EC50 of 16 microM for 1S,3R-ACPD. The facilitation induced by quisqualate was reduced, but not blocked, by 4 microM 6-cyano-7-nitroquinoxaline-2,3-dione. 3. D,L-2-Amino-3-phosphonopropionic acid and D,L-2-amino-4-phosphonobutyric acid antagonized the 1S,3R-ACPD facilitation in a non-competitive manner with IC50 values of 0.24 microM and 4.4 microM respectively. 4. Homologous desensitization of the 1S,3R-ACPD induced facilitation was not observed. The facilitation was not altered by 10 nM staurosporine or 3 microM phorbol diacetate. 5. Substitution of 20 microM 8-bromo-cyclic adenosine monophosphate, 20 microM 8-bromo-cyclic guanosine monophosphate, or 10 microM arachidonic acid for 1S,3R-ACPD did not induce facilitation of the NMDA response. However, the 1S,3R-ACPD facilitation was potentiated by 10 mM myo-inositol and exhibited heterologous desensitization following exposure to 100 microM 5-hydroxytryptamine. 6. The 1S,3R-ACPD-induced facilitation persisted in both 10 microM nifedipine and nominally Ca(2+)-free medium and was only gradually eliminated following addition of 100 microM bis-(-o-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid in Ca(2+)-free medium. Facilitation of the NMDA response induced by carbachol, but not phenylephrine, was also observed in nominally Ca(2+)-free medium. Perfusing 50 microM bis-(-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid aminoethoxy eliminated the 1S,3R-ACPD facilitation. 7. These experiments have shown that mGluR agonists selectively facilitate the NMDA depolarization of cortical wedges, most likely by activating one or more mGluR subtypes that couple to phospholipase C. We conclude the facilitation results from a Ca(2+)-sensitive mechanism dependent on activation of phospholipase C and release of internal Ca2+. The facilitation is not contingent on activation of protein kinase C or entry of Ca2+ through nifedipine-sensitive Ca2+ channels.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Involvement of serotonin in developmental plasticity of kitten visual cortex

During a critical period of postnatal development, neuronal connections in the kitten visual cortex are susceptible to experience-dependent modifications. These modifications are facilitated by the neuromodulators noradrenaline and acetylcholine. To address the question of whether serotonin (5-hydroxytryptamine; 5-HT), the other major neuromodulator in the cerebral cortex, also plays a role in developmental plasticity, we investigated whether interference with serotoninergic transmission in the kitten visual cortex affects ocular dominance (OD) plasticity. The serotonin neurotoxin 5,7-dihydroxytryptamine or the serotonin receptor blockers ketanserin and methysergide were infused into the visual cortex of kittens undergoing monocular deprivation. We found that both methods of disrupting serotoninergic transmission reduced OD plasticity. However, to be effective, the receptor blockers ketanserin and methysergide had to be applied in combination, suggesting that coactivation of serotonin receptor subtypes of both the 5-HT1 and 5-HT2 families have a permissive function in OD plasticity. Since activation of 5-HT2 receptors stimulates phosphoinositide hydrolysis, our data suggest that second messengers from the phospholipid pathway may play an important role in developmental plasticity of visual cortex.

Loss of cortical serotonin2A signal transduction in senescent rats: reversal following inhibition of protein kinase C

Using grease gap recordings, age-related changes in serotonin2A receptors were assessed in sensorimotor regions of the cortex by examining serotonin-induced facilitation of the N-methyl-D-aspartate depolarization in cortical wedges prepared from young adult (3-6 months) and senescent (22-34 months) Fisher 344 rats. Serotonin (10-100 microM) facilitated the N-methyl-D-aspartate depolarization in wedges from young adult rats in a concentration-dependent manner, whereas no facilitation was observed in wedges from senescent rats. Similar results were obtained when +/- 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, a mixed serotonin2A and serotonin2C receptor agonist, was substituted for serotonin. In contrast, agonists at alpha 1A-adrenoceptors, metabotropic glutamate receptors and muscarinic cholinoceptors facilitated the N-methyl-D-aspartate depolarization in wedges from both young adult and senescent rats. Chelerythrine and staurosporine, inhibitors of protein kinase C, but not concanavalin A, myo-inositol or calmodulin antagonists, restored the serotonin facilitation in wedges from senescent animals. In situ hybridization histochemistry revealed that serotonin2A receptor messenger RNA was present in layers II-VI of the cortex, with the highest density of silver grains located in layers III and V of both young adult and senescent rats. Detailed examination of layer V showed that silver grains were significantly higher than background only over pyramidal cells. We conclude that serotonin2A receptors are expressed by pyramidal cells in both young adult and senescent rats and that serotonin acts directly on these receptors to facilitate the N-methyl-D-aspartate depolarization. Moreover, in senescent rats, signal transduction at cortical serotonin2A receptors involved with facilitation of the N-methyl-D-aspartate response is compromised as a result of protein kinase C activation.

Origins of serotonin-like immunoreactivity in the optic tectum of Rana pipiens

We have previously identified a population of serotonin-like immunoreactive (5-HT-ir) retinal ganglion cells in Rana pipiens. In this study, we examined serotonin-like immunoreactivity (5-HTLI) in a probable target of those cells, the optic tectum. We observed both 5-HT-ir fibers and cell bodies in this structure. 5-HT-ir cells were located in the cellular layers of the tectum, layers 2, 4, and 6, and scattered in its superficial layers. 5-HT-ir fibers in the tectum displayed a laminated organization and were located in tectal layers 3, 5, 6, 7, and 9. Retrograde labelling experiments showed that 5-HT-ir retinal ganglion cells projected to the optic tectum. However, these experiments also demonstrated that serotonergic neurons in the midbrain tegmentum, the nucleus isthmi, and the medulla did so as well. 5-HT-ir fibers seen in lamina A of layer 9 were very much reduced in density in animals in which the optic nerve had been lesioned for 3-6 months. Immunoreactive fibers in lamina B of layer 9 were not affected by the lesion. Our results suggest that 5-HT-ir fibers in lamina A of layer 9 are mainly of retinal origin, whereas those in lamina B originate from other brain areas. The 5-HT-ir tectal cells located in the cellular layers probably contribute the 5-HT-ir fibers seen in layers 3, 5, 6, and 7.

Synaptic glutamate release by postnatal rat serotonergic neurons in microculture

Serotonergic neurons are thought to play a role in depression and obsessive compulsive disorder. However, their functional transmitter repertoire is incompletely known. To investigate this repertoire, intracellular recordings were obtained from 132 cytochemically identified rat mesopontine serotonergic neurons that had re-established synapses in microcultures. Approximately 60% of the neurons evoked excitatory glutamatergic potentials in themselves or in target neurons. Glutamatergic transmission was frequently observed in microcultures containing a solitary serotonergic neuron. Evidence for co-release of serotonin and glutamate from single raphe neurons was also obtained. However, evidence for gamma-aminobutyric acid release by serotonergic neurons was observed in only two cases. These findings indicate that many cultured serotonergic neurons form glutamatergic synapses and may explain several observations in slices and in vivo.

Myo-inositol reduces serotonin (5-HT2) receptor induced homologous and heterologous desensitization

The effect of myo-inositol was examined on 5-HT2 receptor mediated facilitation of NMDA depolarization of rat neocortical neurons in vitro. Myo-inositol (1-10 mM) potentiated the 5-HT facilitation, the potentiation increasing linearly with log 5-HT concentration. Myo-inositol also eliminated 5-HT induced heterologous desensitization of muscarinic and alpha 1-adrenergic receptor mediated facilitation. Our findings suggest that 5-HT induced homologous and heterologous desensitization results in part from depleting phosphoinositide substrate.

Sensory responsiveness of brain noradrenergic neurons is modulated by endogenous brain serotonin

Previous results have indicated that application of serotonin (5-HT) onto noradrenergic locus coeruleus (LC) neurons selectively attenuates the response of these cells to excitatory amino acids (EAAs). Other studies revealed that certain sensory responses of LC neurons are mediated by EAA inputs. We examined the role of endogenous 5-HT in modulating sensory responses of LC neurons that are EAA-mediated. LC neurons recorded in rats pretreated with the serotonin (5-HT) depletor, p-chlorophenylalanine (PCPA), exhibited increased responsiveness to electrical stimulation of a rear footpad. Conversely, injection of the 5-HT precursor, 5-hydroxytryptophan (5-HTP), reversed this effect of PCPA and attenuated this sensory response of LC neurons in drug-naive animals. Neither treatment altered the spontaneous discharge rate of LC neurons. These results are consistent with previous findings indicating that 5-HT has potent but selective effects on EAA-mediated responses of LC neurons, and in addition point to a possible functional role for endogenous 5-HT in controlling sensory-evoked LC activity.

Potentiating effects of serotonin and vasoactive intestinal peptide on the action of glutamate on suprachiasmatic neurons in brain slices

Using extracellular single-unit recording in brain slices, we have tested the effects of serotonin and vasoactive intestinal peptide (VIP) on the action of glutamate on hypothalamic suprachiasmatic neurons. We hypothesized that serotonin and VIP may interact with glutamate so that a modulation of the circadian rhythm entrainment to the light-dark may be possible. Given individually, glutamate excited 32% and inhibited 1% (n = 65), serotonin excited 8% and inhibited 27% (n = 73), and VIP excited 7% and inhibited 7% (n = 41) of suprachiasmatic neurons. When administered together, however, serotonin or VIP not only increased the percentage of neurons (up to 50%) that responded to glutamate, but also potentiated the neuronal responses to glutamate. These findings substantiated our hypothesis that both serotonin and VIP may modulate the action of glutamate in the suprachiasmatic nucleus.

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.