Further studies on the action of 5-hydroxytryptamine in the dorsal lateral geniculate nucleus of the cat

Effects of intracerebroventricular corticotropin releasing factor on sensory-evoked responses in the rat visual thalamus

Corticotropin releasing factor (CRF) coordinates the brain׳s responses to stress. Recent evidence suggests that CRF-mediated activation of the locus coeruleus-norepinephrine (LC-NE) system contributes to alterations in sensory signal processing during stress. However, it remains unclear whether these actions are dependent upon the degree of CRF release. Using intracerebroventricular (ICV) infusions, we examine the dose-dependent actions of CRF on sensory-evoked discharges of neurons in the dorsal lateral geniculate nucleus of the thalamus (dLGN). The LGN is the primary relay for visual signals from retina to cortex, receiving noradrenergic modulation from the LC. In vivo extracellular recording in anesthetized rats was used to monitor single dLGN neuron responses to light flashes at three different stimulus intensities before and after administration of CRF (0.1, 0.3, 1.0, 3.0 or 10.0 μg). CRF produced three main effects on dLGN stimulus evoked activity: (1) increased magnitude of sensory evoked discharges at moderate doses, (2) decreased response latency, and (3) dose-dependent increases in the number of cells responding to a previously sub-threshold (low intensity) stimulus. These modulatory actions were blocked or attenuated by intra-LC infusion of a CRF antagonist prior to ICV CRF administration. Moreover, intra-LC administration of CRF (10 ng) mimicked the facilitating effects of moderate doses of ICV CRF on dLGN neuron responsiveness to light stimuli. These findings suggest that stressor-induced changes in sensory signal processing cannot be defined in terms of a singular modulatory effect, but rather are multi-dimensional and dictated by variable degrees of activation of the CRF-LC-NE system.

The impact of hemodynamic stress on sensory signal processing in the rodent lateral geniculate nucleus

Hemodynamic stress via hypotensive challenge has been shown previously to cause a corticotropin-releasing factor (CRF)-mediated increase in tonic locus coeruleus (LC) activity and consequent release of norepinephrine (NE) in noradrenergic terminal fields. Although alterations in LC-NE can modulate the responsiveness of signal processing neurons along sensory pathways, little is understood regarding how continuous CRF-mediated activation of LC-NE output due to physiologically relevant stressor affects downstream target cell physiology. The goal of the present study was to investigate the effects of a physiological stressor [hemodynamic stress via sodium nitroprusside (SNP) i.v.] on stimulus evoked responses of sensory processing neurons that receive LC inputs. In rat, the dorsal lateral geniculate nucleus (dLGN) of the thalamus is the primary relay for visual information and is a major target of the LC-NE system. We used extracellular recording techniques in the anesthetized rat monitor single dLGN neuron activity during repeated presentation of light stimuli before and during hemodynamic stress. A significant decrease in magnitude occurred, as well as an increase in latency of dLGN stimulus-evoked responses were observed during hemodynamic stress. In another group of animals the CRF antagonist DpheCRF12-41 was infused onto the ipsilateral LC prior to SNP administration. This infusion blocked the hypotension-induced changes in dLGN stimulus-evoked discharge. These results show that CRF-mediated increases in LC-NE due to hemodynamic stress disrupts the transmission of information along thalamic-sensory pathways by: (1) initially reducing signal transmission during onset of the stressor and (2) decreasing the speed of stimulus evoked sensory transmission.

Different roles for AMPA and NMDA receptors in transmission at the immature retinogeniculate synapse

The relay of information at the retinogeniculate synapse, the connection between retina and visual thalamus, begins days before eye opening and is thought to play an important role in the maturation of neural circuits in the thalamus and visual cortex. Remarkably, during this period of development, the retinogeniculate synapse is immature, with single retinal ganglion cell inputs evoking an average peak excitatory postsynaptic current (EPSC) of only about 40 pA compared with 800 pA in mature synapses. Yet, at the mature synapse, EPSCs >400 pA are needed to drive relay neuron firing. This raises the question of how small-amplitude EPSCs can drive transmission at the immature retinogeniculate synapse. Here we find that several features of the immature synapse, compared with the mature synapse, contribute to synaptic transmission. First, although the peak amplitude of EPSC is small, the decay time course of both alpha-amino-3-hydroxy-5-methyl-4isoxazolepropionic acid receptor (AMPAR) and N-methyl-d-aspartate receptor (NMDAR) currents is significantly slower. The prolonged time course of NMDAR currents is a result of the presence of both NR2B and NR2C/D subunits. In addition, the extended presence of neurotransmitter released prolongs the synaptic current time course. Second, reduced sensitivity to magnesium block results in significantly greater synaptic charge transfer through NMDAR. Third, AMPAR currents contribute to the spike latency, but not to temporal precision, at the immature synapse. Furthermore, intrinsic excitability is greater. These properties enable immature synapses with predominantly NMDARs and little or no AMPARs to contribute to the relay of information from retina to visual cortex.

Dysregulation of thalamic sensory "transmission" in schizophrenia: neurochemical vulnerability to hallucinations

Cholinergic arousal mechanisms predispose thalamic and cortical neurons to fire action potentials at gamma rhythms, which have a tendency to resonate in thalamocortical networks, thereby forming coherent assemblies under constraints of sensory input to specific thalamic nuclei, on the one hand, and prefrontal and limbic attentional mechanisms, on the other. Perception may be based on sustained assemblies of coherent gamma oscillations in thalamocortical circuits. In schizophrenia, the impact of sensory input on self-organization of thalamocortical activity may be generally reduced. As a result, processes underlying perception can become uncoupled from sensory input, particularly at times of hyperarousal, leading to domination of attentional mechanisms and the emergence of hallucinations. Evidence is reviewed that implicates excessive neuronal noise in specific thalamic nuclei in the generation of hallucinations in schizophrenia. Nicotinic receptor abnormalities, dopaminergic hyperactivity and glutamate-receptor hypofunction are reconciled within a model of psychotic symptom generation that places crucial emphasis on dysfunction of the reticular thalamic nucleus.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

Serotonergic innervation of the isthmo-optic nucleus of the pigeon centrifugal visual system. An immunocytochemical electron microscopic study

The ultrastructural features of serotonergic fibers, terminals and synaptic contacts were studied with the pre-embedding immunocytochemical method in the isthmo-optic nucleus of the pigeon centrifugal visual system. The 5-HT immunoreactive (-ir) profiles were diffusely distributed and their density was low. The labeled axons were thin and unmyelinated (mean diameter=0.21+/-0.03 microm) though a few larger myelinated axons were observed (mean diameter=0.51+/-0.07 microm). The 5-HT-ir terminals or varicosities were small (diameter=0.71+/-0.54 microm) and contained small agranular synaptic vesicles (diameter=28.5+/-6.9 nm) and large granular vesicles (diameter=102.2+/-19.5 nm). The latter only constituted approximately 1% of the total profiles containing synaptic vesicles in the isthmo-optic nucleus. In single thin sections, only 5% of the 5-HT-ir varicosities exhibited an active asymmetrical zone synapsing upon dendritic profiles of centrifugal visual neurons. Calculations indicated that 17% of these 5-HT-ir varicosities were actually engaged in junctional synaptic relationships, whereas the remaining (83%) were nonjunctional. The data suggest that, within the isthmo-optic nucleus, 5-HT acts both at synaptic junctions (wiring transmission) and at a distance via the extracellular space (volume transmission). These 5-HT afferents could thus modulate the activity of the retinopetal neurons and visual information processing.

Electrophysiological and pharmacological properties of interneurons in the cat dorsal lateral geniculate nucleus

We investigated the electrophysiological and pharmacological properties of morphologically identified and putative interneurons within laminae A and A1 of the cat dorsal lateral geniculate nucleus maintained in vitro. These intralaminar interneurons possess unique electrophysiological characteristics, including (1) action potentials of a short duration (average width at half amplitude of 0.34 ms). (2) the ability to generate high-frequency trains of action potentials exceeding 500 Hz, without strong spike frequency adaptation, and (3) a low-threshold regenerative response with variable magnitude of expression, ranging from a subthreshold depolarization towards the generation of one to several action potentials in different cells. The low-threshold regenerative depolarization following a hyperpolarizing current pulse was increased in size by application of 4-aminopyridine, was reduced by nickel, and was not influenced by extracellular cesium. These findings indicate that this event is mediated by an underlying Ca(2+)-dependent mechanism, such as a low-threshold Ca(2+) current, that is regulated by the activation of opposing transient K+ currents. Every interneuron tested responded to glutamate, kainate, quisqualate, or N-methyl-D-aspartate with depolarization and action potential discharge. In contrast, we did not observe a postsynaptic response to activation of the metabotropic receptors with 1S,3R-(+/-)-1-amino-cyclopentane-1,3-dicarboxylate. Application of gamma-amino-butyric acid (GABA) strongly inhibited spike firing through a biphasic hyperpolarization and increase in membrane conductance, a response that reversed close to the presumed chloride equilibrium potential and was imitated by the GABAA receptor agonist muscimol. The GABAB receptor agonist baclofen evoked only a weak membrane hyperpolarization from rest and suppression of spontaneous spike activity. Application of acetylcholine, or the muscarinic agonist acetyl-beta-methylcholine, inhibited spontaneous action potential activity through hyperpolarization of the membrane potential, presumably resulting from an increase in membrane potassium conductance. In contrast, application of serotonin only slightly facilitated tonic activity in a subpopulation of interneurons, histamine induced a small slow depolarization apparently through activation of presynaptic excitatory pathways, and noradrenaline and adenosine had no detectable effect on the spontaneous firing or resting potential of interneurons. We suggest that intralaminar interneurons may function in a relatively linear manner to transform retinal and cortical inputs into a local field of inhibition in the dorsal lateral geniculate and that the excitability of these neurons is largely controlled by retinal, cortical, GABAergic, and cholinergic (brainstem) afferents.

Modulatory effects of acetylcholine, serotonin and noradrenaline on the activity of cat perigeniculate neurons

We studied the modulatory actions of microiontophoretically applied acetylcholine (ACH), serotonin (5-HT, 5-hydroxytryptamine) and noradrenaline (NA), and those of the adrenoceptor agonists phenylephrine (PHE, alpha 1), clonidine (CLO, alpha 2) and isoprenaline (ISO, beta) on spontaneous and visually induced activities in cat perigeniculate (PGN) and thalamic reticular (NRT) neurons (only spontaneous) during extracellular recordings performed in vivo. ACH and 5-HT were found to affect the ongoing (spontaneous) and visually evoked activity of PGN cells and also the spontaneous activity of NRT cells in an opposite fashion. ACH inhibited tonic firing and often induced burst activity. By contrast, 5-HT exerted an excitatory influence, which caused a long-lasting, very regular, high-frequency activity between about 35 and 120 Hz. Spontaneous as well as 5-HT-induced firing was found to prefer three distinct frequency ranges: 35-42 Hz, 60-67 Hz and 80-120 Hz. Opposite actions of ACH and 5-HT were also evident when applied simultaneously. ACH dampened the high-frequency activity elicited with 5-HT, and 5-HT could replace the burst activity induced with ACH application by a regular tonic activity. The absolute strength of visual responses (in spikes per second) was only slightly enhanced or reduced by ACH and 5-HT, respectively, but due to the strong effects on background activity, ACH clearly elevated the signal-to-noise ratio and 5-HT reduced it. Despite its excitatory action, 5-HT did not facilitate visual responses. Spontaneous changes in ongoing activity were found to affect the visual response amplitude in the same way. Noradrenaline, the alpha 1-agonist PHE and the beta-agonist ISO exerted a weak depressant action on high-frequency maintained activity, but during low-frequency single spike activity and/or burst activity a facilitatory effect was evident, which prevented the generation of burst discharges and slightly increased single spike firing. Visually evoked activity was little affected, but signal-to-noise ratio changed with changes in ongoing activity. The alpha 2-agonist CLO clearly attenuated both spontaneous activity and visual responses. We suggest that, in addition to direct effects of ACH and 5-HT on geniculate relay cells, the balance between the opposite actions of ACH and 5-HT on PGN cells determines the mode of operation in the recurrent inhibitory circuit: either a global, tonic inhibition of relay cells during a dominating 5-HT influence or a less tonic but phasic inhibition during increased activity in the cholinergic system.

High-affinity nerve growth factor receptor (Trk) immunoreactivity is localized in cholinergic neurons of the basal forebrain and striatum in the adult rat brain

Trk-immunoreactivity was observed in basal forebrain and striatal cholinergic neurons, whereas low-affinity NGF receptor immunoreactivity was observed in basal forebrain but not striatal cholinergic neurons. Since NGF exerts trophic actions on both basal forebrain and striatal cholinergic populations, the presence of Trk in these neurons lends strong support for an essential role of Trk in NGF-responsive neurons, but suggests that the low affinity receptor is not necessary for NGF actions in the striatum.

The histaminergic innervation of the lateral geniculate complex in the cat

The histaminergic innervation of the thalamic dorsal and ventral lateral geniculate nuclei and the perigeniculate nucleus of the cat was examined immunohistochemically by means of an antibody to histamine. We find histamine-immunoreactive neurons in the cat brain are concentrated in the ventrolateral portion of the posterior hypothalamus, confirming a previous report. However, this cell group also spreads into medial, dorsal, and extreme lateral regions of the posterior hypothalamus and extends as far rostral as the optic chiasm. Histamine-labeled fibers cover all regions of the lateral geniculate complex, but the density of labeling varies. The ventral lateral geniculate nucleus (vLGN) is most densely labeled, the A laminae of the dorsal lateral geniculate are sparsely labeled, and the geniculate C laminae and the perigeniculate nucleus show intermediate amounts of label. Thus, histaminergic fibers demonstrate a predilection for zones innervated by the W-cell system. Labeled fibers exhibit few branchings and numerous en passant swellings, lending a beaded appearance. The vLGN showed more instances of fibers with larger-sized swellings (up to 2 microns). Following injections of biotinylated tracers into the hypothalamus, we find labeled fibers throughout the lateral geniculate complex. The anterogradely labeled fibers resemble the histaminergic fibers in morphology, distribution, and relative bouton size. Thus, the hypothalamus appears to be the source of the histaminergic fibers in the lateral geniculate complex. Histamine-labeled fibers in the dorsal lateral geniculate nucleus (dLGN) exhibit uncommon ultrastructural morphology. Many extremely large, round, or elliptical vesicles fill the fiber swellings. Swellings are directly apposed to a variety of other dendritic and axonal profiles, but thus far no convincing synaptic contacts have been seen. The distribution and appearance of these histaminergic fibers resembles those reported for serotonergic fibers. Our results support the idea that histamine works nonsynaptically as a neuromodulator in the lateral geniculate complex, affecting the level of visual arousal.

Effects of neonatal enucleation on catecholamine and serotonin turnover and amino acid levels in lateral geniculate nucleus and visual cortex of the adult rat

Changes in turnover of dopamine (DA), noradrenaline (NA) and serotonin (5-hydroxytryptamine (5-HT)) and their metabolites, together with amino acid content, have been studied in dorsal lateral geniculate nucleus (LGNd) and visual cortex (VC) of neonatal enucleated rats. Enucleation increases the 5-HT turnover in LGNd and catecholamine turnover in VC. In contrast, enucleation decreases glutamate (and/or aspartate) content in LGNd and gamma-aminobutyric acid (GABA) in VC. These changes suggest an increase of the inhibitory action of the biogenic amines in LGNd after neonatal enucleation. The decrease of GABA in VC may reflect the importance of GABA in intracortical circuitry.

The effects of a 5-HT2 receptor antagonist (ICI 169,369) on changes in waking EEG, pupillary responses and state of arousal in human volunteers

1. ICI 169,369 (2-(2-dimethylamino ethylthio)-3-phenyl quinoline) is a potent selective competitive antagonist of the 5-HT2 receptor in animal models. Effects of ICI 169,369 as single oral doses (80 and 120 mg) separated by 1 week, on the power spectrum of waking EEG, dark adapted pupil responses and sedation score, were studied in a double-blind, placebo controlled, randomised cross over within subject comparison, in six healthy male volunteers. 2. Pupillary responses were measured using a portable infrared pupillometer following 15 min dark adaptation, assessing resting vertical pupil diameter (RPD), light constricted diameter (MPD) and recovered final diameter (FPD) at the end of a 3 s measurement cycle. 3. Both doses of ICI 169,369 produced a mean 36% (range 10-54%) decrease in log 10 power of the waking EEG alpha activity with eyes closed (P less than 0.02), and mean 38% (range 2-86%) increase in theta activity at 2 h compared with placebo. 4. Both 80 and 120 mg doses of ICI 169,369 reduced RPD by approximately 30% from a predose value of 6.25 mm (+/- 0.87; 95% CI) and from placebo values 6.41 mm (+/- 1.06) and 7.48 mm (+/- 1.49) at 3 and 5 h after dosing. MPD was reduced by 50% with the 120 mg dose at 5 h after dosing (placebo 5.2 mm; ICI 169,369 2.7 mm; P less than 0.05). FPD was significantly reduced (P less than 0.01) by both doses at 3 h after dosing.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotoninergic innervation of the thalamus in the primate: an immunohistochemical study

Little is known of the serotoninergic innervation of the thalamus in primates; therefore, we undertook a detailed study of the distribution of 5-hydroxytryptamine (5-HT)-immunoreactive neuronal profiles in the thalamus of the squirrel monkey (Saimiri sciureus) with a specific antibody directly raised against 5-HT. All thalamic nuclei in the squirrel monkey displayed 5-HT-immunoreactive fibers, but none contained immunopositive cell bodies. The 5-HT innervation of the thalamus derived from extrinsic fibers arising mostly from the midbrain raphe nuclei and forming the transtegmental system. Most of the fibers destined to the thalamus collected into a major bundle that swept dorsoventrally within the midbrain tegmentum and coursed beneath the thalamus along its entire caudorostral extent. Several fiber fascicles broke off from this main bundle at different levels and ascended dorsally to innervate the various thalamic nuclei. Overall, the 5-HT innervation of the thalamus in the squirrel monkey was more massive than would have been expected from earlier studies in nonprimate species. Marked differences in the regional density of innervation were noted both between the various nuclei and within single nuclei. The most densely innervated nuclei were those delineating the principal subdivisions of the thalamic mass, that is, the midline, rostral intralaminar, limitans, and reticular nuclei, where very dense fields of isolated axonal varicosities occurred. In contrast to the rostral intralaminar nuclei, which were rather uniformly innervated, the centre médian/parafascicular complex contained immunoreactive fibers and isolated varicosities distributed according to a mediolateral gradient. The habenula and the ventral anterior nucleus were among the most weakly innervated nuclei. In the latter nucleus, as well as in more densely innervated nuclei, thin varicose fibers formed numerous pericellular contacts on cell bodies and proximal dendrites of thalamic neurons. The 5-HT innervation of the lateral nuclear group as well as that of the medial and lateral geniculate nuclei ranged from very weak to dense. The mediodorsal nucleus displayed a highly heterogeneous 5-HT innervation that varied from weak in its central portion to moderate or dense in its medial and lateral borders. A moderate 5-HT innervation was observed in the anterior nuclear group. The surprisingly dense and heterogeneous 5-HT innervation of the thalamus noted in the present study suggests that serotonin may be involved in several specific functions of the thalamus in primates.

Suppressive effects of L-5-hydroxytryptophan in a feline model of photosensitive epilepsy

We recently demonstrated that long-lasting photosensitivity is acquired as a result of kindling of the lateral geniculate nucleus (LGN), and that the LGN-kindled cat pretreated with D, L-allylglycine represents a useful model of epilepsy for drug studies. The present experiments studied anticonvulsant effects of a serotonin precursor, L-5-hydroxytryptophan (5-HTP), on photosensitivity in the LGN-kindled cat under D,L-allylglycine and on LGN-kindled seizures. 5-HTP suppressed both myoclonic responses and paroxysmal EEG discharges induced by photic stimulation in a dose-related manner. Photically-induced seizures were completely blocked 1.5-2 h after injection of 20 mg/kg 5-HTP. 5-HTP was also effective in reducing the afterdischarge duration and behavioral seizure stage in LGN-kindled seizures; following 40 mg/kg administration, no electroclinical seizures were elicited in the LGN-kindled cats. Serotonergic mechanisms may play an important role in epileptic photosensitivity; the 5-HTP suppressive effect on photosensitivity is at least partly due to reduced neuronal activity at the level of the LGN via serotonergic inhibition.

Cholinergic and monoaminergic innervation of the cat's thalamus: comparison of the lateral geniculate nucleus with other principal sensory nuclei

The cholinergic and monoaminergic innervation of the lateral geniculate nucleus (GL) and other thalamic nuclei in the cat was examined by using immunocytochemical and tract-tracing techniques. Cholinergic fibers, identified with an antibody to choline acetyltransferase (ChAT), are present in all layers of the GL. They are fine in caliber and exhibit numerous swellings along their lengths. The A layers, the magnocellular C layer, and the medial interlaminar nucleus are rich in cholinergic fibers that give rise to prominent clusters of boutons, while the parvicellular C layers contain fewer fibers that are more uniformly distributed. The interlaminar zones are largely devoid of ChAT-immunoreactive fibers. Double-label experiments show that cholinergic projections to the GL originate from two sources, the pedunculopontine reticular formation (PPT) and the parabigeminal nucleus (Pbg). The PPT contributes cholinergic fibers to all layers, while Pbg projections are limited to the parvicellular C layers. The lateral geniculate nucleus has a much greater density of cholinergic fibers than the other principal sensory nuclei: the density of fibers in the A layers is more than three times greater than that in the ventral posterior nucleus (VP) or the ventral division of the medial geniculate nucleus (GMv). In contrast, serotonin (5-HT)-immunoreactive fibers are distributed with equal density across the principal thalamic nuclei, while tyrosine hydroxylase (TH)-immunoreactive fibers (presumed to contain norepinephrine) are noticeably less dense in the GL than in the others. Monoaminergic fibers also differ from cholinergic fibers in their laminar distribution within the GL: both TH- and 5HT-immunoreactive fibers are distributed evenly across the layers and interlaminar zones and are slightly more abundant in the parvicellular C layers than in the other layers. Other thalamic nuclei rich in cholinergic fibers include the pulvinar nucleus, the ventral lateral geniculate nucleus, the intermediate nucleus of the lateral group, the lateral medial and suprageniculate nuclei (Graybiel and Berson: Neuroscience 5:1175-1238, '80), and the paracentral and central-lateral components of the intralaminar nuclei. This pattern matches the distribution of projections from the PPT and is similar, but not identical, to the pattern of acetylcholinesterase staining. The fact that most of the nuclei rich in cholinergic fibers have been implicated in visual sensory or visual motor functions suggests that cholinergic projections from the reticular formation play an especially important role in visually guided behavior.

Effects of stimulating the dorsal raphe nucleus of the rat on neuronal activity in the dorsal lateral geniculate nucleus

The effect of stimulating the dorsal raphe nucleus (DRN) on the activity of single relay neurons in the dorsal lateral geniculate nucleus (LGNd) was studied in rats anesthetized with urethane. The position of stimulating electrodes was confirmed on histological sections processed with NADPH-diaphorase histochemistry which could delineate the DRN clearly. During repetitive stimulation of the DRN at 200 Hz for several to 10 seconds no consistent change in firing was observed, but between several and several tens of seconds after the cessation of stimulation spontaneous firing of LGNd neurons was suppressed. In many cases the suppression proceeded concomitantly with augmentation of slow waves in the cortical EEG. The suppression was mimicked by ionophoresis of serotonin, and antagonized by a serotonergic antagonist, methysergide. In addition, in animals in which DRN stimulation became ineffective after it was evoked many times, the suppression could be restored by intraperitoneal administration of a serotonin precursor, 5-hydroxytryptophan. Compilation of peristimulus time histograms revealed that a brief DRN stimulation (5 shocks at 1000 Hz) could also elicit the suppression lasting from 60 to 100 ms or longer after the shocks. These results suggest that serotonin released from terminals of DRN neurons exerts long-latency and long-lasting inhibition of LGNd relay neurons. Along with brainstem noradrenergic and cholinergic systems, the serotonergic projection from the DRN acts to control excitation levels of the forebrain.

p-Chloroamphetamine treatment modifies evoked responses to sinusoidal gratings in the pigeon optic tectum

This study was performed in order to establish whether selective depletion of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the pigeon optic tectum (TeO) induced by p-chloroamphetamine (p-CA) modified tectal evoked potentials (TEPs). TEPs in response to sinusoidal gratings of different contrast, spatial and temporal frequency were recorded in control pigeons and in pigeons intraperitoneally injected with p-CA (10 mg/kg; two administrations in consecutive days). TEPs of p-CA treated pigeons, as compared to those of control pigeons, were reduced in amplitude as a function of contrast, spatial and temporal frequency. In addition, TEPs of p-CA treated pigeons differed from those recorded in controls in their transfer characteristics of contrast and spatial frequency. In particular, TEPs of p-CA treated pigeons did not saturate at moderate contrast, unlike those of controls. Furthermore, the TEP spatial tuning in p-CA treated pigeons is broader than that in controls; it thus suggests a reduction of spatial-frequency selectivity. These findings indicate that a selective neurotoxin for serotonergic systems, such as p-CA, can serve as a useful denervation tool for the study of the serotonergic function in the pigeon TeO. In addition, selective changes of TEP properties suggest the possibility that serotonergic afferents play a modulatory role on the receptive-field characteristics of tectal neurons.

The cellular mechanism of thalamic ponto-geniculo-occipital waves

The cellular mechanisms underlying the genesis of thalamic ponto-geniculo-occipital waves were studied in reserpinized cats under urethane anaesthesia. Simultaneous field potential and intracellular recordings were performed in the lateral geniculate nucleus after acute lesions of retinal and visual cortical inputs. In most relay cells, reserpine-induced ponto-geniculo-occipital waves were associated with a transient depolarization that was often interrupted by a unitary inhibitory postsynaptic potential. The depolarization grew in size with membrane hyperpolarization and was accompanied by an increase in membrane conductance. The inhibitory postsynaptic potential is likely to have resulted from the activation of intrageniculate interneurons since perigeniculate cells were always inhibited during the occurrence of ponto-geniculo-occipital waves. Under reserpine, thalamic ponto-geniculo-occipital waves could also be triggered by peribrachial or auditory stimulation. These evoked ponto-geniculo-occipital waves were associated with intracellular events identical to those occurring spontaneously after reserpine administration. In addition, thalamic spindle oscillations were readily blocked by the occurrence of spontaneous or evoked ponto-geniculo-occipital waves. On the basis of the present results and those already published in the literature, the conclusion is reached that lateral geniculate ponto-geniculo-occipital waves result from a nicotinic activation of relay cells and from a parallel muscarinic inhibition of perigeniculate cells by peribrachial afferents. The functional significance of the ponto-geniculo-occipital activity is discussed on the basis of the antagonistic action of these signals on thalamic oscillations. It is proposed that these signals are the central correlates of orienting reactions elicited by sensory stimuli during waking (the so-called eye movement potentials) and by internally generated drives during paradoxical sleep.

Modulatory effects of serotonin on excitatory amino acid responses and sensory synaptic transmission in the ventrobasal thalamus

Excitatory amino acid receptors are thought to mediate sensory input to the ventrobasal thalamus. There is evidence for a brainstem serotonergic projection to the ventrobasal thalamus which may have a modulatory role. The possibility that serotonin may selectively modulate responses to excitatory amino acid receptor agonists, and its effects on sensory synaptic transmission has been examined in the rat ventrobasal thalamus in vivo. Iontophoretic ejection of serotonin at low currents produced a marked facilitation of responses to excitatory amino acids. In contrast, excitatory responses to cholinomimetic agonists were attenuated. Synaptic transmission was concomitantly enhanced or unchanged in these circumstances. Higher serotonin ejection currents reversed the facilitation, or inhibited excitatory amino acid responses and synaptic transmission. It is concluded that serotonin can modulate responses to excitatory amino acids relatively selectively and that synaptic transmission of somatosensory information through the ventrobasal thalamus may be susceptible to brainstem serotonergic modulation.

Dorsal raphe serotoninergic branching neurons projecting both to the lateral geniculate body and superior colliculus: a combined retrograde tracing-immunohistochemical study in the rat

Injections of HRP into the superior colliculus labelled cells in the lateral cell groups of the dorsal raphe nucleus. The cytoarchitectural features and location of these cells showed remarkable similarities with those known to project to the lateral geniculate body, and, therefore, the possible existence of branching neurons in the dorsal raphe nucleus projecting to these two visual structures was tested. Injections into the lateral geniculate body and the superior colliculus of several fluorescent tracers--namely, Fast Blue, Fluoro-Gold, propidium iodide, rhodamine-B-isothiocyanate, and Diamidino Yellow, used in different combinations, showed single- and double-labelled neurons in the lateral wings of the dorsal raphe nucleus. In order to verify the chemical nature of these cells, the tissue was processed for immunofluorescence with serotonin antibodies. The results obtained showed several triple-labelled cells exhibiting two fluorescent tracers as well as 5-hydroxytryptamine-like immunoreactivity. Some immunonegative tracer-positive cells were also observed, suggesting their nonserotoninergic nature. Finally, electrolytic lesions of the lateral wings of the dorsal raphe nucleus caused a gradual disappearance of serotonin-immunoreactive fibers in these visual areas following different survival times. This correlated well with a decrease in the serotonin content studied by high-pressure liquid chromatography. These results support a role of the serotoninergic dorsal raphe projection to the lateral geniculate body and to the superior colliculus in the processing of visual information, and they suggest that serotonin may have a coordinating influence on primary visual centers.

The innervation density of serotonergic (5-HT) fibers varies in different subdivisions of the cat lateral geniculate nucleus complex

The innervation density of serotonin (5-HT)-immunoreactive fibers, identified using an antibody to 5-HT, was found to differ in the 4 subdivisions of the cat lateral geniculate nucleus complex (LGN). The mean density (fiber length per unit area) of anti-5-HT-stained fibers was highest in the ventral LGN (0.062 micron per micron 2), moderate in the medial interlaminar nucleus (MIN) and the parvicellular C laminae of the dorsal LGN (0.039-0.040 per micron 2), and lowest in the A and magnocellular C laminae of the dorsal LGN (0.020 per micron 2). The fiber density in MIN was particularly dense along the medial edge of the nucleus, a region called the geniculate wing. The heaviest serotonin innervation is thus found in geniculate structures receiving input from W-type retinal ganglion cells and lightest in structures receiving X and Y input.

The brainstem projection to the lateral geniculate nucleus in the cat: identification of cholinergic and monoaminergic elements

The pontomesencephalic projection to the dorsal lateral geniculate nucleus (dLGN) of the cat was analyzed by combining retrograde transport of rhodamine-labeled latex spheres and immunohistochemistry. After injections of latex beads into the dLGN, sections of the brainstem were treated immunohistochemically for choline acetyltransferase (ChAT), serotonin (Ser), tyrosine hydroxylase (TH), and dopamine-beta-hydroxylase (DBH). Essentially, six regions in the brainstem contained retrogradely labeled cells: the superior colliculus, the parabigeminal nucleus, the dorsal raphe nuclei, the parabrachial area of the central tegmental field, the marginal nucleus of the brachium conjunctivum, and the nucleus coeruleus. Furthermore, isolated retrogradely labeled cells were present in the central nucleus of the raphe, in the cuneiform nucleus, and in the periaqueductal gray. Most serotoninergic double-labeled cells were found in the medial and lateral divisions of the dorsal raphe nuclei, but a few were also present in the central nucleus of the raphe. In the sections immunostained for ChAT, double-labeled cells were located in the central tegmental field, in the marginal nucleus of the brachium conjunctivum, and in the nucleus coeruleus. In the sections treated for TH and DBH, double-labeled cells showed a similar distribution, and like the ChAT(+) cells, they were located mainly in the central tegmental field, in the marginal nucleus of the brachium conjunctivum, and in the nucleus coeruleus. In these regions the cholinergic and noradrenergic cells that projected to the lateral geniculate nucleus were intermingled, the former predominating rostrally and the latter caudally. The majority of retrogradely labeled cells were located in the region of the central tegmental field in the vicinity of the brachium conjunctivum, and most of these cells were also ChAT-immunoreactive. We, therefore, conclude that the cholinergic projection is the most important of the central core projections ascending to the dLGN.

The serotoninergic fibers in the dorsal lateral geniculate nucleus of the cat: distribution and synaptic connections demonstrated with immunocytochemistry

The distribution, morphology, and synaptic contacts of serotoninergic fibers were studied with immunocytochemical methods in the lateral geniculate complex of the cat. The serotonin-immunoreactive fibers are diffusely distributed throughout the main laminae of the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN) and reach a particular density in the ventral lateral geniculate nucleus (vLGN). The labeled fibers are in most cases very thin and sometimes varicose. There is no obvious order in their distribution pattern except that they sometimes partially encircle the unlabeled cell bodies of the dLGN. The synaptic connections of the serotoninergic fibers were investigated mainly in the A laminae of the dLGN. Few synaptic complexes were found, most of them with asymmetric morphology. The postsynaptic elements were small dendritic profiles. Perisomatic serotoninergic fibers were seen, but no convincing synaptic contacts were found between labeled fibers and cell somata. In the dLGN, serotoninergic profiles were almost exclusively confined to the extraglomerular neuropile. In the PGN serotoninergic fibers also contacted dendritic profiles and formed asymmetrical synapses, but as in the geniculate, synaptic specializations were very rare.

The distribution of putative neurotransmitters in the lateral geniculate nucleus of the rat

Immunocytochemical techniques were used to examine the distribution of several putative peptidergic and aminergic neurotransmitters within the various subdivisions of the rat lateral geniculate nucleus (LG). Neuronal cell bodies, immunoreactive for enkephalin and neuropeptide Y and neuronal fibers immunoreactive for enkephalin, neuropeptide Y, substance P, vasoactive intestinal polypeptide and 5-HT were each localized within distinct subdivisions of the LG. These results suggest that the anatomical and functional differences of LG neurons are also reflected by differences in the transmitters which they utilize.