A study of the organization of the locus coeruleus projections to the lateral geniculate nuclei in the albino rat

Modulation of the Rat Intergeniculate Leaflet of the Thalamus Network by Norepinephrine

Circadian rhythms are regulated by a set of brain structures, one of which is the Intergeniculate Leaflet of the Thalamus (IGL). The most recognised role of the IGL is the integration of a variety of stimuli affecting rhythmicity, such as lighting conditions, received by the eye, or light-independent (non-photic) cues, the information about which is delivered via the activation of the non-specific projections. One of them is the norepinephrinergic system originating in the brainstem Locus Coeruleus (LC). In order to investigate the effect of norepinephrine (NE) on the IGL neurons we have performed ex vivo recordings using the extracellular multi-electrode array technique as well as the intracellular whole-cell patch clamp. Using both agonists and antagonists of specific NE receptor subtypes, we confirmed the presence of functional α₁-, α₂- and β-adrenergic receptors within the investigated structure, allowing NE to exert multiple types of effects on different IGL neurons, mainly depolarisation of the neurons projecting to the Suprachiasmatic Nuclei - the master circadian pacemaker, and various responses exhibited by the cells creating the connection with the contralateral IGL. Moreover, NE was shown to affect IGL cells both directly and via modulation of the synaptic network, in particular the miniature inhibitory postsynaptic currents. To the best of our knowledge, these are the first studies to confirm the effects of NE on the activity of the IGL network.

Cell type- and layer-specific convergence in core and shell neurons of the dorsal lateral geniculate nucleus

Over 40 distinct types of retinal ganglion cells (RGCs) generate parallel processing pathways in the visual system. In mice, two subdivisions of the dorsal lateral geniculate nucleus (dLGN), the core and the shell, organize distinct parallel channels to transmit visual information from the retina to the primary visual cortex (V1). To investigate how the dLGN core and shell differentially integrate visual information and other modalities, we mapped synaptic input sources to each dLGN subdivision at the cell-type level with G-deleted rabies viral vectors. The monosynaptic circuit tracing revealed that dLGN core neurons received inputs from alpha-RGCs, Layer 6 neurons of the V1, the superficial and intermediate layers of the superior colliculus (SC), the internal ventral LGN, the lower layer of the external ventral LGN (vLGNe), the intergeniculate leaf, the thalamic reticular nucleus (TRN), and the pretectal nucleus (PT). Conversely, shell neurons received inputs from alpha-RGCs and direction-selective ganglion cells of the retina, Layer 6 neurons of the V1, the superficial layer of the SC, the superficial and lower layers of the vLGNe, the TRN, the PT, and the parabigeminal nucleus. The present study provides anatomical evidence of the cell type- and layer-specific convergence in dLGN core and shell neurons. These findings suggest that dLGN core neurons integrate and process more multimodal information along with visual information than shell neurons and that LGN core and shell neurons integrate different types of information, send their own convergent information to discrete populations of the V1, and differentially contribute to visual perception and behavior.

Online LI-rTMS during a Visual Learning Task: Differential Impacts on Visual Circuit and Behavioral Plasticity in Adult Ephrin-A2A5-/- Mice

Repetitive transcranial magnetic stimulation (rTMS) induces plasticity in normal and abnormal neural circuitries, an effect that may be influenced by intrinsic brain activity during treatment. Here, we study potential synergistic effects between low-intensity rTMS (LI-rTMS) and concurrent neural activity in promoting circuit reorganization and enhancing visual behavior. We used ephrin-A2A5^–/– mice, which are known to possess visuotopic mapping errors that are ameliorated by LI-rTMS, and assessed the impact of stimulation when mice were engaged in a visual learning task. A detachable coil was affixed to each mouse, and animals underwent 2 wk of 10-min daily training in a two-choice visual discrimination task with concurrent LI-rTMS or sham stimulation. No-task controls (+LI-rTMS/sham) were placed in the task arena without visual task training. At the end of the experiment, visuomotor tracking behavior was assessed, and corticotectal and geniculocortical pathway organization was mapped by injections of fluorescent tracers into the primary visual cortex. Consistent with previous results, LI-rTMS alone improved geniculocortical and corticotectal topography, but combining LI-rTMS with the visual learning task prevented beneficial corticotectal reorganization and had no additional effect on geniculocortical topography or visuomotor tracking performance. Unexpectedly, there was a significant increase in the total number of trials completed by task + LI-rTMS mice in the visual learning task. Comparison with wild-type mice revealed that ephrin-A2A5^–/– mice had reduced accuracy and response rates, suggesting a goal-directed behavioral deficit, which was improved by LI-rTMS. Our results suggest that concurrent brain activity during behavior interacts with LI-rTMS, altering behavior and different visual circuits in an abnormal system.

Methylphenidate Enhances Early-Stage Sensory Processing and Rodent Performance of a Visual Signal Detection Task

Methylphenidate (MPH) is used clinically to treat attention-deficit/hyperactivity disorder (ADHD) and off-label as a performance-enhancing agent in healthy individuals. MPH enhances catecholamine transmission via blockade of norepinephrine (NE) and dopamine (DA) reuptake transporters. However, it is not clear how this action affects neural circuits performing cognitive and sensorimotor functions driving performance enhancement. The dorsal lateral geniculate nucleus (dLGN) is the primary thalamic relay for visual information from the retina to the cortex and is densely innervated by NE-containing fibers from the locus coeruleus (LC), a pathway known to modulate state-dependent sensory processing. Here, MPH was evaluated for its potential to alter stimulus-driven sensory responses and behavioral outcomes during performance of a visual signal detection task. MPH enhanced activity within individual neurons, ensembles of neurons, and visually-evoked potentials (VEPs) in response to task light cues, while increasing coherence within theta and beta oscillatory frequency bands. MPH also improved reaction times to make correct responses, indicating more efficient behavioral performance. Improvements in reaction speed were highly correlated with faster VEP latencies. Finally, immunostaining revealed that catecholamine innervation of the dLGN is solely noradrenergic. This work suggests that MPH, acting via noradrenergic mechanisms, can substantially affect early-stage sensory signal processing and subsequent behavioral outcomes.

Amygdalar Gating of Early Sensory Processing through Interactions with Locus Coeruleus

Fear- and stress-induced activity in the amygdala has been hypothesized to influence sensory brain regions through the influence of the amygdala on neuromodulatory centers. To directly examine this relationship, we used optical imaging to observe odor-evoked activity in populations of olfactory bulb inhibitory interneurons and of synaptic terminals of olfactory sensory neurons (the primary sensory neurons of the olfactory system, which provide the initial olfactory input to the brain) during pharmacological inactivation of amygdala and locus coeruleus (LC) in mice. Although the amygdala does not directly project to the olfactory bulb, joint pharmacological inactivation of the central, basolateral, and lateral nuclei of the amygdala nonetheless strongly suppressed odor-evoked activity in GABAergic inhibitory interneuron populations in the OB. This suppression was prevented by inactivation of LC or pretreatment of the olfactory bulb with a broad-spectrum noradrenergic receptor antagonist. Visualization of synaptic output from olfactory sensory neuron terminals into the olfactory bulb of the brain revealed that amygdalar inactivation preferentially strengthened the odor-evoked synaptic output of weakly activated populations of sensory afferents from the nose, thus demonstrating a change in sensory gating potentially mediated by local inhibition of olfactory sensory neuron terminals. We conclude that amygdalar activity influences olfactory processing as early as the primary sensory input to the brain by modulating norepinephrine release from the locus coeruleus into the olfactory bulb. These findings show that the amygdala and LC state actively determines which sensory signals are selected for processing in sensory brain regions. Similar local circuitry operates in the olfactory, visual, and auditory systems, suggesting a potentially shared mechanism across modalities.SIGNIFICANCE STATEMENT The affective state is increasingly understood to influence early neural processing of sensory stimuli, not just the behavioral response to those stimuli. The present study elucidates one circuit by which the amygdala, a critical structure for emotional learning, valence coding, and stress, can shape sensory input to the brain and early sensory processing through its connections to the locus coeruleus. One function of this interaction appears to be sensory gating, because inactivating the central, basolateral, and lateral nuclei of the amygdala selectively strengthened the weakest olfactory inputs to the brain. This linkage of amygdalar and LC output to primary sensory signaling may have implications for affective disorders that include sensory dysfunctions like hypervigilance, attentional bias, and impaired sensory gating.

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 noradrenergic projection from the locus coeruleus to the cochlear root neurons in rats

The cochlear root neurons (CRNs) are key components of the primary acoustic startle circuit; mediating auditory alert and escape behaviors in rats. They receive a great variety of inputs which serve to elicit and modulate the acoustic startle reflex (ASR). Recently, our group has suggested that CRNs receive inputs from the locus coeruleus (LC), a noradrenergic nucleus which participates in attention and alertness. Here, we map the efferent projection patterns of LC neurons and confirm the existence of the LC-CRN projection using both anterograde and retrograde tract tracers. Our results show that each LC projects to the CRNs of both sides with a clear ipsilateral predominance. The LC axons terminate as small endings distributed preferentially on the cell body and primary dendrites of CRNs. Using light and confocal microscopy, we show a strong immunoreactivity for tyrosine hydroxylase and dopamine β-hydroxylase in these terminals, indicating noradrenaline release. We further studied the noradrenergic system using gene expression analysis (RT-qPCR) and immunohistochemistry to detect specific noradrenergic receptor subunits in the cochlear nerve root. Our results indicate that CRNs contain a noradrenergic receptor profile sufficient to modulate the ASR, and also show important gender-specific differences in their gene expression. 3D reconstruction analysis confirms the presence of sexual dimorphism in the density and distribution of LC neurons. Our study describes a coerulean noradrenergic projection to the CRNs that might contribute to neural processes underlying sensory gating of the ASR, and also provides an explanation for the gender differences observed in the behavioral paradigm.

Methylphenidate and atomoxetine enhance sensory-evoked neuronal activity in the visual thalamus of male rats

Attention deficits and inappropriate regulation of sensory signal processing are hallmarks of many neuropsychiatric conditions, including attention deficit hyperactivity, for which methylphenidate (MPH) and atomoxetine (ATX) are commonly prescribed therapeutic treatments. Despite their widespread use and known mechanism of blocking reuptake of catecholamine transmitters in the brain, the resultant actions on individual neuron and neural circuit function that lead to therapeutic efficacy are poorly understood. Given the ability of MPH and ATX to improve cognitive performance in humans and rodent assays of attention, we were interested in their influence on early sensory processing in the dorsal lateral geniculate nucleus (dLGN), the primary thalamic relay for visual information from the retina to the visual cortex. In male rats, dLGN neuronal responses to light stimuli were altered in multiple ways after doses of MPH or ATX observed to enhance performance in visually guided assays of attention (MPH = 2 mg/kg; ATX = 0.5 mg/kg). Latencies to response onset and to the peak of the primary response were decreased, while the peak intensity and area of the primary response were increased. In addition, some cells that were unresponsive to light stimuli prior to drug treatment displayed a "gating effect," wherein prominent responses to light stimuli were evident after drug administration. Our results begin to reveal unique effects of MPH and ATX in enhancing sensory signal transmission through visual circuitry, and may yield new insights for understanding the pathophysiology of certain cognitive disorders and inform development of improved therapeutic treatments for these conditions.

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.

The selective neurotoxin DSP-4 impairs the noradrenergic projections from the locus coeruleus to the inferior colliculus in rats

The inferior colliculus (IC) and the locus coeruleus (LC) are two midbrain nuclei that integrate multimodal information and play a major role in novelty detection to elicit an orienting response. Despite the reciprocal connections between these two structures, the projection pattern and target areas of the LC within the subdivisions of the rat IC are still unknown. Here, we used tract-tracing approaches combined with immunohistochemistry, densitometry, and confocal microscopy (CM) analysis to describe a projection from the LC to the IC. Biotinylated dextran amine (BDA) injections into the LC showed that the LC-IC projection is mainly ipsilateral (90%) and reaches, to a major extent, the dorsal and lateral part of the IC and the intercollicular commissure. Additionally, some LC fibers extend into the central nucleus of the IC. The neurochemical nature of this projection is noradrenergic, given that tyrosine hydroxylase (TH) and dopamine beta hydroxylase (DBH) colocalize with the BDA-labeled fibers from the LC. To determine the total field of the LC innervations in the IC, we destroyed the LC neurons and fibers using a highly selective neurotoxin, DSP-4, and then studied the distribution and density of TH- and DBH-immunolabeled axons in the IC. In the DSP-4 treated animals, the number of axonal fibers immunolabeled for TH and DBH were deeply decreased throughout the entire rostrocaudal extent of the IC and its subdivisions compared to controls. Our densitometry results showed that the IC receives up to 97% of its noradrenergic innervations from the LC neurons and only 3% from non-coeruleus neurons. Our results also indicate that TH immunoreactivity in the IC was less impaired than the immunoreactivity for DBH after DSP-4 administration. This is consistent with the existence of an important dopaminergic projection from the substantia nigra to the IC. In conclusion, our study demonstrates and quantifies the noradrenergic projection from the LC to the IC and its subdivisions. The re-examination of the TH and DBH immunoreactivity after DSP-4 treatment provides insights into the source, extent, and topographic distribution of the LC efferent network in the IC, and hence, contributes to our understanding of the role of the noradrenaline (NA) system in auditory processing.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

Thalamo-cortical dysfunction in cocaine abusers: implications in attention and perception

Cocaine affects sensory perception and attention, but little is known about the neural substrates underlying these effects in the human brain. We used functional magnetic resonance imaging (fMRI) and a sustained visuospatial attention task to assess if the visual attention network is dysfunctional in cocaine abusers (n=14) compared to age-, gender-, and education-matched controls (n=14). Compared with controls, cocaine abusers showed (1) hypo-activation of the thalamus, which may reflect noradrenergic and/or dopaminergic deficits; (2) hyper-activation in occipital and prefrontal cortices, which may reflect increased visual cortical processing to compensate for inefficient visual thalamic processing; and (3) larger deactivation of parietal and frontal regions possibly to support the larger hemodynamic supply to the hyper-activated brain regions. These findings provide evidence of abnormalities in thalamo-cortical responses in cocaine abusers that are likely to contribute to the impairments in sensory processing and in attention. The development of therapies that diminish these thalamo-cortical deficits could improve the treatment of cocaine addiction.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Excitatory actions of synaptically released catecholamines in the rat lateral geniculate nucleus

The gating properties of thalamic relay neurons are influenced by the actions of a variety of neuromodulators in concert with the intrinsic properties of these relay neurons. In this study, we have investigated the consequences of synaptically released catecholamines on the excitability of neurons in the rat dorsal lateral geniculate nucleus. Tetanic stimulation of the optic tract, in which catecholamine fibers also course near or through, produced a strong depolarization that consisted of a fast and slow component. The fast excitatory postsynaptic potential was attenuated by ionotropic glutamate receptor antagonists and further unmasked the slow excitatory postsynaptic potential. The amplitude of the slow excitatory postsynaptic potential was dependent on the frequency and intensity of the tetanic stimulation. The alpha1-adrenergic receptor antagonist, prazosin, and the D1-like dopamine receptor antagonist, SCH23390, attenuated the slow excitatory postsynaptic potential; however, the slow excitatory postsynaptic potential was unaltered by metabotropic glutamate, cholinergic, alpha2-adrenergic, and beta-adrenergic receptor antagonists. On the other hand, tetanic stimulation of the optic radiations (corticothalamic axons) evoked a slow excitatory postsynaptic potential that was completely attenuated by metabotropic glutamate receptor antagonists. Our results suggest that tetanic stimulation of catecholamine fibers within the optic tract produces synaptic release of norepinephrine and dopamine that in turn activates both alpha(1)-adrenergic and D1-like dopamine receptors leading to a robust membrane depolarization. By altering the excitability of relay neurons, ascending activating systems may modulate the efficacy of information transfer through the thalamus.

Intergeniculate leaflet and ventral lateral geniculate nucleus afferent connections: An anatomical substrate for functional input from the vestibulo-visuomotor system

The intergeniculate leaflet (IGL) has widespread projections to the basal forebrain and visual midbrain, including the suprachiasmatic nucleus (SCN). Here we describe IGL-afferent connections with cells in the ventral midbrain and hindbrain. Cholera toxin B subunit (CTB) injected into the IGL retrogradely labels neurons in a set of brain nuclei most of which are known to influence visuomotor function. These include the retinorecipient medial, lateral and dorsal terminal nuclei, the nucleus of Darkschewitsch, the oculomotor central gray, the cuneiform, and the lateral dorsal, pedunculopontine, and subpeduncular pontine tegmental nuclei. Intraocular CTB labeled a retinal terminal field in the medial terminal nucleus that extends dorsally into the pararubral nucleus, a location also containing cells projecting to the IGL. Distinct clusters of IGL-afferent neurons are also located in the medial vestibular nucleus. Vestibular projections to the IGL were confirmed by using anterograde tracer injection into the medial vestibular nucleus. Other IGL-afferent neurons are evident in Barrington's nucleus, the dorsal raphe, locus coeruleus, and retrorubral nucleus. Injection of a retrograde, trans-synaptic, viral tracer into the SCN demonstrated transport to cells as far caudal as the vestibular system and, when combined with IGL injection of CTB, confirmed that some in the medial vestibular nucleus polysynaptically project to the SCN and monosynaptically to the IGL, as do cells in other brain regions. The results suggest that the IGL may be part of the circuitry governing visuomotor activity and further indicate that circadian rhythmicity might be influenced by head motion or visual stimuli that affect the vestibular system.

Postnatal development of the noradrenergic system in the dorsal lateral geniculate nucleus of the rat

The noradrenergic innervation of the developing dorsal lateral geniculate nucleus of the rat was examined with light and electron microscopic immunocytochemistry. At birth, few, relatively thick, noradrenergic fibers innervated the nucleus. Their density was steadily increased and they became thinner, tortuous, and varicose with the progression of age. Only a minority (11-15%) of labeled varicosities made synaptic contacts. Most of these synapses were symmetrical and on dendritic shafts. The present findings demonstrate the establishment of the anatomical relationships between noradrenergic afferents and neurons of the dorsal lateral geniculate nucleus during development and may help to understand the role of noradrenaline in the processing of visual information.

Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

Neuronal pathways for the propagation of herpes simplex virus type 1 from one retina to the other in a murine model

Herpetic retinitis in humans is characterized by a high frequency of bilateral localization. In order to determine the possible mechanisms leading to bilateral retinitis, we studied the pathways by which herpes simplex virus type 1 (HSV-1) is propagated from one retina to the other after intravitreal injection in mice. HSV-1 strain SC16 (90 p.f.u.) was injected into the vitreous body of the left eye of BALB/c mice. Animals were sacrificed 1, 2, 3, 4 and 5 days post-inoculation (p.i.). Histological sections were studied by immunochemical staining. Primary retinitis in the inoculated eye (beginning 1 day p.i.) was followed by contralateral retinitis (in the uninoculated eye) starting at 3 days p.i. Infected neurons of central visual pathway nuclei (lateral geniculate nuclei, suprachiasmatic nuclei and pretectal areas) were detected at 4 days p.i. Iris and ciliary body infection was minimal early on, but became extensive thereafter and was accompanied by the infection of connected sympathetic and parasympathetic pathways. The pattern of virus propagation over time suggests that the onset of contralateral retinitis was mediated by local (non-synaptic) transfer in the optic chiasm from infected to uninfected axons of the optic nerves. Later, retinopetal transneuronal propagation of the virus from visual pathways may have contributed to increase the severity of contralateral retinitis.

Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

The cholinergic and histaminergic projections have important neuromodulatory functions in the ascending visual pathways, so we compared the pattern and mode of innervation of the two projections in the lateral geniculate complex (dorsal lateral geniculate nucleus and pregeniculate nucleus) of the macaque monkey. Brain tissue from macaques was immunoreacted by means of antibodies to choline acetyltransferase (ChAT) or to histamine and processed for light and electron microscopy. A dense plexus of thin, highly branched ChAT-immunoreactive axons laden with varicosities was found in all layers of the dLGN including the koniocellular laminae and in the pregeniculate nucleus. ChAT label was more dense in magnocellular layers 1 and 2 than in parvocellular layers 3-6 and relatively sparse in the interlaminar zones. Varicosities associated with the cholinergic axons had an average of three conventional asymmetric synapses per varicosity, and these appeared to contact dendrites of both thalamocortical cells and interneurons. Histamine-immunoreactive axons were distributed homogeneously throughout all laminar and interlaminar zones of the dLGN, but were denser in the pregeniculate nucleus than in the dLGN. Histaminergic axons branched infrequently and were typically larger in caliber than cholinergic axons. The overwhelming majority of varicosities were found en passant and rarely displayed conventional synapses, despite the abundance of synaptic vesicles, and were not associated preferentially with specific cellular structures. The innervation of the macaque dLGN complex by cholinergic and histaminergic systems is consistent with their proposed role in state dependent modulation of thalamic activity. The dense and highly synaptic innervation by cholinergic axons supports the proposal of additional involvement of these axons in functions related to eye movements.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Serotonergic innervation of the lateral geniculate nucleus of the rat during postnatal development: a light and electron microscopic immunocytochemical analysis

The serotonergic innervation of the developing lateral geniculate nucleus of the rat was studied with immunocytochemical techniques at the light and electron microscope levels. A relatively small number of thick serotonergic fibers were observed at the time of birth, distributed more densely in the ventral portion of the nucleus and in the intergeniculate leaflet than in the dorsal lateral geniculate nucleus. By the end of the first postnatal week, this distribution pattern was more clearly established, but the number of immunoreactive fibers was increased. Thereafter, and until the adult pattern was established at the end of the third postnatal week, serotonergic fibers increased further in number and changed morphologically (e.g., they became finer and more ramified with closely spaced varicosities), but their pattern of distribution remained unchanged. Electron microscopical analysis of the dorsal lateral geniculate nucleus revealed that the vast majority of serotonin varicosities formed asymmetrical synapses with dendritic shafts; axosomatic synapses were a feature of the nucleus only at the time of birth. The proportion of serotonin varicosities forming synapses increased gradually from birth to reach a peak at the end of the second postnatal week, then declined markedly in the following week before increasing again at a later stage. It may be speculated that synapses formed during the first two weeks of life may be related to the involvement of serotonin in the morphogenesis of the lateral geniculate nucleus, whereas those formed later in development may be involved in the mediation of neurotransmitter effects.

Lesions of the thalamic intergeniculate leaflet block activity-induced phase shifts in the circadian activity rhythm of the golden hamster

A variety of stimuli, which are associated with acute increases in locomotor activity, induce similar phase-dependent shifts in the circadian rhythm of locomotor activity in golden hamsters. Treatment with the benzodiazepine, triazolam (Tz), or transfer of an animal normally housed without a running wheel to a new cage with 1 h of access to a wheel are both examples of such stimuli. Phase shifts normally induced by injections of Tz can be blocked by lesions of the intergeniculate leaflet (IGL) of the thalamus. Experiments were conducted to determine whether phase shifts induced by transfer to a new cage with a running wheel also require an intact IGL. Animals normally housed without running wheels were transferred to new cages with access to wheels for 1 h a few hours before the expected onset of activity. They then received either lesions of the IGL or sham lesions and, after recovery, they were again transferred to new cages with running wheels for 1 h. Lesions of the IGL blocked phase shifts normally induced by wheel access while sham lesions had no effect. The amount of wheel-running and total locomotor activity which occurred during access to the running wheel were significantly reduced by IGL lesions. These results indicate that the phase-shifting effect of a novel running wheel depends upon an intact IGL.

Phasic stimulation of the locus coeruleus: effects on activity in the lateral geniculate nucleus

Neurons in the locus coeruleus (LC) encode information related to behavioral state in a tonic pattern of firing and information related to the occurrence of a sensory stimulus in a phasic pattern of firing. The effects of phasic stimulation of the LC (6 pulses at 30 Hz), designed to approximate its physiological activation by sensory stimuli, were studied in the lateral geniculate nucleus (LGN) of anesthetized rats. Phasic stimulation of the LC significantly increased neuronal firing in the LGN with a mean latency 320 ms from onset of stimulation. Receiver operating characteristic analyses on a trial-by-trial basis showed that phasic LC stimulation can result in a highly discriminable signal in the LGN. This increased neuronal firing rate in the LGN was specific for the site of stimulation and was reduced by the norepinephrine synthesis inhibitor alpha-methyl-p-tyrosine and by intravenous WB-4101 (alpha 1-receptor antagonist). Neurons in the LGN have a single-spike firing mode when sensory information is faithfully relayed from retina to cortex and a burst-firing mode when the transfer of this information is degraded. Phasic LC stimulation reduced burst firing (2-5 ms interspike intervals, ISIs) at low frequencies (< or = 4 Hz) in the LGN, and for some neurons there was an absolute decrease in burst-like ISIs after LC stimulation, despite an increase in mean firing rate.

Intergeniculate leaflet: an anatomically and functionally distinct subdivision of the lateral geniculate complex

The intergeniculate leaflet (IGL) in the rat is a distinctive subdivision of the lateral geniculate complex that participates in the regulation of circadian function through its projections to the circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The present investigation was undertaken to provide a precise definition of the IGL and a characterization of its neuronal organization including neuronal morphology, chemical phenotype, connections, and synaptic organization. The IGL extends the entire rostrocaudal length of the geniculate complex and contains a distinct population of small to medium neurons. In Golgi preparations, the neurons are multipolar with dendrites largely confined to the IGL. The neurons can be subdivided into three groups on the basis of neurotransmitter content and projections: (1) neurons that contain GABA and neuropeptide Y and project to the SCN; (2) neurons that contain GABA and enkephalin and project to the contralateral IGL; and (3) a small group of neurons that projects to the SCN but not characterized as yet by neurotransmitter content. The IGL receives dense, bilateral input from retinal ganglion cells and dense substance P input of unknown origin. A number of neurons in the anterior hypothalamic area and, particularly, the retrochiasmatic area project to the IGL, and there are sparse projections from brainstem monoamine and cholinergic neurons. The synaptic organization of the IGL is complex with afferents terminating in glomerular complexes that include axoaxonic synaptic interactions. Virtually all IGL afferents synapse upon dendrites and spines, with the densest synaptic input occurring on the distal portions of the dendritic arbor. The organization of the IGL and its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to provide feedback regulation of the SCN pacemaker.

Extent of bilateral collateralization among pontomesencephalic tegmental afferents to dorsal lateral geniculate nuclei of pigmented and albino rats

In adult pigmented and albino rats, small amounts of different fluorescent dyes (Fast Blue and Fluoro-Gold) were pressure-injected into the dorsal lateral geniculate nuclei, each nucleus (right or left) being injected with one dye only. After postinjection survival of three days, the distribution of neurons retrogradely labelled by each dye was analysed. Consistent with previous studies, in each strain each dye labelled a large number of neurons in the several ipsilateral visuotopically or retinotopically organized structures--visual cortices, retino-recipient layers of the superior colliculi and the pretectal nuclei. A substantial number of retrogradely labelled neurons was also found in the contralateral parabigeminal nucleus. A few retrogradely labelled neurons were found in the ipsilateral and (to a lesser extent) contralateral dorsolateral divisions of the periaqueductal gray matter, as well as in the ipsilateral parabigeminal nucleus and the caudal part of the lateral hypothalamus. However, in all the above structures there was a paucity of cells retrogradely labelled with both dyes (double-labelled cells). By contrast, in each strain, several "modulatory" nuclei (containing cholinergic and aminergic cells) of the pontomesencephalic tegmentum--dorsal raphe, pedunculopontine tegmental nucleus, parabrachial nucleus, laterodorsal tegmental nucleus and locus coeruleus--contained significant numbers of cells projecting to both ipsilateral and contralateral dorsal lateral geniculate nuclei. In each nucleus, ipsilaterally and contralaterally projecting cells constituted, respectively, about 65-70% and about 30-35% of retrogradely labelled cells. About 25% of the contralaterally projecting cells (i.e. about 5-10% of all retrogradely labelled tegmental neurons) were double-labelled with both dyes. Double-labelled cells were intermingled with single-labelled cells projecting ipsilaterally or contralaterally. The proportions of the ipsilaterally, contralaterally and bilaterally projecting neurons in the modulatory components of the pontomesencephalic tegmentum were virtually identical in pigmented and albino strains. It appears that in both strains the visuotopically organized structures convey to the dorsal lateral geniculate nuclei information related mainly to the contralateral visual field. The projections from these structures might play an important role in regulating transmission of visual information in the retinotopically distinct parts of each dorsal lateral geniculate nucleus. By contrast, the projections from the modulatory nuclei of the pontomesencephalic tegmentum are likely to contribute to the functional synchronization of both dorsal lateral geniculate nuclei during the sleep-wakefulness cycle and saccadic eye movements.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 3. Selective detection of noradrenaline efflux in the locus coeruleus

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor 'real time' endogenous noradrenaline (NA) efflux in superfused slices of rat locus coeruleus (LC) following local electrical stimulation. When stimulated with a standard train (30 pulses, 100 Hz, 0.2 ms, 10 mA, every 5 min), efflux of monoamine was constant over the experimental period (2.5 h): Amine efflux declined by only 16 +/- 5% while uptake half-life lengthened by only 9 +/- 8%. When calibrated in solutions of NA, peak amine efflux corresponded to 0.31 +/- 0.04 microM (mean +/- S.E.M., n = 28) and was removed by uptake with a half-life of 2.93 +/- 0.28 s (n = 16). The released compound was confirmed as NA on the basis of pharmacological and electrochemical criteria. Stimulated monoamine efflux was reversibly reduced by 78% by omission of Ca2+ from the superfusate for 30 min (P < 0.05). Ro 4-1284 (1 microM), a fast-acting reserpine-like drug, decreased amine efflux by 86% (P < 0.05). The monoamine oxidase inhibitor pargyline (2 microM) increased efflux by 30% (P < 0.05). Desipramine (0.05 microM), a selective NA uptake blocker, significantly increased amine efflux (by 96%, P < 0.05) and uptake half-life (by 314%, P < 0.05). Fluvoxamine (0.5 microM), the selective serotonin (5HT) uptake blocker, increased efflux by 59% (P < 0.05) and the uptake half-life by 122% (P < 0.05). Vanoxerine (GBR 12909: 0.3 microM), the dopamine (DA) uptake blocker, had no effect on amine efflux or uptake half-life. The voltammogram of the released amine had single oxidation and reduction peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of calretinin in the rat lateral geniculate nucleus and its retino-geniculate projection

In the present study, we examined the distribution of calretinin-immunoreactive neuronal cell bodies and fibers in the lateral geniculate nucleus of the rat. In normal rats, clusters of immunoreactive cell bodies were found in: (i) the rostral portion of the ventral lateral geniculate nucleus pars medialis (VLGM), (ii) the intergeniculate leaflet (IGL), (iii) the intermediate region between the VLGM and the ventral lateral geniculate nucleus pars lateralis (VLGL), (iv) the caudomedial portion of the VLGM, and (v) the caudolateral portion of the VLGM. In the dorsal lateral geniculate nucleus (DLG), immunoreactive cell bodies were rarely observed. After uni- or bilateral eye enucleation, no significant alteration in the morphological features or distribution of immunoreactive cell bodies was detected in the lateral geniculate nucleus. In normal rats, immunoreactive fibers formed dense plexuses in: (i) the DLG, (ii) the external layer of the VLGL, (iii) the internal layer of the VLGL, (iv) the IGL, (v) the caudomedial portion of the VLGM, and (vi) the optic tract. After unilateral eye enucleation, immunoreactive fibers in the external layer of the VLG and in the optic tract almost totally disappeared on the contralateral side to the lesion. Unilateral eye enucleation caused a significant decrease of immunoreactive fibers in the DLG and in the internal layer of the VLGL, but a substantial number of immunoreactive fibers still remained there. In the IGL and the caudomedial portion of the VLGM, no observable alteration in the distribution of immunoreactive fibers was detected after uni- or bilateral eye enucleation.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic innervation of the thalamic reticular nucleus: a light and electron microscopic immunohistochemical study in rats

Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.

An immunocytochemical analysis of the lateral geniculate complex in the pigeon (Columba livia)

The lateral geniculate complex (GL) of pigeons was investigated with respect to its immunohistochemical characteristics, retinal afferents, and the putative transmitters/modulators of its neurons. The distributions of serotonin-, choline acetyltransferase-, glutamic acid decarboxylase-, tyrosine hydroxylase-, neuropeptide Y- (NPY), substance P- (SP), neurotensin- (NT), cholecystokinin- (CCK), and leucine-enkephalin- (L-ENK) like immunoreactive perikarya and fibers were mapped. Retinal projections were studied following injections of Rhodamine-B-isothiocyanate into the vitreous. Transmitter-specific projections onto the visual Wulst and the optic tectum were studied by simultaneous double-labelling of retrograde tracer molecules and immunocytochemical labelling. The GL can be divided into three major subdivisions, the n. geniculatus lateralis, pars dorsalis (GLd; previously designated as the n. opticus principalis thalami, OPT), the n. marginalis tractus optici (nMOT), and the n. geniculatus lateralis, pars ventralis (GLv). All three subdivisions are retinorecipient. The GLd can be further subdivided into at least five components differing in their immunohistochemical characteristics: n. lateralis anterior (LA); n. dorsolateralis anterior thalami, pars lateralis (DLL), n. dorsolateralis anterior thalami, pars magnocellularis (DLAmc); n. lateralis dorsalis nuclei optici principalis thalami (LdOPT); and n. suprarotundus (SpRt). The LdOPT consists of an area of dense CCK-like and NT-like terminals of probable retinal origin. Three subnuclei (DLL, DLAmc, SpRt) were shown to project to the visual Wulst. Cholinergic and cholecystokinergic relay neurons participated in this projection. The nMOT occupies a position between the GLd and GLv and encircles the rostral pole of n. rotundus and the LA. It is characterized mainly by medium sized NPY-like perikarya which were shown to project onto the ipsilateral optic tectum. Bands of NPY-like fibers in the tectal layers 2, 4, and 7 could at least in part be due to this projection of the nMOT. Most of the antisera used revealed transmitter/modulator-specific fiber systems in the GLv which often showed a layer-specific distribution. Perikaryal labelling was only obtained with glutamic acid decarboxylase. On the basis of its chemoarchitectonics, topography, and connectional pattern, the GLd complex of pigeons is most directly equivalent to the mammalian GLd. However, although the different subdivisions of the avian GLd may represent functionally different channels within the thalamofugal pathway similar to the lamina-specific differentiation within the mammalian geniculostriate projection, direct comparison of subnuclei of birds and mammals is not justified at this time. The nMOT appears similar to the intergeniculate leaflet (IGL) and the avian GLv clearly corresponds in many features to the mammalian GLv.

Distribution and origin of noradrenergic and serotonergic fibers in the cochlear nucleus and inferior colliculus of the rat

We examined the monoaminergic innervation of the rat cochlear nucleus (CN) and the inferior colliculus (IC) by using retrograde transport of the fluorescent dye Fluoro-Gold combined with immunohistochemistry. We used antisera against the catecholamine synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT), and one against the transmitter serotonin (5-HT). Each substance revealed a distinct pattern of immunoreactive staining in the CN and the IC. In the CN, DBH-immunoreactive (-ir) fibers were present in all subnuclei. The molecular layer of the dorsal CN and the granular layer of the ventral CN, however, were largely devoid of DBH-ir fibers. In contrast, 5-HT-ir fibers were abundant in the molecular layer and the granular cell layer of the CN. In the dorsal CN and the postero- and anteroventral CN, however, this innervation was less dense and evenly distributed across subnuclei. In the IC, the DBH-ir fibers were slightly more numerous in layer 2 of the dorsal cortex than in other subnuclei, while the layer 1 of both the dorsal and the external cortex contained only a few fibers. In contrast, the 5-HT-ir fibers formed a dense network in both the dorsal and external cortices of the IC, while they were less abundant in the remaining subnuclei. PNMT-ir fibers were not found in any of the auditory brainstem nuclei. Following Fluoro-Gold injections into the CN or IC, retrogradely labeled DBH-ir neurons were found in the A6 noradrenergic cell group (locus coeruleus). The CN received additional projections from the A5 noradrenergic cell group, as well as sparse projections from the A4 and A7 cell groups. The serotonergic innervation of the CN and IC originated largely in the B7 serotonergic cell group (dorsal raphe nucleus). Serotonergic neurons in other groups of the raphe nuclei were only occasionally labeled. Our data indicate that both noradrenaline and serotonin may play a role in central auditory processing. Their differential distribution in the IC and CN subnuclei suggests that these transmitter systems might influence different functional circuits.

Luminance coding properties of intergeniculate leaflet neurons in the golden hamster and the effects of chronic clorgyline

Cells in the intergeniculate leaflet (IGL) project to the suprachiasmatic nuclei, a mammalian circadian pacemaker. Chronic treatment with clorgyline alters hamster circadian rhythms in ways similar to alterations seen after ablation of the IGL. Chronic clorgyline also alters the light intensity dependence of phase-shifting. In this study luminance coding properties of IGL cells were measured in control hamsters and in hamsters chronically treated with clorgyline. In control animals three patterns of response to increasing and decreasing luminance were observed. Type I cells showed a monotonic pattern. Type II cells were similar to Type I with additional increases in firing rate at several specific luminance levels. Type III cells only coded increases in luminance. Cells from clorgyline-treated animals did not differ from those from control animals in the pattern of luminance response but IGL cells from these animals showed decreased firing rate in both light and dark conditions. These results suggest that the effects of clorgyline on the photic sensitivity of circadian rhythms may be related to a clorgyline-induced decrease in firing rate of IGL cells. They also indicate that some IGL cells show complex patterns of response to luminance changes in addition to those showing simple monotonic responses.

The mode of axonal regeneration of locus coeruleus neurons in the lateral geniculate nucleus following neonatal 6-hydroxydopamine treatment

The effects of neonatal 6-hydroxydopamine (6-OHDA) treatment on the projection of locus coeruleus (LC) neurons to the dorsal part of the lateral geniculate nucleus (dLGN) were examined using histofluorescence and electrophysiological methods. Histofluorescence studies confirmed the previous results of Gustafson and Moore (1987, Dev. Brain Res. 37: 143-155) that catecholamine fibers in the cerebral cortex almost completely disappeared whereas the density of catecholamine fibers in the dLGN increased in the 6-OHDA-treated rats. To quantify the density of the LC projections, we assessed the percentage of LC neurons activated antidromically (projection index, P-index) from the cortex and dLGN. In the 6-OHDA-treated rats, the P-indices for the frontal and visual cortex were greatly reduced, whereas those for the dLGN were increased to twice the control values. In the control rats, about 50% of the LC neurons exhibited antidromic latencies from the dLGN of more than 50 ms, whereas all latencies obtained in the 6-OHDA-treated rats were shorter than 50 ms. These results suggest two types of axonal regeneration of LC neurons: (i) LC neurons projecting to the cerebral cortex lose their distal axonal arborization in the cortex and project alternatively to more proximal sites in the dLGN with regenerating axons; (ii) LC axons projecting to the dLGN with a long trajectory are lost, and regeneration occurs with axons of a shorter trajectory.

Development of substance P, Leu-enkephalin and serotonin profiles in the lateral geniculate nuclear complex of albino rat

Immunohistochemical studies for analysing the development of the profile of two peptides--substance P (SP) and Leu-enkephalin (Leu-ENK), and serotonin (SER)--have been conducted on the lateral geniculate nuclear (LGN) complex of albino rats at gestation day 18 and various postnatal age periods. SP immunoreactivity is found to increase from 1 day postnatal (DPN) up to 20 DPN and decrease thereafter, whereas the SER and Leu-ENK-immunoreactive fibres and terminals seen as occasional fibres at 1, 5, and 10 DPN are better visualized from 20 DPN and gradually increase up to 40 DPN. The possible role and significance of the changes seen in these putative neurotransmitters/neuromodulators with development are discussed.

An ultrastructural and morphometric study of the effect of removal of retinal input on the development of the dorsal lateral geniculate nucleus

In normal development, cell layers in the dorsal lateral geniculate nucleus (dLGN) segregate from a relatively homogeneous cell group. If all retinal input is removed prior to this segregation, the layers fail to form. In the present study, we used ultrastructural and morphometric analyses to study dLGN development in the tree shrew following neonatal removal of retinal input. The goal of the present study was to determine whether there are differences between normal animals and enucleates in the development of dLGN cells and their interrelationships with each other and/or with the surrounding glia, which might explain the failure of cellular lamination in enucleated animals. The results indicate that although the development in enucleated animals may take place somewhat more slowly, by P90 cell size and density are not significantly different from normal. These results, coupled with the observation that the dLGN in enucleates is smaller than in normals, suggest that the removal of retinal input results in dLGN cell loss. At both the light and electron microscopic level, cells in the developing normal dLGN are arranged in bands of immediately adjacent cells. In enucleates, dLGN cells are less frequently in immediate contact and are arranged in small groups or clumps which may be separated by degenerating cells. The present data suggest that the presence of retinal input may be necessary to allow dLGN cells to maintain the intercellular relationships necessary for laminar segregation to take place.

Distribution and synaptic organization of serotoninergic and noradrenergic axons in the lateral geniculate nucleus of the rat

Antisera raised against the monoamines serotonin (5-HT) and noradrenaline (NA) were employed in a study designed to provide a detailed description of the distribution, morphology, and synaptic organization of the serotoninergic and noradrenergic afferents in the lateral geniculate nucleus (LGN) of the rat. The distribution patterns of the two types of immunoreactive fibers were distinct and largely complementary to each other. NA axons were particularly concentrated in the dorsal lateral geniculate nucleus (LGd), with the ventral lateral geniculate nucleus (LGv) and the intergeniculate leaflet (IGL) receiving substantially fewer fibers. In contrast, 5-HT axons, although present throughout the LGN, were preferentially concentrated in the LGv and IGL. 5-HT and NA axon terminals and axonal varicosities, examined in single and serial ultrathin sections, formed conventional synapses in the extraglomerular neuropil. The types of synapses and the nature of the postsynaptic targets were different for the two monoamines. 5-HT afferents formed asymmetrical synapses on dendritic spines and shafts of both presumptive relay cells and interneurons but established symmetrical synapses on cell bodies. However, NA afferents formed almost exclusively symmetrical synapses on dendritic spines and shafts and made no contacts with cell bodies. The present findings suggest that the 5-HT and NA afferents of the rat LGN, which are likely to influence certain stages of visual processing, exhibit distinct organizational principles and act at restricted sites as do other classical neurotransmitter systems.

Distribution and synaptic organization of dopaminergic axons in the lateral geniculate nucleus of the rat

In the present study, immunocytochemistry with an antiserum against dopamine (DA) revealed hitherto unknown terminal fields of DA axons in the lateral geniculate nucleus (LGN) of the rat. The innervation of all subdivisions of the LGN is achieved by a common set of afferent fibers that branch to form terminal fields of uneven density. The ventral lateral geniculate nucleus (LGv) receives slightly more DA axons than the dorsal lateral geniculate nucleus (LGd), whereas within the latter, DA afferents innervate the lateral part of the nucleus slightly more densely. Labeled axon terminals and varicosities, examined in single and serial ultrathin sections, were found in the extraglomerular neuropil in the LGd and in the neuropil of the LGv characterized by relatively simple synaptic relationships. They formed predominantly asymmetrical synaptic contacts with dendritic profiles. Occasionally, the postsynaptic elements were found to be presynaptic dendrites of presumptive interneurons. Some of the possible roles of this newly demonstrated DA afferent system in the physiology of the LGN and in the pathophysiology of diseases associated with impairment of dopaminergic activity are discussed.

Electrophysiological evidences of a bidirectional communication between the locus coeruleus and the suprachiasmatic nucleus

To assess a possible relation between the suprachiasmatic nucleus (SCN) and the locus coeruleus (LC), a study with evoked potentials was performed in the rat. An evoked potential was recorded in the SCN area after electrical stimulation of the LC. Also, an evoked potential was recorded in the LC after electrical stimulation of the SCN area. The results indicate specificity of the projecting regions and of the activated regions, suggesting the existence of a possible bidirectional communication between the LC and the SCN. The pathways possibly involved are discussed.

Hamster circadian rhythms are phase-shifted by electrical stimulation of the geniculo-hypothalamic tract

The suprachiasmatic nuclei (SCN) contain the major pacemaker for mammalian circadian rhythms. The SCN receive photic input both directly, via the retinohypothalamic tract (RHT), and indirectly, via the geniculohypothalamic tract (GHT), which originates in cells in the intergeniculate leaflet (IGL) and anterior portions of the ventral lateral geniculate nucleus (vLGN). We tested whether electrical stimulation of the GHT would induce phase shifts in wheel-running activity rhythms of Syrian hamsters housed in continuous darkness or continuous illumination. In both lighting conditions, electrical stimulation of the GHT induced mainly phase advances when given during the late subjective day and small phase delays when given during the late subjective night and early subjective day. Stimulation in the thalamus outside the GHT failed to produce similar phase shifts. Repeated daily stimulation had only a weak entraining effect on the activity rhythm. Activation of GHT neurons appears to influence the pacemaker for activity rhythms in a phase-dependent manner.

The organization of the rat lateral geniculate body by immunohistochemical analysis of neuroactive substances

The organization of neuroactive substances in the rat lateral geniculate body (LGB) was studied with available immunohistochemical stainings. In the dorsal lateral geniculate nucleus (DLG), there existed only gamma-aminobutyric acid (GABA)-like immunoreactive neurons. Immunoreactive fiber plexuses for substance P (SP), cholecystokinin-8 (CCK) and vasoactive intestinal polypeptide (VIP) were present in the lateral margin of the DLG, just beneath the optic tract. There were immunoreactive neurons and fibers for GABA, SP, leucine-enkephalin (ENK) and neuropeptide Y (NPY) in the intergeniculate leaflet (IGL). ENK-, NPY- and SP-like immunoreactive neurons in the IGL were mainly medium-sized, and bipolar or spindle-shaped with a few dendrites oriented dorsoventrally. In the IGL, use of double-labeled immunofluorescence demonstrated that a few neurons exhibited both ENK- and SP-like immunoreactivities, and a few neurons had both GABA- and ENK-like immunoreactivities. Although the morphology of ENK-like immunoreactive neurons resembled to NPY-like immunoreactive neurons, both neurons were clearly different neurons. Many GABA-, ENK- and SP-like immunoreactive neurons and fibers were found in the ventral lateral geniculate nucleus (VLG). These immunoreactive neurons were mainly medium-sized, and bipolar in shape, while a few immunoreactive neurons were of multipolar shape. Neurons containing ENK and fibers containing SP mainly existed in the lateral half of the parvocellular part and in the medial half of magnocellular part of the VLG. In this region, about one-third of the GABA-like immunoreactive neurons contained ENK-like immunoreactivity. Many SP neurons mainly existed in the medial half of the parvocellular part of the VLG. CCK- and VIP-like immunoreactive fibers were present in the lateral half of the magnocellular part of the VLG. Immunoreactive fibers for calcitonin gene-related peptide, corticotropin-releasing factor, neurotensin and tyrosine hydroxylase were disseminated throughout the LGB. The subdivisions of the LGB were discussed, based upon the distribution of neuroactive substances.

Adaptive changes in the central noradrenergic system in monocular deprived rats

Noradrenaline and dopamine content, K+-stimulated release of 3H-noradrenaline, 3H-noradrenaline high-affinity uptake, 3H-prazosin and 3H-rauwolscine binding to alpha 1- and alpha 2-adrenoreceptors, respectively, were studied in isolated visual structures (visual cortex, superior colliculus, lateral geniculate nucleus) of three-month-old rats monocularly deprived by unilateral eyelid closure at postnatal day 11. Monocular deprivation resulted in a number of changes in comparison with normal littermates, mainly in the lateral geniculate nucleus, both ipsilateral and contralateral to the closed eye. K+-stimulated release of noradrenaline and high-affinity noradrenaline uptake were significantly increased, 3H-rauwolscine binding to alpha 2-adrenergic receptor sites was decreased in the contralateral and ipsilateral lateral geniculate nucleus. 3H-Prazosin binding to alpha 1-adrenergic receptors as well as the levels or noradrenaline and dopamine in the lateral geniculate nucleus were unaffected by monocular deprivation. The data support the suggestion that the lateral geniculate nucleus plays a key role in modulating visual information processing and that the lack of an adequate visual input within the first weeks of life interferes mainly with the development of the noradrenergic system at the level of the lateral geniculate nucleus.

The modulation of the retinal relay to the cortex in the dorsal lateral geniculate nucleus

The translation of the retinal input through the dorsal lateral geniculate nucleus (dLGN) to the visual cortex is highly dependent on a range of influences. This article reviews the available evidence. One of the influences, the corticofugal projection to the dLGN from layer VI of the visual cortex, provides a synaptic input which in magnitude exceeds that from the retina. This makes direct synaptic contact on relay cells and the intrinsic and perigeniculate inhibitory interneurones influencing their activity. The corticofugal system appears to be spatially organised in such a way that for any given region in the dLGN, there is a central zone comprising an overlying field with facilitatory effect, and a surrounding zone with inhibitory influence. The extent to which these overlap is open to question at present. The inhibitory effect of the corticofugal projection can be clearly seen in its contribution to the length tuning of dLGN cells when tested with drifting bars. On average dLGN cells exhibit a very high degree of length tuning, matching that of cortical hypercomplex cells. Removal of the corticofugal influence causes a radical reduction in this, shifting the mean reduction in peak response with increasing bar length from 71% to 43%. One consequence of this corticofugal effect is that the selectivity of the dLGN cell receptive field towards stimuli spatially restricted to the vicinity of the centre mechanism, is as good for moving bars as it is for stationary flashing spots. The retinal output to dLGN relay cells appears to be mediated by excitatory amino acid receptors, of both NMDA and non-NMDA categories. The non-NMDA receptors appear to provide an initial level of depolarisation which enables the operation of the voltage dependent NMDA receptor channels. The NMDA receptor however sits as a critical gate regulating the transmission of retinal information in the dLGN, when it is blocked visual responses are virtually eliminated. Its voltage dependency makes it crucially dependent on the complex pattern of excitatory and inhibitory influences from the cortex and the "non-specific" modulatory influence of the cholinergic system.