Subcortical projections to lateral geniculate and thalamic reticular nuclei in the hooded rat

Spiking activity in the visual thalamus is coupled to pupil dynamics across temporal scales

The processing of sensory information, even at early stages, is influenced by the internal state of the animal. Internal states, such as arousal, are often characterized by relating neural activity to a single "level" of arousal, defined by a behavioral indicator such as pupil size. In this study, we expand the understanding of arousal-related modulations in sensory systems by uncovering multiple timescales of pupil dynamics and their relationship to neural activity. Specifically, we observed a robust coupling between spiking activity in the mouse dorsolateral geniculate nucleus (dLGN) of the thalamus and pupil dynamics across timescales spanning a few seconds to several minutes. Throughout all these timescales, 2 distinct spiking modes-individual tonic spikes and tightly clustered bursts of spikes-preferred opposite phases of pupil dynamics. This multi-scale coupling reveals modulations distinct from those captured by pupil size per se, locomotion, and eye movements. Furthermore, coupling persisted even during viewing of a naturalistic movie, where it contributed to differences in the encoding of visual information. We conclude that dLGN spiking activity is under the simultaneous influence of multiple arousal-related processes associated with pupil dynamics occurring over a broad range of timescales.

Transport of Prions in the Peripheral Nervous System: Pathways, Cell Types, and Mechanisms

Prion diseases are transmissible protein misfolding disorders that occur in animals and humans where the endogenous prion protein, PrP^C, undergoes a conformational change into self-templating aggregates termed PrP^Sc. Formation of PrP^Sc in the central nervous system (CNS) leads to gliosis, spongiosis, and cellular dysfunction that ultimately results in the death of the host. The spread of prions from peripheral inoculation sites to CNS structures occurs through neuroanatomical networks. While it has been established that endogenous PrP^C is necessary for prion formation, and that the rate of prion spread is consistent with slow axonal transport, the mechanistic details of PrP^Sc transport remain elusive. Current research endeavors are primarily focused on the cellular mechanisms of prion transport associated with axons. This includes elucidating specific cell types involved, subcellular machinery, and potential cofactors present during this process.

Not a one-trick pony: Diverse connectivity and functions of the rodent lateral geniculate complex

Often mislabeled as a simple relay of sensory information, the thalamus is a complicated structure with diverse functions. This diversity is exemplified by roles visual thalamus plays in processing and transmitting light-derived stimuli. Such light-derived signals are transmitted to the thalamus by retinal ganglion cells (RGCs), the sole projection neurons of the retina. Axons from RGCs innervate more than ten distinct nuclei within thalamus, including those of the lateral geniculate complex. Nuclei within the lateral geniculate complex of nocturnal rodents, which include the dorsal lateral geniculate nucleus (dLGN), ventral lateral geniculate nucleus (vLGN), and intergeniculate leaflet (IGL), are each densely innervated by retinal projections, yet, exhibit distinct cytoarchitecture and connectivity. These features suggest that each nucleus within this complex plays a unique role in processing and transmitting light-derived signals. Here, we review the diverse cytoarchitecture and connectivity of these nuclei in nocturnal rodents, in an effort to highlight roles for dLGN in vision and for vLGN and IGL in visuomotor, vestibular, ocular, and circadian function.

Altered functional connectivity in lesional peduncular hallucinosis with REM sleep behavior disorder

Brainstem lesions causing peduncular hallucinosis (PH) produce vivid visual hallucinations occasionally accompanied by sleep disorders. Overlapping brainstem regions modulate visual pathways and REM sleep functions via gating of thalamocortical networks. A 66-year-old man with paroxysmal atrial fibrillation developed abrupt-onset complex visual hallucinations with preserved insight and violent dream enactment behavior. Brain MRI showed restricted diffusion in the left rostrodorsal pons suggestive of an acute ischemic stroke. REM sleep behavior disorder (RBD) was diagnosed on polysomnography. We investigated the integrity of ponto-geniculate-occipital circuits with seed-based resting-state functional connectivity MRI (rs-fcMRI) in this patient compared to 46 controls. Rs-fcMRI revealed significantly reduced functional connectivity between the lesion and lateral geniculate nuclei (LGN), and between LGN and visual association cortex compared to controls. Conversely, functional connectivity between brainstem and visual association cortex, and between visual association cortex and prefrontal cortex (PFC) was significantly increased in the patient. Focal damage to the rostrodorsal pons is sufficient to cause RBD and PH in humans, suggesting an overlapping mechanism in both syndromes. This lesion produced a pattern of altered functional connectivity consistent with disrupted visual cortex connectivity via de-afferentation of thalamocortical pathways.

Light-induced responses of slow oscillatory neurons of the rat olivary pretectal nucleus

Background

The olivary pretectal nucleus (OPN) is a small midbrain structure responsible for pupil constriction in response to eye illumination. Previous electrophysiological studies have shown that OPN neurons code light intensity levels and therefore are called luminance detectors. Recently, we described an additional population of OPN neurons, characterized by a slow rhythmic pattern of action potentials in light-on conditions. Rhythmic patterns generated by these cells last for a period of approximately 2 minutes.

Methodology

To answer whether oscillatory OPN cells are light responsive and whether oscillatory activity depends on retinal afferents, we performed in vivo electrophysiology experiments on urethane anaesthetized Wistar rats. Extracellular recordings were combined with changes in light conditions (light-dark-light transitions), brief light stimulations of the contralateral eye (diverse illuminances) or intraocular injections of tetrodotoxin (TTX).

Conclusions

We found that oscillatory neurons were able to fire rhythmically in darkness and were responsive to eye illumination in a manner resembling that of luminance detectors. Their firing rate increased together with the strength of the light stimulation. In addition, during the train of light pulses, we observed two profiles of responses: oscillation-preserving and oscillation-disrupting, which occurred during low- and high-illuminance stimuli presentation respectively. Moreover, we have shown that contralateral retina inactivation eliminated oscillation and significantly reduced the firing rate of oscillatory cells. These results suggest that contralateral retinal innervation is crucial for the generation of an oscillatory pattern in addition to its role in driving responses to visual stimuli.

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Neurons expressing relaxin 3/INSL 7 in the nucleus incertus respond to stress

Relaxin 3/INSL 7 has recently been identified as a new member of the insulin/relaxin superfamily. Although it was reported to be dominantly expressed in the brain, its detailed distribution and function in the central nervous system are still obscure. In the present study we demonstrated that in the rat relaxin 3 was mainly expressed in neurons of the nucleus incertus (NI) of the median dorsal tegmental pons. Other relaxin 3-expressing neurons were scattered in the pontine raphe nucleus, the periaqueductal gray and dorsal area to the substantia nigra in the midbrain reticular formation. Relaxin 3-immunoreactive fibers projected particularly densely in the septum, hippocampus, lateral hypothalamus and intergeniculate leaflet of the thalamus. Ultrastructural examination revealed that relaxin 3 was localized in the dense-cored vesicles in the perikarya and was also observed in the synaptic terminals of axons. As almost all relaxin 3-containing neurons express corticotropin-releasing factor (CRF) type 1 receptor in the NI, we examined the response of relaxin 3 neurons to intracerebroventricular administration of CRF; 65% of relaxin 3 neurons expressed c-Fos 2 h after intracerebroventricular administration of 1 microg CRF. We then confirmed that c-Fos was induced in 60% of relaxin 3 neurons in the NI and the expression of relaxin 3 mRNA increased significantly in the NI after water-restraint stress. Collectively, these results suggest that relaxin 3 produced in the NI is released from nerve endings and is involved in the regulation of the stress response.

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.

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

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

Changes of immunocytochemical localization of vesicular glutamate transporters in the rat visual system after the retinofugal denervation

To clarify which vesicular glutamate transporter (VGluT) is used by excitatory axon terminals of the retinofugal system, we examined immunoreactivities and mRNA signals for VGluT1 and VGluT2 in the rat retina and compared immunoreactivities for VGluT1 and VGluT2 in the retinorecipient regions using double immunofluorescence method, anterograde tracing, and immunoelectron microscopy. Furthermore, the changes of VGluT1 and VGluT2 immunoreactivities were studied after eyeball enucleation. Intense immunoreactivity and mRNA signal for VGluT2, but not for VGluT1 immunoreactivity, were observed in most perikarya of ganglion cells in the retina. Immunoelectron microscopy revealed that VGluT1- and VGluT2-immunolabeled terminals made asymmetrical synapses, suggesting that they were excitatory synapses, and that VGluT1-immunolabeled terminals were smaller than VGluT2-labeled ones in many retinorecipient regions, such as the dorsal lateral geniculate nucleus (LGd) and superior colliculus (SC). Double immunofluorescence study further revealed that almost no VGluT2 immunoreactivity was colocalized with VGluT1 in the retinorecipient regions. After wheat germ agglutinin (WGA) injection into the eyeballs, WGA immunoreactivity was colocalized in the single axon terminals of LGd and SC with VGluT2 but not VGluT1 immunoreactivity. After unilateral enucleation, VGluT2 immunoreactivity in the LGd, SC, nucleus of the optic tract, and nuclei of the accessory optic tract in the contralateral side of the enucleated eye was clearly decreased. Although only a small change of VGluT2 immunoreactivity was observed in the contra- and ipsilateral suprachiasmatic nuclei, olivary pretectal nucleus, anterior pretectal nucleus, and posterior pretectal nucleus, moderate reduction of VGluT2 was found in these regions after bilateral enucleation. On the other hand, almost no change in VGluT1 immunoreactivity was found in the structures examined in the present enucleation study. Thus, the present results support the notion that the retinofugal pathways are glutamatergic, and indicate that VGluT2, but not VGluT1, is employed for accumulating glutamate into synaptic vesicles of retinofugal axons.

Cytoarchitecture and efferent projections of the nucleus incertus of the rat

The nucleus incertus is located caudal to the dorsal raphe and medial to the dorsal tegmentum. It is composed of a pars compacta and a pars dissipata and contains acetylcholinesterase, glutamic acid decarboxylase, and cholecystokinin-positive somata. In the present study, anterograde tracer injections in the nucleus incertus resulted in terminal-like labeling in the perirhinal cortex and the dorsal endopyriform nucleus, the hippocampus, the medial septum diagonal band complex, lateral and triangular septum medial amygdala, the intralaminar thalamic nuclei, and the lateral habenula. The hypothalamus contained dense plexuses of fibers in the medial forebrain bundle that spread in nearly all nuclei. Labeling in the suprachiasmatic nucleus filled specifically the ventral half. In the midbrain, labeled fibers were observed in the interpeduncular nuclei, ventral tegmental area, periaqueductal gray, superior colliculus, pericentral inferior colliculus, pretectal area, the raphe nuclei, and the nucleus reticularis pontis oralis. Retrograde tracer injections were made in areas reached by anterogradely labeled fibers including the medial prefrontal cortex, hippocampus, amygdala, habenula, nucleus reuniens, superior colliculus, periaqueductal gray, and interpeduncular nuclei. All these injections gave rise to retrograde labeling in the nucleus incertus but not in the dorsal tegmental nucleus. These data led us to conclude that there is a system of ascending projections arising from the nucleus incertus to the median raphe, mammillary complex, hypothalamus, lateral habenula, nucleus reuniens, amygdala, entorhinal cortex, medial septum, and hippocampus. Many of the targets of the nucleus incertus were involved in arousal mechanisms including the synchronization and desynchronization of the theta rhythm.

Quantitative age-related changes in NADPH-diaphorase-positive neurons in the ventral lateral geniculate nucleus

Age-related changes in nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) were examined in the rat ventral lateral geniculate nucleus (vLGN) using histochemical methods. Eighteen rats aged 3, 24, and 26 months were studied using quantitative methods to investigate the number of neurons per mm(2), the cross-sectional area, and the orientation of dendritic processes of NADPH-d-positive neurons. We have described three types of neurons: types A and B are both located in the lateral and medial vLGN (vLGN-l and vLGN-m, respectively), and type C neurons over the optic tract. The number of NADPH-d-positive neurons was significantly reduced in the old rats (-39%) when compared with controls (3-month-old rats). The quantitative analysis of cell areas revealed a significant decrease of somatic size in type B neurons, both in the lateral and medial vLGN, and in C neurons; however, type A neurons did not show significant changes. By quantifying the orientation of dendritic processes, we observed a predominant dorsolateral orientation in type A and B neurons. During aging, there are no changes in the dendritic orientation of neurons located in the vLGN-m; however, vLGN-l neurons show an increase in dendritic processes with dorsoventral orientation. In type C neurons, our results show that 87.4% of dendritic processes are lateromedially oriented at 26 months old. Therefore, the types A and B neurons behave differently during senescence. Type A neurons do not change in size, but those located in the vLGN-l modify the orientation of their dendritic processes; however, type B neurons, reduce their size and those located in the vLGN-l also modify their dendritic process orientation. Finally, the type C neurons modify their size and dendritic process.

Physiological Characterization of Pretectal Neurons Projecting to the Láteral Geniculate Nucleus in the Cat

Single neurons in the pretectal nucleus of the optic tract and posterior pretectal nucleus were extracellularly recorded in anaesthetized cats and tested for antidromic activation after electrical stimulation of the ipsilateral dorsal lateral geniculate nucleus. Cells were further characterized by their response latencies to electrical stimulation of the optic nerve head and the optic chiasm, and by responses to various visual stimuli. 46 out of 188 neurons (24%) were antidromically activated from the lateral geniculate nucleus at response latencies of 0.6 - 2.6 ms. They had low spontaneous activities and preferred fast-moving visual stimuli. 29 of the antidromically activated neurons (63%) could be activated from the optic chiasm with response latencies of 4 - 10 ms. Together with the mean conduction time of 0.8 ms between the optic nerve head and the optic chiasm, this indicates that they receive an indirect retinal input via fast-conducting Y-fibres. Sometimes antidromically activated neurons spontaneously showed irregular burst activity. During unidirectional stimulation with a large moving visual stimulus, burst activity became more regular, and interburst intervals and the duration of single bursts decreased. After the stimulus was stopped, interburst intervals slowly increased until prestimulation activity was restored. The response properties of these neurons could reflect the transfer of saccade-related visual as well as oculomotor signals through the pretectum to the visual thalamus.

Expression of messenger RNAs for glutamic acid decarboxylase, preprotachykinin, cholecystokinin, somatostatin, proenkephalin and neuropeptide Y in the adult rat superior colliculus

The mammalian superior colliculus is an important subcortical integrator of sensorimotor behaviours. It is multi-layered, each layer containing specific neuronal types and possessing distinct input/output relationships. Here we use in situ hybridisation methods to map the distribution of seven neurotransmitters/neuromodulator systems in adult rat superior colliculus. Coronal sections were probed for preprotachykinin, cholecystokinin, somatostatin, proenkephalin, neuropeptide Y and the enzymes glutamic acid decarboxylase and choline acetyltransferase, markers for GABA and acetylcholine respectively. Cells expressing glutamic acid decarboxylase messenger RNA were the most abundant, the highest density being found in the superficial layers. Many cells containing proprotachykinin messenger RNA were found in stratum zonale and the upper two-thirds of stratum griseum superficiale; cells were also located in deeper tectal laminae, particularly caudomedially. Most cholecystokinin messenger RNA expressing cells were located in the superficial layers with a prominent band in the middle third of stratum griseum superficiale. Cells expressing moderate to high levels of somatostatin messenger RNA formed a dense band in the lower third of stratum griseum superficiale/upper stratum opticum; two less distinct tiers of labelling were seen in deeper layers. These in situ hybridisation data reveal three distinct sub-laminae in rat stratum griseum superficiale. Cells expressing moderate to low levels of proenkephalin messenger RNA were located in lower stratum griseum superficiale/upper stratum opticum and intermediate laminae. A cluster of enkephalinergic cells was located medially in the deep tectal laminae. Expression of neuropeptide Y messenger RNA was relatively low and mostly confined to cells in stratum griseum superficiale and stratum opticum. No choline acetyltransferase messenger RNA was detected. This in situ analysis of seven different neurotransmitters/neuromodulator systems sheds new light on the neurochemical organisation of the rat superior colliculus. The data are related to what is known anatomically and physiologically about intrinsic and extrinsic tectal circuitry, and the potential involvement of different neuropeptides in these circuits is discussed. The work forms the basis for future developmental studies examining the effects of transplantation and visual deprivation/deafferentation on tectal neurochemistry and function.

Distribution of galaninergic immunoreactivity in the brain of the mouse

The distribution of galaninergic immunoreactive (-ir) profiles was studied in the brain of colchicine-pretreated and non-pretreated mice. Galanin (GAL)-ir neurons and fibers were observed throughout all encephalic vesicles. Telencephalic GAL-ir neurons were found in the olfactory bulb, cerebral cortex, lateral and medial septum, diagonal band of Broca, nucleus basalis of Meynert, bed nucleus of stria terminalis, amygdala, and hippocampus. The thalamus displayed GAL-ir neurons within the anterodorsal, paraventricular, central lateral, paracentral, and central medial nuclei. GAL-ir neurons were found in several regions of the hypothalamus. In the midbrain, GAL-ir neurons appeared in the pretectal olivary nucleus, oculomotor nucleus, the medial and lateral lemniscus, periaqueductal gray, and the interpeduncular nucleus. The pons contained GAL-ir neurons within the dorsal subcoeruleus, locus coeruleus, and dorsal raphe. In the medulla oblongata, GAL-ir neurons appear in the anterodorsal and dorsal cochlear nuclei, salivatory nucleus, A5 noradrenergic cells, gigantocellular nucleus, inferior olive, solitary tract nucleus, dorsal vagal motor and hypoglossal nuclei. Only GAL-ir fibers were seen in the lateral habenula nucleus, substantia nigra, parabrachial complex, cerebellum, spinal trigeminal tract, as well as the motor root of the trigeminal and facial nerves. GAL-ir was also observed in several circumventricular organs. The widespread distribution of galanin in the mouse brain suggests that this neuropeptide plays a role in the regulation of cognitive and homeostatic functions.

Role of the thalamic intergeniculate leaflet and its 5-HT afferences in the chronobiological properties of 8-OH-DPAT and triazolam in syrian hamster

The 5-HT(1A/7) receptor agonist 8-hydroxy-2-[di-n-propylamino]-tetralin (8-OH-DPAT) has chronobiological effects on the circadian system and, in the Syrian hamster, it is known that serotonergic (5-HT) projections connecting the median raphe nucleus to the suprachiasmatic nuclei (SCN) of the hypothalamus are a prerequisite for the expression of 8-OH-DPAT-induced phase advance of locomotor activity rhythm. We examined the possible involvement of the thalamic intergeniculate leaflet (IGL) in the phase-shifting properties of 8-OH-DPAT injections at CT7. Bilateral electrolytic lesions of the IGL blocked phase-shift responses to 8-OH-DPAT of the activity rhythm. Phase changes induced by injections of 8-OH-DPAT at CT7 and triazolam (Tz), a short-acting benzodiazepine, at CT6 were also studied after bilateral chemical lesion of the 5-HT fibres connecting the dorsal raphe nucleus (DR) to IGL. Destruction of 5-HT fibres within the IGL blocked the phase-shift response to Tz, but not the phase-shift response to 8-OH-DPAT. In conclusion, (a) IGL is essential for the phase-shifting effect of peripheral 8-OH-DPAT injections; (b) 5-HT fibres connecting DR to IGL are necessary for the expression of the phase-shifting effect of Tz but not of 8-OH-DPAT.

Temporal contrast sensitivity in the lateral geniculate nucleus of a New World monkey, the marmoset Callithrix jacchus

1. The temporal contrast sensitivity of koniocellular, parvocellular and magnocellular cells in the lateral geniculate nucleus (LGN) of nine adult marmosets was measured. The receptive fields of the cells were between 0.3 and 70 deg from the fovea. The stimulus was a large spatially uniform field which was modulated in luminance at temporal frequencies between 0.98 and 64 Hz. 2. For each cell group there was a gradual increase in modulation sensitivity, especially for temporal frequencies below 8 Hz, with increasing distance from the fovea. At any given eccentricity, magnocellular cells had the greatest sensitivity. In central visual field, the sensitivity of koniocellular cells lay between that of parvocellular and magnocellular cells. In peripheral visual field (above 10 deg eccentricity) koniocellular and parvocellular cells had similar sensitivity. 3. The contrast sensitivity of each cell class was dependent on the anaesthetic used. Cells from animals anaesthetized with isoflurane were less sensitive than cells from animals anaesthetized with sufentanil. This effect was more marked for temporal frequencies below 4 Hz. 4. These results are incompatible with the notion that the koniocellular pathway is functionally homologous to a sluggish, W-like pathway in other mammals. At least in terms of their temporal transfer properties, many koniocellular cells are more like parvocellular cells.

The ascending serotonergic system in the hamster: comparison with projections of the dorsal and median raphe nuclei

The ascending serotonergic projections are derived largely from the midbrain median and dorsal raphe nuclei, and contribute to the regulation of many behavioral and physiological systems. Serotonergic innervation of the hamster circadian system has been shown to be substantially different from earlier results obtained with other methods and species. The present study was conducted to determine whether similar differences are observed in other brain regions. Ascending projections from the hamster dorsal or median raphe were identified using an anterograde tracer, Phaseolus vulgans leucoagglutinin, injected by iontophoresis into each nucleus. Brains were processed for tracer immunoreactivity, and drawings were made of the median raphe and dorsal raphe efferent projection patterns. The efferents were also compared to the distribution of normal serotonergic innervation of the hamster midbrain and forebrain. The results show widespread, overlapping projection patterns from both the median and dorsal raphe, with innervation generally greater from the dorsal raphe. In several brain regions, including parts of the pretectum, lateral geniculate and basal forebrain, nuclei are innervated by the dorsal, but not the median, raphe. The hypothalamic suprachiasmatic nucleus is the only site innervated exclusively by the median and not by the dorsal raphe. The pattern of normal serotonin fiber and terminal distribution is generally more robust than would be inferred from the anterograde tracer material. However, there is good qualitative similarity between the two sets of data. The oculomotor nucleus and the medial habenula are unusual to the extent that each has a moderately dense serotonin terminal plexus, although neither receives innervation from the median or dorsal raphe. In contrast, the centrolateral thalamic nucleus and lateral habenula have little serotonergic innervation, but receive substantial other neural input from the raphe nuclei. The normal serotonergic innervation of the hamster brain is similar to that in the rat, although there are exceptions. The anterograde tracing of ascending median or dorsal raphe projections reveals a high, but imperfect, degree of correspondence with the serotonin innervation data, and with data from rats derived from immunohistochemical and autoradiographic tract-tracing techniques.

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.

Connections between the reticular nucleus of the thalamus and pulvinar-lateralis posterior complex: a WGA-HRP study

The present study utilises the capacity of wheat germ agglutinin-conjugated horseradish peroxidase to label both afferent and efferent projections from selected regions of the thalamic reticular nucleus (TRN) to the pulvinar lateralis-posterior complex (Pul-LP) of the cat. Fourteen injections into the TRN located between anterior-posterior levels 8.5 and 4.5 were analysed. The projection of the TRN to the Pul-LP complex is roughly organised in a topographic manner and is not widespread within the thalamus. Anterograde labelling in the Pul-LP extended rostrocaudally with a slight oblique dorsoventral orientation. Projections to the medial LP were predominantly but not exclusively from rostral areas of TRN, while projections to the lateral LP were largely from caudal areas of the TRN. Projections to other areas of the Pul-LP were sparse. The connections between TRN and Pul-LP were reciprocal, although the distribution of labelled cells and anterograde labelling was not completely overlapping. Reciprocal connections with the dorsal lateral geniculate nucleus were largely with the C-laminae and the medial interlaminar nucleus. The results are discussed with reference to the corticothalamic projections and the visuotopy of the Pul-LP.

Interactive events subserving the pupillary light reflex in pigmented and albino rats

The consensual pupilloconstrictor response requires the intensity information that is delivered to each eye to be integrated to produce an averaged response such that the pupillary diameter in each eye is equal. Here we have examined how luminance information from each eye is integrated in the rat and where the integration occurs, using a pupillometry system to record pupillary diameter after illumination of one or both eyes. In albino rats the size of the uncrossed optic pathway to the pupilloconstrictor centre, the olivary pretectal nucleus, is reduced three-fold and there is a concomitant but less dramatic reduction in the size of the consensual response. Unilateral lesions of the olivary pretectal nucleus in pigmented rats restrict all optic input to the opposite pretectum. Stimulation of one or both eyes in these animals suggests that integration occurs not only at the level of the olivary pretectal nucleus, but also downstream in the Edinger-Westphal nucleus. Parallels with human studies and the opportunity to perform secondary lesions suggests the rat may provide a good model in which to study the substrates of the pupilloconstrictor response and mechanisms of integration of convergent parallel inputs in general.

Cholinergic projections to the anterior thalamic nuclei in the rat: a combined retrograde tracing and choline acetyl transferase immunohistochemical study

Retrograde transport of horseradish peroxidase (HRP) was combined with choline acetyltransferase (ChAT) immunohistochemistry to study cholinergic projections to the anterior thalamic nuclei in the rat. Small iontophoretic injections of HRP placed into different subdivisions of the anterior thalamic nuclear complex resulted in distinct patterns of retrograde labelling in two major cholinergic cell groups of the mesopontine tegmentum, the laterodorsal tegmental nucleus (LDTg), in which a majority of the labelled cells was located, and the pedunculopontine tegmental nucleus (PPT). After injections into the posterior subdivision of the anteroventral thalamic nucleus (AVp), double-labelled neurons were present predominantly in the ipsilateral LDTg while a smaller number was found in the PPT. In the ipsilateral LDTg, 60-70% of ChAT-positive neurons were HRP-labelled, and 90-95% of the HRP-labelled neurons were ChAT-positive. In the contralateral LDTg, 30-40% of ChAT-positive neurons were HRP-labelled. After injections in the medial subdivision of the anteroventral thalamic nucleus (AVm), the pattern of labelling in LDTg was similar to that detected after injections in the AVp. The number of double-labelled neurons in the LDTg and PPT was much lower after injections into AVm than after injections into AVp. When injections were confined to the anterodorsal thalamic nucleus (AD), no HRP-labelled cells were present in the LDTg or PPT. These results show that the LDTg and PPT are the sources of the cholinergic input to the rat anterior thalamus. The major projection from LDTg and PPT is to the AVp, whereas there is a lighter cholinergic projection to the AVm. The AD does not receive a projection from cholinergic cells in the mesopontine tegmentum.

Independent efferent populations in the nucleus of the optic tract: an anatomical and physiological study in rat and cat

The efferent projections of the nucleus of the optic tract (NOT) and dorsal terminal nucleus of the accessory optic system (DTN) to the contralateral NOT-DTN, ipsilateral inferior olive (IO), ipsilateral nucleus prepositus hypoglossi (NPH), and ipsilateral dorsal lateral geniculate nucleus (LGNd) were examined in pigmented rats and in cats by using anterograde and retrograde tract tracing, as well as extracellular recording and electrical stimulation. Anterograde tracing in the rat revealed a dense termination field of NOT-DTN efferents throughout the homologous contralateral territory. In both species three different cell populations, projecting to the contralateral NOT-DTN, ipsilateral IO, and ipsilateral LGNd, respectively, were distinguished by means of multiple retrograde tracing. No clear topographical segregation of the different NOT-DTN relay cell populations was observed. On the other hand, a large proportion (at least 60%) of NOT-DTN neurons projecting to the ipsilateral NPH were found to bifurcate upon the IO in the rat. Electrophysiologically, NOT-DTN neurons projecting to the IO were identified by their directionally selective responses. Such neurons were never activated by electrical stimulation of either the contralateral NOT-DTN or the ipsilateral LGNd. Neurons antidromically activated from the contralateral NOT-DTN could not be activated from the ipsilateral LGNd. Thus, in both cat and rat the NOT-DTN includes at least three independent relay cell populations. As a consequence, the NOT-DTN must serve functions additional to the generation of eye movements during optokinetic nystagnus, a function subserved by the directionally selective NOT-DTN cells.

Functional anatomy of the thalamic reticular nucleus as revealed with the [14C]deoxyglucose method following electrical stimulation and electrolytic lesion

The [14C]-deoxyglucose quantitative autoradiographic method was used to map the metabolic changes induced by electrical stimulation and electrolytic lesion of the rostral pole of the thalamic reticular nucleus in the rat brain. Unilateral electrical stimulation of the thalamic reticular nucleus induced the following changes in glucose utilization: (i) local enhancement of metabolic activity within the stimulated thalamic reticular nucleus, (ii) increase in glucose consumption in the ipsilateral thalamic mediodorsal, centrolateral, ventromedial and ventrolateral nuclei, as well as in the nucleus accumbens, (iii) bilateral depression of metabolism in the locus coeruleus, periaqueductal gray, ventral tegmental area, and medial habenula, as well as contralateral metabolic depression in the substantia nigra reticulata, compacta and in the ventral pallidum. Unilateral electrolytic lesion of thalamic reticular nucleus elicited metabolic depression in the ipsilateral thalamic mediodorsal, centrolateral, ventrolateral and ventromedial nuclei, and metabolic activation in the dorsal tegmental nucleus bilaterally. The existence of a descending thalamic reticular nucleus input to the periaqueductal gray is supported by the depressed activity measured in brain stem structures after thalamic reticular nucleus stimulation. The similar effects observed in the periaqueductal gray and substantia nigra contralateral to the stimulated thalamic reticular nucleus indicate a possible flow of information from one thalamic reticular nucleus to the contralateral basal ganglia via the periaqueductal gray. The opposite effects induced in the dorsal thalamic nuclei by thalamic reticular nucleus stimulation and lesion support the gating role of the thalamic reticular nucleus in the information flow between thalamus and cortex.

Sources of subcortical afferents to the macaque's dorsal lateral geniculate nucleus

BACKGROUND

The dorsal lateral geniculate nucleus (dLGN) is the thalamic region responsible for transmitting retina signals to cortex. Brainstem pathways to this nucleus have been described in several species and are believed to control the retinocortical pathway depending on the state of the animal (awake, asleep, drowsy, etc.). The purpose of this study was to determine all of the subcortical sources of afferents to the dLGN in a higher primate, the macaque monkey, whose visual system is similar to that of humans.

METHODS

Injections of horseradish peroxidase (HRP), with or without conjugation to wheat germ agglutinin, were made into the dLGNs of seven macaque monkeys, followed by perfusion, brain sectioning, and analyses of neurons in the brainstem, thalamus, and hypothalamus that contained the retrogradely transported marker.

RESULTS

The reticular nucleus of the thalamus, pedunculopontine nucleus, parabigeminal nucleus, pretectal nucleus of the optic tract, superior colliculus, dorsal raphe nucleus, and tuberomammillary region of the hypothalamus contained many retrogradely labeled neurons ipsilateral to the injections. In the contralateral brainstem, HRP-labeled cells were found only in the pedunculopontine nucleus, nucleus of the optic tract, and dorsal raphe nucleus. The number of labeled neurons on the contralateral side was about one-half of that in corresponding ipsilateral nuclei. The locus coeruleus contained no labeled neurons in four of the macaques that had injections limited to the dLGN.

CONCLUSION

There are seven subcortical regions that send afferents to the dLGNs of macaque monkeys. Except for the locus coeruleus, these are the same as observed for other species, such as the cat and rat, and indicate the possible sources of subcortical control over the dLGNs of humans.

Topographic organization of projections from the thalamic reticular nucleus to the anterior thalamic nuclei in the rat

We have investigated connections between the thalamic reticular nucleus (TRN) and the anterior thalamic nuclei (ATN) in the rat, following injections of horseradish peroxidase (HRP) into subnuclei of the ATN and different regions of the rostral TRN. Three nonoverlapping groups of neurons in the dorsal part of the ipsilateral rostral TRN project to, and receive reciprocal projections from, specific subnuclei of the ATN. A vertical sheet of neurons in the most dorsal part of the rostral TRN projects to the dorsal half of the posterior subdivision of the anteroventral thalamic nucleus (AVp), the dorsal region of the medial subdivision of the anteroventral thalamic nucleus (AVm), and the dorsolateral part of the rostral anterodorsal thalamic nucleus (AD). Immediately ventral to this part of TRN, but still within its dorsal portion, are a lateral cluster of neurons and a medially located vertical sheet of neurons. The lateral cluster projects to the ventral part of AVp and to the dorsomedial part of rostral AD. The medial sheet projects to the ventral part of AVm, the ventral part of rostral AD, and to the caudal portions of both AV and AD. There appears to be no input to the anteromedial thalamic nucleus (AM) from the TRN. These findings shed new light on the anatomy of the rostral TRN, the ATN, and the connections between the two, and are relevant to emerging hypotheses about the functional organization of the TRN and reticulo-thalamic projections.

Two types of interneuron in the dorsal lateral geniculate nucleus of the rat: a combined NADPH diaphorase histochemical and GABA immunocytochemical study

The rationale for this study was to provide a comprehensive light microscopical description of the morphology of diaphorase-reactive neurons and neuropil elements in the dorsal lateral geniculate nucleus (dLGN) of the rat. An additional objective was to quantitatively assess whether a subpopulation of the diaphorase-reactive neurons, previously shown to be GABA-immunoreactive, constitute a distinct type of local-circuit neuron in the rat dLGN. Diaphorase activity was localised in a population of predominantly bipolar fusiform neurons. These cells were weak to moderately stained and possessed the morphological features of intrinsic inhibitory neurons, previously called class B neurons in the rat dLGN. Quantitative estimates indicated that the diaphorase-reactive neurons constituted approximately 10% of the total neuron composition of the dLGN. The majority (about 83%) of the diaphorase-reactive cells were located in the lateral half of the nucleus. In addition, a dense plexus of diaphorase-reactive varicose fibres was found throughout the dLGN lying between the oriented fibre bundles coursing dorsoventrally through the LGN. Diaphorase-reactive punctae were found to be closely associated with the somata and proximal dendritic segments of nonreactive neurons and also with the stained proximal dendritic segments of diaphorase-reactive dLGN neurons. The source of the diaphorase-reactive fibres in the dLGN was unknown. Evidence suggests, however, that they are of extrinsic origin. The GABA-immunoreactive nature of the diaphorase neurons in the dLGN was demonstrated by colocalising GABA immunoreactivity within the somata of diaphorase-reactive cells. The majority (> 90%) of diaphorase-reactive dLGN neurons were GABA-immunopositive. Also present was a distinct population of GABA-immunopositive neurons that were not diaphorase-reactive. In this study, cells that were solely GABA-immunopositive have been called class B1 neurons, while cells that were both diaphorase-reactive and GABA-immunoreactive have been called class B2 neurons. Size-frequency distributions of somatic profile areas established that the two populations of GABA-immunoreactive neuron were significantly different. Class B1 neurons constituted 57%, with class B2 cells representing 43% of all GABA-immunostained neurons in the rat dLGN. The characteristic morphological features, neurochemical identity and frequency of the diaphorase-reactive neurons in the rat dLGN indicate that they represent a subpopulation of inhibitory interneurons with the ability to affect intrinsic dLGN operations and thalamocortical interactions using the neuromodulator nitric oxide.

Specific destruction of the serotonergic afferents to the suprachiasmatic nuclei prevents triazolam-induced phase advances of hamster activity rhythms

Administration of Triazolam (Tz)--a short acting benzodiazepine (BZ)--induces permanent phase-shifts in locomotor activity of golden hamsters (Mesocricetus auratus). However, the target area(s) as well as the mechanism involved in the Tz-induced changes are not known. Previous results indicated that raphe nuclei (RN) would appear to be a likely site for Tz-induced phase shifts. Therefore, we specifically destroyed the 5-HT fibers connecting the RN with the SCN--the site of the endogenous mammalian clock--by microinjections of the selective neurotoxin 5,7 dihydroxytryptamine (5,7-DHT) at the level of SCN. Infusion of 5,7-DHT resulted in long lasting damage of the ascending serotonergic projection from RN to the hypothalamus. Subsequently, the phase-shifting effect of Tz was investigated. Only complete or almost complete depletion of the 5-HT input to the SCN was accompanied with a pronounced reduction of the phase shift together with a significant reduction of wheel-running activity during the 6 h following Tz injection. Our present results support the view that the 5-HT innervation of the SCN represents an essential link in the phase-shifting action following peripheral Tz injections.

An oriented framework of neuronal processes in the ventral lateral geniculate nucleus of the rat demonstrated by NADPH diaphorase histochemistry and GABA immunocytochemistry

This study investigated the morphology and quantitative distribution of neurons containing NADPH diaphorase activity in the ventral lateral geniculate nucleus of the rat. The pattern of diaphorase staining revealed a strongly reactive lateral subdivision and a weakly staining medial subdivision. A characteristic feature of the diaphorase staining in the lateral part was its "stripe-like" appearance. These "diaphorase stripes" resulted from regions of strong somatic and neuropil diaphorase activity lying between unstained fibre bundles coursing dorsoventrally through the nucleus. Two distinct populations of diaphorase reactive cell types were present--class A and class B neurons. The ratio of class A to class B diaphorase neurons was approximately 14:1 (A:B). Diaphorase reactive neurons made up 73% of the total neuron population in the lateral subdivision, and 31% in the medial subdivision. A third population of cells was found exclusively in the optic tract--class C neurons. Quantitative analyses in the coronal and sagittal planes indicated that the principal processes of both class A and class B neurons were oriented preferentially--either parallel with, or perpendicular to the outlying optic tract. Diaphorase enzyme histochemistry in combination with GABA immunocytochemistry demonstrated the co-localization of GABA immunoreactivity in the majority of class B neurons, whereas class A and class C neurons were GABA immunonegative. Furthermore a large population of GABA-immunoreactive neurons was present that were not stained for diaphorase activity. From this and previous studies, it can be concluded that a high proportion of the diaphorase reaction class A neurons are geniculotectal projection cells, while diaphorase reaction class B neurons represent a numerically small subpopulation of "local-circuit" inhibitory neurons. Since diaphorase activity co-localizes with nitric oxide synthase, the results indicate the likely involvement of nitric oxide in the neuronal operations of both subpopulations of geniculotectal projection neurons and "local-circuit" GABAergic neurons in the rat's ventral lateral geniculate nucleus.

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.

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Patterns of antigenic expression in the thalamic reticular nucleus of developing rats

The present study describes the development of the thalamic reticular nucleus in rats with the use of Nissl staining and antibodies to parvalbumin and pro-alpha-thyrotropin-releasing hormone (alpha TRH). Two major subdivisions of the reticular nucleus are apparent: 1) the main body, which is itself heterogeneous and lies for the most part between the fibres of the internal capsule and external medullary lamina, and 2) the perireticular nucleus, which lies lateral to the main body and medial to the globus pallidus. In the main body of the reticular nucleus of adults, most cells in all regions are immunoreactive to parvalbumin and alpha TRH. During development there are two waves of parvalbumin and alpha TRH expression. The first wave occurs between postnatal day (P) 0 and P10, and labelled cells are apparent in rostrolateral areas of the main body of the nucleus only. At P10, such cells are not apparent. From P7 to adult, there is a second wave of parvalbumin and alpha TRH expression: labelled cells emerge first in central, then in caudal, and finally in rostral areas of the nucleus. In adults, the perireticular nucleus is made up of a few small cells which are immunostained for parvalbumin and alpha TRH. These cells are more frequent in areas of the internal capsule adjacent to the ventral regions of the main body of the reticular nucleus, rostrodorsal to the entopeduncular nucleus. From E (embryonic day) 17 to about P10, the perireticular nucleus consists of a surprisingly large population of neurones, many of which are parvalbumin and alpha TRH immunoreactive. By about P10, as in adults, there are few perireticular cells.

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.

Selective reduction of glutamate in the rat superior colliculus and dorsal lateral geniculate nucleus after contralateral enucleation

The effects of afferent lesions on the levels of glutamate, aspartate and gamma-aminobutyric acid (GABA) in the laminae of the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN) of the rat were studied, using microassay methods for these amino acids. The analysis was performed 12-14 days after left eye enucleation, or ablation of right visual cortical area, or both left eye enucleation and ablation of right visual cortex. Superficial gray layer (SGL) and deep layers in the SC were dissected out from the thin-sectioned, freeze-dried sample. In the dLGN, the outer and inner laminae were separately dissected. The glutamate contents in the upper half of SGL and outer lamina of dLGN contralateral to eye enucleation decreased significantly (15%). Combination of eye enucleation and visual cortical ablation further decreased the glutamate content in the upper half of the right SGL (29.3%). On the other hand, aspartate and GABA concentrations in the SC and dLGN exhibited no significant reduction after deafferentations. These results indicate that the retino-tectal and retino-geniculate pathway of the rat may be glutamatergic in nature.

A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat

Ascending projections from the dorsal raphe nucleus (DR) were examined in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from the DR ascended through the forebrain within the medial forebrain bundle. DR fibers were found to terminate heavily in several subcortical as well as cortical sites. The following subcortical nuclei receive dense projections from the DR: ventral regions of the midbrain central gray including the 'supraoculomotor central gray' region, the ventral tegmental area, the substantia nigra-pars compacta, midline and intralaminar nuclei of the thalamus including the posterior paraventricular, the parafascicular, reuniens, rhomboid, intermediodorsal/mediodorsal, and central medial thalamic nuclei, the central, lateral and basolateral nuclei of the amygdala, posteromedial regions of the striatum, the bed nucleus of the stria terminalis, the lateral septal nucleus, the lateral preoptic area, the substantia innominata, the magnocellular preoptic nucleus, the endopiriform nucleus, and the ventral pallidum. The following subcortical nuclei receive moderately dense projections from the DR: the median raphe nucleus, the midbrain reticular formation, the cuneiform/pedunculopontine tegmental area, the retrorubral nucleus, the supramammillary nucleus, the lateral hypothalamus, the paracentral and central lateral intralaminar nuclei of the thalamus, the globus pallidus, the medial preoptic area, the vertical and horizontal limbs of the diagonal band nuclei, the claustrum, the nucleus accumbens, and the olfactory tubercle. The piriform, insular and frontal cortices receive dense projections from the DR; the occipital, entorhinal, perirhinal, frontal orbital, anterior cingulate, and infralimbic cortices, as well as the hippocampal formation, receive moderately dense projections from the DR. Some notable differences were observed in projections from the caudal DR and the rostral DR. For example, the hippocampal formation receives moderately dense projections from the caudal DR and essentially none from the rostral DR. On the other hand, virtually all neocortical regions receive significantly denser projections from the rostral than from the caudal DR. The present results demonstrate that dorsal raphe fibers project significantly throughout widespread regions of the midbrain and forebrain.

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.

Cholinergic modulation of responses to glutamate in the thalamic reticular nucleus of the anesthetized rat

Neurons in the thalamic reticular nucleus (TRN) of the chloral hydrate-anesthetized rat were studied with extracellular recording and microiontophoretic application of cholinergic agents. In most cases (63%), the ejection of the agonist, carbachol, had no observable effect on spontaneous activity, and in an additional 33% of cases was observed to inhibit discharge rate. Carbachol ejections with identical current and duration parameters proved capable of antagonizing the uniformly facilitatory responses produced by glutamate ejection in these same cells. The muscarinic nature of cholinergic effects was documented by scopolamine's specific antagonism of the responses. The muscarinic antagonists, pirenzepine and AF-DX-116, both diminished the effects of carbachol. Application of muscarinic agonists, such as McN-A-343 and oxotremorine-M, yielded qualitatively the same results as carbachol, though, with current as a criterion, oxotremorine-M was slightly more and McN-A-343 much less potent than carbachol. The functional implications of cholinergic modulation of the facilitatory inputs to TRN are discussed, with particular emphasis on the role of acetylcholine and the TRN in the sleep/wake-related activity of thalamic neurons.

Immunocytochemical localization of phosphatase inhibitor-1 in rat brain

The localization of phosphatase inhibitor-1 was investigated in rat brain by use of immunocytochemistry. Studies were performed with an affinity purified IgG raised against purified rabbit skeletal muscle inhibitor-1. In rat brain tissue homogenates, this antibody reacted only with a 29 kDa protein corresponding to inhibitor-1. Immunocytochemical studies with this antibody revealed numerous immunoreactive cell bodies and fibers. The highest concentration of immunoreactive perikarya was observed in the caudate-putamen and nucleus accumbens, and these appeared to be exclusively medium-sized neurons. Other areas containing substantial populations of immunoreactive neurons included the suprachiasmatic nucleus of the hypothalamus, lateral hypothalamus, horizontal limb of the diagonal band of Broca, dentate gyrus of the hippocampal formation, habenula, superior colliculus, claustrum, endopiriform nuclei, and neocortex. The distribution of terminals containing inhibitor-1 coincided with the distribution of terminal fields known to originate from the above regions. Thus, plexuses of immunoreactive axons were seen in the globus pallidus, substantia nigra pars reticulata, paraventricular hypothalamus, dorsal thalamus, CA3 region of the hippocampus, and interpeduncular nucleus. These results demonstrate that phosphatase inhibitor-1, a cyclic AMP-regulated inhibitor of phosphatase-1, is differentially distributed in the rat CNS. Given the widespread role of protein phosphorylation and dephosphorylation in intracellular signal transduction, these results suggest that neurons containing high levels of inhibitor-1 may share common, hitherto unrecognized, properties in terms of neurotransmitter regulation and/or responsiveness.

Transient retinal axon collaterals to visual and somatosensory thalamus in neonatal hamsters

We have studied the postnatal development of individual axons in the optic tract and thalamus of the Syrian hamster, concentrating attention on retinal ganglion cell axons that make a transient projection to the main somatosensory nucleus, the ventrobasal complex. We bulk-filled axons with horseradish peroxidase in hemithalami maintained en bloc, in vitro. After processing and reaction with diaminobenzidine, we reconstructed individual axons from serial sections. In hamsters and other rodents, the optic tract is composed of superficial and internal components, either or both being possible sources of the retino-ventrobasal projection. Both project to the midbrain, but in normal adults only the superficial optic tract maintains collaterals in the thalamus. We found that the axons of the internal component bear numerous transient thalamic collaterals on postnatal days 0, 1, and 2, and some of these extend into the ventrobasal complex. Axons in the superficial optic tract also bear collaterals on days 0 to 2, but these are confined to the superficial half of the dorsal lateral geniculate nucleus. Thus the transient retino-ventrobasal projection comprises solely transient collaterals originating from axon trunks in the internal optic tract. On days 1 and 2, some collaterals from the superficial optic tract appear to have begun to arborize in the lateral geniculate nucleus. In contrast, collaterals from internal optic tract axons to the ventrobasal complex branch little if at all as they traverse the lateral geniculate nucleus, and at no time prior to their elimination do they develop an appreciable terminal arbor. These long collaterals often terminate in growth cones that include lamellopodia. Our HRP-impregnation method also revealed some transient non-retinofugal axons that pass medially from the ventral lateral geniculate nucleus to the ventrobasal complex but then return without terminating or branching. By day 4, they are absent, as are collaterals from the internal optic tract to the ventrobasal complex.

Geniculo-hypothalamic tract lesions block chlordiazepoxide-induced phase advances in Syrian hamsters

Administration of the benzodiazepine triazolam at the appropriate time in the circadian cycle has been shown to induce phase shifts in hamster circadian rhythms. These phase shifts can be blocked by geniculo-hypothalamic tract (GHT) ablation or by restraint of activity. The present study examined the effects of the benzodiazepine chlordiazepoxide on running-wheel activity rhythms of hamsters. The phase-advancing effect of intraperitoneal injections of chlordiazepoxide administered at circadian time 6 (CT 6) was dose-dependent. Average shifts ranged from 6 min at a dose of 0.05 mg/kg to 135 min at a dose of 200 mg/kg. Four of twenty hamsters did not show a phase shift to any dose tested. Phase advance shifts to chlordiazepoxide (CT 6; 100 mg/kg) were blocked by GHT lesions. Chlordiazepoxide injections at doses which induced phase shifts were often followed by sedation. These results indicate that chlordiazepoxide is similar to triazolam, in that its ability to induce phase shifts at circadian time 6 is blocked by GHT lesions.

Projection of the mammalian superior colliculus upon the dorsal lateral geniculate nucleus: organization of tectogeniculate pathways in nineteen species

Anterograde and retrograde transport methods have been used to analyze the projection of the superior colliculus upon the dorsal lateral geniculate nucleus in 19 mammalian species. Our retrograde findings reveal that tectogeniculate neurons are relatively small, and lie dorsally within the superficial gray. These small tectogeniculate neurons are spatially related to a dense tier of W-cell retinal input. Our anterograde tracing results show that tectogeniculate axons are visuotopically distributed to small-celled regions of the lateral geniculate in all nineteen species. In the majority of these species, the small-celled, tectally innervated regions of the lateral geniculate lie adjacent to the optic tract and contain W-cell-like neurons. Our findings suggest that neuroanatomical demonstration of the tectogeniculate projection is a relatively simple and straightforward way of revealing regions of the lateral geniculate which contain W-cells. This is true even in species in which the lateral geniculate lacks obvious cellular laminae, and in regions of the lateral geniculate where W-cells are few in number. The present data are especially interesting in light of the cortical projections of tectally innervated, small-celled regions of the lateral geniculate to the patches or puffs within layer III of area 17. Since these regions of small-celled geniculocortical axons are co-extensive with zones ("blobs") rich in cytochrome oxidase, it might be that information carried over the tectogeniculate circuitry plays an important role in the functions of the blob system.