Are the interlaminar zones of the ferret dorsal lateral geniculate nucleus actually part of the perigeniculate nucleus?

Cell-type specific binocular interactions in mouse visual thalamus

Projections from each eye are segregated in separate domains within the dorsal lateral geniculate nucleus (dLGN). Yet, in vivo studies indicate that the activity of single dLGN neurons can be influenced by visual stimuli presented to either eye. In this study we explored whether intrinsic circuits mediate binocular interactions in the mouse dLGN. We employed dual color optogenetics in vitro to selectively activate input from each eye and recorded synaptic responses in thalamocortical (relay) cells as well as inhibitory interneurons, which have extensive dendritic arbors that are not confined to eye specific domains. While most relay cells received monocular retinal input, most interneurons received binocular retinal input; consequently, the majority of dLGN relay cells received binocular retinogeniculate-evoked inhibition. Moreover, in recordings from adjacent pairs of relay cells and interneurons, the most common relationship observed was binocular excitation of interneurons paired with binocular inhibition of adjacent relay cells. Finally, we found that dLGN interneurons are interconnected, displaying both monocular and binocular inhibition in response to retinal activation. In sum, our results indicate that geniculate interneurons provide one of the first locations where signals from the two eyes can be compared, integrated, and adjusted before being transmitted to cortex, shedding new light on the role of the thalamus in binocular vision.

Highlights

In vitro dual color optogenetics examined convergence of eye-specific retinal inputs to thalamocortical (relay) cells and interneurons in the dLGNThe majority of relay cells receive monocular excitatory retinogeniculate input while the majority of interneurons receive binocular inputBinocular relay cells are located in and around the ipsilateral patch whereas binocular interneurons are distributed throughout the dLGNThe majority of relay cells receive binocular retinogeniculate-evoked inhibitiondLGN interneurons are interconnected, receiving both monocular and binocular retinogeniculate-evoked inhibition.

Transient expression of heavy-chain neurofilaments in the perigeniculate nucleus of cats

The perigeniculate nucleus (PGN) is a visual part of the thalamic reticular nucleus modulating the information transfer between the lateral geniculate nucleus and the visual cortex. This study focused on the postnatal development of the PGN in cats, using the SMI-32 antibody, which recognizes non-phosphorylated heavy-chain neurofilaments responsible for neuronal structural maturation and is also used as a marker for motion processing, or Y, stream. We questioned whether transient neuronal populations exist in the PGN and can they possibly be related to the Y processing stream. We uncovered a transient, robust SMI-32 staining in the PGN of kittens aged 0-34 days with the significant decline in the cellular density of labeled cells in older animals. According to the double-labeling, in all examined age groups, perigeniculate SMI-32-immunopositive cells are part of the main parvalbumin-positive population. The maximal cellular density of the double-stained cells appeared in animals aged 10-28 days. We also revealed that the most significant growth of perigeniculate cells's soma occurred at three postnatal weeks. The possible link of our data to the development of the Y visual processing stream and to the heterogeneity of the perigeniculate neuronal population is also discussed.

3D electron microscopy and volume-based bouton sorting reveal the selectivity of inputs onto geniculate relay cell and interneuron dendrite segments

Introduction

The visual signals evoked at the retinal ganglion cells are modified and modulated by various synaptic inputs that impinge on lateral geniculate nucleus cells before they are sent to the cortex. The selectivity of geniculate inputs for clustering or forming microcircuits on discrete dendritic segments of geniculate cell types may provide the structural basis for network properties of the geniculate circuitry and differential signal processing through the parallel pathways of vision. In our study, we aimed to reveal the patterns of input selectivity on morphologically discernable relay cell types and interneurons in the mouse lateral geniculate nucleus.

Methods

We used two sets of Scanning Blockface Electron Microscopy (SBEM) image stacks and Reconstruct software to manually reconstruct of terminal boutons and dendrite segments. First, using an unbiased terminal sampling (UTS) approach and statistical modeling, we identified the criteria for volume-based sorting of geniculate boutons into their putative origins. Geniculate terminal boutons that were sorted in retinal and non-retinal categories based on previously described mitochondrial morphology, could further be sorted into multiple subpopulations based on their bouton volume distributions. Terminals deemed non-retinal based on the morphological criteria consisted of five distinct subpopulations, including small-sized putative corticothalamic and cholinergic boutons, two medium-sized putative GABAergic inputs, and a large-sized bouton type that contains dark mitochondria. Retinal terminals also consisted of four distinct subpopulations. The cutoff criteria for these subpopulations were then applied to datasets of terminals that synapse on reconstructed dendrite segments of relay cells or interneurons.

Results

Using a network analysis approach, we found an almost complete segregation of retinal and cortical terminals on putative X-type cell dendrite segments characterized by grape-like appendages and triads. On these cells, interneuron appendages intermingle with retinal and other medium size terminals to form triads within glomeruli. In contrast, a second, presumed Y-type cell displayed dendrodendritic puncta adherentia and received all terminal types without a selectivity for synapse location; these were not engaged in triads. Furthermore, the contribution of retinal and cortical synapses received by X-, Y- and interneuron dendrites differed such that over 60% of inputs to interneuron dendrites were from the retina, as opposed to 20% and 7% to X- and Y-type cells, respectively.

Conclusion

The results underlie differences in network properties of synaptic inputs from distinct origins on geniculate cell types.

Thalamic subnetworks as units of function

The thalamus engages in various functions including sensory processing, attention, decision making and memory. Classically, this diversity of function has been attributed to the nuclear organization of the thalamus, with each nucleus performing a well-defined function. Here, we highlight recent studies that used state-of-the-art expression profiling, which have revealed gene expression gradients at the single-cell level within and across thalamic nuclei. These gradients, combined with anatomical tracing and physiological analyses, point to previously unappreciated heterogeneity and redefine thalamic units of function on the basis of unique input-output connectivity patterns and gene expression. We propose that thalamic subnetworks, defined by the intersection of genetics, connectivity and computation, provide a more appropriate level of functional description; this notion is supported by behavioral phenotypes resulting from appropriately tailored perturbations. We provide several examples of thalamic subnetworks and suggest how this new perspective may both propel progress in basic neuroscience and reveal unique targets with therapeutic potential.

GABAB receptors: modulation of thalamocortical dynamics and synaptic plasticity

GABA_B-receptors (GABA_B-Rs) are metabotropic, G protein-coupled receptors for the neurotransmitter GABA. Their activation induces slow inhibitory control of the neuronal excitability mediated by pre- and postsynaptic inhibition. Presynaptically GABA_B-Rs reduce GABA and glutamate release inhibiting presynaptic Ca²⁺ channels in both inhibitory and excitatory synapses while postsynaptic GABA_B-Rs induce robust slow hyperpolarization by the activation of K⁺ channels. GABA_B-Rs are activated by non-synaptic or volume transmission, which requires high levels of GABA release, either by the simultaneous discharge of GABAergic interneurons or very intense discharges in the thalamus or by means of the activation of a neurogliaform interneurons in the cortex. The main receptor subunits GABA_B1a, GABA_B1b and GABA_B2 are strongly expressed in neurons and glial cells throughout the central nervous system and GABA_B-R activation is related to many neuronal processes such as the modulation of rhythmic activity in several brain regions. In the thalamus, GABA_B-Rs modulate the generation of the main thalamic rhythm, spindle waves. In the cerebral cortex, GABA_B-Rs also modulate the most prominent emergent oscillatory activity-slow oscillations-as well as faster oscillations like gamma frequency. Further, recent studies evaluating the complexity expressed by the cortical network, a parameter associated with consciousness levels, have found that GABA_B-Rs enhance this complexity, while their blockade decreases it. This review summarizes the current results on how the activation of GABA_B-Rs affects the interchange of information between brain areas by controlling rhythmicity as well as synaptic plasticity.

Receptive field properties of cat perigeniculate neurons correlate with excitatory and inhibitory connectivity to LGN relay neurons

The cat perigeniculate nucleus (PGN) is a visual sector of the thalamic reticular nucleus that consists of GABAergic neurons. It receives excitatory axon-collateral input from relay neurons of the dorsal lateral geniculate nucleus (LGN) to which it provides inhibitory input. Thus, it is usually argued that the PGN works as feedback inhibition to the LGN. At the single neuron level, however, this circuit can also provide lateral inhibition. Which inhibition dominates in the visual circuit of the thalamus has yet to be well characterized. In this study, we conducted cross-correlation analysis of single spike trains simultaneously recorded from PGN and LGN neurons in anesthetized cats. For 12 pairs of functionally connected PGN and LGN neurons with overlapped receptive fields (RF), we quantitatively compared RF properties including the spatial frequency (SF) and temporal frequency (TF) tunings of each neuron. We found the SF and TF tunings of PGN neurons and LGN neurons were similar when there was only excitatory input from the LGN neuron to the PGN neuron, but different when the PGN neuron returned inhibitory inputs back, suggesting the circuit between PGN and LGN neurons works as lateral inhibition for these properties.

Optic nerve, superior colliculus, visual thalamus, and primary visual cortex of the northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus)

The northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus) are members of a diverse clade of carnivorous mammals known as pinnipeds. Pinnipeds are notable for their large, ape-sized brains, yet little is known about their central nervous system. Both the northern elephant seal and California sea lion spend most of their lives at sea, but each also spends time on land to breed and give birth. These unique coastal niches may be reflected in specific evolutionary adaptations to their sensory systems. Here, we report on components of the visual pathway in these two species. We found evidence for two classes of myelinated fibers within the pinniped optic nerve, those with thick myelin sheaths (elephant seal: 9%, sea lion: 7%) and thin myelin sheaths (elephant seal: 91%, sea lion: 93%). In order to investigate the architecture of the lateral geniculate nucleus, superior colliculus, and primary visual cortex, we processed brain sections from seal and sea lion pups for Nissl substance, cytochrome oxidase, and vesicular glutamate transporters. As in other carnivores, the dorsal lateral geniculate nucleus consisted of three main layers, A, A1, and C, while each superior colliculus similarly consisted of seven distinct layers. The sea lion visual cortex is located at the posterior side of cortex between the upper and lower banks of the postlateral sulcus, while the elephant seal visual cortex extends far more anteriorly along the dorsal surface and medial wall. These results are relevant to comparative studies related to the evolution of large brains.

Modulation of GABA release from the thalamic reticular nucleus by cocaine and caffeine: role of serotonin receptors

Serotonin receptors are targets of drug therapies for a variety of neuropsychiatric and neurodegenerative disorders. Cocaine inhibits the re-uptake of serotonin (5-HT), dopamine, and noradrenaline, whereas caffeine blocks adenosine receptors and opens ryanodine receptors in the endoplasmic reticulum. We studied how 5-HT and adenosine affected spontaneous GABAergic transmission from thalamic reticular nucleus. We combined whole-cell patch clamp recordings of miniature inhibitory post-synaptic currents (mIPSCs) in ventrobasal thalamic neurons during local (puff) application of 5-HT in wild type (WT) or knockout mice lacking 5-HT2A receptors (5-HT2A -/-). Inhibition of mIPSCs frequency by low (10 μM) and high (100 μM) 5-HT concentrations was observed in ventrobasal neurons from 5-HT2A -/- mice. In WT mice, only 100 μM 5-HT significantly reduced mIPSCs frequency. In 5-HT2A -/- mice, NAN-190, a specific 5-HT1A antagonist, prevented the 100 μM 5-HT inhibition while blocking H-currents that prolonged inhibition during post-puff periods. The inhibitory effects of 100 μM 5-HT were enhanced in cocaine binge-treated 5-HT2A -/- mice. Caffeine binge treatment did not affect 5-HT-mediated inhibition. Our findings suggest that both 5-HT1A and 5-HT2A receptors are present in pre-synaptic thalamic reticular nucleus terminals. Serotonergic-mediated inhibition of GABA release could underlie aberrant thalamocortical physiology described after repetitive consumption of cocaine. Our findings suggest that both 5-HT1A , 5-HT2A and A1 receptors are present in pre-synaptic TRN terminals. 5-HT1A and A1 receptors would down-regulate adenylate cyclase, whereas 5-HT1A would also increase the probability of the opening of G-protein-activated inwardly rectifying K(+) channels (GIRK). Sustained opening of GIRK channels would hyperpolarize pre-synaptic terminals activating H-currents, resulting in less GABA release. 5-HT2A -would activate PLC and IP3 , increasing intracellular [Ca(2+) ] and thus facilitating GABA release.

Comparison of numbers of interneurons in three thalamic nuclei of normal and epileptic rats

The inhibitory sources in the thalamic nuclei are local interneurons and neurons of the thalamic reticular nucleus. Studies of models of absence epilepsy have shown that the seizures are associated with an excess of inhibitory neurotransmission in the thalamus. In the present study, we used light-microscopic gamma-aminobutyric acid (GABA) immunocytochemistry to quantify the interneurons in the lateral geniculate (LGN), ventral posteromedial (VPM), and ventral posterolateral (VPL) thalamic nuclei, and compared the values from normal Wistar rats and genetic absence epilepsy rats from Strasbourg (GAERS). We found that in both Wistar rats and GAERS, the proportion of interneurons was significantly higher in the LGN than in the VPM and VPL. In the LGN of Wistar rats, 16.4% of the neurons were interneurons and in the GAERS, the value was 15.1%. In the VPM, the proportion of interneurons was 4.2% in Wistar and 14.9% in GAERS; in the VPL the values were 3.7% for Wistar and 11.1% for the GAERS. There was no significant difference between Wistar rats and the GAERS regarding the counts of interneurons in the LGN, whereas the VPM and VPL showed significantly higher counts in GAERS. Comparison of the mean areas of both relay cells and interneuronal profiles showed no significant differences between Wistar rats and GAERS. These findings show that in the VPL and the VPM there are relatively more GABAergic interneurons in GAERS than in Wistar rats. This may represent a compensatory response of the thalamocortical circuitry to the absence seizures or may be related to the production of absence seizures.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Dual chemoarchitectonic lamination of the visual sector of the thalamic reticular nucleus

The chemoanatomical organization of the visual sector of the cat's thalamic reticular nucleus (TRN)-that is at the dorsal lateral geniculate nucleus (dLGN) and at the pulvinar nucleus (Pul)-was investigated with two novel cytoarchitectonic markers. The Wisteria floribunda agglutinin (WFA) binding reaction visualized the extracellular perineuronal net (PN) and the SMI 32 immunoreaction stained intracellular neurofilaments. Two distinct layers of the TRN could be detected, particularly by WFA- but also by SMI 32-staining. The outer tier outlined a canopy of labeling placed a bit detached from the diencephalon dorsolaterally, while the inner TRN tier is very tightly attached to the thalamic lamina limitans externa. The labeled neurons showed typically fusiform morphology with dendrites orienting in the plane of TRN. Additionally, these chemoarchitectural reactions identified a chain of structures in the ventral diencephalon connected to the TRN tiers. One stained string is formed by the subthalamic nucleus bound laterally to the peripeduncular nucleus extending further dorsolateral into the outer TRN tier. The other chain laced up the field of Forel, the zona incerta, the ventral LGN, the perigeniculate nucleus (PGN) and the previously-overlooked peripulvinar nucleus (PPulN) and so formed the inner TRN tier. In the third most distanced TRN tier, in the perireticular nucleus, a very few WFA-binding presenting neuron were found. In addition to the PN possessing TRN neurons, WFA-reactive presumable interneurons were also labeled within the visual thalamus. Following tracer injections into the feline Pul, two stripes of cells were retrogradely labeled in the neighboring visual TRN sector. The location of these reticular neurons coincided precisely with the chemoanatomically identified inner and outer TRN tiers. On the analogy of the PGN-TRN duality at the dLGN, the chemoanatomical and tract tracing findings strongly suggest a similar dual organization in the pulvinoprojecting TRN portion.

Synaptic organization of thalamocortical axon collaterals in the perigeniculate nucleus and dorsal lateral geniculate nucleus

We examined the synaptic targets of large non-gamma-aminobutyric acid (GABA)-ergic profiles that contain round vesicles and dark mitochondria (RLD profiles) in the perigeniculate nucleus (PGN) and the dorsal lateral geniculate nucleus (dLGN). RLD profiles can provisionally be identified as the collaterals of thalamocortical axons, because their ultrastrucure is distinct from all other previously described dLGN inputs. We also found that RLD profiles are larger than cholinergic terminals and contain the type 2 vesicular glutamate transporter. RLD profiles are distributed throughout the PGN and are concentrated within the interlaminar zones (IZs) of the dLGN, regions distinguished by dense binding of Wisteria floribunda agglutinin (WFA). To determine the synaptic targets of thalamocortical axon collaterals, we examined RLD profiles in the PGN and dLGN in tissue stained for GABA. For the PGN, we found that all RLD profiles make synaptic contacts with GABAergic PGN somata, dendrites, and spines. In the dLGN, RLD profiles primarily synapse with GABAergic dendrites that contain vesicles (F2 profiles) and non-GABAergic dendrites in glomerular arrangements that include triads. Occasional synapses on GABAergic somata and proximal dendrites were also observed in the dLGN. These results suggest that correlated dLGN activity may be enhanced via direct synaptic contacts between thalamocortical cells, whereas noncorrelated activity (such as that occurring during binocular rivalry) could be suppressed via thalamocortical collateral input to PGN cells and dLGN interneurons.

Thalamic mechanisms of EEG alpha rhythms and their pathological implications

During relaxed wakefulness, the human brain exhibits pronounced rhythmic electrical activity in the alpha frequency band (8-13 Hz). This activity consists of 3 main components: the classic occipital alpha rhythm, the Rolandic mu rhythm, and the so-called third rhythm. In recent years, the long-held belief that alpha rhythms are strongly influenced by the thalamus has been confirmed in several animal models and, in humans, is well supported by numerous noninvasive imaging studies. Of specific importance is the emergence of 2 key cellular thalamic mechanisms, which come together to generate locally synchronized alpha activity. First, a novel form of rhythmic burst firing, termed high-threshold (HT) bursting, which occurs in a specialized subset of thalamocortical (TC) neurons, and second, the interconnection of this subset via gap junctions (GJs). Because repetitive HT bursting in TC neurons occurs in the range of 2 to 13 Hz, with the precise frequency increasing with increasing depolarization, the same cellular components that underlie thalamic alpha rhythms can also lead to theta (2-7 Hz) rhythms when the TC neuron population is less depolarized. As such, this scenario can explain both the deceleration of alpha rhythms that takes place during early sleep and the chronic slowing that characterizes a host of neurological and psychiatric disorders.

Interneurons and triadic circuitry of the thalamus

The thalamus is strategically placed to control the flow of information to cortex and thus conscious perception. A key player in this control is a local GABAergic interneuron that inhibits relay cells. This interneuron is especially interesting because, in addition to a conventional axonal output, most of its output is via distal dendrites. The latter seem to be electrotonically and thus functionally isolated from the soma and axon, and they enter into complex synaptic arrangements. It is proposed that, because of special synaptic properties of its dendritic outputs, this local GABAergic interneuron of the thalamus provides gain control for the relay cell and thereby keeps relay of information to cortex within a fairly linear regime.

Functional synaptic contacts by intranuclear axon collaterals of thalamic relay neurons

Relay neurons of the lateral geniculate nucleus innervate visual cortex, but they also provide axonal collaterals to neurons in the thalamic reticular nucleus, and these thalamic reticular neurons provide feedback inhibition to relay cells. An alternative source of inhibitory inputs onto geniculate relay neurons arises from intralaminar interneurons that provide feedforward inhibition via retinogeniculate innervation, and perhaps feedback inhibition via the corticothalamic pathway, analogous to that involving thalamic reticular neurons. Several reports indicate that relay neurons may also give rise to axonal collaterals within the lateral geniculate nucleus, constituting another route for feedback or local integration. We now provide new data indicating that collaterals from geniculate relay neurons provide excitatory input to local intralaminar interneurons and that this pathway may serve as a previously unknown means of local feedback inhibition. This circuitry could prove important in such activities as surround inhibition of receptive fields or increasing signal gain over noise.

Laminar and cellular targets of individual thalamic reticular nucleus axons in the lateral geniculate nucleus in the prosimian primate Galago

The visual sector of the thalamic reticular nucleus is the source of the primary inhibitory projection to the visual thalamic relay nucleus, the dorsal lateral geniculate nucleus. The purpose of this study was to investigate laminar and cellular targets of individual thalamic reticular nucleus axons in the highly laminated lateral geniculate nucleus of the prosimian primate Galago to better understand the nature and function of this projection. Thalamic reticular axons labeled anterogradely by means of biotinylated dextran amine were examined by using light microscopic serial reconstruction and electron microscopic analysis in combination with postembedding immunohistochemical labeling for the neurotransmitter gamma-aminobutyric acid (GABA). The synaptic targets of labeled reticular terminal profiles were primarily GABA-negative dendrites (79-84%) of thalamocortical cells, whereas up to 16% were GABA-positive dendritic shafts or F2 terminals of interneurons. Reconstructed thalamic reticular nucleus axons were narrowly aligned along a single axis perpendicular to the geniculate laminar plane, exhibiting a high degree of visuotopic precision. Individual reticular axons targeted multiple or all geniculate laminae, with little laminar selectivity in the distribution of swellings with regard to the eye of origin or to the parvocellular, koniocellular, or magnocellular type neurons contained in the separate layers of the Galago lateral geniculate nucleus. These results suggest that cells in the visual thalamic reticular nucleus influence the lateral geniculate nucleus retinotopically, with little regard to visual functional streams.

Thalamic organization and function after Cajal

Cajal's many contributions to understanding the thalamus have been hidden by his body of work on the cerebral cortex. He delineated many thalamic nuclei in rodents, defined afferent fibers, thalamocortical relay neurons and interneurons, was first to demonstrate thalamocortical fibers and their terminations in the cortex, and recognized the feed-back provided by corticothalamic fibers. This presentation outlines modern methods for identifying classes of thalamic neurons, their chemical characteristics, synaptology and differential connections, and describes the intrinsic circuitry of the thalamus, showing how interactions between GABAergic cells of the reticular nucleus and glutamatergic relay cells underlie rhythmic activities of neurons in the thalamo-cortico-thalamic network, activities associated with changes in the conscious state, and which are generated and maintained by the corticothalamic projection. Corticothalamic fibers interact with reticular nucleus cells and relay cells through NMDA, AMPA and metabotropic receptors while interactions between reticular nucleus cells and relay cells are mediated by GABAA and GABAB receptors. Differing strengths of synaptic input to the two cell types, from which oscillatory behavior commences, depend upon differential expression at individual synapses of specific AMPA receptor subunits which modulate excitatory postsynaptic conductances. Two classes of relay cells can be distinguished by differential staining for calbindin and parvalbumin. The first forms a matrix in the thalamus, unconstrained by nuclear borders; the second is concentrated in certain nuclei in which it forms the topographically organized core. In projecting diffusely to the cortex, calbindin cells provide a substrate for binding together activities of multiple cortical areas that receive focused input from single thalamic nuclei. This, and the presence of specific and diffuse corticothalamic projections may serve to promote coherent activity of large populations of cortical and thalamic neurons in perception, attention and conscious awareness.

Neuromodulatory role of serotonin in the ferret thalamus

Serotonergic fibers broadly innervate the thalamus and may influence the sleep wake cycle, attention, and other processes through modulation of neurons in this structure. However, the actions of serotonin in the dorsal thalamus have been investigated in detail only in the dorsal lateral geniculate nucleus. In the present study, we examined the action of serotonin in several different regions of the ferret dorsal thalamus, including the associative nuclei, using the in vitro slice preparation and intracellular recording techniques. In nearly all nuclei examined, the predominant action of serotonin was one of hyperpolarization and inhibition of the tonic firing mode. The magnitude of the hyperpolarizing response decreased with age and varied greatly across and somewhat within nuclei maintaining the following relationship (in descending order of magnitude): lateral posterior, lateral dorsal, pulvinar, mediodorsal, center median, anteroventral, central lateral, ventral basal, and medial geniculate. This hyperpolarization is elicited through two mechanisms: one direct and the other via local interneurons. The direct action occurs through an increase in potassium conductance mediated through the 5-HT(1A) receptor. This conclusion is supported by the findings that it persists in the presence of tetrodotoxin and block of GABAergic synaptic transmission, the reversal potential shifts in a Nernstian fashion with changes in extracellular potassium concentration, and the response is antagonized by the 5-HT(1A) antagonist WAY100635 and mimicked by the application of the 5-HT(1A)-selective agonist 8-OH DPAT. The second mechanism by which 5-HT evoked a hyperpolarization was through the activation of local interneurons. In slices in which GABA receptors were not blocked, 5-HT application increased the frequency and amplitude of spontaneous inhibitory postsynaptic potentials (IPSPs) occurring in thalamocortical neurons. Application of 5-HT to physiologically or morphologically identified interneurons evoked a prolonged suprathreshold depolarization. Our results suggest that serotonergic inputs act differentially across the thalamus in a complex manner involving direct and indirect mechanisms. It appears that 5-HT has a greater direct postsynaptic inhibitory influence in the posterior, medial, and intralaminar nuclei than in the primary sensory nuclei.

Synaptic targets of thalamic reticular nucleus terminals in the visual thalamus of the cat

A major inhibitory input to the dorsal thalamus arises from neurons in the thalamic reticular nucleus (TRN), which use gamma-aminobutyric acid (GABA) as a neurotransmitter. We examined the synaptic targets of TRN terminals in the visual thalamus, including the A lamina of the dorsal lateral geniculate nucleus (LGN), the medial interlaminar nucleus (MIN), the lateral posterior nucleus (LP), and the pulvinar nucleus (PUL). To identify TRN terminals, we injected biocytin into the visual sector of the TRN to label terminals by anterograde transport. We then used postembedding immunocytochemical staining for GABA to distinguish TRN terminals as biocytin-labeled GABA-positive terminals and to distinguish the postsynaptic targets of TRN terminals as GABA-negative thalamocortical cells or GABA-positive interneurons. We found that, in all nuclei, the TRN provides GABAergic input primarily to thalamocortical relay cells (93-100%). Most of this input seems targeted to peripheral dendrites outside of glomeruli. The TRN does not appear to be a significant source of GABAergic input to interneurons in the visual thalamus. We also examined the synaptic targets of the overall population of GABAergic axon terminals (F1 profiles) within these same regions of the visual thalamus and found that the TRN contacts cannot account for all F1 profiles. In addition to F1 contacts on the dendrites of thalamocortical cells, which presumably include TRN terminals, another population of F1 profiles, most likely interneuron axons, provides input to GABAergic interneuron dendrites. Our results suggest that the TRN terminals are ideally situated to modulate thalamocortical transmission by controlling the response mode of thalamocortical cells.

Activation of GABA(A) receptors in subthalamic neurons in vitro: properties of native receptors and inhibition mechanisms

The subthalamic nucleus (STN) influences the output of the basal ganglia, thereby interfering with motor behavior. The main inputs to the STN are GABAergic. We characterized the GABA(A) receptors expressed in the STN and investigated the response of subthalamic neurons to the activation of GABA(A) receptors. Cell-attached and whole cell recordings were made from rat brain slices using the patch-clamp technique. The newly identified epsilon subunit confers atypical pharmacological properties on recombinant receptors, which are insensitive to barbiturates and benzodiazepines. We tested the hypothesis that native subthalamic GABA(A) receptors contain epsilon proteins. Applications of increasing concentrations of muscimol, a selective GABA(A) agonist, induced Cl(-) and HCO currents with an EC(50) of 5 microM. Currents induced by muscimol were fully blocked by the GABA(A) receptor antagonists, bicuculline and picrotoxin. They were strongly potentiated by the barbiturate, pentobarbital (+190%), and by the benzodiazepines, diazepam (+197%) and flunitrazepam (+199%). Spontaneous inhibitory postsynaptic currents were also significantly enhanced by flunitrazepam. Furthermore, immunohistological experiments with an epsilon subunit-specific antibody showed that the epsilon protein was not expressed within the STN. Native subthalamic GABA(A) receptors did not, therefore, display pharmacological or structural properties consistent with receptors comprising epsilon. Burst firing is a hallmark of Parkinson's disease. Half of the subthalamic neurons have the intrinsic capacity of switching from regular-firing to burst-firing mode when hyperpolarized by current injection. This raises the possibility that activation of GABA(A) receptors might trigger the switch. Statistical analysis of spiking activity established that 90% of intact neurons in vitro were in single-spike firing mode, whereas 10% were in burst-firing mode. Muscimol reversibly stopped recurrent electrical activity in all intact neurons. In neurons held in whole cell configuration, membrane potential hyperpolarized by -10 mV whilst input resistance decreased by 50%, indicating powerful membrane shunting. Muscimol never induced burst firing, even in neurons that exhibited the capacity of switching from regular- to burst-firing mode. These molecular and functional data indicate that native subthalamic GABA(A) receptors do not contain the epsilon protein and activation of GABA(A) receptors induces membrane shunting, which is essential for firing inhibition but prevents switching to burst-firing. They suggest that the STN, like many other parts of the brain, has the physiological and structural features of the widely expressed GABA(A) receptors consisting of alphabetagamma subunits.

The functional influence of burst and tonic firing mode on synaptic interactions in the thalamus

Thalamocortical and perigeniculate (PGN) neurons can generate action potentials either as Ca2+ spike-mediated high-frequency bursts or as tonic trains. Using dual intracellular recordings in vitro in monosynaptically connected pairs of PGN and dorsal lateral geniculate nucleus (LGNd) neurons, we found that the functional effect of synaptic transmission between these cell types was strongly influenced by the membrane potential and hence the firing mode of both the pre- and postsynaptic neurons. Activation of single action potentials or low-frequency spike trains in PGN or thalamocortical neurons resulted in the generation of PSPs that were 0.5-2.0 mV in amplitude. In contrast, the generation of Ca2+ spike-mediated bursts of action potentials in the presynaptic cell increased these PSPs to an average of 4.4 mV for the IPSP and 3.0 mV for the EPSP barrage, because of temporal summation and/or facilitation. If the postsynaptic neuron was at a resting membrane potential (e.g., -65 mV), these PSP barrages could result in the activation of a low-threshold Ca2+ spike and burst of action potentials. These results demonstrate that the burst firing mode of action potential generation is a particularly effective means by which perigeniculate and thalamocortical neurons may influence one another. We propose that the activation of burst discharges in these cell types is essential for the generation of some forms of synchronized rhythmic oscillations of sleep and of epileptic seizures.

Spatiotemporal patterns of ontogenetic expression of parvalbumin in the superior colliculi of rats and rabbits

We have examined the development of parvalbumin immunoreactivity in the superior colliculi (SC) of the perinatal and mature rats and rabbits. In mature animals, parvalbumin-expressing cells (PECs) and neuropil in the retinorecipient layers were distributed in a continuous single band extending throughout the entire extent of the colliculus, whereas those in the intermediate layers formed distinct, radially oriented patches. Parvalbumin was expressed for the first time on postconceptional day 34 (PCD 34, postnatal day 12) and PCD 42 (postnatal day 11) in the SC of rat and rabbit, respectively. During ensuing development, both the thickness of the parvalbumin-expressing band in the retinorecipient layers and the numbers of PECs in this band gradually increased, reaching adultlike values by PCD 44 and PCD 50 in the rat and rabbit, respectively. In the rat, monocular eye enucleations on PCD 23 resulted in approximately 55% reduction in the number of PECs in the retinorecipient layers of the contralateral colliculi examined on PCD 44 or PCD 50. Unilateral ablations of the entire visual cortex on PCD 23 (before the first corticotectal fibers from visual cortices reach the SC) or on PCD 28 (when about half of the corticotectal fibers have reached colliculus) resulted in, respectively, approximately 55% and approximately 25% relative reduction in the number of PECs in the retinorecipient layers of the ipsilateral colliculi examined on PCD 44 or PCD 50. We conclude that the ontogenetic expression of parvalbumin in most of PECs in the retinorecipient collicular layers is induced by the activity of the contralateral retinotectal and/or the activity of the ipsilateral corticotectal afferents.

Functional properties of perigeniculate inhibition of dorsal lateral geniculate nucleus thalamocortical neurons in vitro

The properties of the inhibitory influence of neurons in the perigeniculate (PGN) nucleus on thalamocortical cells were examined with intracellular recordings in the ferret geniculate slice maintained in vitro. Activation of PGN neurons with the local application of glutamate caused IPSPs in thalamocortical neurons that were mediated by both GABAA and GABAB receptors, as well as the activation of spindle waves. With low intensity stimulation of the PGN, local application of bicuculline to the dorsal lateral geniculate nucleus (LGNd) strongly inhibited evoked and spindle-associated IPSPs, indicating that these are largely mediated by GABAA receptors. The generation of GABAB receptor-mediated IPSPs in thalamocortical cells that were large enough to generate rebound low threshold Ca2+ spikes required substantially increased activation of the PGN with glutamate. The activation of synchronous bicuculline-induced slowed oscillations in thalamocortical neurons required the block of GABAA receptors in the LGNd as well as in the PGN. These results indicate that bursts of action potentials in PGN neurons can result in the activation of both GABAA and GABAB receptors in thalamocortical neurons, with the strong activation of GABAB receptors requiring an intense, simultaneous discharge of a number of PGN neurons. Functionally, these results suggest that PGN neurons inhibit thalamocortical cells preferentially through the activation of GABAA receptors, although the strong activation of GABAB receptors may occur under pathological conditions and contribute to the generation of abnormal, synchronous slow oscillations.

Inhibitory interactions between perigeniculate GABAergic neurons

Perigeniculate neurons form an interactive sheet of cells that inhibit one another as well as thalamocortical neurons in the dorsal lateral geniculate nucleus (LGNd). The inhibitory influence of the GABAergic neurons of the perigeniculate nucleus (PGN) onto other PGN neurons was examined with intracellular recordings in vitro. Intracellular recordings from PGN neurons during the generation of spindle waves revealed barrages of EPSPs and IPSPs. The excitation of local regions of the PGN with the local application of glutamate resulted in activation of IPSPs in neighboring PGN neurons. These IPSPs displayed an average reversal potential of -77 mV and were blocked by application of bicuculline methiodide or picrotoxin, indicating that they are mediated by GABAA receptors. In the presence of GABAA receptor blockade, the activation of PGN neurons with glutamate could result in slow IPSPs that were mediated by GABAB receptors in a subset (40%) of cells. Similarly, application of specific agonists muscimol and baclofen demonstrated that PGN neurons possess both functional GABAA and GABAB receptors. Examination of the axon arbors of biocytin-filled PGN neurons often revealed the presence of beaded axon collaterals within the PGN, suggesting that this may be an anatomical substrate for PGN to PGN inhibition. Functionally, activation of inhibition between PGN neurons could result in a shortening or a complete abolition of the low threshold Ca2+ spike or an inhibition of tonic discharge. We suggest that the mutual inhibition between PGN neurons forms a mechanism by which the excitability of these cells is tightly controlled. The activation of a point within the PGN may result in the inhibition of neighboring PGN neurons. This may be reflected in the LGNd as a center of inhibition surrounded by an annulus of disinhibition, thus forming a "center-surround" mechanism for thalamic function.