A systematic topographical relationship between mouse lateral posterior thalamic neurons and their visual cortical projection targets.

Higher-order visual thalamus communicates broadly and bi-directionally with primary and extrastriate cortical areas in various mammals. In primates, the pulvinar is a topographically and functionally organized thalamic nucleus that is largely dedicated to visual processing. Still, a more granular connectivity map is needed to understand the role of thalamocortical loops in visually guided behavior. Similarly, the secondary visual thalamic nucleus in mice (the lateral posterior nucleus, LP) has extensive connections with cortex. To resolve the precise connectivity of these circuits, we first mapped mouse visual cortical areas using intrinsic signal optical imaging and then injected fluorescently tagged retrograde tracers (cholera toxin subunit B) into retinotopically-matched locations in various combinations of seven different visual areas. We find that LP neurons representing matched regions in visual space but projecting to different extrastriate areas are found in different topographically organized zones, with few double-labeled cells (~4-6%). In addition, V1 and extrastriate visual areas received input from the ventrolateral part of the laterodorsal nucleus of the thalamus (LDVL). These observations indicate that the thalamus provides topographically organized circuits to each mouse visual area and raise new questions about the contributions from LP and LDVL to cortical activity.

A New Projection From the Deep Cerebellar Nuclei to the Hippocampus via the Ventrolateral and Laterodorsal Thalamus in Mice.

The cerebellar involvement in cognitive functions such as attention, language, working memory, emotion, goal-directed behavior and spatial navigation is constantly growing. However, an exact connectivity map between the hippocampus and cerebellum in mice is still unknown. Here, we conducted a tracing study to identify the sequence of transsynaptic, cerebellar-hippocampal connections in the mouse brain using combinations of Recombinant adeno-associated virus (rAAV) and pseudotyped deletion-mutant rabies (RABV) viruses. Stereotaxic injection of a primarily anterograde rAAV-WGA (wheat germ agglutinin)-Cre tracer virus in the deep cerebellar nuclei (DCN) of a Cre-dependent tdTomato reporter mouse resulted in strong tdTomato labeling in hippocampal CA1 neurons, retrosplenial cortex (RSC), rhinal cortex (RC) as well as thalamic and cerebellar areas. Whereas hippocampal injections with the retrograde tracer virus rAAV-TTC (tetanus toxin C fragment)-eGFP, displayed eGFP positive cells in the rhinal cortex and subiculum. To determine the sequence of mono-transsynaptic connections between the cerebellum and hippocampus, we used the retrograde tracer RABVΔG-eGFP(EnvA). The tracing revealed a direct connection from the dentate gyrus (DG) in the hippocampus to the RSC, RC and subiculum (S), which are monosynaptically connected to thalamic laterodorsal and ventrolateral areas. These thalamic nuclei are directly connected to cerebellar fastigial (FN), interposed (IntP) and lateral (Lat) nuclei, discovering a new projection route from the fastigial to the laterodorsal thalamic nucleus in the mouse brain. Collectively, our findings suggest a new cerebellar-hippocampal connection via the laterodorsal and ventrolateral thalamus to RSC, RC and S. These results strengthen the notion of the cerebellum's involvement in cognitive functions such as spatial navigation via a polysynaptic circuitry.

Functional correlation of GABA(A) receptor alpha subunits expression with the properties of IPSCs in the developing thalamus.

GABA(A) receptor alpha1 and alpha2 subunits are expressed differentially with ontogenic period in the brain, but their functional roles are not known. We have recorded GABA(A) receptor-mediated IPSCs from laterodorsal (LD) thalamic relay neurons in slices of rat brain at various postnatal ages and found that decay times of evoked IPSCs and spontaneous miniature IPSCs undergo progressive shortening during the first postnatal month. With a similar time course, expression of transcripts and proteins of GABA(A) receptor alpha2 subunit in LD thalamic region declined, being replaced by those of alpha1 subunit. To further address the causal relationship between alpha subunits and IPSC decay time kinetics, we have overexpressed GABA(A) receptor alpha1 subunit together with green fluorescent protein in LD thalamic neurons in organotypic culture using recombinant Sindbis virus vectors. Miniature IPSCs recorded from the LD thalamic neurons overexpressed with alpha1 subunit had significantly faster decay time compared with control expressed with beta-galactosidase. We conclude that the alpha2-to-alpha1 subunit switch underlies the developmental speeding in the decay time of GABAergic IPSCs.

Comparison of the ultrastructure of cortical and retinal terminals in the rat dorsal lateral geniculate and lateral posterior nuclei.

We compared the ultrastructure and synaptic targets of terminals of cortical or retinal origin in the rat dorsal lateral geniculate nucleus (LGN) and lateral posterior nucleus (LPN). Following injections of biotinylated dextran amine (BDA) into cortical area 17, two types of corticothalamic terminals were labeled by anterograde transport. Type I terminals, found throughout the LGN and LPN, were small, drumstick-shaped terminals that extended from thin axons. At the ultrastructural level in both the LGN and LPN, labeled type I corticothalamic terminals were observed to be small profiles that contained densely packed round vesicles (RS profiles) and contacted small-caliber dendrites. In tissue stained for gamma amino butyric acid (GABA) using postembedding immunocytochemical techniques, most dendrites postsynaptic to type I corticothalamic terminals did not contain GABA (97%). Type II corticothalamic terminals, found only in the LPN, were large terminals that sometimes formed clusters. At the ultrastructural level, type II terminals were large profiles that contained round vesicles (RL profiles) and contacted large-caliber dendrites, most of which did not contain GABA (98%). Retinogeniculate terminals, identified by their distinctive pale mitochondria, were similar to type II corticothalamic terminals except that 26% of their postsynaptic targets were vesicle-containing profiles that contained GABA (F2 profiles). Our results suggest that type I corticothalamic terminals are very similar across nuclei but that the postsynaptic targets of RL profiles vary. Comparison of the responses to retinal inputs in the LGN and to layer V cortical inputs in the LPN may provide a unique opportunity to determine the function of interneurons in the modulation of retinal signals and, in addition, may provide insight into the signals relayed by cortical layer V.

Distribution and properties of GABA(B) antagonist [3H]CGP 62349 binding in the rhesus monkey thalamus and basal ganglia and the influence of lesions in the reticular thalamic nucleus.

GABA(B) receptors are believed to be associated with the efferents of the nucleus reticularis thalami, which is implicated in the regulation of activity in the thalamocortical-corticothalamic circuit and plays a role in absence seizures. Yet, the distribution of GABA(B) receptors in the thalamus has only been studied in the rat, and there is no comparable information in primates. The potent GABA(B) receptor antagonist [3H]CGP 62349 was used to study the distribution and binding properties of the receptor in control monkeys and those with small ibotenic acid lesions in the anterodorsal segment of the nucleus reticularis thalami. Eight-micrometer-thick cryostat sections of the fresh frozen brains were incubated in the presence of varying concentrations of the ligand. Autoradiographs were analysed using a quantitative image analysis technique, and binding parameters were calculated for select thalamic nuclei as well as basal ganglia structures present in the same sections. The overall number of GABA(B) binding sites in the monkey thalamus and basal ganglia was several-fold higher than previously reported values for the rat. In the thalamus, the receptors were distributed rather uniformly and the binding densities and affinities were high (Bmax range of 245.5-437.9 fmol/ mg of tissue, Kd range of 0.136-0.604 nM). In the basal ganglia, the number of binding sites and the affinities were lower (Bmax range of 51.1-244.2 fmol/mg of tissue; K(d) range of 0.416-1.394 nM), and the differences between nuclei were more pronounced, with striatum and substantia nigra pars compacta displaying the highest binding densities. Seven days post-lesion, a 20-30% decrease in Bmax values (P < 0.05) was found in the nuclei receiving input from the lesioned nucleus reticularis thalami sector (the mediodorsal nucleus and densicellular and magnocellular parts of the ventral anterior nucleus) without changes in affinity. No significant changes were detected in any other structures. The results of the lesioning experiments suggest that a portion of thalamic GABA(B) receptors is in a presynaptic location on the nucleus reticularis thalami efferents. The overall distribution pattern in the thalamus also suggests a partial association of GABA(B) receptors with corticothalamic terminals presynaptically.

Distribution of angiotensin II binding sites in the mouse thalamus: receptor-binding study with fluorescent coupled peptides and their conversion to a light stable product.

Fluorescence-coupled peptides allow a non-radioactive receptor binding study whereby single cells can be examined under a fluorescence microscope. By the combination of such a method with immunohistochemistry, using an HRP-coupled anti-fluorescein antibody, a permanent labeling can be achieved. By using this method the distribution of angiotensin II binding sites has been examined in the mouse thalamus. The results show that a moderate staining was obvious within the thalamus and that the distribution of binding sites in the thalamus is very homogeneous in the mouse brain. In detail, angiotensin II binding sites were found in the anterodorsal nucleus, in the laterodorsal and posterior nucleus of the thalamus, as well as in the lateral geniculate nucleus, the reticular thalamic nucleus and in the zona incerta.

Subtypes of low voltage-activated Ca2+ channels in laterodorsal thalamic neurons: possible localization and physiological roles.

The macroscopic, low-voltage-activated (LVA or T-type) Ca2+ current in isolated associative (or local-circuit) neurons from the laterodorsal thalamic nucleus of 14-17-day old rats was dissected into two components ("fast" and "slow"), corresponding to the activation of two LVA channel subtypes, based on the difference in the kinetics of inactivation and recovery from inactivation. The steady-state activation and inactivation properties of the channel subtypes endowed slow channels with a substantial window current, whereas fast channels had almost no such current. Fast channels were almost 2 times more sensitive to 30 microM nifedipine (78% inhibition), 10 microM flunarizine (92% inhibition) and 1 microM La3+ (87% inhibition), but about 1.8-fold less sensitive to 100 microM Ni2+ (32% inhibition) than slow channels (40%, 52%, 46% and 56% inhibition respectively). Both channels were almost equally sensitive to 100 microM amiloride (58% and 51% inhibition of fast and slow channels respectively). Comparison of the fast and slow LVA Ca2+ current amplitudes and densities between enzymatically isolated and intact (in brain slices) neurons suggest a predominant localization of the fast channels in soma and the proximal dendrites that remain intact during isolation procedure, whereas the slow channels are more evenly distributed with some preference to the distal areas. These data, together with our previous studies, support the notion of two LVA Ca2+ channel subtypes in associative thalamic neurons and suggest a role for the slow channels in providing the constant Ca2+ influx necessary for the outgrowth of the neurites and for the fast channels in the generation of low-threshold Ca2+ spikes and bursting activity.