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

Immunocytochemical and ultrastructural organization of the taste thalamus of the tree shrew (Tupaia belangeri)

Ventroposterior medialis parvocellularis (VPMP) nucleus of the primate thalamus receives direct input from the nucleus of the solitary tract, whereas the homologous thalamic structure in the rodent does not. To reveal whether the synaptic circuitries in these nuclei lend evidence for conservation of design principles in the taste thalamus across species or across sensory thalamus in general, we characterized the ultrastructural and molecular properties of the VPMP in a close relative of primates, the tree shrew (Tupaia belangeri), and compared these to known properties of the taste thalamus in rodent, and the visual thalamus in mammals. Electron microscopy analysis to categorize the synaptic inputs in the VPMP revealed that the largest-size terminals contained many vesicles and formed large synaptic zones with thick postsynaptic density on multiple, medium-caliber dendrite segments. Some formed triads within glomerular arrangements. Smaller-sized terminals contained dark mitochondria; most formed a single asymmetric or symmetric synapse on small-diameter dendrites. Immuno-EM experiments revealed that the large-size terminals contained VGLUT2, whereas the small-size terminal populations contained VGLUT1 or ChAT. These findings provide evidence that the morphological and molecular characteristics of synaptic circuitry in the tree shrew VPMP are similar to that in nonchemical sensory thalamic nuclei. Furthermore, the results indicate that all primary sensory nuclei of the thalamus in higher mammals share a structural template for processing thalamocortical sensory information. In contrast, substantial morphological and molecular differences in rodent versus tree shrew taste nuclei suggest a fundamental divergence in cellular processing mechanisms of taste input in these two species.

Activation of both Group I and Group II metabotropic glutamatergic receptors suppress retinogeniculate transmission

Relay cells of dorsal lateral geniculate nucleus (LGN) receive a Class 1 glutamatergic input from the retina and a Class 2 input from cortical layer 6. Among the properties of Class 2 synapses is the ability to activate metabotropic glutamate receptors (mGluRs), and mGluR activation is known to affect thalamocortical transmission via regulating retinogeniculate and thalamocortical synapses. Using brain slices, we studied the effects of Group I (dihydroxyphenylglycine) and Group II ((2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine) mGluR agonists on retinogeniculate synapses. We showed that both agonists inhibit retinogeniculate excitatory postsynaptic currents (EPSCs) through presynaptic mechanisms, and their effects are additive and independent. We also found high-frequency stimulation of the layer 6 corticothalamic input produced a similar suppression of retinogeniculate EPSCs, suggesting layer 6 projection to LGN as a plausible source of activating these presynaptic mGluRs.

Activity-dependent regulation of retinogeniculate signaling by metabotropic glutamate receptors

Thalamocortical neurons in dorsal lateral geniculate nucleus (dLGN) dynamically convey visual information from retina to the neocortex. Activation of metabotropic glutamate receptors (mGluRs) exerts multiple effects on neural integration in dLGN; however, their direct influence on the primary sensory input, namely retinogeniculate afferents, is unknown. In the present study, we found that pharmacological or synaptic activation of type 1 mGluRs (mGluR(1)s) significantly depresses glutamatergic retinogeniculate excitation in rat thalamocortical neurons. Pharmacological activation of mGluR(1)s attenuates excitatory synaptic responses in thalamocortical neurons at a magnitude sufficient to decrease suprathreshold output of these neurons. The reduction in both NMDA and AMPA receptor-dependent synaptic responses results from a presynaptic reduction in glutamate release from retinogeniculate terminals. The suppression of retinogeniculate synaptic transmission and dampening of thalamocortical output was mimicked by tetanic activation of retinogeniculate afferent in a frequency-dependent manner that activated mGluR(1)s. Retinogeniculate excitatory synaptic transmission was also suppressed by the glutamate transport blocker TBOA (dl-threo-β-benzyloxyaspartic acid), suggesting that mGluR(1)s were activated by glutamate spillover. The data indicate that presynaptic mGluR(1) contributes to an activity-dependent mechanism that regulates retinogeniculate excitation and therefore plays a significant role in the thalamic gating of visual information.

Abnormalities in thalamic neurophysiology in schizophrenia: could psychosis be a result of potassium channel dysfunction?

Psychosis in schizophrenia is associated with source-monitoring deficits whereby self-initiated behaviors become attributed to outside sources. One of the proposed functions of the thalamus is to adjust sensory responsiveness in accordance with the behavioral contextual cues. The thalamus is markedly affected in schizophrenia, and thalamic dysfunction may here result in reduced ability to adjust sensory responsiveness to ongoing behavior. One of the ways in which the thalamus accomplishes the adjustment of sensory processing is by a neurophysiological shift to post-inhibitory burst firing mode prior to and during certain exploratory actions. Reduced amount of thalamic burst firing may result from increased neuronal excitability secondary to a reported potassium channel dysfunction in schizophrenia. Pharmacological agents that reduce the excitability of thalamic cells and thereby promote burst firing by and large tend to have antipsychotic effects.

Inhibitory circuits for visual processing in thalamus

Synapses made by local interneurons dominate the intrinsic circuitry of the mammalian visual thalamus and influence all signals traveling from the eye to cortex. Here we draw on physiological and computational analyses of receptive fields in the cat's lateral geniculate nucleus to describe how inhibition helps to enhance selectivity for stimulus features in space and time and to improve the efficiency of the neural code. Further, we explore specialized synaptic attributes of relay cells and interneurons and discuss how these might be adapted to preserve the temporal precision of retinal spike trains and thereby maximize the rate of information transmitted downstream.

Dopamine enhances the excitability of somatosensory thalamocortical neurons

The thalamus conveys sensory information from peripheral and subcortical regions to the neocortex in a dynamic manner that can be influenced by several neuromodulators. Alterations in dopamine (DA) receptor function in thalami of Schizophrenic patients have recently been reported. In addition, schizophrenia is associated with sensory gating abnormalities and sleep-wake disturbances, thus we examined the role of DA on neuronal excitability in somatosensory thalamus. The ventrobasal (VB) thalamus receives dopaminergic innervation and expresses DA receptors; however, the action of DA on VB neurons is unknown. In the present study, we performed whole cell current- and voltage-clamp recordings in rat brain slices to investigate the role of DA on excitability of VB neurons. We found that DA increased action potential discharge and elicited membrane depolarization via activation of different receptor subtypes. Activation of D2-like receptors (D(2R)) leads to enhanced action potential discharge, whereas the membrane depolarization was mediated by D1-like receptors (D(1R)). The D(2R-mediated) increase in spike discharge was mimicked and occluded by α-dendrotoxin (α-DTX), indicating the involvement of a slowly inactivating K(+) channels. The D1R-mediated membrane depolarization was occluded by barium, suggesting the involvement of a G protein-coupled K(+) channel or an inwardly rectifying K(+) channel. Our results indicate that DA produces dual modulatory effects acting on subtypes of DA receptors in thalamocortical relay neurons, and likely plays a significant role in the modulation of sensory information.

Metabotropic glutamate receptors differentially regulate GABAergic inhibition in thalamus

Thalamic interneurons and thalamic reticular nucleus (TRN) neurons provide inhibitory innervation of thalamocortical cells that significantly influence thalamic gating. The local interneurons in the dorsal lateral geniculate nucleus (dLGN) give rise to two distinct synaptic outputs: classical axonal and dendrodendritic. Activation of metabotropic glutamate receptors (mGluRs) by agonists or optic tract stimulation increases the output of these presynaptic dendrites leading to increased inhibition of thalamocortical neurons. The present study was aimed to evaluate the actions of specific mGluRs on inhibitory GABA-mediated signaling. We found that the group I mGluR (mGluR(1,5)) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) or optic tract stimulation produced a robust increase in spontaneous IPSCs (sIPSCs) in thalamocortical neurons that was attenuated by the selective mGluR(5) antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP). In contrast, the group II mGluR (mGluR(2,3)) agonists (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) or (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl)glycine (DCG-IV) suppressed the frequency of sIPSCs. In addition, mGluR(1,5) agonist DHPG produced depolarizations and mGluR(2/3) agonists APDC or L-CCG-I [(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine] produced hyperpolarizations in dLGN interneurons. Furthermore, the enhanced sIPSC activity by optic tract stimulation was reduced when paired with corticothalamic fiber stimulation. The present data indicate that activation of specific mGluR subtypes differentially regulates inhibitory activity via different synaptic pathways. Our results suggest that activation of specific mGluR subtypes can upregulate or downregulate inhibitory activity in thalamic relay neurons, and these actions likely shape excitatory synaptic integration and thus regulate information transfer through thalamocortical circuits.

An argument for an olfactory thalamus

The mammalian olfactory system is unique in that sensory receptors synapse directly into the olfactory bulb of the forebrain without the thalamic relay that is common to all other sensory pathways. We argue that the olfactory bulb has an equivalent role to the thalamus, because the two regions have very similar structures and functions. Both the thalamus and the olfactory bulb are the final stage in sensory processing before reaching target cortical regions, at which there is a massive increase in neuron and synapse numbers. Thus, both structures act as a bottleneck that is a target for various modulatory inputs, and this arrangement enables efficient control of information flow before cortical processing occurs.

Modulation of thalamic neuron excitability by orexins

Orexins (hypocretins) are peptides of hypothalamic origin that play an important role in maintaining wakefulness. Reduced orexin levels have been associated with an increased incidence of narcolepsy. Considering thalamic nuclei are interconnected with virtually all neocortical regions and the thalamus has been found to produce distinct activities related to different levels of arousal, we have examined the actions of orexins on thalamic neurons using an in vitro thalamic slice preparation. The orexins (orexin-A and orexin-B) produced distinct actions within different intralaminar nuclei. Orexin-B strongly depolarized the majority of centrolateral nucleus (CL) neurons (71%), but depolarized a significantly smaller population of parafascicular nuclei (Pf) neurons (10%). In the mediodorsal thalamic nucleus (MD), orexin-B depolarized 21% of the neurons tested. Overall, orexin-B was found to be more potent than Orexin-A. Orexin-A depolarized a significantly smaller population of CL neurons (23%), but had no effect on Pf neurons. In addition, orexin-A produced a small depolarization in 28% of neurons in the thalamic reticular nucleus (TRN). Both orexin-A and orexin-B had no effect on neurons in the lateral posterior (LP), lateralodorsal (LD), posterior thalamic (Po), ventrobasal (VB) nucleus and lateral geniculate nucleus (LGN). The depolarizing actions of orexins were sufficient to alter the firing mode of these neurons from a burst- to tonic-firing mode. The excitatory actions of orexin-B result from a decrease in the apparent leak potassium current (Kleak). The orexin-B mediated excitation was also attenuated by bupivacaine suggesting the involvement of Kleak current. Further, the actions of orexin-B were occluded by the classical neurotransmitter dopamine, indicating the orexins may share similar ionic mechanisms. Thus, the depolarizing actions of orexins may play a key role in altering the firing mode of thalamic neurons associated with different states of consciousness.