Immunoelectron microscopic study of glutamate inputs from the retrosplenial granular cortex to identified thalamocortical projection neurons in the anterior thalamus of the rat

Thalamic neuromodulation and its implications for executive networks

The thalamus is a key structure that controls the routing of information in the brain. Understanding modulation at the thalamic level is critical to understanding the flow of information to brain regions involved in cognitive functions, such as the neocortex, the hippocampus, and the basal ganglia. Modulators contribute the majority of synapses that thalamic cells receive, and the highest fraction of modulator synapses is found in thalamic nuclei interconnected with higher order cortical regions. In addition, disruption of modulators often translates into disabling disorders of executive behavior. However, modulation in thalamic nuclei such as the midline and intralaminar groups, which are interconnected with forebrain executive regions, has received little attention compared to sensory nuclei. Thalamic modulators are heterogeneous in regards to their origin, the neurotransmitter they use, and the effect on thalamic cells. Modulators also share some features, such as having small terminal boutons and activating metabotropic receptors on the cells they contact. I will review anatomical and physiological data on thalamic modulators with these goals: first, determine to what extent the evidence supports similar modulator functions across thalamic nuclei; and second, discuss the current evidence on modulation in the midline and intralaminar nuclei in relation to their role in executive function.

Anterior thalamic lesions stop synaptic plasticity in retrosplenial cortex slices: expanding the pathology of diencephalic amnesia

Recent, convergent evidence places the anterior thalamic nuclei at the heart of diencephalic amnesia. However, the reasons for the severe memory loss in diencephalic amnesia remain unknown. A potential clue comes from the dense, reciprocal connections between the anterior thalamic nuclei and retrosplenial cortex, another region vital for memory. We now report a loss of synaptic plasticity [long-term depression (LTD)] in rat retrosplenial cortex slices months following an anterior thalamic lesion. The loss of LTD was lamina-specific, occurring only in superficial layers of the cortex and was associated with a decrease in GABA(A)-mediated inhibitory transmission. As retrosplenial cortex is itself vital for memory, this distal lesion effect will amplify the impact of anterior thalamic lesions. These findings not only provide novel insights into the functional pathology of diencephalic amnesia and have implications for the aetiology of the posterior cingulate hypoactivity in Alzheimer's disease, but also show how distal changes in plasticity could contribute to diaschisis.

A review on electron microscopy and neurotransmitter systems

The purpose of this article is to review the contributions of transmission electron microscopy studies to the understanding of brain circuits and neurotransmitter systems. Our views on the microstructure of connections between neurons have gradually changed, and now we recognize that the classical mental image we had on a chemical synapse is no longer applicable to every neuronal connection. We highlight studies that converge to point out that, while the most prevalent fast transmitters in the brain, glutamate and GABA, are stored in small, clear synaptic vesicles (SSV) and released at synapses, neuropeptides are exclusively stored in large dense core vesicles (LDCV) and released extrasynaptically. Amine transmitters are preferentially, but not exclusively, accumulated in LDCV and may be released at synaptic or extrasynaptic sites. We discuss evidence suggesting that axon terminals from pyramidal cortical neurons and dorsal thalamic neurons lack LDCV and therefore could not use neuropeptides as transmitters. This idea fits with the fast, high temporal resolution information processing that characterizes cortical and thalamic function.

Impaired water maze navigation of Wistar rats with retrosplenial cortex lesions: effect of nonspatial pretraining

Damage to the retrosplenial cortex (RC) impairs the performance of rodents on spatial learning and memory tasks, but the extent of these deficits was previously reported to be influenced by the lesion type, rat strain, and behavioral task used. The present study addressed the issue of whether or not cytotoxic damage to RC impairs place navigation of Wistar rats in the Morris water maze and, if so, whether this is merely attributable to spatial learning deficits or to impaired learning of general (nonspatial) behavioral strategies required to correctly perform this task or both. Behaviorally naive rats with bilateral lesions to RC were significantly impaired relative to sham-lesioned rats both during the period of initial learning of the task and during the later phases of training. In addition, these animals showed enhanced thigmotaxis, indicating that the lesion was associated with considerable abnormalities in nonspatial learning. In contrast, RC-lesioned animals that have been previously familiarized with general task rules in a series of shaping trials did not show more thigmotaxis than did their respective controls. Furthermore, although these rats were still impaired in the middle of the training process, their performance during the period of initial learning as well as by the end of training was found to now be normal. Our results confirm those of earlier studies indicating that RC is important for spatial navigation. The findings herein reported are also consistent with the notion that, in addition to spatial information processing, RC is involved in cognitive processes underlying the ability of subjects to properly respond to general task demands.

Ultrastructure of ascending cholinergic terminals in the anteroventral thalamic nucleus of the rat: a comparison with the mammillothalamic terminals

In this study, to identify the ultrastructure and distribution of ascending cholinergic afferent terminals in the anteroventral thalamic nucleus, we used an anti-vesicular acetylcholine transporter antibody as marker of cholinergic afferents, and characterized the immunoreactive terminals at the ultrastructural level. We then compared the distribution pattern of the cholinergic terminals and that of the mammillothalamic terminals identified by anterograde transport of a tracer injected into the mammillary body. The cholinergic terminals were small, and formed both symmetrical and asymmetrical synaptic contacts throughout the dendritic arborizations, particularly in the distal region. This distribution pattern differed from that of mammillothalamic terminals, that were of LR (large terminal containing round synaptic vesicles) type and were preferentially distributed in the proximal region of dendrites. We also found relatively numerous cholinergic terminals making contact directly with immunonegative excitatory terminals, both LR and SR (small terminal containing round vesicles) terminals, without clear postsynaptic specialization. A few cholinergic terminals even seemed to form a synaptic complex with the LR or SR terminals. These findings suggest that the ascending cholinergic afferents in the anteroventral thalamic nucleus can effectively modulate excitatory inputs from both the mammillothalamic and corticothalamic terminals, in close vicinity to a synaptic site.

The early stages of schizophrenia: speculations on pathogenesis, pathophysiology, and therapeutic approaches

Schizophrenia is commonly considered a neurodevelopmental disorder that is associated with significant morbidity; however, unlike other neurodevelopmental disorders, the symptoms of schizophrenia often do not manifest for decades. In most patients, the formal onset of schizophrenia is preceded by prodromal symptoms, including positive symptoms, mood symptoms, cognitive symptoms, and social withdrawal. The proximal events that trigger the formal onset of schizophrenia are not clear but may include developmental biological events and environmental interactions or stressors. Treatment with antipsychotic drugs clearly ameliorates psychotic symptoms, and maintenance therapy may prevent the occurrence of relapse. The use of atypical antipsychotic agents may additionally ameliorate the pathophysiology of schizophrenia and prevent disease progression. Moreover, if treated properly early in the course of illness, many patients can experience a significant remission of their symptoms and are capable of a high level of recovery following the initial episode. Because the clinical deterioration that occurs in schizophrenia may actually begin in the prepsychotic phase, early identification and intervention may favorably alter the course and outcome of schizophrenia.

Differential immunolocalization of m2 and m3 muscarinic receptors in the anteroventral and anterodorsal thalamic nuclei of the rat

In this study, to identify the precise localization of m2 and m3 muscarinic receptors in the anteroventral and anterodorsal thalamic nuclei of the rat, we used receptor-subtype-specific antibodies and characterized their immunolocalization patterns by light and electron microscopy. Many m2-positive neurons were distributed throughout these nuclei. Ultrastructural analysis showed that more than 30% of m2-positive dendritic profiles in these nuclei are proximal dendritic shafts. Moreover, a few m2-positive fiber terminals were found only in the anterodorsal thalamic nucleus. These m2-positive terminals were large (1.10+/-0.30 microm in diameter) and formed asymmetrical synapses with dendritic profiles. The m3-positive neurons were also distributed in both nuclei, and the m3-positive neuropil exhibited a significant staining gradient, with the most intense staining in the ventrolateral part of the anteroventral thalamic nucleus. This region receives the densest cholinergic input originating from the dorsal tegmental region. At the ultrastructural level, the majority of m3-positive dendritic profiles were more distal regions of the dendrites compared to the m2 receptors in the anteroventral thalamic nucleus. However, no significant difference in the intradendritic distribution pattern between m2 and m3 receptors was found in the anterodorsal thalamic nucleus, which receives no cholinergic input. These findings show the differential localization of m2 and m3 receptors in the anteroventral and anterodorsal thalamic nuclei, and suggest that the m3 receptors are spatially more closely associated with ascending cholinergic afferent fibers in the anteroventral thalamic nucleus.