Individual Subnuclei of the Rat Anterior Thalamic Nuclei Differently affect Spatial Memory and Passive Avoidance Tasks

The role of the anterior thalamic nuclei (ATN) has been proven in different learning and memory tasks. The ATN consist of three main subnuclei, the anterodorsal (AD), anteroventral (AV) and anteromedial (AM), which have different biological characteristics such as distinct circuitry, cell population and neurotransmitter content. The role of ATN subnuclei in learning and memory has been shown in several studies. However, their probable role in different phases of memory including acquisition, consolidation and retrieval are not still well-known. For this purpose, the effect of reversible inactivation of each ATN subnucleus on different memory phases in two behavioral tasks including passive avoidance (PA) and Morris water maze (MWM) was studied. Wister male rats were bilaterally implanted with cannulas above the AD, AV or AM subnucleus in separate experimental groups in order to inject lidocaine (4%) for their temporal inactivation or, equal volume of saline. Animals were trained in the behavioral tasks and different phases of memory were investigated. Our findings indicated that the AV inactivation strongly disrupts all memory phases in the MWM, and consolidation and retrieval phases in the PA tasks. The AM inactivation had no effect on acquisition of both tasks while it impaired the PA consolidation and MWM retrieval. However, the AD inactivation could not disrupt memory phases in the PA task but impaired the MWM retrieval. In conclusion, it seems that the ATN distinct subnuclei differently affect different phases of memory in these two tasks.

The effect of pharmacological inactivation of the mammillary body and anterior thalamic nuclei on hippocampal theta rhythm in urethane-anesthetized rats

The mammillary body (MB) and the anterior thalamic nuclei (ATN) are closely related structures, which take part in learning and memory processes. However, the exact role of these structures has remained unclear. In both structures neurons firing according to hippocampal theta rhythm have been found, mainly in the medial mammillary nucleus (MM) and anteroventral thalamic nucleus (AV). These neurons are driven by descending projections from the hippocampal formation and are thought to convey theta rhythm back to the hippocampus (HP). We argue that the MB-ATN axis not only relays theta signal, but may also modulate it. To examine it, we performed a pharmacological inactivation of the MM and AV by local infusion of procaine, and measured changes in theta activity in selected structures of the extended hippocampal system in urethane-anesthetized rats. The inactivation of the MM resulted in decrease in EEG power in the HP and AV, the most evidently in the lower theta frequency bands, i.e. 3-5Hz in the HP (down to 9.2% in 3- to 4-Hz band and 37.6% in 4- to 5-Hz band, in comparison to the power in the control conditions) and 3-4Hz in the AV (down to 24.9%). After the AV inactivation, hippocampal EEG power decreased in theta frequency bands of 3-8Hz (down to 61.6% in 6- to 7-Hz band and 69.4% in 7- to 8-Hz band). Our results suggest that the role of the MB-ATN axis in regulating theta rhythm signaling may be much more important than has been speculated so far.

A subset of thalamocortical projections to the retrosplenial cortex possesses two vesicular glutamate transporter isoforms, VGluT1 and VGluT2, in axon terminals and somata

Vesicular glutamate transporter isoforms, VGluT1-VGluT3, accumulate glutamate into synaptic vesicles and are considered to be important molecules in glutamatergic transmission. Among them, VGluT2 mRNA is expressed predominantly throughout the dorsal thalamus, whereas VGluT1 mRNA is expressed in a few thalamic nuclei. In the thalamic nuclei that project to the retrosplenial cortex (RSC), VGluT1 mRNA is expressed strongly in the anterodorsal thalamic nucleus (AD), is expressed moderately in the anteroventral and laterodorsal thalamic nuclei, and is not expressed in the anteromedial thalamic nucleus. Thus, it has been strongly suggested that a subset of thalamocortical projections to RSC possesses both VGluT1 and VGluT2. In this study, double-labeled neuronal somata showing both VGluT1 and VGluT2 immunolabelings were found exclusively in the ventral region of AD (vAD). Many double-labeled axon terminals were also found in two major targets of vAD, the rostral part of the reticular thalamic nucleus and layers Ia and III-IV of the retrosplenial granular b cortex (RSGb). Some were also found in layer Ia of the retrosplenial granular a cortex (RSGa). These axon terminals contain significant amounts of both VGluTs. Because the subset of thalamocortical projections to RSC has a unique molecular basis in the glutamatergic transmission system, it might play an important role in the higher cognitive functions processed in the RSC. Furthermore, double-labeled axon terminals of a different type were distributed in RSGb and RSGa. Because they are small and the immunoreactivity of VGluT2 is significantly weaker than that of VGluT1, they seemed to be a subset of corticocortical terminals.