Role of Reuniens Nucleus Projections to the Medial Prefrontal Cortex and to the Hippocampal Pyramidal CA1 Area in Associative Learning

We studied the interactions between short- and long-term plastic changes taking place during the acquisition of a classical eyeblink conditioning and following high-frequency stimulation (HFS) of the reuniens nucleus in behaving mice. Synaptic changes in strength were studied at the reuniens-medial prefrontal cortex (mPFC) and the reuniens-CA1 synapses. Input/output curves and a paired-pulse study enabled determining the functional capabilities of the two synapses and the optimal intensities to be applied at the reuniens nucleus during classical eyeblink conditioning and for HFS applied to the reuniens nucleus. Animals were conditioned using a trace paradigm, with a tone as conditioned stimulus (CS) and an electric shock to the trigeminal nerve as unconditioned stimulus (US). A single pulse was presented to the reuniens nucleus to evoke field EPSPs (fEPSPs) in mPFC and CA1 areas during the CS-US interval. No significant changes in synaptic strength were observed at the reuniens-mPFC and reuniens-CA1 synapses during the acquisition of eyelid conditioned responses (CRs). Two successive HFS sessions carried out during the first two conditioning days decreased the percentage of CRs, without evoking any long-term potentiation (LTP) at the recording sites. HFS of the reuniens nucleus also prevented the proper acquisition of an object discrimination task. A subsequent study revealed that HFS of the reuniens nucleus evoked a significant decrease of paired-pulse facilitation. In conclusion, reuniens nucleus projections to prefrontal and hippocampal circuits seem to participate in the acquisition of associative learning through a mechanism that does not required the development of LTP

Modification des propriétés neurochimiques des noyaux faciaux après axotomie et des noyaux vestibulaires après déafférentation

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Amino acid transporter (VIAAT, VGLUT2) and chloride cotransporter (KCC1, KCC2 and NKCC1) expression in the vestibular nuclei of intact and labyrinthectomized rat.

We report the first investigation of whether unilateral labyrinthectomy in adult rats affects the expression of two amino acid transporters, vesicular glutamate transporter 2 (VGLUT2) and vesicular inhibitory amino acid transporter (VIAAT) and of chloride cotransporters (KCC1, KCC2 and NKCC1) in the intact and deafferented medial vestibular nuclei (MVN). In situ hybridization with specific radioactive oligonucleotide probes and immunofluorescent methods were used in normal and unilaterally labyrinthectomized rats at various times following the lesion: 5 h, and 1, 3 and 8 days. In normal animals, several brainstem regions including the lateral, medial, superior and inferior vestibular nuclei contained VGLUT2, VIAAT and KCC2 mRNA. In contrast, no or a very faint labeling was observed with KCC1 and NKCC1 probes. In unilaterally lesioned rats, there was no asymmetry between the two MVN with any of the oligonucleotide probes at any time after the lesion. Similarly, there were no differences in the intensity of MVN labeling between controls and lesioned animals. Finally, no over-expression of the cotransporter KCC1 and NKCC1 was found in ipsilateral or controlateral MVN in lesioned rats at any time. Immunohistochemical experiments gave similar conclusions. Our findings suggest that the recovery of the resting discharge of the deafferented MVN neurons, and consequently the functional compensation of the deficits, are not dependent on changes in the expression of amino acid transporters (VIAAT, VGLUT2), and chloride cotransporters (KCC1, KCC2 and NKCC1) or on their mRNAs

Effects of two HFS sessions on the reuniens-CA1 and reuniens-mPFC synapses.

<p>(A, B) Evolution of fEPSPs evoked in the PFC (A) and in the CA1 area (B) by paired-pulse stimulation of reuniens nucleus before and after two HFS sessions. Each animal was presented with two HFS sessions (see shaded areas) each consisting of five 200 Hz, 100 ms trains of pulses at a rate of 1/s. This protocol was presented six times, at intervals of 1 min. The 100 µs, square, biphasic pulses used to evoke LTP were applied at the same intensity used for the single pulse presented following HFS presentation. The evolution of LTP was checked using a pair of pulses (1st, black circles; 2nd, white circles) with an interstimulus interval of 40 ms. Recording was carried out for 72 h. Note that fEPSP amplitudes evoked by the 1st and the 2nd pulses reached values below baseline following the two HFS sessions for both synapses. *, <i>P</i><0.05 for fEPSPs evoked by the 1st pulse; #, <i>P</i><0.05 for fEPSPs evoked by the 2nd pulse.</p

Experimental design.

<p>(A) EMG recording electrodes were implanted in the orbicularis oculi (O.O.) muscle of the upper left eyelid. In addition, bipolar stimulating electrodes were implanted on the ipsilateral supraorbital nerve for presentation of unconditioned stimulus (US). The conditioned stimulus (CS) consisted of a tone delivered from a loudspeaker located 30 cm from the animal's head. Animals were also implanted with stimulating electrodes in the thalamic reuniens nucleus and with recording electrodes in the medial prefrontal cortex (top diagram) or the hippocampal CA1 area (bottom diagram). (B–D) Photomicrographs illustrating the location of recording electrodes in the mPFC (B) and in the hippocampal CA1 area (D), as well as the stimulation (C) site (arrows). Calibration bars 500 µm. Abbreviations: D, L, M, V, dorsal, lateral, medial, and ventral. E, A schematic representation of the conditioning paradigm, illustrating CS and US stimuli, and the moment at which a single pulse (100 µs, square, biphasic) was presented to the reuniens nucleus (St. Reu.). An example of an EMG record from the orbicularis oculi (O.O.) muscle obtained from the 9th conditioning session is illustrated, as well as an extracellular record of hippocampal activity from the same animal, session, and trial. Note the fEPSPs evoked by the pulse presented to the reuniens nucleus.</p