Spatial memory formation is enabled through synaptic information processing, in the form of persistent strengthening and weakening of synapses, within the hippocampus. It is, however, unclear how relevant spatial information is selected for encoding, in preference to less pertinent information. As the noradrenergic locus coeruleus (LC) becomes active in response to novel experiences, we hypothesized that the LC may provide the saliency signal required to promote hippocampal encoding of relevant information through changes in synaptic strength. Test pulse stimulation evoked stable basal synaptic transmission at Schaffer collateral (SC)–CA1 stratum radiatum synapses in freely behaving adult rats. Coupling of these test pulses with electrical stimulation of the LC induced long-term depression (LTD) at SC–CA1 synapses and induced a transient suppression of theta-frequency oscillations. Effects were N-methyl-D-aspartate and β-adrenergic receptor dependent. Activation of the LC also increased CA1 noradrenalin levels and facilitated the encoding of spatial memory for a single episode via a β-adrenoceptor–dependent mechanism. Our results demonstrate that the LC plays a key role in the induction of hippocampal LTD and in promoting the encoding of spatial information. This LC–hippocampal interaction may reflect a means by which salient information is distinguished for subsequent synaptic processing.

The aim of this study was to investigate the effect of chronic restraint stress (RS) on spatial learning and memory. Fifty healthy male Wistar rats, aged three months were used. They were equally divided into five groups--C: Control, W: Water Maze, CS-1: Restrained for 21 days (1 h/day) + water maze protocol following stress period, CS-2: Restrained for 28 days (1 h/day) + water maze protocol during last 7 days of stress period, CS-3: Restrained for 21 days and allowed to recovery for 7 days (1 h/day). Corticosterone levels were higher in all stress groups than in C and W groups. Nitrite levels of frontal cortex and hippocampus were found to be elevated in chronic stress groups with respect to C and W groups. Thiobarbituric acid reactive substances (TBARS) of both tissues were increased significantly in CS1 and CS2 groups compared with C, W, and CS3 groups. Escape latencies of CS1 and CS2 groups were longer than those of the W group on each day of acquisition. In transfer test, CS1 and CS2 groups stayed significantly shorter in target quadrant according to the W group. Significant correlations between corticosterone and either nitrite or TBARS of hippocampus and frontal cortex were found. Both acquisition and memory performances were negatively correlated with plasma corticosterone level, nitrite, and TBARS levels of hippocampus and frontal cortex. The results of this study suggest that stress-induced lipid peroxidation may affect the acquisition and memory performances.

Repetitive transcranial magnetic stimulation (rTMS) has been shown to alter cortical excitability that lasts beyond the duration of rTMS application itself. High‐frequency rTMS leads primarily to facilitation, whereas low‐frequency rTMS leads to inhibition of the treated cortex. However, the contribution of rTMS train duration is less clear. In this study, we investigated the effects of nine different rTMS protocols, including low and high frequencies, as well as short and long applications (1, 3 and 10 Hz applied for 1, 5 and 20 min), on visual cortex excitability in anaesthetized and paralysed cats by means of visual evoked potential (VEP) and electroencephalography (EEG) recordings. Our results show that 10 Hz rTMS applied for 1 and 5 min significantly enhanced early VEP amplitudes, while 1 and 3 Hz rTMS applied for 5 and 20 min significantly reduced them. No significant changes were found after 1 and 3 Hz rTMS applied for only 1 min, and 10 Hz rTMS applied for 20 min. EEG activity was only transiently (<20 s) affected, with increased delta activity after 1 and 3 Hz rTMS applied for 1 or 5 min. These findings indicate that the effects of rTMS on cortical excitability depend on the combination of stimulus frequency and duration (or total number of stimuli): short high‐frequency trains seem to be more effective than longer trains, and low‐frequency rTMS requires longer applications. Changes in the spectral composition of the EEG were not correlated to changes in VEP size.