Septal serotonin depletion in rats facilitates working memory in the radial arm maze and increases hippocampal high-frequency theta activity

Older rats show slow modulation of hippocampal theta rhythm during voluntary running

Aging causes brain function degeneration and slows many motor and behavioural responses. The hippocampal theta rhythm (4-12 Hz) is related to cognition and locomotion. However, the findings on aging-related changes in the frequency and amplitude of hippocampal theta oscillations have been inconsistent. We hypothesized that older rats have slower responses in terms of hippocampal theta rhythm during voluntary wheel running than do young adult rats. By simultaneously recording electroencephalography and physical activity (PA), we evaluated theta oscillations in 8-week-old (young adult) and 60-week-old (middle-aged) rats before and during wheel running, which was conducted only during the rats' 12-h dark period. To test the alterations of hippocampal theta rhythm in voluntary wheel running, we analyzed the signals without (8-s) or with (2-s) chronological order. No significant difference was observed in total frequency (TP, 4-12 Hz), low-frequency (LT, 4-6.5 Hz), or high-frequency (9.5-12 Hz) theta activity between active waking and overall running in either group. The theta oscillations were slower in the middle-aged rats than in the young adult rats during wheel running but increased during running for both age groups. During wheel running, the middle-aged rats exhibited an increased LT, which was related to PA. On the basis of the chronological order of running, the young adult rats exhibited increased TP, and the middle-aged rats exhibited significant increases in middle-frequency (MT, 6.5-9.5 Hz) theta activity. The dominant modulations of MT in the middle-aged rats may have caused nonsignificant changes in total activity. These between-group differences in theta rhythm characteristics during voluntary running provide insights into age-related brain function decline.

Septal medial/diagonal band of Broca citalopram infusion reduces place learning efficiency and alters septohippocampal theta learning-related activity in rats

Increases in power and frequency of hippocampal theta activity have been related to efficient place learning and memory acquisition in hippocampal-dependent tests. The complex medial septum-diagonal band of Broca (MS/DBB) is the pacemaker of hippocampal theta activity, influenced by the ascending synchronizing system, and modulated by serotonergic raphe medial afferents, acting on cholinergic and GABAergic septal neurons. The suppression of hippocampal theta expression and the modulation of hippocampal learning and memory are attributed to serotonin. To simultaneously test these hypotheses, a daily local serotonin increase was induced by citalopram (CIT) infusion (100 µM, 0.88 µl, 0.2 µl/m) 15 min before training in the Morris water maze. The theta activity was recorded in the MS/DBB, dentate gyrus (DG) and CA1 of one group infused with artificial cerebrospinal liquid (ACL) and the other with CIT on Days 1-6 of training. After a probe trial (Day 7) and one resting day, the treatments were reversed (Days 8-11). The CIT MS/DBB infusion in the first 6 training days reduced the efficiency of spatial learning in association with reduced power in the DG, reduced MS/DBB-DG coherence, increased DG-CA1 coherence, and a lack of a negative correlation between MS/DBB power and swam distances. No effect of the CIT occurred once the information was acquired under ACL training. These results support a role of serotonin, in acting on the MS/DBB in the fine tuning of hippocampal learning and memory efficiency through the modulation of learning-related theta activity power and septohipocampal synchronization.

Neurobiology of Anorexia Nervosa: Serotonin Dysfunctions Link Self-Starvation with Body Image Disturbances through an Impaired Body Memory

The etiology of anorexia nervosa (AN) is still unclear, despite that it is a critical and potentially mortal illness. A recent neurobiological model considers AN as the outcome of dysfunctions in the neuronal processes related to appetite and emotionality (Kaye et al., 2009, 2013). However, this model still is not able to answer a critical question: What is behind body image disturbances (BIDs) in AN? The article starts its analysis from reviewing some of the studies exploring the effects of the serotonin systems in memory (episodic, working, and spatial) and its dysfunctions. The review suggests that serotonin disturbances may: (a) facilitate the encoding of third person (allocentric) episodic memories; (b) facilitate the consolidation of emotional episodic memories (e.g., teasing), if preceded by repeated stress; (c) reduce voluntary inhibition of mnestic contents; (d) impair allocentric spatial memory. If we discuss these results within the interpretative frame suggested by the “Allocentric Lock Hypothesis” (Riva, 2012, 2014), we can hypothesize that altered serotoninergic activity in AN patients: (i) improves their ability to store and consolidate negative autobiographical memories, including those of their body, in allocentric perspective; (ii) impairs their ability to trigger voluntary inhibition of the previously stored negative memory of the body; (iii) impairs their capacity to retrieve/update allocentric information. Taken together, these points suggest a possible link between serotonin dysfunctions, memory impairments and BIDs: the impossibility of updating a disturbed body memory using real time experiential data—I'm locked to a wrong body stored in long term memory—pushes AN patients to control body weight and shape even when underweight.

Hippocampal strata theta oscillations change their frequency and coupling during spatial learning

The theta rhythm is necessary for hippocampal-dependent spatial learning. It has been proposed that each hippocampal stratum can generate a current theta dipole. Therefore, considering that each hippocampal circuit (CA1, CA3, and Dentate Gyrus (DG)) contributes differently to distinct aspects of a spatial memory, the theta oscillations on each stratum and their couplings may exhibit oscillatory dynamics associated with different stages of learning. To test this hypothesis, the theta oscillations from five hippocampal strata were recorded in the rat during different stages of learning in a Morris maze. The peak power, the relative power (RP) and the coherence between hippocampal strata were analyzed. The early acquisition stage of the Morris task was characterized by the predominance of slow frequency theta activity and high coupling between specific hippocampal strata at slow frequencies. However, on the last training day, the theta oscillations were faster in all hippocampal strata, with tighter coupling at fast frequencies between the CA3 pyramidal stratum and other strata. Our results suggest that modifications to the theta frequency and its coupling can be a means by which the hippocampus differentially operates during acquisition and retrieval states.

Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze

Hippocampal theta activity is related to spatial information processing, and high-frequency theta activity, in particular, has been linked to efficient spatial memory performance. Theta activity is regulated by the synchronizing ascending system (SAS), which includes mesencephalic and diencephalic relays. The supramamillary nucleus (SUMn) is located between the reticularis pontis oralis and the medial septum (MS), in close relation with the posterior hypothalamic nucleus (PHn), all of which are part of this ascending system. It has been proposed that the SUMn plays a role in the modulation of hippocampal theta-frequency; this could occur through direct connections between the SUMn and the hippocampus or through the influence of the SUMn on the MS. Serotonergic raphe neurons prominently innervate the hippocampus and several components of the SAS, including the SUMn. Serotonin desynchronizes hippocampal theta activity, and it has been proposed that serotonin may regulate learning through the modulation of hippocampal synchrony. In agreement with this hypothesis, serotonin depletion in the SUMn/PHn results in deficient spatial learning and alterations in CA1 theta activity-related learning in a Morris water maze. Because it has been reported that SUMn inactivation with lidocaine impairs the consolidation of reference memory, we asked whether changes in hippocampal theta activity related to learning would occur through serotonin depletion in the SUMn, together with deficiencies in memory. We infused 5,7-DHT bilaterally into the SUMn in rats and evaluated place learning in the standard Morris water maze task. Hippocampal (CA1 and dentate gyrus), septal and SUMn EEG were recorded during training of the test. The EEG power in each region and the coherence between the different regions were evaluated. Serotonin depletion in the SUMn induced deficient spatial learning and altered the expression of hippocampal high-frequency theta activity. These results provide evidence in support of a role for serotonin as a modulator of hippocampal learning, acting through changes in the synchronicity evoked in several relays of the SAS.