A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Long-term depression of excitatory transmission in the lateral septum

Neurons in the lateral septum (LS) integrate glutamatergic synaptic inputs, primarily from hippocampus, and send inhibitory projections to brain regions involved in reward and the generation of motivated behavior. Motivated learning and drugs of abuse have been shown to induce long-term changes in the strength of glutamatergic synapses in the LS, but the cellular mechanisms underlying long-term synaptic modification in the LS are poorly understood. Here, we examined synaptic transmission and long-term depression (LTD) in brain slices prepared from male and female C57BL/6 mice. No sex differences were observed in whole cell patch-clamp recordings of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)- and N-methyl-d-aspartate receptor (NMDA-R)-mediated currents. Low-frequency stimulation of the fimbria fiber bundle (1 Hz 15 min) induced LTD of the LS field excitatory postsynaptic potential (fEPSP). Induction of LTD was blocked by the NMDA-R antagonist (d)-2-amino-5-phosphonovaleric acid (APV), but not the selective antagonist of GluN2B-containing NMDA-Rs ifenprodil. These results demonstrate the NMDA-R dependence of LTD in the LS. The LS is a sexually dimorphic structure, and sex differences in glutamatergic transmission have been reported in vivo; our results suggest sex differences observed in vivo result from network activity rather than intrinsic differences in glutamatergic transmission.NEW & NOTEWORTHY The lateral septum (LS) integrates information from hippocampus and other regions to provide context-dependent (top down or higher order) regulation of mood and motivated behavior. Learning and drugs of abuse induce long-term changes in the strength of glutamatergic projections to the LS; however, the cellular mechanisms underlying such changes are poorly understood. Here, we demonstrate there are no apparent sex differences in fast excitatory transmission and that long-term synaptic depression in the LS is NMDA-R dependent.

Characterization of age-related changes in synaptic transmission onto F344 rat basal forebrain cholinergic neurons using a reduced synaptic preparation

Basal forebrain (BF) cholinergic neurons participate in a number of cognitive processes that become impaired during aging. We previously found that age-related enhancement of Ca(2+) buffering in rat cholinergic BF neurons was associated with impaired performance in the water maze spatial learning task (Murchison D, McDermott AN, Lasarge CL, Peebles KA, Bizon JL, and Griffith WH. J Neurophysiol 102: 2194-2207, 2009). One way that altered Ca(2+) buffering could contribute to cognitive impairment involves synaptic function. In this report we show that synaptic transmission in the BF is altered with age and cognitive status. We have examined the properties of spontaneous postsynaptic currents (sPSCs) in cholinergic BF neurons that have been mechanically dissociated without enzymes from behaviorally characterized F344 rats. These isolated neurons retain functional presynaptic terminals on their somata and proximal dendrites. Using whole cell patch-clamp recording, we show that sPSCs and miniature PSCs are predominately GABAergic (bicuculline sensitive) and in all ways closely resemble PSCs recorded in a BF in vitro slice preparation. Adult (4-7 mo) and aged (22-24 mo) male rats were cognitively assessed using the water maze. Neuronal phenotype was identified post hoc using single-cell RT-PCR. The frequency of sPSCs was reduced during aging, and this was most pronounced in cognitively impaired subjects. This is the same population that demonstrated increased intracellular Ca(2+) buffering. We also show that increasing Ca(2+) buffering in the synaptic terminals of young BF neurons can mimic the reduced frequency of sPSCs observed in aged BF neurons.

Implications for treatment: GABAA receptors in aging, Down syndrome and Alzheimer's disease

In addition to progressive dementia, Alzheimer's disease (AD) is characterized by increased incidence of seizure activity. Although originally discounted as a secondary process occurring as a result of neurodegeneration, more recent data suggest that alterations in excitatory-inhibitory (E/I) balance occur in AD and may be a primary mechanism contributing AD cognitive decline. In this study, we discuss relevant research and reports on the GABA(A) receptor in developmental disorders, such as Down syndrome, in healthy aging, and highlight documented aberrations in the GABAergic system in AD. Stressing the importance of understanding the subunit composition of individual GABA(A) receptors, investigations demonstrate alterations of particular GABA(A) receptor subunits in AD, but overall sparing of the GABAergic system. In this study, we review experimental data on the GABAergic system in the pathobiology of AD and discuss relevant therapeutic implications. When developing AD therapeutics that modulate GABA it is important to consider how E/I balance impacts AD pathogenesis and the relationship between seizure activity and cognitive decline.

GABAAreceptors in aging and Alzheimer’s disease

In this article we present a comprehensive review of relevant research and reports on the GABA(A) receptor in the aged and Alzheimer's disease (AD) brain. In comparison to glutamatergic and cholinergic systems, the GABAergic system is relatively spared in AD, but the precise mechanisms underlying differential vulnerability are not well understood. Using several methods, investigations demonstrate that despite resistance of the GABAergic system to neurodegeneration, particular subunits of the GABA(A) receptor are altered with age and AD, which can induce compensatory increases in GABA(A) receptor subunits within surrounding cells. We conclude that although aging- and disease-related changes in GABA(A) receptor subunits may be modest, the mechanisms that compensate for these changes may alter the pharmacokinetic and physiological properties of the receptor. It is therefore crucial to understand the subunit composition of individual GABA(A) receptors in the diseased brain when developing therapeutics that act at these receptors.

Age-related alterations in GABAA receptor subunits in the nonhuman primate hippocampus

Pharmacological studies have documented that altered drug responses, particularly to benzodiazepines, are common in elderly populations. While numerous factors may contribute to changes in drug response, age-related alterations in the molecular composition of GABA(A) receptors may be a key factor in regulating these responses. We employed quantitative densitometry to examine the cytological features and density of highly prevalent hippocampal GABA(A) receptor subunits (alpha1 and beta2/3) in young and aged rhesus monkeys. alpha1 and beta2/3 subunit immunostaining was differentially distributed throughout the hippocampus. In addition, beta2/3 immunolabeling in aged monkeys was characterized by marked intersubject variability in labeling intensity, with dramatic reductions present in 3 of 5 samples. alpha1 immunolabeling in aged monkeys was significantly reduced in the CA2 and CA3 subregions, and in hilus/polymorphic layer of the dentate gyrus. Collectively, our findings demonstrate that not only are GABA(A) receptor subunits differentially distributed throughout the hippocampus, but they are also differentially altered with increased age--changes that may have an important impact on the binding properties of GABA(A) receptor pharmacological agents.

Neurotoxicity of NMDA antagonists: a glutamatergic theory of schizophrenia based on selective impairment of local inhibitory feedback circuits

Modulation of recurrent inhibition is critical not only for the normal function of highly excitable regions of the brain, especially the limbic system, but may also be a primary determining factor for the viability of neurons in these regions. Standard extracellular and intracellular recordings from in vitro brain slices of rat hippocampi were employed to show that recurrent inhibition onto CA1 neurons can be modulated by N-methyl-D-aspartate (NMDA) antagonists. Besides reducing the amplitude of inhibitory postsynaptic potentials (IPSPs) at resting membrane potential conditions, different NMDA antagonists, including the endogenous substance N-acetyl-L-aspartyl-L-glutamic acid (NAAG), are able to block long-term potentiation (LIP) of recurrent inhibition completely at concentrations that are not sufficient to block LTP of the excitatory drive onto pyramidal neurons. This LTP of recurrent inhibition may play a significant role in stimulus discrimination and learning, as simulated in a biophysical computer model of a basic neuronal circuit. Both the amplitude of the IPSP and LTP of the recurrent inhibitory circuit also undergo developmental changes showing their highest expression and vulnerability to chronic NMDA antagonist injections in juvenile rats. Finally, blocking NMDA receptor-dependent transmission in the recurrent inhibition loop may lead to an overall increased excitability of the neuronal network. This may resemble the positive schizophrenic symptoms observed in man, presumably caused by elevated levels of the endogenous NMDA antagonist NAAG.

Age-related changes in rhythmically bursting activity in the medial septum of rats

The effects of aging on the firing of septohippocampal neurons were estimated in unanesthetized, restrained young, old and very old rats (respectively 3, 23 and 30 months). Extracellular recordings were obtained during various states of arousal. The mean spontaneous activity for the overall neuronal population was not modified by aging. In contrast, the percentage of rhythmically bursting neurons was significantly lower in aged rats. During wakefulness, decrease of bursting activity was observed in old and very old rats (P<0.01 and P<0.001) whereas during rapid eye movement sleep it appeared only in the oldest group (P<0.01). The frequency of the bursts decreased in 30-month-old rats during wakefulness while it remained unchanged in both aged groups during rapid eye movement sleep. In old rats, at a time when the cholinergic septal neurons already deteriorated, a third of neurons recorded during rapid eye movement sleep exhibited a pattern of activity composed of long duration bursts with higher intraburst frequency than in young or very old rats. Our study shows that rhythmically bursting septal activity is impaired in aged rats and that the amplitude of the changes depends on advancing age and on states of arousal. Our findings suggest that age-induced loss and atrophy of cholinergic septal neurons contribute to the disorganization of the rhythmic activity but that functional alterations, influenced by the states of arousal, may also be considered.

Pretraining tetanic fimbrial stimulation impairs the expression but not the acquisition of contextual fear conditioning in mice

We recently reported that the pretraining induction of long-term potentiation in the lateral septum by fimbrial tetanic stimulation altered contextual fear conditioning in mice. The aim of the present study was to examine at which stage of fear conditioning (i.e. either acquisition or expression) this impairment takes place. Mice implanted with stimulating electrodes in the fimbria and recording electrodes in the lateral septal were conditioned to acquire fear towards a novel context using a footshock procedure. Twenty-four hours after conditioning, animals were re-exposed to the conditioning environment and the level of freezing behavior served as the measure of conditioned fear. The level of fimbrial-lateral septal synaptic neurotransmission was manipulated using either fimbrial tetanic stimulation (which induced septal long-term potentiation) alone, or followed by fimbrial low-frequency stimulation producing depotentiation of the previously established long-term potentiation. The results showed that (i) septal long-term potentiation induced either prior to acquisition or only prior to retention testing impaired conditioned freezing; and (ii) the impairing effect of pretraining induction of long-term potentiation on conditioned freezing was not only abolished by fimbrial low-frequency stimulation administered prior to retention testing but actually produced enhanced conditioned freezing with respect to controls. These data suggest that the level of fimbrial-lateral septal synaptic neurotransmission may influence the expression, but not the acquisition, of contextual fear conditioning.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

AGE-RELATED DEFICITS IN MOTOR LEARNING AND DIFFERENCES IN FEEDBACK PROCESSING DURING THE PRODUCTION OF A BIMANUAL COORDINATION PATTERN

Learning and transfer of a new bimanual coordination pattern were investigated in a group of adolescents and elderly subjects. The pattern consisted of continuous horizontal flexionextension movements with a 90 phase offset between the upper limbs. All subjects practised the task under augmented feedback conditions, involving a real-time orthogonal display of both limb movements. Three different transfer test conditions were administered at regular intervals during practice, i.e. blindfolded, with normal vision, and with augmented visual feedback. Findings showed that the performance levels of the elderly group were lower than the group of adolescents and their rate of improvement was also smaller. The observed learning deficits in the elderly are hypothesised to be a consequence of a decreased capability to overcome the preferred coordination modes, as required for developing new coordination modes. This reduced capability to suppress prepotent response tendencies may reflect an age-related decrease in the efficiency of inhibitory processes in the central nervous system and may be associated with changes in frontal lobe functioning.

Influences of the bed nucleus of the stria terminalis and of the paraventricular nucleus of the hypothalamus on the excitability of hippocampal-lateral septal synapses in mice

Previous experiments have shown that conditioning in aversive situations is associated with specific changes in excitability of hippocampal-septal synaptic transmission and that these changes might be related to a modulation of this synaptic transmission by afferents originating from the bed nucleus of the stria terminalis (BNST) and from the paraventricular nucleus (PVN) of the hypothalamus. Accordingly, the aim of the present experiment was to assess changes in excitability of hippocampal-septal synapses by varying the interval between the application of a conditioning pulse in either the BNST or the PVN, and a test pulse in fimbria fibers (FF). Electrical stimulation of FF, induces in the lateral septum (LS) a field potential characterized by two negative waves (N2 and N3) the magnitude of which is an index of excitability of two populations of target cells located in the ventral and dorsal lateral septum, respectively. Results showed that prestimulation of both the BNST and the PVN produced an increase in the amplitude of the N3 wave, although the optimal interpulse interval required for producing maximal increase was different as a function of the two structures. Only prestimulation of the BNST induced a significant increase in the amplitude of the N2 wave. These results suggest that the PVN projects mainly to the dorsal aspect of the LS, while the BNST projects to both dorsal and ventral parts of the LS. Together with results from previous experiments conducted in behaving mice exposed to conditioned aversive stimuli, it is concluded that these projections might play a role in the relief of contextual conditioned fear.

NMDA receptor activation in the aged rat: electrophysiological investigations in the CA1 area of the hippocampal slice ex vivo

The effects of aging on activation of N-methyl-D-aspartate (NMDA) receptors were studied in the CA1 field of hippocampal slices from young (2-4 months old) and aged (25-32 months old) Sprague-Dawley rats with the use of ex vivo extra- and intracellular electrophysiological recording techniques. No significant age-related changes of the unitary NMDA-receptor mediated excitatory postsynaptic potentials (EPSPs), recorded from the pyramidal cells after stimulation of the stratum radiatum in a magnesium-free medium and isolated in the presence of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, were found. Simultaneously, the magnitude of synaptic plasticity which involved NMDA receptor activation was not altered. No significant age-related modifications in the mechanisms controlling glutamate release and of postsynaptic NMDA receptor responsiveness were revealed. Considering the 30-40% decrease in NMDA binding sites in the aged hippocampus, our results suggest the occurrence of compensatory mechanisms which are discussed.

Contextual conditioned fear blocks the induction but not the maintenance of lateral septal LTP in behaving mice

High-frequency stimulation (HFS) of the fimbria induces long-term potentiation (LTP) in the lateral septum. This study was aimed at investigating the effect of contextual fear conditioning on septal LTP with the use of behaving C57 BL/6 mice as subjects. For the acquisition of contextual fear conditioning, animals were placed in a conditioning chamber, where they were subjected to footshocks (FSs, 0.6 mA); the following day (retention), animals were reexposed to the chamber. Animals from the first group received HFS in their home cages before being submitted to conditioning; animals from the second group were first submitted to conditioning before receiving HFS during reexposure to the conditioning chamber; animals from the third group were submitted to the same regimen as those from the second group, except that no FS was delivered in the conditioning chamber; and animals from the fourth group received FS in the conditioning chamber but were maintained in their home cages the day after for LTP induction. Before conditioning, animals from the first group, placed in a familiar context (home cage), displayed an LTP of the N3 wave of septal field potential. After conditioning, reexposure of these animals to the conditioning chamber produced a transient decrease in the amplitude of N3 but did not interfere with the duration of maintenance of LTP. Conversely, in animals from the second group, when HFS was applied during reexposure to the conditioning chamber the induction of LTP was totally blocked. However, mice from the two other groups (3rd and 4th) displayed normal levels of LTP. Taken together with previous findings, these data suggest that contextual conditioned fear may interfere with certain forms of learning via blockade of hippocampal-septal LTP.

Polysynaptic potentiation in the lateral septum following stimulation of the fimbria in anesthetized rats

In anesthetized rats, electrical stimulation of fimbria fibers evoked, in the ipsilateral lateral septum (LS), a field potential consisting of two negative components: an initial negativity (N2-3 complex wave) of high amplitude at 6.7 ms (+/- 0.8 ms; peak latency) and a slow negative wave (N4 wave) of small amplitude at 14.4 ms (+/- 2.4 ms). The N2-3 complex wave represents the monosynaptic activation of LS neurons while the N4 wave corresponds to polysynaptic activation of neurons in the mediolateral part of the LS. In this study, we investigated the effects of high-frequency stimulation of fimbria fibers on LS field potentials and compared them with those observed in the CA3 area. Tetanic stimulation of the fimbria did not change the characteristics of the N2-3 wave but induced a long-lasting increase in amplitude and slope of the N4 wave. A positive correlation was found between the magnitude of CA3 LTP and lateral septal polysynaptic potentiation of the N4 component. These results indicate that patterns of stimulation delivered to the same input fibers (fimbria fibers) produce similar changes in a polysynaptic input to the LS and in a monosynaptic input to the CA3 and emphasize the complexity of signal processing in serial networks.

Changes in synaptic excitability in the lateral septum associated with contextual and auditory fear conditioning in mice

Synaptic excitability in the lateral septum (LS) was assessed electrophysiologically in freely moving mice either submitted to a painful stimulus (shock) or tested on two forms of conditioned fear: contextual conditioning and auditory cue conditioning. Only the amplitude of the N3 component of the two negative waves (N2 and N3) evoked by fimbrial stimulation displayed significant changes in these tests. Experiment 1 showed that both the painful stimulus and subsequent re-exposure (24 h later) to the conditioning foreground context induced significant and context specific decrease in the N3 amplitude. In Experiment 2, a phasic tone (conditioned stimulus: CS) was paired (paired group) with the footshock (unconditioned stimulus: US) or not (unpaired group) and, 24 h later, animals were re-exposed successively to the auditory cue and to the context. During the auditory cue test, only the paired group displayed significant freezing and this occurred only during presentation of the CS. In this group, however, a significant reduction in the N3 amplitude was only observed immediately after the cessation of the CS. During the context test, the percentage and time-course of freezing across the 10 mm session were similar in each group. However, in the unpaired group the N3 amplitude reduction was significant and outlasted the duration of the freezing behaviour itself. These results show that alterations in LS synaptic excitability may be dissociated from fear-induced freezing behaviour. We suggest that LS synapses are part of a brain circuit that predict if and when the US is going to occur.

Long-term potentiation and long-term depression in the lateral septum in spatial working and reference memory

We report two experiments conducted on a radial arm maze in the mouse showing that training could either enhance or reduce the efficacy of the fimbria-lateral septal synapses. It is suggested that the direction of change is determined by the kind of situation the animal is faced with (ie trial-dependent, respectively).