Interactions between septal and entorhinal inputs to the rat dentate gyrus: facilitation effects

Deep Brain Stimulation of the Medial Septal Area Can Modulate Gene Expression in the Hippocampus of Rats under Urethane Anesthesia

We studied the effects of stimulation of the medial septal area on the gene expression in the dorsal and ventral hippocampus. Rats under urethane anesthesia were implanted with a recording electrode in the right hippocampus and stimulating electrode in the dorsal medial septum (dMS) or medial septal nucleus (MSN). After one-hour-long deep brain stimulation, we collected ipsi- and contralateral dorsal and ventral hippocampi. Quantitative PCR showed that deep brain stimulation did not cause any changes in the intact contralateral dorsal and ventral hippocampi. A comparison of ipsi- and contralateral hippocampi in the control unstimulated animals showed that electrode implantation in the ipsilateral dorsal hippocampus led to a dramatic increase in the expression of immediate early genes (c-fos, arc, egr1, npas4), neurotrophins (ngf, bdnf) and inflammatory cytokines (il1b and tnf, but not il6) not only in the area close to implantation site but also in the ventral hippocampus. Moreover, the stimulation of MSN but not dMS further increased the expression of c-fos, egr1, npas4, bdnf, and tnf in the ipsilateral ventral but not dorsal hippocampus. Our data suggest that the activation of medial septal nucleus can change the gene expression in ventral hippocampal cells after their priming by other stimuli.

Levetiracetam has no significant gamma-aminobutyric acid-related effect on paired-pulse interaction in the dentate gyrus of rats

Gamma-aminobutyric acid (GABA)ergic mechanisms of the novel antiepileptic drug, levetiracetam (Keppra), have been both favored and rejected. Since paired-pulse interaction is accepted in functionally assessing GABAergic mechanisms, we investigated whether levetiracetam affects the paired-pulse inhibition/facilitation of the field potentials, evoked in the dentate gyrus of urethane-anesthesized rats. This model revealed a strong paired-pulse inhibition at 20-ms interstimulus interval, a noteworthy paired-pulse facilitation at 80-ms interstimulus interval, and a moderate paired-pulse inhibition at 500-ms interstimulus interval. Bicuculline (3 mg/kg/h, i.v.) and baclofen (10 mg/kg, i.v.) markedly depressed paired-pulse inhibition at 20-ms interstimulus interval, while clonazepam (1 mg/kg, i.p.), diazepam (10 mg/kg, i.v.), and phenobarbital (40 mg/kg, i.v.) enhanced it. Bicuculline also depressed paired-pulse inhibition at 500-ms interstimulus interval. Bicuculline, baclofen, and diazepam reduced paired-pulse facilitation at 80-ms interstimulus interval. Distinct from these GABA(A) receptor- and GABA(B) receptor-related drugs, levetiracetam (17 and 540 mg/kg, i.v.) had no significant effect on either paired-pulse interaction in this model, a result not favoring any major role of GABAergic mechanisms in its antiseizure action.

Glutamatergic activation of the medial septum complex: an enhancement of the dentate gyrus population spike and accompanying EEG and unit changes

A large number of cells from the medial septum complex (MSC) innervate the dentate gyrus of the hippocampus. Electrical prestimulation of the MSC enhances perforant path-dentate gyrus evoked field potentials. Considering the large number of fibres that pass through this region, the effects glutamatergic stimulation of the MSC had on dentate gyrus field potentials, and accompanying changes in units, and EEG, was investigated in urethane-anaesthetized rats. The perforant path was stimulated at a rate of 0.1 Hz, evoking an EPSP and a population spike recorded in the dentate gyrus granule cell layer. L-glutamate was delivered by pressure ejection. Glutamate ejection to the MSC produced a significant enhancement of the population spike. The duration of enhancement ranged from 1 to 49 min ( approximately =10.5 min). A consistent, but relatively short increase in the EPSP slope was also demonstrated. MSC activation induced a theta rhythm in 7 of 10 animals (duration=20-112 s). Theta rhythm induction preceded spike enhancement and occurred for a shorter duration than the enhancement. The effects on spontaneous unit activity were mixed. However, all changes in firing rate preceded spike enhancement, and their duration rarely coincided with the duration of the spike enhancement. The population spike enhancement usually occurred without evidence of a change in paired-pulse inhibition.

Differential presynaptic and postsynaptic expression of m1-m4 muscarinic acetylcholine receptors at the perforant pathway/granule cell synapse

A family of muscarinic acetylcholine receptor proteins mediates diverse pre- and postsynaptic functions in the hippocampus. However the roles of individual receptors are not understood. The present study identified the pre- and postsynaptic muscarinic acetylcholine receptors at the perforant pathway synapses in rat brain using a combination of lesioning, immunocytochemistry and electron microscopic techniques. Entorhinal cortex lesions resulted in lamina-specific reductions of m2, m3, and m4 immunoreactivity in parallel with the degeneration of the medial and lateral perforant pathway terminals in the middle and outer thirds of the molecular layer, respectively. In contrast, granule cell lesions selectively reduced m1 and m3 receptors consistent with degeneration of postsynaptic dendrites. Direct visualization of m1-m4 by electron microscopic immunocytochemistry confirmed their differential pre- and postsynaptic localizations. Together, these findings provide strong evidence for both redundancy and spatial selectivity of presynaptic (m2, m3 and m4) and postsynaptic (m1 and m3) muscarinic acetylcholine receptors at the perforant pathway synapse.

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis

Converging inputs to the entorhinal cortex from the piriform cortex and medial septum: facilitation and current source density analysis. J. Neurophysiol. 78: 2602-2615, 1997. The entorhinal cortex receives sensory inputs from the piriform cortex and modulatory inputs from the medial septum. To examine short-term synaptic facilitation effects in these pathways, current source density (CSD) analysis was used first to localize the entorhinal cortex membrane currents, which generate field potentials evoked by stimulation of these afferents. Field potentials were recorded at 50-micron intervals through the medial entorhinal cortex in urethan-anesthetized rats and the one-dimensional CSD was calculated. Piriform cortex stimulation evoked a surface-negative, deep-positive field potential component in the entorhinal cortex with mean onset and peak latencies of 10.4 and 18.4 ms. The component followed brief 100-Hz stimulation, consistent with a monosynaptic response. CSD analysis linked the component to a current sink, which often began in layer I before peaking in layer II. A later, surface-positive field potential component peaked at latencies near 45 ms and was associated with a current source in layer II. Medial septal stimulation evoked positive and negative field potential components which peaked at latencies near 7 and 16 ms, respectively. A weaker and more prolonged surface-negative, deep-positive component peaked at latencies near 25 ms. The early components were generated by currents in the hippocampal formation, and the late surface-negative component was generated by currents in layers II to IV of the entorhinal cortex. Short-term facilitation effects in conscious animals were examined using electrodes chronically implanted near layer II of the entorhinal cortex. Paired-pulse stimulation of the piriform cortex at interpulse intervals of 30 and 40 ms caused the largest facilitation (248%) of responses evoked by the second pulse. Responses evoked by medial septal stimulation also were facilitated maximally (59%) by a piriform cortex conditioning pulse delivered 30-40 ms earlier. Paired pulse stimulation of the medial septum caused the largest facilitation (149%) at intervals of 70 ms, but piriform cortex evoked responses were facilitated maximally (46%) by a septal conditioning pulse 100-200 ms earlier. Frequency potentiation effects were maximal during 12- to 18-Hz stimulation of either the piriform cortex or medial septum. Occlusion tests suggested that piriform cortex and medial septal efferents activate the same neurons. The CSD analysis results show that evoked field potential methods can be used effectively in chronically prepared animals to examine synaptic responses in the converging inputs from the piriform cortex and medial septum to the entorhinal cortex. The short-term potentiation phenomena observed here suggest that low-frequency activity in these pathways during endogenous oscillatory states may enhance entorhinal cortex responsivity to olfactory inputs.

Chronic intraventricular infusion with NGF improves LTP in old cognitively-impaired rats

Aged (21 months) cognitively-impaired male Sprague-Dawley rats received intraventricular infusion of nerve growth factor (NGF) or cytochrome C (Cit C) for 14 or 28 days using miniosmotic pumps and were evaluated either 1 week or 3 months after treatment. Groups of untreated young, aged-impaired and aged non-impaired rats were also evaluated. Under narcose recording and stimulating electrodes were stereotactically implanted in the dentate gyrus and the perforant path. The stimulation intensity was individually adjusted to obtain a half-maximal population spike (P) for test stimuli and a quarter-maximal for tetanization. The amplitude and latency of P and the slope (S) of the field EPSP were determined before and at 2, 5, 15, 30 and 60 min after tetanization at 400 Hz. Paired stimuli at 30 ms interval were also applied before and after tetanization. Aged, cognitively impaired rats showed an absent S potentiation and a delayed P potentiation, both in amplitude and latency, while non-impaired rats behaved like the young controls. Paired pulse inhibition showed no difference among groups before or after tetanization suggesting that the impaired potentiation is not due to an increased retroactive inhibition. NGF treatment ameliorates LTP deficits to levels equivalent to non-impaired rats, while Cit C controls showed no improvement. No differences appear among NGF treated groups, but evidence suggest that the animals evaluated 3 months after treatment developed a stronger potentiation.

Attention, configuration, and hippocampal function

We present a neural network that characterizes a remarkably large number of classical conditioning paradigms and describes the effects of many neurophysiological manipulations. First, the network 1) describes behavior in real time 2) comprises simple and configural stimulus representations, and 3) includes attentional control of storage and retrieval. Second, mapping of the network onto the brain can be summarized by several information processing loops: 1) a hippocampal-cortical configural loop, 2) a hippocampal-cerebellar conditioned-response loop, 3) a hippocampal-accumbens-thalamic attentional loop, and 4) a hippocampal-medial raphe-medial septum error loop. Third, within this global view of brain function, it is assumed that the hippocampal formation computes 1) the aggregate prediction of environmental events and 2) the error signals for cortical learning. These assumptions are supported by rigorous computer simulations consistent with a large body of data on hippocampal and septal neural activity, induction and blockade of hippocampal long-term potentiation, administration of cholinergic agonists and antagonists, administration of haloperidol, and selective and nonselective hippocampal and cortical lesions.

Amygdala kindling-induced seizures selectively impair spatial memory. 2. Effects on hippocampal neuronal and glial muscarinic acetylcholine receptor

The muscarinic acetylcholine receptor is linked via hydrolysis of phosphoinositides to the protein kinase C pathway. In a preceding paper (Beldhuis, H. J. A., H. G. J. Everts, E. A. Vander Zee, P. G. M. Luiten, and B. Bohus (1992) Amygdala kindling-induced seizures selectively impair spatial memory. 1. Behavioral characteristics and effects on hippocampal neuronal protein kinase C isoforms. Hippocampus 2:397-410), the role of different isoforms of protein kinase C in neurobiological processes associated with plasticity was studied using both a spatial learning paradigm and amygdala kindling in the rat. This study extended the findings on protein kinase C activity to the level of the muscarinic acetylcholine receptor. Rats were trained in a spatial learning paradigm and kindled simultaneously in the amygdala to develop generalized motor convulsions. Control rats were trained only in the spatial learning paradigm to acquire stable working and reference memory performance. Alteration in the expression of the muscarinic acetylcholine receptor was investigated using a monoclonal antibody to muscarinic acetylcholine receptor proteins. Trained control rats that were exposed repeatedly to the spatial learning paradigm showed an increase in immunoreactivity for the muscarinic acetylcholine receptor located in the same hippocampal regions in which the protein kinase C activity was increased. In fully kindled rats, however, this increase located in principal neurons was absent, whereas expression of muscarinic acetylcholine receptor proteins was increased in hippocampal astrocytes. Moreover, fully kindled rats showed an impairment in reference memory performance as compared to trained control rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal learning of the rabbit nictitating membrane response following kindling-induced potentiation within the hippocampal dentate gyrus

The following experiment examined the effects of kindled seizures on reversal learning and the effects of both kindling and classical conditioning of the rabbit nictitating membrane on granule cell responsivity to perforant path input. Kindling resulted in significant potentiation of the population spike, the excitatory postsynaptic potential (EPSP) and the magnitude of twin pulse inhibition. Following kindling, rabbits were trained in a discrimination-reversal paradigm with either a tone or light paired with a corneal airpuff. Kindling did not affect acquisition of the initial discriminative response but did retard the rate of reversal learning. Kindling-induced potentiation, within dentate excitatory and inhibitory circuits, persisted for the duration of training. Thus, these results do not distinguish between the contribution of kindling-induced potentiation within dentate excitatory and inhibitory circuits to discrimination-reversal training. Spikes evoked during tone presentations were of reduced amplitude compared to spikes evoked either between trials or during light trials. The EPSP was not affected by stimulus conditions. In control rabbits, the magnitude of both the spike and EPSP increased across training. Training-related potentiation, in kindled rabbits, could not be separated from kindling-induced potentiation. These results demonstrate that an LTP-like effect of both the population spike and EPSP occurs with discrimination-reversal training.

Acetylcholine modulates averaged sensory evoked responses and perforant path evoked field potentials in the rat dentate gyrus

The effect of localized application of acetylcholine (ACh) on well characterized components of sensory evoked and electrically induced potentials in the dentate gyrus was investigated in rats while performing a tone discrimination task. Local pressure application of ACh to the granule cell layer of the dentate gyrus through the recording pipette increased the amplitude of perforant path evoked population spikes without changing the amplitude of the field EPSP. When the pipette was relocated to the outer molecular layer of the dentate gyrus (OM), ACh application decreased the amplitude of the perforant path field EPSP. Two major components of the averaged auditory evoked potential (AEP) recorded during criterion performance of the discrimination task were significantly changed by dendritic application of ACh. The N1 component of the OM AEP which has been shown to reflect perforant path synaptic activity decreased in amplitude while the N2 component which represents activity from septal connections, was significantly increased. These effects were not due to the pressure ejection procedure nor drug related changes in behavioral performance of the task. The results suggest that ACh may act to differentially modulate the synaptic excitability of dentate granule cells, allowing them to acquire responses to sensory stimulation during the establishment and maintenance of discrimination learning.

Behaviour-dependent changes in acetylcholine release in normal and graft-reinnervated hippocampus: evidence for host regulation of grafted cholinergic neurons

Grafted neurons obtained from the fetal basal forebrain can provide a functional cholinergic reinnervation of the hippocampal formation in rats with a lesion of the intrinsic septal cholinergic afferents. In the present experiments graft-derived acetylcholine release in the hippocampus was studied by microdialysis in awake rats during different types of behaviours which are known to activate the innate septohippocampal cholinergic system and during different activity periods of the day-night cycle. Two types of basal forebrain grafts were studied: cell suspensions implanted into the hippocampus in rats with an aspirative lesion of the fimbria-fornix, and grafts of solid tissue implanted as a tissue bridge into the fimbria-fornix lesion cavity. Increased acetylcholine overflow was seen in both groups with grafts during sensory stimulation (by handling). The strongest response (50% increase in acetylcholine release) was seen in rats with solid basal forebrain grafts (equivalent to two-thirds of that seen in intact rats). Immobilization stress and motor activity (swimming) also resulted in increased, but more variable, acetylcholine release (+ 30%; about one-third of the normal response). None of these effects was seen in the control rats with fimbria-fornix lesion only. The two-fold difference in hippocampal acetylcholine release in normal animals between day and night was absent in both types of grafted rats. An acute knife-cut, transecting the connections between the solid basal forebrain graft and the host hippocampus, caused an immediate 75% reduction in acetylcholine release (similar to the effect of an acute fimbria-fornix transection in the normal rats) and the response to swimming was no longer evident. The results show that grafted cholinergic neurons can be functionally integrated into the host brain, allowing the grafted neurons to be activated in the correct behavioural contexts, although the changes in acetylcholine overflow were overall smaller and more variable than normal. The ability of the host to influence cholinergic graft activity, most probably mediated via activation of afferent host-graft connections, may contribute to the efficacy of basal forebrain grafts in the amelioration of behavioural impairments in animals with lesions of the forebrain cholinergic system.

Role of the medial septum and hippocampal theta rhythm in exploration-related synaptic efficacy changes in rat fascia dentata

Animals transferred from their home cages to a different environment exhibited an increase in exploratory behavior which was accompanied by a substantial increase in perforant path-evoked population excitatory postsynaptic potentials and decreases in both the areas and the onset latencies of population spikes. As reported previously, these changes substantially outlasted the exploratory behaviors that induced them. Electrolytic lesions of the medial septum severely attenuated the theta rhythm of the hippocampal EEG, but had no significant effect on the exploration related changes in the synaptic and postsynaptic components of the evoked response. In urethane-anesthetized animals, long trains of hippocampal theta produced by sensory stimulation failed to affect the amplitude of evoked responses. These results show that the information critical for the exploration-related alterations in dentate evoked responses does not originate in or pass through the medial septum, and that the changes are not linked to hippocampal EEG states.

Long-lasting facilitation of excitatory postsynaptic potentials in the rat hippocampus by acetylcholine

1. The effects of acetylcholine (ACh) on excitatory postsynaptic potentials (EPSPs) evoked by stimulating Schaffer-commissural afferents and on ionophoretically applied L-glutamate ligands, were investigated in CA1 neurones of hippocampal slices using current- and voltage-clamp techniques. 2. ACh produced a transient suppression followed by a long-lasting facilitation of EPSPs. The facilitation was also seen in Cs(+)-filled cells under voltage-clamp conditions. Both suppressing and facilitating effects were blocked by atropine. 3. All components of the EPSP were reduced in the initial phase of ACh action, while only the slow component was enhanced during the later phase. The facilitation was blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist, d-2-amino-5-phosphonovalerate (2-APV) and by hyperpolarization. 4. ACh also facilitated responses to ionophoretically applied NMDA in voltage-clamped, Cs(+)-filled cells in Ba2(+)-treated slices. ACh facilitated responses to L-glutamate which was blocked by 2-APV. ACh failed to affect responses to kainate or quisqualate. 5. We conclude that ACh, acting on muscarinic receptors, exerts a primary effect in the hippocampus to specifically amplify NMDA receptor-mediated synaptic responses and thereby facilitate EPSPs.

Changes in septo-hippocampal projections after lateral entorhinal or combined entorhinal-raphé lesions as studied by anterograde tracing methods

Septal and entorhinal projections to the hippocampus show a considerable overlap in their target structures in the molecular layer of the dentate gyrus (DG) and stratum lacunosum-moleculare of the cornu ammonis (CA). Employing anterograde tracing methods, it was investigated in which way the morphological pattern of the septohippocampal projections were influenced by lateral entorhinal cortex (LEA) lesions. Anterograde filling of neurons from soma to axonal terminals with Phaseolus vulgaris leucoagglutinin (PHA-L) revealed lesion-induced changes in innervation patterns in the DG but not in CA fields. LEA lesions provoke an impressive shift of septo-dentate projections from a predominant middle molecular layer innervation to the outer molecular layer, whereas septal projections to the CA remain unchanged. Comparison with concurrent acetylcholinesterase (AChE) staining and immunocytochemical demonstration of choline acetyltransferase (ChAT) confirm the cholinergic nature of this plasticity response. This response was equally strong in unilateral or bilateral damage to the LEA and was neither enhanced nor inhibited by simultaneous injury to the median raphé nuclei.

Enhanced long-term potentiation induced in rat dentate gyrus by coactivation of septal and entorhinal inputs: temporal constraints

High-frequency activation of the entorhinal cortical (perforant path) inputs to the rat dentate gyrus can produce a long-term potentiation (LTP) of perforant path-dentate evoked responses. In this paper we examined the enhanced LTP effects produced by coactivation of septal and entorhinal inputs to the dentate gyrus. Trains of electrical stimulation applied to the two inputs were found to increase the magnitude of LTP to a level above that produced by trains applied to the perforant path alone. The largest LTP increments were observed when the septal trains were applied less than 100 ms prior to the perforant path trains. If the septal trains followed the perforant path trains there was no additional increment in LTP magnitude, regardless of the intertrain interval. The relationship of this cooperativity effect to mechanisms of associative learning is discussed.