Selective destruction of medial septal cholinergic neurons attenuates pyramidal cell suppression, but not excitation in dorsal hippocampus field CA1 induced by subcutaneous injection of formalin

Neurokinin1 - cholinergic receptor mechanisms in the medial Septum-Dorsal hippocampus axis mediates experimental neuropathic pain

The neurokinin-1 receptors (NK1Rs) in the forebrain medial septum (MS) region are localized exclusively on cholinergic neurons that partly project to the hippocampus and the cingulate cortex (Cg), regions implicated in nociception. In the present study, we explored the hypothesis that neurotransmission at septal NK1R and hippocampal cholinergic mechanisms mediate experimental neuropathic pain in the rodent chronic constriction injury model (CCI). Our investigations showed that intraseptal microinjection of substance P (SP) in rat evoked a peripheral hypersensitivity (PH)-like response in uninjured animals that was attenuated by systemic atropine sulphate, a muscarinic-cholinergic receptor antagonist. Conversely, pre-emptive destruction of septal cholinergic neurons attenuated the development of PH in the CCI model that also prevented the expression of cellular markers of nociception in the spinal cord and the forebrain. Likewise, anti-nociception was evoked on intraseptal microinjection of L-733,060, an antagonist at NK1Rs, and on bilateral or unilateral microinjection of the cholinergic receptor antagonists, atropine or mecamylamine, into the different regions of the dorsal hippocampus (dH) or on bilateral microinjection into the Cg. Interestingly, the effect of L-733,060 was accompanied with a widespread decreased in levels of CCI-induced nociceptive cellular markers in forebrain that was not secondary to behaviour, suggesting an active modulation of nociceptive processing by transmission at NK1R in the medial septum. The preceding suggest that the development and maintenance of neuropathic nociception is facilitated by septal NK1R-dH cholinergic mechanisms which co-ordinately affect nociceptive processing in the dH and the Cg. Additionally, the data points to a potential strategy for pain modulation that combines anticholinergics and anti-NKRs.

A regulatory role of the medial septum in the chloroquine-induced acute itch through local GABAergic system and GABAergic pathway to the anterior cingulate cortex

Itch, a common somatic sensation, serves as a crucial protective system. Recent studies have unraveled the neural mechanisms of itch at peripheral, spinal cord as well as cerebral levels. However, a comprehensive understanding of the central mechanism governing itch transmission and regulation remains elusive. Here, we report the role of the medial septum (MS), an integral component of the basal forebrain, in modulating the acute itch processing. The increases in c-Fos+ neurons and calcium signals within the MS during acute itch processing were observed. Pharmacogenetic activation manipulation of global MS neurons suppressed the scratching behaviors induced by chloroquine or compound 48/80. Microinjection of GABA into the MS or pharmacogenetic inhibition of non-GABAergic neurons markedly suppressed chloroquine-induced scratching behaviors. Pharmacogenetic activation of the MS-ACC GABAergic pathway attenuated chloroquine-induced acute itch. Hence, our findings reveal that MS has a regulatory role in the chloroquine-induced acute itch through local increased GABA to inhibit non-GABAergic neurons and the activation of MS-ACC GABAergic pathway.

Ventral Hippocampal CA1 Pyramidal Neurons Encode Nociceptive Information

As a main structure of the limbic system, the hippocampus plays a critical role in pain perception and chronicity. The ventral hippocampal CA1 (vCA1) is closely associated with negative emotions such as anxiety, stress, and fear, yet how vCA1 neurons encode nociceptive information remains unclear. Using in vivo electrophysiological recording, we characterized vCA1 pyramidal neuron subpopulations that exhibited inhibitory or excitatory responses to plantar stimuli and were implicated in encoding stimuli modalities in naïve rats. Functional heterogeneity of the vCA1 pyramidal neurons was further identified in neuropathic pain conditions: the proportion and magnitude of the inhibitory response neurons paralleled mechanical allodynia and contributed to the confounded encoding of innocuous and noxious stimuli, whereas the excitatory response neurons were still instrumental in the discrimination of stimulus properties. Increased theta power and theta-spike coupling in vCA1 correlated with nociceptive behaviors. Optogenetic inhibition of vCA1 pyramidal neurons induced mechanical allodynia in naïve rats, whereas chemogenetic reversal of the overall suppressed vCA1 activity had analgesic effects in rats with neuropathic pain. These results provide direct evidence for the representations of nociceptive information in vCA1.

Medial septum glutamatergic neurons modulate nociception in chronic neuropathic pain via projections to lateral hypothalamus

The medial septum (MS) contributes in pain processing and regulation, especially concerning persistent nociception. However, the role of MS glutamatergic neurons in pain and the underlying neural circuit mechanisms in pain remain poorly understood. In this study, chronic constrictive injury of the sciatic nerve (CCI) surgery was performed to induce thermal and mechanical hyperalgesia in mice. The chemogenetic activation of MS glutamatergic neurons decreased pain thresholds in naïve mice. In contrast, inhibition or ablation of these neurons has improved nociception thresholds in naïve mice and relieved thermal and mechanical hyperalgesia in CCI mice. Anterograde viral tracing revealed that MS glutamatergic neurons had projections to the lateral hypothalamus (LH) and supramammillary nucleus (SuM). We further demonstrated that MS glutamatergic neurons regulate pain thresholds by projecting to LH but not SuM, because the inhibition of MS-LH glutamatergic projections suppressed pain thresholds in CCI and naïve mice, yet, optogenetic activation or inhibition of MS-SuM glutamatergic projections had no effect on pain thresholds in naïve mice. In conclusion, our results reveal that MS glutamatergic neurons play a significant role in regulating pain perception and decipher that MS glutamatergic neurons modulate nociception via projections to LH.

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

The Medial Septum as a Potential Target for Treating Brain Disorders Associated With Oscillopathies

The medial septum (MS), as part of the basal forebrain, supports many physiological functions, from sensorimotor integration to cognition. With often reciprocal connections with a broad set of peers at all major divisions of the brain, the MS orchestrates oscillatory neuronal activities throughout the brain. These oscillations are critical in generating sensory and emotional salience, locomotion, maintaining mood, supporting innate anxiety, and governing learning and memory. Accumulating evidence points out that the physiological oscillations under septal influence are frequently disrupted or altered in pathological conditions. Therefore, the MS may be a potential target for treating neurological and psychiatric disorders with abnormal oscillations (oscillopathies) to restore healthy patterns or erase undesired ones. Recent studies have revealed that the patterned stimulation of the MS alleviates symptoms of epilepsy. We discuss here that stimulus timing is a critical determinant of treatment efficacy on multiple time scales. On-demand stimulation may dramatically reduce side effects by not interfering with normal physiological functions. A precise pattern-matched stimulation through adaptive timing governed by the ongoing oscillations is essential to effectively terminate pathological oscillations. The time-targeted strategy for the MS stimulation may provide an effective way of treating multiple disorders including Alzheimer's disease, anxiety/fear, schizophrenia, and depression, as well as pain.

Modulation of Septo-Hippocampal Neural Responses in Anesthetized and Behaving Rats by Septal AMPA Receptor Mechanisms

This study explored the effects of septal glutamatergic transmission on septal-hippocampal theta activity via intraseptal microinjection of antagonist at AMPA receptors (AMPAR). The current results showed that microinjection of AMPAR antagonist, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione, 20 μg/μl, 0.5 μl), evoked a decrease in the frequency of theta activity evoked by various means in anesthetized and behaving rat. Theta wave activity was induced on: (a) intraseptal microinjection of carbachol, an agonist at cholinergic receptors, (b) reticular stimulation, (c) exploration in novel open field (OF), and (d) hind paw (HP) injection of the algogen, formalin. The effect on frequency in the formalin test was observed in an early period on injection of formalin, which was novel to the animal, but not in the later more sustained phase of the formalin test. The effect of NBQX, being seen in both anesthetized and behaving animals, suggests that the modulation of theta wave frequency, including in novelty, is a function of AMPAR in MS. The effect of the antagonist on theta power was less apparent, being observed only in anesthetized animals. In addition to theta power and frequency, intraseptal NBQX also attenuated suppression of CA1 population spike (PS) induced by intraseptal carbachol, thus suggesting that septal glutamate neurotransmission is involved in the spectrum of MS-mediated network responses. Indeed, in the context of behavior, formalin injection induced an increase in the level of septal glutamate, while NBQX attenuated nociceptive behaviors. Notably, MS is involved in the modulation of formalin nociception. These findings suggest that AMPA receptors are a key modulator of septal physiological function.

Forebrain medial septum sustains experimental neuropathic pain

The present study explored the role of the medial septal region (MS) in experimental neuropathic pain. For the first time, we found that the MS sustains nociceptive behaviors in rodent models of neuropathic pain, especially in the chronic constriction injury (CCI) model and the paclitaxel model of chemotherapy-induced neuropathic pain. For example, inactivation of the MS with intraseptal muscimol (2 μg/μl, 0.5 μl), a GABA mimetic, reversed peripheral hypersensitivity (PH) in the CCI model and induced place preference in a conditioned place preference task, a surrogate measure of spontaneous nociception. The effect of intraseptal muscimol on PH was comparable to that seen with microinjection of the local anesthetic, lidocaine, into rostral ventromedial medulla which is implicated in facilitating experimental chronic nociception. Cellular analysis in the CCI model showed that the MS region sustains nociceptive gain with CCI by facilitating basal nociceptive processing and the amplification of stimulus-evoked neural processing. Indeed, consistent with the idea that excitatory transmission through MS facilitates chronic experimental pain, intraseptal microinjection of antagonists acting at AMPA and NMDA glutamate receptors attenuated CCI-induced PH. We propose that the MS is a central monitor of bodily nociception which sustains molecular plasticity triggered by persistent noxious insult.

Inhibition shapes the organization of hippocampal representations

Hippocampal neurons become tuned to stimuli that predict behaviorally salient outcomes. This plasticity suggests that memory formation depends upon shifts in how different anatomical inputs can drive hippocampal activity. Here, I present evidence that inhibitory neurons can provide such a mechanism for learning-related changes in the tuning of pyramidal cells. Inhibitory currents arriving on the dendrites of pyramidal cells determine whether an excitatory input can drive action potential output. Specificity and plasticity of this dendritic modulation allows for precise, modifiable changes in how afferent inputs are integrated, a process that defines a neuron's receptive field. In addition, feedback inhibition plays a fundamental role in biasing which excitatory neurons may be co-active. By defining the rules of synchrony and the rules of input integration, interneurons likely play an important role in the organization of memory representation within the hippocampus.

The forebrain medial septal region and nociception

The forebrain medial septum, which is an integral part of the septo-hippocampal network, is implicated in sensorimotor integration, fear and anxiety, and spatial learning and memory. A body of evidence also suggests that the septal region affects experimental pain. Indeed, some explorations in humans have raised the possibility that the region may modulate clinical pain as well. This review explores the evidence that implicates the medial septum in nociception and suggests that non-overlapping circuits in the region facilitate acute nociceptive behaviors and defensive behaviors that reflect affect and cognitive appraisal, especially in relation to persistent nociception. In line with a role in nociception, the region modulates nociceptive responses in the neuraxis, including the hippocampus and the anterior cingulate cortex. The aforementioned forebrain regions have also been implicated in persistent/long-lasting nociception. The review also weighs the effects of the medial septum on nociception vis-à-vis the known roles of the region and emphasizes the fact that the region is a part of network of forebrain structures which have been long associated with reward, cognition and affect-motivation and are now implicated in persistent/long-lasting nociception.

GABAergic neurons of the medial septum play a nodal role in facilitation of nociception-induced affect

The present study explored the functional details of the influence of medial septal region (MSDB) on spectrum of nociceptive behaviours by manipulating intraseptal GABAergic mechanisms. Results showed that formalin-induced acute nociception was not affected by intraseptal microinjection of bicuculline, a GABAA receptor antagonist, or on selective lesion of septal GABAergic neurons. Indeed, the acute nociceptive responses were dissociated from the regulation of sensorimotor behaviour and generation of theta-rhythm by the GABAergic mechanisms in MSDB. The GABAergic lesion attenuated formalin-induced unconditioned cellular response in the anterior cingulate cortex (ACC) and blocked formalin-induced conditioned place avoidance (F-CPA), and as well as the contextual fear induced on conditioning with brief footshock. The effects of lesion on nociceptive-conditioned cellular responses were, however, variable. Interestingly, the lesion attenuated the conditioned representation of experimental context in dorsal hippocampus field CA1 in the F-CPA task. Collectively, the preceding suggests that the MSDB is a nodal centre wherein the GABAergic neurons mediate nociceptive affect-motivation by regulating cellular mechanisms in ACC that confer an aversive value to the noxious stimulus. Further, in conjunction with a modulatory influence on hippocampal contextual processing, MSDB may integrate affect with context as part of associative learning in the F-CPA task.

Zolpidem reduces hippocampal neuronal activity in freely behaving mice: a large scale calcium imaging study with miniaturized fluorescence microscope

Therapeutic drugs for cognitive and psychiatric disorders are often characterized by their molecular mechanism of action. Here we demonstrate a new approach to elucidate drug action on large-scale neuronal activity by tracking somatic calcium dynamics in hundreds of CA1 hippocampal neurons of pharmacologically manipulated behaving mice. We used an adeno-associated viral vector to express the calcium sensor GCaMP3 in CA1 pyramidal cells under control of the CaMKII promoter and a miniaturized microscope to observe cellular dynamics. We visualized these dynamics with and without a systemic administration of Zolpidem, a GABAA agonist that is the most commonly prescribed drug for the treatment of insomnia in the United States. Despite growing concerns about the potential adverse effects of Zolpidem on memory and cognition, it remained unclear whether Zolpidem alters neuronal activity in the hippocampus, a brain area critical for cognition and memory. Zolpidem, when delivered at a dose known to induce and prolong sleep, strongly suppressed CA1 calcium signaling. The rate of calcium transients after Zolpidem administration was significantly lower compared to vehicle treatment. To factor out the contribution of changes in locomotor or physiological conditions following Zolpidem treatment, we compared the cellular activity across comparable epochs matched by locomotor and physiological assessments. This analysis revealed significantly depressive effects of Zolpidem regardless of the animal's state. Individual hippocampal CA1 pyramidal cells differed in their responses to Zolpidem with the majority (∼ 65%) significantly decreasing the rate of calcium transients, and a small subset (3%) showing an unexpected and significant increase. By linking molecular mechanisms with the dynamics of neural circuitry and behavioral states, this approach has the potential to contribute substantially to the development of new therapeutics for the treatment of CNS disorders.

Somatosensory stimulation suppresses the excitability of pyramidal cells in the hippocampal CA1 region in rats

The hippocampal region of the brain is important for encoding environment inputs and memory formation. However, the underlying mechanisms are unclear. To investigate the behavior of individual neurons in response to somatosensory inputs in the hippocampal CA1 region, we recorded and analyzed changes in local field potentials and the firing rates of individual pyramidal cells and interneurons during tail clamping in urethane-anesthetized rats. We also explored the mechanisms underlying the neuronal responses. Somatosensory stimulation, in the form of tail clamping, chan-ged local field potentials into theta rhythm-dominated waveforms, decreased the spike firing of pyramidal cells, and increased interneuron firing. In addition, somatosensory stimulation attenuated orthodromic-evoked population spikes. These results suggest that somatosensory stimulation suppresses the excitability of pyramidal cells in the hippocampal CA1 region. Increased inhibition by local interneurons might underlie this effect. These findings provide insight into the mechanisms of signal processing in the hippocampus and suggest that sensory stimulation might have therapeutic potential for brain disorders associated with neuronal hyperexcitability.

Exacerbation of tonic but not phasic pain by entorhinal cortex lesions

The hippocampus is actively involved in pain modulation. Previous studies have shown that inhibition, resection or pharmacological interference of the hippocampus or its subcortical afferent sources such as the medial septum and amygdala produce anti-nociceptive effects. But how the cortical connections of the hippocampus modulate pain remains unexplored. The entorhinal cortex (EC) constitutes the major gateway between the hippocampus and the neocortex. In the present study, rats with medial (MEC), lateral (LEC) or sham EC lesions and received the hot plate and the intra-plantar formalin injection tests. Neither MEC nor LEC lesions affected the hot plate test and the first phase of the formalin test. In contrast, paw licking responses in the second phase of the formalin test significantly increased with both MEC and LEC lesions. These results suggested that that the hippocampal-cortical interactions channeled by the EC were involved in tonic but not phasic pain conditions, and that cortical and sub-cortical connections of the hippocampus played independent roles in pain modulation.

Dendritic inhibition in the hippocampus supports fear learning

Fear memories guide adaptive behavior in contexts associated with aversive events. The hippocampus forms a neural representation of the context that predicts aversive events. Representations of context incorporate multisensory features of the environment, but must somehow exclude sensory features of the aversive event itself. We investigated this selectivity using cell type-specific imaging and inactivation in hippocampal area CA1 of behaving mice. Aversive stimuli activated CA1 dendrite-targeting interneurons via cholinergic input, leading to inhibition of pyramidal cell distal dendrites receiving aversive sensory excitation from the entorhinal cortex. Inactivating dendrite-targeting interneurons during aversive stimuli increased CA1 pyramidal cell population responses and prevented fear learning. We propose subcortical activation of dendritic inhibition as a mechanism for exclusion of aversive stimuli from hippocampal contextual representations during fear learning.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Nicotinic receptor mechanism in supramammillary nucleus mediates physiological regulation of neural activity in dorsal hippocampal field CA1 of anaesthetized rat

Evidence suggests that stimulation of the region of the rostral pontine oralis (RPO) nucleus and the peripheral application of a noxious stimulus activates an ascending system that also modulates hippocampal neural responses during behavioral arousal. Indeed, the two stimuli and behavioral arousal elicit theta activation and the suppression of population spikes (PS) in dorsal hippocampus field CA1. Interestingly, such neural responses in CA1 are also elicited by microinjection of the cholinergic agonist carbachol into the hypothalamic supramammillary nucleus (SuM). In the present in vivo electrophysiological study, we tested the hypothesis that cholinergic neural elements in the SuM modulate the neural drive to CA1 on RPO stimulation or the peripheral application of a noxious stimulus. Pharmacological investigation showed that intra-SuM microinjection of either a muscarinic or a nicotinic receptor antagonist attenuated the SuM carbachol-induced neural effects in CA1, namely, theta activation and PS suppression. However, neither antagonist attenuated the CA1 effects of intra-SuM microinjection of the excitatory neurotransmitter glutamate. Subsequent investigations revealed that microinjection of only the nicotinic antagonist, mecamylamine, into the lateral SuM selectively attenuated the responses elicited in CA1 by stimulation of the RPO or on nociceptive stimulation with hind paw injection of formalin (5%, 0.05 ml); whereas, microinjection of mecamylamine into the medial SuM did not affect the hippocampal responses elicited by either type of stimulation. Furthermore, application of mecamylamine into the lateral SuM attenuated the CA1 responses induced by injection of formalin into the contralateral, but not the ipsilateral hind paw. The lateralization of drug effect is consistent with the predominant unilateral anatomical connections between the SuM and the septohippocampal region. These findings provide novel evidence that nicotinic cholinoceptive neurons in the lateral SuM are common elements of the neural drive(s) to the hippocampus on RPO activation and noxious stimulation.

Roles of the hippocampal formation in pain information processing

Pain is a complex experience consisting of sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Now it has been gradually known that noxious information is processed by a widely-distributed, hierarchically- interconnected neural network, referred to as neuromatrix, in the brain. Thus, identifying the multiple neural networks subserving these functional aspects and harnessing this knowledge to manipulate the pain response in new and beneficial ways are challenging tasks. Albeit with elaborate research efforts on the cortical responses to painful stimuli or clinical pain, involvement of the hippocampal formation (HF) in pain is still a matter of controversy. Here, we integrate previous animal and human studies from the viewpoint of HF and pain, sequentially representing anatomical, behavioral, electrophysiological, molecular/biochemical and functional imaging evidence supporting the role of HF in pain processing. At last, we further expound on the relationship between pain and memory and present some unresolved issues.

Intra-hippocampal tonic inhibition influences formalin pain-induced pyramidal cell suppression, but not excitation in dorsal field CA1 of rat

It has been hypothesized that intra-hippocampal GABAergic inhibitory interneurons mediate formalin pain-induced suppression of dorsal hippocampal CA1 pyramidal cell discharge. The present study performed on anaesthetized rats tested the hypothesis by disrupting GABAergic mechanisms with intra-hippocampal administration of the GABA(A) receptor antagonist bicuculline methiodide, applied either dorsally into the pyramidal cell layer and stratum oriens (dorsal-bicuculline) or ventrally into the region of apical dendrites (ventral-bicuculline). It was found that ventral-, but not dorsal-bicuculline attenuated formalin-induced suppression of pyramidal cell extracellular discharge. The antagonism was selective in such a way that the excitation of pyramidal cell was unaffected. Interestingly, ventral-bicuculline strongly disinhibited CA1 pyramidal cells and shifted the distribution of their spontaneous discharge to values higher than the control group. However, dorsal-bicuculline disinhibited the local CA1 interneurons that were strongly excited on injection of formalin. Overall, the findings favour the notion that tonic GABA(A) receptor mechanisms located in the region of apical dendrites facilitate formalin-induced pyramidal cell suppression by masking the background excitatory drive impinging on the pyramidal cells. Interestingly, both the attenuation of formalin-induced inhibition and facilitation of basal discharge of CA1 pyramidal cells by ventral-bicuculline are similar to the effects seen previously with the destruction of medial septal cholinergic neurons. This convergence of effects strengthens the proposal that the network of medial septal cholinergic neurons and hippocampal GABAergic interneurons influence formalin pain-induced CA1 pyramidal cell suppression. In addition, the data point to a non-overlapping excitatory drive whose strength is unaffected by the inhibitory drive that underpins formalin suppression.

In vivo MRI identifies cholinergic circuitry deficits in a Down syndrome model

In vivo quantitative magnetic resonance imaging (MRI) was employed to detect brain pathology and map its distribution within control, disomic mice (2N) and in Ts65Dn and Ts1Cje trisomy mice with features of human Down syndrome (DS). In Ts65Dn, but not Ts1Cje mice, transverse proton spin-spin (T(2)) relaxation time was selectively reduced in the medial septal nucleus (MSN) and in brain regions that receive cholinergic innervation from the MSN, including the hippocampus, cingulate cortex, and retrosplenial cortex. Basal forebrain cholinergic neurons (BFCNs) in the MSN, identified by choline acetyltransferase (ChAT) and nerve growth factor receptors p75(NTR) and TrkA immunolabeling were reduced in Ts65Dn brains and in situ acetylcholinesterase (AChE) activity was depleted distally along projecting cholinergic fibers, and selectively on pre- and postsynaptic profiles in these target areas. T(2) effects were negligible in Ts1Cje mice that are diploid for App and lack BFCN neuropathology, consistent with the suspected relationship of this pathology to increased App dosage. These results establish the utility of quantitative MRI in vivo for identifying Alzheimer's disease-relevant cholinergic changes in animal models of DS and characterizing the selective vulnerability of cholinergic neuron subpopulations.

Hippocampal acetylcholine efflux increases during negative patterning and elemental discrimination in rats

The purpose of this paper is to examine whether hippocampal acetylcholine (ACh) efflux increases during negative patterning (NP) discrimination tasks. For these tasks, a rat's response was rewarded when either a single stimulus A (tone) or stimulus B (light) was presented, but was not rewarded when the compound stimulus AB (tone+light) was presented to the NP group of rats. An elemental discrimination (E) task was given to another group (E group). In the E group, the rat's response was rewarded when one of two stimuli (e.g., tone) was presented, but not rewarded when the other stimulus (e.g., light) was presented. After reaching a learning criterion, a guide cannula was implanted into dorsal hippocampus under anesthesia. In test sessions, rats were given the same task as before the guide cannula implantation, and ACh efflux was measured. Hippocampal ACh efflux increased during both NP and E tasks. In addition, the magnitude of increase was higher in the NP group than in the E group. Thus, over all our results demonstrate that task difficulty is a critical factor that relates to the difference in ACh efflux in the hippocampus.

Inhibition of platelet-activating factor receptors in hippocampal plasma membranes attenuates the inflammatory nociceptive response in rats

Evidence suggests that platelet-activating factor (PAF) is a mediator in inflammatory-based pain. Using the biphasic formalin model in rats, we recently demonstrated that PAF antagonists which were selective for either intracellular or plasma membrane PAF receptors decreased the late-phase of the nociceptive response. Inasmuch as both of the PAF antagonists previously used were administered systemically, and reportedly are able to cross the blood-brain barrier, the anatomic locations at which PAF affects pain processing remained to be elucidated. Since PAF is required for hippocampal-dependent memory consolidation, and since the hippocampus has been shown to mediate the late-phase of formalin-induced nociception, the present study investigated the effects on nociception of administration of PAF antagonists within the hippocampus, and of using agents specific for either plasma membrane (BN 52021) or intracellular (BN 50730) PAF binding sites. Intrahippocampal injections of BN 52021 decreased the late-phase of the nociceptive response in a concentration-dependent manner. In contrast, intrahippocampal administration of BN 50730 had no effect on inflammatory nociception. These findings suggest that hippocampal plasma membrane PAF receptors, but not intracellular PAF binding sites, mediate tonic inflammatory pain processing in rats.

Hippocampal theta state in relation to formalin nociception

In the present study using extracellular electrophysiological recording techniques, we explored the temporal characteristics of hippocampal theta activation in relation to formalin nociception. Results indicate that, compared to hind paw injection of saline, formalin injection in behaving rat evoked biphasic increase in duration of dorsal CA1 theta. Such an increase broadly paralleled animal biphasic behavioral activation, especially lick and moment-to-moment agitated behaviors. Correspondingly, theta-modulated cell firing was observed following formalin injection in anesthetized rat. The formalin-induced theta activation in behaving rat was most marked during peak of theta activation in the 2nd theta state (11-40 min post-injection) comprising 73% of the time in the 5 min block. An increase in theta peak frequency was also observed with respect to pre-injection control. However, the peak of theta in the 2nd theta state mostly preceded the peak of lick and flinch of the affected paw. In the 41-60 min, following formalin injection while the animals displayed robust nociceptive flinching and lifting, the theta activity approached control levels. Furthermore, the theta peak frequency at peak of theta was higher than the corresponding values of sustained theta observed in correlation with the nociceptive behaviors; in contrast, high frequency theta rhythm was observed during formalin-induced other moment-to-moment agitated behaviors. These findings favor the notion that in the formalin model the theta state of the hippocampus reflects a neural drive that is dissociated from the duration of nociceptive experience and is not selective to the typical nociceptive indices of lick, flinch, and lift of the injured paw.

Microinjection of carbachol in the supramammillary region suppresses CA1 pyramidal cell synaptic excitability

Previous studies have established that the posterior hypothalamus-supramammillary (SUM) region is involved in the control of the hippocampal theta rhythm and also modulates the synaptic excitation of hippocampal neurons. Particularly, the medial but not lateral SUM region mediates reticular stimulation-induced suppression of CA1 pyramidal cell synaptic excitation to Schaffer collateral stimulation. In the present study using urethane anesthetized rats, we have investigated the effect of direct chemical stimulation of the posterior hypothalamus-SUM region on CA1 pyramidal cell excitability. It was observed that microinjection of the cholinergic muscarinic receptor agonist, carbachol (0.1 microl, concentration of either 0.0052, 0.156, or 0.625 microg/microl), evoked concentration-dependent suppression of CA1 pyramidal cell excitability that was dissociated from theta activation. Further, carbachol microinjection preferentially recruited the lateral SUM region when compared with the medial SUM and the posterior hypothalamic regions. In this context, the shortest latencies to suppression at the lowest concentration of carbachol and the strongest suppression at higher concentrations were observed with lateral microinjections. The carbachol-induced suppression was attenuated by inactivation of the medial septal region by microinjection of procaine (0.5 microl, 20% w/v). These results underscore a possible role for cholinergic mechanisms in the lateral SUM region in modulation of CA1 pyramidal cell synaptic excitation via the medial septal region. Furthermore, the present findings when juxtaposed with the medial SUM mediation of reticularly-elicited suppression suggest a medial-lateral topographic organization of the SUM region in modulation of CA1 excitability.

The medial septum mediates impairment of prepulse inhibition of acoustic startle induced by a hippocampal seizure or phencyclidine

The involvement of the septohippocampal system on the impaired sensorimotor gating induced by phencyclidine (PCP) or by an electrically induced hippocampal seizure was examined in behaving rats. An impaired sensorimotor gating, measured by prepulse inhibition (PPI) of the acoustic startle response, was observed following a hippocampal afterdischarge (AD) or systemic injection of PCP and was accompanied with an increase in hippocampal gamma waves (30-70 Hz). The medial septum infusion with muscimol (0.25 microg), a GABA(A) receptor agonist, 15 min prior to PCP or a hippocampal AD, prevented the impairment of sensorimotor gating and the increase in gamma waves. By itself, muscimol (0.25 microg) injection into the medial septum did not affect PPI, although it significantly suppressed spontaneous gamma waves. In order to identify subpopulations of neurons mediating the sensorimotor gating deficit and the hippocampal gamma wave increase, 0.14-0.21 microg of p75 antibody conjugated to saporin (192 IgG-saporin) was injected into the medial septum to selectively lesion the septohippocampal cholinergic neurons. Neither the PPI deficit nor the gamma wave increase induced by PCP or a hippocampal AD was affected by 192 IgG-saporin lesion of the medial septum. It is concluded that increase in neural activity in the medial septum participates in the impairment of sensorimotor gating and the increase in hippocampal gamma waves induced by PCP or a hippocampal AD. It is suggested that the GABAergic but not the cholinergic septohippocampal neurons mediate the sensorimotor gating deficit.

Nociception-driven decreased induction of Fos protein in ventral hippocampus field CA1 of the rat

To test the hypothesis that the hippocampus field CA1 is recruited in nociceptive intensity-dependent fashion in the formalin model of inflammatory pain, we determined the effect of injection of formalin (0.625-2.5%) on the induction of Fos protein along the length of the hippocampus. Compared to injection of saline, injection of formalin (0.625-2.5%) evoked a concentration-dependent increase in nociceptive behavior and a significant linear increase in the number of Fos-positive cells in the spinal cord, especially in the deeper laminae. Injection of saline also increased induction of Fos along the length of hippocampus. On the other hand, injection of formalin decreased the number of Fos-positive cells in whole CA1, CA3 and dentate gyrus, with a greater significant effect in the posterior-ventral regions of the hippocampus. Indeed, a formalin concentration-dependent decrease was observed in the ventral CA1. A systematic pattern of change in Fos induction was not observed in the medial septum region. Of the regions examined, only the formalin-induced changes in Fos cell counts in the posterior and ventral CA1 were tightly correlated with the changes observed in the spinal cord. The foregoing findings suggest that nociceptive information is processed in distributed fashion by the hippocampus, and at least the ventral CA1 is implicated in nociceptive intensity-dependent integrative functions.

Reticular stimulation evokes suppression of CA1 synaptic responses and generation of theta through separate mechanisms

The induction of hippocampal theta by reticular stimulation involves a relay to the hippocampus via the posterior hypothalamic-supramammillary region and then the medial septum. Interestingly, sensory- or behaviour-induced theta is accompanied by suppression of hippocampal field CA1 synaptic responses. In the present study, performed on anaesthetized rats, we observed that reticular stimulation also induced a suppression of the CA1 pyramidal cell population spike and the corresponding dendritic field excitatory postsynaptic potential evoked by field CA3 stimulation. This suppression was observed at stimulation intensity below the threshold for generation of CA1 theta and was maximal at stimulation intensities at the threshold for theta. The frequency and amplitude of theta waves, by contrast, increased further with increasing reticular stimulation voltage. Neural inactivation by microinjection of the local anaesthetic procaine (20% w/v, 0.1-0.2 microL) or the inhibitory ligand gamma aminobutyric acid (0.8 m, 0.5 micro L) in the posterior hypothalamic regions, especially the ipsilateral medial supramammillary region, or the medial septum attenuated both the suppression of CA1 pyramidal cell synaptic excitability and theta generation. However, the effects of microinjection on suppression and theta were not always in parallel. Furthermore, the effect of microinjection of gamma aminobutyric acid on reticularly elicited suppression was observed from relatively fewer sites in the posterior hypothalamus as compared with that on theta activation. These results suggest that reticular stimulation evokes an inhibition of CA1 pyramidal cell excitability that (i) is mediated, at least in part, via medial supramammillary and septal regions, but (ii) involves a separate neural pathway from theta generation.

Cholinergic activity enhances hippocampal long-term potentiation in CA1 during walking in rats

Long-term potentiation (LTP) at the basal-dendritic synapses of CA1 pyramidal cells was induced by a single 200 Hz stimulation train (0.5-1 sec duration) in freely behaving rats during one of four behavioral states: awake-immobility (IMM), walking, slow-wave sleep (SWS), and rapid eye movement sleep (REM). Field EPSPs generated by basal-dendritic excitation of CA1 were recorded before and up to 1 d after the tetanus. After a tetanus during any behavioral state, basal-dendritic LTP was >170% of the baseline for the first hour after the tetanus and decayed to approximately 120% 1 d after. LTP induced during walking was significantly larger than that induced during IMM, SWS, or REM, which had similar LTP magnitudes. To test the hypothesis that septohippocampal cholinergic activity enhanced LTP during walking as compared with IMM, rats were either pretreated with muscarinic cholinergic antagonist scopolamine or injected with IgG192-saporin in the medial septum to selectively lesion cholinergic septohippocampal neurons. Pretreatment with scopolamine decreased the LTP induced during walking but did not affect that induced during IMM, such that the difference between the LTP induced during walking and IMM was abolished after scopolamine. Rats injected with IgG192-saporin showed no difference in the LTP induced during walking and IMM, and scopolamine did not reduce the LTP during walking. In contrast, sham-lesion rats showed larger LTP induced during walking than IMM, and the LTP induced during walking was attenuated by scopolamine. This is the first demonstration of an enhancement of hippocampal LTP by physiologically activated cholinergic inputs.