Cholinergic facilitation and inhibition of long-term potentiation of CA1 in the urethane-anaesthetized rats

The Induction of Long-Term Potentiation by Medial Septum Activation under Urethane Anesthesia Can Alter Gene Expression in the Hippocampus

We studied changes in the expression of early genes in hippocampal cells in response to stimulation of the dorsal medial septal area (dMSA), leading to long-term potentiation in the hippocampus. Rats under urethane anesthesia were implanted with stimulating electrodes in the ventral hippocampal commissure and dMSA and a recording electrode in the CA1 area of the hippocampus. We found that high-frequency stimulation (HFS) of the dMSA led to the induction of long-term potentiation in the synapses formed by the ventral hippocampal commissure on the hippocampal CA1 neurons. One hour after dMSA HFS, we collected the dorsal and ventral hippocampi on both the ipsilateral (damaged by the implanted electrode) and contralateral (intact) sides and analyzed the expression of genes by qPCR. The dMSA HFS led to an increase in the expression of bdnf and cyr61 in the ipsilateral hippocampi and egr1 in the ventral contralateral hippocampus. Thus, dMSA HFS under the conditions of degeneration of the cholinergic neurons in the medial septal area prevented the described increase in gene expression. The changes in cyr61 expression appeared to be dependent on the muscarinic M1 receptors. Our data suggest that the induction of long-term potentiation by dMSA activation enhances the expression of select early genes in the hippocampus.

In Vivo Plasticity at Hippocampal Schaffer Collateral-CA1 Synapses: Replicability of the LTP Response and Pharmacology in the Long-Evans Rat

Broad issues associated with non-replicability have been described in experimental pharmacological and behavioral cognitive studies. Efforts to prevent biases that contribute to non-replicable scientific protocols and to improve experimental rigor for reproducibility are increasingly seen as a basic requirement for the integrity of scientific research. Synaptic plasticity, encompassing long-term potentiation (LTP), is believed to underlie mechanisms of learning and memory. The present study was undertaken in Long-Evans (LE) rats, a strain of rat commonly used in cognitive behavioral tests, to (1) compare three LTP tetanisation protocols, namely, the high-frequency stimulation (HFS), the theta-burst stimulation (TBS), and the paired-pulse facilitation (PPF) at the Schaffer collateral-CA1 stratum radiatum synapse and to (2) assess sensitivity to acute pharmacology. Results: (1) When compared to Sprague-Dawley (SD) rats, the HFS using a stimulus intensity of 50% of the maximum slope obtained from input/output (I/O) curves elicited lower and higher thresholds of synaptic plasticity responses in SD and LE rats, respectively. The 2-train TBS protocol significantly enhanced the LTP response in LE rats over the 5- and 10-train TBS protocols in both strains, and the 5 × TBS protocol inducing a subthreshold LTP response was used in subsequent pharmacological studies in LE rats. The PPF protocol, investigating the locus of the LTP response, showed no difference for the selected interstimulus intervals. (2) Scopolamine, a nonspecific muscarinic antagonist, had a subtle effect, whereas donepezil, an acetylcholinesterase inhibitor, significantly enhanced the LTP response, demonstrating the sensitivity of the TBS protocol to enhanced cholinergic tone. MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist, significantly reduced LTP response, while memantine, another NMDA antagonist, had no effect on LTP response, likely associated with a weaker TBS protocol. PQ10, a phosphodiesterase-10 inhibitor, significantly enhanced the TBS-induced LTP response, but did not change the PPF response. Overall, the results confirm the strain-dependent differences in the form of synaptic plasticity, replicate earlier plasticity results, and report effective protocols that generate a relatively subthreshold margin of LTP induction and maintenance, which are suitable for pharmacology in the LE rat strain mainly used in cognitive studies.

Long-term potentiation in the hippocampal CA3 to CA1 synapses may be induced in vivo by activation of septal cholinergic inputs

PURPOSE/AIM

The role of cholinergic neurotransmission in the hippocampus remains controversial since different studies showed either no influence or its modulatory effect on glutamatergic hippocampal synapses. It remains unclear whether septal cholinergic input can modulate plasticity of synapses formed by CA3 pyramids on CA1 neurons. The aim of the study was to clarify the role of septal input in the development of LTP in this synapse.

MATERIALS AND METHODS

We recorded in vivo in rats under urethane anesthesia focal excitatory postsynaptic potential (fEPSP) characteristics in CA1 area after stimulation of the ventral hippocampal commissure (VHC), which contains both CA3 axons innervating CA1 neurons and cholinergic axons coming from the medial septum. We performed two series of experiments in which LTP was induced by tetanization of either VHC or medial septal area (MSA). Degeneration of cholinergic neurons in MSA was induced by intraseptal injection of 192IgG-saporin.

RESULTS

In both experimental series, tetanization induced an increase in fEPSP amplitude which lasted for at least 40 min after tetanic stimulation, although tetanization of VHC induced a larger increase in fEPSP amplitude compared to MSA tetanization. Elimination of septal cholinergic neurons by 192IgG-saporin abolished LTP development in both experimental series. This suppression of LTP in animals with cholinergic deficit was not due to loss of hippocampal neurons.

CONCLUSIONS

Our data suggest that activation of septal cholinergic fibers during tetanization is a critical factor of LTP induction in the hippocampal CA3 to CA1 synapses.

Muscarinic Regulation of Spike Timing Dependent Synaptic Plasticity in the Hippocampus

Long-term changes in synaptic transmission between neurons in the brain are considered the cellular basis of learning and memory. Over the last few decades, many studies have revealed that the precise order and timing of activity between pre- and post-synaptic cells ("spike-timing-dependent plasticity; STDP") is crucial for the sign and magnitude of long-term changes at many central synapses. Acetylcholine (ACh) via the recruitment of diverse muscarinic receptors is known to influence STDP in a variety of ways, enabling flexibility and adaptability in brain network activity during complex behaviors. In this review, we will summarize and discuss different mechanistic aspects of muscarinic modulation of timing-dependent plasticity at both excitatory and inhibitory synapses in the hippocampus to shape learning and memory.

Lasting modulation of in vitro oscillatory activity with weak direct current stimulation

Transcranial direct current stimulation (tDCS) is emerging as a versatile tool to affect brain function. While the acute neurophysiological effects of stimulation are well understood, little is know about the long-term effects. One hypothesis is that stimulation modulates ongoing neural activity, which then translates into lasting effects via physiological plasticity. Here we used carbachol-induced gamma oscillations in hippocampal rat slices to establish whether prolonged constant current stimulation has a lasting effect on endogenous neural activity. During 10 min of stimulation, the power and frequency of gamma oscillations, as well as multiunit activity, were modulated in a polarity specific manner. Remarkably, the effects on power and multiunit activity persisted for more than 10 min after stimulation terminated. Using a computational model we propose that altered synaptic efficacy in excitatory and inhibitory pathways could be the source of these lasting effects. Future experimental studies using this novel in vitro preparation may be able to confirm or refute the proposed hypothesis.

Cooperation between cholinergic and glutamatergic receptors are essential to induce BDNF-dependent long-lasting memory storage

The induction of long-lasting memory storage depends on the behavioral state of humans and animals. This behavioral state is mediated by neuromodulatory systems, like the cholinergic-septum-hippocampal circuit. Cholinergic neurotransmission is known to affect short-term activity-dependent plasticity in various brain areas, including the hippocampus. We could show here that a chemical late-long-term potentiation (LTP) could be induced in the basal dendrites by the coapplication of the cholinergic receptor agonist, carbachol, and the phosphodiesterase type 4 (PDE4)-inhibitor, rolipram at a concentration that by itself has no effect on basal synaptic transmission. This chemical late-LTP was similar to electrical late-LTP in that it is dependent on protein synthesis, cAMP, and NMDA-receptor activation. Occlusion experiments demonstrated that saturation of three tetanus (TET) late-LTP occluded carbachol-rolipram-LTP, indicating that they share similar properties. This cholinergic modulation of LTP in the basal dendrites was mediated by both muscarinic and nicotinic receptors. Carbachol also reinforced an early form of LTP into a long-lasting LTP. Most interestingly, these two forms of LTP could participate in the functional plasticity processes like synaptic tagging and capture (STC). In addition, we studied whether a cooperation between cholinergic and glutamatergic receptors is essential to induce functional synaptic-plasticity. Indeed, we could show that coactivation of acetylcholine/PDE4 inhibition must coincide with the release of glutamate to induce a long-lasting plasticity, showing a functional convergence of the two neuromodulatory systems. Moreover, we could also show that both chemical late-LTP and carbachol-reinforced early-LTP-induced STC processes are mediated by the neurotrophin BDNF.

The cholinergic system and hippocampal plasticity

Acetylcholine is an essential excitatory neurotransmitter in the central nervous system and undertakes a vital role in cognitive function. Consequently, there is ample evidence to suggest the involvement of both nicotinic and muscarinic acetylcholine receptors in the modulation of synaptic plasticity, which is believed to be the molecular correlate of learning and memory. In the hippocampus in particular, multiple subtypes of both nicotinic and muscarinic receptors are present at presynaptic and postsynaptic loci of both principal neurons and inhibitory interneurons, where they exert profound bi-directional influences on synaptic transmission. Further evidence points to a role for cholinergic activation in the induction and maintenance of synaptic plasticity, and key influences on hippocampal network oscillations. The present review examines these multiple roles of acetylcholine in hippocampal plasticity.

Cholinergic afferents to the locus coeruleus and noradrenergic afferents to the medial septum mediate LTP-reinforcement in the dentate gyrus by stimulation of the amygdala

Transient long-term potentiation (E-LTP) can be transformed into a long-lasting LTP (L-LTP) in the dentate gyrus (DG) by behavioral stimuli with high motivational content. Previous research from our group has identified several brain structures, such as the basolateral amygdala (BLA), the locus coeruleus (LC), the medial septum (MS) and transmitters as noradrenaline (NA) and acetylcholine (ACh) that are involved in these processes. Here we have investigated the functional interplay among brain structures and systems which result in the conversion of a E-LTP into a L-LTP (reinforcement) by stimulation of the BLA (BLA-R). We used topical application of specific drugs into DG, and other targets, while following the time course of LTP induced by stimulation of the perforant pathway (PP) to study their specific contribution to BLA-R. One injection cannula, a recording electrode in the DG and stimulating electrodes in the PP and the BLA were stereotactically implanted one week before electrophysiological experiments. Topical application of atropine or propranolol into the DG blocked BLA-R in both cases, but the effect of propranolol occurred earlier, suggesting a role of NA within the DG during an intermediate stage of LTP maintenance. The injection of lidocaine into the LC abolished BLA-R indicating that the LC is part of the functional neural reinforcing system. The effect on the LC is mediated by cholinergic afferents because application of atropine into the LC produced the same effect. Injection of lidocaine inactivating the MS also abolished BLA-R. This effect was mediated by noradrenergic afferents (probably from the LC) because the application of propranolol into the MS prevented BLA-R. These findings suggest a functional loop for BLA-R involving cholinergic afferents to the LC, a noradrenergic projection from the LC to the DG and the MS, and finally, the cholinergic projection from the MS to the DG.

Endogenous acetylcholine lowers the threshold for long-term potentiation induction in the CA1 area through muscarinic receptor activation: in vivo study

Little is known how synaptically released endogenous ACh affects hippocampal synaptic plasticity in vivo. Here, we examined the role of cholinergic drive in the regulation of the induction of long-term potentiation (LTP) at basal dendrites in the CA1 area of the anaesthetized rat hippocampus. The non-subtype selective muscarinic acetylcholine receptor antagonist, scopolamine, (0.3 mg/kg, i.p.) inhibited the induction of LTP by weak, but not strong, high frequency conditioning stimulation. A relatively M1 subtype-selective receptor antagonist, pirenzepine, (50 nmol/5 microL, i.c.v.) also inhibited LTP induction by the weak protocol. As the medial septum (MS) is a major source of endogenous ACh in the hippocampus, we also examined the effect of high frequency pre-conditioning stimulation of the MS on LTP induction. The pre-conditioning MS tetanus reduced the threshold for LTP induction at basal synapses in a narrow time window. Such an effect of MS pre-conditioning was prevented by scopolamine, strong evidence of a direct MS control of LTP threshold through a mechanism dependent on muscarinic receptor activation. These results suggest that the cholinergic drive to the hippocampus is critically involved in the control of the LTP induction threshold in vivo. To the extent that LTP mechanisms may underlie certain types of learning and memory, the septo-hippocampal cholinergic regulation of synaptic plasticity may constitute an important target for the treatment of cognitive disorders associated with ACh deficits.

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.

Administration of aggregated beta-amyloid peptide (25-35) induces changes in long-term potentiation in the hippocampus in vivo

Intracereroventricular administration of aggregated beta-amyloid protein fragment (25-35) (7.5 nmol/ventricle) was followed one month later by significant changes in the dynamics of long-term potentiation in the hippocampus in vivo, expressed as powerful and stable increases in the amplitude of evoked potentials. This phenomenon may be associated with oxidative stress in the hippocampus, which has previously been demonstrated in this model, and, thus, with disturbances in ion homeostasis.

The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

The morphology of the mature motor neuron dendritic arbor is determined by activity-dependent processes occurring during a critical period in early postnatal life. The abundance of the AMPA receptor subunit GluR1 in motor neurons is very high during this period and subsequently falls to a negligible level. To test the role of GluR1 in dendrite morphogenesis, we reintroduced GluR1 into rat motor neurons at the end of the critical period and quantitatively studied the effects on dendrite architecture. Two versions of GluR1 were studied that differed by the amino acid in the "Q/R" editing site. The amino acid occupying this site determines single-channel conductance, ionic permeability, and other essential electrophysiologic properties of the resulting receptor channels. We found large-scale remodeling of dendritic architectures in a manner depending on the amino acid occupying the Q/R editing site. Alterations in the distribution of dendritic arbor were not prevented by blocking NMDA receptors. These observations suggest that the expression of GluR1 in motor neurons modulates a component of the molecular substrate of activity-dependent dendrite morphogenesis. The control of these events relies on subunit-specific properties of AMPA receptors.

Cholinergic modulation of neocortical long-term potentiation in the awake, freely moving rat

The neocortex has proven resistant to LTP induction using standard in vitro and acute, in vivo preparations. Because the neocortex is widely thought to be involved in long-term information storage, this resistance raises questions about the validity of LTP as a memory model. Recently, we have shown that the neocortex of freely moving rats reliably supports LTP, provided that the stimulation is spaced and repeated over days. The following experiments were designed to evaluate the neuromodulatory role played by cholinergic systems in the induction of LTP in this preparation. Chronically implanted rats received either low- or high-intensity LTP-inducing tetani in combination with the administration of either a cholinergic agonist or antagonist injected systemically. Potentiation was evidenced as amplitude changes in both early and late components of the evoked field potential, the former including population spikes. The cholinergic agonist facilitated LTP induction in the late component of both high- and low-intensity groups. The cholinergic antagonist blocked LTP induction in the early component of the high-intensity group. The possibility that there are component-specific modulatory effects of cholinergic agents on the induction of neocortical LTP is discussed.

Spinal plasticity of acute opioid tolerance

Spinal acute opioid tolerance remains mechanistically undercharacterized. Expanded clinical use of direct spinal administration of opioids and other analgesics indicates that studies to further understand spinal mechanisms of analgesic tolerance are warranted. Rodent models of spinal administration facilitate this objective. Specifically, acute spinal opioid tolerance in mice presents a plasticity-dependent, rapid, and efficient opportunity for evaluation of novel clinical agents. Similarities between the pharmacology of acute and chronic spinal opioid tolerance, neuropathic pain, and learning and memory suggest that this model may serve as a high through-put predictor of bioactivity of novel plasticity-modifying compounds.