Balázs Lendvai

Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo

Do changes in neuronal structure underlie cortical plasticity? Here we used time-lapse two-photon microscopy of pyramidal neurons in layer 2/3 of developing rat barrel cortex to image the structural dynamics of dendritic spines and filopodia. We found that these protrusions were highly motile: spines and filopodia appeared, disappeared or changed shape over tens of minutes. To test whether sensory experience drives this motility we trimmed whiskers one to three days before imaging. Sensory deprivation markedly (∼40%) reduced protrusive motility in deprived regions of the barrel cortex during a critical period around postnatal days (P)11–13, but had no effect in younger (P8–10) or older (P14–16) animals. Unexpectedly, whisker trimming did not change the density, length or shape of spines and filopodia. However, sensory deprivation during the critical period degraded the tuning of layer 2/3 receptive fields. Thus sensory experience drives structural plasticity in dendrites, which may underlie the reorganization of neural circuits.

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.

Nonsynaptic communication in the central nervous system

Classical synaptic functions are important and suitable to relatively fast and discretely localized processes, but the nonclassical receptorial functions may be providing revolutionary possibilities for dealing at the cellular level with many of the more interesting and seemingly intractable features of neural and cerebral activities. Although different forms of nonsynaptic communication (volume transmission) often appear in different studies, their importance to modulate and mediate various functions is still not completely recognized. To establish the existence and the importance of nonsynaptic communication in the nervous system, here we cite pieces of evidence for each step of the interneuronal communication in the nonsynaptic context including the release into the extracellular space (ECS) and the extrasynaptic receptors and transporters that mediate nonsynaptic functions. We are now faced with a multiplicity of chemical communication. The fact that transmitters can even be released from nonsynaptic varicosities without being coupled to frequency-coded neuronal activity and they are able to diffuse over large distances indicates that there is a complementary mechanism of interneuronal communication to classical synaptic transmission. Nonconventional mediators that are also important part of the nonsynaptic world will also be overviewed.

Role of nitric oxide in modulating neurotransmitter release from rat striatum

The effect of the nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) on the basal and stimulation-evoked release of dopamine (DA) and acetylcholine (ACh) was investigated in rat striatum. The experiments were carried out in isolated superfused striatal slices, loaded with either [3H]-dopamine or [3H]-choline. We have found that L-NAME reduced the electrical field stimulation-evoked release of DA, while its enantiomer N-nitro-D-arginine methyl ester (D-NAME) was ineffective. In the presence of the nitric oxide (NO) precursor L-arginine, L-NAME failed to influence DA release. Furthermore, treatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 completely reversed the effect of L-NAME on striatal DA release. In contrast, L-NAME had no effect on either the basal or the stimulation-evoked ACh release in any experimental conditions studied. Our data indicate that endogenously produced NO is involved in the modulation of striatal DA, but not in ACh release. Furthermore, it seems likely that the modulatory effect of NO is linked to activation of presynaptic NMDA receptors located on the striatal dopaminergic nerve terminals.

Distance-Dependent Scaling of Calcium Transients Evoked by Backpropagating Spikes and Synaptic Activity in Dendrites of Hippocampal Interneurons

Although interactions between backpropagating action potentials and synaptic stimulations have been extensively studied in pyramidal neurons, dendritic propagation and the summation of these signals in interneurons are not nearly as well known. In this study, two-photon imaging was used to explore the basic properties of dendritic calcium signaling in CA1 stratum radiatum interneurons. In contrast to hippocampal pyramidal neurons, the backpropagating action potential-evoked calcium transients in dendrites of interneurons underwent a distance-dependent increment. Although, in proximal dendrites, an increment could be attributed to a smaller dendrite diameter, distal dendrites did not show such dependence. Calcium responses in interneurons had a smaller amplitude, slower rise time, and decay than in pyramidal neurons. To explore the factors underlying the difference, we compared the calcium-binding capacity in interneurons and in pyramidal neurons. Our finding that endogenous calcium buffers had a higher level in interneurons may primarily explain the different kinetics and amplitudes of calcium transients. Synaptic stimulation-evoked calcium transients were also larger at distant dendritic locations. The spread of these signals was restricted to 12-13 μm long dendritic compartments. Supporting the reported lack of long-term potentiation in these interneurons, we found only sublinear or linear summations of calcium responses to coincident synaptic inputs and backpropagating spikes.

Differential Mechanisms Involved in the Effect of Nicotinic Agonists DMPP and Lobeline to Release [3H]5-HT from Rat Hippocampal Slices

In the present study we investigated the effect of different nicotinic agonists (dimethylphenyl-piperazinium-iodide (DMPP), (-)nicotine, cytisine, (-)-lobeline, and (-)epibatidine) and antagonists (mecamylamine and dihydro-beta-erythroidine) on the release of [3H]5-HT from hippocampal slices. The nicotinic agonists DMPP and lobeline and electrical field stimulation, released [3H]5-HT from the hippocampus; other nicotinic agonists, such as (-)-nicotine, cytisine, and (-)-epibatidine had no effect. Unlike lobeline-induced release of [3H]5-HT, the effect of DMPP (10 and 40 microM) was antagonized by mecamylamine (20 and 10 microM). The effect of DMPP was [Ca2+]o-independent. In experiments carried out at 7 degrees C, i.e. the membrane carrier proteins are inhibited and the release by lobeline was abolished while the DMPP-induced release of 5-HT was rather potentiated. It is proposed that the effect of DMPP and lobeline, to enhance the release of [3H]5-HT from the hippocampus, was mediated by two different mechanisms. While DMPP-induced 5-HT release can be linked to a non-classical nAChR activation ([Ca2+]o-independence), the effect of lobeline was likely mediated by uptake carriers.

Neurochemical evidence of dopamine release by lateral olivocochlear efferents and its presynaptic modulation in guinea-pig cochlea

In this study, using an in vitro superfusion technique for the first time, we provide direct neurochemical evidence of the transmitter role of dopamine at the level of lateral olivocochlear efferent fibres of the guinea-pig cochlea. Our results revealed that nerve terminals are able to take up and release dopamine upon axonal stimulation. Since dopamine is thought to protect the afferent nerve fibres from damage due to acoustic trauma or ischaemia, enhancement of the release of dopamine, a potential therapeutic site of these injuries, was investigated. Positive modulation of dopamine release has been shown by a D1 dopamine receptor agonist, an antagonist and piribedil. Furthermore, negative feedback on the stimulation-evoked release of dopamine via D2 dopamine receptors has been excluded. Electrical stimulation of the cochlear tissue produced a significant and reproducible release of [3H]dopamine, which could be blocked by tetrodotoxin (1 microM) and cadmium (100 microM), proving that axonal activity releases dopamine and its dependence on Ca2+ influx verifies its neuronal origin. Nomifensine, a high-affinity dopamine uptake blocker, prevented the tissue from taking up [3H]dopamine from the bathing solution, also indicating the neural origin of dopamine released in response to stimulation. SKF-38393 (a selective D1 agonist) increased both the resting and electrically evoked release of dopamine. Piribedil (a D3/D2/D1 agonist), a drug under investigation, known to prevent acoustic trauma or ischaemia-induced hearing loss, had a similar and concentration-dependent increasing effect on both resting and evoked release of dopamine. The effect of both drugs on stimulation-evoked release could be prevented by SKF-83566 (a selective D1 antagonist). However, SKF-83566 alone enhanced the resting and axonal conduction-associated release of dopamine. D2 agonists and antagonists failed to modulate the release of dopamine, indicating the lack of negative feedback modulation of dopamine release. Our results suggest that the release of dopamine was subjected to modulation by a D1 receptor agonist and an antagonist. In addition, it is concluded that D2 receptors are not involved in the modulation of dopamine release. This observation may have clinical relevance in the prevention or therapy of particular types of hearing loss, because enhanced dopaminergic input into the primary auditory neuron may inhibit the (over)excitation of this neuron by glutamatergic input from inner hair cells.

α7 nicotinic acetylcholine receptors and their role in cognition.

The precise role of nicotinic acetylcholine receptors (nAChRs) in central cognitive processes still remains incompletely understood almost 150 years after its initial discovery. Central nAChRs are activated by acetylcholine, which functions in the extracellular space as a nonsynaptic messenger. Recently, a novel concept in the nAChR mode of operation has been described as a fast-type nonsynaptic transmission. In this review, we attempt to summarise the experimental findings that support the role of one of the most distributed receptor subtypes, the α7 nAChRs, and particularly focus on its procognitive effects following receptor activation. The basic characteristics of α7 nAChRs are discussed, from receptor homology to cellular-level functions. Synaptic plasticity is often implicated with α7 nAChRs on the basis of several diverse studies. Here, we provide a summary of the plastic features of the α7 receptor subtype and its role in higher level cognitive function. Finally, recent clinical evidence is reviewed, which demonstrates with increasing confidence the promise α7 nAChRs as a molecular target in future pharmacotherapy to prevent cognitive decline in various types of dementia, specifically, via the development of positive allosteric modulator compounds.

Layer-specific differences in reactive oxygen species levels after oxygen–glucose deprivation in acute hippocampal slices

The major role of reactive oxygen species (ROS) in the pathomechanism of ischemia have been widely recognized. Still, measurements of the precise time course and regional distribution of ischemia-induced ROS level changes in acute brain slices have been missing. By using acute hippocampal slices and the fluorescent dye CM-H2DCFDA, we showed that reoxygenation after in vitro ischemia (oxygen-glucose deprivation; OGD) increased ROS levels in the hippocampal CA1 layers vulnerable to ischemia but did not have significant effects in the resistant stratum granulosum in the dentate gyrus (DG). Production of ROS started during OGD, but, contrary to reoxygenation, it manifested as a ROS level increase exclusively in the presence of catalase and glutathione peroxidase inhibition. The mechanism of ROS production involves the activation of NMDA receptors and nitric oxide synthases. The inhibition of ROS response by either AP-5 or L-NAME together with the ROS sensitivity profile of the dye suggest that peroxynitrite, the reaction product of superoxide and nitric oxide, plays a role in the response. Direct visualization of layer-specific effects of ROS production and its scavenging, shown for the first time in acute hippocampal slices, suggests that distinct ROS homeostasis may underlie the different ischemic vulnerability of CA1 and DG.

Effect of selective opiate antagonists on striatal acetylcholine and dopamine release

We investigated the effect of selective opiate antagonists on striatal acetylcholine (ACh) and dopamine (DA) release. The mu-receptor antagonist beta-funaltrexamine (beta-FNA), the delta-antagonist naltrindole (NTI), and the kappa-antagonist norbinaltorphimine (nor-BNI) were used to selectively block different subtypes of opiate receptors. The experiments were carried out on isolated superfused striatal slices of rats, loaded with [3H]choline or [3H]dopamine. beta-FNA and NTI significantly enhanced the electrical field stimulation-evoked release of ACh but only if the dopaminergic input had been impaired either by chemical denervation or D2 dopamine receptor blockade. By contrast, neither the selective nor nonselective antagonists had any modulatory effect on the release of dopamine. It is concluded, therefore, that the release of ACh is tonically controlled by endogenous opioid peptide(s) through the stimulation of mu- and delta-opiate receptors located on cholinergic axon terminals, in addition to the tonic control by DA.