Working memory features are embedded in hippocampal place fields

Hippocampal principal neurons display both spatial tuning properties and memory features. Whether this distinction corresponds to separate neuron types or a context-dependent continuum has been debated. We report here that the task-context ("splitter") feature is highly variable along both trial and spatial position axes. Neurons acquire or lose splitter features across trials even when place field features remain unaltered. Multiple place fields of the same neuron can individually encode both past or future run trajectories, implying that splitter fields are under the control of assembly activity. Place fields can be differentiated into subfields by the behavioral choice of the animal, and splitting within subfields evolves across trials. Interneurons also differentiate choices by integrating inputs from pyramidal cells. Finally, bilateral optogenetic inactivation of the medial entorhinal cortex reversibly decreases the fraction of splitter fields. Our findings suggest that place or splitter features are different manifestations of the same hippocampal computation.

Aversive stimulus-tuned responses in the CA1 of the dorsal hippocampus

Throughout life animals inevitably encounter unforeseen threatening events. Activity of principal cells in the hippocampus is tuned for locations and for salient stimuli in the animals' environment thus forming a map known to be pivotal for guiding behavior. Here, we explored if a code of threatening stimuli exists in the CA1 region of the dorsal hippocampus of mice by recording neuronal response to aversive stimuli delivered at changing locations. We have discovered a rapidly emerging, location independent response to innoxious aversive stimuli composed of the coordinated activation of subgroups of pyramidal cells and connected interneurons. Activated pyramidal cells had higher basal firing rate, more probably participated in ripples, targeted more interneurons than place cells and many of them lacked place fields. We also detected aversive stimulus-coupled assemblies dominated by the activated neurons. Notably, these assemblies could be observed even before the delivery of the first aversive event. Finally, we uncovered the systematic shift of the spatial code from the aversive to, surprisingly, the reward location during the fearful stimulus. Our results uncovered components of the dorsal CA1 circuit possibly key for re-sculpting the spatial map in response to abrupt aversive events.

The medial septum controls hippocampal supra-theta oscillations

Hippocampal theta oscillations orchestrate faster beta-to-gamma oscillations facilitating the segmentation of neural representations during navigation and episodic memory. Supra-theta rhythms of hippocampal CA1 are coordinated by local interactions as well as inputs from the entorhinal cortex (EC) and CA3 inputs. However, theta-nested gamma-band activity in the medial septum (MS) suggests that the MS may control supra-theta CA1 oscillations. To address this, we performed multi-electrode recordings of MS and CA1 activity in rodents and found that MS neuron firing showed strong phase-coupling to theta-nested supra-theta episodes and predicted changes in CA1 beta-to-gamma oscillations on a cycle-by-cycle basis. Unique coupling patterns of anatomically defined MS cell types suggested that indirect MS-to-CA1 pathways via the EC and CA3 mediate distinct CA1 gamma-band oscillations. Optogenetic activation of MS parvalbumin-expressing neurons elicited theta-nested beta-to-gamma oscillations in CA1. Thus, the MS orchestrates hippocampal network activity at multiple temporal scales to mediate memory encoding and retrieval.

Novel expression system based on enhanced permeability of Vibrio natriegens cells induced by D,D- carboxypeptidase overexpression

Vibrio natriegens is a fast-growing, non-pathogenic marine bacterium with promising features for biotechnological applications such as high-level recombinant protein production or fast DNA propagation. A remarkable short generation time (< 10 min), robust proteosynthetic activity and versatile metabolism with abilities to utilise wide range of substrates contribute to its establishment as a future industrial platform for fermentation processes operating with high productivity.D,D-carboxypeptidases are membrane-associated enzymes involved in peptidoglycan biosynthesis and cell wall formation. This study investigates the impact of overexpressed D,D-carboxypeptidases on membrane integrity and the increased leakage of intracellular proteins into the growth medium in V. natriegens. Our findings confirm that co-expression of these enzymes can enhance membrane permeability, thereby facilitating the transport of target proteins into the extracellular environment, without the need for secretion signals, tags, or additional permeabilization methods. Using only a single step IMAC chromatography, we were able to purify AfKatG, MDBP or Taq polymerase in total yields of 117.9 ± 56.0 mg/L, 36.5 ± 12.9 mg/L and 26.5 ± 6.0 mg/L directly from growth medium, respectively. These results demonstrate the feasibility of our V. natriegens based system as a broadly applicable extracellular tag-less recombinant protein producer.

Activity and Coupling to Hippocampal Oscillations of Median Raphe GABAergic Cells in Awake Mice

Ascending serotonergic/glutamatergic projection from the median raphe region (MRR) to the hippocampal formation regulates both encoding and consolidation of memory and the oscillations associated with them. The firing of various types of MRR neurons exhibits rhythmic modulation coupled to hippocampal oscillatory activity. A possible intermediary between rhythm-generating forebrain regions and entrained ascending modulation may be the GABAergic circuit in the MRR, known to be targeted by a diverse array of top-down inputs. However, the activity of inhibitory MRR neurons in an awake animal is still largely unexplored. In this study, we utilized whole cell patch-clamp, single cell, and multichannel extracellular recordings of GABAergic and non-GABAergic MRR neurons in awake, head-fixed mice. First, we have demonstrated that glutamatergic and serotonergic neurons receive both transient, phasic, and sustained tonic inhibition. Then, we observed substantial heterogeneity of GABAergic firing patterns but a marked modulation of activity by brain states and fine timescale coupling of spiking to theta and ripple oscillations. We also uncovered a correlation between the preferred theta phase and the direction of activity change during ripples, suggesting the segregation of inhibitory neurons into functional groups. Finally, we could detect complementary alteration of non-GABAergic neurons' ripple-coupled activity. Our findings support the assumption that the local inhibitory circuit in the MRR may synchronize ascending serotonergic/glutamatergic modulation with hippocampal activity on a subsecond timescale.

Uncovering spatial representations from spatiotemporal patterns of rodent hippocampal field potentials

Spatiotemporal patterns of large-scale spiking and field potentials of the rodent hippocampus encode spatial representations during maze runs, immobility, and sleep. Here, we show that multisite hippocampal field potential amplitude at ultra-high-frequency band (FPAuhf), a generalized form of multiunit activity, provides not only a fast and reliable reconstruction of the rodent's position when awake, but also a readout of replay content during sharp-wave ripples. This FPAuhf feature may serve as a robust real-time decoding strategy from large-scale recordings in closed-loop experiments. Furthermore, we develop unsupervised learning approaches to extract low-dimensional spatiotemporal FPAuhf features during run and ripple periods and to infer latent dynamical structures from lower-rank FPAuhf features. We also develop an optical flow-based method to identify propagating spatiotemporal LFP patterns from multisite array recordings, which can be used as a decoding application. Finally, we develop a prospective decoding strategy to predict an animal's future decision in goal-directed navigation.

Do not waste your electrodes-principles of optimal electrode geometry for spike sorting

Objective. This study examines how the geometrical arrangement of electrodes influences spike sorting efficiency, and attempts to formalise principles for the design of electrode systems enabling optimal spike sorting performance.Approach. The clustering performance of KlustaKwik, a popular toolbox, was evaluated using semi-artificial multi-channel data, generated from a library of real spike waveforms recorded in the CA1 region of mouse Hippocampusin vivo.Main results. Based on spike sorting results under various channel configurations and signal levels, a simple model was established to describe the efficiency of different electrode geometries. Model parameters can be inferred from existing spike waveform recordings, which allowed quantifying both the cooperative effect between channels and the noise dependence of clustering performance.Significance. Based on the model, analytical and numerical results can be derived for the optimal spacing and arrangement of electrodes for one- and two-dimensional electrode systems, targeting specific brain areas.

Recombinant Enzymatic Redox Systems for Preparation of Aroma Compounds by Biotransformation

The aim of this study was to develop immobilized enzyme systems that reduce carbonyl compounds to their corresponding alcohols. The demand for natural aromas and food additives has been constantly growing in recent years. However, it can no longer be met by extraction and isolation from natural materials. One way to increase the availability of natural aromas is to prepare them by the enzymatic transformation of suitable precursors. Recombinant enzymes are currently being used for this purpose. We investigated trans-2-hexenal bioreduction by recombinant Saccharomyces cerevisiae alcohol dehydrogenase (ScADH1) with simultaneous NADH regeneration by recombinant Candida boidinii formate dehydrogenase (FDH). In a laboratory bioreactor with two immobilized enzymes, 88% of the trans-2-hexenal was transformed to trans-2-hexenol. The initial substrate concentration was 3.7 mM. The aldehyde destabilized ScADH1 by eluting Zn²⁺ ions from the enzyme. A fed-batch operation was used and the trans-2-hexenal concentration was maintained at a low level to limit the negative effect of Zn²⁺ ion elution from the immobilized ScADH1. Another immobilized two-enzyme system was used to reduce acetophenone to (S)-1-phenylethanol. To this end, the recombinant alcohol dehydrogenase (RrADH) from Rhodococcus ruber was used. This biocatalytic system converted 61% of the acetophenone to (S)-1-phenylethanol. The initial substrate concentration was 8.3 mM. All enzymes were immobilized by poly-His tag to Ni²⁺, which formed strong but reversible bonds that enabled carrier reuse after the loss of enzyme activity.

Cause analysis and risk evaluation of the occurrence of Campylobacter spp. in the broiler production chain

Tendencies of foodborne outbreaks show that the number of illnesses caused by Campylobacter spp. has been increasing recently in the European Union and in Hungary as well. However, the epidemiological statuses of Member States are diverse. There are several aspects to be investigated by competent authorities before the introduction of interventions. Methods supporting food safety decision making range from quick and easy techniques to complex, resource consuming approaches. The aim of the present study was the implementation of an evaluation and ranking system for a risk and its causes occurring in the broiler production chain. Data and information available in scientific literature were converted to a structured easy-to-use evaluation that supports decision making and helps structured data processing.

Median raphe controls acquisition of negative experience in the mouse

Adverse events need to be quickly evaluated and memorized, yet how these processes are coordinated is poorly understood. We discovered a large population of excitatory neurons in mouse median raphe region (MRR) expressing vesicular glutamate transporter 2 (vGluT2) that received inputs from several negative experience-related brain centers, projected to the main aversion centers, and activated the septohippocampal system pivotal for learning of adverse events. These neurons were selectively activated by aversive but not rewarding stimuli. Their stimulation induced place aversion, aggression, depression-related anhedonia, and suppression of reward-seeking behavior and memory acquisition-promoting hippocampal theta oscillations. By contrast, their suppression impaired both contextual and cued fear memory formation. These results suggest that MRR vGluT2 neurons are crucial for the acquisition of negative experiences and may play a central role in depression-related mood disorders.

Brainstem nucleus incertus controls contextual memory formation

Hippocampal pyramidal cells encode memory engrams, which guide adaptive behavior. Selection of engram-forming cells is regulated by somatostatin-positive dendrite-targeting interneurons, which inhibit pyramidal cells that are not required for memory formation. Here, we found that γ-aminobutyric acid (GABA)-releasing neurons of the mouse nucleus incertus (NI) selectively inhibit somatostatin-positive interneurons in the hippocampus, both monosynaptically and indirectly through the inhibition of their subcortical excitatory inputs. We demonstrated that NI GABAergic neurons receive monosynaptic inputs from brain areas processing important environmental information, and their hippocampal projections are strongly activated by salient environmental inputs in vivo. Optogenetic manipulations of NI GABAergic neurons can shift hippocampal network state and bidirectionally modify the strength of contextual fear memory formation. Our results indicate that brainstem NI GABAergic cells are essential for controlling contextual memories.

Molecular hydrogen affords neuroprotection in a translational piglet model of hypoxic-ischemic encephalopathy

Hypoxic-ischemic encephalopathy (HIE) is the major consequence of perinatal asphyxia (PA) in term neonates. Although the newborn piglet is an accepted large animal PA/HIE model, there is no consensus on PA-induction methodology to produce clinically relevant HIE. We aimed to create and to characterize a novel PA model faithfully reproducing all features of asphyxiation including severe hypercapnia resulting in HIE, and to test whether H₂ is neuroprotective in this model. Piglets were anaesthetised, artificially ventilated, and intensively monitored (electroencephalography, core temperature, O₂ saturation, arterial blood pressure and blood gases). Asphyxia (20 min) was induced by ventilation with a hypoxic-hypercapnic (6%O₂ - 20%CO₂) gas mixture. Asphyxia-induced changes in the cortical microcirculation were assessed with laser-speckle contrast imaging and analysis. Asphyxia was followed by reventilation with air or air containing hydrogen (2.1%H₂, 4 hours). After 24 hours survival, the brains were harvested for neuropathology. Our PA model was characterized by the development of severe hypoxia (pO2 = 27 ± 4 mmHg), and combined acidosis (pH = 6.76 ± 0.04; pCO₂ = 114 ± 11 mmHg; lactate = 12.12 ± 0.83 mmol/L), however, cortical ischemia did not develop during the stress. Severely depressed electroencephalography (EEG), and marked neuronal injury indicated the development of HIE. H₂ was neuroprotective shown both by the enhanced recovery of EEG and by the significant preservation of neurons in the cerebral cortex, hippocampus, basal ganglia, and the thalamus. H2 appeared to reduce oxidative stress shown by attenuation of 8-hydroxy-2'-deoxyguanosine immunostaining. In summary, this new PA piglet model is able to induce moderate/severe HIE, and the efficacy of hydrogen post-treatment to preserve neuronal activity/function in this PA/HIE model suggests the feasibility of this safe and inexpensive approach in the treatment of asphyxiated babies.

Divergent in vivo activity of non-serotonergic and serotonergic VGluT3-neurones in the median raphe region

KEY POINTS

The median raphe is a key subcortical modulatory centre involved in several brain functions, such as regulation of the sleep-wake cycle, emotions and memory storage. A large proportion of median raphe neurones are glutamatergic and implement a radically different mode of communication compared to serotonergic cells, although their in vivo activity is unknown. We provide the first description of the in vivo, brain state-dependent firing properties of median raphe glutamatergic neurones identified by immunopositivity for the vesicular glutamate transporter type 3 (VGluT3) and serotonin (5-HT). Glutamatergic populations (VGluT3+/5-HT- and VGluT3+/5-HT+) were compared with the purely serotonergic (VGluT3-/5-HT+ and VGluT3-/5-HT-) neurones. VGluT3+/5-HT+ neurones fired similar to VGluT3-/5-HT+ cells, whereas they significantly diverged from the VGluT3+/5-HT- population. Activity of the latter subgroup resembled the spiking of VGluT3-/5-HT- cells, except for their diverging response to sensory stimulation. The VGluT3+ population of the median raphe may broadcast rapidly varying signals on top of a state-dependent, tonic modulation.

ABSTRACT

Subcortical modulation is crucial for information processing in the cerebral cortex. Besides the canonical neuromodulators, glutamate has recently been identified as a key cotransmitter of numerous monoaminergic projections. In the median raphe, a pure glutamatergic neurone population projecting to limbic areas was also discovered with a possibly novel, yet undetermined function. In the present study, we report the first functional description of the vesicular glutamate transporter type 3 (VGluT3)-expressing median raphe neurones. Because there is no appropriate genetic marker for the separation of serotonergic (5-HT+) and non-serotonergic (5-HT-) VGluT3+ neurones, we utilized immunohistochemistry after recording and juxtacellular labelling in anaesthetized rats. VGluT3+/5-HT- neurones fired faster, more variably and were permanently activated during sensory stimulation, as opposed to the transient response of the slow firing VGluT3-/5-HT+ subgroup. VGluT3+/5-HT- cells were also more active during hippocampal theta. In addition, the VGluT3-/5-HT- population, comprising putative GABAergic cells, resembled the firing of VGluT3+/5-HT- neurones but without any significant reaction to the sensory stimulus. Interestingly, the VGluT3+/5-HT+ group, spiking slower than the VGluT3+/5-HT- population, exhibited a mixed response (i.e. the initial transient activation was followed by a sustained elevation of firing). Phase coupling to hippocampal and prefrontal slow oscillations was found in VGluT3+/5-HT- neurones, also differentiating them from the VGluT3+/5-HT+ subpopulation. Taken together, glutamatergic neurones in the median raphe may implement multiple, highly divergent forms of modulation in parallel: a slow, tonic mode interrupted by sensory-evoked rapid transients, as well as a fast one capable of conveying complex patterns influenced by sensory inputs.

Determination of Soil Endangerment by Wind Erosion with Consideration of Legislative Changes in Acceptable Soil Loss

Value tightening of acceptable soil loss by wind erosion in amendment to the Act No. 220/2004 on Protection and Use of Agricultural Land in the Slovak Republic from 1st of April 2013 is necessary to reconsider wind erosion intensity in agricultural territories. The paper presents results of wind erosion intensity calculation by using Wind Erosion Equation (WEQ) that is recommended by Act No. 220/2004. As observed we choose cadastral area Moèenok territory and had determined and compared changes in levels of soil endangerment of arable land by wind and spatial delamination of wind erosion in specific territory of Moèenok. According to WEQ calculation, we determined that soil loss from 3778.85 ha arable land is 1220.52 ha, which is highly endangerment by wind erosion. By defining levels of soil erosion endangerment (LSEE), we found out that area in 3rd class of endangerment rose from 1.48% to 43.37% after changing acceptable soil from 40 to 15 t ha-1 year-1. Results enable us to specify priority areas where to implement erosion control measures in according to sustainable use and protection of arable land in model area.

The c.-133A > G polymorphism in NPC1L1 gene influences the efficacy of ezetimibe monotherapy on apolipoprotein A1 in hyperlipidemic patients

Niemann-Pick C1-like 1 protein (NPC1L1) plays a critical role in intestinal cholesterol absorption. Previous studies found that the NPC1L1 c.-133A > G SNP, but not other NPC1L1 SNPs, was associated with response to statin treatment and statin-ezetimibe combinations. To date effect of NPC1L1 c.-133A > G SNP on ezetimibe monotherapy has not been studied. Our objective was to examine whether SNP c.-133A > G at the NPC1L1 gene has effects on lipid levels and on the efficacy of 3, 6 and 12 months of 10 mg daily ezetimibe monotherapy in hyperlipidemic patients with statin induced adverse effects. One hundred and one type IIa and IIb hyperlipidemic patients (72 females, 29 males; age: 61.23 +/- 9.87 ys; BMI: 28.18 +/- 4.29 kg/m2) were enrolled. The genotype frequencies were conformed to Hardy-Weinberg equilibrium. We could not find significant differences in initial lipid levels between AA and AG + GG patients. While plasma levels of apolipoprotein A1 (ApoA1) did not significantly decrease after ezetimibe treatment (1.96; 3.39 and 2.74%) in AA patients, a significant elevation in ApoA1 levels has been found after treatment in AG + GG patients (9.15; 8.54 and 13.58%). The effect of NPC1L1 c.-133A > G on the ApoA1 levels was found significant (p < 0.05). Efficacy of treatment with ezetimibe on other plasma lipid parameters after 3, 6 or 12 months did not differ significantly. NPC1L1-133A > G SNP influences the ApoA1 response to ezetimibe monotherapy, therefore, may alter the effect of ezetimibe on the structure and function of the high-density lipoprotein particles.

Fast synaptic subcortical control of hippocampal circuits

Cortical information processing is under state-dependent control of subcortical neuromodulatory systems. Although this modulatory effect is thought to be mediated mainly by slow nonsynaptic metabotropic receptors, other mechanisms, such as direct synaptic transmission, are possible. Yet, it is currently unknown if any such form of subcortical control exists. Here, we present direct evidence of a strong, spatiotemporally precise excitatory input from an ascending neuromodulatory center. Selective stimulation of serotonergic median raphe neurons produced a rapid activation of hippocampal interneurons. At the network level, this subcortical drive was manifested as a pattern of effective disynaptic GABAergic inhibition that spread throughout the circuit. This form of subcortical network regulation should be incorporated into current concepts of normal and pathological cortical function.

The presence of pacemaker HCN channels identifies theta rhythmic GABAergic neurons in the medial septum

The medial septum (MS) is an indispensable component of the subcortical network which synchronizes the hippocampus at theta frequency during specific stages of information processing. GABAergic neurons exhibiting highly regular firing coupled to the hippocampal theta rhythm are thought to form the core of the MS rhythm-generating network. In recent studies the hyperpolarization-activated, cyclic nucleotide-gated non-selective cation (HCN) channel was shown to participate in theta synchronization of the medial septum. Here, we tested the hypothesis that HCN channel expression correlates with theta modulated firing behaviour of MS neurons by a combined anatomical and electrophysiological approach. HCN-expressing neurons represented a subpopulation of GABAergic cells in the MS partly overlapping with parvalbumin (PV)-containing neurons. Rhythmic firing in the theta frequency range was characteristic of all HCN-expressing neurons. In contrast, only a minority of HCN-negative cells displayed theta related activity. All HCN cells had tight phase coupling to hippocampal theta waves. As a group, PV-expressing HCN neurons had a marked bimodal phase distribution, whereas PV-immunonegative HCN neurons did not show group-level phase preference despite significant individual phase coupling. Microiontophoretic blockade of HCN channels resulted in the reduction of discharge frequency, but theta rhythmic firing was perturbed only in a few cases. Our data imply that HCN-expressing GABAergic neurons provide rhythmic drive in all phases of the hippocampal theta activity. In most MS theta cells rhythm genesis is apparently determined by interactions at the level of the network rather than by the pacemaking property of HCN channels alone.

Childhood-onset mild cutaneous porphyria with compound heterozygotic mutations in the uroporphyrinogen decarboxylase gene

Three children (two boys and one girl) from the same family presented with photosensitivity, hyperpigmentation, hypertrichosis, mild skin fragility, blistering and scarring in childhood. On examination, the cutaneous lesions were found to have improved since their previous examinations. Laboratory tests showed raised plasma and urine carboxyporphyrins and decreased uroporphyrinogen decarboxylase enzyme activity in red blood cells. Triggering factors for porphyria were not detected except for a hepatitis C virus infection in the younger boy. The girl's clinical symptoms recurred in late adolescence, after iron and oestrogen treatments. Mutation analysis of the UROD gene detected two missense mutations, 19 A-->G M1V (novel) and 703C-->T P235S (previously reported), in an uncommon compound heterozygous manner in the three siblings.

Cortical control of zona incerta

The zona incerta (ZI) is at the crossroad of almost all major ascending and descending fiber tracts and targets numerous brain centers from the thalamus to the spinal cord. Effective ascending drive of ZI cells has been described, but the role of descending cortical signals in patterning ZI activity is unknown. Cortical control over ZI function was examined during slow cortical waves (1-3 Hz), paroxysmal high-voltage spindles (HVSs), and 5-9 Hz oscillations in anesthetized rats. In all conditions, rhythmic cortical activity significantly altered the firing pattern of ZI neurons recorded extracellularly and labeled with the juxtacellular method. During slow oscillations, the majority of ZI neurons became synchronized to the depth-negative phase ("up state") of the cortical waves to a degree comparable to thalamocortical neurons. During HVSs, ZI cells displayed highly rhythmic activity in tight synchrony with the cortical oscillations. ZI neurons responded to short epochs of cortical 5-9 Hz oscillations, with a change in the interspike interval distribution and with an increase in spectral density in the 5-9 Hz band as measured by wavelet analysis. Morphological reconstruction revealed that most ZI cells have mediolaterally extensive dendritic trees and very long dendritic segments. Cortical terminals established asymmetrical synapses on ZI cells with very long active zones. These data suggest efficient integration of widespread cortical signals by single ZI neurons and strong cortical drive. We propose that the efferent GABAergic signal of ZI neurons patterned by the cortical activity can play a critical role in synchronizing thalamocortical and brainstem rhythms.

Automated virtual microscopy of gastric biopsies

BACKGROUND

Automated virtual microscopy of specimens from gastrointestinal biopsies is based on cytometric parameters of digitized histological sections. To our knowledge, cytometric parameters of gastritis and of adenocarcinoma have yet to be fully characterized. Our objective was to classify gastritis and adenocarcinoma based on cytometric parameters. We hypothesized that automated virtual microscopy using this novel classification can reliably diagnose gastritis and adenocarcinoma.

METHODS

Routinely processed hematoxylin-and-eosin-stained histological sections from specimens that showed normal mucosa (14 cases), gastritis (35 cases), and adenocarcinoma (30 cases) diagnosed by conventional optical microscopy were scanned and digitized at high resolution. Thirty-eight cytometric parameters based on density and morphometry were applied to glands and superficial epithelium. Twelve cytometric parameters based on cytologic detail were applied to individual cells.

RESULTS

Statistically significant differences in cytometric parameters for normal mucosa, gastritis, and adenocarcinoma were found. The most discriminatory parameter was the ratio of the total number of cells to the number of interstitial cells. These differences correctly classified adenocarcinoma at 100% accuracy and overall correctness was 86%.

CONCLUSIONS

We describe a novel method of analyzing gastric mucosal histology based on cytometric parameters. Automated virtual microscopy can be used to classify gastric mucosa as normal, gastritis, or adenocarcinoma with reasonable accuracy. Further research is necessary to determine whether automated virtual microscopy can subclassify gastric mucosal histology in greater detail.

Serotonergic neuron diversity: identification of raphe neurons with discharges time-locked to the hippocampal theta rhythm

The serotonergic system plays a key role in the regulation of brain states, and many of the known features of serotonergic neurons appear to match this function. Midbrain raphe nuclei provide a diffuse projection to all regions of the forebrain, and raphe neurons exhibit a slow metronome-like activity that sets the ambient levels of serotonin across the sleep-wake cycle. Serotonergic cells have also been implicated, however, in a variety of more specific functions that can hardly be related to their low-rate monotonous patterns of discharges. The amazing variety of serotonergic receptors and their type-specific distribution on cortical neurons also raise the possibility of a more intimate coordination between the activity of serotonergic neurons and their target cortical circuits. Here we report an unexpected diversity in the behavior of immunohistochemically identified serotonergic neurons. Two outstanding subpopulations were identified by using the in vivo juxtacellular recording and labeling technique. The first subpopulation of serotonergic cells exhibited the classic clock-like activity with no apparent short timescale interaction with the hippocampal electroencephalogram. The other subpopulation discharged action potentials that were phase-locked to the hippocampal theta rhythm, the oscillatory pattern associated with acquisition of information and memory formation. These results indicate that the ascending serotonergic system comprises cells involved in complex information processing beyond the regulation of state transitions. The heterogeneity of serotonergic neuron behavior can also help to explain the complexity of symptoms associated with serotonergic dysfunction.

GABAergic control of the ascending input from the median raphe nucleus to the limbic system

The median raphe nucleus (MRN) is the primary source of serotonergic afferents to the limbic system that are generally considered to suppress hippocampal theta oscillations. GABA receptors are expressed in the MRN by serotonergic and nonserotonergic cells, including GABAergic and glutamatergic neurons. This study investigated the mechanisms by which the fluctuating GABA tone in the MRN leads to induction or suppression of hippocampal theta rhythm. We found that MRN application of the GABA(A) agonist muscimol (0.05-1.0 mM) or GABA(B) agonist baclofen (0.2 mM) by reverse microdialysis had strong theta promoting effects. The GABA(A) antagonist bicuculline infused in low concentrations (0.1, 0.2 mM) eliminated theta rhythm. A short period of theta activity of higher than normal frequency preceded hippocampal desynchronization in 46% of rats. Bicuculline in larger concentrations (0.5, 1.0, 2.0 mM) resulted in a biphasic response of an initial short (<10 min) hippocampal desynchronization followed by stable theta rhythm that lasted as long as the infusion continued. The frequency and amplitude of theta waves were higher than in control recordings and the oscillations showed a conspicuous intermittent character. Hippocampal theta rhythm elicited by MRN administration of bicuculline could be completely (0.5 mM bicuculline) or partially (1.0 mM bicuculline) blocked by simultaneous infusion of the GABA(B) antagonist CGP35348. Our findings suggest that the GABAergic network may have two opposing functions in the MRN: relieving the theta-generators from serotonergic inhibition and regulating the activity of a theta-promoting circuitry by the fluctuating GABA tone. The two mechanisms may be involved in different functions.

Phase segregation of medial septal GABAergic neurons during hippocampal theta activity

Septo-hippocampal GABAergic neurons immunoreactive for parvalbumin are thought to play a crucial role in the generation of hippocampal theta oscillations associated with a specific stage of memory formation. Here we use in vivo juxtacellular recording and filling in the medial septum followed by immunocytochemical identification of the recorded cells containing parvalbumin to determine their firing pattern, phase relationship with hippocampal theta, morphology, and to thereby reveal their involvement in the generation of hippocampal theta activity. We have demonstrated that GABAergic medial septal neurons form two distinct populations exhibiting highly regular bursting activity that is tightly coupled to either the trough (178 degrees ) or the peak (330 degrees ) of hippocampal theta waves. Additionally, different types of bursting as well as nonbursting activity patterns were also observed. The morphological reconstruction of theta-bursting neurons revealed extensive axon arbors of these cells with numerous local collaterals establishing symmetrical synapses; thus, synchrony among the septal pacemaker units may be brought about by their recurrent collateral interactions. Long projecting axons could also be found running dorsally toward the hippocampus and ventrally in the direction of basal forebrain regions. We conclude that GABAergic neurons in the medial septum, which are known to selectively innervate hippocampal interneurons, are in a position to induce rhythmic disinhibition in the hippocampus and other theta-related subcortical areas at two different phases of hippocampal theta.

In vivo inhibition of neuronal activity in the rat ventromedial prefrontal cortex by midbrain-raphe nuclei: role of 5-HT1A receptors

The ventral part of the medial prefrontal cortex (mPFC) plays an important role in mood and cognition. This study examined the effect of the 5-HT in this region by measuring the electrophysiological response of ventral mPFC neurones to electrical stimulation of the dorsal and median raphe nuclei (DRN and MRN), which are the source of the 5-HT input. DRN or MRN stimulation evoked a consistent, short-latency, post-stimulus inhibition in the majority of ventral mPFC neurones tested (DRN: 44/73 neurones; MRN: 24/31 neurones). Some neurones responded to DRN or MRN stimulation with antidromic spikes indicating that they were mPFC-raphe projection neurones. Both DRN- and MRN-evoked inhibitions were attenuated by systemic administration of the 5-HT1A antagonist WAY 100635 (0.1 mg/kg i.v.). DRN-evoked inhibition was also attenuated by iontophoretic application of WAY 100635 and by systemic administration of the 5-HT1A antagonist, NAD-299 (4 mg/kg i.v.) but not the 5-HT2 antagonist ketanserin (4 mg/kg, i.v.). These data suggest that DRN and MRN 5-HT neurones inhibit neurones in the ventral mPFC via activation of 5-HT1A receptors. Some of these mPFC neurones may be part of a 5-HT1A receptor-controlled postsynaptic feedback loop to the DRN and MRN.

Electrophysiological evidence for convergence of inputs from the medial prefrontal cortex and lateral habenula on single neurons in the dorsal raphe nucleus

Neuronal projections to the dorsal raphe nucleus (DRN) from the medial prefrontal cortex (mPFC) and lateral habenula nucleus (LHb) provide the two key routes by which information processed by mood regulatory, cortico-limbic-striatal circuits input into the 5-HT system. These two projections may converge as it appears that both activate local GABAergic neurons to inhibit 5-HT neurons in the DRN. Here we have tested this hypothesis by measuring the effect of stimulation of the mPFC and LHb on the activity of 5-HT and non-5-HT, putative gamma-amino butyric acid (GABA) neurons in the DRN using extracellular recordings in anaesthetized rats. A total of 119 5-HT neurons (regular, slow firing, broad spike width) and 21 non-5-HT, putative GABA neurons (fast-firing, narrow spike width) were tested. Electrical stimulation of the mPFC or LHb caused a poststimulus inhibition (30 ms latency) of 101/119 5-HT neurons, of which 61 (60%) were inhibited by both the mPFC and LHb. Electrical stimulation of the mPFC or LHb also caused a short latency (12-20 ms) poststimulus facilitation of 10/21 non-5-HT neurons, of which 5 (50%) were activated by both the mPFC and LHb. These data indicate that a significant number of 5-HT neurons and non-5-HT neurons in the DRN are influenced by both the mPFC and LHb. Moreover, the data are compatible with the hypothesis and that there is a convergence of mPFC and LHb inputs on local circuit GABAergic neurons in the DRN which in turn inhibit the activity of 5-HT neurons.

GABA(B) receptors in the median raphe nucleus: distribution and role in the serotonergic control of hippocampal activity

Previous studies have shown that serotonergic neurons of the median raphe nucleus have a suppressive effect on theta synchronization in the hippocampus. Median raphe lesion, suppression of 5-HT neuronal activity by administration of GABA(A) receptor antagonist or by glutamate blockade or depletion produced long-lasting non-interrupted hippocampal theta in freely behaving rats independent of behavior and in rats anesthetized with urethane. Serotonergic neurons show a characteristic sleep-wake pattern of activity and there is evidence that GABAergic mechanisms play an important role in their regulation. In this study we analyzed the distribution and subcellular localization of GABA(B) receptors in the midbrain raphe complex using combined 5-HT/GABA(B) receptor immunohistochemistry at the light and electron microscopic levels and studied the effects of their pharmacological manipulation on hippocampal electroencephalographic activity in urethane-anesthetized rats. We found that sustained infusion of the GABA(B) receptor agonist baclofen into the median raphe nucleus, using the microdialysis technique, elicited lasting theta activity in the hippocampus. The effect was antagonized by selective GABA(B) receptor antagonists. The predominant localization of GABA(B) receptors in the median, as well as in dorsal raphe was found on serotonergic neurons which strongly indicates that the increase in theta occurrence after baclofen injection resulted from suppression of the serotonergic output originating from the median raphe. On the electron microscopic level, we found GABA(B) receptors located extrasynaptically indicating that these receptors are preferentially activated by strong inputs, i.e. when GABA released from the synaptic terminals is sufficient to spill over from the synaptic cleft. Such conditions might be satisfied during rapid eye movement sleep when GABAergic neurons in the raphe are firing at their highest rate and in rhythmic synchronized bursts. Our data indicate that midbrain raphe GABA(B) mechanisms play an important role in behavioral state control and in hippocampal activity, in particular.

Evidence for a role of GABA interneurones in the cortical modulation of midbrain 5-hydroxytryptamine neurones

Recent electrophysiological studies demonstrate that the ventral medial prefrontal cortex has a powerful inhibitory influence on 5-hydroxytryptamine (5-HT) neurones in the dorsal raphe nucleus. Here we utilised a combination of anatomical and electrophysiological methods to characterise the cellular substrate underlying this effect.Anterograde tracing (Phaseolus vulgaris leucoagglutinin) using electron microscopy demonstrated a pathway from the ventral medial prefrontal cortex that makes neuronal contacts throughout the dorsal raphe nucleus. These contacts were predominantly asymmetrical synapses adjoining GABA immunoreactive dendrites and spines. In vivo extracellular recordings were made in the dorsal raphe nucleus of the anaesthetised rat from a subpopulation of non-5-HT neurones. These neurones were fast-firing, irregular and with short spike width, properties strongly reminiscent of immunochemically identified GABA interneurones in other brain regions. Recordings of classical 5-HT neurones were also included. Electrical stimulation of the ventral medial prefrontal cortex elicited a rapid onset (16 ms latency), orthodromic excitation of the non-5-HT neurones (13/25 neurones). This stimulation also caused a pronounced inhibition of most 5-HT neurones tested, with a longer latency (30 ms), and this was partially blocked by locally applied bicuculline. These data provide the first evidence that the ventral medial prefrontal cortex influences the activity of large numbers of raphe 5-HT neurones by targeting a local network of GABA neurones. This circuitry predicts that physiological and pathological changes in the ventral medial prefrontal cortex will impact on significant parts of the forebrain 5-HT system.

[Porphyria cutanea tarda and hepatitis C virus]

The development of clinically overt porphyria cutanea tarda (PCT) can be attributed to joint effects of genetic predisposition and environmental factors. Regarding exogen factors, studies from several countries published in the last years gave an account of significantly higher frequency of chronic hepatitis C virus (HCV) infection in PCT patients compared to the normal population. At the Department of Dermatology of University of Debrecen the prevalence of positive anti-HCV antibodies has been found in approximately 55% of PCT patients diagnosed from 1990 to 1999, which is comparable to the average prevalence rate seen in Southern-European countries. The majority of male patients were anti-HCV positive and consumed regularly alcohol, whereas every female patient had taken contraceptives. Liver enzymes were only slightly elevated in the majority of the patients and liver biopsy had to be performed only in three patients duo to chronic hepatitis. Our findings emphasise how important the screening of PCT patients for anti-HCV antibody considering that it might be important quo ad vitam for young men.

Effect of glycerol-induced acute renal failure and di-2-ethylhexyl phthalate on the enzymes involved in biotransformation of xenobiotixs

The effects of di-(2-ethylhexyl)-phthalate (DEPH) on the levels of cytochrome P-450 and b5 monooxygenases were studied in the rat kidney and liver in acute renal failure induced by glycerol. Intramuscular injection of glycerol (50%,10 ml x kg(-1)) to rats produced proximal tubular damage and acute renal failure. The indicators of renal function, serum urea and creatinine significantly increased (480 and 350 percent, respectively). In control and glycerol-treated animals DEPH had no significant effect on the concentrations of serum urea and creatinine. Twenty-four hours after glycerol injection the total amount of cytochrome P-450 and b5 significantly decreased in renal but increased in liver microsomal fractions. Moreover, 48 and 72 hours after glycerol injection the level of cytochrome P-450 and b5 significantly increased in both organs. A single dose of DEPH (2 ml x kg(-1), i.p.) also elevated the total cytochrome P-450 and b5 in control animals. This enhancing effect of DEPH was additive to that of glycerol in glycerol-induced acute renal failure. These results indicate that DEPH and glycerol evoked pathological changes may affect the metabolism of xenobiotics plus endogenous hormones in the liver and in kidney.

Mechanisms of L-cysteine neurotoxicity

We review here the possible mechanisms of neuronal degeneration caused by L-cysteine, an odd excitotoxin. L-Cysteine lacks the omega carboxyl group required for excitotoxic actions via excitatory amino acid receptors, yet it evokes N-methyl-D-aspartate (NMDA) -like excitotoxic neuronal death and potentiates the Ca2+ influx evoked by NMDA. Both actions are prevented by NMDA antagonists. One target for cysteine effects is thus the NMDA receptor. The following mechanisms are discussed now: (1) possible increase in extracellular glutamate via release or inhibition of uptake/degradation, (2) generation of cysteine alpha-carbamate, a toxic analog of NMDA, (3) generation of toxic oxidized cysteine derivatives, (4) chelation of Zn2+ which blocks the NMDA receptor-ionophore, (5) direct interaction with the NMDA receptor redox site(s), (6) generation of free radicals, and (7) formation of S-nitrosocysteine. In addition to these, we describe another new alternative for cytotoxicity: (8) generation of the neurotoxic catecholamine derivative, 5-S-cysteinyl-3,4-dihydroxyphenylacetate (cysdopac).

Modulation of glutamate receptor functions by glutathione

In addition to its well-known antioxidant effects, glutathione apparently has an additional double role in the central nervous system as a neurotransmitter and neuromodulator. A number of recent neurochemical, neuropharmacological and electrophysiological studies have yielded evidence on both functions. As an excitatory neurotransmitter, glutathione depolarizes neurons by acting as ionotropic receptors of its own which are different from any other excitatory amino acid receptors. As a neuromodulator, it displaces ionotropic glutamate receptor ligands from their binding sites and regulates calcium influx through N-methyl-D-aspartate receptor-governed ionophores. In brain slices glutathione has been shown to regulate the release of other transmitters, e.g., gamma-aminobutyrate and dopamine, mediated by N-methyl-D-aspartate receptors. In the present article, we review recent findings on the neuromodulatory actions of glutathione and discuss possible physiological and pathophysiological consequences.

Interference of S-nitrosoglutathione with the binding of ligands to ionotropic glutamate receptors in pig cerebral cortical synaptic membranes

The interactions of S-nitrosoglutathione (GSNO) with the ionotropic glutamate receptors were studied on synaptic membranes isolated from the pig cerebral cortex. GSNO displaced the binding of [3H]glutamate, 3-[(R)-2-carboxypiperazin-4-yl] [3H]propyl-1-phosphonate ([3H]CPP), a competitive N-methyl-D-aspartate (NMDA) antagonist, and [3H]kainate, with IC50 values in the low micromolar range. It failed to displace (S)-5-fluoro-[3H]willardiine, a selective agonist of 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. Reduced and oxidized glutathione were almost as effective as GSNO in glutamate and CPP binding. Of the three, GSNO was the most potent in kainate binding. They all stimulated [3H]dizocilpine binding in a concentration-dependent manner. This effect was additive to that of glycine and not mimicked by NO donors such as S-nitroso-N-acetylpenicillamine, 5-amino-3-morpholinyl-1,2,3-oxadiazolium chloride (SIN-1) and nitroglycerin. We assume that GSNO may act as an endogenous ligand at the NMDA and non-NMDA classes of glutamate receptors. In this manner it may facilitate NO transfer and target its delivery to specific sites in these receptors.

Specific glutathione binding sites in pig cerebral cortical synaptic membranes

Glutathione (gamma-glutamylcysteinylglycine) is a neuromodulator at glutamate receptors, but may also act as a neurotransmitter at sites of its own. The Na+-independent binding of [3H]glutathione to pig cortical synaptic membranes was characterized here using glycine, cysteine analogs, dipeptides and glutathione derivatives, and ligands selective for known glutamate receptors. L-Glutamate, pyroglutamate, quinolinate, (S)-5-fluorowillardiine and 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione were weak inhibitors at concentrations of 0.5 or 1 mM. D-Glutamate, L- and D-aspartate, glutamine, quisqualate, kynurenate, other N-methyl-D-aspartate receptor ligands and non-N-methyl-D-aspartate receptor ligands failed to displace [3H]glutathione. Except for weak inhibition by D-serine (0.5 mM), glycine and other ligands of the glycine co-activatory site in the N-methyl-D-aspartate receptors had no displacing effect. Similarly, metabotropic glutamate group I, II and III receptor agonists and antagonists and compounds acting at the glutamate uptake sites were generally inactive. Glutathione, oxidized glutathione, S-nitrosoglutathione, gamma-L-glutamylcysteine, cysteinylglycine, cysteine, cysteamine and cystamine were the most potent displacers (IC50 values in the micromolar range), followed by dithiothreitol, glutathione sulfonate and the S-alkyl derivatives of glutathione (S-methyl-, -ethyl-, -propyl-, -butyl- and -pentylglutathione). L-Homocysteinate and aminomethanesulfonate exhibited a moderate efficacy. Thiokynurenate, a cysteine analog and an antagonist at the N-methyl-D-aspartate receptor glycine co-activatory site, was a potent activator of glutathione binding. At 1 mM, some dipeptides also slightly activated the binding, gamma-L-glutamylleucine and gamma-L-glutamyl-GABA being the most effective. The specific binding sites for glutathione in brain synaptic membranes are not identical to any known excitatory amino acid receptor. The cysteinyl moiety is crucial in the binding of glutathione. The oxidation or alkylation of the cysteine thiol group reduces the binding affinity. The strong activation by thiokynurenate may indicate that the glutathione receptor protein contains a modulatory site to which co-agonists may bind and allosterically activate glutathione binding. The novel population of specific binding sites of glutathione gives rise to the possibility that they may have profound effects on synaptic functions in the mammalian central nervous system. The glutathione binding sites may be an important, and for the most part unrecognized, component in signal transduction in the brain.

Lamotrigine and carbamazepine affect differently the release of D-[3H]aspartate from mouse cerebral cortex slices: involvement of NO

The effects of lamotrigine and carbamazepine on the release of preloaded D-[3H]aspartate and the involvement of nitric oxide were studied with mouse cerebral cortical slices in a superfusion system. Lamotrigine inhibited the veratridine-evoked release, whereas the K+-stimulated release was attenuated more strongly by carbamazepine than by lamotrigine. These effects were accentuated by the N-methyl-D-aspartate receptor antagonist L-2-amino-5-phosphonovalerate and the nitric oxide synthase inhibitor L-nitroarginine, but diminished by the nitric oxide donor sodium nitroprusside. The results show that in addition to the blockade of voltage-sensitive Na+ (and Ca2+) channels, NO-mediated mechanisms are probably involved in the anticonvulsant actions of carbamazepine and, in particular, those of lamotrigine.

[A tularemia epidemic in western Slovakia 1995-1996]

The activation of natural foci of tularaemia in West Slovakia during 1994-1996 led to an epidemic outbreak in 1995-1996--mean annual morbidity 6.2 per 10(5) population. In comparison with the mean annual morbidity rate in the preceding period (1980-1994), a more than sevenfold increase was recorded. Of 213 notified cases of the disease 156 cases occurred in 1995--morbidity 8.9 per 10(5) population--being the highest morbidity recorded in this endemic region since the period of epidemic occurrence in the 60s. The highest proportion of cases (59.2%) was recorded in the districts of Nitra and Nové Zámky. The activation of natural foci along with changing social conditions, caused also marked changes of some epidemiological characteristics of tularaemia in Slovakia, such as seasonal and professional occurrence, as well as clinical forms of the disease. The impaired epidemiological situation in the occurrence of tularaemia is pointing to the importance of systematic surveillance to improve the diagnosis of the disease and ensure effective preventive measures.

[The effect of sodium nitroprusside and lamotrigine on D-[3H]aspartate release from mouse cortex slices]

The work deals with the study of the effect of the nitrous oxide and the antiepileptic agent lamotridgin on nonstimulated and K(+)- and veratridine-stimulated release of D-[3H]aspartate from sections of the brain cortex of mice. Sodium nitroprusside (0.1 mM) intensified nonstimulated (by 38-52%) and K(+)-stimulated (by 86%) release of labeled D-aspartate. Lamotridgin (0.1 mM) inhibited nonstimulated and veratridine-stimulated release of the label (by 50 and 70%, respectively). Sodium nitroprusside completely turned the inhibiting affect of lamotridgin on spontaneous and veratridine-stimulated release of D-aspartate. It is suggested that NMDA-subtype presynaptic receptors contribute to the regulation of D-aspartate release and are modulated by nitrous oxide.

Interference of S-alkyl derivatives of glutathione with brain ionotropic glutamate receptors

The effects of glutathione, glutathione sulfonate and S-alkyl derivatives of glutathione on the binding of glutamate and selective ligands of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors were studied with mouse synaptic membranes. The effects of glutathione and its analogues on 45Ca2+ influx were also estimated in cultured rat cerebellar granule cells. Reduced and oxidized glutathione, glutathione sulfonate, S-methyl-, -ethyl-, -propyl-, -butyl- and -pentylglutathione inhibited the Na+-independent binding of L-[3H]glutamate. They strongly inhibited also the binding of (S)-2-amino-3-hydroxy-5-[3H]methyl-4-isoxazolepropionate [3H]AMPA (IC50 values: 0.8-15.9 microM). S-Alkylation of glutathione rendered the derivatives unable to inhibit [3H]kainate binding. The NMDA-sensitive binding of L-[3H]glutamate and the binding of 3-[(R)-2-carboxypiperazin-4-yl][1,2-(3)H]propyl-1-phosphonate ([3H]CPP, a competitive antagonist at NMDA sites) were inhibited by the peptides at micromolar concentrations. The strychnine-insensitive binding of the NMDA coagonist [3H]glycine was attenuated only by oxidized glutathione and glutathione sulfonate. All peptides slightly enhanced the use-dependent binding of [3H]dizocilpine (MK-801) to the NMDA-gated ionophores. This effect was additive with the effect of glycine but not with that of saturating concentrations of glutamate or glutamate plus glycine. The glutamate- and NMDA-evoked influx of 45Ca2+ into cerebellar granule cells was inhibited by the S-alkyl derivatives of glutathione. We conclude that besides glutathione the endogenous S-methylglutathione and glutathione sulfonate and the synthetic S-alkyl derivatives of glutathione act as ligands of the AMPA and NMDA receptors. In the NMDA receptor-ionophore these glutathione analogues bind preferably to the glutamate recognition site via their gamma-glutamyl moieties.

NADPH-diaphorase activity in the nervous system of the embryonic and juvenile pond snail, Lymnaea stagnalis

Nicotinamide-adenine-dinucleotide-phosphate-diaphorase (NADPH-d) histochemistry has been applied in the present study to determine the distribution of putative nitric oxide (nitric oxide synthase)-producing cells during embryonic and early postembryonic development in the pond snail, Lymnaea stagnalis L., with special reference to the nervous system. The first NADPH-d-positive structures appear as early as 18% of development (E18, trochophore stage) and correspond to the pair of protonephridia. These structures later show disintegration, although after metamorphosis (E26=75%) staining of their individually spreading cells can be observed until hatching. Peripheral sensory neurons in the foot, mantle edge and lips, and their afferents projecting to the central nervous system reveal NADPH-d activity in the postmetamorphosis period (E25-E27=E60%-E80%) of embryogenesis. After hatching (P1-P3), a number of stained sensory cells appear in the pharynx and esophagus. Some NADPH-d positive neuronal perikarya occur in the pedal and pleural ganglia, and a few weakly stained cells in the cerebral and buccal ganglia of juvenile snails. At the same time, a continuous bundle of reactive fibers is formed in the neuropil both through and through around the circumesophageal ganglion ring. The localization of NADPH-d activity in the developing nervous system of Lymnaea suggests that nitric oxide participates mainly in sensory processes. However, its role in specific intraganglionic integrative events cannot be excluded following embryonic metamorphosis.