Psychotropic profile of S 17092, a prolyl endopeptidase inhibitor, using quantitative EEG in young healthy volunteers

The central activity of S 17092, a prolyl endopeptidase (PEP) inhibitor, was investigated by quantitative electroencephalography (qEEG) in 48 young healthy men participating in a double-blind, randomized, placebo-controlled, cross-over study. S 17092 (100, 200, 400 or 600 mg) and placebo were administered once daily for 10 days in a rising multiple-dose scheme. EEG recordings were performed before and repeatedly from 0.5 to 24 h after dose on day 1 and day 10. PEP activity in plasma was also measured for the same periods. S 17092 appeared as a potent inhibitor of PEP activity at all doses, after both single and repeated administrations. EEG changes after acute doses were slight and of short duration, mainly characterized by increased relative alpha 1 power, suggesting a vigilance-promoting EEG profile. After repeated doses and more strikingly after a superimposed dose, increases in relative alpha 1 power were still present with additional increase in relative delta power and decreases in absolute fast alpha, fast beta, theta powers and total power at all doses. These EEG findings suggest that S 17092 might possess some mood-stabilizing potential in addition to its cognition-enhancing properties.

Effects of TC-1734 (AZD3480), a selective neuronal nicotinic receptor agonist, on cognitive performance and the EEG of young healthy male volunteers

OBJECTIVES

The aim of this study was to get insight into the central effects of TC-1734 (renamed AZD3480), a selective agonist at the neuronal nicotinic receptor of the alpha4beta2 subtype.

MATERIALS AND METHODS

Electroencephalography (EEG) techniques and computerized cognitive tests were performed in young, healthy male volunteers during two double-blind and placebo-controlled studies: a rising single dose crossover study (from 2 to 320 mg) and a rising repeated dose study with a parallel group design (50, 100, and 200 mg).

RESULTS

In contrast to acute administration, administration of AZD3480 over 10 days produced statistically significant enhancement of several cognitive measures (attention and episodic memory) compared to placebo. Regarding EEG data, AZD3480 showed acceleration of the alpha centroid and of the alpha peak in the single-dose study. This EEG profile of the acceleration type was confirmed in the repeated dose study on both day 1 and day 10, with the greatest effect observed with the highest dose. The EEG pattern shown for AZD3480 was consistent with that previously described with other drugs known to improve attention and vigilance (including nicotine). In addition, subjects dosed with AZD3480 showed a statistically significant increase in mismatch negativity (MMN) amplitude at 50 and 200 mg while reducing MMN latency (200 mg only), suggesting an improvement of pre-attentional mechanisms.

CONCLUSION

These early data in healthy subjects provide encouragement to consider development of AZD3480 as a novel agent for the treatment of cognitive decline in the elderly, including age-associated memory impairment and/or dementia of the Alzheimer's type.

Pharmacodynamic effects of acamprosate on markers of cerebral function in alcohol-dependent subjects administered as pretreatment and during alcohol abstinence

Animal studies suggested that acamprosate modulates neuronal hyperexcitability of acute alcohol withdrawal, acting through the glutamatergic neurotransmission. In the present study, we further investigated whether treatment with acamprosate could attenuate the post-alcohol withdrawal hyperexcitability or hyperarousal in humans using brain magnetoencephalography mapping of spontaneous fields. A double-blind, randomised, placebo-controlled study with a parallel group design comparing 2,000 mg/day of acamprosate versus placebo was conducted in alcohol-dependent subjects meeting DSM-IV criteria for alcohol dependence. Treatments were initiated 8 days before alcohol withdrawal and prolonged during the 15 following (abstinence) days. The study demonstrated that during alcohol withdrawal, acamprosate decreased the arousal level as reflected by alpha slow-wave index (ASI) measurement. This effect was mostly evidenced in left parietotemporal regions and, to a lesser extent, in the contiguous anterior, posterior and right-sided regions. In the placebo group, on the contrary, ASI measures increased between day 2 (acute withdrawal) and day 14 (prolonged withdrawal). The present results suggest a sustained effect of acamprosate on the hyperexcitability state due to alcohol withdrawal in alcohol-dependent patients and that acamprosate may have a protective effect when administered 8 days before alcohol withdrawal.

Characterization of the CNS effects of naftidrofuryl (Praxilène) by quantitative EEG and functional MRI: a study in healthy elderly subjects

In the present study, we investigated the effects of a single and a repeated (5 days) administration of naftidrofuryl, a serotonin 5-HT2 receptor inhibitor having neuroprotective properties, on functional brain physiology in male healthy elderly subjects, using quantitative electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Twelve subjects aged 60 +/- 3.8 years completed the quantitative EEG study, where the effects of 400 and 600 mg were assessed, and 12 other subjects (aged 56 +/- 4.7 years) completed the fMRI study, where the effect of 400 mg was assessed on the brain activation induced by the continuous performance test (CPT). Naftidrofuryl induced a transient reduction in alpha activity followed by a specific synchronisation of the 9.5- to 11-Hz EEG activity most pronounced after repeated administration. Such regimen also increased the CPT-induced brain activation visualized by way of fMRI. The results of the present study can be interpreted at the functional level that naftidrofuryl induced an improved level of vigilance or an increased capacity of alertness in healthy elderly subjects.

Planning of pharmacodynamic trials: I. Specificity and possible solutions. II. Stability considerations under placebo and interpretation of drug effects on EEG

This workshop deals with the concept of quantitative electroencephalography (QEEG) to characterize the central effects of drugs. For proper interpretation, the circumstances under which data are obtained play an important role. To infer the size of variability in standard practice, we elaborated some computations for "placebo-treatment" in healthy volunteers, which helps to determine the threshold of drug effect detection. Simple rules for interpretation of multiple statistical comparisons were proposed and validation of dose effects were carried out with accepted reference compounds. Furthermore, psychotropic agents with comparable therapeutic indications often present similar modifications in EEG spectral composition (pharmaco-EEG profile). To extrapolate this concept, quantified wake-EEG is a rapid, validated technique for early psychopharmacological investigation of new psychotropic compounds (Phase I, healthy volunteers). Classification of drug-induced changes in cerebral activity at this stage forms a useful decision instrument in planning the long-term clinical scenario of drug development (Phases II and higher).

Involvement of calmodulin-dependent protein kinase II in carbachol-induced rhythmic activity in the hippocampus of the rat

The role of calcium and protein kinases in rhythmic activity induced by muscarinic receptor activation in the CA1 area in rat hippocampal slices was investigated. Extracellular recording showed that carbachol (20 microM) induced synchronized field potential activity with a dominant frequency of 7.39+/-0.68 Hz. Pretreatment with the membrane permeable Ca(2+) chelator BAPTA-AM (50 microM) or with thapsigargin (1 microM), a compound which depletes intracellular calcium stores, reduced the dominant power of carbachol-induced theta-like activity by 83% and 78%, respectively. Inhibition of calmodulin-dependent protein kinase II (CaMKII) by the cell permeable inhibitor KN-93 (10 microM) reduced the power of carbachol-induced theta-like activity by 80%. In contrast the protein kinase C (PKC) inhibitor calphostin C did not significantly (P>0.05) affect the effect of carbachol. Whole-cell recording indicated that KN-93 also blocked carbachol-induced suppression of slow I(AHP) and strongly inhibited the carbachol-induced plateau potential. Our data suggest that activation of CaMKII by carbachol is crucial for local theta-like activity in the CA1 area of the rat hippocampus in vitro. Furthermore, involvement of CaMKII in carbachol-induced suppression of the slow I(AHP) and the induction of plateau potentials could play a role in the induction of theta-like rhythmic activity by carbachol.

Effects of serotonin-selective and classical antidepressants on the auditory P300 cognitive potential

The cognitive potential, P300, is a phenomenon frequently studied in relation to template matching of the brain. To understand the neurochemical mechanisms of its generation, we compared the effects of three antidepressants, fluoxetine, tianeptine and clomipramine after single and repeated application as well as after 1 week of withdrawal on the P300 and N200 waves in an auditory 'odd-ball' paradigm in three parallel groups of 10 healthy volunteers. Following single administration, both fluoxetine and clomipramine reduced (-39 +/- 14%, p < 0.01) the peak amplitude of P300 at the Pz electrode. For fluoxetine and tianeptine, reduced amplitudes of 19 +/- 7% and 24 +/- 11%, respectively, were found following 8 days of treatment, 2 h after administration. However, for clomipramine no additional diminution was found on day 8 with respect to day 1. Topographic distributions tended to be significantly modified at the frontal scalp area 1 h after the tianeptine administration on day 8, whereas the postdosing changes induced by fluoxetine were localised in the midline and right centrotemporal scalp regions. Only minor reductions in peak latencies have been observed. It can be concluded that serotonin selective drugs have a slower onset of P300 amplitude decrease than clomipramine, which has additional effects on monoaminergic and on cholinergic systems. These results suggest that serotonin has a regulatory function in the neurotransmission of cerebral structures which are involved in the evaluation of stimulus relevance.

Synchronous 20 Hz rhythmic activity in hippocampal networks induced by activation of metabotropic glutamate receptors in vitro

High frequency synchronized neuronal activity plays an important role in spatiotemporal processing of sensory inputs and possibly cognitive function. This activity, in the so called beta and gamma electroencephalographic frequency range is found in cortical areas and in the hippocampus. Here we show that activation of metabotropic glutamate receptors by the agonist 1-aminocyclopentane-(1S,3R)-dicarboxylate evokes synchronous 20 Hz rhythmic activity in rat hippocampal slices. Quisqualate was more potent than 1-aminocyclopentane-(1S,3R)-dicarboxylate which suggests involvement of either the metabotropic glutamate receptor subtypes I, V or both. The 20 Hz rhythmic activity was different from the theta-rhythmic effect induced by the cholinergic muscarinic receptor agonist carbachol which showed a dominant frequency of 9.0 +/- 0.5 Hz. The 20 Hz rhythmic activity is correlated with an increased firing rate of pyramidal neurons. The 20 Hz rhythmic activity was reduced by the GABA(A) receptor antagonist bicuculline, thus interneuron activity plays a major role in the 20 Hz oscillations. The 20 Hz rhythmic activity induced by activation of metabotropic glutamate receptors represents a new aspect of synchronized activity in the hippocampus and may be involved in the binding and representation of converging sensory inputs.

Activation of 5-HT1B receptors suppresses low but not high frequency synaptic transmission in the rat subicular cortex in vitro

We have shown previously that activation of 5-HT1B serotonin receptors mediates suppression of the amplitude of evoked potentials in the subiculum [2]. Here we show that after application of 5-HT (10 microM), excitatory postsynaptic potentials of subicular neurons have reduced amplitudes with no change in membrane potential, input resistance and presynaptic fiber volleys. These results suggest that activation of 5-HT1B receptors reduces the release of glutamate from incoming fibers originating from CA1 pyramidal cells. In presence of 5-HT (10 microM), theta patterned stimulation still induced LTP, similar to that observed in control slices. Application of similar concentrations of 5-HT during double pulse stimulation (interval 10-15 ms) reduced the response to the first pulse (repetition interval 30 s), but the response to the second pulse of the pair was unaffected. We propose that 5-HT1B receptor activity suppresses subicular transmission at low but not at high frequencies.

The amyloid precursor protein fragment His 657-Lys 676 inhibits noradrenaline- and enkephaline-induced suppression of voltage sensitive calcium currents in NG108-15 hybrid cells

We have investigated the effects of the C-terminal amyloid precursor protein fragment His 657-Lys 676 upon calcium currents in NG108-15 neuroblastoma x glioma hybrid cells. The amyloid precursor protein fragment His 657-Lys 676 (1-10 microM) did not affect calcium currents per se, but clearly blocked the calcium current suppression mediated by both adrenergic alpha 2B- and opioid delta receptors in a concentration-dependent manner. The reverse amyloid precursor protein fragment Lys 676-His 657 and the shorter amyloid precursor protein fragment Gly 659-Lys 676 did not affect calcium current suppression by adrenergic alpha 2B- and opioid delta receptors. The similar interaction of C-terminal amyloid precursor protein with adrenergic alpha 2B- and opioid delta receptors suggest that the effect occurs downstream of the receptor, possibly via the GTP binding protein Go.

Differential effects of centrally-active antihypertensives on 5-HT1A receptors in rat dorso-lateral septum, rat hippocampus and guinea-pig hippocampus

1. The electrophysiological responses elicited by 5-hydroxytryptamine1A-(5-HT1A) receptor agonists in rat and guinea-pig CA1 pyramidal neurones and rat dorso-lateral septal neurones were compared in vitro by use of conventional intracellular recording techniques. 2. In the presence of 1 microM tetrodotoxin (TTX), to prevent indirect effects, 5-HT, N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT) and 8-hydroxy-2(di-n-propylamino) tetralin (8-OH-DPAT) hyperpolarized the neurones from rat and guinea-pig brain. 3. The hypotensive drug flesinoxan, a selective 5-HT1A receptor agonist, hyperpolarized neurones in all three areas tested; however, another hypotensive agent with high affinity at 5-HT1A-receptors, 5-methyl-urapidil, hyperpolarized only the neurones in rat hippocampus and septum. 4. In guinea-pig hippocampal neurones, 5-methyl-urapidil behaved as a 5-HT1A-receptor antagonist. 5. The relative efficacies (5-HT = 1) of DP-5-CT, 8-OH-DPAT, flesinoxan and 5-methyl-urapidil at the three sites were: rat hippocampus, 1.09, 0.7, 0.5 and 0.24; rat septum, 0.88, 0.69, 0.82 and 0.7; guinea-pig hippocampus, 1.0, 0.69, 0.89 and 0, respectively. 6. It is concluded that the hypotensive agents flesinoxan and 5-methyl-urapidil appear to have different efficacies at 5-HT1A receptors located in different regions of the rodent brain. Whether these regional and species differences arise from receptor plurality or variability in intracellular transduction mechanisms remains to be elucidated.

Serotonergic modulation of neurotransmission in the rat subicular cortex in vitro: a role for 5-HT1B receptors

We have studied the effect of serotonin on synaptic transmission in rat hippocampal subiculum slices. Electrical stimulation of the alveus induced a field potential in the subiculum. The non-NMDA glutamate receptor antagonist, NBQX (3 x 10(-6) mol/l) suppressed the response by 78%, indicating that the signal involves glutamatergic neurons. Application of serotonin suppressed (EC50 = 3.6 x 10(-6) mol/l) the amplitude of the evoked potentials in a reversible, concentration-dependent manner. The responses to 5-HT were not altered after pretreatment with the 5-HT uptake blocker, fluvoxamine (10(-5) mol/l) or a combination of the MAO inhibitor pargyline (10(-5) mol/l) and ascorbic acid (10(-4) mol/l). The responses to 5-HT were also unaffected by pretreatment with the 5-HT1A selective antagonist NAN-190 (10(-6) mol/l), the 5-HT2A antagonist ketanserin (10(-6) mol/l) or the 5-HT3/5-HT4 antagonist ICS 205-930 (10(-6) mol/l). The 5-HT1B selective agonist CP 93,129 mimicked the effects of serotonin, but was more potent (EC50 4.1 x 10(-7) mol/l). The 5-HT1B receptor antagonist, (+/-)21-009 (3 x 10(-7) mol/l), antagonized the response to 5-HT and CP 93,129 with a pKB value of 7.1 and 7.2, respectively. These results suggest that the effect of 5-HT in the rat subiculum is mediated by 5-HT1B receptors.

Responses of the nucleus accumbens following fornix/fimbria stimulation in the rat. Identification and long-term potentiation of mono- and polysynaptic pathways

The nucleus accumbens occupies a strategic position as an interface between limbic cortex and midbrain structures involved in motor performance. The fornix-fimbria carries limbic inputs to the ventral striatum, namely by way of fibers originating in the CA1/subiculum and projecting to the nucleus accumbens. It also carries fibers arising in the septal area that project to the hippocampal formation, and projection fibers to other areas of the rostral forebrain from Ammon's horn. Electrical stimulation of this bundle causes characteristic field potentials both in the nucleus accumbens and in the subiculum. In rats, under halothane anesthesia, the responses evoked by fornix/fimbria stimulation in the nucleus accumbens consist of two main positive peaks (at 10 and 25 ms, referred to as P10 and P25, respectively). P10 represents monosynaptic activation. We hypothesized that P25 reflects the activation of a polysynaptic loop, i.e. a fornix-fimbria hippocampal loop in series with the fibers that arise in the subiculum and project to the nucleus accumbens. To test this hypothesis, we reversibly blocked the fibers projecting caudally to the hippocampus by a local anesthetic (lidocaine) and the glutamatergic transmission through the CA1/subiculum by a local injection of kynurenic acid. Both manipulations yielded a reversible depression of about 90% of the P25 component while P10 remained unaffected as expected. In concert a strong reduction (to 24-31%) of control values of the responses evoked in the subiculum was seen. The dynamics of the mono- and polysynaptic pathways differ markedly. The synaptic responses through both pathways are enhanced by paired-pulse stimulation, but the polysynaptic pathway is facilitated in a much stronger way. Following a tetanus (50 Hz, 2 s duration) applied to the fornix/fimbria, the P10 component of the nucleus accumbens responses showed an immediate increase by a factor of about 2 followed by a phase of gradual decrement with half-decay time of about 10 min, after which a persistent long-term potentiation of about 25% above control level was maintained for the rest of the experiment (max 90 min). The P25 component showed a transient 10-fold potentiation with return to control values after about 10 min. In contrast to the P25 elicited by a conditioning stimulus, the P25 component elicited by a second stimulus delivered at an interval of 100 ms (test stimulus) showed a persistent long-term potentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the novel 5-HT3 antagonists MDL 73147EF (dolasetron mesilate) and MDL 74156 in NG108-15 neuroblastoma x glioma cells

In radioligand binding experiments, MDL 73147EF and MDL 74156 inhibited the binding of [3H]GR65630 to 5-hydroxy-tryptamine3 (5-HT3) binding sites on membranes prepared from NG108-15 neuroblastoma x glioma cells. The calculated dissociation constants (KI) were 20.03 +/- 6.58 and 0.44 +/- 0.18 nM, respectively (means +/- S.E.M., n = 6 and 9, respectively). Application of 5-HT (10-50 microM) to voltage-clamped NG108-15 cells elicited a rapidly desensitizing inward membrane current, characteristic for the activation of 5-HT3 receptors. The 5-HT-induced membrane current was suppressed in a reversible, concentration-dependent manner by MDL 73147EF and MDL 74156EF. The concentrations required to block half the 5-HT response (IC50) were 3.8 and 0.1 nM, respectively. It is concluded that both compounds are potent and reversible antagonists at 5-HT3 receptors in this neuroblastoma cell line.

Contribution of NMDA receptors to postsynaptic potentials and paired-pulse facilitation in identified neurons of the rat nucleus accumbens in vitro

The principal aim of this study was to characterize the transmitter mechanisms mediating fast postsynaptic potentials in identified neurons of the rat nucleus accumbens. Using the biocytin-avidin labeling technique, impaled neurons were identified as medium spiny neurons. The basic membrane characteristics of these neurons were determined. Local electrical stimulation or stimulation of the corpus callosum elicited a depolarizing postsynaptic potential consisting of an EPSP often followed by an IPSP. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (4 microM) abolished most of the depolarizing postsynaptic potential. The N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid depressed a small part of the decay phase of the depolarizing postsynaptic potential. Paired-pulse facilitation of postsynaptic potentials was found using interstimulus-intervals between 10 and 150 ms. N-methyl-D-aspartate receptors were found to contribute only slightly to the facilitation of the decay phase of the depolarizing postsynaptic potential, but not to its rising phase. This contribution was particularly clear under conditions of reduced GABAA receptor mediated inhibition. The present study indicates that postsynaptic responses of medium spiny neurons in the nucleus accumbens to local stimulation or stimulation of neocortical afferents are primarily mediated by quisqualate/kainate receptors. The contribution of NMDA receptors is normally limited to a portion of the decay phase of these responses, but is enlarged in the absence of GABAergic inhibition and following paired-pulse stimulation.

Locally evoked potentials in slices of the rat nucleus accumbens: NMDA and non-NMDA receptor mediated components and modulation by GABA

In a slice preparation of the rat nucleus accumbens (Acb), local electrical stimulation elicited a field potential composed of two negative peaks, followed by a positive wave. The early negative peak was identified as a non-synaptic compound action potential, the late negative peak as a monosynaptic population spike (PS) and the positive wave as a mixture of an excitatory and an inhibitory postsynaptic potential (PSP). Both the PS and the PSP exhibited a marked degree of paired-pulse facilitation. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2 microM) and the broadly acting glutamate receptor antagonist kynurenic acid (300 microM) reversibly abolished or reduced both the PS and PSP. In contrast, nicotinic, muscarinic and N-methyl-D-aspartate (NMDA) receptor antagonists had no suppressive action. Washout of Mg2+ from the superfusion medium reversibly enhanced and prolonged the PSP and this effect was blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5). The gamma-aminobutyric acid antagonist picrotoxin (60 microM) enhanced the PS and induced secondary spikes which were superimposed on a prolonged PSP. Most of this prolongation was abolished by D-AP-5. It is concluded that locally evoked synaptic responses in the Acb are mediated by glutamate or aspartate, and that NMDA receptor mediated activity evoked by low frequency stimulation is substantial in Mg2(+)-free medium or during reduced GABAA receptor activity, but not under normal conditions.

Paired-pulse facilitation in the nucleus accumbens following stimulation of subicular inputs in the rat

Anatomical tracing studies indicate that the nucleus accumbens receives inputs from limbic structures, and projects to the ventral pallidum. In order to get more fundamental insight into how information from the limbic areas is relayed via the nucleus accumbens, electrophysiological experiments were carried out in rats under halothane anaesthesia. Inputs originating in the subiculum were activated by electrical stimulation of the fornix fibres, and both field potentials and extracellular unit activity were recorded from the medial and lateral aspects of the nucleus accumbens. Evoked potentials consisted of two positive peaks (P1 at 10 ms and P2 at 25-30 ms). In between a negative-going wave (N1) was present. These initial components were followed by a complex negative wave (N2) with variable duration of 30-100 ms. The P2 and N2 components showed a conspicuous paired-pulse facilitation at stimulus intervals between 80 and at least 200 ms. When responses were recorded at increasing stimulus intensity, the second response emerged at lower threshold than the first response. The mechanisms underlying these phenomena were investigated by analysing the extracellularly recorded unit activity. Primarily, excitatory responses were found. Onset-latencies could be divided roughly into two clusters, one around 10 ms, representing monosynaptic inputs, and a second around 24-26 ms. Inhibitory responses were also found. Stimulation of the ventral pallidum was carried out in order to test whether the cells that could be driven by stimulation of the subicular inputs were projection cells. Latencies of antidromic action potentials ranged from 9 to 13 ms. A minority of the identified projection cells were activated by limbic inputs. The projection cells were found in the core region of the nucleus accumbens. Units that were inhibited by stimulation of the limbic inputs were found in the shell only, whereas excitatory responses were measured in both subdivisions of the nucleus accumbens. For the latter responses a significant enhancement, by a factor of four, was found using double pulse stimulation of the fornix at intervals of 100 ms. The basic electrophysiological properties are compared with those described in the literature, and speculations about the possible mechanisms responsible for the paired-pulse facilitation phenomena are put forward.

A new method to estimate time delays between EEG signals applied to beta activity of the olfactory cortical areas

A method based on coherence and phase spectra was developed to characterize the transmission of signals through neural networks that were assumed to have linear filter properties. The system is thought to consist of a frequency independent delay in series with a network that gives rise to a frequency dependent lag, the so-called minimum phase shift. The latter was estimated from the gain of the transfer function by means of Hilbert transform pairs. By subtracting the minimum phase shift from the phase differences between input and output, a corrected phase spectrum was obtained that represents the frequency independent delay. The slope of this spectrum was calculated and converted into the time delay. This method was applied to EEG signals recorded from different parts of the olfactory basal forebrain structures of the cat, in order to determine time delays for the beta activity. The results showed that the beta activity was propagated in a rostro-caudal direction. Along this rostro-caudal axis the conduction velocity slowed down from approximately 3 m/sec for the pathways between olfactory bulb and prepyriform cortex to 0.5 m/sec within the EC. These findings were compared to estimates of the conduction velocities in the lateral olfactory tract fibres and collaterals using transient responses. It is concluded that, to a first approximation, the linear approach gives physiologically meaningful results.

Modulations of EEG activity in the entorhinal cortex and forebrain olfactory areas during odour sampling

With the aim of determining a possible role of the entorhinal cortex (EC) in the processing of olfactory information, its electrical activity was recorded during different types of odour sampling behaviour. The EC spontaneous field potentials (EEGs) were related to those recorded simultaneously from the more rostrally lying olfactory bulb (OB) and prepiriform cortex (PPC), using coherence functions. The coherence values are measures of coupling between two neuronal populations. Auto- and coherence spectra were characterized by a peak in the beta-band (35-36 Hz) and another in the 16-20 Hz band. For the beta-peak of the PPC-EC coherence it was shown that both the maximal values and the median frequency decrease during the first 0.5-1.0 s of a trial in a two-choice odour discrimination task. These transients differed significantly for CS+ and CS- trials. However, no such difference was observed during exploratory sniffing at the same pair of odorants. It was concluded that during sniffing the degree of coupling of the EC with the PPC depends on the behavioural context but not on the quality of the odorants. As regards the 16-20 Hz components it was shown, using bispectral analysis, that these components represent the 1/2 subharmonic of the beta-components. The subharmonic components were enhanced during the initiation of sniffing. This means that the neural networks of OB, PPC and EC have non-linear dynamic properties. These networks show different modes of oscillatory behaviour, characteristic of the restful state and of the active sniffing state. Theoretical implications of these experimental results are discussed. In conclusion, it was demonstrated that modulations with sniffing behaviour are present in the beta-EEG components and their subharmonics in the olfactory brain areas including the EC. This indicates that the latter is involved in processes of the evaluation of olfactory cues in relation to the animal's behaviour.

Differential distribution of beta and theta EEG activity in the entorhinal cortex of the cat

The basic characteristics of the electroencephalograms (EEGs) of the entorhinal cortex (EC) were compared to those of the olfactory bulb (OB) and prepiriform cortex (PPC) in the awake cat, using methods that take into account linear (coherence functions) and non-linear relationships (mutual information). The frequency spectra of EC signals differ from those of EEG signals recorded from OB or PPC, but in the beta frequency band (35-40 Hz) coherent activity between these latter two areas and the EC could be put in evidence. Taking the values of coherence as a measure of linear coupling between populations of neurons, this coupling is strong between PPC and the rostral part of the EC, and becomes weaker towards more lateral, caudal and medial areas. This decrease in coupling is confirmed by non-linear analysis methods. On the basis of intracortical depth profiles it was shown that the beta activity is locally generated in layer II of the EC. Another rhythmic activity was also recorded in EC in the theta frequency range. Theta rhythm is generated mainly in the caudal part of EC. This area has the strongest coupling with the hippocampal formation. These analyses lead to the conclusion that in the EC olfactory and limbic domains have different topographical distributions but with a certain degree of overlap.

Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat. II. Physiological studies

Field potentials and unit activity elicited by electrical stimulation of the olfactory bulb (OB) and anterior and posterior prepiriform cortex (PPCa and PPCp) were measured extracellularly in the entorhinal cortex (EC) of the cat. Different topographic distributions of the amplitude and peak latency of average evoked potentials (AEPs) were obtained depending on the stimulated area. The maximal evoked activity in the EC showed a gradient in a latero-medial direction with the extremes corresponding to the stimulation of OB and PPCp respectively. Analysis of firing patterns of units in the EC in response to stimulation of the OB, PPCa and PPCp showed that an appreciable number of units responded to stimulation of different areas, mainly PPCa and PPCp. It was found that the pathways being stimulated differed in conduction velocities with the PPCp-EC being the slowest. Most responding units lay in layer I and II of the EC. The AEPs to PPCa- and PPCp-stimulation presented different types of depth profiles. Stimulation of the PPCa evoked an initial surface-negative depth-positive potential whereas the PPCp evoked a different type of AEP with an initial positive component at the surface and negative in depth. It is assumed that the stimulated fibres have their active synapses at different levels within the superficial layers of the EC. The possibility of direct influence of olfactory inputs on the hippocampus mediated by one synapse in the EC is discussed.