Characterization of synaptically mediated fast and slow inhibitory processes in piriform cortex in an in vitro slice preparation

Pre- and Postsynaptic Activation of GABAB Receptors Modulates Principal Cell Excitation in the Piriform Cortex

The piriform cortex (PC), like other cortical regions, normally operates in a state of dynamic equilibrium between excitation and inhibition. Here we examined the roles played by pre- and postsynaptic GABA_B receptors in maintaining this equilibrium in the PC. Using whole-cell recordings in brain slices from the anterior PC of mice, we found that synaptic activation of postsynaptic GABA_B receptors hyperpolarized the two major classes of layer 2 principal neurons and reduced the intrinsic electrical excitability of these neurons. Presynaptic GABA_B receptors are expressed on the terminals of associational (intracortical) glutamatergic axons in the PC. Heterosynaptic activation of these receptors reduced excitatory associational inputs onto principal cells. Presynaptic GABA_B receptors are also expressed on the axons of GABAergic interneurons in the PC, and blockade of these autoreceptors enhanced inhibitory inputs onto principal cells. Hence, presynaptic GABA_B autoreceptors produce disinhibition of principal cells. To study the functional consequences of GABA_B activation in vivo, we used 2-photon calcium imaging to simultaneously monitor the activity of ~200 layer 2 neurons. Superfusion of the GABA_B agonist baclofen reduced spontaneous random firing but also promoted synchronous epileptiform activity. These findings suggest that, while GABA_B activation can dampen excitability by engaging pre- and postsynaptic GABA_B heteroreceptors on glutamatergic neurons, it can also promote excitability by disinhibiting principal cells by activating presynaptic GABA_B autoreceptors on interneurons. Thus, depending on the dynamic balance of hetero- and autoinhibition, GABA_B receptors can function as variable modulators of circuit excitability in the PC.

Fifty shades of inhibition

Inhibitory circuits are essential for brain function. Our understanding of their synaptic organization has advanced extensively with the identification and classification of an impressive variety of neuron groups, receptor types, and patterns of connectivity. However, the conceptual discussion regarding the role of in neural circuits still revolves around the idea that its primary role is to regulate circuit excitability. Here, I will focus on recent findings from cortical circuits and argue that inhibitory circuits are central to the integration of incoming inputs and can promote sophisticated fine-scale control of local circuits. I propose that inhibitory circuits should not be viewed so much as brakes on principal neurons activity, but as primary contributors to a variety of neural network functions.

Matching of feedback inhibition with excitation ensures fidelity of information flow in the anterior piriform cortex

Odor-evoked responses in mitral cells of the olfactory bulb are characterized by prolonged patterns of action potential (spike) activity. If downstream neurons are to respond to each spike in these patterns, the duration of the excitatory response to one spike should be limited, enabling cells to respond to subsequent spikes. To test for such mechanisms, we performed patch-clamp recordings in slices of the mouse anterior piriform cortex. Mitral cell axons in the lateral olfactory tract (LOT) were stimulated electrically at different intensities and with various frequency patterns to mimic changing input conditions that the piriform cortex likely encounters in vivo. We found with cell-attached measurements that superficial pyramidal (SP) cells in layer 2 consistently responded to LOT stimulation across conditions with a limited number (1-2) of spikes per stimulus pulse. The key synaptic feature accounting for the limited spike number appeared to be somatic inhibition derived from layer 3 fast-spiking cells. This inhibition tracked the timing of the first spike in SP cells across conditions, which naturally limited the spike number to 1-2. These response features to LOT stimulation were, moreover, not unique to SP cells, also occurring in a population of fluorescently labeled interneurons in glutamic acid decarboxylase 65-eGFP mice. That these different cortical cells respond to incoming inputs with 1-2 spikes per stimulus may be especially critical for relaying bulbar information contained in synchronized oscillations at beta (15-30Hz) or gamma (30-80Hz) frequencies.

Learning-induced modulation of the GABAB-mediated inhibitory synaptic transmission: mechanisms and functional significance

Complex olfactory-discrimination (OD) learning results in a series of intrinsic and excitatory synaptic modifications in piriform cortex pyramidal neurons that enhance the circuit excitability. Such overexcitation must be balanced to prevent runway activity while maintaining the efficient ability to store memories. We showed previously that OD learning is accompanied by enhancement of the GABAA-mediated inhibition. Here we show that GABAB-mediated inhibition is also enhanced after learning and study the mechanism underlying such enhancement and explore its functional role. We show that presynaptic, GABAB-mediated synaptic inhibition is enhanced after learning. In contrast, the population-average postsynaptic GABAB-mediated synaptic inhibition is unchanged, but its standard deviation is enhanced. Learning-induced reduction in paired pulse facilitation in the glutamatergic synapses interconnecting pyramidal neurons was abolished by application of the GABAB antagonist CGP55845 but not by blocking G protein-gated inwardly rectifying potassium channels only, indicating enhanced suppression of excitatory synaptic release via presynaptic GABAB-receptor activation. In addition, the correlation between the strengths of the early (GABAA-mediated) and late (GABAB-mediated) synaptic inhibition was much stronger for each particular neuron after learning. Consequently, GABAB-mediated inhibition was also more efficient in controlling epileptic-like activity induced by blocking GABAA receptors. We suggest that complex OD learning is accompanied by enhancement of the GABAB-mediated inhibition that enables the cortical network to store memories, while preventing uncontrolled activity.

Evidence for loss of synaptic AMPA receptors in anterior piriform cortex of aged mice

It has been suggested that age-related impairments in learning and memory may be due to age-related deficits in long-term potentiation of glutamatergic synaptic transmission. For example, olfactory discrimination learning is significantly affected by aging in mice and this may be due, in part, to diminished synaptic plasticity in piriform cortex. In the present study, we tested for alterations in electrophysiological properties and synaptic transmission in this simple cortical network. Whole-cell recordings were made from principal neurons in slices of anterior piriform cortex from young (3–6 months old) and old (24–28 months) C57Bl/6 mice. Miniature excitatory postsynaptic currents (mEPSCs) mediated by AMPA receptors were collected from cells in presence of tetrodotoxin (TTX) and held at -80 mV in voltage-clamp. Amplitudes of mEPSCs were significantly reduced in aged mice, suggesting that synaptic AMPA receptor expression is decreased during aging. In a second set of experiments, spontaneous excitatory postsynaptic currents (s/mEPSCs) were recorded in slices from different cohorts of young and old mice, in the absence of TTX. These currents resembled mEPSCs and were similarly reduced in amplitude in old mice. The results represent the first electrophysiological evidence for age-related declines in glutamatergic synaptic function in the mammalian olfactory system.

Adult depression-like behavior, amygdala and olfactory cortex functions are restored by odor previously paired with shock during infant's sensitive period attachment learning

Maltreatment from the caregiver induces vulnerability to later life psychopathologies, yet attraction and comfort is sometimes provided by cues associated with early life maltreatment. We used a rat model of early life maltreatment with odor-0.5 mA shock conditioning to produce depressive-like behaviors and questioned whether stimuli associated with maltreatment would restore emotional neurobehavioral function to control levels. Pups received daily novel odor-0.5 mA shock conditioning from postnatal day 8 to 12. This procedure produces a new maternal odor that controls pups' attachment behaviors. In adulthood, either with or without the infant odor, animals received a Forced Swim Test, Sucrose Preference Test or assessment of amygdala and olfactory system functioning using field potential signal evoked by olfactory bulb paired-pulse electrical stimulation. Following neonatal odor-shock pairings, but not unpaired controls, adults without the odor present showed increased depression-like behavior in the Forced Swim Test and Sucrose Preference Test and a deficit in paired-pulse inhibition in amygdala and piriform (olfactory) cortex. All effects were brought to control levels when the infant conditioned odor was presented during behavioral and neural tests. The ability of cues associated with early life maltreatment to normalize behavior and amygdala activity suggests these cues provide adaptive value in adulthood.

Microcircuits mediating feedforward and feedback synaptic inhibition in the piriform cortex

Local inhibition by GABA-releasing neurons is important for the operation of sensory cortices, but the details of these inhibitory circuits remain unclear. We addressed this question in the olfactory system by making targeted recordings from identified classes of inhibitory and glutamatergic neurons in the piriform cortex (PC) of mice. First, we looked for feedforward synaptic inhibition provided by interneurons located in the outermost layer of the PC, layer Ia, which is the unique recipient of afferent fibers from the olfactory bulb. We found two types of feedforward inhibition: a fast-rising, spatially restricted kind that was generated by horizontal cells, and a slow-rising, more diffuse kind generated by neurogliaform cells. Both cell types targeted the distal apical dendrites of layer II principal neurons. Next, we studied feedback synaptic inhibition in isolation by making a tissue cut across layer I to selectively remove feedforward inhibitory connections. We identified a powerful type of feedback inhibition of layer II neurons, mostly generated by soma-targeting fast-spiking multipolar cells in layer III, which in turn were driven by feedforward excitation from layer II semilunar cells. Dynamic clamp simulation of feedback inhibition revealed differential effects of this inhibition on the two main types of layer II principal neurons. Thus, our results articulate the connectivity and functions of two important classes of inhibitory microcircuits in the PC. Feedforward and feedback inhibition generated by these circuits is likely to be required for the operation of this sensory paleocortex during the processing of olfactory information.

Microcircuits mediating feedforward and feedback synaptic inhibition in the piriform cortex

Local inhibition by GABA-releasing neurons is important for the operation of sensory cortices, but the details of these inhibitory circuits remain unclear. We addressed this question in the olfactory system by making targeted recordings from identified classes of inhibitory and glutamatergic neurons in the piriform cortex (PC) of mice. First, we looked for feedforward synaptic inhibition provided by interneurons located in the outermost layer of the PC, layer Ia, which is the unique recipient of afferent fibers from the olfactory bulb. We found two types of feedforward inhibition: a fast-rising, spatially restricted kind that was generated by horizontal cells, and a slow-rising, more diffuse kind generated by neurogliaform cells. Both cell types targeted the distal apical dendrites of layer II principal neurons. Next, we studied feedback synaptic inhibition in isolation by making a tissue cut across layer I to selectively remove feedforward inhibitory connections. We identified a powerful type of feedback inhibition of layer II neurons, mostly generated by soma-targeting fast-spiking multipolar cells in layer III, which in turn were driven by feedforward excitation from layer II semilunar cells. Dynamic clamp simulation of feedback inhibition revealed differential effects of this inhibition on the two main types of layer II principal neurons. Thus, our results articulate the connectivity and functions of two important classes of inhibitory microcircuits in the PC. Feedforward and feedback inhibition generated by these circuits is likely to be required for the operation of this sensory paleocortex during the processing of olfactory information.

Effects of corticoliberin on synaptic transmission in rat olfactory cortex slices in a water immersion model of depression

Corticoliberin (corticotrophin-releasing factor, CRF, CRH) is an active regulator of endocrine, autonomic, and immune functions in stress, as well as a mediator of anxiety, determining the behavioral stress response. The present report describes studies of its action on neuron activity evoked by microstimulation of olfactory cortex slices. Behavioral testing in a T maze was used to select individuals with a passive behavioral strategy from a population of Wistar rats, and the animals were subjected to water immersion. Olfactory cortex slices were prepared 10 days later and evoked focal potentials were recorded on perfusion with medium containing corticoliberin (0.1 microM). Among active rats, 60% of slices retained high excitability after stress, and corticoliberin produced only insignificant reductions in the amplitudes of excitatory potentials in these slices, simultaneously increasing the amplitudes of inhibitory potentials. Low excitability was found in 40% of slices from active stressed rats, and corticoliberin had a significant inhibitory effect in these slices. Addition of corticoliberin to the incubation medium used for slices from passive rats with initially low excitability led to complete blockade of synaptic transmission. These data support the involvement of corticoliberin in the development of depression.

Investigation of the effects of the novel anticonvulsant compound carisbamate (RWJ-333369) on rat piriform cortical neurones in vitro

BACKGROUND AND PURPOSE

Carisbamate is being developed for adjuvant treatment of partial onset epilepsy. Carisbamate produces anticonvulsant effects in primary generalized, complex partial and absence-type seizure models, and exhibits neuroprotective and antiepileptogenic properties in rodent epilepsy models. Phase IIb clinical trials of carisbamate demonstrated efficacy against partial onset seizures; however, its mechanisms of action remain unknown. Here, we report the effects of carisbamate on membrane properties, evoked and spontaneous synaptic transmission and induced epileptiform discharges in layer II-III neurones in piriform cortical brain slices.

EXPERIMENTAL APPROACH

Effects of carisbamate were investigated in rat piriform cortical neurones by using intracellular electrophysiological recordings.

KEY RESULTS

Carisbamate (50-400 micromol x L(-1)) reversibly decreased amplitude, duration and rise-time of evoked action potentials and inhibited repetitive firing, consistent with use-dependent Na+ channel block; 150-400 micromol x L(-1) carisbamate reduced neuronal input resistance, without altering membrane potential. After microelectrode intracellular Cl(-) loading, carisbamate depolarized cells, an effect reversed by picrotoxin. Carisbamate (100-400 micromol x L(-1)) also selectively depressed lateral olfactory tract-afferent evoked excitatory synaptic transmission (opposed by picrotoxin), consistent with activation of a presynaptic Cl(-) conductance. Lidocaine (40-320 micromol x L(-1)) mimicked carisbamate, implying similar modes of action. Carisbamate (300-600 micromol x L(-1)) had no effect on spontaneous GABA(A) miniature inhibitory postsynaptic currents and at lower concentrations (50-200 micromol x L(-1)) inhibited Mg2+-free or 4-aminopyridine-induced seizure-like discharges.

CONCLUSIONS AND IMPLICATIONS

Carisbamate blocked evoked action potentials use-dependently, consistent with a primary action on Na+ channels and increased Cl(-) conductances presynaptically and, under certain conditions, postsynaptically to selectively depress excitatory neurotransmission in piriform cortical layer Ia-afferent terminals.

GABAergic circuits control input-spike coupling in the piriform cortex

Odor coding in mammals is widely believed to involve synchronized gamma frequency (30-70 Hz) oscillations in the first processing structure, the olfactory bulb. How such inputs are read in downstream cortical structures however is not known. Here we used patch-clamp recordings in rat piriform cortex slices to examine cellular mechanisms that shape how the cortex integrates inputs from bulb mitral cells. Electrical stimulation of mitral cell axons in the lateral olfactory tract (LOT) resulted in excitation of pyramidal cells (PCs), which was followed approximately 10 ms later by inhibition that was highly reproducible between trials in its onset time. This inhibition was somatic in origin and appeared to be driven through a feedforward mechanism, wherein GABAergic interneurons were directly excited by mitral cell axons. The precise inhibition affected action potential firing in PCs in two distinct ways. First, by abruptly terminating PC excitation, it limited the PC response to each EPSP to exactly one, precisely timed action potential. In addition, inhibition limited the summation of EPSPs across time, such that PCs fired action potentials in strong preference for synchronized inputs arriving in a time window of <5 ms. Both mechanisms would help ensure that PCs respond faithfully and selectively to mitral cell inputs arriving as a synchronized gamma frequency pattern.

GABAergic circuits control input-spike coupling in the piriform cortex

Odor coding in mammals is widely believed to involve synchronized gamma frequency (30-70 Hz) oscillations in the first processing structure, the olfactory bulb. How such inputs are read in downstream cortical structures however is not known. Here we used patch-clamp recordings in rat piriform cortex slices to examine cellular mechanisms that shape how the cortex integrates inputs from bulb mitral cells. Electrical stimulation of mitral cell axons in the lateral olfactory tract (LOT) resulted in excitation of pyramidal cells (PCs), which was followed approximately 10 ms later by inhibition that was highly reproducible between trials in its onset time. This inhibition was somatic in origin and appeared to be driven through a feedforward mechanism, wherein GABAergic interneurons were directly excited by mitral cell axons. The precise inhibition affected action potential firing in PCs in two distinct ways. First, by abruptly terminating PC excitation, it limited the PC response to each EPSP to exactly one, precisely timed action potential. In addition, inhibition limited the summation of EPSPs across time, such that PCs fired action potentials in strong preference for synchronized inputs arriving in a time window of <5 ms. Both mechanisms would help ensure that PCs respond faithfully and selectively to mitral cell inputs arriving as a synchronized gamma frequency pattern.

Neonatal odor-shock conditioning alters the neural network involved in odor fear learning at adulthood

Adult learning and memory functions are strongly dependent on neonatal experiences. We recently showed that neonatal odor-shock learning attenuates later life odor fear conditioning and amygdala activity. In the present work we investigated whether changes observed in adults can also be observed in other structures normally involved, namely olfactory cortical areas. For this, pups were trained daily from postnatal (PN) 8 to 12 in an odor-shock paradigm, and retrained at adulthood in the same task. (14)C 2-DG autoradiographic brain mapping was used to measure training-related activation in amygdala cortical nucleus (CoA), anterior (aPCx), and posterior (pPCx) piriform cortex. In addition, field potentials induced in the three sites in response to paired-pulse stimulation of the olfactory bulb were recorded in order to assess short-term inhibition and facilitation in these structures. Attenuated adult fear learning was accompanied by a deficit in 2-DG activation in CoA and pPCx. Moreover, electrophysiological recordings revealed that, in these sites, the level of inhibition was lower than in control animals. These data indicate that early life odor-shock learning produces changes throughout structures of the adult learning circuit that are independent, at least in part, from those involved in infant learning. Moreover, these enduring effects were influenced by the contingency of the infant experience since paired odor-shock produced greater disruption of adult learning and its supporting neural pathway than unpaired presentations. These results suggest that some enduring effects of early life experience are potentiated by contingency and extend beyond brain areas involved in infant learning.

INHIBITORY INTERNEURONS IN THE PIRIFORM CORTEX

1. The piriform cortex (PC) is the largest subdivision of the olfactory cortex and the first cortical destination of olfactory information. Despite the relatively simple anatomy of the PC and its obvious appeal as a model system for the study of cortical sensory processing, there are many outstanding questions about its basic cell physiology. In the present article, we review what is known about GABAergic inhibitory interneurons in the PC. 2. The GABA-containing neurons in the PC are morphologically diverse, ranging from small neurogliaform cells to large multipolar forms. Some of these classes are distributed across all three main layers of the PC, whereas others have a more restricted laminar expression. 3. Distinct and overlapping populations of GABAergic basket cells in Layers II and III of the PC express different combinations of calcium-binding proteins and neuropeptides. Few Layer I interneurons express any of the molecular markers so far examined. 4. The intrinsic firing properties of one or two types of putative PC interneurons have been measured and inhibitory post-synaptic responses have been recorded in PC pyramidal cells following extracellular stimulation. However, little is known about the physiology of the subtypes of interneurons identified. 5. In view of the likely importance of PC interneurons in olfactory learning, olfactory coding and epileptogenesis, further investigation of their properties is likely to be highly informative.

Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback

Substances such as acetylcholine and glutamate act as both neurotransmitters and neuromodulators. As neuromodulators, they change neural information processing by regulating synaptic transmitter release, altering baseline membrane potential and spiking activity, and modifying long-term synaptic plasticity. Slice physiology research has demonstrated that many neuromodulators differentially modulate afferent, incoming information compared to intrinsic and recurrent processing in cortical structures such as piriform cortex, neocortex, and the hippocampus. The enhancement of afferent (external) pathways versus the suppression at recurrent (internal) pathways could cause cortical dynamics to switch between a predominant influence of external stimulation to a predominant influence of internal recall. Modulation of afferent versus intrinsic processing could contribute to the role of neuromodulators in regulating attention, learning, and memory effects in behavior.

Developmental changes in presynaptic muscarinic modulation of excitatory and inhibitory neurotransmission in rat piriform cortex in vitro: relevance to epileptiform bursting susceptibility

Suppression of depolarizing postsynaptic potentials and isolated GABA-A receptor-mediated fast inhibitory postsynaptic potentials by the muscarinic acetylcholine receptor agonist, oxotremorine-M (10 microM), was investigated in adult and immature (P14-P30) rat piriform cortical (PC) slices using intracellular recording. Depolarizing postsynaptic potentials evoked by layers II-III stimulation underwent concentration-dependent inhibition in oxotremorine-M that was most likely presynaptic and M2 muscarinic acetylcholine receptor-mediated in immature, but M1-mediated in adult (P40-P80) slices; percentage inhibition was smaller in immature than in adult piriform cortex. In contrast, compared with adults, layer Ia-evoked depolarizing postsynaptic potentials in immature piriform cortex slices in oxotremorine-M, showed a prolonged multiphasic depolarization with superimposed fast transients and spikes, and an increased 'all-or-nothing' character. Isolated N-methyl-d-aspartate receptor-mediated layer Ia depolarizing postsynaptic potentials (although significantly larger in immature slices) were however, unaffected by oxotremorine-M, but blocked by dl-2-amino-5-phosphonovaleric acid. Fast inhibitory postsynaptic potentials evoked by layer Ib or layers II-III-fiber stimulation in immature slices were significantly smaller than in adults, despite similar estimated mean reversal potentials ( approximately -69 and -70 mV respectively). In oxotremorine-M, only layer Ib-fast inhibitory postsynaptic potentials were suppressed; suppression was again most likely presynaptic M2-mediated in immature slices, but M1-mediated in adults. The degree of fast inhibitory postsynaptic potential suppression was however, greater in immature than in adult piriform cortex. Our results demonstrate some important physiological and pharmacological differences between excitatory and inhibitory synaptic systems in adult and immature piriform cortex that could contribute toward the increased susceptibility of this region to muscarinic agonist-induced epileptiform activity in immature brain slices.

Entorhinal cortex stimulation modulates amygdala and piriform cortex responses to olfactory bulb inputs in the rat

The rodent olfactory bulb sends direct projections to the piriform cortex and to two structures intimately implicated in memory processes, the entorhinal cortex and the amygdala. The piriform cortex has monosynaptic projections with the amygdala and the piriform cortex and is therefore in a position to modulate olfactory input either directly in the piriform cortex, or via the amygdala. In order to investigate this hypothesis, field potential signals induced in anesthetized rats by electrical stimulation of the olfactory bulb or the entorhinal cortex were recorded simultaneously in the piriform cortex (anterior part and posterior part) and the amygdala (basolateral nucleus and cortical nucleus). Single-site paired-pulse stimulation was used to assess the time courses of short-term inhibition and facilitation in each recording site in response to electrical stimulation of the olfactory bulb and entorhinal cortex. Paired-pulse stimulation of the olfactory bulb induced homosynaptic inhibition for short interpulse interpulse intervals (20-30 ms) in all the recording sites, with a significantly lower degree of inhibition in the anterior piriform cortex than in the other structures. At longer intervals (40-80 ms), paired-pulse facilitation was observed in all the structures. Paired-pulse stimulation of the entorhinal cortex mainly resulted in inhibition for the shortest interval duration (20 ms) in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. Double-site paired-pulse stimulation was then applied to determine if stimulation of the entorhinal cortex can modulate responses to olfactory bulb stimulation. For short interpulse intervals (20 ms) heterosynaptic inhibition was observed in anterior piriform cortex, posterior piriform cortex and amygdala basolateral but not cortical nucleus. The level of inhibition was greater in the basolateral nucleus than in the other structures. Taken together these data suggest that the entorhinal cortex exerts a main inhibitory effect on the olfactory input via the amygdala basolateral nucleus and to a lesser extent the piriform cortex. The potential role of these effects on the processing of olfactory information is discussed.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Cortical injury affects short-term plasticity of evoked excitatory synaptic currents

The hypothesis that plastic changes in the efficacy of excitatory neurotransmission occur in areas of chronic cortical injury was tested by assessing short-term plasticity of evoked excitatory synaptic currents (EPSCs) in neurons of partially isolated neocortical islands (undercut cortex). Whole cell recordings were obtained from layer V pyramidal neurons of sensorimotor cortical slices prepared from P36-P43 control and undercut rats. AMPA/kainate receptor-mediated EPSCs elicited by stimuli delivered at 40 to 66.7 Hz exhibited more paired-pulse depression (PPD) in undercut cortex than control, the time constant of depression evoked by trains of 20- to 66.7-Hz stimuli was faster, and the steady-state amplitude of EPSCs reached after five to seven EPSCs was lower. An antagonist of the glutamate autoreceptor, group II mGluR, increased the steady-state amplitude of EPSCs from undercut but not control cortex, suggesting that activation of presynaptic receptors by released glutamate is more prominent in undercut cortex. In contrast, the GABA(B) receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid had no effect. Increasing [Ca(2+)](o) from 2 to 4 mM increased PPD, with a smaller effect in neurons of the undercut. The I-V relationship of AMPA/kainate receptor-mediated EPSCs was close to linear in both control and undercut neurons, and spermine had no significant effect on the EPSCs, suggesting that decreases in postsynaptic glutamate receptors containing the GluR2 subunit were not involved in the alterations in short-term plasticity. Results are compatible with an increase in the probability of transmitter release at excitatory synapses in undercut cortex due to functional changes in presynaptic terminals.

Learning-induced enhancement of postsynaptic potentials in pyramidal neurons

We studied the effect of olfactory learning-induced modifications in piriform (olfactory) cortex pyramidal neurons on the propagation of postsynaptic potentials (PSPs). Rats were trained to distinguish between odors in pairs, in an olfactory discrimination task. Three days after training completion, PSPs were evoked in layer II pyramidal cells in piriform cortex brain slices by electrical stimulation of two pathways. Stimulation of layer Ib activated the intra-cortical fibers that terminate on the proximal region of the apical and basal dendrites. Stimulation of layer Ia activated the afferent axons that originate from the olfactory bulb and terminate on the distal apical dendrites. We have previously shown that olfactory training is accompanied by enhanced synaptic transmission in the intrinsic pathway, but not in the afferent pathway at 3 days after training. Here we show that at this stage, in both pathways PSPs evoked in neurons from trained rats had significantly faster rise time measured at the soma compared with PSPs in neurons from pseudo-trained and naive rats. Activation of the slow afterhyperpolarization (AHP), which is generated by potassium channels probably located at the proximal region of both apical and basal dendrites, reduced the amplitude measured at the soma of the proximal intrinsic pathway PSPs more effectively than PSPs that were generated distally by the afferent fibers. Thus the amount of reduction by AHP was used as a measure for the relative distance of PSP-generating sites from the soma. In neurons from trained rats, despite the previously reported reduction in AHP amplitude, AHP conductance shunted the PSPs from both synaptic pathways more efficiently compared with neurons from the control rats. We suggest that in neurons from trained rats PSPs are electrotonicly closer to the soma.

Physiological characterization of layer III non-pyramidal neurons in piriform (olfactory) cortex of rat

We performed whole-cell recordings of layer III non-pyramidal neurons in the piriform cortex of Sprague-Dawley rats. For comparison purposes, recordings were made from deep pyramidal cells, which are also present in layer III. These two cell types could be distinguished both anatomically and physiologically. Anatomically, the layer III non-pyramidal neuron displayed smooth beady dendrites, while deep pyramidal cells showed thicker dendrites with spines. The dendrites of the layer III non-pyramidal neuron also tended to be restricted to layer III while deep pyramidal cells had long apical dendrites that spanned layers I and II. Although the resting membrane potentials of both cell types were very similar, significant differences were noted in other physiological measures. Layer III non-pyramidal neurons typically displayed higher input resistances, faster time constants, smaller spike amplitudes, shorter spike widths, and higher spike thresholds. In addition, layer III non-pyramidal neurons were able to spike at much higher rates when stimulated with the same level of threshold normalized current injection. The most dramatic differences in physiology were seen in the pattern of spiking in response to increasing levels of positive constant current pulses. Layer III non-pyramidal neurons showed qualitatively different responses at low and high levels of stimulation. At low levels, spikes occurred with long latency and the firing frequency increased throughout the duration of the current pulse. At high levels, non-pyramidal neurons started spiking with short latency, followed by a decrease in firing frequency, which in turn was followed by an increase in firing frequency. Deep pyramidal neurons differed dramatically from this pattern, displaying a qualitatively similar response at all levels of current injection. This response was characterized by short latency spikes and spike adaptation for the duration of the current pulse.

Layer-specific immunocytochemical localization of GABA(B)R1a and GABA(B)R1b receptors in the rat piriform cortex

A peculiar, layer-segregated immunoreactive distribution of GABABR1a and GABABR1b receptor antibodies is present in the piriform cortex of adult rats. The GABABR1a antibody selectively marked the neuropile in layer Ia, where afferent olfactory fibres and intrinsic GABAergic (gamma-aminobutyric acid) axons terminate on the distal apical dendrites of pyramidal neurons. The GABABR1b antibody was detected in the soma and the large basal dendrites of layer II and III neurons. The pattern of distribution observed supports the hypothesis that (presynaptic) GABABR1a receptors in the superficial molecular layer modulate neurotransmitter release in a feedforward synaptic circuit, whereas GABABR1b (postsynaptic) receptors mediate feedback inhibitory potentials on principal cells.

GABAB receptors mediate frequency-dependent depression of excitatory potentials in rat perirhinal cortex in vitro

Excitatory synaptic transmission in the perirhinal cortex exhibits marked homosynaptic paired pulse depression (PPD) at inter-pulse intervals between 100 and 1000 ms, being maximal at 200 ms. Additionally, there is greater PPD with stimulation of the pathway from the temporal cortex side than with stimulation of the pathway from the entorhinal cortex side. We establish that this frequency-dependent depression relies on the activation of GABAB (gamma-aminobutyric acid) receptors. PPD in both temporal and entorhinal pathways is abolished by either of the selective GABAB receptor antagonists, 3-N[1-(S)-(3, 4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-p-benzyl-phosphinic acid (CGP55845A) or 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP35348). Barium which blocks G-protein-coupled, inwardly rectifying potassium channels, does not block PPD. Heterosynaptic depression mediated by GABAB receptors was also observed. The depression of the entorhinal pathway by stimulation of the temporal pathway is greater than depression of the temporal pathway by stimulation of the entorhinal pathway. Moreover, PPD increases with stimulus strength and the depression is enhanced by short trains of stimuli, consistent with stronger stimulation resulting in more GABA reaching GABAB receptors on excitatory glutamatergic synapses. Synaptic activation of GABAB receptors may be important in regulating excitability in a frequency-dependent manner with maximal depression occurring at approximately 5 Hz, which approximates to the theta rhythm. That homosynaptic and heterosynaptic depression by stimulation of the temporal pathway is greater than by stimulation of the entorhinal pathway suggests that activation of temporal feedforward connections to the perirhinal cortex can dominate the GABAergic control of synaptic activity within the perirhinal cortex.

Reduced synaptic facilitation between pyramidal neurons in the piriform cortex after odor learning

Learning-related cellular modifications were studied in the rat piriform cortex after operand conditioning. Rats were trained to discriminate positive cues in pairs of odors. In one experimental paradigm, rats were trained to memorize 35-50 pairs of odors ("extensive training"). In another paradigm, training was continued only until rats acquired the rule of the task, usually after learning the first two pairs of odors ("short training"). "Pseudotrained" and "naive" rats served as controls. We have previously shown that "rule learning" of this task was accompanied by reduced spike afterhyperpolarization in pyramidal neurons in brain slices of the piriform cortex. In the present study, synaptic inputs to the same cells were examined. Pairs of electrical stimuli applied to the intrinsic fibers that interconnect layer II pyramidal neurons revealed significant reduction in paired-pulse facilitation (PPF) in this pathway even after short training. PPF in shortly trained rats was reduced to the same extent as in extensively trained rats. PPF reduction did not result from modification of membrane properties in the postsynaptic cells, change in postsynaptic inhibition, or impairment of the facilitation mechanism. Extracellular field potential recordings showed enhanced synaptic transmission in these synapses. The reduction in PPF became apparent only 3 d after task acquisition and returned to control value 5 d later. PPF evoked by stimulating the afferent fibers to the same neurons was increased 1 d after training for 2 d. We suggest that the transient enhancement in connectivity in the intrinsic pathway is related to the enhanced learning capability and not to memory for specific odors, which lasts for weeks.

Brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling without affecting paired-pulse measures of neuronal inhibition in adult rats

Kindling is an animal model of human temporal lobe epilepsy in which excitability in limbic structures is permanently enhanced by repeated stimulations. Kindling also increases the expression of nerve growth factor, brain-derived neurotrophic factor, and brain-derived neurotrophic factor receptor messenger RNAs in both the hippocampus and cerebral cortex and causes structural changes in the hippocampus including hilar hypertrophy. We have recently shown that intraventricular nerve growth factor infusion enhances the development of kindling, whereas blocking nerve growth factor activity retards amygdaloid kindling. Furthermore, we have shown that nerve growth factor protects against kindling-induced hilar hypertrophy. The physiological role of brain-derived neurotrophic factor in kindling is not as clear. Acute injection of brain-derived neurotrophic factor increases neuronal excitability and causes seizures, whereas chronic brain-derived neurotrophic factor infusion in rats slows hippocampal kindling. In agreement with the latter, we show here that intrahilar brain-derived neurotrophic factor infusion delays amygdala and perforant path kindling. In addition, we show that brain-derived neurotrophic factor, unlike nerve growth factor, does not protect against kindling-induced increases in hilar area. To test the hypothesis that brain-derived neurotrophic factor suppresses kindling by increasing inhibition above normal levels, we performed paired-pulse measures in the perforant path-dentate gyrus pathway. Brain-derived neurotrophic factor infused into the hippocampus had no effect on the stimulus intensity function (input/output curves); there was also no significant effect on paired-pulse inhibition. We then kindled the perforant path 10 days after the end of brain-derived neurotrophic factor treatment. Once again, kindling was retarded, showing that the brain-derived neurotrophic factor effect is long-lasting. These results indicate that prolonged in vivo infusion of brain-derived neurotrophic factor reduces, rather than increases, excitability without increasing inhibitory neuron function, at least as assessed by paired-pulse protocols. This effect may be mediated by long-lasting effects on brain-derived neurotrophic factor receptor regulation.

Pain and functional imaging

Functional neuroimaging has fundamentally changed our knowledge about the cerebral representation of pain. For the first time it has been possible to delineate the functional anatomy of different aspects of pain in the medial and lateral pain systems in the brain. The rapid developments in imaging methods over the past years have led to a consensus in the description of the central pain responses between different studies and also to a definition of a central pain matrix with specialized subfunctions in man. In the near future we will see studies where a systems perspective allows for a better understanding of the regulatory mechanisms in the higher-order frontal and parietal cortices. Also, pending the development of experimental paradigms, the functional anatomy of the emotional aspects of pain will become better known.

[Evoked motor potentials obtained with double magnetic cortical stimulation: techniques and interpretation]

The technique of motor evoked potentials (MEP) obtained with single and double magnetic stimulation of the motor cortex in man has considerably improved over the past decade. We present the techniques and parameters involved in double magnetic stimulation for clinical purposes.

METHOD

The conditioning-test design is used to study modifications in the amplitudes of the muscular responses to the "test" shock, recorded on the first dorsal interosseus muscle. Enhanced amplitudes of conditioned responses indicate facilitation, reduced response inhibition.

RESULTS

The effects vary according to the shock intensity, the delay between shocks and the position of the conditioning coil. The latter may be located at the same place as the test shock (to test interneural circuitry related to pyramidal tract), on the hand area opposite the test shock (to test interhemispheric influences), or over the cerebellar area contralateral to the test side (to test the effect of cerebellar stimulations over the motor cortex). When the coils were located on the same cortical hand area there was facilitation when the intensities were both set at the threshold with an interstimulus interval (ISI) between 1 and 2.5-3 ms. At conditioning shock intensities below the threshold and the test shock 150% above, inhibition occurred at ISI 1-5 ms followed by facilitation at ISI 15-35 ms. When the intensities of both shocks were 150% above threshold, there were two clear cut individual responses at ISI above 10 ms; facilitation was recorded at ISI 15-35 ms, and inhibition between 55 and 255 ms. When the conditioning coil was located on the opposite hand area from the test shock (conditioning shock intensity supramaximal, test shock intensity above the threshold), ISI 1-5 ms facilitation occurred followed by inhibition up to ISI 30 ms. When the conditioning shock (intensity supramaximal) was located on the cerebellar area contralateral to the test side (intensity above the threshold), inhibition occurred at ISI 5 ms.

CONCLUSIONS

Double magnetic stimulations delivered over the same cortical area reflect facilitatory and inhibitory influences over the pyramidal tract controlled by interneurons, i.e., these tests investigate the intrinsic circuitry of the motor strip. Double magnetic stimulations delivered on each motor area study interhemispheric influences mediated by the corpus callosum, which are facilitatory and inhibitory. Double magnetic stimulations delivered on the cerebellar area demonstrates inhibitory influences over the contralateral cerebral motor cortex.

Modulation of inhibitory synaptic potentials in the piriform cortex

Modulation of inhibitory synaptic potentials in the piriform cortex. Intracellular recordings from pyramidal neurons in brain slice preparations of the piriform cortex were used to test results from a computational model about the effects of cholinergic agonists on inhibitory synaptic potentials induced by stimulation of afferent fibers in layer Ia and association/intrinsic fibers in layer Ib. A simple model of piriform cortex as an associative memory was used to analyze how suppression of inhibitory synaptic transmission influenced performance of the network. Levels of suppression of excitatory synaptic transmission were set at levels determined in previous experimental work. Levels of suppression of inhibitory synaptic transmission were then systematically varied within the model. This modeling work demonstrated that suppression of inhibitory synaptic transmission in layer Ib should be stronger than suppression of inhibitory synaptic transmission in layer Ia to keep activity levels high enough for effective storage. Experimental data showed that perfusion of the cholinergic agonist carbachol caused a significant suppression of inhibitory postsynaptic potentials (IPSPs) in the pyramidal neurons that were induced by stimulation of layer Ib, with a weaker effect on IPSPs induced by stimulation of layer Ia. As previously described, carbachol also selectively suppressed excitatory postsynaptic potentials (EPSPs) elicited by intrinsic but not afferent fiber stimulation. The decrease in amplitude of IPSPs induced by layer Ib stimulation did not appear to be directly related to the decrease in EPSP amplitude induced by layer Ib stimulation. The stimulation necessary to induce neuronal firing with layer Ia stimulation was reduced in the presence of carbachol, whereas that necessary to induce neuronal firing with layer Ib stimulation was increased, despite the depolarization of resting membrane potential. Thus physiological data on cholinergic modulation of inhibitory synaptic potentials in the piriform cortex is compatible with the functional requirements determined from computational models of piriform cortex associative memory function.

Voltage imaging of epileptiform activity in slices from rat piriform cortex: onset and propagation

The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.

Spatiotemporal distribution of a late synchronized activity in olfactory pathways following stimulation of the olfactory bulb in rats

The evoked potential recorded in the rat piriform cortex in response to electrical stimulation of the olfactory bulb is composed of an early component occasionally followed by a late component (60-70 ms). We previously showed that the late component occurrence was enhanced following an olfactory learning. In the present study carried out in naive rats, we investigated the precise conditions of induction of this late component, and its spatiotemporal distribution along the olfactory pathways. In the anaesthetized rat, a stimulating electrode was implanted in the olfactory bulb. Four recording electrodes were positioned, respectively, in the olfactory bulb, the anterior and posterior parts of the piriform cortex, and the entorhinal cortex. Simultaneous recording of signals evoked in the four sampled structures in response to stimulation of the olfactory bulb revealed that the late component was detected in anterior and posterior piriform cortex as well as in entorhinal cortex, but not in the olfactory bulb. The late component occurred reliably for a narrow range of low intensities of stimulation delivered at frequencies not exceeding 1 Hz. Comparison of late component amplitude and latency across the different recorded sites showed that this component appeared first and with the greatest amplitude in the posterior piriform cortex. In addition to showing a functional dissociation between anterior and posterior parts of the piriform cortex, these data suggest that the posterior piriform cortex could be the locus of generation of this late high amplitude synchronized activity, which would then propagate to the neighbouring regions.

Low-voltage activated T-type calcium currents are differently expressed in superficial and deep layers of guinea pig piriform cortex

A variety of voltage-dependent calcium conductances are known to control neuronal excitability by boosting peripheral synaptic potentials and by shaping neuronal firing patterns. The existence and functional significance of a differential expression of low- and high-voltage activated (LVA and HVA, respectively) calcium currents in subpopulations of neurons, acutely isolated from different layers of the guinea pig piriform cortex, were investigated with the whole cell variant of the patch-clamp technique. Calcium currents were recorded from pyramidal and multipolar neurons dissociated from layers II, III, and IV. Average membrane capacitance was larger in layer IV cells [13.1 +/- 6.2 (SD) pF] than in neurons from layers II and III (8.6 +/- 2.8 and 7.9 +/- 3.1 pF, respectively). Neurons from all layers showed HVA calcium currents with an activation voltage range positive to -40 mV. Neurons dissociated from layers III and IV showed an LVA calcium current with the biophysical properties of a T-type conductance. Such a current displayed the following characteristics: 1) showed maximal amplitude of 11-16 pA/pF at -30 mV, 2) inactivated rapidly with a time constant of approximately 22 ms at -30 mV, and 3) was completely steady-state inactivated at -60 mV. Only a subpopulation of layer II neurons (group 2 cells; circa 18%) displayed an LVA calcium current similar to that observed in deep layers. The general properties of layer II-group 2 cells were otherwise identical to those of group 1 neurons. The present study demonstrates that LVA calcium currents are differentially expressed in neurons acutely dissociated from distinct layers of the guinea pig piriform cortex.

GABAA-mediated IPSCs in piriform cortex have fast and slow components with different properties and locations on pyramidal cells

GABAA-mediated IPSCs in piriform cortex have fast and slow components with different properties and locations on pyramidal cells. J. Neurophysiol. 78: 2531-2545, 1997. A recent study in piriform (olfactory) cortex provided evidence that, as in hippocampus and neocortex, gamma-aminobutyric acid-A (GABAA)-mediated inhibition is generated in dendrites of pyramidal cells, not just in the somatic region as previously believed. This study examines selected properties of GABAA inhibitory postsynaptic currents (IPSCs) in dendritic and somatic regions that could provide insight into their functional roles. Pharmacologically isolated GABAA-mediated IPSCs were studied by whole cell patch recording in slices. To compare properties of IPSCs in distal dendritic and somatic regions, local stimulation was carried out with tungsten microelectrodes, and spatially restricted blockade of GABAA-mediated inhibition was achieved by pressure-ejection of bicuculline from micropipettes. The results revealed that largely independent circuits generate GABAA inhibition in distal apical dendritic and somatic regions. With such independence, a selective decrease in dendritic-region inhibition could enhance integrative or plastic processes in dendrites while allowing feedback inhibition in the somatic region to restrain system excitability. This could allow modulatory fiber systems from the basal forebrain or brain stem, for example, to change the functional state of the cortex by altering the excitability of interneurons that mediate dendritic inhibition without increasing the propensity for regenerative bursting in this highly epileptogenic system. As in hippocampus, GABAA-mediated IPSCs were found to have fast and slow components with time constants of decay on the order of 10 and 40 ms, respectively, at 29 degrees C. Modeling analysis supported physiological evidence that the slow time constant represents a true IPSC component rather than an artifactual slowing of the fast component from voltage clamp of a dendritic current. The results indicated that, whereas both dendritic and somatic-region IPSCs have both fast and slow GABAA components, there is a greater proportion of the slow component in dendrites. In a companion paper, the hypothesis is explored that the resulting slower time course of the dendritic IPSC increases its capacity to regulate the N-methyl--aspartate component of EPSPs. Finally, evidence is presented that the slow GABAA-mediated IPSC component is regulated by presynaptic GABAB inhibition whereas the fast is not. Based on the requirement for presynaptic GABAB-mediated block of inhibition for expression of long-term potentiation, this finding is consistent with participation of the slow GABAA component in regulation of synaptic plasticity. The lack of susceptibility of the fast GABAA component to the long-lasting, activity-induced suppression mediated by presynaptic GABAB receptors is consistent with a protective role for this process in preventing seizure activity.

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex

Regulation of the NMDA component of EPSPs by different components of postsynaptic GABAergic inhibition: computer simulation analysis in piriform cortex. J. Neurophysiol. 78: 2546-2559, 1997. Physiological analysis in the companion paper demonstrated that gamma-aminobutyric acid-A (GABAA)-mediated inhibition in piriform cortex is generated by circuits that are largely independent in apical dendritic and somatic regions of pyramidal cells and that GABAA-mediated inhibitory postsynaptic currents (IPSCs) in distal dendrites have a slower time course than those in the somatic region. This study used modeling methods to explore these characteristics of GABAA-mediated inhibition with respect to regulation of the N-methyl--aspartate (NMDA) component of excitatory postsynaptic potentials. Such regulation is relevant to understanding NMDA-dependent long-term potentiation (LTP) and the integration of repetitive synaptic inputs that can activate the NMDA component as well as pathological processes that can be activated by overexpression of the NMDA component. A working hypothesis was that the independence and differing properties of IPSCs in apical dendritic and somatic regions provide a means whereby the NMDA component and other dendritic processes can be controlled by way of GABAergic tone without substantially altering system excitability. The analysis was performed on a branched compartmental model of a pyramidal cell in piriform cortex constructed with physiological and anatomic data derived by whole cell patch recording. Simulations with the model revealed that NMDA expression is more effectively blocked by the slow GABAA component than the fast. Because the slow component is present in greater proportion in apical dendritic than somatic regions, this characteristic would increase the capacity of dendritic IPSCs to regulate NMDA-mediated processes. The simulations further revealed that somatic-region GABAergic inhibition can regulate the generation of action potentials with little effect on the NMDA component generated by afferent fibers in apical dendrites. As a result, if expression of the NMDA component or other dendritic processes were enabled by selective block of dendritic inhibition, for example, by centrifugal fiber systems that may regulate learning and memory, the somatic-region IPSC could preserve system stability through feedback regulation of firing without counteracting the effect of the dendritic-region block. Simulations with paired inputs revealed that the dendritic GABAA-mediated IPSC can regulate the extent to which a strong excitatory input facilitates the NMDA component of a concurrent weak input, providing a possible mechanism for control of "associative LTP" that has been demonstrated in this system. Postsynaptic GABAB-mediated inhibition had less effect on the NMDA component than either the fast or slow GABAA components. Depolarization from a concomitant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component also was found to have comparatively little effect on current through the NMDA channel because of its brief time course.

Acetylcholine and associative memory in the piriform cortex

The significance of cholinergic modulation for associative memory performance in the piriform cortex was examined in a study combining cellular neurophysiology in brain slices with realistic biophysical network simulations. Three different physiological effects of acetylcholine were identified at the single-cell level: suppression of neuronal adaptation, suppression of synaptic transmission in the intrinsic fibers layer, and activity-dependent increase in synaptic strength. Biophysical simulations show how these three effects are joined together to enhance learning and recall performance of the cortical network. Furthermore, our data suggest that activity-dependent synaptic decay during learning is a crucial factor in determining learning capability of the cortical network. Accordingly, it is predicted that acetylcholine should also enhance long-term depression in the piriform cortex.

Optical recording of the rat piriform cortex activity

The piriform cortex (PCx) is a phylogenetically old brain structure which presents characteristics of a content-addressable memory. Taking into account its particular anatomo-functional organization, we hypothesized that this cortex could behave rather as an assembly of different functional units than as a functionally homogeneous structure. This hypothesis was tested by using both anatomical and functional approaches. Immunohistological and tracing experiments demonstrated that both the connections of the PCx with the higher nervous centres, and its monoaminergic and cholinergic modulatory afferents exhibited a heterogeneous distribution. Then, optical monitoring of its neuronal activity with a voltage-sensitive dye pointed out that the PCx is a functionally heterogeneous structure. Electrical stimulations of the olfactory bulb showed that the inhibitory processes which control the cortical responsiveness were not identical in all the PCx area. Two different functional areas at least could be distinguished: in the ventromedial PCx, the afferent activity is privileged since the level of inhibition of disynaptic activation remained large during repetitive stimuli. Contrarily, in the posterior PCx, the disynaptic activity remained unchanged in response to successive stimulations and the responses of neighbouring sites were statistically more synchronized than in its anterior part. Moreover, a late depolarization wave was significantly larger in the posterior PCx. These data are in good agreement with the results provided by computational models of the PCx. In the future, theoretical and experimental investigations of this cortex will be useful for understanding olfactory information processing and as a model of brain functioning at the neocortical level as well.

Noradrenergic suppression of synaptic transmission may influence cortical signal-to-noise ratio

Norepinephrine has been proposed to influence signal-to-noise ratio within cortical structures, but the exact cellular mechanisms underlying this influence have not been described in detail. Here we present data on a cellular effect of norepinephrine that could contribute to the influence on signal-to-noise ratio. In brain slice preparations of the rat piriform (olfactory) cortex, perfusion of norepinephrine causes a dose-dependent suppression of excitatory synaptic potentials in the layer containing synapses among pyramidal cells in the cortex (layer Ib), while having a weaker effect on synaptic potentials in the afferent fiber layer (layer Ia). Effects of norepinephrine were similar in dose-response characteristics and laminar selectivity to the effects of the cholinergic agonist carbachol, and combined perfusion of both agonists caused effects similar to an equivalent concentration of a single agonist. In a computational model of the piriform cortex, we have analyzed the effect of noradrenergic suppression of synaptic transmission on signal-to-noise ratio. The selective suppression of excitatory intrinsic connectivity decreases the background activity of modeled neurons relative to the activity of neurons receiving direct afferent input. This can be interpreted as an increase in signal-to-noise ratio, but the term noise does not accurately characterize activity dependent on the intrinsic spread of excitation, which would more accurately be described as interpretation or retrieval. Increases in levels of norepinephrine mediated by locus coeruleus activity appear to enhance the influence of extrinsic input on cortical representations, allowing a pulse of norepinephrine in an arousing context to mediate formation of memories with a strong influence of environmental variables.

The role of the piriform cortex in kindling

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC ("primary olfactory cortex") is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) The PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neuron at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC c

Odor coding properties of frog olfactory cortical neurons

Until now, in amphibians, response odor properties of primary cortical neurons had never been investigated. Furthermore, very few data on this subject are available in other species. This prompted us to explore the functional properties of olfactory cortical neurons at rest and in response to odors. To achieve this, our experience with odor coding in the first two stages of the frog olfactory system, the olfactory mucosa and the olfactory bulb, led us to use odor stimuli which were chemical compounds with known stimulating properties, delivered to the mucosa in controlled conditions over a wide concentration range. Most of the cortical neurons were found to be very silent at rest, their average spontaneous activity being significantly lower than that of bulb neurons recorded previously in the same conditions. Cortical cells displayed, with all odors combined, 35% excitatory responses and 8% inhibitory responses. The excitatory response rate was similar to that of the bulb, while the inhibitory response rate was about 4.5-fold lower. Interestingly, two functional groups of cortical cells emerged based both on differences in response temporal patterning to odors delivered at increasing concentrations and in qualitative discrimination power. Regarding intensity coding, group 1 cells (53%) displayed "classical" temporal pattern evolution, increase of discharge frequencies and decrease of latency and burst duration, over the concentration range. The responses of group 2 cells (47%) were clearly original, since they consisted of a single spike (or more rarely two spikes) occurring with a strictly reproducible latency at a given concentration and a decreased latency as a function of increasing concentration. The dynamics of cell recruitment in the cortex showed that group 1 cell recruitment mimicked that of mitral cells, group 2 cells being recruited at higher concentrations. The analysis of qualitative discrimination properties of cortical cells regarding the eight-odor set revealed that the discrimination power of group 2 cells was similar to that of mitral cells. By contrast, the qualitative discrimination power of group 1 cells was found to be similar to that of neuroreceptor cells. In conclusion, this pioneer approach leads us to report that olfactory cortical neurons of the frog are responsive to odors and can be clearly divided into two groups based on functional criteria. Group 1 cells, which were relatively selective, poorly discriminating but sensitive, may be mainly devoted to intensity coding. By contrast, group 2 cells, which were not very sensitive but were selective and discriminating, were hypothesized to provide minimal intensity coding and thus to be mainly devoted to qualitative discrimination tasks.

Olfactory bulb repetitive stimulations reveal non-homogeneous distribution of the inhibitory processes in the rat piriform cortex

Optical signals were recorded in the in vivo rat piriform cortex in response to a burst of seven electrical stimulations (100 ms interval) delivered in the olfactory bulb. Based on the recorded responses, three types of signal could be identified according to the relative amplitude of their monosynaptic and disynaptic components. The dysynaptic component had a larger (type 1) or an equal amplitude (type 2) compared with the monosynaptic one. Type 3 exhibited only the monosynaptic component. Type 1 represented 96% of the first response. The second response was characterized by an increase in type 3 signals (39%). The remaining type 1 signals were lower in amplitude when compared with the first response. The responses to the last five stimulations did not differ from one another but were different from the first two (type 1, 74%; type 2, 7.8%; type 3, 18.2% on average). The spatial distribution of these three types of signal was analysed by dividing the piriform cortex into several areas. These areas were not homogeneous in the percentage of each signal type: the percentage of type 3 signals was highest (approximately 30%) in the area near the lateral olfactory tract and < 10% in the most posterodorsal area. Thus the level of inhibition remained high in some piriform areas whereas it decreased rapidly in others, suggesting that the inhibitory processes were not homogeneously distributed in the whole piriform cortex. Functional implications are discussed.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.