Hippocampal gamma-frequency oscillations: from interneurones to pyramidal cells, and back

GABAergic interneurones are necessary for the emergence of hippocampal gamma-frequency network oscillations, during which they play a key role in the synchronization of pyramidal cell firing. However, it remains to be resolved how distinct interneurone subtypes contribute to gamma-frequency oscillations, in what way the spatiotemporal pattern of interneuronal input affects principal cell activity, and by which mechanisms the interneurones themselves are synchronized. Here we summarize recent evidence from cholinergically induced gamma-frequency network oscillations in vitro , showing that perisomatic-targeting GABAergic interneurones provide prominent rhythmic inhibition in pyramidal cells, and that these interneurones are synchronized by recurrent excitation. We conclude by presenting a minimal integrate-and-fire network model which demonstrates that this excitatory-inhibitory feedback loop is sufficient to explain the generation of intrahippocampal gamma-frequency oscillations.     

Stratum oriens horizontal interneurone diversity and hippocampal network dynamics

In this last decade, the combination of differential interference contrast infrared video technology and patch-clamp techniques applied to slices in vitro has allowed the routine electrophysiological recording of visually identified central neurones. This has opened the way to the possibility of preselecting GABAergic interneurones of the hippocampus on the basis of some peculiar morphological characteristics. In particular, stratum oriens 'horizontal' interneurones are easily recognizable in living hippocampal slices because of their location and bipolar/bitufted appearance. Thus, this class of cells has rapidly risen as one of the most studied in the entire hippocampus. In this review, I will try to assemble the vast electrophysiological knowledge on these interneurones into a more focused picture, which is relevant for network activity in vitro and in vivo .     

Synaptic interactions between pyramidal cells and interneurone subtypes during seizure-like activity in the rat hippocampus

We have recently reported that excitatory GABAergic and glutamatergic mechanisms may be involved in the generation of seizure-like (ictal) rhythmic synchronization (afterdischarge), induced by a strong synaptic stimulation of the CA1 pyramidal cells in the mature rat hippocampus in vitro . To clarify the network mechanism of this neuronal synchronization, dual whole-cell patch-clamp recordings of the afterdischarge responses were performed simultaneously from a variety of interneurones and their neighbouring pyramidal cells in hippocampal CA1-isolated slice preparations. According to morphological and electrophysiological criteria, the recorded interneurones were then classified into distinct subtypes. The non-fast-spiking interneurones located in the strata lacunosum-moleculare and radiatum hardly discharged during the afterdischarge, whereas most of the fast-spiking and non-fast-spiking interneurones in the strata oriens and pyramidale, including the basket, chandelier and bistratified cells, exhibited prominent firings that were precisely synchronous with oscillatory responses in the pyramidal cells. Field potential recordings showed that excitatory synaptic transmissions might take place primarily in the strata oriens and pyramidale during the afterdischarge. Restricted lesions in the strata oriens and pyramidale, but not in the other layers, resulted in the complete desynchronization of afterdischarge activity, and also, local application of glutamate receptor antagonists to these layers blocked the expression of afterdischarge. The present findings indicate that the neuronal synchronization of epileptic afterdischarge may be accomplished in a 'positive feedback circuit' formed by the excitatory GABAergic interneurones and the glutamatergic pyramidal cells within the strata oriens and/or pyramidale of the hippocampal CA1 region.     

Profound regulation of neonatal CA1 rat hippocampal GABAergic transmission by functionally distinct kainate receptor populations

Neonatal hippocampus exhibits distinct patterns of network activity that are dependent on the interaction between inhibitory and excitatory transmission. Kainate receptors are ideally positioned to regulate this activity by virtue of their ability to regulate presynaptic function in GABAergic interneurones. Indeed, kainate receptors are highly expressed in neonatal hippocampal interneurones, yet the role and mechanisms by which they might regulate neonatal circuitry are unexplored. To address this we investigated the kainate receptor-dependent regulation of GABAergic transmission onto neonatal CA1 pyramidal neurones. Kainate receptor activation produced two distinct opposing effects, a very large increase in the frequency of spontaneous IPSCs, and a robust depression of evoked GABAergic transmission. The up-regulation of spontaneous transmission was due to activation of somatodendritic and axonal receptors while the depression of evoked transmission could be fully accounted for by a direct regulation of GABA release by kainate receptors located at the terminals. None of the effects of kainate receptor agonists were sensitive to GABA_B receptor antagonists, nor was there any postsynaptic kainate receptor-dependent effects observed in CA1 pyramidal cells that could account for our findings. Our data demonstrate that kainate receptors profoundly regulate neonatal CA1 GABAergic circuitry by two distinct opposing mechanisms, and indicate that these two effects are mediated by functionally distinct populations of receptors. Thus kainate receptors are strategically located to play a critical role in shaping early hippocampal network activity and by virtue of this have a key role in hippocampal development.     

Visual identification of two kinds of nerve cells and their synaptic contacts in a living autonomic ganglion of the mudpuppy (Necturus maculosus).

1. Many of the nerve cells comprising the cardiac parasympathetic ganglion of the mudpuppy are spread out in a thin, transparent sheet of tissue, enabling one to see cellular details in living preparations with differential interference contrast optics. The aim of this study was twofold: to establish the morphology of the nerve cells and their synaptic connections by light and electron microscopy, and to determine which aspects of the ganglion's structure could be reliably identified in the living tissue. 2. There are two types of neurones in the ganglion: (a) principal cells that send post-ganglionic axons to cardiac muscle fibres, and (b) interneurones whose processes are confined to the ganglion. 3. Interneurones are distinguished from principal cells by the presence of numerous granular vesicles seen with the electron microscope, and by intense formaldehyde-induced fluorescence. The interneurones are thus similar to catecholamine-containing interneurones in autonomic ganglia of other vertebrates. 4. Principal cells are innervated by processes that terminate mainly on the cell body, forming up to forty-five synaptic boutons and covering, on the average, 5% of the perikaryal surface. The synaptic terminals are derived from three sources: (a) axons from the vagus nerves, (b) interneurones and (c) other principal cells. Vagal terminals contacting principal cells contain agranular vesicles typical of preganglionic cholinergic endings. At regions of contact between processes of interneurones and principal cells, the interneurones have granular vesicles focused at membrane specializations; in addition there are small areas of close plasma membrane apposition, probably gap junctions. Some of the contacts between principal cells are characterized by gap junctions; others are structurally similar to vagal endings but persist after vagal degeneration. 6. Interneurones are innervated by axons that make contact mainly with their processes. The axon terminals on processes of interneurones contain agranular vesicles similar to vagal terminals on principal cells. 7. In live preparations principal cells are distinguished from interneurones by their size and the appearance of their organelles. Synaptic contacts on principal cells could often be identified and, in some cases, large contacts from interneurones or those from other nearby principal cells could be traced back to their cell bodies of origin. The validity of these identifications was confirmed by subsequent electron microscopic examination of the same cells.     

Distribution and expression of TREK-1, a two-pore-domain potassium channel, in the adult rat CNS

TREK-1 is a member of the two-pore-domain potassium channel family which is expressed predominantly in the CNS. Using an anti-peptide polyclonal antiserum, we have determined the distribution of TREK-1 in the brain and spinal cord of adult rats. Specificity of the antiserum was tested using a TREK-1-transfected cell line and confirmed with c-myc-tagged TREK-1. In thin tissue sections, immunoreactivity was widespread throughout the rat brain and spinal cord. TREK-1-like signals were observed in the cerebral cortex, basal ganglia, hippocampus, and various other subcortical nuclei in the hypothalamus, thalamus, mesencephalon and rhombencephalon. TREK-1 labelling appeared to be over the entire cell membrane, including the cell body and processes. Cells that morphologically resembled projection neurones and interneurones but not glial cells were labelled. As interneurones and known GABAergic projection neurones were the predominant population labelled, we investigated the possibility that TREK-1 is expressed in GABA-containing neurones using a specific anti-GABA antiserum. Expression of TREK-1 in GABA-containing neurones was observed in a number of areas, including the isocortex, hippocampus and thalamus. Thus, TREK-1 expression defines a unique and specific subset of interneurones and principal cells.These studies indicate a widespread distribution of TREK-1 potassium channels throughout the rat brain and spinal cord, with expression in a number of areas being demonstrated to be present on GABA-containing neurones.     

Modulation of network behaviour by changes in variance in interneuronal properties

Interneurones are important regulators of neuronal networks. The conventional approach to interneurones is to focus on the mean values of various parameters. Here we tested the hypothesis that changes in the variance of interneuronal properties (e.g. in the degree of scattering of parameter values of individual cells around the population mean) may modify the behaviour of networks. Biophysically based multicompartmental models of principal cells and interneurones showed that changes in the variance in the electrophysiological and anatomical properties of interneurones significantly alter the input-output functions, rhythmicity and synchrony of principal cells, even if the mean values were unchanged. In most cases, increased heterogeneity in interneurones resulted in stronger inhibition of principal cell firing; however, there were parameter ranges where increased interneuronal variance decreased the inhibition of principal cells. Electrophysiological recordings showed that the variance in the resting membrane potential of CA1 stratum oriens interneurones persistently increased following experimental complex febrile seizures in developing rats, without a change in the mean resting membrane potential, indicating that lasting alterations in interneuronal heterogeneity can take place in real neuronal systems. These computational and experimental data demonstrate that modifications in interneuronal population variance influence the behaviour of neuronal networks, and suggest a physiological role for interneuronal diversity. Furthermore, the results indicate that interneuronal heterogeneity can change in neurological diseases, and raise the possibility that neuromodulators may act by regulating the variance of key parameters in interneuronal populations.     

Plasticity of GABAA receptor subunit expression during the oestrous cycle of the rat: implications for premenstrual syndrome in women

Many women experience psychological changes during the luteal phase of their menstrual cycle. The late luteal (premenstrual) phase, when symptoms become most severe, is characterized by declining levels of ovarian progesterone. In female rats, withdrawal from prolonged dosing with progesterone leads to upregulation of alpha4 and delta subunits of the GABAA receptor in several brain regions. During the oestrous cycle of the rat, the natural fall in progesterone that occurs in late dioestrus is associated with a parallel increase in expression of alpha4, beta1 and delta GABAA receptor subunits in neurones in the periaqueductal grey matter (PAG), suggesting that new receptors of the alpha4beta1delta composition have been formed. Recombinant alpha4beta1delta receptors display a low EC50 for GABA, which is consistent with activation by extracellular levels of GABA. They are also likely to be located extrasynaptically and to carry tonic currents. In the PAG, a region involved in mediating panic-like anxiety, alpha4, beta1 and delta GABAA receptor subunits are located principally on GABAergic interneurones. On-going GABAergic neuronal activity normally limits and controls the excitability of the panic circuitry. During late dioestrus, when expression of alpha4, beta1 and delta subunits on GABAergic interneurones is upregulated, the increase in tonic current would be expected to lead to a reduction in the activity of the GABAergic population. Thus the panic circuitry would become intrinsically more excitable. It is suggested that during the menstrual cycle in women, plasticity of GABAA receptor subunit expression in brain regions such as the PAG, which are involved in mediating anxiety behaviour, may underlie some of the changes in mood that occur during the premenstrual period.     

Interaction between inhibitory pathways to principal cells in the lateral geniculate nucleus of the cat

Summary Inhibitory interactions between interneurones of the lateral geniculate nucleus (LGN) of the cat were studied with an indirect method based on intracellular recordings of synaptic responses in principal cells. Recurrent inhibitory postsynaptic potentials (IPSPs), evoked by antidromic activation of principal cell axons in the visual cortex, were depresse by a preceding stimulation of the optic tract or the visual cortex. Disynaptic feed-forward IPSPs, evoked by optic tract stimulation, were likewise depressed after cortex stimulation. The duration of the depression was in both cases about 100 ms. The effect was not due to conductance changes in the recorded principal cells or to activation of corticogeniculate fibres. The observations indicate that perigeniculate neurones, the recurrent inhibitory interneurones of the LGN, have mutual inhibitory connexions and that they also project to intrageniculate interneurones, the inhibitory cells in the feed-forward pathway to principal cells. These conclusions were supported by intracellular recordings from a few interneurones. No evidence was found for interaction between feed-forward interneurones activated from separate eyes or for a projection from intrageniculate interneurones to perigeniculate cells. The results point to an unexpected similarity in the organization of the recurrent inhibitory system of principal cells in the LGN and of spinal motoneurones. It is suggested that the recurrent system of the LGN serves as a variable gain regulator in analogy with a recently proposed model for the spinal system.     

Differential involvement of oriens/pyramidale interneurones in hippocampal network oscillations in vitro

Using whole-cell patch-clamp recordings in conjunction with post hoc anatomy we investigated the physiological properties of hippocampal stratum oriens and stratum pyramidale inhibitory interneurones, before and following the induction of pharmacologically evoked gamma frequency network oscillations. Prior to kainate-induced transient epochs of gamma activity, two distinct classes of oriens interneurones, oriens lacunosum-moleculare (O-LM) and trilaminar cells, showed prominent differences in their membrane and firing properties, as well as in the amplitude and kinetics of their excitatory postsynaptic events. In the active network both types of neurone received a phasic barrage of gamma frequency excitatory inputs but, due to their differential functional integration, showed clear differences in their output patterns. While O-LM cells fired intermittently at theta frequency, trilaminar interneurones discharged on every gamma cycle and showed a propensity to fire spike doublets. Two other classes of fast spiking interneurones, perisomatic targeting basket and bistratified cells, in the active network discharged predominantly single action potentials on every gamma cycle. Thus, within a locally excited network, O-LM cells are likely to provide a theta-frequency patterned output to distal dendritic segments, whereas basket and bistratified cells are involved in the generation of locally synchronous gamma band oscillations. The anatomy and output profile of trilaminar cells suggest they are involved in the projection of locally generated gamma rhythms to distal sites. Therefore a division of labour appears to exist whereby different frequencies and spatiotemporal properties of hippocampal rhythms are mediated by different interneurone subtypes.     

Differential expression of Na+/K+-ATPase α-subunits in mouse hippocampal interneurones and pyramidal cells

The sodium pump (Na+/K+-ATPase), maintains intracellular and extracellular concentrations of sodium and potassium by catalysing ATP. Three sodium pump α subunits, ATP1A1, ATP1A2 and ATP1A3, are expressed in brain. We compared their role in pyramidal cells and a subset of interneurones in the subiculum. Interneurones were identified by their expression of GFP under the GAD-65 promoter. We used the sensitivity to the cardiac glycoside, ouabain, to discriminate between different α subunit isoforms. GFP-positive interneurones were depolarized by nanomolar doses of ouabain, but higher concentrations were needed to depolarize pyramidal cells. Comparison of pump currents in these cells revealed a current sensitive to low doses of ouabain in interneurones, while micromolar doses of ouabain were needed to suppress the pump current in subicular pyramidal cells. As predicted, nanomolar doses of ouabain increased the frequency but not the amplitudes of IPSPs in pyramidal cells. Immunostaining confirmed a differential distribution of α-subunits of the Na+/K+-ATPase in subicular interneurones and pyramidal cells. In conclusion, these data suggest that while ATP1A3-isoforms regulate sodium and potassium homeostasis in subicular interneurones, ATP1A1-isoforms assume this function in pyramidal cells. This differential expression of sodium pump isoforms may contribute to differences in resting membrane potential of subicular interneurones and pyramidal cells.     

Ca2+ imaging of mouse neocortical interneurone dendrites: Ia-type K+ channels control action potential backpropagation

GABAergic interneurones are essential in cortical processing, yet the functional properties of their dendrites are still poorly understood. In this first study, we combined two-photon calcium imaging with whole-cell recording and anatomical reconstructions to examine the calcium dynamics during action potential (AP) backpropagation in three types of V1 supragranular interneurones: parvalbumin-positive fast spikers (FS), calretinin-positive irregular spikers (IS), and adapting cells (AD). Somatically generated APs actively backpropagated into the dendritic tree and evoked instantaneous calcium accumulations. Although voltage-gated calcium channels were expressed throughout the dendritic arbor, calcium signals during backpropagation of both single APs and AP trains were restricted to proximal dendrites. This spatial control of AP backpropagation was mediated by Ia-type potassium currents and could be mitigated by by previous synaptic activity. Further, we observed supralinear summation of calcium signals in synaptically activated dendritic compartments. Together, these findings indicate that in interneurons, dendritic AP propagation is synaptically regulated. We propose that interneurones have a perisomatic and a distal dendritic functional compartment, with different integrative functions.     

GABAergic synaptic communication in the GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) are the final integrative units of the cerebellar network. The strongest single afferent to the DCN is formed by GABAergic Purkinje neuron axons whose synapses constitute the majority of all synapses in the DCN, with their action strongly regulating the intrinsic activity of their target neurons. Although this is well established, it remains unclear whether all DCN cell groups receive a functionally similar inhibitory input. We previously characterized three types of mouse DCN neurons based on the expression of glutamic acid decarboxylase isoform 67 (GAD67), their active membrane properties and morphological features. Here we describe the GABAergic synapses in these cell groups and show that spontaneous GABAergic synaptic activity can be seen in all three cell types. Since the majority of DCN neurons fire action potentials spontaneously at high frequencies both in vivo and in vitro, we expected that spontaneous GABAergic synaptic activities mediated by intra-DCN synaptic connections could be uncovered by their sensitivity to TTX. However, TTX had little effect on spontaneous synaptic activity. It seems, therefore that functional GABAergic connectivity within the DCN is sparse and/or weak at least under our experimental conditions. Even though present in all cell types, the spontaneous GABAergic events showed significant differences between the cell types. The synaptic currents in GABAergic cells had lower amplitude, lower frequency and slower kinetics than those of non-GABAergic cells. These differences could not be sufficiently explained by considering only cell size differences or a differential GABA(A)-receptor alpha-subunit composition. Rather, the main differentiating factor appears to be the dendritic localization of GABAergic synapses in the GABAergic cells.     

Cholecystokinin-immunopositive basket and Schaffer collateral-associated interneurones target different domains of pyramidal cells in the CA1 area of the rat hippocampus.

Two types of GABAergic interneurone are known to express cholecystokinin-related peptides in the isocortex: basket cells, which preferentially innervate the somata and proximal dendrites of pyramidal cells; and double bouquet cells, which innervate distal dendrites and dendritic spines. In the hippocampus, cholecystokinin immunoreactivity has only been reported in basket cells. However, at least eight distinct GABAergic interneurone types terminate in the dendritic domain of CA1 pyramidal cells, some of them with as yet undetermined neurochemical characteristics. In order to establish whether more than one population of cholecystokinin-expressing interneurone exist in the hippocampus, we have performed whole-cell current clamp recordings from interneurones located in the stratum radiatum of the hippocampal CA1 region of developing rats. Recorded neurones were filled with biocytin to reveal their axonal targets, and were tested for the presence of pro-cholecystokinin immunoreactivity.The results show that two populations of cholecystokinin-immunoreactive interneurones exist in the CA1 area (n=15 positive cells). Cholecystokinin-positive basket cells (53%) preferentially innervate stratum pyramidale and adjacent strata oriens and radiatum. A second population of cholecystokinin-positive cells, previously described as Schaffer collateral-associated interneurones [Vida et al. (1998) J. Physiol. 506, 755-773], have axons that ramify almost exclusively in strata radiatum and oriens, overlapping with the Schaffer collateral/commissural pathway originating from CA3 pyramidal cells. Two of seven of the Schaffer collateral-associated cells were also immunopositive for calbindin. Soma position and orientation in stratum radiatum, the number and orientation of dendrites, and the passive and active membrane properties of the two cell populations are only slightly different. In addition, in stratum radiatum and its border with lacunosum of perfusion-fixed hippocampi, 31.6+/-3.8% (adult) or 26.8+/-2.9% (postnatal day 17-20) of cholecystokinin-positive cells were also immunoreactive for calbindin.Therefore, at least two populations of pro-cholecystokinin-immunopositive interneurones, basket and Schaffer collateral-associated cells, exist in the CA1 area of the hippocampus, and are probably homologous to cholecystokinin-immunopositive basket and double bouquet cells in the isocortex. It is not known if the GABAergic terminals of double bouquet cells are co-aligned with specific glutamatergic inputs. However, in the hippocampal CA1 area, it is clear that the terminals of Schaffer collateral-associated cells are co-stratified with the glutamatergic input from the CA3 area, with as yet unknown functional consequences. The division of the postsynaptic neuronal surface by two classes of GABAergic cell expressing cholecystokinin in both the hippocampus and isocortex provides further evidence for the uniform synaptic organisation of the cerebral cortex.     

Induced sharp wave-ripple complexes in the absence of synaptic inhibition in mouse hippocampal slices

The characteristic, behaviour-related network oscillations of the mammalian hippocampus (θ, γ and ripples) are accompanied by strongly phase-coupled action potentials in specific subsets of GABAergic interneurones. It has been suggested that the resulting phasic, repetitive inhibition shapes rhythmic coherent activity of the neuronal network. Here, we examined whether synaptic inhibition entrains ∼200 Hz network ripples by applying the GABA_A receptor antagonist gabazine to CA1 minislices of mouse hippocampus. Gabazine blocked spontaneously occurring sharp wave–ripple (SPW–R) activity. However, local application of KCl to the dendritic layer elicited excitatory sharp waves on which ∼200 Hz ripple oscillations were superimposed with equal temporal properties of native SPW–R. The activity also persisted in the additional presence of blockers of glutamatergic synaptic transmission. In contrast, synchrony was largely abolished after addition of gap junction blockers. Thus, GABAergic transmission appears to be involved in the generation of sharp waves but phasic inhibition is no prerequisite for the precise synchronization of hippocampal neurones during high-frequency oscillations at ∼200 Hz. Gap junctions on the other hand seem to be necessary to orchestrate coordinated activity within the ripple frequency domain.     

Disruption of GABA(A) receptors on GABAergic interneurons leads to increased oscillatory power in the olfactory bulb network.

Synchronized neural activity is believed to be essential for many CNS functions, including neuronal development, sensory perception, and memory formation. In several brain areas GABA(A) receptor-mediated synaptic inhibition is thought to be important for the generation of synchronous network activity. We have used GABA(A) receptor beta3 subunit deficient mice (beta3-/-) to study the role of GABAergic inhibition in the generation of network oscillations in the olfactory bulb (OB) and to reveal the role of such oscillations in olfaction. The expression of functional GABA(A) receptors was drastically reduced (>93%) in beta3-/- granule cells, the local inhibitory interneurons of the OB. This was revealed by a large reduction of muscimol-evoked whole-cell current and the total current mediated by spontaneous, miniature inhibitory postsynaptic currents (mIPSCs). In beta3-/- mitral/tufted cells (principal cells), there was a two-fold increase in mIPSC amplitudes without any significant change in their kinetics or frequency. In parallel with the altered inhibition, there was a significant increase in the amplitude of theta (80% increase) and gamma (178% increase) frequency oscillations in beta3-/- OBs recorded in vivo from freely moving mice. In odor discrimination tests, we found beta3-/- mice to be initially the same as, but better with experience than beta3+/+ mice in distinguishing closely related monomolecular alcohols. However, beta3-/- mice were initially better and then worse with practice than control mice in distinguishing closely related mixtures of alcohols. Our results indicate that the disruption of GABA(A) receptor-mediated synaptic inhibition of GABAergic interneurons and the augmentation of IPSCs in principal cells result in increased network oscillations in the OB with complex effects on olfactory discrimination, which can be explained by an increase in the size or effective power of oscillating neural cell assemblies among the mitral cells of beta3-/- mice.     

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

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

On the Purkinje cell activity increase induced by suppression of inferior olive activity

Summary Previous experiments performed in rats under barbiturate anaesthesia have shown a remarkable increase of simple spike firing rate in cerebellar Purkinje cells following inferior olive lesion or inactivation. The increase is due, at least in part, to the withdrawal of the tonic background activity of corticocerebellar interneurones, which have GABA as a chemical transmitter. Since barbiturates potentiate GABAergic inhibition, it is possible that the effect is due to the presence of this type of anaesthesia. In absence of general anaesthesia, we have compared the simple spike firing rate of the Purkinje cells in rats with intact inferior olive and 3–5 days after inferior olive lesion by 3-acetylpyridine. In the latter condition, the firing rate is significantly higher. In other rats, under urethane anaesthesia, which is not known to interfere with GABAergic transmission, the inferior olive has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Following cooling of the inferior olive on one side, a remarkable increase of simple spike activity, parallel to the disappearance of complex spike activity, has been observed in the Purkinje cells of the contralateral side. These results show that the presence of the simple spike firing increase, which follows the removal of the climbing fibre activity, does not depend on an anaesthetic which potentiates GABAergic transmission, although its amplitude is affected by the same anaesthetic. They suggest, therefore, that the tonic inhibition exerted by the olivocerebellar pathway on the Purkinje cells operates also in physiological conditions. By analysing the pattern of discharge of the Purkinje cell simple spikes in intact rats and following suppression of inferior olive activity, we have seen that, in the latter condition, the highest firing rate is accompanied by a higher degree of regularity. This change of pattern does not depend significantly on the removal of an irregularity induced by the complex spikes, but it is mainly the consequence of the firing rate increase.     

Adenosine down-regulates giant depolarizing potentials in the developing rat hippocampus by exerting a negative control on glutamatergic inputs

Adenosine is a widespread neuromodulator that can be directly released in the extracellular space during sustained network activity or can be generated as the breakdown product of adenosine triphosphate (ATP). Whole cell patch-clamp recordings were performed from CA3 principal cells and interneurons in hippocampal slices obtained from P2-P7 neonatal rats to study the modulatory effects of adenosine on giant depolarizing potentials (GDPs) that constitute the hallmark of developmental networks. We found that GDPs were extremely sensitive to the inhibitory action of adenosine (IC(50) = 0.52 microM). Adenosine also contributed to the depressant effect of ATP as indicated by DPCPX-sensitive changes of ATP-induced reduction of GDP frequency. Similarly, adenosine exerted a strong inhibitory action on spontaneous glutamatergic synaptic events recorded from GABAergic interneurons and on interictal bursts that developed in CA3 principal cells after blockade of gamma-aminobutyric acid type A (GABA(A)) receptors with bicuculline. All these effects were prevented by DPCPX, indicating the involvement of inhibitory A1 receptors. In contrast, GABAergic synaptic events were not changed by adenosine. Consistent with the endogenous role of adenosine on network activity, DPCPX per se increased the frequency of GDPs, interictal bursts, and spontaneous glutamatergic synaptic events recorded from GABAergic interneurons. Moreover, the adenosine transport inhibitor NBTI and the adenosine deaminase blocker EHNA decreased the frequency of GDPs, thus providing further evidence that endogenous adenosine exerts a powerful control on GDP generation. We conclude that, in the neonatal rat hippocampus, the inhibitory action of adenosine on GDPs arises from the negative control of glutamatergic, but not GABAergic, inputs.