Synaptic activation in guinea-pig dentate area: dependence on the stimulation site

Dissecting deep brain stimulation evoked neural activity in the basal ganglia

Deep brain stimulation (DBS) is an established therapeutic tool for the treatment of Parkinson's disease (PD). The mechanisms of DBS for PD are likely rooted in modulation of the subthalamo-pallidal network. However, it can be difficult to electrophysiologically interrogate that network in human patients. The recent identification of large amplitude evoked potential (EP) oscillations from DBS in the subthalamic nucleus (STN) or globus pallidus internus (GPi) are providing new scientific opportunities to expand understanding of human basal ganglia network activity. In turn, the goal of this review is to provide a summary of DBS-induced EPs in the basal ganglia and attempt to explain various components of the EP waveforms from their likely network origins. Our analyses suggest that DBS-induced antidromic activation of globus pallidus externus (GPe) is a key driver of these oscillatory EPs, independent of stimulation location (i.e. STN or GPi). This suggests a potentially more important role for GPe in the mechanisms of DBS for PD than typically assumed. And from a practical perspective, DBS EPs are poised to become clinically useful electrophysiological biomarker signals for verification of DBS target engagement.

GABA(B) receptors couple to potassium and calcium channels on identified lateral perforant pathway projection neurons

Activation of presynaptic GABA(B) receptors inhibits neurotransmitter release at most cortical synapses, at least in part because of inhibition of voltage-gated calcium channels. One synapse where this is not the case is the lateral perforant pathway synapse onto dentate granule cells in the hippocampus. The current study was conducted to determine whether the neurons that make these synapses express GABA(B) receptors that can couple to ion channels. Perforant pathway projection neurons were labeled by injecting retrograde tracer into the dorsal hippocampus. The GABA(B) receptor agonist baclofen (10 microM) activated inwardly rectifying potassium channels and inhibited currents mediated by voltage-gated calcium channels in retrogradely labeled neurons in layer II of the lateral entorhinal cortex. These effects were reversed by coapplication of the selective GABA(B) receptor antagonist CGP 55845A (1 microM). Equivalent effects were produced by 100 microM adenosine, which inhibits neurotransmitter release at lateral perforant pathway synapses. The effects of baclofen and adenosine on inward currents were largely occlusive. These results suggest that the absence of GABA(B) receptor-mediated presynaptic inhibition at lateral perforant pathway synapses is not simply due to a failure to express these receptors and imply that GABA(B) receptors can either be selectively localized or regulated at terminal versus somatodendritic domains.

Effects of variations in hippocampal slice preparation protocol on the electrophysiological stability, epileptogenicity and graded hypoxia responses of CA1 neurons

Evoked, extracellularly recorded field potentials and whole-cell current-clamp recordings were used to assay the effects of variations in dissection method and incubation temperature on the electrophysiology of CA1 neurons in hippocampal slices. Slices were cut with either a vibratome or a tissue chopper, and incubated at 28-30 degrees C, room temperature (19-21 degrees C), or in cool solution (13-15 degrees C) which was allowed to passively warm to room temperature while the slices were incubating ('cold-shock', CS). Although no effects of dissection method were observed, it was found that incubation temperature had profound effects on synaptically, but not non-synaptically evoked field potentials. Cold-shocked slices, cut with either a vibratome or a tissue chopper, exhibited epileptiform and spontaneously potentiating orthodromic field potentials. Slices incubated at warmer temperatures demonstrated responses that were larger in amplitude, more stable and much less epileptiform. In response to orthodromic stimulation, CS neurons fired more action potentials than did neurons in slices incubated at room temperature. Further, CS neurons generated smaller inhibitory postsynaptic potentials. Field potential changes resulting from graded hypoxia were not significantly affected by the temperature at which the slices were incubated. These data suggest that the electrophysiology of the hippocampal slice can be altered by the methods used to prepare the tissue. This finding may account for some of the discrepancies that exist between laboratories, and serves to underscore the importance of accurately reporting detailed protocols. Further, CS hippocampal tissue may represent a novel in vitro model of epileptiform activity.

Kainic acid-induced seizures enhance dentate gyrus inhibition by downregulation of GABA(B) receptors

Seizures cause a persistent enhancement in dentate synaptic inhibition concurrent with, and possibly compensatory for, seizure-induced hippocampal hyperexcitability. To study this phenomenon, we evoked status epilepticus in rats with systemic kainic acid (KA), and 2 weeks later assessed granule cell inhibition with paired-pulse stimulation of the perforant path (PP) in vitro. Controls demonstrated three components of paired-pulse inhibition: early inhibition (10-30 msec), intermediate facilitation (30-120 msec), and late inhibition (120 msec to 120 sec). After seizures, inhibition in all components was enhanced significantly. The GABA(A) antagonist bicuculline blocked only early enhanced inhibition, demonstrating that both GABA(A) and GABA(B) postsynaptic receptors contribute to seizure-induced enhanced inhibition. In controls, the GABA(B) antagonist CGP 35348 increased both GABA(A) and GABA(B) responses in granule cells, suggesting that CGP 35348 acts presynaptically, blocking receptors that suppress GABA release. In contrast, slices from KA-treated rats were markedly less sensitive to CGP 35348. To test the hypothesis that GABA(B) receptors regulating GABA release are downregulated after seizures, we measured paired-pulse suppression of recurrent IPSPs, or disinhibition, using mossy fiber stimuli. Early disinhibition (< 200 msec) was reduced after seizures, whereas late disinhibition remained intact. CGP 35348 blocked the early component of disinhibition in controls and, to a lesser extent, reduced disinhibition in KA slices. However, paired monosynaptic IPSPs recorded intracellularly showed no difference in disinhibition between groups. Our findings indicate that seizure-induced enhancement in dentate inhibition is caused, at least in part, by reduced GABA(B) function in the polysynaptic recurrent inhibitory circuit, resulting in reduced disinhibition and heightened GABA release.

Spontaneous and synaptic input from granule cells and the perforant path to dentate basket cells in the rat hippocampus

To characterize excitatory inputs to dentate basket cells from dentate granule cells and the perforant path, the whole-cell recording technique was used in neonatal rat hippocampal slices. Spontaneous excitatory input to basket cells was also examined and compared to that of other interneurons in the dentate gyrus. Basket cells were separable from other neurons in the dentate gyrus based on morphology and location, as determined by biocytin staining following recording, and by resting membrane potential, propensity to fire action potentials spontaneously, and miniature excitatory postsynaptic current (EPSC) characteristics. Minimal electrical stimulation of the granule cell layer evoked in basket cells short latency EPSCs that were composed of both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) components as judged by their time course, voltage dependence, and blockade by selective antagonists. Perforant path EPSCs exhibited slower kinetics than EPSCs evoked by granule cell stimulation. Like granule cell evoked EPSCs, however, perforant path EPSCs were composed of both NMDA and AMPA components. Minimal electrical stimulation of the granule cell layer and perforant path evoked monosynaptic EPSCs in only 67% and 62% of the trials, respectively, suggesting that these inputs are as unreliable as previously determined inputs from CA3 pyramidal cells (48%). Tetrodotoxin-insensitive spontaneous miniature EPSCs were frequent in basket cells and non-basket interneurons residing either at the border between the granule cell layer and the hilus or deep within the hilus. Miniature EPSCs recorded from all cells held at -70 mV were blocked completely by 3 microM 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). Though a component of the miniature EPSCs recorded from border and deep hilar interneurons at +40 mV was blocked by the NMDA receptor antagonist D-2-amino-phosphonovaleric acid (D-APV), miniature EPSCs in basket cells were insensitive to D-APV. We conclude that input from granule cells and the perforant path results in activation of basket cells via glutamatergic synapses that employ both NMDA and AMPA receptors. These inputs to basket cells likely contribute to feedback and feedforward inhibition of granule cells. The absence of an NMDA receptor component in spontaneous miniature EPSCs of dentate basket cells implies a difference in organization of excitatory synapses made onto basket cells compared with other hilar interneurons.

Muscarinic amplification of fast excitation in hilar neurones and inhibition in granule cells in the guinea-pig hippocampus

1. Effects of the cholinergic agonist, carbachol (CCh), or the acetylcholinesterase inhibitor, eserine, on presumed inhibitory hilar neurones and on inhibition in granule cells were studied by intracellular recording in guinea-pig hippocampal slices. 2. CCh (1-5 microM) strongly enhanced the discharge activity of hilar neurones and spontaneous and evoked IPSPs in granule cells. 3. Eserine, in an atropine-sensitive manner, increased the excitability of hilar neurones through effects on membrane properties and on excitatory synaptic barrage. EPSPs readily triggered long-duration burst discharges. In granule cells, the amplitudes of evoked GABAA and GABAB receptor-mediated IPSPs were enhanced. 4. In the presence of eserine and antagonists for glutamatergic and GABAergic synaptic transmission, train stimulation evoked atropine-sensitive slow EPSPs. In contrast to those in granule cells, slow EPSPs in hilar neurones were invariably preceded by a strong burst-after-hyperpolarization. 5. We suggest that acetylcholine, released from septo-hippocampal fibres, amplifies fast synaptic excitation of inhibitory hilar neurones and inhibition of granule cells. In the dentate area, muscarinic receptor-mediated effects are faster than anticipated from the time course of the slow EPSP.

Ipsilateral associational pathway in the dentate gyrus: an excitatory feedback system that supports N-methyl-D-aspartate-dependent long-term potentiation

Axons from granule cells in the dentate gyrus of the rat hippocampus project to cells in the hilar region, including mossy cells, which project along the longitudinal axis of the hippocampus and synapse in the inner (proximal) one-third of the molecular layer of the dentate gyrus. To study this feedback system, multiple recording electrodes were located along the longitudinal (septo-temporal) axis in the dorsal leaf of the dentate gyrus in urethane-anesthetized rats. Single pulse electrical stimuli delivered to the hilar region evoked negative-going, monosynaptic field potentials that were largest in the inner one-third of the molecular layer (commissural zone). These evoked field potentials (EFPs) were recorded simultaneously at three to five locations. The latency to onset and peak amplitude of the EFP varied linearly with distance from point of stimulation, and EFPs were elicited in both directions along the longitudinal axis. The transmission speed was estimated to be 1.4 m/s. Tetanic stimulation of the hilar region potentiated the EFP slopes (mean = 26%). Potentiation lasted at least 2 hours and was specific to responses from the tetanized stimulating electrode; the responses to other stimulating electrodes in the hilus and the angular bundle of the perforant path changed less than 4%. Combined stimulation of the hilus and the medial perforant path increased the magnitude of recorded field potentials and population spikes, demonstrating that both pathways are excitatory. NMDA antagonist NPC-17742 blocked potentiation of EFP slopes in both the medial perforant path and hilus pathways. The results suggest that the ipsilateral associational system of the dentate gyrus is excitatory and capable of supporting long-lasting NMDA-dependent, synapse-specific plasticity.

Divergence of hippocampal mossy fibers

By connecting the fascia dentata with the hippocampus proper, the axons of the granule cells, the mossy fibers, represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. In this review the various synaptic connections of the mossy fibers are discussed. It turns out that the mossy fibers do not only establish synapses with the pyramidal neurons of regio inferior as traditionally assumed, but a variety of local circuit neurons as well as projection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the "trisynaptic" pathway at the level of the mossy fibers. Similarly, the pattern of afferent input to the granule cells, especially that of GABAergic neurons, is more complex than previously assumed. In this respect the concept of a unidirectional "trisynaptic" pathway certainly is an oversimplification. In particular, the hilus of the fascia dentata, that the mossy fibers traverse on their way to regio inferior, is often neglected in this concept. The hilar region comprises a large variety of morphologically and functionally distinct neuronal types that, to a large extent, are targets of hilar mossy fiber collaterals. By focusing on the mossy fiber system, an attempt is made in this review to summarize new data on hippocampal circuitries that have been accumulated since the original description of the trisynaptic pathway. This concept, which originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neurons, and the synapses of the Schaffer collaterals on CA1 pyramidal cells, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.