Expansion of receptive fields in motor cortex by local blockade of GABAA receptors

Experiments were done in -choralose anesthetized cats to determine whether local disinhibition would expand the sensory receptive field (RF) of motor cortical neurons. Most of the neurons (n = 17) responded only to a rapid high velocity "tap" of the paw or forearm, often requiring movement of a joint, while four cells responded to light touch of the skin. The receptive field of single neurons was re-examined after microiontophoretic ejection of bicuculline (BIC). In all 21 neurons examined, BIC produced an expansion of the RF (mean 4 times before drug). Expansion was seen most often in the proximal-distal axis (17 neurons) but was also commonly seen in the mediolateral axis (9 neurons). The expansion was usually restricted to the dorsal or ventral surface that the original RF was on; in only three neurons in which the pre-drug RF was on the dorsal surface of the paw did the expansion include part or the entire ventral surface. Response thresholds could only be tested in those neurons with touch RFs and showed no evidence of a change within the original RF of these cells. Local disinhibition has previously been shown to allow for the functional linking of motor cortical points, a mechanism that may be involved in the recruitment of movement related muscle synergies. The present results suggest that this may be also accompanied by expansion of the receptive fields. Such a receptive field expansion may be of functional value since motor cortical output neurons would receive sensory input integrated over a larger area of the limb. The role of local inhibitory control of sensory inputs to motor cortex neurons may thus be different than that in sensory cortex where it is thought to restrict receptive field size.     

Long-term potentiation of evoked and spontaneous neuronal activity in the grafted hippocampus

Summary Adult rats with unilateral fimbria-fornix lesion received fetal hippocampal grafts into the lesion cavity. Seven to ten months after the transplantation the graft was examined for long-term potentiation (LTP) in response to host hippocampus and direct graft stimulation. High frequency tetanizing trains delivered to either the host hippocampus or the graft resulted in augmented field potentials and prolonged neuronal discharges in the graft lasting several hours. Very low currents (10–30 A) were required to induce LTP by direct graft stimulation. In addition to the enhancement of evoked responses, the frequency of occurrence of spontaneously occurring EEG spikes and concurrent population neuronal bursts in the graft increased significantly after the tetanizing trains. These findings suggest that the physiological-biochemical mechanisms required for plastic changes of synaptic efficacy are present in the grafted hippocampus.     

Potent depressant effects of adenosine analogs on hippocampal slow-wave activity in the unanesthetized rat

Summary Adenosine and its analogs have previously been shown to exert a depressant effect on several measures of hippocampal excitability in the hippocampal slice and intact anesthetized preparation. In the present report, we examined the effects of intraventricular injections of adenosine analogs on hippocampal slow-wave activity in the freely moving rat. Each of three adenosine analogs— 5-N-ethylcarboxamidoadenosine (NECA) and N⁶-(phenylisopropyl) adenosine (L- and D-PIA) — were found to strongly suppress hippocampal electroencephalographic (EEG) activity. For instance, low doses of NECA (0.5 g) produced an 80–90% decrease in the amplitude of the hippocampal EEG. NECA was approximately 20-fold more potent than L-PIA, and L-PIA was twice the potency of D-PIA. In separate experiments in the anesthetized rat, NECA and L-PIA were found to block completely the activation of the hippocampal theta rhythm elicited with brainstem stimulation. The effects of adenosine analogs on both the hippocampal EEG and theta rhythm were very effectively reversed with methylxanthine, 8-para-sulphophenyl-theophylline (8-PSPT). The present findings demonstrate that adenosine analogs exert a powerful depressant effect on the hippocampal EEG in the natural unanesthetized state, and suggest that changes in the levels of endogenous adenosine may play a significant role in modulating the normal activity and function of the hippocampus.This work was supported in part by National Science Foundation grant BNS-8403544 to RPV     

The effect of a GABAA agonist muscimol on acoustic injury of the mouse cochlea

Gamma-aminobutyric acid (GABA) is one of major inhibitory neurotransmitter in the central nervous system and constitutes the cochlear efferent system. Glutamate excitotoxicity is implicated in the pathogenesis of acoustic injury of the cochlea. The present work investigated whether GABA(A) agonist muscimol can alleviate acoustic injury. Mice were exposed to a 4 kHz pure tone of 128 dB SPL for 4h. Muscimol and/or bicuculline, a GABA(A) antagonist, were intraperitoneally administered immediately before the onset of acoustic overexposure. The threshold shifts of the auditory brainstem response (ABR) and cochlear morphology after acoustic overexposure were then evaluated. Muscimol significantly decreased the ABR threshold shift and inhibited swelling of the afferent dendrites induced by acoustic overexposure. In addition, bicuculline inhibited the effects of muscimol. These findings suggest that activation of GABA(A) receptors reduces acoustic injury of the cochlea.     

Involvement of non-NMDA receptors in picrotoxin-induced epileptiform activity in the hippocampus

The ability of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to suppress picrotoxin-induced epileptiform burst activity was examined. Intracellular recordings were obtained from hippocampal CA1 and CA3 pyramidal neurons maintained in vitro. Bath application of CNQX (5 microM) significantly reduced or abolished evoked paroxysmal depolarizing shifts (PDSs) in all CA1 and CA3 neurons tested. In cells where a CNQX-insensitive component in the PDS was manifest, this remaining activity was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovaleric acid (20 microM), suggesting the existence of a NMDA-mediated synaptic potential. Our results indicate that non-NMDA receptor antagonists are capable of markedly reducing picrotoxin-induced epileptiform activity and that these receptors play an important role in generation of PDSs.     

Hippocampal theta rhythms from CA1 and dentate generators during paradoxical sleep of the rat: Differential alterations after septal lesion

Hippocampal theta (θ) rhythm was recorded from CA1 and dentate generators respectively, before and after septal lesion in the freely moving rat. It was observed that θ recorded from CA1 generator and θ recorded from dentate generator can be differentially affected by the lesion. In agreement with our previous data, these findings strongly support the hypothesis that CA1 and dentate generators producing the hippocampal θ are functionally independent. The existence of two independent septo-hippocampal neural systems which might mediate θCA1 and θ dentate separately is discussed.     

Propagation velocity of epileptiform activity in the hippocampus

Summary The propagation of epileptiform burst activity was investigated in the CA1 area of the in-vitro hippocampal slice preparation of the guinea pig. This activity was provoked by 0.1 mM 4-aminopyridine in the bathing medium and was recorded in the pyramidal layer with an array of eight electrodes. The delay between the first population spike of a burst recorded with different electrodes was calculated using the cross-correlation function. The propagation velocity was estimated from the delays and the electrode intervals. It was found that the velocity of spontaneous and evoked epileptiform bursts varies between 0.15 and 5 m/s and is not confined to the range of conduction velocities of the fibre systems in CA1 (0.3–0.55 and 1.0–1.8 m/s). Different velocities can be present in different parts of the CA1 area and the initiation of spontaneous bursts is not confined to the CA2–3 areas, but can also occur in CA1. Burst activity also propagated in a low calcium-high magnesium medium. Different mechanisms of propagation are discussed and it is argued that the propagation velocity due to ephaptic interaction may vary largely. It is concluded that epileptiform activity can be propagated not only by synaptic connections at or near the pyramidal layer, but also by way of electrical field effects of population spikes.     

Firing relations of medial septal neurons to the hippocampal theta rhythm in urethane anesthetized rats

Summary On the basis of spontaneous firing patterns and relations to the hippocampal theta rhythm, three cell types were identified within the medial septal nucleus and vertical limb of the nucleus of the diagonal band of Broca (MSN-NDB). In addition to the well known rhythmically bursting cells that fired in bursts on each cycle of the hippocampal theta rhythm, two other cell types are distinguished. Clock cells fired at high rates with a very regular, periodic firing pattern that was unrelated to the theta rhythm. Irregular cells fired at much lower rates, especially during theta rhythm, and had a pseudo-random firing pattern. The firing of irregular cells was often significantly phase-locked to the hippocampal theta rhythm. Crude estimates of the relative proportions of these cell types suggest that the rhythmically bursting cells comprise about 75% of the cells of the MSN-NDB. These three cell types bear a remarkable resemblance, in firing patterns and relative proportions, to the three principal cell types of the medial septal nuclei described in the freely moving rat (Ranck 1976). Measurements of the preferred phases of firing of 128 rhythmically bursting septal neurons (including 22 atropine-resistant and 11 atropine-sensitive cells) indicate that there is no single preferred phase of firing for the population. Rather the distribution of phases over the theta cycle is statistically flat. Variations in recording locations cannot account for this distribution since large differences in preferred phase were found for pairs of cells at the same location. Similarly, plotting only the group of cells identified as projection cells by antidromic activation from the fimbria/fornix, failed to reveal a peak in the distribution. In contrast to the rhythmically bursting cells, the distribution of preferred firing phases for the irregular cells with a significant phase-locking to the theta rhythm did have a clear peak. The peak occurred near the dentate theta rhythm positivity, consistent with the hypothesis that they are driven by feedback from CA1 complex-spike cells.     

Midbrain central gray GABAA receptor activation enhances,and blockade reduces,sexual behavior in the female rat

Summary The inhibitory neurotransmitter, GABA, has been implicated in the control of lordosis behavior. Previous studies indicate that modulation of GABA_A transmission can have dual effects on lordosis, being facilitative in the ventromedial hypothalamus and inhibitory in the preoptic area. The midbrain central gray (MCG) is also known to be an important neural site for regulating lordosis as well as defensive and escape behaviors, and plays an integral role in the control of nociception. Because of the multitude of behaviors regulated at the level of the MCG, we utilized a two-chamber testing apparatus that allowed simultaneous measurement of the females' proceptive (hopping and darting), receptive and rejection behaviors, as well as an index of nociception and general motor activity. We found that microinfusion of the GABA_A antagonist, bicuculline, into the MCG of steroid-primed female rats resulted in a significant decrease in lordosis and proceptive behaviors at 5 min post-infusion. There was full recovery to pretest levels by 60 min. Furthermore, microinfusion of the GABA_A agonist, muscimol, to estrogen-treated females that displayed low levels of receptivity and high levels of rejection behavior during a pretest, resulted in a significant increase in lordosis responding and a decrease in rejection behaviors. Neither drug significantly affected time spent in the vicinity of the male, motor activity or vocalizations. It is concluded that the decrease in lordosis resulting from blockade of GABA transmission is not solely due to the induction of antagonistic behaviors since there was no increase in rejections after bicuculline administration. The current findings are consistent with the interpretation that GABA facilitates lordosis in the MCG via disinhibition. When the retrograde tracer, Fluoro-gold, was infused into the same cannula sites in the MCG as the GABA_A drugs it demonstrated the presence of strong projections from the ventromedial nucleus, zona incerta, medullary reticular formation and spinal cord. These projections to the MCG may be important for the integration of the diverse behaviors regulated at the level of the MCG and GABAergic transmission may play a role in this integration.     

The effects of selective septal lesions on theta production in CA1 and the dentate gyrus of the hippocampus

The effects of selective lesions of the lateral, medial or entire septum on theta production in CA1 and dentate were studied in the chronically implanted rat. It was found that lateral septal lesions disrupted theta production in the CA1 area of the hippocampus but did not affect dentate theta. Small medial septal lesions affected both CA1 and dentate theta. Total septal lesions abolished both CA1 and dentate theta. These results suggest that the pathways mediating the theta inputs to the CA1 and dentate generators are at one point anatomically distinct.     

Sensory evoked neuronal activity in the hippocampus before and after lesions of the medial septal nuclei

The evoked activity of 142 single neurons in the hippocampus of 44 rats was recorded following presentation of one of several modalities of peripheral stimulation. In the intact rat, 41% of the neurons encountered showed some period of inhibition of neuronal firing following peripheral stimulation. In subjects that had received lesions of the medial septal nuclei, only 21% of the cells encountered showed such inhibition. Medial septal lesions were made in 23 animals while recording the activity of cells which exhibited inhibition following stimulation. Of these 23 cells, only 5 continued to exhibit response inhibition after the lesion, while 13 displayed only response excitation, and 5 showed a complete loss of responsiveness. These findings suggest that the integrity of the medial septal nucleus is necessary for the inhibition of neuronal firing observed in many hippocampal neurons following peripheral stimulation.     

谷氨酸受体与γ-氨基丁酸_A受体在APPswe转基因小鼠海马结构中的表达

目的探讨谷氨酸受体和γ-氨基丁酸A(GABAA)受体在阿尔茨海默病(AD)发生、发展中的作用。方法利用免疫荧光技术标记正常及APPswe转基因小鼠P0至P360各年龄段海马CA3区N-甲基-D-天门冬氨酸受体亚单体1(NMDAR1)、GluR2/3和GABAARα1-6阳性细胞,并利用免疫印迹法对海马组织内NMDAR1和GABAARα1-6的活化片段进行半定量分析。结果在模型组与对照组小鼠海马CA3区,NMDAR1、GluR2/3和GABAARα1-6在出生后P7时明显增加,P30达到高峰,以后逐渐下降至成年水平;与同龄对照组相比,老龄(P360)模型鼠NMDAR1和GluR2/3阳性细胞密度明显减少(P0.05),而GABAARα1-6阳性细胞密度变化不明显(P0.05)。免疫印迹法检测结果与免疫荧光技术统计结果吻合。结论 NMDAR1和GluR2/3表达下降与AD相关的神经退行性疾病有关;而GABAARα1-6在AD发病中的作用尚不能确定。     

Disturbance of the correlation between hippocampal and septal EEGs during epileptogenesis

Field potentials of the hippocampus and the medial septal-diagonal band complex (MSDB) were recorded in the control and during the kindling stimulation of the perforant path in waking guinea pigs. Changes in the correlation of activities of these structures during stimulation-evoked seizures (model of acute epilepsy) and during epileptogenesis elicited by the kindling (model of chronic epilepsy) were analysed. In the control, a high correlation between the background activities of the hippocampus and MSDB was observed. In the first days of stimulation at the parameters that evoked seizure discharges in the hippocampus, the MSDB did not show the epileptiform activity; however, repeated daily stimulation gave rise to epileptiform discharges, which increased with time. As a result of kindling, the MSDB became capable of generating seizure activity irrespective of the hippocampus. The degree of correlation between the activities of the two structures sharply decreased during "acute" and "chronic" seizures. In the process of kindling, a progressive disintegration of activities of the hippocampus and MSDB was revealed, indicating the disturbance of the functioning of septohippocampal network during epileptogenesis. The data obtained add to the knowledge about the mechanisms of temporal lobe epilepsy and may help to develop new approaches to the therapy of this disease.     

Expert system approach to detection of epileptiform activity in the EEG

An expert system for the automated detection of spikes and sharp waves in the EEG has been developed. The system consists of two distinct stages. The first is a feature extractor, written in the conventional procedural language Fortran, which uses parts of previously published spike-detection, algorithms to produce a list of all spike-like occurrences in the EEG. The second stage, written in the production system language OPS5, reads the list and uses rules incorporating knowledge elicited from an electroencephalographer (EEGer) to confirm or exclude each of the possible spikes. Information such as the time of occurrence, polarity and channel relationship are used in this process. A summary of thedetected epileptiform events is produced which is available to the EEGer in interpreting the EEG. The performance of the expert system is compared with an EEGer using a 320s segment from an EEG containing epileptiform activity. The system detected 19 events and missed seven (false negative) which the EEGer considered epileptiform. There were no false positive detections.     

Effects of GABAergic inhibition on neocortical long-term potentiation in the chronically prepared rat

In this study we investigated the hypothesis whether P2-related differences tested in a visual priming paradigm are associated with theta phase-locking. We recorded the EEG from 31 electrodes and calculated phase-locking index and total power differences for frequencies between 2 and 20 Hz. ERPs (event-related potentials) were analyzed for P1, N1 and P2 components. P2 showed strongest task-related amplitude differences between congruent and incongruent targets. A source analyses was performed for the P2 component using sLoreta that revealed local generators of the P2 in parieto-occipital regions. Phase-locking analyses showed specific effects in the theta range (4-6 Hz) appearing in time windows at around the P2 component. We draw the conclusion that phase-locked theta reflect top-down regulation processes mediating information between memory systems and is in part involved in the modulation of the P2 component.     

Effects of intracellular calcium on GABAA receptors in mouse cortical neurons

 Using the patch-clamp technique, we studied the effect of intracellular Ca²⁺ on Cl^– current gated by type A γ-aminobutyric acid receptors (GABA_A) in mouse cortical neurons. When the rapid Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N’,N’-tetraacetic acid (BAPTA) was in the pipette solution, the GABA-activated Cl^– current amplitude decreased over time to 49 ± 7% of control. In contrast, equimolar replacement of BAPTA with ethylenebis(oxonitrilo)tetraacetate (EGTA) caused a 60 ± 10% increase in GABA current. An increased intracellular Ca²⁺ concentration caused a transient augmentation of the GABA current. This effect of Ca²⁺ was concentration dependent (10 nM to 34 μM). Ca²⁺ increased the amplitude of the current by enhancing the maximal response to GABA rather than by changing the affinity of the receptor to GABA (EC₅₀ = 5 ± 0.4 μM vs. 7 ± 0.3 μM). Both calmodulin (CaM) and a CaM kinase II inhibitor (200 μM) blocked the potentiating effect of Ca²⁺ suggesting that it was mediated by activation of CaM kinase II. We found that regulation of GABA_A receptors by intracellular Ca²⁺ in cortical neurons has important physiological implications since the potentiating effect of increasing the intracellular Ca²⁺ on responses to GABA was mimicked by activating excitatory receptors with 100 μM N-methyl-D-aspartate (NMDA). These findings suggest that modulation of GABA_A receptor activity by glutamate may be brought about via changes in intracellular Ca²⁺. Received: 20 May 1997 / Received after revision: 12 August 1997 / Accepted: 1 September 1997     

Action potentials and relations to the theta rhythm of medial septal neurons in vivo

The influence of the medial septal nucleus and the nucleus of the diagonal band of Broca (MS-DB) on the hippocampal theta rhythm includes both cholinergic and γ-aminobutyric acid (GABAergic) components. To understand the intrinsic septal interactions and the separate contributions of the cholinergic and GABAergic septohippocampal neurons to the theta rhythm in behaving animals, it is essential to be able to identify these two classes from extracellular recordings. Here the durations of extracellularly recorded action potentials are compared with the other characteristics of the neurons. Extracellular recordings were taken from neurons of the MS-DB both in freely moving rats (114 cells) and in urethane-anesthetized rats (112 cells). These were compared with intracellular recordings taken from MS-DB neurons in urethane-anesthetized rats (58 cells). Hippocampal EEG was recorded from above the CA1 pyramidal cell layer (CA1 theta) and near the hippocampal fissure (dentate theta) to compare the firing phase across cells. Here it is shown that two major types of rhythmically bursting cells in the MS-DB that had been distinguished previously in intracellular recordings in vivo are also separable in extracellular recordings in vivo on the basis of the durations of their action potentials. In both awake and anesthetized rats the main properties of the two cell types were found to differ: firing rate, phase-relation to the hippocampal theta rhythm and sensitivity of their rhythmicity to blockade of muscarinic transmission. As was previously shown for intracellular recordings in anesthetized rats, it is shown here that in awake rats, too, the more rapidly firing brief-spike (putative GABAergic) cells fired with highest probability on the negative phase of the dentate theta, whereas the more slowly firing long-spike (putative cholinergic) cells fired mostly on the positive phase. Previous work showed that in intracellular recordings from anesthetized rats the rhythmic firing of most brief-spike cells was still retained even during muscarinic blockade, but that of most long-spike cells was lost. Here we also report a recategorization according to spike duration of existing extracellular recordings taken from anesthetized rats, confirming the above observation with much larger numbers of cells. Three additional major new findings are also reported here. (1) In awake rats, muscarinic blockade has relatively little effect on either cell type. (2) Under anesthesia, the firing rates of both cell types are lower than in awake rats, but the effect is greater on the long-spike cells, where the anesthesia also reduces the rhythmicity of the cell firing. (3) Rhythmicity of the putative GABAergic cells is also retained after local injection of GABA-A antagonist, whereas that of the putative cholinergic cells is eliminated. We conclude that either systemic muscarinic blockade or urethane anesthesia alone have relatively little effect on neurons in the defined above MS-DB, but a combination of the two has profound effects on the rhythmicity of the cholinergic cells, largely sparing the GABAergic cells. Taken together, the results suggest that generation of theta rhythm requires a background of excitatory influences on the hippocampus (that can be maintained by either muscarinic or glutamatergic inputs) in combination with the phasic disinhibitory action mediated by the GABAergic MS-DB projection. They also provide additional support for the notion that the phasic activity in local collaterals of GABAergic MS-DB cells contributes to the phasic modulation of the firing of cholinergic septohippocampal neurons. Received: 13 October 1998 / Accepted: 15 March 1999     

The role of orexin-1 receptors in physiologic responses evoked by microinjection of PgE2 or muscimol into the medial preoptic area

The medial preoptic area (mPOA) of the hypothalamus has long been thought to play an important role in both fever production and thermoregulation. Microinjections of prostaglandin E2 (PgE2) or the GABA(A) agonist muscimol into the mPOA cause similar increases in body temperature, heart rate, and blood pressure. Microinjections of these compounds however evoke different behavioral responses with muscimol increasing and PgE2 having no effect on locomotion. The purpose of this study was to determine the role of orexin-1 receptors in mediating these dissimilar responses. Systemic injections of the orexin-1 receptor antagonist SB-334867 reduced temperature and cardiovascular responses produced by microinjections of muscimol, but had no effect on either response produced by PgE2. SB-334867 did not significantly decrease locomotion evoked by microinjections of muscimol into the mPOA. These data suggest that there are two central nervous system circuits involved in increasing body temperature, heart rate and blood pressure: one circuit activated by muscimol, involving orexin neurons, and a separate orexin-independent circuit activated by PgE2.     

Induction and maintenance of epileptiform activity in the rabbit olfactory bulb depends on centrifugal input

Summary A technique of cryogenic blockade was used in waking rabbits to produce complete and reversible isolation of the olfactory bulb from the rest of the brain. During cooling of the olfactory peduncle epileptiform activity occurred spontaneously in the pyriform cortex in 3 out of 20 sessions, but never in the bulb. Following removal of the cryoblockade, during the seizure state, epileptiform discharges appeared simultaneously in the bulb and pyriform cortex. In the control state, without cooling of the peduncle, epileptiform activity could be evoked in the bulb and cortex by intense electrical stimulation of either the bulb or the lateral olfactory tract. During the cryoblockade, however, intense stimulation of the bulb failed to evoke seizure-like discharges. The results demonstrate a dependency on more central olfactory structures for the induction and maintenance of epileptiform activity in the olfactory bulb.This project was supported by a grant no. HL31164 from NIH     

Ischemia-induced changes in the electrical activity of the hippocampus

Summary Consequences of transient (15–20 min) ischemia on the neuronal activity of the dentate gyrus and hippocampal CA 1 region were investigated in chronically implanted Sprague-Dawley rats. Forebrain ischemia was produced by occlusion of the carotids for 15 or 20 min, following cauterization of the vertebral arteries. Following the release of the carotids, both spontaneous and evoked activity showed a steady but partial recovery, reaching a maximum 12 to 24 h after the ischemic insult. From this plateau, both the power of rhythmic slow activity recorded during walking and the power of slow delta activity obtained during alert immobility decreased monotonically, with large changes occuring between postischemic days 2 and 4. The changes in spontaneous activity were accompanied by a decrease and eventual disappearance of the Schaffer collateral evoked responses in CA 1. Perforant path volleys were less efficient in activating the granule cells following ischemia compared to baseline levels. This decreased responsiveness was paralleled by a relative impairment of paired pulse depression. Neurophysiological signs of spontaneous or evoked neuronal hyperexcitability were not observed at any time point during the 8 postischemic days. Neuronal damage in the CA 1 region varied from moderate to complete loss of pyramidal cells. In addition, degenerating neurons were also observed in the hilus of the dentate gyrus. These findings do not support the overwork version of the excitoxic hypothesis of delayed neuronal damage and indicate that the cause of ischemic cell death should be sought in factors other than neuronal hyperactivity.