Striatal, pallidal, and pars reticulata evoked inhibition of nigrostriatal dopaminergic neurons is mediated by GABA(A) receptors in vivo

Dopaminergic neurons express both GABA(A) and GABA(B) receptors and GABAergic inputs play a significant role in the afferent modulation of these neurons. Electrical stimulation of GABAergic pathways originating in neostriatum, globus pallidus or substantia nigra pars reticulata produces inhibition of dopaminergic neurons in vivo. Despite a number of prior studies, the identity of the GABAergic receptor subtype(s) mediating the inhibition evoked by electrical stimulation of neostriatum, globus pallidus, or the axon collaterals of the projection neurons from substantia nigra pars reticulata in vivo remain uncertain. Single-unit extracellular recordings were obtained from substantia nigra dopaminergic neurons in urethane anesthetized rats. The effects of local pressure application of the selective GABA(A) antagonists, bicuculline and picrotoxin, and the GABA(B) antagonists, saclofen and CGP-55845A, on the inhibition of dopaminergic neurons elicited by single-pulse electrical stimulation of striatum, globus pallidus, and the thalamic axon terminals of the substantia nigra pars reticulata projection neurons were recorded in vivo. Striatal, pallidal, and thalamic induced inhibition of dopaminergic neurons was always attenuated or completely abolished by local application of the GABA(A) antagonists. In contrast, the GABA(B) antagonists, saclofen or CGP-55845A, did not block or attenuate the stimulus-induced inhibition and at times even increased the magnitude and/or duration of the evoked inhibition. Train stimulation of globus pallidus and striatum also produced an inhibition of firing in dopaminergic neurons of longer duration. However this inhibition was largely insensitive to either GABA(A) or GABA(B) antagonists although the GABA(A) antagonists consistently blocked the early portion of the inhibitory period indicating the presence of a GABA(A) component. These data demonstrate that dopaminergic neurons of the substantia nigra pars compacta are inhibited by electrical stimulation of striatum, globus pallidus, and the projection neurons of substantia nigra pars reticulata in vivo. This inhibition appears to be mediated via the GABA(A) receptor subtype, and all three GABAergic afferents studied appear to possess inhibitory presynaptic GABA(B) autoreceptors that are active under physiological conditions in vivo.     

Mechanisms involved in tetanus-induced potentiation of fast IPSCs in rat hippocampal CA1 neurons

In the present study, possible mechanisms involved in the tetanus-induced potentiation of gamma-aminobutyric acid-A (GABA-A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were investigated using the whole cell voltage-clamp technique on CA1 neurons in rat hippocampal slices. Stimulations (100 Hz) of the stratum radiatum, while voltage-clamping the membrane potential of neurons, induces a long-term potentiation (LTP) of evoked fast IPSCs while increasing the number but not the amplitude of spontaneous IPSCs (sIPSCs). The potentiation of fast IPSCs was input specific. During the period of IPSC potentiation, postsynaptic responses produced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride and baclofen, GABA-A and GABA-B agonists respectively, were not significantly different from control. CGP 36742, a GABA-B antagonist, blocked the induction of tetanus-induced potentiation of evoked and spontaneous IPSCs, while GTPgammaS, an activator of G proteins, substitution for GTP in the postsynaptic recording electrode did not occlude potentiation. Since GABA-B receptors work through G proteins, our results suggest that pre- but not postsynaptic GABA-B receptors are involved in the potentiation of fast IPSCs. A tetanus delivered when GABA-A responses were completely blocked by bicuculline suggests that GABA-A receptor activation during tetanus is not essential for the induction of potentiation. Rp-cAMPs, an antagonist of protein kinase A (PKA) activation, blocks the induction of potentiation of fast IPSCs. Forskolin, an activator of PKA, increases baseline evoked IPSCs as well as the number of sIPSCs, and a tetanic stimulation during this enhancement uncovers a long-term depression of the evoked IPSC. Sulfhydryl alkylating agents, N-ethylmaleimide and p-chloromercuribenzoic acid, which have been found to presynaptically increase GABA release and have been suggested to have effects on proteins involved in transmitter release processes occurring in nerve terminals, occlude tetanus-induced potentiation of evoked and spontaneous IPSCs. Taken together our results suggest that LTP of IPSCs originates from a presynaptic site and that GABA-B receptor activation, cyclic AMP/PKA activation and sulfhydryl-alkylation are involved. Plasticity of IPSCs as observed in this study would have significant implications for network behavior in the hippocampus.     

Thalamic nociceptive mechanisms in cats, influenced by central conditioning stimuli.

Field potentials and single cell activity evoked by tooth pulp (TP) stimulation were studied in the ventrobasal (VB) complex of the cat. The experiments were performed using a conditioning-test paradigm. Evoked cell activity or field potentials following TP stimulation was used as a test. Conditioning stimulus was given to different regions of the thalamic central lateral nucleus (CL). Conditioning electrical stimulation in medial (ML 2.8-3.6 mm) parts of CL induced a depression of the TP evoked response in 10 cells. Stimulation sites in lateral CL (ML 3.6-4.2 mm) induced facilitation in eight cells and decreased activity in seven cells. Tooth pulp evoked field potentials in thalamus were facilitated by a preceding stimulation in lateral CL. Cells in the lateral parts of CL are suggested to induce an increased activity in cells in the VB complex which mediate nociceptive information. This effect is suggested to be mediated via a CL induced disinhibition at a reticular thalamic (RE) or at a VB complex level. The medial parts of CL seem to give a traditional feedback inhibition on VB cells. Such an effect is also suggested to be mediated via the RE complex. The importance of these findings are discussed with relation to changes in the thalamus that may occur following long lasting nociceptive stimulation.     

GABA in pedunculopontine tegmentum increases rapid eye movement sleep in freely moving rats: possible role of GABA-ergic inputs from substantia nigra pars reticulata

Pedunculopontine tegmentum (PPT) has GABA-ergic neurons and receives GABA-ergic projections from substantia nigra pars reticulata (SNrpr). Based on the recent studies from our and other laboratories, it was hypothesized that GABA in PPT promotes rapid eye movement (REM) sleep. In order to further study the role of GABA in PPT in REM sleep regulation, we microinjected GABA-A agonist, muscimol (200 nL, 3.5 mM), into the PPT. Muscimol in PPT significantly enhanced the amount of REM sleep by increasing the mean number of REM sleep bouts. Besides the local interneurons, GABA-ergic afferents from SNrpr are another source of GABA in PPT. In order to understand the contribution of GABA-ergic inputs from SNrpr into PPT for REM sleep regulation, SNrpr was electrically stimulated either alone or simultaneously along with the infusion of GABA-A antagonist, picrotoxin (200 nL, 0.86 mM), into the PPT. The experiment was designed with the premise that stimulation of SNrpr should increase GABA levels in PPT which should increase REM sleep comparable to that after muscimol microinjection in PPT. Further, the effect of stimulation of SNrpr on REM sleep should be antagonized by simultaneous infusion of picrotoxin into PPT. The electrical stimulation of SNrpr did not produce any significant change in sleep–wake states although it was sufficient to counter the effect of picrotoxin injection into the PPT. To overcome the limitations and confounds of electrical stimulation, SNrpr was pharmacologically stimulated by glutamate microinjection (200 nL, 5.34 mM). Infusion of glutamate into SNrpr enhanced REM sleep by increasing the mean number of REM sleep bouts, which was similar and comparable to the effect of muscimol injection into the PPT. The results confirm that GABA in PPT either from local neurons or from SNrpr promotes REM sleep.     

Brain creatine functions to attenuate acute stress responses through GABAnergic system in chicks

The involvement of brain creatine in the adaptation to acute stress responses was investigated in chicks. In experiment 1, brain creatine content of chicks exposed to social separation stress was significantly increased compared with control chicks. The effects of i.c.v. injection of creatine (2 mug) on vocalizations, spontaneous activity and plasma corticosterone concentration in chicks under social separation stress were investigated in experiment 2. All measurements were attenuated by the i.c.v. injection of creatine compared with the controls under separation stress. Creatine also significantly decreased the active posture, but increased the motionless eye-opened posture, compared with the control. To clarify the relationship between creatine function and GABA receptors, the i.c.v. co-injection of creatine with picrotoxin, a GABA-A receptor antagonist, or CGP54626, a GABA-B receptor antagonist, was investigated in experiments 3 and 4. The effects of creatine on vocalizations and spontaneous activity were attenuated by co-injection of picrotoxin. In this case, active postures decreased by creatine were recovered by co-injection with picrotoxin. However, these effects were not obtained with CGP54626. The results suggest that central creatine functions within the CNS to attenuate the acute stress response by acting through GABA-A receptors in chicks.     

Presence of α-1 norepinephrinergic and GABA-A receptors on medial preoptic hypothalamus thermosensitive neurons and their role in integrating brainstem ascending reticular activating system inputs in thermoregulation in rats

Thermal messages are relayed to the medial preoptic O-anterior hypothalamus (mPOAH) via the ascending reticular activating system (ARAS). According to previous findings that norepinephrine (NE)-ergic and GABA (gamma-amino butyric acid)-ergic inputs convey thermal information to the CNS, those neurotransmitters may be responsible for reciprocal correlation between body temperature and mPOAH warm-(WSNs) and cold-(CSNs) sensitive neuronal firing rates for thermoregulation. In this study on Wistar rats, we have characterized in vivo the role of α-1 NE-ergic and GABA-A receptors in the possible modulation of ARAS inputs to the thermosensitive neurons in the mPOAH. Nine WSNs, 7 CSNs and 19 thermo-insensitive neurons were recorded from mPOAH and effects of ARAS stimulation and iontophoretic application of prazosin as well as picrotoxin on those neurons were evaluated. The WSNs were excited by ARAS stimulation but inhibited by both prazosin and picrotoxin; whereas the CSNs were inhibited by ARAS stimulation and prazosin, but excited by picrotoxin. The NE excited the WSNs as well as the CSNs, while GABA had opposite effects on them, suggesting that NE and GABA interact in the mPOAH for thermoregulation. The findings unravel an intriguing possibility that in the mPOAH, GABA simultaneously acts on hetero-receptors located at pre-and post-synaptic sites, modulating the release of NE on the WSNs and CSNs for thermoregulation. Further, ARAS stimulation-induced similar excitatory and inhibitory responses of the WSNs and the CSNs support such converging inputs on these neurons for thermoregulation.     

Craniovascular application of capsaicin activates nociceptive thalamic neurones in the cat.

In cats anaesthetized with alpha-chloralose and urethane, extracellular recordings were made in ventrobasal thalamus from cells responding to electrical stimulation of the superior sagittal sinus and middle meningeal artery. Capsaicin, but not vehicle, evoked an increase in the firing rate of nociceptive cells (5 of 6 wide dynamic range and the only nociceptive specific cell). Non-nociceptive cells did not respond to either capsaicin or vehicle. Cells with long latencies to electrical stimulation were excited by capsaicin but cells with short latencies were not. Capsaicin-responsive cells were found in the ventroposteromedial nucleus and the medial nucleus of the posterior complex and mostly had receptive fields involving the first trigeminal division.     

GABA(A) receptor facilitation of neurokinin release from primary afferent terminals in the rat spinal cord

Our goal was to test the following hypotheses: 1) GABA_A receptors facilitate neurokinin release from primary afferent terminals; 2) they do this by suppressing an inhibitory effect of GABA_B receptors; 3) the activation of these two receptors is controlled by the firing frequency of primary afferents. We evoked neurokinin release by stimulating the dorsal root attached to spinal cord slices, and measured it using neurokinin 1 receptor (NK1R) internalization. Internalization evoked by root stimulation at 1 Hz (but not at 100 Hz) was increased by the GABA_A receptor agonists muscimol (effective concentration of drug for 50% of the increase [EC₅₀] 3 μM) and isoguvacine (EC₅₀ 4.5 μM). Internalization evoked by root stimulation at 100 Hz was inhibited by the GABA_A receptor antagonists bicuculline (effective concentration of drug for 50% of the inhibition [IC₅₀] 2 μM) and picrotoxin (IC₅₀ 243 nM). Internalization evoked by incubating the root with capsaicin (to selectively recruit nociceptive fibers) was increased by isoguvacine and abolished by picrotoxin. Therefore, GABA_A receptors facilitate neurokinin release. Isoguvacine-facilitated neurokinin release was inhibited by picrotoxin, low Cl⁻, low Ca²⁺, Ca²⁺ channel blockers and N-methyl-d-aspartate receptor antagonists. Bumetanide, an inhibitor of the Na⁺-K⁺-2Cl⁻ cotransporter, inhibited isoguvacine-facilitated neurokinin release, but this could be attributed to a direct inhibition of GABA_A receptors. The GABA_B agonist baclofen inhibited NK1R internalization evoked by 100 Hz root stimulation (IC₅₀ 1.5 μM), whereas the GABA_B receptor antagonist (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid (CGP-55845) increased NK1R internalization evoked by 1 Hz root stimulation (EC₅₀ 21 nM). Importantly, baclofen inhibited isoguvacine-facilitated neurokinin release, and CGP-55845 reversed the inhibition of neurokinin release by bicuculline. In conclusion, 1) GABA_B receptors located presynaptically in primary afferent terminals inhibit neurokinin release; 2) GABA_A receptors located in GABAergic interneurons facilitate neurokinin release by suppressing GABA release onto these GABA_B receptors; 3) high frequency firing of C-fibers stimulates neurokinin release by activating GABA_A receptors and inhibiting GABA_B receptors, whereas low frequency firing inhibits neurokinin release by the converse mechanisms.     

Mechanisms underlying LTP of inhibitory synaptic transmission in the deep cerebellar nuclei

Whole-cell recordings were used to investigate long-term potentiation of inhibitory synaptic currents (IPSCs) in neurons of deep cerebellar nuclei (DCN) in slices. IPSCs were evoked by electrical stimulation of the white matter surrounding the DCN in the presence of non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (20 microM). High-frequency stimulation induced a long-term potentation (LTP) of the IPSC amplitude without changing its reversal potential, rise time, and decay-time constant. This LTP did not require the activation of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors but depended on the activation of NMDA receptors. LTP of IPSCs in DCN neurons could also be induced by voltage-depolarizing pulses in postsynaptic neurons and appeared to depend on an increase in intracellular calcium as the LTP was blocked when the cells were loaded with a calcium chelator, 1,2-bis-(2-amino-phenoxy)-N,N,N', N'-tetraacetic acid (BAPTA, 10 mM). LTP of IPSCs was accompanied by an increase in the frequency of spontaneous IPSCs and miniature IPSCs (recorded in the presence of tetrodotoxin 1 microM), but there was no significant change in their amplitude. In addition, during the LTP, the amplitude of response to exogenously applied GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride was increased. Intracellular application of tetanus toxin, a powerful blocker of exocytosis, in DCN neuron prevented the induction of LTP of IPSCs. Our results suggest that the induction of LTP of IPSCs in the DCN neurons likely involves a postsynaptic locus. Plasticity of inhibitory synaptic transmission in DCN neurons may play a crucial role in cerebellar control of motor coordination and learning.     

Tooth pulp stimulation advances both medullary off-cell pause and tail flick

It has been postulated that the so-called off-cells of nucleus raphe magnus and adjacent structures in the rat are the output elements of a system which inhibits nociceptive transmission at the spinal cord. Off-cells stop firing about 0.4 s before the tail flick reflex (TF) elicited by the application of noxious heat to the tail. When continuous off-cell activity is induced by either morphine injection or periaqueductal gray stimulation, the TF is delayed. The present results show that electrical stimulation of the tooth pulp (TP) causes the off-cells to stop firing. Furthermore, when TP is stimulated during tail heating and before the expected time for TF, off-cells stop firing earlier and the TF occurs also earlier. This supports the notion that off-cells inhibit nociceptive transmission.     

GABAergic modulation of striatal peptide expression in rats and the alterations induced by dopamine antagonist treatment

GABAergic modulation of enkephalin, substance P and glutamic acid decarboxylase (GAD67) gene expression and the alterations induced by dopamine receptor blockade were studied in the rat striatum. Following subchronic treatment with the GABA-A agonist muscimol, the GABA-B agonist baclofen or the GABA transaminase inhibitor γ-vinyl GABA there were no significant changes in striatal peptide and GAD67 gene expression. Following repeated administration of the D-2 antagonists, eticlopride and haloperidol, there was an increase in enkephalin and GAD67 mRNA levels and parallel decrease in that of substance P. These were unaffected by co-administration of γ-vinyl GABA. The D-1 antagonist, SCH 23390 administered alone or together with γ-vinyl GABA did not alter peptide or GAD67 mRNA levels. It seems that pharmacological stimulation of GABA receptors has little effect on enkephalin, substance P or GAD67 mRNA expression in striatal output neurons.     

Electrophysiological properties of neurons in the rat subiculum in vitro

The present study determined the membrane and synaptic properties of neurons in the rat subiculum. Using the in vitro hippocampal slice preparation, intracellular recordings were obtained from 91 subicular neurons. Membrane properties and morphological characteristics were similar to those reported for hippocampal pyramidal neurons. Two categories of subicular neurons were distinguished based on their response to a depolarizing current pulse. One type of neuron showed bursting behavior and the second type was characterized as regular firing. Analysis of the charging functions during hyperpolarizing current pulses yielded a mean ₀ and ₁ for subicular neurons of about 13 ms and 0.60 ms, respectively. Using the model of an equivalent cylinder, the mean dendrite-to-soma conductance ratio () was estimated at 6.0 and electrotonic length constant (L) at 0.7. There was no difference in these values between bursting and regular firing neurons. Tetrodotoxin-resistant potentials (presumed calcium hump/spike) were evoked from bursting subicular neurons at lower current intensities than CA1 pyramidal neurons. Calcium humps could only be evoked from about half the regular firing subicular neurons. Subicular cells showed an excitatory/inhibitory postsynaptic potential (EPSP/IPSP) sequence in response to electrical stimulation in different layers of the CA1 area. An EPSP could also be evoked from stimulation of the superficial or deep layers of the presubiculum and was attributed to activation of entorhinal fibers of passage. At high stimulation intensity, an antidromic spike was often evoked following stimulation in the presubicular area or CA1 alveus. The evoked EPSPs were blocked by addition of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to the bathing medium. In magnesium-free, CN-QX bathing solution, a longer lasting depolarization was recorded; this response was blocked by application of a N-methyl-d-aspartate (NMDA) receptor antagonist (AP5). Iontophoretic application of glutamate or quisqualate (10 mM) along the soma-dendritic axis of subicular neurons leads to either a short-latency depolarization or a burst of action potentials. Application of 10 mM GABA near the recording site usually produced a hyperpolarization, which, at times, was mixed with a depolarization. Mixed hyperpolarizing/depolarizing responses were observed when GABA was applied to the basal or apical dendritic areas. There were no significant differences in the synaptic properties or responses to drug application between bursting and regular firing neurons. These results indicate that subicular neurons (1) are composed of a heterogeneous population of cell types, (2) have similar electrical properties to other hippocampal principal neurons, (3) receive glutaminergic synapses from CA1 and entorhinal cortical neurons, (4) project to the presubicular area and fornix (via the alveus), (5) are inhibited by local circuit neurons, and (6) display complex responses to GABA.     

Inhibitory postsynaptic potentials evoked in thalamic neurons by stimulation of the reticularis nucleus evoke slow spikes in isolated rat brain slices--I

In isolated slices of rat thalamus, inhibitory postsynaptic potentials evoked by electrical stimulation of the nucleus reticularis, were recorded intracellularly in relay neurons in the anterior part of the thalamus. These inhibitory postsynaptic potentials were found to have reversal potentials close to the resting potential of the recorded cell, to reduce neuronal excitability and to be sensitive to electrophoretic application of the GABA antagonists bicuculline and picrotoxin, indicating that they were GABA-activated, chloride mediated events. Voltage sensitive responses of relay neurons evoked by current injection and by inhibitory postsynaptic potentials were then compared. Hyperpolarizing current pulses and hyperpolarizing inhibitory postsynaptic potential trains elicited from membrane potentials positive to -70 mV resulted in rebound slow spike activation on repolarization. Depolarizing current pulses and depolarizing inhibitory postsynaptic potential trains evoked slow spikes when elicited from membrane potentials negative to -60 mV. There was, however, one major difference, the slow spikes evoked by inhibitory postsynaptic potentials were always delayed to the end of the train. Reversal potentials of evoked inhibitory postsynaptic potentials were found to depend on the potential at which the membrane was held immediately before the inhibitory postsynaptic potential was evoked, indicating that passive distribution of chloride ions contributes to their equilibrium potential. Evoked inhibitory postsynaptic potentials consisted of at least two components with different reversal potentials although current voltage relations indicated that similar decreases in membrane resistance were associated with both components and that they shifted approximately in parallel when inhibitory postsynaptic potentials were evoked from different holding potentials. Trains of GABA-mediated inhibitory postsynaptic potentials, similar to those recorded during spindling, will evoke slow spikes in almost all thalamic relay neurons irrespective of other synaptic inputs. This response will effectively synchronize burst firing in all cells receiving the same inhibitory input.     

Modulatory role of glutamic acid on the electrical activities of pain-related neurons in the hippocampal CA3 region

Glutamic acid (Glu) participates in pain modulation of the central nervous system. The CA3 region of the hippocampal formation has been suggested to be involved in nociceptive perception. However, it is unknown whether Glu could modulate the electrical activities of pain-related neurons in the hippocampal CA3 region. The present study aimed to determine the effects of Glu and its receptor antagonist MK-801 in the pain-evoked response of both pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the hippocampal CA3 region of normal rats. We used a train of electric impulses applied to the sciatic nerve as noxious stimulation. The electrical activities of either PENs or PINs in the hippocampal CA3 region were recorded by a glass microelectrode. The results revealed that intra-CA3 region microinjection of Glu (0.5 μg/1 μl) increased the evoked firing frequency and shortened the firing latency of PEN, while decreased the evoked firing frequency and prolonged the inhibitory duration of PIN in the hippocampal CA3 region of rat evoked by the noxious stimulation. Intra-CA3 region administration of MK-801 (0.25 μg/1 μl) produced the opposite response. These results suggest that Glu and its receptors in hippocampal CA3 region are involved in the modulation of nociceptive information transmission by affecting the electric activities of PENs and PINs.     

Synaptic and extrasynaptic GABA-A and GABA-B receptors in the globus pallidus: an electron microscopic immunogold analysis in monkeys

GABA-A and GABA-B receptors mediate differential effects in the CNS. To better understand the role of these receptors in regulating pallidal functions, we compared their subcellular and subsynaptic localization in the external and internal segments of the globus pallidus (GPe and GPi) in monkeys, using pre- and post-embedding immunocytochemistry with antibodies against GABA-A (alpha1, beta2/3 subunits) and GABA-BR1 receptor subtype. Our results demonstrate that GABA-A and GABA-B receptors display a differential pattern of subcellular and subsynaptic localization in both segments of the globus pallidus. The majority of GABA-BR1 immunolabeling is intracellular, whereas immunoreactivity for GABA-A receptor subunits is mostly bound to the plasma membrane. A significant proportion of both GABA-BR1 and GABA-A receptor immunolabeling is extrasynaptic, but GABA-A receptor subunits also aggregate in the main body of putative GABAergic symmetric synapses established by striatal- and pallidal-like terminals. GABA-BR1 immunoreactivity is expressed presynaptically in putative glutamatergic terminals, while GABA-A alpha1 and beta2/3 receptor subunits are exclusively post-synaptic and often coexist at individual symmetric synapses in both GPe and GPi. In conclusion, our findings corroborate the concept that ionotropic and metabotropic GABA receptors are located to subserve different effects in pallidal neurons. Although the aggregation of GABA-A receptors at symmetric synapses is consistent with their role in fast inhibitory synaptic transmission, the extrasynaptic distribution of both GABA-A and GABA-B receptors provides a substrate for complex modulatory functions that rely predominantly on the spillover of GABA.     

Biphasic responses of thalamic neurons to GABA in isolated rat brain slices--II

In isolated thalamic slices, responses of relay neurons to electrophoretically applied GABA were recorded intracellularly and compared with inhibitory postsynaptic potentials evoked by electrical stimulation of the reticularis nucleus of the thalamus. Both reduced the excitability of thalamic neurons and were biphasic in the majority of neurons studied, consisting of an early, negative-going and a later, positive-going component, when recorded close to reversal potential (mean reversal potentials -66.6 and -57.7 mV). Bicuculline and picrotoxin applied electrophoretically reduced conductance increases evoked by GABA in all neurons. The later, positive-going component was more sensitive to these antagonists (applied with submaximal doses) than the early component. Current-voltage relations for responses to GABA, like those for inhibitory postsynaptic potentials, were non-linear in the majority of neurons. In particular, there was a region of reduced slope resistance close to the reversal potential. Holding the membrane at a conditioning potential was found to change the subsequent response and its reversal potential. Positive holding potentials shifted reversal potentials in the positive direction only when GABA was applied during the conditioning period. Negative holding potentials were effective whether GABA was applied during the conditioning period or not. Recovery from these effects followed a similar time course at all membrane potentials tested. Injection of Cl- produced a positive shift in the reversal potential for both components of the response to GABA and of the evoked inhibitory postsynaptic potential. Inhibitory postsynaptic potentials evoked in thalamic relay neurons by stimulation of the nucleus reticularis resembled responses to GABA in their biphasic nature, reversal potentials and sensitivity to antagonists and to changes in intracellular chloride.     

成年大鼠骶髓后连合核神经元对与伤害性信息传递和抑制有关的神经递质的反应──脊髓片盲膜片钳全细胞记录法研究(英文)

以成年大鼠脊髓片骶髓后连合核（DCN）为模型，应用盲膜片钳全细胞记录法，研究了DCN神经元对与伤害性信息传递和抑制有关的递质的反应。证明：谷氨酸在DCN神经元引起了由NMDA和非NMDA受体介导的内向电流；P物质激发的内向电流可被spantide或CP－99994阻断；GABA激发的外向电流由GABAA和GABAB受体介导；甘氨酸激发的外向电流可被士的宁完全阻断。本研究结果提示DCN神经元既表达兴奋性神经递质受体又表达抑制性神经递质受体，即谷氨酸和P物质介导的外用伤害性信息与GABA和甘氨酸介导的抑制性信息可能在DCN神经元水平进行整合，从而起到了抑制伤害性信息传入的作用。本研究为进一步探索DCN参与伤害性信息的传递和镇痛作用的机理提供了电生理学基础。     

Functional GABA(A)-receptor-mediated inhibition in the neonatal dorsal horn

Neonatal nociceptive circuits and dorsal horn cells are characterized by an apparent lack of inhibitory control: receptive fields are large and thresholds low in the first weeks of life. It has been suggested that this may reflect immature GABA(A)-receptor (GABA(A)R) signaling whereby an early developmental shift in transmembrane anion gradient is followed by a longer period of low Cl- extrusion capacity. To investigate whether functional GABA(A)R-mediated inhibition does indeed undergo postnatal regulation at the level of dorsal horn circuits, we applied the selective GABA(A)R antagonist gabazine to the spinal cord in anesthetized rat pups [postnatal day (P) 3 or 21] while recording spike activity in single lumbar dorsal horn cells in vivo. At both ages, blockade of GABA(A)R activity resulted in enlarged hind paw receptive field areas and increased activity evoked by low- and high-intensity cutaneous stimulation, revealing comparable inhibition of dorsal horn cell firing by spinal GABA(A)Rs at P3 and P21. This inhibition did not require descending pathways to the spinal cord because perforated patch-clamp recordings of deep dorsal horn neurons in P3 spinal cord slices also showed an increase in evoked spike activity after application of gabazine. We conclude that spinal GABAergic inhibitory transmission onto single dorsal horn cells "in vivo" is functional at P3 and that low Cl- extrusion capacity does not restrict GABAergic function over the normal range of evoked sensory activity. The excitability of neonatal spinal sensory circuits could reflect immaturity in other intrinsic or descending inhibitory networks rather than weak spinal GABAergic inhibition.     

Membrane and action potential responses evoked by excitatory amino acids acting at N-methyl-D-aspartate receptors and non-N-methyl-D-aspartate receptors in the rat thalamus in vivo.

The membrane potential responses and firing patterns of rat thalamic neurons evoked by iontophoretically applied excitatory amino acids were recorded in vivo. All excitatory amino acids, including N-methyl-D,L-aspartate, evoked a membrane depolarization and a repetitive, regular pattern of action potential firing in the thalamus. Both non-nociceptive and nociceptive thalamic neurons responded to all agonists tested. Iontophoretic application of magnesium ions selectively antagonized responses to N-methyl-D,L-aspartate but did not convert the repetitive firing pattern into a burst firing pattern. In contrast, in the hippocampus, N-methyl-D,L-aspartate evoked a burst pattern of action potential firing associated with rhythmic depolarizing membrane potential shifts, similar to those seen by other workers in the hippocampus and in other brain regions. These findings are discussed in relation to the possibility that the regular firing pattern of spikes evoked by excitatory amino acids in the thalamus is primarily determined by the intrinsic membrane properties of thalamic neurons.     

Widely distributed GABA-mediated afferent inhibition processes within the ventrobasal thalamus of rat and their possible relevance to pathological pain states and somatotopic plasticity

Summary We have recently described extensive inhibitory interactions between inputs to the ventroposterolateral (VPL) (Roberts and Wells 1990, 1991) and ventropos-teromedial (VPM) (Salt 1989) portions of the ventrobasal nucleus of the thalamus (VB). We wished to determine whether (i) the inhibition observed in the VPL was operating at the thalamic level, (ii) was dependant on GABA receptors, (iii) was demonstrable on neurons of the ventro-posteromedial nucleus of the thalamus (VPM) and (iv) was operant on test responses evoked by natural stimuli. Conditioning stimulation of sciatic nerve afferents caused inhibition of air jet evoked test responses of single VB neurons in urethane-anaesthetized rats. Both VPM and VPL neurons were subject to inhibition by conditioning stimulation of hindlimb afferents, indicating the widespread nature of the inhibitory process. This inhibition was reduced by the iontophoretic application of SR95531, a GABA_A receptor antagonist. We conclude that there is a widely distributed inhibitory system operating in the somatic thalamus which involves both the medial and lateral portions of the nucleus and is, at least in part, mediated by GABA_A receptors. The possible involvement of inhibitory processes and intrinsic membrane properties of thalamic neurones in the somatotopic plasticity of the sensory thalamus following deafferentation and in deaf-ferentation pain is discussed.