gamma-Aminobutyric acid-induced response in acutely isolated nucleus solitarii neurons of the rat

Hippocampal µ-opioid receptors on GABAergic neurons mediate stress-induced impairment of memory retrieval

Stressful life events induce abnormalities in emotional and cognitive behaviour. The endogenous opioid system plays an essential role in stress adaptation and coping strategies. In particular, the µ-opioid receptor (μR), one of the major opioid receptors, strongly influences memory processing in that alterations in μR signalling are associated with various neuropsychiatric disorders. However, it remains unclear whether μR signalling contributes to memory impairments induced by acute stress. Here, we utilized pharmacological methods and cell-type-selective/non-cell-type-selective μR depletion approaches combined with behavioural tests, biochemical analyses, and in vitro electrophysiological recordings to investigate the role of hippocampal μR signalling in memory-retrieval impairment induced by acute elevated platform (EP) stress in mice. Biochemical and molecular analyses revealed that hippocampal μRs were significantly activated during acute stress. Blockage of hippocampal μRs, non-selective deletion of μRs or selective deletion of μRs on GABAergic neurons (μR_GABA) reversed EP-stress-induced impairment of memory retrieval, with no effect on the elevation of serum corticosterone after stress. Electrophysiological results demonstrated that stress depressed hippocampal GABAergic synaptic transmission to CA1 pyramidal neurons, thereby leading to excitation/inhibition (E/I) imbalance in a μR_GABA-dependent manner. Pharmaceutically enhancing hippocampal GABA_A receptor-mediated inhibitory currents in stressed mice restored their memory retrieval, whereas inhibiting those currents in the unstressed mice mimicked the stress-induced impairment of memory retrieval. Our findings reveal a novel pathway in which endogenous opioids recruited by acute stress predominantly activate μR_GABA to depress GABAergic inhibitory effects on CA1 pyramidal neurons, which subsequently alters the E/I balance in the hippocampus and results in impairment of memory retrieval.

An Immunohistochemical Study of Osteopontin in Pigment Gallstone Formation

Mucin glycoproteins from the gallbladder epithelium are thought to contribute to the matrix or nucleus of gallstones and other biomineralization systems. The involved acidic glycoproteins have been reported in bile and gallstones. In addition, osteopontin (Opn) is a noncollagenous acidic bone matrix glycoprotein that possesses calcium-binding properties. To investigate the role of Opn in pigment gallstone formation, the involvement of Opn in pigment gallstone formation was studied immunohistochemically in the gallbladder wall and in the stones. Staining for Opn was strongly positive in the epithelium of stone-laden gallbladders and in their stones. The stone-laden gallbladders were infiltrated by macrophages, which intensely stained for Opn. Sections of the pigment stones, under low magnification, showed a lamellar pattern of Opn immunolabeling and showed a reticular pattern under high magnification. Our results indicate that Opn, an acidic glycoprotein from the gallbladder epithelium, seems to be involved in lithiasis. Opn from macrophages and/or the epithelium seems to help form the matrix protein.

Both GABAA and GABAB receptors mediate vagal inhibition in nucleus tractus solitarii neurones in anaesthetized rats

GABA receptors in the nucleus tractus solitarius (NTS) are known to play an important role in the mediation/modulation of various cardiovascular/respiratory functions. Vagal afferent activation of these neurones usually evokes an initial excitation followed by a long lasting inhibition. The present study examines the role of GABA(B) as well as GABA(A) receptors in the mediation of this inhibition in anaesthetized rats, using CGP 35348 and bicuculline, respective antagonists at these receptors, applied topically or by ionophoresis. Bicuculline delayed the onset and reduced the duration of this inhibition. The duration of this inhibition was further and significantly decreased when CGP 35348 was administered along with bicuculline but the delay in onset was unaffected. CGP 35348 application alone, had no effect on the vagal afferent-evoked inhibition. From intracellular recordings the early and late components of this inhibition were found to have reversal potentials close to E(Cl) and E(K), respectively. Therefore it is concluded that this inhibition is mediated by both GABA(A) and GABA(B) receptors, although GABA(B) mediated inhibition can only be unmasked when GABA(A) receptors are blocked.

Differential sensitivity of excitatory and inhibitory synaptic transmission to modulation by nitric oxide in rat nucleus tractus solitarii

The nucleus tractus solitarii (NTS) is a key central link in control of multiple homeostatic reflexes. A number of studies have demonstrated that exogenous and endogenous nitric oxide (NO) within NTS regulates visceral function, but further understanding of the role of NO in the NTS is hampered by the lack of information about its intracellular actions. We studied effects of NO in acute rat brainstem slices. Aqueous NO solution (NO(aq)) potentiated electrically evoked excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) in different neuronal subpopulations and, in some neurones, caused a depolarization. Similar effects were observed using the NO donor diethylamine NONOate (DEA/NO). The threshold NO concentration as determined using an NO electrochemical sensor was estimated as approximately 0.4 nm (EC(50) approximately 0.9 nm) for potentiating glutamatergic EPSPs but approximately 3 nm for monosynaptic GABAergic IPSPs. Bath application of the soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) abolished NO(aq)- and DEA/NO-induced potentiation of evoked EPSPs, IPSPs and depolarization. All NO actions were mimicked by the non-NO-dependent guanylate cyclase activator Bay 41-2272. The effects of NO on EPSPs and IPSPs persisted in cells where postsynaptic sGC was blocked by ODQ and therefore were presynaptic, owing to a direct modulation of transmitter release combined with depolarization of presynaptic neurones. Therefore, while lower concentrations of NO may be important for fine tuning of glutamatergic transmission, higher concentrations are required to directly engage GABAergic inhibition. This differential sensitivity of excitatory and inhibitory connections to NO may be important for determining the specificity of the effects of this freely diffusible gaseous messenger.

GABA-induced response in spiral ganglion cells acutely isolated from guinea pig cochlea

The physiological and pharmacological properties of gamma-aminobutyric acid (GABA)-induced responses were investigated in acutely isolated spiral ganglion cells (SGCs) of guinea pig by using either a nystatin-perforated patch recording configuration or a conventional whole-cell patch recording mode combined with rapid drug application. GABA and GABA(A) subtype receptor agonist, muscimol, induced inward currents in a concentration-dependent manner in 74% of all cells. The current-voltage relationship for the GABA response indicated the GABA-induced current in SGCs is carried by Cl-. Bicuculline (BIC), strychnine (STR), and picrotoxin (PTX) suppressed the GABA response in a concentration-dependent manner. BIC and STR, and PTX blocked the GABA response in a competitive manner and in a non-competitive manner, respectively. For inorganic antagonists, Cd2+ and Ni2+ also inhibited the GABA response. On the other hand, Zn2+ failed to suppress the GABA response in SGCs. An antibiotic, benzylpenicillin, suppressed the GABA response. The GABA response was augmented by both a barbiturate derivative, pentobarbital (PB), and a benzodiazepine derivative, diazepam. The results suggest clearly that the physiological and pharmacological characteristics of GABA(A) receptor on acutely isolated guinea pig SGCs are quite similar to the common GABA(A) receptor found in other sensory ganglion cells.

Early GABA(A) receptor clustering during the development of the rostral nucleus of the solitary tract

While there is an abundance of gamma-aminobutyric acid (GABA) in the gustatory zone of the nucleus of the solitary tract of the perinatal rat, we know that GABAergic synapse formation is not complete until well after birth. Our recent results have shown that GABA(B) receptors are present at birth in the cells of the nucleus; however, they do not redistribute and cluster at synaptic sites until after PND10. The present study examined the time course of appearance and redistribution of GABA(A) receptors in the nucleus. GABA(A) receptors were also present at birth. However, in comparison to GABA(B) receptors, GABA(A) receptors underwent an earlier translocation to synaptic sites. Extrasynaptic label, for example, of GABA(A) receptors was non-existent compared to GABA(B) receptors at PND10 and well-defined clusters of GABA(A) receptors could be seen as early as PND1. We propose that while GABA(A), receptors may play an early neurotransmitter role at the synapse, GABA(B) receptors may play a non-transmitter neurotrophic role.

Na(+)/Ca(2+) exchanger in GABAergic presynaptic boutons of rat central neurons

Rat Meynert neurons were acutely isolated using a dissociation technique that maintains functional GABAergic presynaptic boutons. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded under voltage-clamp conditions using whole cell patch-clamp recordings. Using the frequency of mIPSCs as a measure of presynaptic terminal excitability, the existence of a Na(+)/Ca(2+) exchanger (NCX) in these GABAergic nerve terminals was clearly demonstrated. Both the frequency and the amplitude of mIPSCs were unaffected by replacement of extracellular Na(2+). However, in this Na(+)-free external solution, ouabain could now induce a transient increase of mIPSCs frequency, which was not inhibited by adding Cd(2+) or cyclopiazonic acid but was inhibited by removing external Ca(2+). This indicates that this transient potentiation was dependent on external Ca(2+), but that this Ca(2+) influx was not via voltage-dependent Ca(2+) channels. KB-R7943, an inhibitor of NCX, at a concentration of 3 x 10(-6) M, reduced this transient increase of mIPSCs frequency without affecting mIPSCs amplitude and the response to exogenous GABA. These results demonstrate the existence of NCX in these GABAergic nerve terminals. In zero external Na(+), ouabain causes an accumulation of intraterminal Na(+) and a resultant influx of Ca(2+) through the reversed mode operation of NCX. However, under more physiological conditions, NCX may also operate in a forward mode and serve to maintain low intracellular [Ca(2+)] in nerve terminals.

Functional analysis of GABA(A) receptors in nucleus tractus solitarius neurons from neonatal rats

To gain insight into specific GABA(A) receptor configurations functionally expressed in the nucleus tractus solitarius (NTS), we conducted several physiological and pharmacological assessments. NTS neurons were characterized in thin brain slices from 1-14 day old rats using whole-cell patch clamp recordings. GABA(A-) receptor-mediated currents were detected in all neurons tested, with an average EC(50) of 22.2 microM. GABA currents were consistently stimulated by diazepam (EC(50)=63 nM), zolpidem (EC(50)=85 nM), loreclezole (EC(50)=10.1 microM) and the neurosteroid 5alpha-pregnan-3alpha-hydroxy-20-one (3alpha-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn(2+) (IC(50)=33.6 microM) picrotoxin (IC(50)=2.4 microM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10-1000 microM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant alpha subunit isoform present in GABA(A) receptors of the NTS appears to be the alpha1 and/or alpha2 subunit. beta2 and/or beta3 subunits are the major beta isoform, while the predominant gamma subunit is likely gamma2. Our data suggest the contribution to NTS GABA currents by alpha3-alpha6, beta1, gamma1 and delta subunits, if present, is minor by comparison.

GABA(A) receptor epsilon-subunit may confer benzodiazepine insensitivity to the caudal aspect of the nucleus tractus solitarii of the rat

1. Benzodiazepines (BZ) and barbiturates both potentiate chloride currents through GABA(A) receptors to enhance inhibition. However, unlike barbiturates BZ do not impair autonomic control of heart rate. We hypothesised that BZ might not significantly potentiate GABAergic transmission in the caudal nucleus of the solitary tract (cNTS), which is critically important for mediating the baroreceptor reflex. 2. In rat brain slices the BZ agonists chlordiazepoxide and midazolam (2 and 50 microM) did not significantly enhance currents evoked by GABA in voltage-clamped cNTS neurones. Chlordiazepoxide (50 microM) reversibly increased electrically evoked IPSPs in 5/10 rostral NTS (rNTS) neurones but only in 2/10 cNTS neurones. Pentobarbitone (50-100 microM) was effective in enhancing GABA(A)-mediated responses in all NTS neurones. An inverse BZ agonist, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM; 1 or 10 microM), failed to depress GABA-induced currents in the cNTS. 3. Microinjections of midazolam (10 and 100 microM solutions) into the cNTS did not affect the baroreceptor reflex (P > 0.2) while pentobarbitone (100 microM) significantly and reversibly depressed it (gain decrease to 53 +/- 11 % of control, P < 0.01). 4. Reverse transcriptase polymerase chain reaction revealed the presence of alpha(1), alpha(2), beta(2), beta(3) and gamma(2) GABA(A) receptor subunit mRNA in the cNTS. No alternatively spliced variants of the alpha(1)- and gamma(2)-subunits were revealed. Moreover, GABA(A) epsilon-unit mRNA was found in both the cNTS and rNTS as two alternatively spliced transcripts. 5. Immunocytochemical analysis revealed numerous GABA(A) epsilon-subunit-positive neurones within the cNTS with significantly fewer epsilon-subunit-positive cells in the rNTS. 6. As incorporation of the epsilon-subunit in recombinant GABA(A) receptors may confer BZ insensitivity we propose that the paucity of BZ actions in the cNTS is due to a high level of epsilon-subunit expression. This is the first demonstration of a possible physiological impact of the epsilon-subunit on native GABA(A) receptors.

Redistribution and increased specificity of GABA(B) receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of gamma-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA(B) receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA(B) receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA(B) receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.

Heterogeneity of functional GABA(A) receptors in rat dentate gyrus neurons revealed by a change in response to drugs during the whole-cell current time-course

We examined if the drug sensitivity of GABA(A) receptors in dentate gyrus granule neurons changed during the whole-cell current time-course. Effects of drugs on currents evoked immediately (the peak current) upon drug application and currents remaining about two seconds later (semi-plateau current) were compared. The apparent affinity for GABA (EC(50)) of the peak and the semi-plateau current were 14 and 4 microM, respectively. Bicuculline inhibited 50% of the peak and the semi-plateau current (IC(50)) at 7 and 36 microM, respectively, while 100 microM was required for full inhibition of the 100 microM GABA-evoked current. Zinc inhibited about 50% of the peak current with an IC(50) value of 94 microM whereas biphasic, but complete inhibition of the semi-plateau current was recorded with IC(50) values of 3 and 558 microM. The decay phase of the 100 microM GABA-evoked current was fitted by a fast (tau(1), 100-300 ms) and a slow (tau(2), 1-2 s) time-constants in all cells. The relative current amplitude associated with the fast (A1) and the slow (A2) component varied. The A1 current amplitude appeared more sensitive to bicuculline than the A2 current while the opposite was true for zinc. The results are consistent with heterogenous population of functional GABA(A) receptors in the dentate gyrus granule neurons.

Postnatal differentiation of local networks in the nucleus of the tractus solitarius

Whole-cell voltage-clamp recordings from rat brain slice preparation were used to investigate a possible developmental change in the patterns of synaptic interactions among the nucleus tractus solitarii neurons by analysing spontaneous postsynaptic current activity. Three types of patterns of spontaneous postsynaptic current activity were distinguished in the nucleus tractus solitarii neurons which showed high activities in terms of current frequency and amplitude. The first type was characterized by the presence in an individual cell of high frequencies and large amplitudes of both spontaneous glutamatergic and GABAergic postsynaptic currents, and observed exclusively in postnatal day 0-7 rats. The second and third types of cells showed predominant either inhibitory or excitatory postsynaptic currents, respectively. After postnatal day 5, nucleus tractus solitarii neurons with high background activity were shown to differentiate into either the second or the third type, with the latter of about 70% in the adult caudal/intermediate nucleus tractus solitarii. Axon collaterals of some medium to large neurons seemed to be decreased by pruning during postnatal development. The early postnatal differentiation of background synaptic activity observed in the nucleus tractus solitarii presumably reflects the local network reorganization and may be related to maturational changes in cardiovascular and respiratory functions.

Pharmacology of GABA(A) receptors of retinal dopaminergic neurons

When the vertebrate retina is stimulated by light, a class of amacrine or interplexiform cells release dopamine, a modulator responsible for neural adaptation to light. In the intact retina, dopamine release can be pharmacologically manipulated with agonists and antagonists at GABA(A) receptors, and dopaminergic (DA) cells receive input from GABAergic amacrines. Because there are only 450 DA cells in each mouse retina and they cannot be distinguished in the living state from other cells on the basis of their morphology, we used transgenic technology to label DA cells with human placental alkaline phosphatase, an enzyme that resides on the outer surface of the cell membrane. We could therefore identify DA cells in vitro after dissociation of the retina and investigate their activity with whole cell voltage clamp. We describe here the pharmacological properties of the GABA(A) receptors of solitary DA cells. GABA application induces a large inward current carried by chloride ions. The receptors are of the GABA(A) type because the GABA-evoked current is blocked by bicuculline. Their affinity for GABA is very high with an EC(50) value of 7.4 microM. Co-application of benzodiazepine receptor ligands causes a strong increase in the peak current induced by GABA (maximal enhancement: CL-218872 220%; flunitrazepam 214%; zolpidem 348%) proving that DA cells express a type I benzodiazepine-receptor (BZ1). GABA-evoked currents are inhibited by Zn(2+) with an IC(50) of 58.9 +/- 8.9 microM. Furthermore, these receptors are strongly potentiated by the modulator alphaxalone with an EC(50) of 340 +/- 4 nM. The allosteric modulator loreclezole increases GABA receptor currents by 43% (1 microM) and by 107% (10 microM). Using outside-out patches, we measured in single-channel recordings a main conductance (29 pS) and two subconductance (20 and 9 pS) states. We have previously shown by single-cell RT-PCR and immunocytochemistry that DA cells express seven different GABA(A) receptor subunits (alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L)) and by immunocytochemistry that all subunits are expressed in the intact retina. We show here that at least alpha1, beta3 and gamma2 subunits are assembled into functional receptors.

Presynaptic serotonergic inhibition of GABAergic synaptic transmission in mechanically dissociated rat basolateral amygdala neurons

1. The basolateral amygdala (ABL) nuclei contribute to the process of anxiety. GABAergic transmission is critical in these nuclei and serotonergic inputs from dorsal raphe nuclei also significantly regulate GABA release. In mechanically dissociated rat ABL neurons, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) arising from attached GABAergic presynaptic nerve terminals were recorded with the nystatin-perforated patch method and pharmacological isolation. 2. 5-HT reversibly reduced the GABAergic mIPSC frequency without affecting the mean amplitude. The serotonergic effect was mimicked by the 5-HT1A specific agonist 8-OH DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) and blocked by the 5-HT1A antagonist spiperone. 3. The GTP-binding protein inhibitor N-ethylmaleimide removed the serotonergic inhibition of mIPSC frequency. In either K+-free or Ca2+-free external solution, 5-HT could inhibit mIPSC frequency. 4. High K+ stimulation increased mIPSC frequency and 8-OH DPAT inhibited this increase even in the presence of Cd2+. 5. Forskolin, an activator of adenylyl cyclase (AC), significantly increased synaptic GABA release frequency. Pretreatment with forskolin prevented the serotonergic inhibition of mIPSC frequency in both the standard and high K+ external solution. 6. Ruthenium Red (RR), an agent facilitating the secretory process in a Ca2+-independent manner, increased synaptic GABA release. 5-HT also suppressed RR-facilitated mIPSC frequency. 7. We conclude that 5-HT inhibits GABAergic mIPSCs by inactivating the AC-cAMP signal transduction pathway via a G-protein-coupled 5-HT1A receptor and this intracellular pathway directly acts on the GABA-releasing process independent of K+ and Ca2+ channels in the presynaptic nerve terminals.

Structure and function of gustatory neurons in the nucleus of the solitary tract. IV. The morphology and synaptology of GABA-immunoreactive terminals

In the visual, auditory and somatosensory systems, insight into the synaptic arrangements of specific types of neurons has proven useful in understanding how sensory processing within that system occurs. The neurotransmitter GABA is present in the nucleus of the solitary tract and based on the fact that the vast majority of cells respond to GABA, its agonists and antagonists, and that over 45% of synaptic terminals in the rostral subdivision of the nucleus of the solitary tract are GABA-immunoreactive, GABA is thought to play an important role in gustatory processing. The following study was carried out to establish the distribution of GABA-immunoreactive terminals within the nucleus of the solitary tract. Specifically, the distribution on to physiologically-identified gustatory neurons was determined using post-embedding electron immuno-histochemistry. GABA-immunoreactive terminals synapse with gustatory neuronal somata and all portions of their dendrites, but non-GABAergic terminals synapse only with distal dendrites of the gustatory cells and on to correspondingly small unidentified dendritic profiles in the neuropil. There is a differential distribution of two subtypes of GABA-immunoreactive terminals on to proximal and distal portions of the gustatory neurons as well. Finally, a model for the synaptic arrangements involving gustatory and GABAergic neurons is proposed.

Characterization of Ca(2+)-binding sites in the kidney stone inhibitor glycoprotein nephrocalcin using vanadyl ions: different metal binding properties in strong and weak inhibitor proteins revealed by EPR and ENDOR

Nephrocalcin (NC), a calcium-binding glycoprotein of 14,000 molecular weight as a monomer, is known to inhibit the growth of calcium oxalate monohydrate (COM) crystals in renal tubules. We have isolated NC from bovine kidney tissue and purified into four isoforms, fractions A-D. NC-A and NC-B strongly inhibit the growth of COM crystals, and NC-C and NC-D inhibit crystal growth weakly. The strongly inhibitor proteins are abundant in normal subjects, whereas stone formers excrete less of NC-A and NC-B and more of NC-C and NC-D. NC-C was characterized with respect to its metal binding sites by using vanadyl ion (VO2+) as a paramagnetic probe in electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic studies. We demonstrated that VO2+ binds to NC-C with a stoichiometry of metal:protein binding of 4:1 and that VO2+ competes with Ca2+ in binding to NC-C. In NC-C, the metal ion is exposed to solvent water molecules and two water molecules are detected in the inner coordination sphere of the metal ion by ENDOR. In the metal binding environment of NC-A, as reported previously (Mustafi, D., & Nakagawa, Y. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 11323-11327), inner sphere coordinated water is completely excluded. Based on the results of the metal binding properties in both strong and weak inhibitor proteins, a probable mechanism of inhibition of COM crystal growth by NC has been outlined.

Organization of excitatory and inhibitory local networks in the caudal nucleus of tractus solitarius of rats revealed in in vitro slice preparation

Morphological and physiological properties of neurons in the caudal nucleus of tractus solitarius (NTS) of rats were studied in vitro by whole-cell recording and intracellular staining with biocytin. Synaptic responses following the solitary tract stimulation were also investigated to elucidate anatomical substrates of the underlying local circuits. Biocytin-filled NTS cells were divided into three groups according to the pattern of their axonal arborization: (1) local circuit neurons whose axon collaterals were extensively distributed within the NTS with the main axons leaving the NTS; (2) presumed interneurons whose axon collaterals seemed to be restricted within the NTS; and (3) projection neurons whose axons had few, if any, collaterals. Both local circuit neurons and presumed interneurons had small cell bodies (< 150 microns2 in somal area) and exhibited tonic regular spiking at depolarized membrane potentials. Polysynaptic excitatory background activity was increased and lasted for 300-1000 msec in these neurons following solitary tract stimulation. The projection neurons had medium to large cell bodies (> 150 microns2 in somal area). Inhibitory postsynaptic responses produced by an increased CI-conductance were recorded in these projection neurons. These findings suggest that excitatory local networks are organized by an assembly of the local circuit neurons in the caudal NTS, and that the interneurons are arranged to connect the excitatory local network with medium to large projection neurons via inhibitory synapses. Visceral afferent information is probably processed in the highly organized excitatory and inhibitory local networks within the caudal NTS and conveyed to other brain regions.

Heterogeneous distribution of benzodiazepine receptors among rat neostriatal neurones

1. The effects of benzodiazepine receptor (BZR) agonist were investigated in dissociated rat neostriatal neurones by a conventional whole-cell patch recording configuration at room temperature. 2. The dissociated neurones, with a longest somatic diameter of larger than 25 microns, were classified as 'large neurones', while those having soma measuring less than 15 microns were described as 'small neurones'. Large neurones were intensely positive for acetylcholinesterase staining, whereas the small ones were not. 3. CL218,872 enhanced the GABA response in both the large and small neurones with similar EC50S. However, the potentiation efficacy of CL218,872 in large neurones was larger than that of small ones. 4. Zolpidem also potentiated the GABA response in both neuronal populations with similar EC50S. This compound also enhanced the GABA response more strongly in large neurones than in small ones. 5. Zopiclone exerted a prominent potentiation in large neurones, although no difference was seen in the EC50S in the large and small neurones. 6. It was concluded that the BZR in large neurones had a different pharmacological property from that in small ones and that the BZR agonists showed a prominent difference, not in EC50, but in the potentiation efficacy between these neuronal populations.

Proton sensitivity of the GABA(A) receptor is associated with the receptor subunit composition

1. Modulation of GABA(A) receptors by external H(+) was examined in cultured rat sympathetic neurones, and in Xenopus laevis oocytes and human embryonic kidney (HEK) cells expressing recombinant GABA(A) receptors composed of combinations of alpha 1, beta 1, beta 2, gamma 2S and delta subunits. 2. Changing the external pH from 7.4 reduced GABA-activated currents in sympathetic neurones. pH titration of the GABA-induced current was fitted with a pH model which predicted that H(+) interact with two sites (PK(a) values of 6.4 and 7.2). 3. For alpha 1 beta 1 GABA(A) receptors, low external pH (< 7.4) enhanced responses to GABA. pH titration predicted the existence of two sites with PK(a) values of 6.6 and 7.5. The GABA concentration-response curve was shifted to the left by low pH and non-competitively inhibited at high pH (> 7.4). 4. alpha 1 beta 1 gamma 2S receptor constructs were not affected by external pH, whereas exchanging the beta 1 subunit for beta 2 conferred a sensitivity to pH, with predicted PK(a) values of 5.16 and 9.44. 5. Low pH enhanced the responses to GABA on alpha 1 beta 1 delta subunits, whilst high pH caused an inhibition (PK(a) values of 6.6 and 9.9). The GABA concentration-response curves were enhanced (pH 5.4) or reduced (pH 9.4) with no changes in the GABA EC(50). 6. Immunoprecipitation with subunit and epitope-specific antisera to alpha 1, beta 1 and delta subunits demonstrated that these subunits could co-assemble in cell membranes. 7. Expression of alpha 1 beta 1 gamma 2S delta constructs resulted in a 'bell-shaped' pH titration relationship. Increasing or decreasing external pH inhibited the responses to GABA. 8. The pH sensitivity of recombinant GABA(A) receptors expressed in HEK cells was generally in accordance with data accrued from Xenopus oocytes. However, rapid application of GABA to alpha 1 beta 1 constructs at high pH (> 7.4) caused an increased peak and reduced steady-state current, with a correspondingly increased rate of desensitization. 9. Modulation of GABA(A) receptor function was apparently unaffected by the internal pH. Moreover, pH values between 5 and 9.5 did not significantly affect the charge distribution on the zwitterionic GABA molecules. 10. In conclusion, this study demonstrates that external pH can either enhance, have little effect, or reduce GABA-activated responses, and this is apparently dependent on the receptor subunit composition. The potential importance of H(+) sensitivity of GABA(A) receptors is discussed.

Zolpidem functionally discriminates subtypes of native GABAA receptors in acutely dissociated rat striatal and cerebellar neurons

The whole-cell patch-clamp technique was used to compare the properties of native GABAA receptors in Purkinje and striatal neurons acutely dissociated from neonatal rat brains (7-11 days old). In symmetrical chloride concentrations and at a negative holding voltage, GABA induced inward currents in a concentration-dependent manner with EC50 values of 4 and 8 uM in Purkinje and striatal neurons, respectively. Diazepam potentiated the current induced by 1 uM GABA in Purkinje and striatal neurons with EC50 values of 28 and 42 nM and maximal potentiations of 128 and 182%, respectively. Zolpidem potentiated this GABA-induced current in Purkinje and striatal neurons with EC50 values of 33 and 195 nM and maximal potentiations of 189 and 236%, respectively. These results show that zolpidem, in contrast to diazepam, functionally discriminates subtypes of native GABAA receptors. Zolpidem has greater affinity for GABAA receptors containing omega 1 (Purkinje cells) than for those with omega 2 (striatum) sites and has higher intrinsic activity at these receptors than diazepam. These properties of zolpidem may contribute to its hypnoselective profile.

Functional heterogeneity of hippocampal GABAA receptors

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.

Influence of GABA on neurons of the gustatory zone of the rat nucleus of the solitary tract

The role of gamma-aminobutyric acid (GABA) as an inhibitory neurotransmitter in the rostral, gustatory zone of the nucleus of the solitary tract (rNST) was examined using whole cell recordings in brain slices of the adult rat medulla. Superfusion of GABA resulted in a concentration-dependent reduction in input resistance in 68% of the neurons in rNST. The change in input resistance was often accompanied by membrane hyperpolarization. The effect of GABA was a direct action on the postsynaptic membrane since it could be elicited when synaptic transmission was blocked by tetrodotoxin or in a low Ca2+ and high Mg2+ perfusing solution. The mean reversal potential of the GABA effect was about -60 mV, determined by applying GABA at different holding potentials, or from the intersection of current-voltage curves measured in control saline and saline containing GABA. When neurons were separated into groups based on intrinsic membrane properties, some neurons in each group responded to GABA. Superfusion of the slices with either the GABAA agonist, muscimol, or the GABAB agonist, baclofen, caused a decrease in input resistance accompanied by membrane hyperpolarization. The GABAA antagonist bicuculline either totally or partially blocked the neuronal response to GABA and blocked the response to muscimol but did not antagonize responses to baclofen. Superfusion of the GABAB antagonist phaclofen depressed the membrane responses to GABA. The use of the GABAA and GABAB agonists and antagonists demonstrates that some neurons in rNST have both GABAA and GABAB receptors. Since most rNST neurons studied respond to GABA, inhibition probably plays a major role in sensory processing by the rNST.

Caffeine and related compounds block inhibitory amino acid-gated Cl- currents in freshly dissociated rat hippocampal neurones

1. The effects of caffeine and related compounds on responses mediated by inhibitory amino acids were investigated in freshly dissociated rat hippocampal pyramidal neurones by conventional and nystatin perforated patch-clamp techniques. 2. Glycine and gamma-aminobutyric acid (GABA) evoked Cl- currents in hippocampal neurones. The half-maximum effective concentrations (EC50) of glycine and GABA were 8.5 x 10(-5) and 5 x 10(-6) M, respectively. 3. Caffeine reversibly inhibited both 10(-4) M glycine- and 10(-5) M GABA-induced Cl-currents in a concentration-dependent manner. The half-maximum inhibitory concentrations (IC50) of caffeine were 4.5 x 10(-4) M for the glycine response and 3.6 x 10(-3) M for the GABA response. 4. Caffeine shifted the concentration-response curve of IGly to the right without affecting the maximum response. 5. The inhibitory action of caffeine did not show voltage-dependency. 6. The blocking action of caffeine was not affected by intracellular perfusion with 5 mM BAPTA or by pretreatment with the protein kinase A inhibitor, H-8. This excludes the participation of Ca2+ or cyclic AMP in the inhibitory action of caffeine. 7. Caffeine failed to inhibit the augmentations of aspartate- and N-methyl-D-aspartate (NMDA) -gated current by glycine, suggesting that caffeine has no effect on the allosteric glycine binding site on the NMDA receptor. 8. The inhibitory effects of some xanthine derivatives on IGly were compared. The inhibitory potency of those compounds on IGly was in the order of pentoxifylline > theophylline > or = caffeine > paraxanthine > IBMX > or = theobromine > dyphylline. Xanthine had no effect. 9. The results indicate that methylxanthines including caffeine may act directly on the glycine receptor Cl- channel complex in rat hippocampal pyramidal neurones. The blockade of the inhibitory amino acid response by methylxanthines may be involved in the excitatory side effects of methylxanthines in the mammalian central nervous system.

Novel GABA responses from rod-driven retinal horizontal cells

gamma-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the central nervous system. Two classes of GABA receptors (GABAA and GABAB) have been identified. GABAA receptors are ligand-gated chloride channels that are competitively antagonized by bicuculline, noncompetitively blocked by picrotoxin, and often allosterically modulated by barbiturates and benzodiazepines. GABAB receptors regulate potassium and calcium channels through G-protein and intracellular second-messenger pathways, are selectively activated by baclofen, and are antagonized by phaclofen and 2-hydroxysaclofen. For some years, evidence has accumulated that there are GABA receptors, especially prominent along visual pathways, which are neither antagonized by bicuculline nor activated by baclofen, but are activated by certain conformationally restricted analogues of GABA, including cis-4-aminocrotonic acid (CACA). These receptors have been designated GABAC receptors. As yet, membrane current responses from isolated neurons that reflect this novel pharmacology have not been reported, although such responses have been recorded from oocytes injected with retinal messenger RNA. Here we describe a chloride-mediated current response from isolated rod-driven horizontal cells (H4) of the white perch retina that has this novel pharmacology.

Nicotinic acetylcholine receptor in dissociated rat nucleus tractus solitarii neurons

The ACh-activated response of the acutely dissociated neuron from the rat nucleus tractus solitarii (NTS) was investigated using conventional and perforated-patch techniques. In the present preparation, ACh and nicotine evoked inward transient currents in approximately 30% of NTS neurons tested. The ACh-activated inward current reversed the direction near 0 mV and was inhibited by D-tubocurarine in a dose-dependent manner. In contrast, muscarine resulted in no detectable changes in the NTS neurons. Some populations of the NTS had nicotinic but no muscarinic acetylcholine receptors.

A calcium-binding protein in bile and gallstones

Calcium salts are often present in the center of all types of gallstones. Matrix proteins are known to be essential for biomineralization and may therefore also be important in the formation and growth of gallstones. Other researchers have described an anionic peptide fraction of a biliary lipoprotein complex in bile and a low-molecular weight acidic glycoprotein present in gallstones. Our goal was to determine whether such a protein was present in bile and whether this protein has any calcium-binding properties. We identified a pigment-associated, highly acidic protein that precipitates from bile on addition of CaCl2 0.5 mol/L. In addition, the protein is selectively concentrated in cholesterol and pigment stones. We have, therefore, confirmed the findings of these other researchers, and we have extended the study of this protein's interactions with calcium. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrates a single band (molecular weight < or = 14 kD) that reacts positively with cationic stains. The protein was shown to inhibit the precipitation of CaCO3 from a supersaturated solution. The capacity to bind calcium was further confirmed by autoradiography with 45Ca++ and by a membrane adsorption-binding assay. Calcium-induced aggregation was demonstrated by equilibrium dialysis and by quasielastic light scattering studies. Protein measured by Lowry's assay method and amino acid analysis constitutes only 2% to 4% of the harvested material. We speculate that a substantial lipid component may also be present.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny and distribution of GABAA receptors in rat brainstem and rostral brain regions

Previous studies from our laboratory and others have shown that there are major age-related differences in brainstem neuronal function. Since GABAA receptors are major targets for GABA-mediated inhibitory modulation and play a key role in regulating cardiorespiratory function, especially during O2 deprivation, we examined differences in GABAA receptor density and distribution during postnatal development. Using quantitative receptor autoradiography, the present study was performed to examine the postnatal expression of GABAA receptors in the rat brainstem and rostral brain areas at five ages, i.e. postnatal day 1 (P1), P5, P10, P21 and P120. Ten-micrometer brain sections at different brain levels were labelled with [3H]muscimol in Tris-citrate buffer. We found that (i) GABAA receptors appeared very early in almost all the brainstem as well as rostral areas; (ii) at P1, the brainstem had a higher GABAA receptor binding density than rostral areas and its density peaked at P5 or P10; and (iii) receptor densities of the cerebellum and rostral brain areas such as cortex, thalamus and dentate gyrus increased with age, especially between P10 and P21, but most other subcortical areas like caudate-putamen and hippocampal CA1 area did not increase remarkably after birth. We conclude that: (i) GABAA receptors exist in most brain areas at birth; (ii) there are several patterns of postnatal development of GABAA receptors in the CNS with dramatic differences between the brainstem and cortex; (iii) brainstem functions rely more on GABAA receptors in early postnatal life than at more mature stages. We speculate that GABAA receptors develop earlier in phylogenetically older structures (such as brainstem) than in newer brain regions (such as cortex).