D1 and D2 receptor-mediated dopaminergic modulation of visual responses in cat dorsal lateral geniculate nucleus

The modulatory effects of dopamine (DA) on the visual responses of relay cells of the dorsal aspect of cat lateral geniculate nucleus (dLGN) were tested using local micro-iontophoretic application of DA and application of the receptor-specific agonists SKF38393 (SKF, D₁/D₅) and quinpirole (QUIN, D₂/D₃/D₄) in the anaesthetized alcuronium-treated cat. The effects of DA and QUIN were clearly dose-dependent: small amounts caused a weak and transient facilitation of visual activity (10–30 % increase) preferentially in Y-type relay cells, which changed to a moderate reduction of visual responses when the dose was increased (50 %, maximal 70 %). The effect of SKF was mainly suppressive and increased with the amount of drug applied (up to 90 % reduction). The selective antagonists SCH23390 (SCH, D₁) and sulpiride (SULP, D₂) reduced the effects of co-applied DA agonists. We found little evidence for a specific dopaminergic modulation of the surround inhibition (stimulus-driven lateral inhibition) although DA slightly facilitated the transmission of weak signals (small stimuli). Nevertheless, some dopaminergic effects seem to be mediated via inhibitory interneurons regulating the strength of sustained or recurrent inhibition. Application of DA agonists during blockade of GABA_A receptors indicates a direct suppression of relay cells via D₁ receptors, an excitation of relay cells via D₂ receptors and - with increasing amounts of D₂ agonist - probably also an excitation of inhibitory interneurons, which results in an indirect inhibition of dLGN relay cells (predominantly of the X-type). The results are discussed in relation to the impairment of visual functions in Parkinson's disease.     

Convergent control of synaptic GABA release from rat dorsal horn neurones by adenosine and GABA autoreceptors

Perforated patch clamp recordings were performed on cultured superficial neonatal rat dorsal horn (DH) spinal cord neurones in order to study the presynaptic modulation of GABA release at unitary synaptic connections. Since ATP can be coreleased with GABA at about two-thirds of GABAergic synapses between DH neurones, and can be rapidly metabolized to adenosine in the extracellular space, we investigated the potential role of A1 adenosine receptors and GABA_B receptors which might function as inhibitory autoreceptors. Adenosine and GABA_B receptor agonists reduced the amplitude of electrically evoked GABAergic inhibitory postsynaptic currents (eIPSCs) as well as the frequency of GABAergic miniature IPSCs, suggesting a presynaptic action of these substances. The actions of adenosine were blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). The effects of adenosine and GABA_B agonists were occlusive, indicating a functional convergence of the signalling pathways engaged by A1 and GABA_B receptors. A1 and GABA_B antagonists increased the amplitude of eIPSCs in a supra-additive manner, suggesting a tonic activation of these receptors by ambient adenosine and GABA. Moreover, using trains of electrical stimulations, we were able to unravel a phasic (activity-dependent) activation of presynaptic A1 and GABA_B autoreceptors only in the case of neurones coreleasing ATP and GABA, despite the presence of functional presynaptic A1 and GABA_B receptors on all GABAergic DH neurones. This selective, convergent and activity-dependent inhibition of GABA release by A1 and GABA_B autoreceptors might modulate the integrative properties of postsynaptic DH neurones under physiological conditions and/or during the development of pathological pain states.     

Dopamine presynaptically depresses fast inhibitory synaptic transmission via D4 receptor-protein kinase A pathway in the rat dorsolateral septal nucleus

The lateral septal nucleus receives a diffuse dopaminergic input originating from the ventral tegmental area of the brain stem. We examined whether dopamine (DA) modulates synaptic transmission in the slice preparation of the rat dorsolateral septal nucleus (DLSN). Bath application (10-15 min) of DA (30 muM) markedly depressed the amplitude of fast and slow inhibitory postsynaptic potentials (IPSPs) in DLSN neurons, while it produced only a minor depression of the amplitude of excitatory postsynaptic potentials (EPSPs) obtained in the presence of bicuculline. DA (30 muM) depressed the monosynaptic fast IPSP to approximately 50% of control, but did not depress the inward current (I(GABA)) induced by exogenous gamma-aminobutyric acid (GABA). DA decreased the frequency of miniature fast IPSPs (m-fIPSPs) without significantly changing their amplitude. PD 168077, a selective D4 receptor agonist, depressed the fast and slow IPSPs but not the EPSP and decreased the frequency of m-fIPSPs. Both DA and PD 168077 increased the paired-pulse ratio of the monosynaptic fast IPSP. The inhibitory effect of DA on the fast IPSP was significantly attenuated by L-741,742, an antagonist at D4 receptors, but not by SCH 23390 and sulpiride, a D1-like and a D2-like receptor antagonist, respectively. N-ethylmaleimide, a blocker of pertussis toxin (PTX)-sensitive G protein (G(i/o)), attenuated the DA-induced depression of the fast IPSP. N-[2-((p-bromocinnamyl) amino)ethyl]-5-isoquinoline sulfonamide, a protein kinase A (PKA) inhibitor, attenuated the DA-induced depression of the fast IPSP. These results suggest that DA inhibits spontaneous and evoked release of GABA via the D4 receptor-G(i)-protein-PKA system in DLSN neurons.     

GABAB receptor activation desensitizes postsynaptic GABAB and A1 adenosine responses in rat hippocampal neurones

Whole-cell recordings of EPSCs and G-protein-activated inwardly rectifying (GIRK) currents were made from cultured hippocampal neurones to determine the effect of long-term agonist treatment on the presynaptic and postsynaptic responses mediated by GABA_B receptors (GABA_BRs). GABA_BR-mediated presynaptic inhibition was unaffected by agonist (baclofen) treatment for up to 48 h, and was desensitized by about one-half after 96 h. In contrast, GABA_BR-mediated GIRK currents were desensitized by a similar amount after only 2 h of agonist treatment. In addition, presynaptic inhibition mediated by A₁ adenosine receptors (A₁Rs) was unaffected by prolonged GABA_BR activation, whereas A₁R-mediated GIRK currents were desensitized. Desensitization of postsynaptic GABA_BR and A₁R responses was blocked by the GABA_BR antagonist (1-(S)-3,4-dichlorophenylethyl)amino-2-(S) hydroxypropyl-p-benzyl-phosphonic acid (CGP 55845A), but not by the A₁R antagonist cyclopentyldipropylxanthine (DPCPX). GIRK current amplitude could be partially restored after baclofen treatment by either coapplication of baclofen and adenosine, or intracellular infusion of the non-hydrolysable GTP analog 5'-guanylylimidodiphosphate (Gpp(NH)p). Short-term (4-24 h) baclofen treatment also significantly desensitized the inhibition of postsynaptic voltage-gated calcium channels by activation of GABA_BRs or A₁Rs. These results show that responses mediated by GABA_BRs and A₁Rs desensitize differently in presynaptic and postsynaptic compartments, and demonstrate the heterologous desensitization of postsynaptic A1R responses.     

Direct inhibition of substantia gelatinosa neurones in the rat spinal cord by activation of dopamine D2-like receptors

Dopaminergic innervation of the spinal cord is largely derived from the brain. To understand the cellular mechanisms of antinociception mediated by descending dopaminergic pathways, we examined the actions of dopamine (DA) on nociceptive transmission by using behavioural studies and whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in the spinal cord. Intrathecal administration of DA increased the mechanical nociceptive threshold and this effect was mimicked by a D2-like receptor agonist, quinpirole, but not by a D1-like receptor agonist, SKF 38393. In current-clamp mode of patch-clamp recordings, bath application of DA hyperpolarized the membrane potential of SG neurones and suppressed action potentials evoked by electrical stimulation of a dorsal root. In voltage-clamp mode, DA induced an outward current that was resistant to TTX, was blocked by the addition of Cs⁺ or GDP-β-S in the pipette solution, and was inhibited in the presence of Ba⁺. The DA-induced current reversed its polarity at a potential close to the equilibrium potential of the K⁺ channel calculated from the Nernst equation. The DA-induced outward current was mimicked by quinpirole, but not by SKF 38393. The DA-induced outward current was suppressed by a D2-like receptor antagonist, sulpiride, but not by a D1-like receptor antagonist, SCH 23390. In contrast, DA did not cause any significant change in amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs). These results indicate that DA mainly acts on postsynaptic SG neurones to induce an outward current via G-protein-mediated activation of K⁺ channels through D2-like receptors. This may be a possible mechanism for antinociception by the descending dopaminergic pathway.     

Parallel decrease in ω-conotoxin-sensitive transmission and dopamine-induced inhibition at the striatal synapse of developing rats

Whole-cell patch-clamp recordings of GABAergic IPSCs were made from cholinergic interneurones in slices of striatum from developing rats aged 21-60 days postnatal. In addition, the Ca²⁺ channel subtypes involved in synaptic transmission, as well as dopamine (DA)-induced presynaptic inhibition, were investigated pharmacologically with development by bath application of Ca²⁺ channel blockers and DA receptor agonists. The IPSC amplitude was reduced by ω-conotoxin GVIA (ω-CgTX) or ω-agatoxin TK (ω-Aga-TK) across the whole age range, suggesting that multiple types of Ca²⁺ channels mediate transmission of the synapse. The IPSC fraction reduced by ω-CgTX significantly decreased, whereas that reduced by ω-Aga-TK remained unchanged with development. DA or quinpirole, a D₂-like receptor agonist, presynaptically reduced the IPSC amplitude throughout development. The DA-induced inhibition decreased with age in parallel with the decrease in N-type Ca²⁺ channels. DA showed no further inhibition of IPSCs after the inhibitory effect of ω-CgTX had reached steady state throughout development. These results demonstrate that there is a functional link between presynaptic N-type Ca²⁺ channels and D₂-like DA receptors at inhibitory synapses in the striatum. They also demonstrate that the suppression of GABAergic transmission by D₂-like receptors is mediated by modulation of N-type Ca²⁺ channels and decreases in parallel with the developmental decline in the contribution of N-type Ca²⁺ channels to exocytosis.     

GABA Mediates Autoreceptor Feedback Inhibition in the Rat Carotid Body Via Presynaptic GABAB Receptors and TASK-1

Background K⁺ channels exert control over neuronal excitability by regulating resting potential and input resistance. Here, we show that GABA_B receptor-mediated activation of a background K⁺ conductance modulates transmission at rat carotid body chemosensory synapses in vitro . Carotid body chemoreceptor (type I) cells expressed GABA_B(1) and GABA_B(2) subunits as well as endogenous GABA. The GABA_B receptor agonist baclofen activated an anandamide- and Ba²⁺-sensitive TASK-1-like background K⁺ conductance in chemoreceptor cell clusters, but was without effect on voltage-gated Ca²⁺ channels. Hydroxysaclofen (50 μ m ), 5-aminovaleric acid (100 μ m ) and CGP 55845 (100 n m ), selective GABA_B receptor blockers, potentiated the hypoxia-induced receptor potential; this effect was abolished by pre-treatment with pertussis toxin (PTX; 500 ng ml^-1), an inhibitor of G_i, or by H-89 (50 μ m ), a selective inhibitor of protein kinase A. The protein kinase C inhibitor chelerythrine chloride (100 μ m ) was without effect on this potentiation. GABA_B receptor blockers also caused depolarisation of type I cells in clusters, and enhanced spike discharge in spontaneously firing cells. In functional co-cultures of type I clusters and petrosal sensory neurones, GABA_B receptor blockers potentiated hypoxia-induced postsynaptic chemosensory responses mediated by the fast-acting transmitters ACh and ATP. Thus GABA_B receptor-mediated activation of TASK-1 or a related channel provides a presynaptic autoregulatory feedback mechanism that modulates fast synaptic transmission in the rat carotid body.     

GABAB Receptor- and Metabotropic Glutamate Receptor-Dependent Cooperative Long-Term Potentiation of Rat Hippocampal GABAA Synaptic Transmission

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABA_B receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca²⁺ rise by postsynaptic injection of GDP-β-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca²⁺-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABA_BR, mGluR, G-proteins and intracellular Ca²⁺, which lead to a sustained potentiation of GABA_A synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.     

Dopamine mediates circadian clock regulation of rod and cone input to fish retinal horizontal cells

A circadian (24-hour) clock regulates the light responses of fish cone horizontal cells, second order neurones in the retina that receive synaptic contact from cones and not from rods. Due to the action of the clock, cone horizontal cells are driven by cones in the day, but primarily driven by rods at night. We show here that dopamine, a retinal neurotransmitter, acts as a clock signal for the day by increasing cone input and decreasing rod input to cone horizontal cells. The amount of endogenous dopamine released from in vitro retinae was greater during the subjective day than the subjective night. Application of dopamine or quinpirole, a dopamine D₂-like agonist, during the subjective night increased cone input and eliminated rod input to the cells, a state usually observed during the subjective day. In contrast, application of spiperone, a D₂-like antagonist, or forskolin, an activator of adenylyl cyclase, during the subjective day reduced cone input and increased rod input. SCH23390, a D₁ antagonist, had no effect. Application of R_p-cAMPS, an inhibitor of cAMP-dependent protein kinase, or octanol, an alcohol that uncouples gap junctions, during the night increased cone input and decreased rod input. Because D₂-like receptors are on photoreceptor cells, but not horizontal cells, the results suggest that the clock-induced increase in dopamine release during the day activates D₂-like receptors on photoreceptor cells. The resultant decrease in intracellular cyclic AMP and protein kinase A activation then mediates the increase in cone input and decrease in rod input.     

Slow excitatory synaptic transmission mediated by P2Y1 receptors in the guinea-pig enteric nervous system

Electrophysiological recording was used to study a type of slow excitatory postsynaptic potential (slow EPSP) that was mediated by release of ATP and its action at P2Y₁ receptors on morphologically identified neurones in the submucosal plexus of guinea-pig small intestine. MRS2179, a selective P2Y₁ purinergic receptor antagonist, blocked both the slow EPSP and mimicry of the EPSP by exogenously applied ATP. Increased conductance accounted for the depolarization phase of the EPSP, which occurred exclusively in neurones with S-type electrophysiological behaviour and uniaxonal morphology. The purinergic excitatory input to the submucosal neurones came from neighbouring neurones in the same plexus, from neurones in the myenteric plexus and from sympathetic postganglionic neurones. ATP-mediated EPSPs occurred coincident with fast nicotinic synaptic potentials evoked by the myenteric projections and with noradrenergic IPSPs evoked by sympathetic fibres that innervated the same neurones. The P2Y₁ receptor on the neurones was identified as a metabotropic receptor linked to activation of phospholipase C, synthesis of inositol 1,4,5-trisphosphate and mobilization of Ca²⁺ from intracellular stores.     

Dynamic regulation of glycinergic input to spinal dorsal horn neurones by muscarinic receptor subtypes in rats

Activation of spinal muscarinic acetylcholine receptors (mAChRs) inhibits nociception. However, the cellular mechanisms of this action are not fully known. In this study, we determined the role of mAChR subtypes in regulation of synaptic glycine release in the spinal cord. Whole-cell voltage-clamp recordings were performed on lamina II neurones in the rat spinal cord slices. The mAChR agonist oxotremorine-M significantly increased the frequency of glycinergic sIPSCs but not mIPSCs. Surprisingly, the effect of oxotremorine-M on sIPSCs was largely attenuated at a higher concentration. On the other hand, 1–10 μ m oxotremorine-M dose-dependently increased the frequency of sIPSCs in rats pretreated with intrathecal pertussis toxin. Furthermore, oxotremorine-M also dose-dependently increased the frequency of sIPSCs in the presence of himbacine (an M₂/M₄ mAChR antagonist) or AF-DX116 (an M₂ mAChR antagonist). The M₃ mAChR antagonist 4-DAMP abolished the stimulatory effect of oxotremorine-M on sIPSCs. Interestingly, the GABA_B receptor antagonist CGP55845 potentiated the stimulatory effect of oxotremorine-M on sIPSCs. In the presence of CGP55845, both himbacine and AF-DX116 similarly reduced the potentiating effect of oxotremorine-M on sIPSCs. Collectively, these data suggest that the M₃ subtype is present on the somatodendritic site of glycinergic neurones and is mainly responsible for muscarinic potentiation of glycinergic input to spinal dorsal horn neurones. Concurrent stimulation of mAChRs on adjacent GABAergic interneurones attenuates synaptic glycine release through presynaptic GABA_B receptors on glycinergic interneurones. This study illustrates a complex dynamic interaction between GABAergic and glycinergic synapses in the spinal cord dorsal horn.     

GABAB receptor transduction mechanisms, and cross-talk between protein kinases A and C, in GABAergic terminals synapsing onto neurons of the rat nucleus basalis of Meynert

The transduction mechanisms underlying presynaptic GABA_B receptor-mediated inhibition of transmitter release have been characterized for a variety of synapses in the central nervous system (CNS). These studies have suggested a range of transduction mechanisms, including a role for second messengers such as protein kinases A (PKA) and C (PKC). In the present study, we have examined the intracellular signalling pathways underlying baclofen-induced inhibition of GABA release from terminals synapsing onto rat basalis of Meynert neurons using patch-clamp recordings. Baclofen, a selective GABA_B receptor agonist, reversibly decreased both evoked and spontaneous, miniature, GABAergic inhibitory postsynaptic currents (eIPSCs and mIPSCs, respectively). Such baclofen actions were completely abolished by CGP55845A, a selective GABA_B receptor antagonist, and by staurosporine, a non-selective PKA and PKC inhibitor. The mIPSC frequency was still decreased by baclofen even in the presence of 4 AP, a K⁺ channel blocker, and Cd²⁺, a voltage-dependent calcium channel blocker. Pharmacological activation or inhibition of PKC activity affected basal GABA release and mildly affected the response to baclofen. Inhibition of the cAMP/PKA cascade also affected basal GABA release and, in a subset of neurons, occluded the effects of baclofen, suggesting that the GABA_B receptor-mediated inhibitory action on GABA release was mediated via decreases in PKA activity. In addition, PKA inhibition occluded the effects of PKC modulation on both basal GABA release and on the response to baclofen. Our results characterize the transduction pathway of baclofen at these nucleus basalis of Maynert (nBM) synapses and show, for the first time, some cross-talk between the cAMP/PKA and PKC pathways in mammalian presynaptic nerve terminals.     

Presynaptic N-type and P/Q-type Ca2+ channels mediating synaptic transmission at the calyx of Held of mice

At the nerve terminal, both N- and P/Q-type Ca²⁺ channels mediate synaptic transmission, with their relative contribution varying between synapses and with postnatal age. To clarify functional significance of different presynaptic Ca²⁺ channel subtypes, we recorded N-type and P/Q-type Ca²⁺ currents directly from calyces of Held nerve terminals in α_1A-subunit-deficient mice and wild-type (WT) mice, respectively. The most prominent feature of P/Q-type Ca²⁺ currents was activity-dependent facilitation, which was absent for N-type Ca²⁺ currents. EPSCs mediated by P/Q-type Ca²⁺ currents showed less depression during high-frequency stimulation compared with those mediated by N-type Ca²⁺ currents. In addition, the maximal inhibition by the GABA_B receptor agonist baclofen was greater for EPSCs mediated by N-type channels than for those mediated by P/Q-type channels. These results suggest that the developmental switch of presynaptic Ca²⁺ channels from N- to P/Q-type may serve to increase synaptic efficacy at high frequencies of activity, securing high-fidelity synaptic transmission.     

Differential ontogeny of GABABreceptor-mediated pre- and postsynaptic modulation of GABA and glycine transmission in respiratory rhythm-generating network in mouse

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABA_A-receptor- and glycine-receptor-mediated inhibition is not present, thus the question arises as to whether GABA_B-receptor-mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0-P15), we analysed the role of GABA_B-mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25–2 m m ) reduced the respiratory frequency. This reduction was prevented by the selective GABA_B receptor antagonist CGP55845A (CGP) alone at P0-P3, but by bicuculline as well as CGP at P7-P15. Blockade of GABA_B receptors by CGP increased the respiratory frequency at P0-P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABA_B-receptor-mediated pre- and postsynaptic modulation was examined by analysing the effect of GABA_B receptors on spontaneous and miniature IPSCs. In younger animals (P0-P3), the GABA_B receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7-P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABA_B-receptor-mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABA_B-receptor-mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.     

Synaptic GABAergic and glutamatergic mechanisms underlying alcohol sensitivity in mouse hippocampal neurons

This study was designed to examine the neuronal mechanisms of ethanol sensitivity by utilizing inbred short sleep (ISS) and inbred long sleep (ILS) mouse strains that display large differences in sensitivity to the behavioural effects of ethanol. Comparisons of whole-cell electrophysiological recordings from CA1 pyramidal neurons in hippocampal slices of ISS and ILS mice indicate that ethanol enhances GABA_A receptor-mediated inhibitory postsynaptic currents (GABA_A IPSCs) and reduces NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) in a concentration- and strain-dependent manner. In ILS neurons, these receptor systems are significantly more sensitive to ethanol than those in ISS neurons. To further examine the underlying mechanisms of differential ethanol sensitivities in these mice, GABA_B activity and presynaptic and postsynaptic actions of ethanol were investigated. Inhibition of GABA_B receptor function enhances ethanol-mediated potentiation of distal GABA_A IPSCs in ILS but not ISS mice, and this blockade of GABA_B receptor function has no effect on the action of ethanol on NMDA EPSCs in either mouse strain. Thus, subregional differences in GABA_B activity may contribute to the differential ethanol sensitivity of ISS and ILS mice. Moreover, analysis of the effects of ethanol on paired-pulse stimulation, spontaneous IPSC events, and brief local GABA or glutamate application suggest that postsynaptic rather than presynaptic mechanisms underlie the differential ethanol sensitivity of these mice. Furthermore, these results provide essential information to focus better on appropriate target sites for more effective drug development for the treatment of alcohol abuse.     

Interplay between neuromodulator-induced switching of short-term plasticity at sensorimotor synapses in the neonatal rat spinal cord

In the present study, we investigated the modulation of short-term depression (STD) at synapses between sensory afferents and rat motoneurons by serotonin, dopamine and noradrenaline. STD was elicited with trains of 15 stimuli at 1, 5 and 10 Hz and investigated using whole-cell voltage-clamp recordings from identified motoneurons in the neonatal rat spinal cord in vitro . STD was differentially modulated by the amines. Dopamine was effective at all stimulation frequencies, whereas serotonin affected STD only during 5 and 10 Hz stimulus trains and noradrenaline during 1 and 5 Hz trains. Dopamine and serotonin homogenized the degree of depression observed with the different stimulation modalities, in contrast to noradrenaline, which amplified the rate differences. The different modulatory profiles observed with the amines were partly due to GABAergic interneuron activity. In the presence of GABA_A and GABA_B receptor antagonists, the rate and/or kinetics of STD did not vary with the stimulation frequency in contrast to the control condition, and noradrenaline failed to alter either synaptic amplitude or STD, suggesting indirect actions. Dopamine and serotonin strongly decreased STD and converted depression to facilitation at 5 and 10 Hz during the blockade of the GABAergic receptors in 50% of the neurons tested. Altogether, these results show that STD expressed at sensorimotor synapses in the neonatal rat not only is a function of the frequency of afferent firing but also closely depends on the neuromodulatory state of these connections, with a major contribution from GABAergic transmission.     

GABA and glycine co-release optimizes functional inhibition in rat brainstem motoneurons in vitro

Whole-cell patch clamp recordings of miniature inhibitory postsynaptic currents (mIPSCs) were obtained in identified abducens motoneurons (aMns) from young rats (P5-P13). Three types of mIPSC were distinguished according to their kinetics and their sensitivity to receptor antagonists: faster decaying events mediated by glycine receptors (glyRs), slower decaying events mediated by GABA_A receptors (GABA_ARs), and mIPSCs displaying two components corresponding to GABA and glycine co-release. Dual component events accounted for ≈30 % of mIPSCs, independently of the rat's age and were also identified during evoked transmitter release. In contrast, the kinetics of glyR- and GABA_AR-mediated mIPSCs became faster during development. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were able to fully inhibit motoneuron discharge elicited by current pulses. When the GABA_AR-mediated component or the glyR-mediated component of the IPSP was blocked, the inhibition of motoneuron firing was reduced. The 20-80 % rise time and duration of GABA_AR-mediated IPSPs were significantly longer than those mediated by glyRs. The time window of inhibition for each component was determined using single postsynaptic action potentials elicited with various delays from the onset of the IPSP. GlyR-mediated IPSPs induced fast transient inhibition whereas GABA_AR-mediated IPSPs induced slow sustained suppression of firing. Using a modelling approach, we found that the two components summated non-linearly. We conclude that in developing aMns, co-release of GABA and glycine determines the strength and timing of inhibition through non-linear interactions between the two components, thus optimizing inhibition of motoneuron function.     

Enhanced G protein-dependent modulation of excitatory synaptic transmission in the cerebellum of the Ca2+ channel-mutant mouse, tottering

Tottering , a mouse model for absence epilepsy and cerebellar ataxia, carries a mutation in the gene encoding class A (P/Q-type) Ca²⁺ channels, the dominant exocytotic Ca²⁺ channel at most synapses in the mammalian central nervous system. Comparing tottering to wild-type mice, we have studied glutamatergic transmission between parallel fibres and Purkinje cells in cerebellar slices. Results from biochemical assays and electrical field recordings demonstrate that glutamate release from parallel fibre terminals of the tottering mouse is controlled largely by class B Ca²⁺ channels (N-type), in contrast to the P/Q-channels that dominate release from wild-type terminals. Since N-channels, in a variety of assays, are more effectively inhibited by G proteins than are P/Q-channels, we tested whether synaptic transmission between parallel fibres and Purkinje cells in tottering mice was more susceptible to inhibitory modulation by G protein-coupled receptors than in their wild-type counterparts. GABA_B receptors and α₂-adrenergic receptors (activated by bath application of transmitters) produced a three- to fivefold more potent inhibition of transmission in tottering than in wild-type synapses. This increased modulation is likely to be important for cerebellar transmission in vivo , since heterosynaptic depression, produced by activating GABAergic interneurones, greatly prolonged GABA_B receptor-mediated presynaptic inhibition in tottering as compared to wild-type slices. We propose that this enhanced modulation shifts the balance of synaptic input to Purkinje cells in favour of inhibition, reducing Purkinje cell output from the cerebellum, and may contribute to the aberrant motor phenotype that is characteristic of this mutant animal.     

THE ROLE OF 25-HYDROXY-VITAMIN D3 IN THE INDUCTION OF ATHEROSCLEROSIS IN SWINE and RABBIT BY HYPERVITAMINOSIS D

Fibromuscular intimal thickening was seen in the ascending and thoracic aorta of the swine fed 62,500 IU of vitamin D₃/kg of diet for three months duration; and after 3 months of vitamin D₃ withdrawal, atherosclerotic lesions were found. In rabbits, pronounced aortic smooth muscle cell necrosis developed with the forced feeding of 10,000 IU vitamin D₃ dissolved in corn oil/kg body weight/day for 14 days. Serum analyses indicated that blood calcium did not differ from that of animals fed corn oil alone, but that the level of serum 25-hydroxy- vitamin D₃ measured by HPLC was 30 times that in the control animals. These data suggest that choleclaciferol (the oxidized sterol with vitamin D₃ activity) has a very destructive influence on the integrity of arterial wall, and that smooth muscle cell necrosis could be caused by enhanced membrane permeability to Ca²⁺ following 25-hydroxy-vitamin D₃ incorporation into smooth muscle cell membranes.     

Striatal Dopamine D1 receptors in narcolepsy: A PET study with [11C]NNC 756

SUMMARY We investigated dopamine D, receptors in the putamen and caudate nucleus with positron emission tomography in six patients with narcolepsy and five healthy controls using [¹¹C]NNC 756 as ligand. The caudate-to-cerebellum and putamen-to-cerebellum ratios of [¹¹C]NNC 756 were within normal limits in patients with narcolepsy. No evidence of increased D₁, receptor binding in narcolepsy was found.