Noradrenaline-stimulated inositol phospholipid breakdown in rat dorsal lateral geniculate nucleus neurones

Differential phospholipase C-dependent modulation of TASK and TREK two-pore domain K+ channels in rat thalamocortical relay neurons

KEY POINTS

During the behavioural states of sleep and wakefulness thalamocortical relay neurons fire action potentials in high frequency bursts or tonic sequences, respectively. The modulation of specific K(+) channel types, termed TASK and TREK, allows these neurons to switch between the two modes of activity. In this study we show that the signalling lipids phosphatidylinositol 4,5-bisphosphate (PIP2) and diacylglycerol (DAG), which are components of their membrane environment, switch on and shut off TREK and TASK channels, respectively. These channel modulations contribute to a better understanding of the molecular basis of the effects of neurotransmitters such as ACh which are released by the brainstem arousal system. The present report introduces PIP2 and DAG as new elements of signal transduction in the thalamus. The activity of two-pore domain potassium channels (K2P ) regulates the excitability and firing modes of thalamocortical (TC) neurons. In particular, the inhibition of two-pore domain weakly inwardly rectifying K(+) channel (TWIK)-related acid-sensitive K(+) (TASK) channels and TWIK-related K(+) (TREK) channels, as a consequence of the stimulation of muscarinic ACh receptors (MAChRs) which are coupled to phosphoinositide-specific phospholipase C (PLCβ), induces a shift from burst to tonic firing. By using a whole cell patch-clamp approach, the contribution of the membrane-bound second messenger molecules phosphatidylinositol 4,5-bisphosphate (PIP2 ) and diacylglycerol (DAG) acting downstream of PLCβ was probed. The standing outward current (ISO ) was used to monitor the current through TASK and TREK channels in TC neurons. By exploiting different manoeuvres to change the intracellular PIP2 level in TC neurons, we here show that the scavenging of PIP2 (by neomycin) results in an increased muscarinic effect on ISO whereas increased availability of PIP2 (inclusion to the patch pipette; histone-based carrier) decreased muscarinic signalling. The degree of muscarinic inhibition specifically depends on phosphatidylinositol phosphate (PIP) and PIP2 but no other phospholipids (phosphatidic acid, phosphatidylserine). The use of specific blockers revealed that PIP2 is targeting TREK but not TASK channels. Furthermore, we demonstrate that the inhibition of TASK channels is induced by the application of the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Under current clamp conditions the activation of MAChRs and PLCβ as well as the application of OAG resulted in membrane depolarization, while PIP2 application via histone carrier induced a hyperpolarization. These results demonstrate a differential role of PIP2 and DAG in K2P channel modulation in native neurons which allows a fine-tuned inhibition of TREK (via PIP2 depletion) and TASK (via DAG) channels following MAChR stimulation.

Identification of the muscarinic pathway underlying cessation of sleep-related burst activity in rat thalamocortical relay neurons

Modulation of the standing outward current (I (SO)) by muscarinic acetylcholine (ACh) receptor (MAChR) stimulation is fundamental for the state-dependent change in activity mode of thalamocortical relay (TC) neurons. Here, we probe the contribution of MAChR subtypes, G proteins, phospholipase C (PLC), and two pore domain K(+) (K(2P)) channels to this signaling cascade. By the use of spadin and A293 as specific blockers, we identify TWIK-related K(+) (TREK)-1 channel as new targets and confirm TWIK-related acid-sensitve K(+) (TASK)-1 channels as known effectors of muscarinic signaling in TC neurons. These findings were confirmed using a high affinity blocker of TASK-3 and TREK-1, namely, tetrahexylammonium chloride. It was found that the effect of muscarinic stimulation was inhibited by M(1)AChR-(pirenzepine, MT-7) and M(3)AChR-specific (4-DAMP) antagonists, phosphoinositide-specific PLCβ (PI-PLC) inhibitors (U73122, ET-18-OCH(3)), but not the phosphatidylcholine-specific PLC (PC-PLC) blocker D609. By comparison, depleting guanosine-5'-triphosphate (GTP) in the intracellular milieu nearly completely abolished the effect of MAChR stimulation. The block of TASK and TREK channels was accompanied by a reduction of the muscarinic effect on I (SO). Current-clamp recordings revealed a membrane depolarization following MAChR stimulation, which was sufficient to switch TC neurons from burst to tonic firing under control conditions but not during block of M(1)AChR/M(3)AChR and in the absence of intracellular GTP. These findings point to a critical role of G proteins and PLC as well as TASK and TREK channels in the muscarinic modulation of thalamic activity modes.

Neurotensin excitation of serotonergic neurons in the dorsal raphe nucleus of the rat in vitro

Neurotensin-containing terminals and radioligand binding sites are present in the dorsal raphe nucleus. The purpose of this study was to test, in brain slices containing this nucleus, the effect of neurotensin on the electrical activity of serotonergic neurons. In extracellular recordings, the cells were identified by the ability of the alpha 1-adrenoceptor agonist phenylephrine to induce firing, and serotonin to reduce this effect. After washout of phenylephrine, neurotensin (10 nM to 10 microM) induced a concentration-dependent increase in the firing rate of serotonergic neurons (EC50 = 142 nM; maximum effect approximately 1 microM). The neurotensin excitation, which was mimicked by neurotensin fragments 8-13 but not neurotensin peptide fragment 1-8 and selectively blocked by SR 48692 (100 nM), was observed mainly in the ventral part of the nucleus. Most serotonergic neurons showed marked desensitization to neurotensin, even at low concentrations. The neurotensin response was occluded by supramaximal concentrations of phenylephrine. In intracellular recordings using KCl-containing electrodes, neurotensin induced an inward current associated in some cases with a decrease in apparent input conductance. In conclusion, neurotensin was found to have an excitatory action on serotonergic neurons in the ventral part of the dorsal raphe nucleus, an effect which could be subject to desensitization and was occluded by phenylephrine. This occlusion phenomenon may be important for the physiological role of neurotensin in the dorsal raphe nucleus.

Increase in cytoplasmic free Ca2+ elicited by noradrenalin and serotonin in cultured local interneurons of mouse olfactory bulb

Effects of noradrenalin and serotonin on cytoplasmic free Ca2+ concentrations ([Ca2+]i) were studied by using the fluorescent indicator fura-2 in cultured local interneurons of mouse olfactory bulb. Application of noradrenalin (0.1-100 microM) caused a rapid and concentration-dependent rise in [Ca2+]i, while isoproterenol was ineffective at concentrations up to 100 microM. The noradrenalin (1 microM)-induced increase in [Ca2+]i was completely inhibited by pretreatment with alpha 1-antagonist, prazosin (100 nM), whereas the inhibitory effect of alpha 2-antagonist, yohimbine, was about 100-times less potent. Serotonin (0.1-100 microM) also caused the dose-dependent rise in [Ca2+]i, which was inhibited by serotonin2 antagonist, ketanserin. Even in the absence of the extracellular calcium, the noradrenalin- or serotonin-induced increase in [Ca2+]i was observed. These results indicate that both noradrenalin and serotonin elicit the rise in [Ca2+]i in local interneurons of the olfactory bulb. They also suggest that the rise in [Ca2+]i is mediated by alpha 1-adrenergic and serotonin2 receptors, and that the increased calcium is mainly derived from intracellular calcium storage sites. The above results provide evidence to suggest that in the olfactory bulb, noradrenergic and serotonergic centrifugal fibers exert modulatory influences on synaptic interactions between mitral/tufted cells and local interneurons by increasing cytoplasmic Ca2+ in local interneurons.

Alterations of functional glucose use and ligand binding to second messenger systems following unilateral orbital enucleation

Quantitative autoradiography was used to examine the effect of lesioning a well-defined glutamatergic system (retinofugal fibres) on [3H]forskolin binding to Gs-adenylate cyclase and [3H]PDBu (phorbol-12,13-dibutyrate) binding to protein kinase C (PKC) in the rat visual system at 1, 5, 10 and 20 days after unilateral orbital enucleation. Local cerebral glucose utilisation was determined in the same animals using quantitative [14C]2-deoxyglucose autoradiography. At 5 days post-lesion, [3H]forskolin binding sites were significantly reduced in the visually-deprived superior colliculus (-14 +/- 1%) and dorsal lateral geniculate body (-8 +/- 2%), and these reductions persisted until 20 days post-lesion. There were no significant alterations in the amount of [3H]PDBu binding in any region in the visually-deprived hemisphere following enucleation. Function-related glucose use was significantly reduced throughout the visual pathway after enucleation. In this study, there was no conclusive evidence of plastic modifications of second messenger systems in the rat visual system despite a general depression of visual function following lesion of retinofugal fibres.

Noradrenergic potentiation of excitatory transmitter action in cerebrocortical slices: evidence for mediation by an alpha 1 receptor-linked second messenger pathway

Considerable evidence from intact, anesthetized preparations suggests that norepinephrine (NE) can modulate the efficacy of synaptic transmission within local circuits of the mammalian neocortex; i.e. both iontophoretic application of NE and activation of the coeruleocortical pathway are capable of facilitating cortical neuronal responses to non-noradrenergic synaptic inputs and putative transmitter agents. In the present study, the effects of NE on somatosensory cortical neuronal responses to putative excitatory transmitters were characterized using in vitro tissue slice preparations. Somatosensory unit responses to iontophoretic pulses of acetylcholine (ACh) or glutamate (Glu) (10-60 nA; 5-25 s duration) were examined before, during and after a period of continuous NE (1-35 nA; 4-25 min duration) microiontophoresis. Quantitative analysis of per-event histograms indicated that both Glu- and ACh-evoked excitatory discharges were routinely (Glu 94%, n = 54; ACh 67%, n = 9) potentiated above control levels during NE administration. In 8 cells, NE revealed robust excitatory discharges to otherwise subthreshold iontophoretic doses of Glu. The alpha-specific agonist, phenylephrine, mimicked (n = 3), NE-induced potentiation of Glu-evoked discharges whereas the alpha antagonist phentolamine blocked (n = 5) enhancement of these responses. Moreover, activation of protein kinase C by iontophoretic application of phorbol 12,13-diacetate (5-15 nA, n = 4) mimicked the potentiating actions of NE on Glu-evoked excitatory responses. Results from other experiments further indicated that these facilitating actions of NE on Glu-evoked responses do not involve beta receptor activation or intracellular increases in cyclic AMP. In summary, these results demonstrate that NE can facilitate cortical neuronal responses to threshold and subthreshold level applications of putative excitatory transmitter agents. Moreover, it appears that, unlike noradrenergic facilitating influences on GABA-induced inhibition, these actions are mediated by an alpha adrenoceptor mechanism which may be linked to intracellular activation of protein kinase C. Overall, these findings reinforce the idea that noradrenergic modulatory actions on excitatory and inhibitory neuronal responses may involve the activation of separate receptor-linked second messenger systems.

Autoradiographic distribution of forskolin and phorbol ester binding sites in the retina

We have localized the distribution of [3H]forskolin and [3H]phorbol dibutyrate binding sites autoradiographically in the rat, monkey, and human retina. In the rat and monkey retina, forskolin binding was enriched in the inner plexiform layer, in the inner and outer segments of the photoreceptors, and in the retinal pigment epithelium. In the human retina, forskolin binding sites were uniformly distributed and higher in density. Forskolin binding was also detected over the ciliary body, the ciliary epithelium, and the iris sphincter. The distribution of phorbol ester binding sites was similar in the rat, monkey, and human retina. The inner plexiform layer contained the highest density followed by the inner nuclear and outer plexiform layers, and the ganglion cell layer. In the rat, phorbol ester binding was present in the iris, the ciliary body, and the ciliary epithelium. The monkey and human ciliary body also contained a low density of phorbol ester binding sites.

Muscarinic receptor subclassification and G-proteins: significance for lithium action in affective disorders and for the treatment of the extrapyramidal side effects of neuroleptics

The classification of muscarinic receptors into M1 and M2 subtypes and the involvement of guanine nucleotide binding proteins (G-proteins) as major mediators of receptor information transduction in the cholinergic and other neurotransmitter systems have prompted us to undertake studies both at receptor and postreceptor levels that may shed light on the importance of these new findings to the pharmacotherapy of manic-depressive illness and of extrapyramidal syndromes. We searched for patterns of muscarinic selectivity among the commonly used anticholinergics (biperiden, procyclidine, trihexyphenidyl, benztropine, and methixen) through radioligand receptor studies in various rat tissues. The drugs showed a range of selectivity, from the totally nonselective methixen to the highly M1-selective biperiden. Sinus arrhythmia measurements were undertaken in psychiatric patients treated with different antiparkinsonian anticholinergics. The extent of sinus arrhythmia suppression was inversely correlated with the degree of M1 selectivity of the drugs used, advocating the use of M1-selective antiparkinsonian anticholinergics like biperiden in the treatment of extrapyramidal side effects. The implications of muscarinic receptor subclassification were further extended to include postreceptor phenomena. We have directly studied G-protein function by measuring cholinergic agonist-induced increases in guanosine triphosphate (GTP) binding to these proteins. This cholinergic agonistic effect was shown to be exerted by G-proteins other than Gs (the adenylate cyclase stimulatory G-protein), i.e., Gi (the adenylate cyclase inhibitory G-protein) or Gp [the G-protein activating phosphatidylinositol (PI) turnover], as ribosylation by pertussis toxin abolished this cholinergic effect, whereas it was unaffected by cholera toxin. Pertussis toxin-blockable, carbamylcholine-induced increases in GTP binding capacity were found to be mediated through M1 muscarinic receptors, as M1-selective antagonists were 100-fold more effective than M2 selective antagonists in blocking carbamylcholine effects. Moreover, carbamylcholine effect was exclusively detected in tissues predominantly populated by M1 receptors. Our results thus suggest that carbamylcholine-induced increases in GTP binding are exerted through M1 receptors interacting with Gp. At therapeutically efficacious concentrations, lithium completely blocked carbamylcholine-induced increases in GTP binding capacity in both in vitro and in vivo experiments.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of inositol trisphosphate as a second messenger in signal transduction processes: an essay

This essay attempts to summarize some of the best evidence for the role of inositol trisphosphate as a second messenger in signal transduction processes. The following aspects are addressed in the essay: (a) The synthesis of inositol trisphosphate and other inositol lipids, (b) Receptor-phosphatidylinositol bisphosphate phospholipase C coupling and the N-ras protooncogene, (c) Inositol trisphosphate and intracellular calcium, (d) Cell growth and oncogenes, (e) Receptors linked to the phosphatidylinositol cycle, (f) Phototransduction and (g) Interactions between inositol trisphosphate and other second messengers.

Beyond receptors: multiple second-messenger systems in brain

Second-messenger systems play a major role in mediating neurotransmitter actions. In recent years our understanding of the organization and function of two prominent second-messenger systems has progressed rapidly--the adenylate cyclase and phosphoinositide systems. Guanosine triphosphate-binding proteins, which are especially abundant in brain, couple transmitter receptors to the key second-messenger generating enzymes in both of these systems. Whereas activation of adenylate cyclase produces a single intracellular messenger, cyclic AMP, stimulation of the phosphoinositide system generates at least two, inositol trisphosphate and diacylglycerol. Inositol trisphosphate mobilizes calcium from intracellular stores, and diacylglycerol, like cyclic adenosine monophosphate, activates a phosphorylating enzyme, protein kinase C. These second-messenger systems are particularly enriched in the brain where they modulate many aspects of synaptic transmission.