Whole-cell recordings of inwardly rectifying K+ currents activated by 5-HT1A receptors on dorsal raphe neurones of the adult rat

Gender-dependent regulation of G-protein-gated inwardly rectifying potassium current in dorsal raphe neurons in knock-out mice devoid of the 5-hydroxytryptamine transporter

Agonists at G-protein-coupled receptors in neurons of the dorsal raphe nucleus (DRN) of knock-out mice devoid of the serotonin transporter (5-HTT(-/-)) exhibit lower efficacy to inhibit cellular discharge than in wild-type counterparts. Using patch-clamp whole-cell recordings, we found that a G-protein-gated inwardly rectifying potassium (GIRK) current is involved in the inhibition of spike discharge induced by 5-HT1A agonists (5-carboxamidotryptamine (5-CT) and (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (8-OH-DPAT); 50 nM-30 microM) in both wild-type and 5-HTT(-/-) female and male mice. These effects were mimicked by 5'-guanylyl-imido-diphosphate (Gpp(NH)p; 400 microM) dialysis into cells with differences between genders. The 5-HTT(-/-) knock-out mutation reduced the current density induced by Gpp(NH)p in females but not in males. These data suggest that the decreased response of 5-HT1A receptors to agonists in 5-HTT(-/-) mutants reflects notably alteration in the coupling between G-proteins and GIRK channels in females but not in males. Accordingly, gender differences in central 5-HT neurotransmission appear to depend-at least in part-on sex-related variations in corresponding receptor-G protein signaling mechanisms.

Cell-specific repressor or enhancer activities of Deaf-1 at a serotonin 1A receptor gene polymorphism

The serotonin-1A (5-HT1A) receptor is the primary somatodendritic autoreceptor that inhibits the activity of serotonergic raphe neurons and is also expressed in nonserotonergic cortical and limbic neurons. Alterations in 5-HT1A receptor levels are implicated in mood disorders, and a functional C(-1019)G 5-HT1A promoter polymorphism has been associated with depression, suicide, and panic disorder. We examined the cell-specific activity of identified transcription factors, human nuclear deformed epidermal autoregulatory factor-1 (DEAF-1)-related (NUDR)/Deaf-1 and Hes5, at the 5-HT1A C(-1019) site. In serotonergic raphe RN46A cells, Deaf-1 and Hes5 repressed the 5-HT1A receptor gene at the C(-1019)-allele but not the G(-1019)-allele. However, in nonserotonergic cells that express 5-HT1A receptors (septal SN48, neuroblastoma SKN-SH, and neuroblastoma/glioma NG108-15 cells), Deaf-1 enhanced 5-HT1A promoter activity at the C(-1019)-allele but not the G-allele, whereas Hes5 repressed in all cell types. The enhancer activity of Deaf-1 was orientation independent and competed out Hes5 repression. To test whether Deaf-1 activity is intrinsic, the activity of a Gal4DBD (DNA binding domain)-Deaf-1 fusion protein at a heterologous Gal4 DNA element was examined. Gal4DBD-Deaf-1 repressed transcription in RN46A cells but enhanced transcription in SN48 cells, indicating that these opposite activities are intrinsic to Deaf-1. Repressor or enhancer activities of Deaf-1 or Gal4DBD-Deaf-1 were blocked by histone deacetylase inhibitor trichostatin A. Thus, the intrinsic activity of Deaf-1 at the 5-HT1A promoter is opposite in presynaptic versus postsynaptic neuronal cells and requires deacetylation. Cell-specific regulation by Deaf-1 could underlie region-specific alterations in 5-HT1A receptor expression in different mood disorders.

The role of serotonin in impulsive and aggressive behaviors associated with epilepsy-like neuronal hyperexcitability in the amygdala

Neuronal hyperexcitability in limbic areas, especially the amygdala, is a significant underlying mechanism associated with complex partial seizures (CPS). CPS may be comorbid with emotional disturbances, especially major mood disorders, anxiety, and aggression. Anticonvulsant medications such as phenytoin are also mood-stabilizing, and have been used for treatment of behavioral dyscontrol in impulsive aggressive individuals. Because the amygdala has important functional roles in epilepsy, emotion, and behavioral control, there may be common biological mechanisms involving neuronal excitability that contribute to both seizure activity and psychopathology. This review examines physiological mechanisms in the amygdala that regulate neuronal excitability and discusses how this may underlie, in part, disturbances in emotional behavior.

Coupling of 5-HT1A autoreceptors to inhibition of mitogen-activated protein kinase activation via G beta gamma subunit signaling

The 5-HT1A receptor is expressed presynaptically as the primary somatodendritic autoreceptor on serotonergic raphe neurons, and postsynaptically in several brain regions. Signaling of the 5-HT1A autoreceptor was studied in RN46A cells, a model of serotonergic raphe neurons that express endogenous 5-HT1A receptors. In undifferentiated RN46A cells stably transfected with the wild-type 5-HT1A receptor, 5-HT1A receptor activation inhibited forskolin-induced cyclic adenosine monophosphate (cAMP) formation (by 50%), increased [Ca2+]i, and induced a novel inhibition (up to 60%) of phospho-p42/p44-mitogen-activated protein kinase (MAPK). Upon differentiation of non-transfected or 5-HT1A-transfected RN46A cells, agonist-mediated inhibition of MAPK was enhanced. These actions were blocked by pretreatment with pertussis toxin indicating mediation via Gi/Go proteins and the calcium response was blocked by preactivation of protein kinase C (PKC). In cells overexpressing the G beta gamma scavenger carboxyl-terminal domain of G protein receptor kinase 2 (GRK-CT), 5-HT1A receptor activation inhibited cAMP formation, but coupling to calcium mobilization and inhibition of MAPK was abolished. The activity of 5-HT1A receptors containing mutations of PKC sites in the second (i2: T149A) or third intracellular loop (i3: T229A/S253G/T343A) was tested. At comparable levels of receptor expression, the signaling of the 5-HT1A i3 mutant was similar to the 5-HT1A wild-type receptor, while the i2 and quadruple (i2/i3) mutants failed to couple to G beta gamma-mediated increase in [Ca2+]i or inhibition of MAPK, but did couple to G alpha i-mediated inhibition of cAMP. Thus, the i2-domain of the 5-HT1A autoreceptor is crucial for coupling to G beta gamma subunits and their subsequent responses (e.g. calcium mobilization and inhibition of MAPK activity).

Differential effects of the 5-hydroxytryptamine (5-HT)1A receptor inverse agonists Rec 27/0224 and Rec 27/0074 on electrophysiological responses to 5-HT1A receptor activation in rat dorsal raphe nucleus and hippocampus in vitro

The pharmacological properties of cyclohexanecarboxylic acid, {2-[4-(2-bromo-5-methoxybenzyl)piperazin-1-yl]ethyl}-(2-trifluoromethoxyphenyl)amide (Rec 27/0224), and cyclohexanecarboxylic acid, (2-methoxy-phenyl)-{2-[4-(2-methoxyphenyl)-piperazin-1-yl]ethyl}amide (Rec 27/0074), were characterized using radioligand displacement and guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding assays, as well as electrophysiological experiments, in rat hippocampal and dorsal raphe nucleus (DRN) slices. Both compounds showed a high affinity (Ki, approximately 1 nM) and selectivity (>70-fold) at human 5-hydroxytryptamine (5-HT)1A receptors versus other 5-HT receptors. In [35S]GTPgammaS binding assays on HeLa cells stably expressing human 5-HT1A receptors, Rec 27/0224 and Rec 27/0074 inhibited basal [35S]GTPgammaS binding by 44.8 +/- 1.7% (pEC50 = 8.58) and 25 +/- 2.5% (pEC50 = 8.86), respectively. In intracellularly recorded CA1 pyramidal cells, 5-HT1A (hetero)receptor-mediated hyperpolarization, elicited by 100 nM 5-carboxamidoytryptamine (5-CT), was partially antagonized by Rec 27/0224 (approximately 50%; IC50 = 18.0 nM) and Rec 27/0074 (74%; IC50 = 0.8 nM). In extracellularly recorded DRN serotonergic neurons, Rec 27/0224 and Rec 27/0074 fully antagonized the inhibition of firing caused by the activation of 5-HT1A (auto)receptors by 30 nM 5-CT with an IC50 of 34.9 nM and 16.5 nM, respectively. The antagonism had a slow time course, reaching a steady state within 60 min. Both compounds also antagonized the citalopram-elicited, endogenous 5-HT-mediated inhibition of cell firing. In conclusion, Rec 27/0224 and Rec 27/0074 exhibited inverse agonism in [35S]GTPgammaS binding assays and differential antagonistic properties on 5-HT1A receptor-mediated responses in the hippocampus but not in the DRN. Whether this differential effect is causally related to inverse agonist activity is unclear. The qualitatively different nature of the antagonism in the hippocampus versus the DRN clearly distinguishes the compounds from neutral antagonists, such as N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-2-pyridinylcyclo-hexanecarboxamide (WAY-100635).

Tandospirone activates neuroendocrine and ERK (MAP kinase) signaling pathways specifically through 5-HT1A receptor mechanisms in vivo

Tandospirone, an azapirone, is a selective serotonin(1A) (5-HT(1A)) receptor agonist. The effects of tandospirone on plasma hormones and on mitogen-activated protein (MAP) kinase activity in the brain of male rats were studied. Tandospirone produced a time- and dose-dependent increase in plasma levels of oxytocin, adrenocorticotropin (ACTH), corticosterone, and prolactin. The minimal dose of tandospirone that led to a significant elevation of plasma oxytocin, ACTH, and prolactin levels was 1.0 mg/kg (s.c.), while the minimal dose for corticosterone release was 3.0 mg/kg (s.c.). The ED(50) of tandospirone was 1.3 mg/kg for oxytocin, 1.2 mg/kg for ACTH, 3.0 mg/kg for corticosterone, and 0.24 mg/kg for prolactin. Pretreatment with the specific 5-HT(1A) receptor antagonist WAY 100,635 (0.3 mg/kg, s.c.) completely blocked the effects of tandospirone on plasma levels of oxytocin, ACTH, and corticosterone but shifted the dose-response curve for prolactin to the right. Tandospirone injection (10 mg/kg, s.c.) stimulated the MAP kinase signaling cascade, specifically the phosphorylation of p42/44 extracellular signal-regulated kinase (ERK). Western blot analysis revealed a significant increase in phosphorylated ERK (p-ERK) levels in the hypothalamic paraventricular nucleus (PVN) as well as the dorsal raphe nucleus 5 min following tandospirone injection. These increases were blocked by pretreatment with WAY 100,635 (0.3 mg/kg). The results are the first evidence that systemic 5-HT(1A) receptor agonist administration produces a rapid increase in p-ERK levels in vivo, providing further insight into the signaling mechanisms of the 5-HT(1A) receptor.

delta-Opioid receptor antagonists inhibit GIRK channel currents in acutely dissociated brainstem neurons of rat

In this study, we investigated the effects of delta-opioid receptor antagonists on the G protein-coupled inwardly rectifying potassium (GIRK) channel currents induced by serotonin (5-HT) and noradrenaline (NAd) in the dorsal raphe and the locus coeruleus neurons, respectively. Perforated patch and conventional whole-cell patch clamp recording techniques were used for the study. Neurons were acutely dissociated from neonatal rats. Both naltrindole (NTI) and naltriben (NTB), which are selective delta-antagonists possessing antitussive activity in in vivo animal studies, reversibly inhibited the 5-HT-induced GIRK channel currents (I(5-HT)) in dorsal raphe neurons. This inhibition was concentration-dependent and voltage-independent. The half-maximum inhibitory concentration (IC(50)) on I(5-HT) was 9.84x10(-5) M for NTI and 1.28x10(-5) M for NTB. The inhibition was not reversed by 10(-5) M DPDPE, a selective delta-opioid receptor agonist. NTI did not affect 50% effective concentration (EC(50)) on the concentration-response relationship for 5-HT but inhibited the maximum response. In neurons internally perfused with GTPgammaS, both NTI and NTB also inhibited the GIRK channel currents irreversibly activated by 5-HT. Furthermore, these antagonists concentration dependently inhibited 10(-6) M NAd-induced currents (I(NAd)) in locus coeruleus neurons. The IC(50) of NTI on I(NAd) was 8.44x10(-5) M, which was close to that on I(5-HT). The results suggest that NTI and NTB, which are delta-opioid receptor antagonists possessing antitussive activity, may inhibit GIRK channel currents through a non-opioid action, and give further support to our idea previously proposed that centrally acting non-narcotic antitussives have a common characteristic of the inhibitory action on GIRK channels.

Cell type-dependent recruitment of trichostatin A-sensitive repression of the human 5-HT1A receptor gene

Regulation of serotonin (5-HT)1A receptor expression in brain is implicated in mood disorders such as depression and anxiety. Transcriptional activity of the human 5-HT1A receptor gene was strongly repressed by a negative regulatory region containing a consensus repressor element-1 (RE-1) and two copies of the dual repressor element (DRE) identified in the rat 5-HT1A receptor gene. REST/NRSF, a silencer of neuronal genes, bound the 5-HT1A RE-1 and repressed the 5-HT1A promoter. Inactivation of RE-1 completely abolished REST-mediated repression, but resulted in only partial (15-50%) de-repression of basal 5-HT1A promoter activity. The human 5-HT1A DRE sequences bound specifically to the novel repressor Freud-1 (5'repressor element under dual repression binding protein-1) and conferred repressor activity at 5-HT1A or SV40 promoters. In 5-HT1A-negative cells [L6, human embryonic kidney (HEK) 293], the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) abolished repression mediated by both RE-1/REST and DRE/Freud-1, and induced almost complete de-repression of the 5-HT1A gene. By contrast, in 5-HT1A-expressing neuronal cells (RN46A, SN-48) TSA blocked RE-1/REST repression, but did not affect DRE/Freud-1-mediated repression. Thus in contrast to REST, Freud-1 mediates HDAC-independent repression of the 5-HT1A receptor promoter in neuronal 5-HT1A-positive cells, suggesting that HDAC recruitment might influence neuron-specific gene expression by further silencing expression in non-neuronal tissue.

Contribution of Kir3.1, Kir3.2A and Kir3.2C subunits to native G protein-gated inwardly rectifying potassium currents in cultured hippocampal neurons

G protein-gated inwardly rectifying potassium (GIRK) channels are found in neurons, atrial myocytes and neuroendocrine cells. A characteristic feature is their activation by stimulation of Gi/o-coupled receptors. In central neurons, for example, they are activated by adenosine and GABA and, as such, they play an important role in neurotransmitter-mediated regulation of membrane excitability. The channels are tetrameric assemblies of Kir3.x subunits (Kir3.1-3.4 plus splice variants). In this study I have attempted to identify the channel subunits which contribute to the native GIRK current recorded from primary cultured rat hippocampal pyramidal neurons. Reverse transcriptase-polymerase chain reaction revealed the expression of mRNA for Kir3.1, 3.2A, 3.2C and 3.3 subunits and confocal immunofluorescence microscopy was used to investigate their expression patterns. Diffuse staining was observed on both cell somata and dendrites for Kir3.1 and Kir3.2A yet that for Kir3.2C was weaker and punctate. Whole-cell patch clamp recordings were used to record GIRK currents from hippocampal pyramidal neurons which were identified on the basis of inward rectification, dependence of reversal potential on external potassium concentration and sensitivity to tertiapin. The GIRK currents were enhanced by the stimulation of a number of Gi/o-coupled receptors and were inhibited by pertussis toxin. In order to ascertain which Kir3.x subunits were responsible for the native GIRK current I compared the properties with those of the cloned Kir3.1 + 3.2A and Kir3.1 + 3.2C channels heterologously expressed in HEK293 cells.

Short-Term Desensitization of G-Protein-Activated, Inwardly Rectifying K+ (GIRK) Currents in Pyramidal Neurons of Rat Neocortex

Whole cell recordings from acutely isolated rat neocortical pyramidal cells were performed to study the kinetics and the mechanisms of short-term desensitization of G-protein-activated, inwardly rectifying K+ (GIRK) currents during prolonged application (5 min) of baclofen, adenosine, or serotonin. Most commonly, desensitization of GIRK currents was characterized by a biphasic time course with average time constants for fast and slow desensitization in the range of 8 and 120 s, respectively. The time constants were independent of the agonist used to evoke the current. The biphasic time course was preserved in perforated-patch recordings, indicating that neither component of desensitization is attributable to cell dialysis. Desensitization of GIRK currents displayed a strong heterologous component in that application of a second agonist substantially reduced the responsiveness to a test agonist. Fast desensitization, but not slow desensitization, was lost in cells loaded with GDP, suggesting that the hydrolysis cycle of G proteins might underlie the initial, rapid current decline. Hydrolysis of phosphatidylinositol biphosphate is an unlikely candidate underlying short-term desensitization, because both components of desensitization were preserved in the presence of the phospholipase C inhibitor U73122. We conclude that short-term desensitization does neither result from receptor downregulation nor from altered channel gating but might involve modifications of the G-protein-dependent pathway that serves to translate receptor activation into channel opening.

Effects of ginsenoside on G protein-coupled inwardly rectifying K+ channel activity expressed in Xenopus oocytes

Recently, we provided evidence that ginsenoside, the active component of Panax ginseng, uses the pertussis toxin-insensitive Galpha(q/11)-phospholipase C-beta3 signal transduction pathway to increase Ca(2+)-activated Cl(-) currents in the Xenopus oocyte. Other investigators have shown that stimulation of receptors linked to the Galpha(q)-phospholipase C pathway inhibits the activity of G protein-coupled inwardly rectifying K(+) (GIRK) channels. In the present study, we sought to determine whether ginsenoside influenced the activity of GIRK 1 and GIRK 4 (GIRK 1/4) channels expressed in the Xenopus oocyte, and if so, the underlying signal transduction mechanism. In oocytes injected with GIRK 1/4 channel cRNA, bath-applied ginsenoside inhibited the high K(+) solution-elicited GIRK current (EC(50): 4.9+/-4.3 microg/ml). Pretreatment of the oocyte with pertussis toxin reduced the high K(+) solution-elicited GIRK current by 49%, but it did not alter the inhibitory effect of ginsenoside on the GIRK current. Prior intraoocyte injection of cRNA(s) coding Galpha(q), Galpha(11) or Galpha(q)/Galpha(11), but not Galpha(i2) or Galpha(oA), attenuated the inhibitory ginsenoside effect. Injection of cRNAs coding Gbeta(1)gamma(2) also attenuated the ginsenoside effect. Preincubation of GIRK channel-expressing oocytes with phospholipase C inhibitor, [1-[6-((17b-3-Methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione] (U73122), or protein kinase C inhibitor, staurosporine or chelerythrine, blocked the inhibitory ginsenoside effect on the GIRK current. Intraoocyte injection of bis (o-aminophenoxy)ethane-N,N,N',N'-tetracetic acid (BAPTA), a free Ca(2+) chelator, had no significant effect on the action of ginsenoside. Taken together, these results suggest that ginsenoside inhibits the activity of the GIRK 1/4 channel expressed in the Xenopus oocyte through a pertussis toxin-insensitive and Galpha(q/11)-, phospholipase C- and protein kinase C-mediated signal transduction pathway.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

Postsynaptic action mechanism of somatostatin on the membrane excitability in spinal substantia gelatinosa neurons of juvenile rats

We used tight-seal, whole-cell recording in juvenile rat spinal slices to investigate the action of somatostatin on substantia gelatinosa neurons. Bath application of somatostatin caused a robust and repeatable hyperpolarization or outward current in substantia gelatinosa neurons. Somatostatin inhibited spontaneous action potentials in subpopulation of substantia gelatinosa neurons. The amplitude of dorsal root-evoked excitatory postsynaptic currents and the frequency of spontaneous excitatory postsynaptic currents were not affected by somatostatin. The current induced by somatostatin developed almost instantaneously and did not show any time-dependent inactivation. The current-voltage relationship exhibited inward rectification. The conductance of somatostatin-sensitive current increased with the concentration of external K(+). The reversal potentials in different external K(+) concentrations were close to the K(+) equilibrium potentials. The effect of somatostatin was dose-dependent, with an EC(50) of 113 nM. The somatostatin-sensitive current was blocked by low concentration of extracellular Ba(2+) but not by glibenclamide, an inhibitor of ATP-sensitive K(+) channels. Hyperpolarization-activated cation current in a subpopulation of substantia gelatinosa neurons was not affected by somatostatin. In neurons recorded with an internal solution containing GTPgammaS, somatostatin induced outward current and hyperpolarization that did not reverse on washing. When the spontaneous induction of outward current with GTPgammaS was greatest, somatostatin did not induce any outward currents. Furthermore, intracellular dialysis of GDPbetaS, a G-protein antagonist, abolished the effect of somatostatin. In addition, SST-sensitive neurons were fewer in slices incubated with pertussis toxin than in adjacent control slices incubated without pertussis toxin. These results suggest that somatostatin decreases the postsynaptic membrane excitability of substantia gelatinosa neurons by a pertussis toxin-sensitive G-protein-mediated activation of an inwardly rectifying K(+) conductance.

Perospirone, a Novel Antipsychotic Agent, Hyperpolarizes Rat Dorsal Raphe Neurons via 5-HT1A Receptor

To investigate the effect of cis-N-[4-[4-(1,2-benz-isozole-3-yl)-1-piperazinyl]butyl] cyclohexane-1,2-dicarboximide hydrochloride (perospirone), a novel antipsychotic agent with high affinities for D(2)/5-HT(2) receptors, on the rat dorsal raphe (DR) neurons, an electrophysiological study was performed using the tight-seal whole-cell patch-clamp technique. Applications of perospirone at the concentration between 10(-)(9) and 10(-)(5) M hyperpolarized the membrane potential and inhibited spontaneous action potentials of the DR neurons in a concentration-dependent manner. This effect of perospirone on DR neurons is similar to that of typical 5HT(1A)-receptor agonists, including 8-OH-DPAT or tandospirone. In addition, WAY100635, a 5-HT(1A)-receptor antagonist, inhibited this perospirone-induced hyperpolarization of DR neurons, suggesting that perospirone physiologically acts on DR neurons as a 5HT(1A)-receptor agonist. These results provide new profiles of perospirone as an antipsychotic drug.

Convergent excitation of dorsal raphe serotonin neurons by multiple arousal systems (orexin/hypocretin, histamine and noradrenaline)

Dorsal raphe serotonin neurons fire tonically at a low rate during waking. In vitro, however, they are not spontaneously active, indicating that afferent inputs are necessary for tonic firing. Agonists of three arousal-related systems impinging on the dorsal raphe (orexin/hypocretin, histamine and the noradrenaline systems) caused an inward current and increase in current noise in whole-cell patch-clamp recordings from these neurons in brain slices. The inward current induced by all three agonists was significantly reduced in extracellular solution containing reduced sodium (25.6 mm). In extracellular recordings, all three agonists increased the firing rate of serotonin neurons; the excitatory effects of histamine and orexin A were occluded by previous application of phenylephrine, suggesting that all three systems act via common effector mechanisms. The dose-response curve for orexin B suggested an effect mediated by type II (OX2) receptors. Single-cell PCR demonstrated the presence of both OX1 and OX2 receptors in tryptophan hydroxylase-positive neurons. The effects of histamine and the adrenoceptor agonist, phenylephrine, were blocked by antagonists of histamine H1 and alpha1 receptors, respectively. The inward current induced by orexin A and phenylephrine was not blocked by protein kinase inhibitors or by thapsigargin. Three types of current-voltage responses were induced by all three agonists but in no case did the current reverse at the potassium equilibrium potential. Instead, in many cases the orexin A-induced current reversed in calcium-free medium at a value (-23 mV) consistent with the activation of a mixed cation channel (with relative permeabilities for sodium and potassium of 0.43 and 1, respectively).

Convergent excitation of dorsal raphe serotonin neurons by multiple arousal systems (orexin/hypocretin, histamine and noradrenaline)

Dorsal raphe serotonin neurons fire tonically at a low rate during waking. In vitro, however, they are not spontaneously active, indicating that afferent inputs are necessary for tonic firing. Agonists of three arousal-related systems impinging on the dorsal raphe (orexin/hypocretin, histamine and the noradrenaline systems) caused an inward current and increase in current noise in whole-cell patch-clamp recordings from these neurons in brain slices. The inward current induced by all three agonists was significantly reduced in extracellular solution containing reduced sodium (25.6 mm). In extracellular recordings, all three agonists increased the firing rate of serotonin neurons; the excitatory effects of histamine and orexin A were occluded by previous application of phenylephrine, suggesting that all three systems act via common effector mechanisms. The dose-response curve for orexin B suggested an effect mediated by type II (OX2) receptors. Single-cell PCR demonstrated the presence of both OX1 and OX2 receptors in tryptophan hydroxylase-positive neurons. The effects of histamine and the adrenoceptor agonist, phenylephrine, were blocked by antagonists of histamine H1 and alpha1 receptors, respectively. The inward current induced by orexin A and phenylephrine was not blocked by protein kinase inhibitors or by thapsigargin. Three types of current-voltage responses were induced by all three agonists but in no case did the current reverse at the potassium equilibrium potential. Instead, in many cases the orexin A-induced current reversed in calcium-free medium at a value (-23 mV) consistent with the activation of a mixed cation channel (with relative permeabilities for sodium and potassium of 0.43 and 1, respectively).

Mechanisms of nicotine actions on dorsal raphe serotoninergic neurons

Nicotine, locally administered into the dorsal raphe nucleus (DRN) of rat midbrain slices, increased the discharge rate of 70% of serotoninergic neurons, decreased it in 30% and induced reciprocal oscillatory increases in serotonin (5-hydroxytryptamine, 5-HT) and gamma-aminobutyric acid (GABA) release. All of nicotine's stimulatory effects were maximal at 2.15 microM. Bicuculline, a GABA(A) receptor antagonist, increased the firing rate in 64% of serotoninergic neurons, decreased it in 36% and augmented serotonin and GABA release. Bicuculline increased nicotine's stimulatory effects on firing rate but did not reverse the inhibitory ones. N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinil-cyclohexanecarboxamide (WAY-100635), a 5-HT(1A) receptor antagonist, increased the firing rate of 88% of serotoninergic neurons, as well as serotonin and GABA release and reversed nicotine's inhibitory action on serotoninergic neurons. These data suggest that nicotine decreases the firing rate of one third of serotoninergic neurons through serotonin release and increases the firing rate of the remaining two thirds, due to stronger stimulatory than indirect inhibitory effects.

Effect of pertussis toxin and N-ethylmaleimide on voltage-dependent and -independent calcium current modulation in serotonergic neurons

Introduction of GTP-gamma-S into a neuronal cell spontaneously results in G-protein activation. A possible contribution to this mechanism is that some receptors have a constitutive activity that stimulates GDP/GTP exchange resulting in increased GTPase activity of G-protein alpha subunits, leading to a facilitation of GTP-gamma-S binding. It follows that partial or complete uncoupling of receptors and G-proteins could inhibit Ca(2+) current modulation by GTP-gamma-S. This possibility was tested in acutely isolated rat dorsal raphe neurons by uncoupling the receptor and G-protein using N-ethylmaleimide and pertussis toxin. Since these compounds have been suggested to differentially block voltage-dependent inhibition, relative to voltage-independent, we investigated whether the apparent voltage-independent component of Ca(2+) channel modulation by 5-hydroxytryptamine (5-HT) shares the same mechanism as the voltage-dependent component. N-ethylmaleimide inhibited the response to 5-HT by about 50% but had no effect on the response to GTP-gamma-S. In dorsal raphe neurons 28.9% of the total response to 5-HT was voltage-independent. N-ethylmaleimide had identical effects on the voltage-dependent and -independent components as measured by tail current inhibition. The response to 5-HT was completely sensitive to pertussis toxin, and completely uncoupling the receptors and G-proteins did not affect the maximal response to GTP-gamma-S. Our results suggest that the apparent voltage-independent component of Ca(2+) channel modulation by 5-HT in dorsal raphe neurons might share the same mechanism as does the voltage-dependent component. In addition, these experiments provided evidence that partial or even complete uncoupling of receptors and G-proteins did not affect Ca(2+) current modulation by direct activators of G-proteins.

Cell death in weaver mouse cerebellum

Mice with the weaver mutation exhibit an uneven weave to their gait, ataxia, mild locomotor hyperactivity and, occasionally, tonic-clonic seizures. A single amino acid mutation in a G-protein coupled, inwardly rectifying K+ channel, GIRK2, gives rise to the symptoms seen in the weaver mice. Two areas of the brain are primarily affected. Cerebellar granule cell neurons die soon after birth and dopaminergic neurons are severely depleted in the substantia nigra. In this article we review recent studies of wild-type and mutant GIRK channels found in native cells or introduced into expression systems. We also review two models that explain some of the details leading to the neuronal cell death observed in weaver mice.

Rate-independent inhibition of 5-HT release by 5-HT in the somadendritic regions of raphe neurons

The somadendritic regions of raphe neurons respond to exogenous 5-hydroxytryptamine (5-HT) with an inhibition of spontaneous rate and a consequent reduction in local transmitter release, providing evidence for the operation of negative feedback regulation of spontaneous rate. Experiments were done to determine if a release process for 5-HT might also operate in the somadendritic regions that is independent of negative feedback and rate regulation. Slices of rabbit brain containing medullary or midbrain raphe nuclei, were stimulated in vitro at predetermined frequencies and the efflux of 3H-transmitter determined. The stimulation-induced pattern of transmitter release was independent of frequency, pointing to the absence of feedback. Further, exogenous 5-HT (1 x 10(-6)M) depressed the release of 3H-transmitter, but the inhibition, monitored over a range of frequencies, did not reflect competition with endogenous 5-HT for receptor sites. The antagonist methiothepin (3 x 10(-6)M) attenuated the inhibitions by 5-HT but did not by itself potentiate transmitter release, as expected if feedback inhibition were operative. Labeled transmitter release was antagonized by pretreatment with fluoxetine prior to 3H-HT incubation, and was severely curtailed in a calcium deficient medium, confirming that a neuronally relevant pool of transmitter was involved. It is concluded that serotonergic somadendritic sites contain inhibitory receptors for 5-HT release that operate independently of rate regulation and feedback. These findings could explain how other transmitters, and 5-HT itself (through dendritic release of transmitter), could exert synaptic effects on serotonergic and other neurons without being promptly countermanded by a somadendritic feedback-induced rate correction.

Agonist-specific maturation of GIRK current responses in acutely isolated pyramidal neurons of rat neocortex

We performed whole-cell recordings from acutely isolated pyramidal cell somata of rat neocortex to measure and compare G protein-activated, inwardly rectifying K+ (GIRK) currents induced by adenosine, serotonin and baclofen at different postnatal stages (postnatal days 3-19). In about two thirds of neurons, baclofen-induced GIRK currents were already detected at postnatal days 3 and 4 (P3-P4) and almost all neurons between P5 and P19 were responsive. This robust response suggests that postsynaptic effects of baclofen occur much earlier than previously thought. Sensitivity to adenosine was around 70% during the first two postnatal weeks. Given the late maturation of functional synaptic inhibition in neocortex, we propose that phasic and/or tonic activation of GIRK current by baclofen and adenosine might serve as a mechanism to control neuronal excitability during early postnatal development. In marked contrast to the pronounced early sensitivity to baclofen and adenosine, only 20% of the neurons displayed a GIRK current response to serotonin during the first postnatal week. After that, about half of the neurons tested positive for serotonin. GIRK current densities for baclofen and adenosine attained a maximum at the end of the second postnatal week, whereas the serotonin-induced current showed a linear increase during the second and third week of life. Set in relationship with previous data on the postnatal expression of receptor protein and GIRK channel mRNA, our findings suggest that the maturation of GIRK current responses is determined predominantly by the different postnatal patterns of receptor expression.

Serotonergic raphe neurons express TASK channel transcripts and a TASK-like pH- and halothane-sensitive K+ conductance

The recently described two-pore-domain K+ channels, TASK-1 and TASK-3, generate currents with a unique set of properties; specifically, the channels produce instantaneous open-rectifier (i.e., "leak") K+ currents that are modulated by extracellular pH and by clinically useful anesthetics. In this study, we used histochemical and in vitro electrophysiological approaches to determine that TASK channels are expressed in serotonergic raphe neurons and to show that they confer a pH and anesthetic sensitivity to these neurons. By combining in situ hybridization for TASK-1 or TASK-3 with immunohistochemical localization of tryptophan hydroxylase, we found that a majority of serotonergic neurons in both dorsal and caudal raphe cell groups contain TASK channel transcripts (approximately 70-90%). Whole-cell voltage-clamp recordings were obtained from raphe cells that responded to 5-HT in a manner characteristic of serotonergic neurons (i.e., with activation of an inwardly rectifying K+ current). In those cells, we isolated an endogenous K+ conductance that had properties expected of TASK channel currents; raphe neurons expressed a joint pH- and halothane-sensitive open-rectifier K+ current. The pH sensitivity of this current (pK approximately 7.0) was intermediate between that of TASK-1 and TASK-3, consistent with functional expression of both channel types. Together, these data indicate that TASK-1 and TASK-3 are expressed and functional in serotonergic raphe neurons. The pH-dependent inhibition of TASK channels in raphe neurons may contribute to ventilatory and arousal reflexes associated with extracellular acidosis; on the other hand, activation of raphe neuronal TASK channels by volatile anesthetics could play a role in their immobilizing and sedative-hypnotic effects.

Serotonergic raphe neurons express TASK channel transcripts and a TASK-like pH- and halothane-sensitive K+ conductance

The recently described two-pore-domain K+ channels, TASK-1 and TASK-3, generate currents with a unique set of properties; specifically, the channels produce instantaneous open-rectifier (i.e., "leak") K+ currents that are modulated by extracellular pH and by clinically useful anesthetics. In this study, we used histochemical and in vitro electrophysiological approaches to determine that TASK channels are expressed in serotonergic raphe neurons and to show that they confer a pH and anesthetic sensitivity to these neurons. By combining in situ hybridization for TASK-1 or TASK-3 with immunohistochemical localization of tryptophan hydroxylase, we found that a majority of serotonergic neurons in both dorsal and caudal raphe cell groups contain TASK channel transcripts (approximately 70-90%). Whole-cell voltage-clamp recordings were obtained from raphe cells that responded to 5-HT in a manner characteristic of serotonergic neurons (i.e., with activation of an inwardly rectifying K+ current). In those cells, we isolated an endogenous K+ conductance that had properties expected of TASK channel currents; raphe neurons expressed a joint pH- and halothane-sensitive open-rectifier K+ current. The pH sensitivity of this current (pK approximately 7.0) was intermediate between that of TASK-1 and TASK-3, consistent with functional expression of both channel types. Together, these data indicate that TASK-1 and TASK-3 are expressed and functional in serotonergic raphe neurons. The pH-dependent inhibition of TASK channels in raphe neurons may contribute to ventilatory and arousal reflexes associated with extracellular acidosis; on the other hand, activation of raphe neuronal TASK channels by volatile anesthetics could play a role in their immobilizing and sedative-hypnotic effects.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Kava pyrones exert effects on neuronal transmission and transmembraneous cation currents similar to established mood stabilizers--a review

1. Antiepileptic drugs that are successful as mood stabilizers, e.g. carbamazepine, valproate and lamotrigine, exhibit a characteristic pattern of action on ion fluxes. As a common target, they all affect Na+- and Ca2+ inward and K+ outward currents. 2. Furthermore, they have a variety of interactions with the metabolism and receptor occupation of biogenic amines and excitatory and inhibitory amino acids, and, by this, also influence long- term potentiation (LTP) to different degrees. 3. The kava pyrones (+/-)-kavain and dihydromethysticin are constituents of Piper methysticum. Anticonvulsant, analgesic and anxiolytic properties have been described in small open trials. 4. In the studies summarized in this article the effects mainly of (+/-)-kavain were tested on neurotransmission and especially on voltage gated ion channels. It is assumed that effects on ion channels may significantly contribute to clinical efficacy. 5. Experimental paradigms included current and voltage clamp recordings from rat hippocampal CA 1 pyramidal cells and dorsal root ganglia as well as field potential recordings in guinea pig hippocampal slices. 6. The findings suggest that (i) kava pyrones have a weak Na+ antagonistic effect that may contribute to their antiepileptic properties (ii) that they have pronounced L- type Ca2+ channel antagonistic properties and act as an positive modulator of the early K+ outward current. These two actions may be of importance for mood stabilization. (iii) Furthermore, kava pyrones have additive effects with the serotonin-1A agonist ipsapirone probably contributing to their anxiolytic and sleep- inducing effects. (iv) Finally, they show a distinct pattern of action on glutamatergic and GABAergic transmission without affecting LTP. The latter, however, seems not to be true for the spissum extract of Kava where suppression of LTP was observed. 7. In summary, kava pyrones exhibit a profile of cellular actions that shows a large overlap with several mood stabilizers, especially lamotrigine.

5-HT1A receptor-mediated activation of G-protein-gated inwardly rectifying K+ current in rat periaqueductal gray neurons

5-hydroxytryptamine (5-HT) has been reported to modulate analgesia produced by opioids or electrical stimulation of the periaqueductal gray (PAG). 5-HT increases K+ conductance and inhibits the firing activity of the PAG neurons. We examined the electrophysiological and pharmacological characteristics of the K+ current involved in 5-HT-induced hyperpolarization of dissociated rat PAG neurons. Among the neurons tested, 5-HT activated inward K+ currents in 30-40%, whilst the remaining 60-70% did not respond to 5-HT. 5-HT activated an inwardly rectifying K+ current (I5-HT) in a concentration- and voltage-dependent manner. I5-HT was mimicked by a 5-HT1A receptor selective agonist, 8-OH-DPAT, and was reversibly blocked by a 5-HT1A receptor antagonist, piperazine maleate, but not by a 5-HT2 receptor antagonist, ketanserin. I5-HT was sensitive to K+ channel blockers such as quinine and Ba2+, but insensitive to 4-aminopyridine, Cs+ and tetraethylammonium. I5-HT was inhibited by GDP(beta)s and was irreversibly activated by GTP(gamma)s. I5-HT was significantly suppressed by N-ethylmaleimide and pertussis toxin, but not by cholera toxin. Second messenger modulators such as staurosporin, forskolin, and phorbol-12-myristate-13-acetate did not alter I5-HT. The present study indicates that 5-HT-induced hyperpolarization of the PAG neurons results from activation of the pertussis toxin-sensitive G-protein-coupled inwardly rectifying K+ currents through 5-HT1A receptors.

5-HT1A and 5-HT2 receptors differentially regulate the excitability of 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro

We have used intracellular recording techniques to examine the effects of 5-hydroxytryptamine (5-HT, serotonin) on 5-HT-containing neurones of the guinea pig dorsal raphe nucleus in vitro. Bath-applied 5-HT (30-300 microM) had two opposing effects on the membrane excitability of these cells, reflecting the activation of distinct 5-HT receptor subtypes. As demonstrated previously in the rat, 5-HT evoked a hyperpolarization and inhibition of 5-HT neurones, which appeared to involve the activation of an inwardly rectifying K(+) conductance. This hyperpolarizing response was blocked by the 5-HT(1A) receptor-selective antagonist WAY-100635 (30-100 nM). In the presence of WAY-100635, 5-HT induced a previously unreported depolarizing, excitatory response of these cells, which was often associated with an increase in the apparent input resistance of the neurone, likely due to the suppression of a K(+) conductance. Like the hyperpolarizing response to 5-HT, this depolarization could be recorded in the presence of the Na(+) channel blocker tetrodotoxin. In addition, the response was not significantly attenuated by the alpha(1)-adrenoceptor antagonist prazosin (500 nM), indicating that it is not due to the release of noradrenaline, or to the direct activation of alpha(1)-adrenoceptors by 5-HT. The 5-HT(3) receptor antagonist granisetron (1 microM) and the 5-HT(4) receptor antagonist SB 204070 (100 nM) failed to reduce the depolarizing response to 5-HT; however, ketanserin (100 nM), mesulergine (100 nM) and lysergic acid diethylamide (1 microM) significantly reduced or abolished the depolarization, indicating that this effect of 5-HT is mediated by 5-HT(2) receptors.

Characterization of outward currents induced by 5-HT in neurons of rat dorsolateral septal nucleus

Properties of the 5-hydroxytryptamine (5-HT)-induced current (I(5-HT)) were examined in neurons of rat dorsolateral septal nucleus (DLSN) by using whole cell patch-clamp techniques. I(5-HT) was associated with an increase in the membrane conductance of DLSN neurons. The reversal potential of I(5-HT) was -93 +/- 6 (SE) mV (n = 7) in the artificial cerebrospinal fluid (ACSF) and was changed by 54 mV per decade change in the external K(+) concentration, indicating that I(5-HT) is carried exclusively by K(+). Voltage dependency of the K(+) conductance underlying I(5-HT) was investigated by using current-voltage relationship. I(5-HT) showed a linear I-V relation in 63%, inward rectification in 21%, and outward rectification in 16% of DLSN neurons. (+/-)-8-Hydroxy-dipropylaminotetralin hydrobromide (30 microM), a selective 5-HT(1A) receptor agonist, also produced outward currents with three types of voltage dependency. Ba(2+) (100 microM) blocked the inward rectifier I(5-HT) but not the outward rectifier I(5-HT). In I(5-HT) with linear I-V relation, blockade of the inward rectifier K(+) current by Ba(2+) (100 microM) unmasked the outward rectifier current in DLSN neurons. These results suggest that I(5-HT) with linear I-V relation is the sum of inward rectifier and outward rectifier K(+) currents in DLSN neurons. Intracellular application of guanosine-5'-O-(3-thiotriphosphate) (300 microM) and guanosine-5'-O-(2-thiodiphosphate) (5 mM), blockers of G protein, irreversibly depressed I(5-HT). Protein kinase C (PKC) 19-36 (20 microM), a specific PKC inhibitor, depressed the outward rectifier I(5-HT) but not the inward rectifier I(5-HT). I(5-HT) was depressed by N-ethylmaleimide, which uncouples the G-protein-coupled receptor from pertussis-toxin-sensitive G proteins. H-89 (10 microM) and adenosine 3',5'-cyclic monophosphothioate Rp-isomer (300 microM), protein kinase A inhibitors, did not depress I(5-HT). Phorbol 12-myristate 13-acetate (10 microM), an activator of PKC, produced an outward rectifying K(+) current. These results suggest that both 5-HT-induced inward and outward rectifying currents are mediated by a G protein and that PKC is probably involved in the transduction pathway of the outward rectifying I(5-HT) in DLSN neurons.

Effects of ethanol on the dorsal raphe nucleus and its projections to the caudate putamen

The objective of this study was to examine the effects of intraperitoneal injection of ethanol on the activity of the dorsal raphe nucleus (DRN) serotonin (5-hydroxytryptamine [5-HT]) system and its projections to the rostral caudate putamen (CPu) and determine whether rapid tolerance to the effects of ethanol develops in this system. Adult, male, Wistar rats were used in these experiments. In experiment 1, a microdialysis procedure was used to determine (a) the effects of acute intraperitoneal administration of ethanol (1.75 and 2.5 g/kg) on the extracellular levels of 5-HT in the rostral CPu and (b) whether rapid tolerance develops to these effects. In experiment 2, firing rates of 5-HT neurons were determined in the DRN after intraperitoneal administration of 2.5 g/kg of ethanol. The results of the microdialysis experiments indicated that the 2.5-g/kg dose significantly (P < .005) increased the extracellular levels of 5-HT to 150%-160% of baseline. Compared with findings for rats pretreated with saline 24 h earlier, prior treatment 24 h earlier with 2.5 g/kg of ethanol had no effect on the extracellular levels of 5-HT produced by a challenge dose of 2.5 g/kg of ethanol. Contrary to the effects in the CPu, intraperitoneal administration of 2.5 g/kg of ethanol significantly (P<.005) decreased the firing rates of 5-HT neurons in the DRN to approximately 50% of control. Overall, the results suggest to us that there is a dissociation between the effects of acute administration of ethanol on 5-HT cell body neuronal activity and 5-HT synaptic activity. The higher extracellular levels of 5-HT in the CPu may be due to increased release of 5-HT from a direct or an indirect action of ethanol, a result of inhibiting 5-HT reuptake, or related to both of these mechanisms. In addition, the findings suggest to us that rapid tolerance did not develop to the effects of ethanol on the 5-HT system within the CPu.

Inhibition of the 5-HT(1A) receptor-mediated inwardly rectifying K(+) current by dextromethorphan in rat dorsal raphe neurones

The effect of dextromethorphan (DM) on the inwardly rectifying K(+) currents mediated by 5-HT(1A) receptors in acutely dissociated dorsal raphe (DR) neurones of rats was studied using nystatin-perforated patch and conventional whole-cell patch recording configurations under voltage-clamp conditions. DM rapidly and reversibly inhibited the K(+) currents induced by 10(-7) M 5-HT in a concentration-dependent manner with a half-maximum inhibitory concentration of 1.43 x 10(-5) M. The inhibitory effect of DM was neither voltage- nor use-dependent. DM caused a suppression of the maximum response of the 5-HT concentration-response curve, thus suggesting a non-competitive type of inhibition. In neurones perfused intracellularly with a pipette-solution containing the nonhydrolyzable GTP analog GTPgammaS, 5-HT activated K(+) currents in an irreversible manner. DM suppressed the current irreversibly activated by intracellular GTPgammaS even in the absence of the agonist. DM also inhibited the inwardly rectifying K(+) currents regulated by alpha(2)-adrenoceptors in freshly isolated rat locus coeruleus neurones. These results suggest that DM may inhibit the G-protein coupled inwardly rectifying K(+) channels, but not the neurotransmitter receptors, in the central nervous system.

Serotonin 5-HT(2) receptors activate local GABA inhibitory inputs to serotonergic neurons of the dorsal raphe nucleus

The purpose of the present study was to characterize the synaptic currents induced by bath-applied serotonin (5-HT) in 5-HT cells of the dorsal raphe nucleus (DRN) and to determine which 5-HT receptor subtypes mediate these effects. In rat brain slices, 5-HT induced a concentration-dependent increase in the frequency of inhibitory postsynaptic currents (IPSCs) in 5-HT neurons recorded intracellularly in the ventral part of the DRN (EC(50): 86 microM); 5-HT also increased IPSC amplitude. These effects were blocked by the GABA(A) receptor antagonist, bicuculline (10 microM) and by the fast sodium channel blocker, TTX, suggesting that 5-HT had increased impulse flow in local GABAergic neurons. DAMGO (300 nM), a selective mu-agonist, markedly suppressed the increase in IPSC frequency induced by 5-HT (100 microM) in the DRN. A near maximal concentration of the selective 5-HT(2A) antagonist, MDL100,907 (30 nM), produced a large reduction ( approximately 70%) in the increase in IPSC frequency induced by 100 microM 5-HT; SB242,084 (30 nM), a selective 5-HT(2C) antagonist, was less effective ( approximately 24% reduction). Combined drug application suppressed the increase in 5-HT-induced IPSC frequency almost completely, suggesting involvement of both 5-HT(2A) and 5-HT(2C) receptors. Unexpectedly, the phenethylamine hallucinogen, DOI, a partial agonist at 5-HT(2A/2C) receptors, caused a greater increase (+334%) in IPSC frequency than did 5-HT 100 microM (+80%). This result may be explained by an opposing 5-HT(1A) inhibitory effect since the selective 5-HT(1A) antagonist, WAY-100635, enhanced the 5-HT-induced increase in IPSCs. These results indicate that within the DRN-PAG area there may be a negative feedback loop in which 5-HT induces an increase in IPSC frequency in 5-HT cells by exciting GABAergic interneurons in the DRN via 5-HT(2A) and, to a lesser extent, 5-HT(2C) receptors. Increased GABA tone may explain the previous observation of an indirect suppression of firing of a subpopulation of 5-HT cells in the DRN induced by phenethylamine hallucinogens in vivo.

5-Hydroxytryptamine-induced outward currents mediated via 5-HT(1A) receptors in neurons of the rat dorsolateral septal nucleus

Effects of 5-hydroxytryptamine (5-HT) on neurons of the rat dorsolateral septal nucleus (DLSN) were examined by intracellular and whole-cell patch-clamp recording techniques. An outward current was induced by 5-HT (1-100 microM) in a concentration-dependent manner. The EC(50) for 5-HT was 4.8 microM. Also, 8-OH-DPAT (10-100 microM) produced the outward current an EC(50) of 17 microM. Amplitudes of the outward currents produced by 5-HT (100 microM) and 8-OH-DPAT (100 microM) were 117+/-4 (n=6) and 58+/-8 pA (n=6), respectively. Fluvoxamine (200 nM), a specific serotonin re-uptake inhibitor, enhanced the 5-HT (1 microM)-induced outward current: the EC(50) for 5-HT was 0.5 microM in the presence of fluvoxamine (200 nM). L-694247 (100 microM) and CP 93129 (100 microM) also produced outward currents with amplitudes of 33+/-3 (n=4) and 18+/-5 pA (n=4), respectively in DLSN neurons. DOI (100 microM) and RS 67333 (100 microM) did not produce outward currents. NAN-190 shifted, in a parallel manner, the concentration-response relationship of 5-HT to the right. The Lineweaver-Burk plot of the concentration-response curve showed that NAN-190 depressed the 5-HT-induced current in a competitive manner. The current-voltage relationship indicates that the 5-HT-induced current reversed polarity at a potential close to the equilibrium potential of K(+). Ba(2+) (100 microM-1 mM) partially depressed the outward current produced by 5-HT. These results suggest that 5-HT induces multiple K(+) currents via 5-HT(1A) receptors in DLSN neurons.

Estrogen desensitizes 5-HT(1A) receptors and reduces levels of G(z), G(i1) and G(i3) proteins in the hypothalamus

The present study investigated whether estrogen would desensitize hypothalamic serotonin(1A) (5-HT(1A)) receptors by examining the neuroendocrine response to 8-OH-DPAT, a 5-HT(1A) agonist. Rats were ovariectomized, allowed to recover for 5 days, then given 2 daily injections of estradiol benzoate or vehicle (10 microg/day, s.c.). Twenty-four hours after the second injection, rats were challenged with a sub-maximal dose of 8-OH-DPAT (50 microg/kg, sc) or saline 15 min prior to sacrifice. 8-OH-DPAT produced a significant increase in plasma oxytocin, ACTH and corticosterone levels in ovariectomized rats. While estrogen treatment for 2 days did not alter basal hormone levels, it did significantly reduce the magnitude of oxytocin, ACTH and corticosterone responses to 8-OH-DPAT. The reduction in hormone responses was accompanied by a significant reduction in hypothalamic levels of G(z), G(i1) and G(i3) proteins (by 50%, 30% and 50%, respectively). These findings suggest that a reduction in these G proteins may contribute to the mechanisms underlying estrogen-induced desensitization of 5-HT(1A) receptors. The desensitization of 5-HT(1A) receptors has been suggested to underlie the therapeutic effects of antidepressant 5-HT uptake inhibitors (SSRIs). Thus, the present results suggest that estrogen or estrogen-like substances in combination with SSRIs may prove effective in developing novel therapeutic strategies for neuropsychiatric disorders in women.

Evidence of elevated intracellular calcium levels in weaver homozygote mice

1. A mutation in the G-protein-linked, inwardly rectifying K+ channel GIRK2 leads to the loss of cerebellar and dopaminergic mesencephalic neurons in weaver mice. The steps leading to cell death are not well understood but may involve constitutive influx of Na+ and Ca2+ into the neurons. 2. We found that resting [Ca2+]i was dramatically higher in cerebellar neurons from weaver mice compared to wild-type neurons. 3. High-K+ stimuli elicited much smaller changes in [Ca2+]i in weaver cerebellar neurons compared to wild-type neurons. 4. weaver cerebellar granule cells could be rescued from cell death by the GIRK2wv cationic channel blocker, QX-314. 5. QX-314 lowered resting intracellular Ca2+ levels in weaver cerebellar granule cells. 6. These results suggest that changes in resting [Ca2+]i levels and alterations in K+ channel function are most likely to contribute to the developmental abnormalities and increased cerebellar cell death observed in weaver mice.

Novel dual repressor elements for neuronal cell-specific transcription of the rat 5-HT1A receptor gene

The level of expression of the 5-HT1A receptor in the raphe and limbic systems is implicated in the etiology and treatment of major depression and anxiety disorders. The rat 5-HT1A receptor gene is regulated by a proximal TATA-driven promoter and by upstream repressors that inhibit gene expression. Deletion of a 71-base pair (bp) segment between -1590/-1519 bp of the 5-HT1A receptor gene induced over 10-fold enhancement of transcriptional activity in both 5-HT1A receptor-expressing (RN46A raphe and SN48 septal) cells and receptor-negative (L6 myoblast and C6 glioma) cells. A 31-bp segment of the repressor was protected from DNase I digestion by RN46A or L6 nuclear extracts. Within the 31-bp segment, a single protein complex was present in receptor-expressing cells that bound a novel 14-bp DNA element; in receptor-negative cells, an additional complex bound an adjacent 12-bp sequence. In receptor-positive but not receptor-negative cells, mutation of the 14-bp element to eliminate protein binding abrogated repression to nearly the same extent as deletion of the -1590/-1519 bp segment. Additional mutation of both 14-bp and 12-bp elements abolished protein binding and repressor activity in receptor-negative cells. Thus a single protein-DNA complex at the 14-bp element represses the 5-HT1A receptor gene in 5-HT1A receptor-positive neuronal cells, whereas adjacent DNA elements provide a dual repression mechanism in 5-HT1A receptor-negative cells.

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

The 5HT(1A) receptor ligand, S15535, antagonises G-protein activation: a [35S]GTPgammaS and [3H]S15535 autoradiography study

4-(Benzodioxan-5-yl)1-(indan-2-yl)piperazine (S15535) is a highly selective ligand at 5-HT(1A) receptors. The present study compared its autoradiographic labelling of rat brain sections with its functional actions, visualised by guanylyl-5'-[gamma-thio]-triphosphate ([35S]GTPgammaS) autoradiography, which affords a measure of G-protein activation. [3H]S15535 binding was highest in hippocampus, frontal cortex, entorhinal cortex, lateral septum, interpeduncular nucleus and dorsal raphe, consistent with specific labelling of 5-HT(1A) receptors. In functional studies, S15535 (10 microM) did not markedly stimulate G-protein activation in any brain region, but abolished the activation induced by the selective 5-HT(1A) agonist, (+)-8-hydroxy-dipropyl-aminotetralin ((+)-8-OH-DPAT, 1 microM), in structures enriched in [3H]S15535 labelling. S15535 did not block 5-HT-stimulated activation in caudate nucleus or substantia nigra, regions where (+)-8-OH-DPAT was ineffective and [3H]S15535 binding was absent. Interestingly, S15535 attenuated (+)-8-OH-DPAT and 5-HT-stimulated G-protein activation in dorsal raphe, a region in which S15535 is known to exhibit agonist properties in vivo [Lejeune, F., Millan, M.J., 1998. Induction of burst firing in ventral tegmental area dopaminergic neurons by activation of serotonin (5-HT)(1A) receptors: WAY100,635-reversible actions of the highly selective ligands, flesinoxan and S15535. Synapse 30, 172-180.]. The present data show that (i) [3H]S15535 labels pre- and post-synaptic populations of 5-HT(1A) sites in rat brain sections, (ii) S15535 exhibits antagonist properties at post-synaptic 5-HT(1A) receptors in corticolimbic regions, and (iii) S15535 also attenuates agonist-stimulated G-protein activation at raphe-localised 5-HT(1A) receptors.

Molecular mechanism for sodium-dependent activation of G protein-gated K+ channels

1. G protein-gated inwardly rectifying K+ (GIRK) channels are activated independently by Gbetagamma and internal Na+ via mechanisms requiring phosphatidylinositol phosphates. An aspartate (Asp) at position 226 in GIRK2 is crucial for Na+-dependent activation of GIRK1-GIRK2 heteromeric channels. We expressed wild-type and mutant GIRK1-GIRK2 channels in Xenopus oocytes and tested the effects of Na+ and neutralizing Asp226 on the functional interactions of the channels with phosphatidylinositol 4, 5-bisphosphate (PIP2). 2. The rate of inhibition of GIRK1-GIRK2 currents by application of anti-PIP2 antibody to inside-out membrane patches was slowed > 2-fold by the D226N mutation in GIRK2 and by increasing internal [Na+]. The reverse mutation in GIRK1 (N217D) increased the rate of inhibition. 3. The dose-response relationship for activation by purified PIP2 was shifted to lower concentrations in the presence of 20 mM Na+. 4. Three synthetic isoforms of PIP2, PI(4,5)P2, PI(3,4)P2 and PI(3,5)P2, activated GIRK channels with similar potencies. 5. We conclude that Na+ directly interacts with Asp226 of GIRK2 to reduce the negative electrostatic potential and promote the functional interaction of the channels with PIP2.

Neuropeptide FF selectively attenuates the effects of nociceptin on acutely dissociated neurons of the rat dorsal raphe nucleus

Intracellular Ca2+ concentration ([Ca2+]i) was measured in neurons, acutely dissociated from the rat dorsal raphe nucleus (DRN), with the fluorescent calcium probe Fluo3. Nociceptin (300 nM) had no effect on resting [Ca2+]i but reduced the magnitude of the [Ca2+]i transient triggered by depolarization in 90% of neurons having polygonal or fusiform perikarya. In 94% of neurons with the same morphology 5-HT (30 microM) also reduced the magnitude of the [Ca2+]i transient. The selective 5-HT(1A) receptor antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-ben zamide hydrochloride (p-MPPI) (0.4 microM) strongly attenuated (by 72+/-7%, n=4) this effect. The responses to nociceptin and 5-HT were not affected by BaCl2 (100 microM). The neuropeptide FF analog [D-Tyr1, (N-Me)Phe3]NPFF (1DMe) altered neither the resting [Ca2+]i nor the [Ca2+]i transient triggered by depolarization but dose-dependently decreased the effect of nociceptin (EC50=1.8 nM, maximal reduction: 68+/-5%). 1DMe had no effect on the response to 5-HT. Another neuropeptide FF analog, exhibiting a different pharmacological activity in mice and rats, [D-Tyr1, D-Leu2, D-Phe3]NPFF (1 microM) also reduced the effect of nociceptin by 74+/-11% (n=4). Few neurons (5 out of 42), either with polygonal/fusiform or smaller ovoid cell bodies, responded to the mu-opioid receptor agonist [D-Ala2, (N-Me)Phe4, Gly-ol5]-enkephalin (DAGO) with a decrease in the depolarization-induced [Ca2+]i transient. 1DMe (100 nM) attenuated this response by 69+/-14%. These results suggest that, at the cellular level, neuropeptide FF selectively counteracts the effects of opioid receptor activation.

TATA-driven transcriptional initiation and regulation of the rat serotonin 5-HT1A receptor gene

The transcriptional initiation and regulation of the rat serotonin 5-HT1A receptor gene were characterized. By three types of analyses, a single brain-specific site of transcriptional initiation was localized to -967 bp upstream of the translation initiation codon that is utilized both in hippocampus and in the rat raphe RN46A cell line. This major site of transcriptional initiation was located 58 bp downstream from a consensus TATA element, suggesting TATA-driven transcription of the rat 5-HT1A receptor. To identify the promoter activity of the receptor gene, progressive 5' deletions of the -2,719/-117-bp fragment of the 5-HT1A promoter linked to luciferase gene were transfected into 5-HT1A-negative (pituitary GH4C1, L6 myoblast, and C6 glioma) and 5-HT1A-positive (septal SN-48 and raphe RN46A) cell lines. Enhancer regions were identified within a fragment between nucleotides -426 and -117 that selectively enhanced transcription in 5-HT1A-positive cells. A nonselective enhancer/promoter that mediated expression in all cell lines was located upstream between -1,519 and -426 bp in a DNA segment containing consensus TATA, CCAAT, SP-1, and AP-1 elements as well as a poly-GT26 dinucleotide repeat. Strong repression of transcription in all cell lines was conferred by the region upstream of -1,519 bp that contains a 152-bp DNA segment with >80% identity to RANTES, tumor necrosis factor-beta, and other immune system genes. Our results indicate that TATA-driven expression of the 5-HT1A receptor is regulated by a novel proximal tissue-specific enhancer region, a nonselective promoter, and an upstream repressor region that is distinct from previously identified neuron-specific repressors.

5-HT modulates multiple conductances in immature rat rostral ventrolateral medulla neurones in vitro

1. Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0.5 microM), 5-HT (50 microM) elicited an outward current (I5-HT,outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I5-HT,inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. 2. The steady-state I-V relationship of I5-HT,outward showed an inward rectification; the 5-HT-induced current, which reversed at -87.9 +/- 3.0 mV, was suppressed by 0.1 mM Ba2+. 3. Two types of steady-state I-V relationship for I5-HT,inward were noted: type I I5-HT,inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (EK), -94 mV, in 8/17 neurones; type II I5-HT,inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. 4. Both type I and type II I5-HT,inward were significantly reduced in a low [Na+]o solution. In this solution, I5-HT,inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I5-HT,inward shifted to about -80 mV as the [K+]o was increased from 3.1 to 7.0 mM in low [Na+]o solution. The type II I5-HT,inward did not reverse at the estimated EK in the same solution. 5. While not affected by externally applied Cs+ (1 mM), I5-HT,inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11.9 +/- 6.4 mV in 8/15 responsive neurones. 6. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I5-HT, outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I5-HT,inward.

The 5-HT1A receptor agonist Bay x 3702 inhibits apoptosis induced by serum deprivation in cultured neurons

We examined whether the highly selective 5-HT1A receptor agonist (-)-(R)-2-[4-[[(3,4-dihydro-2H-1-benzopyran-2-yl)methyl]-amino]butyl]-11 ,2-benz-isothiazol-3(2H)-one 1,1-dioxide monohydrochloride (Bay x 3702) could inhibit neuronal apoptosis induced by serum deprivation. In primary cultures of chick embryonic neurons and in mixed neuronal/glial cultures from neonatal rat hippocampus, Bay x 3702 (1 microM) rescued serum-deprived neurons from apoptosis. The antiapoptotic effect of Bay x 3702 (1 microM) was blocked in chick neurons by the selective 5-HT1A receptor antagonists 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazin]ethyl]-N-2-pyridinyl-be nzamide hydrochloride (p-MPPI, 10 microM) and 4-[3-benzotriazol-1-propyl]-1-(2-methoxyphenyl)-piperazine (BPMP, 10 microM) as well as by anti-nerve growth factor (anti-NGF) antibodies and in rat neurons by N-[2-4-(2-methoxy)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexane-carbo xamide trihydrochloride (WAY 100635, 10 microM). We found only under control conditions (medium with serum), but not in serum-deprived cultures, that NGF secretion was 6-fold increased by Bay x 3702 (1 microM) compared to untreated cultures. Additionally, Bay x 3702 (4 microg/kg i.v.), infused within a period of 4 h, significantly increased the NGF content of the rat hippocampus, but not of the striatum. In summary, our data suggest that Bay x 3702 inhibited growth factor withdrawal-induced apoptosis by the stimulation of 5-HT1A receptors and that the NGF signalling pathway is involved in the mechanism of action.

Role of the medial prefrontal cortex in 5-HT1A receptor-induced inhibition of 5-HT neuronal activity in the rat

1. We examined the involvement of the frontal cortex in the 5-HT2A receptor-induced inhibition of 5-HT neurones in the dorsal raphe nucleus (DRN) of the anaesthetized rat using single-unit recordings complemented by Fos-immunocytochemistry. 2. Both transection of the frontal cortex as well as ablation of the medial region of the prefrontal cortex (mPFC) significantly attenuated the inhibition of 5-HT neurones induced by systemic administration of the 5-HT1A receptor agonist, 8-OH-DPAT (0.5-16 microg kg(-1), i.v.). In comparison, the response to 8-OH-DPAT was not altered by ablation of the parietal cortex. The inhibitory effect of 8-OH-DPAT was reversed by the 5-HT1A receptor antagonist, WAY 100635 (0.1 mg kg(-1), i.v.) in all neurones tested. 3. In contrast, cortical transection did not alter the sensitivity of 5-HT neurones to iontophoretic application of 8-OH-DPAT into the DRN. Similarly, cortical transection did not alter the sensitivity of 5-HT neurones to systemic administration of the selective 5-HT reuptake inhibitor, paroxetine (0.1-0.8 mg kg(-1) , i.v.). 4. 8-OH-DPAT evoked excitation of mPFC neurones at doses (0.5-32 microg kg(-1), i.v.) in the range of those which inhibited 5-HT cell firing. At higher doses (32-512 microg kg(-1), i.v.) 8-OH-DPAT inhibited mPFC neurones. 8-OH-DPAT (0.1 mg kg(-1), s.c.) also induced Fos expression in the mPFC. The neuronal excitation and inhibition, as well as the Fos expression, were antagonized by WAY 100635. 5. These data add further support to the view that the inhibitory effect of 5-HT1A receptor agonists on the firing activity of DRN 5-HT neurones involves, in part, activation of a 5-HT1A receptor-mediated postsynaptic feedback loop centred on the mPFC.

A putative alpha-helical G beta gamma-coupling domain in the second intracellular loop of the 5-HT1A receptor

We have identified a conserved threonine residue in the second intracellular (i2) loop of the 5-HT1A receptor that when mutated to alanine prevents coupling to G beta gamma-mediated signaling, while preserving G alpha i-induced actions. In this review, we investigate the characteristics and potential role of the i2 domain in the coupling of the 5-HT1A receptor and other receptors to G proteins. The i2 domain, as well as portions of the i3 domain, is predicted to form an amphipathic alpha-helix with a positively charged face and a hydrophobic face. Mutagenesis experiments support a model in which the hydrophobic faces of these alpha-helical domains form an intracellular binding "pocket" for interaction with G proteins. Embedded in the hydrophobic face, Thr 149 is crucial for signaling through G beta gamma subunits, perhaps via interaction with its hydroxyl side-chain. Mutation of other residues of the i2 domain of Gi-coupled receptors is required to substantiate the importance of the alpha-helical i2 domain in receptor-G beta gamma signaling. If confirmed in other receptors, these results support a general model in which activated receptor and G beta gamma subunits remain associated to interact with effectors in a receptor-specific manner.

Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors

We characterized potassium current activated by G-protein-coupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba2+. The conductance had identical properties to that activated by GABAB receptors in the same cells. In one-half of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba2+-sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABAB-activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The half-maximally effective agonist concentrations were approximately 75 nM for somatostatin, approximately 100 nM for serotonin, and approximately 400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-to-channel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors

We characterized potassium current activated by G-protein-coupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba2+. The conductance had identical properties to that activated by GABAB receptors in the same cells. In one-half of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba2+-sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABAB-activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The half-maximally effective agonist concentrations were approximately 75 nM for somatostatin, approximately 100 nM for serotonin, and approximately 400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-to-channel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

5-Hydroxytryptamine responses in immature rat rostral ventrolateral medulla neurons in vitro

Whole cell patch recordings were made from rostral ventrolateral medullar (RVLM) neurons of brain-stem slices from 8- to 12-day-old rats. By superfusion or pressure ejection to RVLM neurons, 5-hydroxytryptamine (5-HT) elicited three types of membrane potential changes: a slow hyperpolarization (5-HTH), a slow depolarization (5-HTD) and a biphasic response, which persisted in a tetrodotoxin (TTX, 0.3 microM)-containing solution. 5-HTH were accompanied by a decrease of input resistance in the majority of responsive neurons. Hyperpolarization reduced and depolarization increased the 5-HTH; the mean reversal potential was -92.3 mV in 3.1 mM and shifted to -69.3 mV in 7 mM [K+]o. Barium (Ba2+, 0.1 mM) but not tetraethylammonium (TEA, 10 mM) suppressed 5-HTH. The 5-HT1A receptor agonist (+/-)-8-hydroxy-dipropylamino-tetralin (8-OH-DPAT; 5-50 microM) hyperpolarized RVLM neurons. The 5-HT1A antagonist pindobind-5-HT1A (PBD; 1-3 microM) and the 5-HT2/5-HT1 receptor antagonist spiperone (1-10 microM) suppressed 5-HTH and the hyperpolarizing phase of biphasic responses; the 5-HT2 receptor antagonist ketanserin (3 microM) was without significant effect. 5-HTD were associated with an increase or no apparent change of input resistance in RVLM neurons. Hyperpolarization of the membrane decreased or caused no apparent change in 5-HTD. 5-HTD were reduced in an elevated [K+]o (7.0 mM) solution and > 60% in a low Na+ (26 mM) solution and were not significantly changed in a low Cl- (6.7 mM) or Ca(2+)-free/high Mg2+ (10.9 mM) solution. The 5-HT2 receptor agonist alpha-methyl-5-HT (50 microM) depolarized RVLM neurons, and the 5-HT2 antagonist ketanserin (1-10 microM) attenuated the 5-HTD and the depolarizing phase of biphasic responses, whereas the 5-HT1A receptor antagonist PBD (2 microM) was without effect. Inclusion of the hydrolysis resistant guanine nucleotide GDP-beta-S in patch solution significantly reduced the 5-HTH as well as the 5-HTD. The present study shows that, in the immature rat RVLM neurons, 5-HT causes a slow hyperpolarization and depolarization probably by interacting with 5-HT1A and 5-HT2 receptors, which are G-proteins coupled. 5-HTH may involve an increase of an inwardly rectifying K+ conductance, and 5-HTD appear to be caused by a decrease of K+ conductance and/or increase of nonselective cation conductance.

Tandospirone-induced K+ current in acutely dissociated rat dorsal raphe neurones

1. The effects of tandospirone (TDS) on dissociated rat dorsal raphe neurones were investigated using the patch-clamp method. 2. Under current-clamp conditions, TDS hyperpolarized the cell membrane, resulting in the reduction of firing rates. 3. Under voltage-clamp conditions, TDS induced an inward rectifying K+ current in a concentration-dependent manner. 4. The TDS-induced K+ currents (I(TDS)) were mimicked by 8-OH-DPAT, a 5-HT1A agonist. The I(TDS) was blocked by spiperone, a 5-HT1A receptor antagonist, in a concentration-dependent manner. 5. N-Ethylmaleimide, an agent which uncouples between the receptor and the G-protein, irreversibly blocked the I(TDS). 6. In neurones perfused intracellularly with a pipette-solution containing GTP using the conventional whole-cell patch recording, the I(TDS) showed a gradual rundown. When the neurones were perfused with GTPgammaS, TDS activated the inwardly rectifying K+ current in an irreversible manner. 7. In the inside-out patch recording mode, TDS-activated single K+ channel currents (i(TDS)) which also showed an inward rectification. When the GDP in cytosolic side was completely replaced with GTP, the open probability of i(TDS) significantly increased. 8. These results indicate that the activation of 5-HT1A receptors by TDS directly opens the inward rectifying K+ channels via a G-protein mediated process.

Metabotropic glutamate group II receptors activate a G protein-coupled inwardly rectifying K+ current in neurones of the rat cerebellum

1. The effects of the group II metabotropic glutamate receptor (mGluR) agonists DCG-IV and LY354740 were examined in neurones freshly dissociated from the rat cerebellum and olfactory bulb, using the whole-cell configuration of the patch-clamp technique. 2. Under experimental conditions in which K+ currents would be inward, rapid application of DCG-IV and LY354740 to interneurones expressing the group II mGluRs induced an inward current in a subpopulation of interneurones of the cerebellum, the unipolar brush cells. 3. The currents induced by DCG-IV and LY354740 had the major characteristics of a G protein-coupled inwardly rectifying K+ channel (GIRK) current; namely, rapid activation and deactivation upon agonist application and removal, G protein dependence, strong inward rectification, Cs+ and Ba2+ sensitivity, and K+ selectivity. 4. In Golgi cells of the cerebellum and interneurones of the accessory olfactory bulb, which also express group II mGluRs, LY354740 did not induce GIRK activation but inhibited voltage-gated Ca2+ channel currents. 5. These results demonstrate that, in unipolar brush cells, native group II mGluRs can functionally couple to activation of GIRKs. Thus, the absence of coupling in the majority of CNS neurones examined to date may be due to restricted cellular co-localization or co-expression of the appropriate proteins.