Two signal transduction mechanisms of substance P-induced depolarization in locus coeruleus neurons

Ionic signalling mechanisms involved in neurokinin-3 receptor-mediated augmentation of fear-potentiated startle response in the basolateral amygdala

The tachykinin peptides include substance P (SP), neurokinin A and neurokinin B, which interact with three G-protein-coupled neurokinin receptors, NK1Rs, NK2Rs and NK3Rs, respectively. Whereas high densities of NK3Rs have been detected in the basolateral amygdala (BLA), the functions of NK3Rs in this brain region have not been determined. We found that activation of NK3Rs by application of the selective agonist, senktide, persistently excited BLA principal neurons. NK3R-elicited excitation of BLA neurons was mediated by activation of a non-selective cation channel and depression of the inwardly rectifying K+ (Kir) channels. With selective channel blockers and knockout mice, we further showed that NK3R activation excited BLA neurons by depressing the G protein-activated inwardly rectifying K+ (GIRK) channels and activating TRPC4 and TRPC5 channels. The effects of NK3Rs required the functions of phospholipase Cβ (PLCβ), but were independent of intracellular Ca2+ release and protein kinase C. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate was involved in NK3R-induced excitation of BLA neurons. Microinjection of senktide into the BLA of rats augmented fear-potentiated startle (FPS) and this effect was blocked by prior injection of the selective NK3R antagonist SB 218795, suggesting that activation of NK3Rs in the BLA increased FPS. We further showed that TRPC4/5 and GIRK channels were involved in NK3R-elicited facilitation of FPS. Our results provide a cellular and molecular mechanism whereby NK3R activation excites BLA neurons and enhances FPS. KEY POINTS: Activation of NK3 receptors (NK3Rs) facilitates the excitability of principal neurons in rat basolateral amygdala (BLA). NK3R-induced excitation is mediated by inhibition of GIRK channels and activation of TRPC4/5 channels. Phospholipase Cβ and depletion of phosphatidylinositol 4,5-bisphosphate are necessary for NK3R-mediated excitation of BLA principal neurons. Activation of NK3Rs in the BLA facilitates fear-potentiated startle response. GIRK channels and TRPC4/5 channels are involved in NK3R-mediated augmentation of fear-potentiated startle.

Activation of NK₁ receptors in the locus coeruleus induces analgesia through noradrenergic-mediated descending inhibition in a rat model of neuropathic pain

BACKGROUND AND PURPOSE

The locus coeruleus (LC) is a major source of noradrenergic projections to the dorsal spinal cord, and thereby plays an important role in the modulation of nociceptive information. The LC receives inputs from substance P (SP)-containing fibres from other regions, and expresses the NK(1) tachykinin receptor, a functional receptor for SP. In the present study, we investigated the roles of SP in the LC in neuropathic pain.

EXPERIMENTAL APPROACH

Chronic constriction injury (CCI) of the left sciatic nerve was performed in rats to induce neuropathic pain. After development of neuropathic pain, SP was injected into the LC and the nocifensive behaviours were assessed. The involvement of noradrenergic descending inhibition in SP-induced analgesia was examined by i.t. administration of yohimbine, an α(2) -adrenoceptor antagonist. NK(1) receptor expression in the LC was examined by immunohistochemistry.

KEY RESULTS

In CCI rats, mechanical allodynia was alleviated by SP injection into the LC. These effects were abolished by prior injection of WIN 51708, an NK(1) receptor antagonist, into the LC or i.t. treatment with yohimbine. NK(1) receptor-like immunoreactivity was observed in noradrenergic neurons throughout the LC in intact rats, and remained unchanged after CCI.

CONCLUSION AND IMPLICATIONS

SP in the LC exerted analgesic effects on neuropathic pain through NK(1) receptor activation and resulted in facilitation of spinal noradrenergic transmission. Accordingly, manipulation of the SP/NK(1) receptor signalling pathway in the LC may be a promising strategy for effective treatment of neuropathic pain.

Neurokinin 1 receptor activates transient receptor potential-like currents in noradrenergic A7 neurons in rats

Noradrenergic (NAergic) A7 neurons are involved in modulating nociception by releasing noradrenaline in the dorsal spinal cord. Since NAergic A7 neurons receive dense Substance P (Sub-P) releasing terminals from ventromedial medulla, here we tested the effect of Sub-P on them. Bath application of Sub-P induced an inward current (I(Sub-P)) in NAergic neurons, which was significantly blocked by Neurokinin 1 (NK1) receptor antagonist. The I(Sub-P) was reversed at approximately -20 mV, blocked by several TRP channel blockers, enhanced by OAG and negatively regulated by PKC. Immunohistochemistry staining showed that NAergic A7 neurons express high level of TRPC6 channel proteins, which is consistent with pharmacological properties of I(Sub-P) shown above, as TRPC6 channel is shown to be augmented by OAG and inhibited by PKC. In conclusion, the above results provide mechanism underlying postsynaptic action of Sub-P on NAergic A7 neurons and a role for TRPC6 channel in NAergic pain modulation.

Regulation of a family of inwardly rectifying potassium channels (Kir2) by the m1 muscarinic receptor and the small GTPase Rho

Inwardly rectifying potassium channels Kir2.1-Kir2.3 are important regulators of membrane potential and, thus, control cellular excitability. However, little is known about the regulation of these channels. Therefore, we studied the mechanisms mediating the regulation of Kir2.1-Kir2.3 by the G-protein-coupled m1 muscarinic receptor using the whole-cell patch-clamp technique and recombinant expression in the tsA201 mammalian cell line. Stimulation of the m1 muscarinic receptor inhibited all subtypes of inward rectifier tested, Kir2.1-Kir2.3. The inhibition of each channel subtype was reversible and was attenuated by the muscarinic receptor antagonist, atropine. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) mimicked the effects of m1 receptor activation by inhibiting Kir2.1 currents. However, PMA had no effect on Kir2.2 or Kir2.3. Inclusion of 200-microM guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) in the patch pipette solution prevented the effects of m1 muscarinic receptor stimulation on all three of the channel subtypes tested, confirming the mediation of the responses by G-proteins. Cotransfection with the activated mutant of the small GTPase Rho reduced current density, while C3 exoenzyme, a selective inhibitor of Rho, attenuated the m1 muscarinic receptor-induced inhibition of Kir2.1-Kir2.3. Also, buffering the intracellular calcium concentration with a high concentration of EGTA abolished the m1 receptor-induced inhibition of Kir2.1-Kir2.3, implicating a role for calcium in these responses. These results indicate that all three of the Kir2 channels are similarly inhibited by m1 muscarinic receptor stimulation through calcium-dependent activation of the small GTPase Rho.

Postsynaptic actions of substance P on rat periaqueductal grey neurons in vitro

The postsynaptic actions of substance P on rat midbrain periaqueductal grey (PAG) neurons were examined using whole-cell patch-clamp recordings in brain slices. Substance P produced an inward current in a subpopulation (60%) of PAG neurons. The substance P induced current was concentration dependent (EC50=27 nM) and was reduced by the NK1, NK2 and NK3 antagonists L-732,138 (20 microM), GR 159897 (3 microM) and SB 218795 (3 microM). The selective NK1, NK2 and NK3 agonists [Sar9,Met(O2)11]-Substance P (100 nM), GR 64349 (300-500 nM) and senktide (300 nM) also produced inward currents in subpopulations of neurons. A greater proportion of substance P-sensitive neurons (70%) than substance P-insensitive neurons (31%) responded to the mu/delta opioid agonist met-enkephalin (10 microM). Substance P reduced the outward current produced by met-enkephalin. The reversal potential of the substance P induced current varied from -5 mV to below -140 mV in the absence of met-enkephalin, and was -105 mV in the presence of met-enkephalin. These results indicate that substance P acts via NK1, NK2 and NK3 receptors to excite subpopulations of opioid-sensitive and insensitive PAG neurons by increasing a non-selective cation conductance and by reducing a K+ current. In addition, substance P has anti-opioid actions that are largely mediated by a reduction in the opioid induced K+ current.

Corticotropin-releasing hormone directly activates noradrenergic neurons of the locus ceruleus recorded in vitro

The neuropeptide corticotropin-releasing hormone (CRH) activates locus ceruleus (LC) neurons, thereby increasing norepinephrine levels throughout the CNS. Despite anatomical and physiological evidence for CRH innervation of the LC, the mechanism of CRH-evoked activation of LC neurons is unknown. Moreover, given the apparent absence of mRNA for CRH receptors in LC neurons, the exact location of action of CRH within the cerulear region is debated. Using in vitro intracellular recordings from rat brainstem, we examined whether CRH exerts a direct effect on LC neurons and which ionic currents are likely affected by CRH. We demonstrate that CRH dose-dependently increases the firing rate of LC neurons through a direct (TTX- and cadmium-insensitive) mechanism by decreasing a potassium conductance. The CRH-evoked activation of LC neurons is, at least in part, mediated by CRH1 receptors and a cAMP-dependent second messenger system. These data provide additional support that CRH functions as an excitatory neurotransmitter in the LC and the hypothesis that dysfunction of the CRH peptidergic and noradrenergic systems observed in patients with mood and anxiety disorders are functionally related.

Substance P-mediated modulation of pacemaker properties in the mammalian respiratory network

Neuromodulators are integral parts of a neuronal network, and unraveling how these substances alter neuronal activity is critical for understanding how networks generate patterned activity and, ultimately, behavior. In this study, we examined the cellular mechanisms underlying the excitatory action of substance P (SP) on the respiratory network isolated in spontaneously active transverse slice preparation of mice. SP produced a slow depolarization in all recorded inspiratory pacemaker and non-pacemaker neurons. Ion exchange experiments and blockers for different ion channels suggest that the slow depolarization is caused by the activation of a low-threshold TTX-insensitive cationic current that carries mostly Na+. The SP-induced slow depolarization increased tonic discharge in non-pacemaker neurons and primarily enhanced the frequency of bursting in Cd2+-insensitive pacemaker neurons. In the Cd2+-sensitive pacemaker neuron, the burst frequency was not significantly affected, whereas burst duration and amplitude were more enhanced than in Cd2+-insensitive pacemaker neurons. In a subset of non-pacemaker neurons that produced NMDA-dependent subthreshold oscillations, SP caused the production of bursts of action potentials. We conclude that the degree of pacemaker activity in the respiratory network is not fixed but dynamically regulated by neuromodulators such as SP. This finding may have clinical implications for Rett syndrome in which SP levels along with other neuromodulators are decreased in the brainstem.

Effects of orexin (hypocretin) on GIRK channels

Orexins (hypocretins) are recently discovered excitatory transmitters implicated in arousal and sleep. Yet, their ionic and signal transduction mechanisms have not been fully clarified. Here we show that orexins suppress G-protein-coupled inward rectifier (GIRK) channel activity, and this suppression is likely to lead to neuronal excitation. Cultured neurons from the locus coeruleus (LC) and the nucleus tuberomammillaris (TM) were used, as well as HEK293A cells transfected with GIRK1 and 2, either human orexin receptor type 1 (OX1R) or type 2 (OX2R), mu opioid receptor and GFP cDNAs. In GTPgammaS-loaded cells, orexin A (OXA, 3 microM) inhibited GIRK currents that had previously been activated by somatostatin (in LC cells), nociceptin (TM cells), or the mu opioid agonist DAMGO (HEK cells). In guanosine triphosphate (GTP)-loaded HEK cells, in which GIRK currents were not preactivated, OXA induced a biphasic response through both types of orexin receptors: an initial current increase and a subsequent decrease to below resting levels. Current-voltage (I-V) relationships revealed that both the OXA-induced and suppressed currents are inwardly rectifying with reversal potentials around EK. The OXA-induced initial current was partially pertussis toxin (PTX) sensitive and partially PTX insensitive, whereas the OXA-suppressed current was PTX insensitive. These data suggest that orexin receptors couple with more than one type of G-protein, including PTX-sensitive (such as Gi/o) and PTX-insensitive (such as Gq/11) G-proteins. The modulation of GIRK channels by orexins may be one of the cellular mechanisms for the regulation of brain nuclei (e.g., LC and TM) that are crucial for arousal, sleep, and appetite.

Two different inward rectifier K+ channels are effectors for transmitter-induced slow excitation in brain neurons

Substance P (SP) excites large neurons of the nucleus basalis (NB) by inhibiting an inward rectifier K(+) channel (Kir). The properties of the Kir in NB (KirNB) in comparison with the G protein-coupled Kir (GIRK) were investigated. Single-channel recordings with the cell-attached mode showed constitutively active KirNB channels, which were inhibited by SP. When the recording method was changed from the on-cell to the inside-out mode, the channel activity of KirNB remained intact with its constitutive activity unaltered. Application of Gbeta(1gamma2) to inside-out patches induced activity of a second type of Kir (GIRK). Application of Gbeta(1gamma2), however, did not change the KirNB activity. Sequestering Gbeta(1gamma2) with Galpha(i2) abolished the GIRK activity, whereas the KirNB activity was not affected. The mean open time of KirNB channels (1.1 ms) was almost the same as that of GIRKs. The unitary conductance of KirNB was 23 pS (155 mM [K(+)](o)), whereas that of the GIRK was larger (32-39 pS). The results indicate that KirNB is different from GIRKs and from any of the classical Kirs (IRKs). Whole-cell current recordings revealed that application of muscarine to NB neurons induced a GIRK current, and this GIRK current was also inhibited by SP. Thus, SP inhibits both KirNB and GIRKs. We conclude that the excitatory transmitter SP has two types of Kirs as its effectors: the constitutively active, Gbetagamma-independent KirNB channel and the Gbetagamma-dependent GIRK.

Expression of mRNA and functional alpha(1)-adrenoceptors that suppress the GIRK conductance in adult rat locus coeruleus neurons

1. Locus coeruleus neurons in adult rats express binding sites and mRNA for alpha(1)-adrenoceptors even though the depolarizing effect of alpha(1)-adrenoceptor agonists on neonatal neurons disappears during development. 2. In this study intracellular microelectrodes were used to record from locus coeruleus neurons in brain slices of adult rats and reverse transcription-polymerase chain reaction (RT - PCR) was used to investigate the mRNA expression of alpha(1)- and alpha(2)-adrenoceptors in juvenile and adult rats. 3. The alpha(1)-adrenoceptor agonist phenylephrine had no effect on the membrane conductance of locus coeruleus neurons (V(hold) -60 mV) but decreased the G protein coupled, inward rectifier potassium (GIRK) conductance induced by alpha(2)-adrenoceptor or mu-opioid agonists. The GIRK conductance induced by noradrenaline was increased in amplitude when alpha(1)-adrenoceptors were blocked with prazosin. 4. RT - PCR of total cellular RNA isolated from microdissected locus coeruleus tissue demonstrated strong mRNA expression of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in both juvenile and adult rats. However, only mRNA transcripts for the alpha(1b)-adrenoceptors were consistently detected in cytoplasmic samples taken from single locus coeruleus neurons of juvenile rats, suggesting that this subtype may be responsible for the physiological effects seen in juvenile rats. 5. Juvenile and adult locus coeruleus tissue expressed mRNA for the alpha(2a)- and alpha(2c)-adrenoceptors while the alpha(2b)-adrenoceptor was only weakly expressed in juveniles and was not detected in adults. 6. The results of this study show that alpha(1)-adrenoceptors expressed in adult locus coeruleus neurons function to suppress the GIRK conductance that is activated by mu-opioid and alpha(2)-adrenoceptors.

Substance P in the descending cholinergic projection to REM sleep-induction regions of the rat pontine reticular formation: anatomical and electrophysiological analyses

Release of acetylcholine within the pontine reticular formation (PRF) from the axon terminals of mesopontine cholinergic neurons has long been hypothesized to play an important role in rapid eye movement (REM) sleep generation. As some of these cholinergic neurons are known to contain substance P (SP), we used anatomical, electrophysiological and pharmacological techniques to characterize this projection in the rat. Double immunofluorescence demonstrated that 16% of all cholinergic neurons within the mesopontine tegmentum contained SP; this percentage increased to 27% in its caudal regions. When double immunofluorescence was combined with retrograde tracing techniques, it was observed that up to 11% of all SP-containing cholinergic neurons project to the PRF. Whole-cell patch-clamp recordings from in vitro brainstem slices revealed that SP administration depolarized or evoked an inward current in a dose-dependent manner in all PRF neurons examined, and that these effects were antagonized by a SP antagonist. The amplitude of the SP-induced inward current varied with changes in the Na+ concentration, did not reverse at the calculated K+ or Cl- equilibrium potentials, and was not attenuated in the presence of tetrodotoxin, low Ca2+ concentration or caesium ions. These data suggest that activation of a tetrodotoxin-insensitive cation channel(s) permeable to Na+ is responsible for a SP-induced inward current at resting membrane potentials. The depolarizing actions of SP appeared to be primarily due to activation of the adenylate cyclase pathway, and were additive with cholinergic receptor activation even at maximal concentrations. These data indicate that SP is colocalized in a subpopulation of mesopontine tegmental cholinergic neurons projecting to REM sleep-induction regions of the PRF, and that actions of these two neuroactive substances on PRF neurons are additive. If SP is coreleased with acetylcholine, the additive actions of the two neurotransmitters might heighten the excitability of postsynaptic PRF neurons and ensure the initiation and maintenance of REM sleep.

Regulation of a G protein-gated inwardly rectifying K+ channel by a Ca(2+)-independent protein kinase C

1. Members of the Kir3.0 family of inwardly rectifying K(+) channels are expressed in neuronal, atrial and endocrine tissues and play key roles in generating late inhibitory postsynaptic potentials (IPSPs), slowing heart rate and modulating hormone release. They are activated directly by G(betagamma) subunits released in response to G(i/o)-coupled receptor stimulation. However, it is not clear to what extent this process can be dynamically regulated by other cellular signalling systems. In this study we have explored pathways activated by the G(q/11)-coupled M(1) and M(3) muscarinic receptors and their role in the regulation of Kir3.1+3.2A neuronal-type channels stably expressed in the human embryonic kidney cell line HEK293. 2. We describe a novel biphasic pattern of behaviour in which currents are initially stimulated but subsequently profoundly inhibited through activation of M(1) and M(3) receptors. This contrasts with the simple stimulation seen through activation of M(2) and M(4) receptors. 3. Channel stimulation via M(1) but not M(3) receptors was sensitive to pertussis toxin whereas channel inhibition through both M(1) and M(3) receptors was insensitive. In contrast over-expression of the C-terminus of phospholipase Cbeta1 or a G(q/11)-specific regulator of G protein signalling (RGS2) essentially abolished the inhibitory phase. 4. The inhibitory effects of M(1) and M(3) receptor stimulation were mimicked by phorbol esters and a synthetic analogue of diacylglycerol but not by the inactive phorbol ester 4alphaphorbol. Inhibition of the current by a synthetic analogue of diacylglycerol effectively occluded any further inhibition (but not activation) via the M(3) receptor. 5. The receptor-mediated inhibitory phenomena occur with essentially equal magnitude at all intracellular calcium concentrations examined (range, 0-669 nM). 6. The expression of endogenous protein kinase C (PKC) isoforms in HEK293 cells was examined by immunoblotting, and their translocation in response to phorbol ester treatment by cellular extraction. The results indicated the expression and translocation of the novel PKC isoforms PKCdelta and PKCepsilon. 7. We also demonstrate that activation of such a pathway via both receptor-mediated and receptor-independent means profoundly attenuated subsequent channel stimulation by G(i/o)-coupled receptors. 8. Our data support a role for a Ca(2+)-independent PKC isoform in dynamic channel regulation, such that channel activity can be profoundly reduced by M(1) and M(3) muscarinic receptor stimulation.

Selective blockade of neurokinin (NK)(1) receptors facilitates the activity of adrenergic pathways projecting to frontal cortex and dorsal hippocampus in rats

The selective NK(1) receptor antagonist, GR205,171 (2.5-40.0 mg/kg, i.p.), dose-dependently elevated dialysate levels of noradrenaline (NA), but not serotonin (5-HT), in the frontal cortex of freely moving rats. This action was exerted stereospecifically inasmuch as its less active isomer, GR226,206, was ineffective. In the dorsal hippocampus, GR205,171 (but not GR226,206) also significantly increased dialysate levels of NA, whereas levels of 5-HT were unaffected. Further, in anaesthetized rats, GR205,171 dose-dependently (1.0-4.0 mg/kg, i.v.) increased the firing rate of adrenergic perikarya in the locus coeruleus. In contrast, their activity was not modified by GR226,206. These findings indicate that selective blockade of NK(1) receptors enhances the activity of ascending adrenergic pathways in rats. Adrenergic mechanisms may, thus, be involved in the potential antidepressant and other functional properties of NK(1) receptor antagonists.

Periaqueductal gray neurons monosynaptically innervate extranuclear noradrenergic dendrites in the rat pericoerulear region

Previous reports using light microscopy have provided anatomical evidence that neurons in the ventrolateral periaqueductal gray (PAG) innervate the medial pericoerulear dendrites of noradrenergic neurons in the nucleus locus coeruleus (LC). The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with the somata or dendrites of noradrenergic LC neurons. Deposits of either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the rat ventrolateral PAG labeled a moderate to high number of axons in the region of the medial pericoerulear region and Barrington's nucleus, but a relatively low number were labeled in the nuclear core of the LC. Ultrastructural analysis of anterogradely labeled terminals at the levels of the rostral (n = 233) and caudal (n = 272) subdivisions of the LC indicated that approximately 20% of these form synapses with tyrosine hydroxylase-immunoreactive dendrites; most of these were located in the medial pericoerulear region. In rostral sections, about 12% of these were symmetric synapses, 9% were asymmetric synapses, and 79% were membrane appositions without clear synaptic specializations. In caudal sections, about 30% were symmetric synapses, 11% were asymmetric synapses, and 59% were appositions. In both rostral and caudal sections, 60% of the anterogradely labeled terminals formed synapses with noncatecholamine dendrites, and 20% formed axoaxonic synapses. These results provide direct evidence for monosynaptic projections from neurons in the ventrolateral PAG to the extranuclear dendrites of noradrenergic LC neurons. This monosynaptic pathway may mediate in part the analgesia, reduced responsiveness to external stimuli, and decreased excitability of somatic motoneurons produced by stimulation of neurons in the ventrolateral PAG.

G-protein-independent signaling by G-protein-coupled receptors

Two classes of receptors transduce neurotransmitter signals: ionotropic receptors and heptahelical metabotropic receptors. Whereas the ionotropic receptors are structurally associated with a membrane channel, a mediating mechanism is necessary to functionally link metabotropic receptors with their respective effectors. According to the accepted paradigm, the first step in the metabotropic transduction process requires the activation of heterotrimeric G-proteins. An increasing number of observations, however, point to a novel mechanism through which neurotransmitters can initiate biochemical signals and modulate neuronal excitability. According to this mechanism metabotropic receptors induce responses by activating transduction systems that do not involve G-proteins.

Noradrenergic neurons expressing substance P receptor (NK1) in the locus coeruleus complex: a double immunofluorescence study in the rat

By using a double immunofluorescence method we examined the distribution of noradrenergic neurons expressing substance P receptor (NK1) or neuromedin K receptor (NK3) in the rat brainstem. The distribution of SPR-like immunoreactive (-LI) neurons completely overlapped that of tyrosine hydroxylase (TH)-LI neurons in the locus coeruleus (A6), ventrolateral and lateral reticular formation of pons (A5 and A7). Partially overlapping distribution of SPR- and TH-LI neurons were found in certain regions of the medulla oblongata (A1-A4). Neurons showing both SPR- and TH-like immunoreactivities, however, were only found in the locus coeruleus complex (A5-A7): 100% of these TH-LI neurons displayed SPR-like immunoreactivity. Neurons showing both NKR- and TH-like immunoreactivities were not detected in the aforementioned areas of brainstem. The present study has provided morphological evidence for direct physiological modulation of noradrenergic neurons by tachykinins through SPR in locus coeruleus complex (A5-A7).

Neuropeptides in drug research

Neuropeptides have been a subject of considerable interest in the pharmaceutical industry over the last 20 years or more. Many drug discovery teams have contributed to our understanding of neuropeptide biology but no significant drugs that act selectively upon neuropeptide receptors have yet emerged from the clinic. There are, however, a plethora of clinically useful drugs that act at other classes of neurotransmitter and neuromodulator receptors, many of them discovered over the last 20 years. Nevertheless, we think that the future for the discovery of novel drugs acting at neuropeptide receptors looks bright for two reasons: (1) there has been a substantial increase in our understanding of the function of neuropeptides; and (2) high-throughput screening (HTS) against neuropeptide receptors has now begun to yield many interesting drug-like molecules, rather than peptides, that have the potential to become clinically useful drugs. The objective of this review is to summarise our current understanding of specific areas of neuropeptide biology and pharmacology in the CNS as well as the PNS. We will also speculate on where we think the new generation of neuropeptide agonists and antagonists could emerge from the clinic.

Stress-induced C-fos expression in the rat locus coeruleus is dependent on neurokinin 1 receptor activation

These experiments examined the role of substance P-selective neurokinin 1 receptors in the restraint-induced activation of the rat locus coeruleus. Immunohistochemistry revealed high levels of neurokinin 1 receptor expression in the plasma membrane of tyrosine hydroxylase-positive locus coeruleus neurons. The selective neurokinin 1 receptor antagonists, RP 67580 (5 nmol) and L-760,735 (3.4 nmol), were administered intracerebroventricularly prior to restraint stress, and c-fos protein was measured as an index of locus coeruleus activation. Both antagonists attenuated the restraint-induced increase in locus coeruleus c-fos expression, whereas their inactive enantiomers were ineffective. These results suggest that neurokinin 1 receptors may mediate activation of locus coeruleus neurons during stress. Neurokinin 1 receptor antagonists may prove to be novel therapeutic compounds in the treatment of anxiety and depression.

Action of substance P (neurokinin-1) receptor activation on rat neostriatal projection neurons

Substance P (SP) acts as a neurotransmitter in the neostriatum through the axon collaterals of spiny projection neurons. However, possible direct or indirect actions of SP on the neostriatal output neurons have not been described. Targets of SP terminals within the neostriatum include interneurons, spiny neurons, afferent fibers and boutons. SP induces the release of both dopamine (DA) and acetylcholine (ACh). Since some postsynaptic actions of both DA and ACh on spiny neurons are known, we asked if activation of neostriatal NK1-class receptors is able to reproduce them. The SP NK1-receptor agonist, GR73632 (1 microM), had both excitatory and inhibitory actions on virtually all spiny neurons tested at resting potential. The excitatory action was blocked by atropine and coursed with an increase in firing rate and input resistance (R(N)). The inhibitory action was blocked by haloperidol and coursed with a reduction in firing rate and R(N). Therefore, the release of both DA and ACh induced by NK1-receptor activation modulates indirectly the excitability of the projection neurons. SP facilitates the actions of these transmitters on the spiny neuron. A residual excitatory response to the NK1-receptor agonist was observed in 30% of a sample of neurons tested in the presence of both haloperidol and atropine. The increase in R(N) that accompanied this response could be observed in the presence of 1 microM TTX or 100 microM Cd2+, suggesting a direct effect. Double labeling showed that only SP-immunoreactive neurons were facilitated by NK1-receptor activation in these conditions.

Characterization of the mechanism of action of tachykinins in rat striatal cholinergic interneurons

The mechanism by which substance P depolarizes cholinergic interneurons in the rat striatum was studied using whole-cell recording techniques. In all cases the effects of substance P were mimicked by the neurokinin1 receptor agonist [Sar9, Met(O2)11] substance P and were antagonized by the neurokinin1 receptor antagonist SR140333. [Sar9, Met(O2)11] substance P was found to depolarize cholinergic interneurons by the induction of a calcium-activated inward current at -60 mV. This inward current was irreversibly potentiated by photolysis of caged GTPgammaS within neurons implicating the involvement of a G-protein. The [Sar9, Met(O2)11] substance P-induced inward current was inhibited by the phospholipase C inhibitor U-73122, and by the inclusion of the inositol-1,4,5-triphosphate receptor antagonist heparin in the electrode solution. These findings suggest that neurokinin1 receptors depolarize cholinergic interneurons in the rat striatum primarily through a phosphoinositide signalling pathway.

Substance P-induced inward current in identified auditory efferent neurons in rat brain stem slices

The effects of substance P (SP) on whole cell currents were studied in neurons of the medial olivocochlear efferent system (MOCS) in the ventral nucleus of the trapezoid body (VNTB) of brain stem slices from neonatal rats. Each neuron was identified by retrograde labeling with Fast Blue injected into the cochlea. Bath application of SP (0.1-10 microM) reversibly induced an apparent inward current in 49 of 63 labeled neurons when voltage clamped at near resting voltages. This apparent inward current was consistent with the SP-induced membrane depolarization observed in current-clamp mode. The SP-induced change in current was dose dependent with a half-maximal response dose of 200 nM. It was mimicked by [Cys3,6, Tyr8, Pro9]-SP, a neurokinin (NK1) receptor selective agonist, whereas [Succinyl-Asp6, MePhe8]-SP 6-11 (Senktide), a NK3 receptor agonist, had no detectable effect. The SP effect was not blocked by 10(-6) M tetrodotoxin (TTX) and persisted when the perfusate contained 30 mM tetraethylammonium (TEA) or 100 microM Cd2+ or was in a 0-Ca solution. In a TTX-containing solution, SP caused a voltage-dependent decrease of membrane conductance, and the SP-evoked current reversed at a potential at around -105 mV. The predicted K+ equilibrium potential was -93.8 mV under the experimental conditions. The SP-induced inward current was attenuated by 66% when the perfusate contained 3 mM Cs+. We conclude that the apparent inward current is partly caused by SP decreasing an outward current normally maintained by the inward rectifier K+ channels in these cells. In the presence of Cs solution in the recording pipette and with a perfusate containing 3 mM Cs+, 0.1 mM Cd2+ and 10(-6) M TTX, a residual SP-induced inward current was observed at test voltages ranging from -120 to 40 mV. This subcomponent reversed its polarity at approximately 20 mV. This inward current was reduced substantially (but not abolished) when all NaCl in the external solution was replaced by TEA-Cl. The results indicate that SP also opens an unknown cation channel, which the available data suggests may be relatively nonselective. The results suggest that MOCS neurons are subject to modulation by SP, which depolarizes the cell membrane by decreasing the activity of inward rectifier K+ channels as well as concurrently activating a separate cation conductance. It also was found that in MOCS neurons responsive to both SP and norepinephrine, the norepinephrine effect was abolished by TTX, suggesting that an interneuronal population excited by norepinephrine converges selectively onto SP-sensitive MOCS neurons in the VNTB.

The tachykinin NK1 receptor. Part I: ligands and mechanisms of cellular activation

The tachykinin NK1 receptor is widely expressed in the mammalian central and peripheral nervous system. Powerful pharmacological tools (agonists and antagonists) are now available to elucidate the physiological role of NK1 receptors at these levels, as well as to understand their role in diseases and establish the possible therapeutic usefulness of NK1 receptor antagonists for treatment of human diseases. The structure-activity studies that have led to the development of potent peptide and non-peptide ligands for the tachykinin NK1 receptor are here reviewed. Among the peptide agonists and antagonists, linear and cyclic sequences have been developed. The non peptide antagonists belong to different chemical classes, i.e. steroids, perhydroisoindolones, quinuclidines, piperidines and tryptophane derivatives. The first non peptide antagonists for NK1 receptors have been obtained by random screening of chemical compounds large collections. The resulting leads were optimized with 'classic' structure activity approaches, aiming at identifying 'common' motifs for interaction with the receptor by ligands of different chemical classes. The results derived from the recent application of molecular biology techniques were useful to drive the design of new ligands toward a precise structural definition of ligand-receptor bi-molecular interactions. Studies on mutant receptors have established that the sites of interaction of peptide agonists and non peptide antagonists with the tachykinin NK1 receptor are largely non overlapping. Moreover, data obtained from mutagenesis of the NK1 receptor further indicate that some amino acid residues in the NK1 receptor sequence are critical for determining the binding affinity of some but not all ligands. Therefore, different antagonists discovered from random screening may not possess common points of interaction or common structural and conformational characteristics for their interaction with the tachykinin NK1 receptor. The tachykinin NK1 receptor couples with G-proteins to determine its biological effects in target cells. Several G-proteins both sensitive (Go, Gi) and insensitive (Gq, G11) to pertussis toxin can mediate the action of NK1 receptors. Moreover, several second messanger signalling systems (elevation of intracellular calcium, stimulation of phosphoinositol turnover, arachidonic acid mobilization, cAMP accumulation) have to be activated following NK1 receptor signalling. Also a direct modulation of certain ion channels at membrane level has been proposed. The NK1 receptor undergoes prompt and significant tachyphylaxis upon exposure to the agonist: this has been shown to be linked with receptor internalization which also occurs physiologically when the NK1 receptor is stimulated by endogenous tachykinins.

Tachykinin-induced activation of non-specific cation conductance via NK3 neurokinin receptors in guinea-pig intracardiac neurones

1. Whole mount preparations from guinea-pig hearts were used to characterize the receptors and ionic mechanisms mediating the substance P (SP)-induced depolarization of parasympathetic postganglionic neurones of the cardiac ganglion. 2. Measurement of the amplitude of depolarization in response to superfusion of different tachykinin agonists (neurokinins A (NKA) and B (NKB), SP, and senktide) gave a rank-order potency of NKB = senktide > NKA > SP, indicating involvement of an NK3 receptor. The use of the selective tachykinin receptor antagonists SR 140333, SR 48986, and SR 142801 demonstrated that only the NK3 receptor antagonist SR 142801 inhibited the SP-induced depolarization. 3. The SP-induced depolarization was not inhibited by Ba2+, TEA, or niflumic acid, or altered by reduced Cl- solutions, but was attenuated in reduced Na+ solutions. Single electrode voltage clamp studies demonstrated that the SP-induced inward current increased in amplitude at more negative potentials, had a reversal potential of approximately 0 mV, and was reduced in amplitude in reduced Na+ solutions. 4. We conclude that the SP-induced depolarization in guinea-pig postganglionic parasympathetic neurones of the cardiac ganglion is due to NK3-mediated activation of a non-selective cation conductance.

Low-threshold Na+ currents: a new family of receptor-operated inward currents in mammalian nerve cells

In the mammalian nervous system, various neurotransmitters can modulate cell excitability by inducing slow membrane potential changes. In the last decade, inhibition of potassium currents has been characterized as the primary mechanism by which neurones can undergo sustained depolarization. More recently (1990s), a new class of inward currents, which are voltage-dependent and mainly carried by sodium ions, has been found to be activated by various neurotransmitter receptors in mammalian central and peripheral neurones. Because the channels involved pass depolarizing current, are open at more negative membrane potentials than the resting potential, and are voltage-gated and persistent, these currents are capable of producing regenerative and maintained depolarizations and play an important role in neuronal signalling.

Actions of substance P on rat neostriatal neurons in vitro

Actions of substance P (SP) on the neostriatal neurons in in vitro rat slice preparations were studied via whole-cell patch-clamp recording. Almost all large aspiny neurons (cholinergic cells) and half of the low-threshold spike (LTS) cells (somatostatin/ NOS-positive cells) showed depolarization or an inward shift of the holding currents in response to bath-applied SP in a dose-dependent manner. In contrast, no responses were observed in fast-spiking (FS) cells (parvalbumin-positive cells) and medium spiny cells. Spike discharges followed by slow EPSPs/EPSCs were evoked by intrastriatal electrical stimulation in the large aspiny neurons. Pretreatment with [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-SP, an antagonist of the SP receptor, reversibly suppressed the induction of the slow EPSPs/EPSCs and unmasked slow IPSCs. The SP-induced inward current, although almost unchanged even after the blockade of Ih channels and voltage-dependent Na+, Ca2+, and K+ channels, changed its amplitude according to the Na+ concentration used in both the large aspiny neurons and LTS cells. Thus, the cation current could account for virtually all of the inward current at resting levels in both neurons. These results suggest that the firing of afferent neurons such as striatonigral medium spiny neurons, one of the possible sources of SP, would increase the firing probability of the two types of interneurons of the neostriatum by SP-receptor-mediated opening of tetrodotoxin-insensitive cation channels.

Single-channel properties of the nonselective cation conductance induced by neurotensin in dopaminergic neurons

Slow nonselective cation conductances play a central role in determining the excitability of many neurons, but heretofore this channel type has not been analyzed at the single-channel level. Neurotensin (NT) excites cultured dopaminergic neurons from the ventral tegmental area primarily by increasing such a cation conductance. Using the outside-out configuration of the patch clamp, we elicited single-channel activity of this NT-induced cation channel. Channel activity was blocked by the nonpeptide NT antagonist SR48692, indicating that the response was mediated by NT receptors. The channel opened in both solitary form and in bursts. The reversal potential was -4.2 +/- 1.7 mV, and the elementary conductance was 31 pS at -67 mV with [Na+]o = 140 mM, [Cs+]o = 5 mM, [Na+]i = 88 mM, and [Cs+]i = 74 mM. Thus, the channel was permeable to both Na+ and Cs+. From these characteristics, it is likely that this channel is responsible for the whole-cell current we studied previously. In guanosine 5'-[gamma-thio]triphosphate-loaded cells, NT irreversibly activated about half of the channel activity, suggesting that at least part of the response was mediated by a G protein. Similar channel activity could be induced occasionally in the cell-attached configuration by applying NT outside the patch region.

Increased level of neurokinin-1 tachykinin receptor gene expression during early postnatal development of rat brain

Substance P is known to elicit diverse actions via activating multiple subtypes of tachykinin receptors, and these actions appear to be involved not only in synaptic transmission but also in synaptic plasticity during development of the mammalian central nervous system. The availability of sensitive quantitation of individual tachykinin receptor subtypes is crucial for elucidating the physiological function specifically mediated by activation of a particular receptor subtype. We thus attempted to develop an assay to determine the level of messenger RNA molecule encoding the neurokinin-1-type tachykinin receptor and apply it for assessment of developmental changes in the neurokinin-1 receptor gene expression in the rat brain to explore the role of tachykinin receptors during ontogeny. The assay was designed to use a competitive reverse transcription-polymerase chain reaction co-amplifying endogenous neurokinin-1 receptor messenger RNA and internal standard, which enabled specific quantification of the number of neurokinin-1 receptor transcripts, ranging from 3.1 x 10(3) to 1.3 x 10(5) molecules/microgram total RNA. The levels of neurokinin-1 receptor gene expression were examined in three different brain regions of the rat aged 0-56 days after birth. The order of neurokinin-1 receptor messenger RNA expression was hippocampus > cerebral cortex > > cerebellum at all ages examined except postnatal day 0, where its expression was more abundant in the cerebral cortex than in the hippocampus. From postnatal day 3 onward, the hippocampus contained 140-160% of the cortical levels. Although the tachykinin receptor expression in the cerebellum was too low to be accurately assessed by conventional techniques, our assay enabled us to determine the amount of cerebellar neurokinin-1 receptor messenger RNA that changed in the range 7-23% of the cortical level during postnatal development. A prominent feature revealed by this assay is that the neurokinin-1 receptor gene expression in the rat brain is developmentally regulated. The hippocampus displayed a transient peak of neurokinin-1 receptor messenger RNA at postnatal day 3 and a subsequent gradual decrease. In the cerebral cortex, the amount of the message was highest at birth, and was followed by a moderate decrease during postnatal development. At 56 days after birth, the expression levels in both brain regions were down-regulated to approximately 50% of their maximal levels. The transitory pattern of gene expression was also observed in the cerebellum. The results of this study demonstrate that the reverse transcription-polymerase chain reaction-based assay is useful to quantitate precisely the neurokinin-1 tachykinin receptor message in limited tissue samples derived from discrete brain regions. Together with previous findings, the increased level of neurokinin-1 receptor messenger RNA expression in immature rat brain shown by the present analysis suggests that the neurokinin-1-type tachykinin receptor may play a role in the synaptic plasticity associated with morphological and functional development of the mammalian CNS.

Families of ion channels with two hydrophobic segments

The family of epithelial sodium channels and nematode degenerins has expanded recently to include a member found only in brain, and another that functions in molluscs as a ligand-gated channel. A new gene family of mammalian ATP-gated channels has been discovered; one of its seven members plays a role in the lysis of macrophages. The mechanosensitive channel of bacteria has emerged as the simplest form of a channel protein subunit with two hydrophobic domains.

Effects of substance P on medial vestibular nucleus neurons in guinea-pig brainstem slices

The undecapeptide substance P (SP) has been recently implicated in the control of vestibular function. In particular, it seems to be co-localized with glutamate in approximately half of the primary vestibular afferents in mammals. Using intracellular recordings in guinea-pig brainstem slices, we have investigated the effects of SP and of several agonists of the three known tachykinin receptor subtypes (NK1, NK2 and NK3) on the three main types (A, B and B+LTS) of guinea-pig medial vestibular nucleus neurons (MVNn) that we had previously described. SP could induce two distinct kinds of effects on all types of MVNn. Whereas around half of them were depolarized and had their membrane resistance increased by SP, approximately 10% of all MVNn were in contrast hyperpolarized and inhibited while their membrane resistance was decreased. Both responses persisted under conditions of blockade of synaptic transmission, and were thus due to the activation of postsynaptic binding sites. The SP-induced membrane depolarization could not be reproduced with any one of the specific agonists of the three tachykinin receptor subtypes, nor was it blocked by the specific NK1 receptor antagonists GR 82664 and CP 99994. This effect might therefore be due to the activation of a new, pharmacologically distinct, 'NK1-like' receptor. Only the hyperpolarizing effects, which were in contrast mimicked by the specific NK1 receptor agonists GR 73632 and [Sar9, Met (O2)11]-SP, would be mediated by the few typical NK1 receptors which have been demonstrated in the medial vestibular nucleus.

G protein-coupled inward rectifier modulated by dopamine agonists in cultured substantia nigra neurons

Dopamine release from dopaminergic neurons in the substantia nigra plays an important role in regulating their activity. In the present experiments the whole-cell clamp technique was used to elucidate the modulatory role of dopamine on K+ conductance in substantia nigra neurons in culture. In pars compacta neurons held near the resting potential, dopamine and quinpirole (a D2 receptor agonist) increased membrane conductance. This response was blocked by S(-)-sulpiride, a D2 receptor antagonist. The current-voltage relation of the quinpirole-induced current showed an inward rectification with a reversal potential that approximately equaled the K+ equilibrium potential. Intracellular application of a non-hydrolysable GTP analogue, guanosine 5'-O-(3-thiotriphosphate; 0.1-0.5 mM) into pars compacta neurons produced an initial spontaneous increase in an inwardly rectifying K+ conductance. Quinpirole markedly accelerated this conductance increase. In cells loaded with a poorly metabolized GDP analogue, guanosine 5'-O-(2-thiodiphosphate), the quinpirole response was diminished. Treatment of pars compacta neurons with pertussis toxin abolished the D2 response. Intracellular application of cyclic AMP (1 mM) neither mimicked nor occluded the D2 response. These results indicate that D2 receptor agonists produce neuronal inhibition by activating an inwardly rectifying K+ current, that this D2 agonist effect is mediated by a pertussis toxin-sensitive G protein, and that cyclic AMP does not mediate the D2 response. Unexpectedly, in cells loaded with guanosine 5'-O-(3-thiotriphosphate) after the inwardly rectifying K+ conductance spontaneously reached a maximum, it started to decline slowly. In cells loaded with guanosine 5'-O-(2-thiodiphosphate), the quinpirole response became diminished at first, but then the membrane conductance slowly increased, together with an increase in the quinpirole response. These results were explained by an interplay of two G proteins, one producing an increase and the other producing a decrease in the K+ conductance. In some pars reticulata neurons, R(+)-SKF-38393 (a D1 receptor agonist) reduced an inwardly rectifying K+ conductance; this would excite the neuron under current clamp conditions. The present study shows that cultured substantia nigra neurons produce functional responses to dopamine agonists. Using these neurons, the ionic mechanisms and signal transduction mechanisms of the responses were elucidated in a more critical manner than in previous studies.

Protein kinase C-mediated inhibition of an inward rectifier potassium channel by substance P in nucleus basalis neurons

In nucleus basalis neurons, substance P (SP) causes a slow excitation, mediated through a pertussis toxin-insensitive G protein, by suppressing an inward rectifier K+ channel. Here we report that SP applied outside the patch pipette inhibited the single-channel activity, recorded on-cell, of the inward rectifier. The PKC inhibitors staurosporine and PKC(19-36) suppressed this effect in whole-cell mode and in on-cell single-channel mode. A diacylglycerol analog mimicked the SP effect, and PKC(19-36) suppressed this analog effect. SP irreversibly suppressed the inward rectifier in neurons treated with okadaic acid. These results indicate that a diffusible messenger mediates the SP effect, that its signal transduction involves phosphorylation by PKC, and that dephosphorylation by a serine/threonine protein phosphatase mediates its recovery.

Opposing mechanisms of regulation of a G-protein-coupled inward rectifier K+ channel in rat brain neurons

In locus coeruleus neurons, substance P (SP) suppresses an inwardly rectifying K+ current via a pertussis toxin-insensitive guanine nucleotide binding protein (G protein; GnonPTX), whereas somatostatin (SOM) or [Met]enkephalin (MENK) enhances it via a pertussis toxin-sensitive G protein (GPTX). The interaction of the SP and the SOM (or MENK) effects was studied in cultured locus coeruleus neurons. In neurons loaded with guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), application of SOM (or MENK) evoked a persistent increase in the inward rectifier K+ conductance. A subsequent application of SP suppressed this conductance to a level less than that before the SOM (or MENK) application; the final conductance level was independent of the magnitude of the SOM (or MENK) response. This suppression by SP was persistent, and a subsequent SOM (or MENK) application did not reverse it. When SP was applied to GTP[gamma S]-loaded cells first, subsequent SOM elicited only a small response. In GTP-loaded neurons, application of SP temporarily suppressed the subsequent SOM- (or MENK)-induced conductance increase. These results suggest that the same inward rectifier molecule that responds to an opening signal from GPTX also responds to a closing signal from GnonPTX. The closing signal is stronger than the opening signal.

Antagonism of nociceptive responses of cat spinal dorsal horn neurons in vivo by the NK-1 receptor antagonists CP-96,345 and CP-99,994, but not by CP-96,344

Extracellular and intracellular studies were undertaken to test the effects of the non-peptide, substance P (NK-1) receptor antagonists CP-96,345 and CP-99,994, and of CP-96,344, the inactive enantiomer of CP-96,345, on the responses of spinal dorsal horn neurons to peripheral noxious and non-noxious cutaneous stimuli in spinalized cats anesthetized with alpha-chloralose. The effect of these agents on the response of dorsal horn neurons to iontophoretic application of substance P was also tested in extracellular studies. The substance P-induced slow, prolonged discharge of dorsal horn neurons was blocked by administration (0.5 mg/kg, i.v.) of CP-96,345 (n = 10) or CP-99,994 (n = 9), but was unaffected by CP-96,344 (n = 9). The response of substance P-sensitive neurons to noxious thermal stimulation of the cutaneous receptive field, especially the late afterdischarge phase, was also significantly inhibited by CP-96,345 (n = 10) and by CP-99,994 (n = 7). The response of such neurons to noxious pinch stimulation of the receptive field was also significantly inhibited by CP-96,345 (n = 7) and CP-99,994 (n = 8), but the response of three other substance P-sensitive neurons to pinch was unaffected by CP-96,345. CP-96,344 did not affect the response of any neuron tested to either of these noxious stimuli (noxious thermal, n = 7; pinch, n = 6). The response to hair afferent stimulation was unaffected by any of these compounds (CP-96,345, n = 16; CP-96,344, n = 5; CP-99,994, n = 6). In intracellular studies, the effect of these antagonists was tested on responses of dorsal horn neurons to noxious pinch stimulation or to a train of high intensity electrical stimulation of the superficial peroneal nerve. Both stimuli produced an initial fast depolarization followed by a slow and prolonged depolarization with corresponding discharge patterns. CP-96,345 (n = 3) and CP-99,994 (n = 6) selectively blocked the late, slow components of the stimulus-evoked response without affecting the early components. Responses to single electrical pulses of the same intensity and duration were not affected. CP-96,344 did not affect any of the responses tested (n = 5). The data indicate that nociceptive responses of a subset of spinal dorsal horn cells are selectively blocked by the NK-1 receptor antagonists, CP-96,345 and CP-99,994, thus confirming the involvement of NK-1 receptors in these responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Fast inhibition of inwardly rectifying K+ channels by multiple neurotransmitter receptors in oligodendroglia

An essential function of myelinating oligodendroglia in the mammalian central nervous system is the regulation of extracellular potassium levels by means of a prominent inwardly rectifying K+ current. Cardiac and neuronal K+ inward rectifiers are either activated by hyperpolarizing voltages or controlled by neurotransmitters through the action of receptor-activated G proteins. Neuromodulation of inward rectifiers has not previously been considered as a way to regulate oligodendrocyte function. Here we report the expression of serotonin, somatostatin and muscarinic acetylcholine G protein-coupled receptors in rat brain oligodendrocytes. Activation of these receptors leads to pertussis toxin-sensitive inhibition of inwardly rectifying K+ channels within < 1 s. By contrast, in the heart and in neurons, similar pathways activate an inwardly rectifying conductance. Thus, transmitter-mediated blockade of inward rectifiers appears to be an oligodendrocyte-specific variation of a common motif for convergent signalling pathways. In vivo, expression of this mechanism, which may be dependent on neuron-glia signalling, may have a regulatory role in K+ homeostasis during neuron activity in the central nervous system.

The role of arachidonic acid metabolism in somatostatin and substance P effects on inward rectifier K conductance in rat brain neurons

Somatostatin enhances an inward rectifier K conductance in cultured locus coeruleus neurons, while substance P reduces an inward rectifier K conductance in cultured nucleus basalis and locus coeruleus neurons. The role of arachidonic acid metabolites in these responses was studied. The somatostatin-induced response was reduced by phospholipase A2 inhibitors, non-specific lipoxygenase inhibitors and specific 5-lipoxygenase inhibitors. A cyclooxygenase inhibitor and a 12-lipoxygenase inhibitor had no effect. 5(S)-HPETE occasionally increased the K conductance, but failed to occlude the somatostatin response. The substance P response was suppressed by a 5-lipoxygenase inhibitor but not by a 12-lipoxygenase inhibitor. These results suggest that the 5-lipoxygenase pathway is not a specific messenger of either one of these responses, but that it plays a more general role in maintaining or enhancing the effectiveness of both somatostatin and substance P responses.

Neurotensin excites basal forebrain cholinergic neurons: ionic and signal-transduction mechanisms

The whole-cell patch-clamp technique was used to investigate the effect of neurotensin on cholinergic neurons cultured from the rat nucleus basalis of Meynert. Neurotensin excited the neurons by inducing an initial inward current carried, at least in part, by Na+ and by reducing inwardly rectifying K+ conductance. Reduction of the inwardly rectifying K+ conductance was mediated by a pertussis toxin-insensitive G protein.