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

Ionic and signaling mechanisms involved in neurotensin-mediated excitation of central amygdala neurons

Neurotensin (NT) serves as a neuromodulator in the brain where it regulates a variety of physiological functions. Whereas the central amygdala (CeA) expresses NT peptide and NTS1 receptors and application of NT has been shown to excite CeA neurons, the underlying cellular and molecular mechanisms have not been determined. We found that activation of NTS1 receptors increased the neuronal excitability of the lateral nucleus (CeL) of CeA. Both phospholipase Cβ (PLCβ) and phosphatidylinositol 4,5-bisphosphate (PIP₂) depletion were required, whereas intracellular Ca²⁺ release and PKC were unnecessary for NT-elicited excitation of CeL neurons. NT increased the input resistance and time constants of CeL neurons, suggesting that NT excites CeL neurons by decreasing a membrane conductance. Depressions of the inwardly rectifying K⁺ (Kir) channels including both the Kir2 subfamily and the GIRK channels were required for NT-elicited excitation of CeL neurons. Activation of NTS1 receptors in the CeL led to GABAergic inhibition of medial nucleus of CeA neurons, suggesting that NT modulates the network activity in the amygdala. Our results may provide a cellular and molecular mechanism to explain the physiological functions of NT in vivo.

The actions of neurotensin in rat bladder detrusor contractility

This study assessed the expression, distribution and function of neurotensin (NTs) and two main neurotensin receptors (NTSR), NTSR1 and NTSR2 in normal rat urinary bladders. NTs is primarily located in the suburothelium and the interstitium of smooth muscle bundles. The NTSR1 and NTSR2 receptor subtypes are found to co-localize with smooth muscle cells (SMCs). NTs not only can directly act on bladder SMCs to induce intracellular calcium mobilization by activating the phospholipase C/inositol triphosphate (PLC/IP3) pathway, promoting extracellular calcium influx through a non-selective cation channels, but may be also involved in the modulation of the cholinergic system. Nowadays, the selective antimuscarinic drugs (solifenacin) and the selective beta 3-adrenergic agonist (mirabegron) are used as the first-line pharmacotherapy for overactive bladder (OAB), but without satisfactory treatment benefits in some patients. This study provided evidence suggesting that bladder NTs may play an important role in the regulation of micturition. Further research is needed to investigate the effects of NTs on bladder contractility and the underlying mechanism, which might reveal that the administration of NTSR antagonists can potentially relieve the symptoms of OAB by coordination with antimuscarinic pharmacotherapy.

Sleep-wake sensitive mechanisms of adenosine release in the basal forebrain of rodents: an in vitro study

Adenosine acting in the basal forebrain is a key mediator of sleep homeostasis. Extracellular adenosine concentrations increase during wakefulness, especially during prolonged wakefulness and lead to increased sleep pressure and subsequent rebound sleep. The release of endogenous adenosine during the sleep-wake cycle has mainly been studied in vivo with microdialysis techniques. The biochemical changes that accompany sleep-wake status may be preserved in vitro. We have therefore used adenosine-sensitive biosensors in slices of the basal forebrain (BFB) to study both depolarization-evoked adenosine release and the steady state adenosine tone in rats, mice and hamsters. Adenosine release was evoked by high K⁺, AMPA, NMDA and mGlu receptor agonists, but not by other transmitters associated with wakefulness such as orexin, histamine or neurotensin. Evoked and basal adenosine release in the BFB in vitro exhibited three key features: the magnitude of each varied systematically with the diurnal time at which the animal was sacrificed; sleep deprivation prior to sacrifice greatly increased both evoked adenosine release and the basal tone; and the enhancement of evoked adenosine release and basal tone resulting from sleep deprivation was reversed by the inducible nitric oxide synthase (iNOS) inhibitor, 1400 W. These data indicate that characteristics of adenosine release recorded in the BFB in vitro reflect those that have been linked in vivo to the homeostatic control of sleep. Our results provide methodologically independent support for a key role for induction of iNOS as a trigger for enhanced adenosine release following sleep deprivation and suggest that this induction may constitute a biochemical memory of this state.

Neurotensin inhibits background K+ channels and facilitates glutamatergic transmission in rat spinal cord dorsal horn

Neurotensin (NT) is a neuropeptide involved in the modulation of nociception. We have investigated the actions of NT on cultured postnatal rat spinal cord dorsal horn (DH) neurons. NT induced an inward current associated with a decrease in membrane conductance in 46% of the neurons and increased the frequency of glutamatergic miniature excitatory synaptic currents in 37% of the neurons. Similar effects were observed in acute slices. Both effects of NT were reproduced by the selective NTS1 agonist JMV449 and blocked by the NTS1 antagonist SR48692 and the NTS1/NTS2 antagonist SR142948A. The NTS2 agonist levocabastine had no effect. The actions of NT persisted after inactivation of G(i/o) proteins by pertussis toxin but were absent after inactivation of protein kinase C (PKC) by chelerythrine or inhibition of the MAPK (ERK1/2) pathway by PD98059. Pre- and postsynaptic effects of NT were insensitive to classical voltage- and Ca(2+) -dependent K(+) channel blockers. The K(+) conductance inhibited by NT was blocked by Ba(2+) and displayed no or little inward rectification, despite the presence of strongly rectifying Ba(2+) -sensitive K(+) conductance in these neurons. This suggested that NT blocked two-pore domain (K2P) background K(+) -channels rather than inwardly rectifying K(+) channels. Zn(2+) ions, which inhibit TRESK and TASK-3 K2P channels, decreased NT-induced current. Our results indicate that in DH neurons NT activates NTS1 receptors which, via the PKC-dependent activation of the MAPK (ERK1/2) pathway, depolarize the postsynaptic neuron and increase the synaptic release of glutamate. These actions of NT might modulate the transfer and the integration of somatosensory information in the DH.

Measurement of orexin (hypocretin) and substance P effects on constitutively active inward rectifier K(+) channels in brain neurons

Electrophysiological experiments in our laboratory have led to the discovery that the cholinergic neurons in the nucleus basalis in the rat forebrain possess constitutively active inward rectifier K(+) channels. Unlike cloned inward rectifier K(+) channels, these constitutively active inward rectifier K(+) channels were found to have unique properties, and thus were named "KirNB" (inward rectifier K(+) channels in the nucleus basalis). We found that slow excitatory transmitters, such as orexin (hypocretin) and substance P, suppress the KirNB channel, resulting in neuronal excitation. Furthermore, it was discovered that suppression of KirNB channels by these transmitters is through protein kinase C (PKC). This chapter describes detailed electrophysiological techniques for investigating the effects of orexin and substance P on constitutively active KirNB channels. For this purpose, we also present a method for culturing nucleus basalis cholinergic neurons in which KirNB channels exist. Then, we describe the procedures through which PKC has been determined to mediate inhibition of KirNB channels by orexin and substance P. There are probably many other transmitters which may produce effects on KirNB channels. This chapter will enable researchers to investigate the effects of such transmitters on KirNB channels and their roles in neuronal functions.

cGMP activates a pH-sensitive leak K+ current in the presumed cholinergic neuron of basal forebrain

In an earlier study, we demonstrated that nitric oxide (NO) causes the long-lasting membrane hyperpolarization in the presumed basal forebrain cholinergic (BFC) neurons by cGMP-PKG-dependent activation of leak K+ currents in slice preparations. In the present study, we investigated the ionic mechanisms underlying the long-lasting membrane hyperpolarization with special interest in the pH sensitivity because 8-Br-cGMP-induced K+ current displayed Goldman-Hodgkin-Katz rectification characteristic of TWIK-related acid-sensitive K+ (TASK) channels. When examined with the ramp command pulse depolarizing from -130 to -40 mV, the presumed BFC neurons displayed a pH-sensitive leak K+ current that was larger in response to pH decrease from 8.3 to 7.3 than in response to pH decrease from 7.3 to 6.3. This K+ current was similar to TASK1 current in its pH sensitivity, whereas it was highly sensitive to Ba(2+), unlike TASK1 current. The 8-Br-cGMP-induced K+ currents in the presumed BFC neurons were almost completely inhibited by lowering external pH to 6.3 as well as by bath application of 100 microM Ba(2+), consistent with the nature of the leak K+ current expressed in the presumed BFC neurons. After 8-Br-cGMP application, the K+ current obtained by pH decrease from 7.3 to 6.3 was larger than that obtained by pH decrease from pH 8.3 to 7.3, contrary to the case seen in the control condition. These observations strongly suggest that 8-Br-cGMP activates a pH- and Ba(2+)-sensitive leak K+ current expressed in the presumed BFC neurons by modulating its pH sensitivity.

Nonselective cation channels are essential for maintaining intracellular Ca2+ levels and spontaneous firing activity in the midbrain dopamine neurons

Intracellular Ca2+ and Ca2+-permeable ion channels are important in regulating the firing activity and pattern of midbrain dopamine neurons, but the role of Ca2+-permeable nonselective cation channels (NSCCs) on spontaneous firing activity is unclear. Therefore, we investigated how Ca2+-permeable NSCCs modulate spontaneous firing activity and cytosolic Ca2+ concentration ([Ca2+]c) in acutely isolated midbrain dopamine neurons of the rat. Applications of voltage-dependent Ca2+ channels antagonists failed to abolish spontaneous firing activity completely, but they decreased firing rate and [Ca2+]c. However, a blockade of NSCCs by 2-APB or SKF96365 more potently suppressed spontaneous firings with a depolarization of membrane potential and strong decreases in basal [Ca2+]c levels. The depolarization of membrane potentials was attenuated by intracellular dialysis with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). NSCCs blockers inhibited oscillatory potentials and decreased basal [Ca2+]c in the presence of tetrodotoxin. Apamin, a small-conductance Ca2+-activated K+ channel inhibitor, depolarized membrane potentials and enhanced firing rates. From these data, we conclude that NSCCs not only make up the tonic Ca2+ entry pathways to uphold basal [Ca2+]c levels but also contribute to generation of spontaneous firings, thereby regulating spontaneous firing activities of the midbrain dopamine neurons.

The role of neurotensin in central nervous system pathophysiology: what is the evidence?

The peptide neurotensin has been studied for more than 30 years. Although it is widely distributed in the central and peripheral nervous systems, neurotensin has been more intensely studied with regard to its interactions with the central dopamine system. A number of claims have been made regarding its possible implication in many diseases of the central nervous system, including schizophrenia. In this review, we describe briefly the basic biology of this neuropeptide, and then we consider the strengths and the weaknesses of the data that suggest a role for neurotensin in schizophrenia, drug abuse, Parkinson's disease, pain, central control of blood pressure, eating disorders, cancer, neurodegenerative disorders and inflammation.

Neurotensin modulates synaptic transmission in the nucleus of the solitary tract of the rat

Whole-cell patch clamp recordings were made from neurons of the rat subpostremal region of the nucleus tractus solitarius (NTS) in transverse brainstem slices. Neurotensin (NT) enhanced the firing rate of action potentials from 0.8 +/- 0.4 Hz in control to 1.9 +/- 1.3 Hz (n = 9) and increased their decay time. The peak amplitude of the after-hyperpolarization was decreased by 34+/-5% (n = 9). These effects were associated with a depolarization of 4 +/- 1 mV (n = 10) in the resting membrane potential and an increase in the input resistance (from 768 +/- 220 MOmega to 986+/-220 MOmega; n = 5) and were compensated by manually hyperpolarizing the cell to control values. In voltage clamp experiments NT decreased an outward current (from 488 +/- 161 to 340 +/- 96 pA at +40 mV; n = 5) which reversed near the potassium equilibrium potential. In addition, NT increased the frequency of both excitatory and inhibitory spontaneous synaptic currents, an effect blocked by tetrodotoxin, and did not change the evoked excitatory or inhibitory postsynaptic currents. The selective NTR1 receptor antagonist SR48692 reversibly blocked the effects of NT on both action potentials and spontaneous synaptic currents. Our results suggest that NTR1 receptors can modulate post-synaptic responses in neurons of the subpostremal NTS by increasing cell excitability as a result of blockade of a potassium conductance.

High-affinity neurotensin receptor is involved in phosphoinositide hydrolysis stimulation by carbachol in neonatal rat brain

Ontogenetic studies indicate that inositol phosphate accumulation in rodent brain tissue by cholinergic muscarinic agonists as well as expression of high-affinity neurotensin receptor (NTS1) peak at 7 days after birth. Herein, potential participation of this receptor in such effect was investigated. Cerebral cortex prisms of 7-day-old rats were preloaded with [3H]myoinositol and later incubated during 60 or 20 min in the presence of muscarinic agonist carbachol plus neurotensin and SR 48692, a non-peptide NTS1 antagonist. In 60-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was roughly 320%, an effect which remained unaltered plus 10(-6) M to 10(-4) M neurotensin but partially decreased with equimolar SR 48692 concentration. In 20-min incubation experiments, inositol phosphate accumulation by 10(-3) M carbachol was circa 240%, a value which attained 320-360% plus 10(-7) M neurotensin; this effect was totally blocked by 10(-7) M SR 48692. It was concluded that in inositol phosphate accumulation by carbachol, besides the cholinergic muscarinic receptor, the NTS1 receptor is likewise involved; findings at 60 min are attributable to the effect of endogenous neurotensin whereas those at 20 min most likely involve both endogenous and exogenously added peptide.

Endogenous neurotensin attenuates dopamine-dependent locomotion and stereotypy

The neuropeptide neurotensin (NT) is highly sensitive to changes in dopaminergic signaling in the striatum, and is thought to modulate dopamine-mediated behaviors. To explore the interaction of NT with the dopamine system, we utilized mice with a targeted deletion of dopamine synthesis specifically in dopaminergic neurons. Dopamine levels in dopamine-deficient (DD) mice are less than 1% of control mice, and they require daily administration of the dopamine precursor L-dihydroxyphenylalanine (L-DOPA) for survival. DD mice are supersensitive to the effects of dopamine, becoming hyperactive relative to control mice in the presence of L-DOPA. We show that 24 h after L-DOPA treatment, when DD mice are in a "dopamine-depleted" state, Nt mRNA levels in the striatum of DD mice are similar to those in control mice. Administration of L-DOPA or L-DOPA plus the L-amino acid decarboxylase inhibitor, carbidopa, (C/L-DOPA) induced Nt expression in the striatum of DD mice. The dopamine D1 receptor antagonist, SCH23390, blocked C/L-DOPA-induced Nt. To test the hypothesis that this striatal Nt expression modulated dopamine-mediated behavior in DD mice, we administered SR 48692, an antagonist of the high affinity NT receptor, together with L-DOPA or C/L-DOPA. L-DOPA-induced hyperlocomotion and C/L-DOPA-induced stereotypy were potentiated by peripheral administration of SR 48692. Furthermore, intrastriatal microinjections of SR 48692 augmented L-DOPA-induced hyperlocomotion. These results demonstrate a dynamic regulation of striatal Nt expression by dopamine via D1 receptors in DD mice, and point to a physiological role for endogenous striatal NT in counteracting motor behaviors induced by an overactive dopamine system.

The neurotensin agonist PD149163 increases Fos expression in the prefrontal cortex of the rat

Dopaminergic axons innervating the prefrontal cortex (PFC) target both pyramidal cells and GABAergic interneurons. Many of these dopamine (DA) axons in the rat coexpress the peptide neurotransmitter neurotensin. Previous electrophysiological data have suggested that neurotensin activates GABAergic interneurons in the PFC. Activation of D2-like DA receptors increases extracellular GABA levels in the PFC, as opposed to the striatum, where D2 receptor activation inhibits GABAergic neurons. Because activation of presynaptic D2 release-modulating autoreceptors in the PFC suppresses DA release but increases release of the cotransmitter neurotensin, D2 agonists may enhance the activity of GABAergic interneurons via release of neurotensin. In order to determine if neurotensin can activate GABAergic interneurons, we treated rats with the peptide neurotensin agonist, PD149163, and examined Fos expression in PFC neurons. Systemic administration of PD149163 increased overall Fos expression in the PFC, but not in the dorsal striatum. PD149163 induced Fos in PFC interneurons, as defined by the presence of calcium-binding proteins, and in pyramidal cells. Pretreatment with the high-affinity neurotensin antagonist, SR48692, blocked neurotensin agonist-induced Fos expression. These data suggest that neurotensin activates interneurons in the PFC of the rat.

Orexin (hypocretin) effects on constitutively active inward rectifier K+ channels in cultured nucleus basalis neurons

Orexins are excitatory transmitters implicated in sleep disorders. Because orexins were discovered only recently, their ionic and signal transduction mechanisms have not been well clarified. We recently reported that orexin A (OXA) inhibits G protein-coupled inward rectifier K+ (GIRK) channels in cultured locus coeruleus and nucleus tuberomammillaris neurons. Other work in our laboratory revealed the existence of a novel inward rectifier K+ channel (KirNB), which is located in cholinergic neurons of the nucleus basalis (NB) and possesses unique single-channel characteristics. The mean open time is considerably shorter in KirNB than in Kir2.0 channels. Constitutive activity and a smaller unitary conductance set KirNB apart from cloned Kir3.0 channels. Previously, we found that substance P excites NB neurons by inhibiting KirNB channels. Here we show that orexins suppress KirNB channel activity, likely leading to neuronal excitation. Electrophysiological studies were performed on cultured NB neurons from the basal forebrain. OXA application decreased whole cell conductance through a pertussis toxin (PTX)-insensitive G protein. The OXA-suppressed current was inwardly rectifying with a reversal potential around E(K). Single-channel recordings of NB neurons revealed that constitutively active KirNB channels were transiently inhibited by OXA. Okadaic acid pretreatment abolished the recovery. The results suggest that OXA inhibition of KirNB is mediated by a PTX-insensitive G protein (i.e., G(q/11)), which eventually results in channel phosphorylation. Recovery from this inhibition is by dephosphorylation. These results, taken together with our previous study, suggest that orexin receptors can elicit neuronal excitation through at least two families of inward rectifier K+ channels: GIRK and KirNB channels.

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.

Neurotensin excitation of serotonergic neurons in the rat nucleus raphe magnus: ionic and molecular mechanisms

To understand the cellular and molecular mechanisms by which neurotensin (NT) induces an analgesic effect in the nucleus raphe magnus (NRM), whole-cell patch-clamp recordings were performed to investigate the electrophysiological effects of NT on acutely dissociated NRM neurons. Two subtypes of neurons, primary serotonergic and secondary non-serotonergic cells, were identified from acutely isolated NRM neurons. During current-clamp recordings, NT depolarized NRM serotonergic neurons and evoked action potentials. Voltage-clamp recordings showed that NT excited serotonergic neurons by enhancing a voltage-insensitive and non-selective cationic conductance. Both SR48692, a selective antagonist of subtype 1 neurotensin receptor (NTR-1), and SR 142948A, a non-selective antagonist of NTR-1 and subtype 2 neurotensin receptor (NTR-2), failed to prevent neurotensin from exciting NRM serotonergic neurons. NT-evoked cationic current was inhibited by the intracellular administration of GDP-beta-S. NT failed to induce cationic currents after dialyzing serotonergic neurons with the anti-G(alphaq/11) antibody. Cellular Ca(2+) imaging study using fura-2 showed that NT induced the calcium release from the intracellular store. NT-evoked current was blocked after the internal perfusion of heparin, an IP(3) receptor antagonist, or BAPTA, a fast Ca(2+) chelator. It is concluded that neurotensin enhancement of the cationic conductance of NRM serotonergic neurons is mediated by a novel subtype of neurotensin receptors. The coupling mechanism via G(alphaq/11) proteins is likely to involve the generation of IP(3), and subsequent IP(3)-evoked Ca(2+) release from intracellular stores results in activating the non-selective cationic conductance.

Neurotensin excites periaqueductal gray neurons projecting to the rostral ventromedial medulla

Microinjection of neurotensin into the midbrain periaqueductal gray (PAG) produces a potent and naloxone-insensitive analgesic effect. To test the hypothesis that neurotensin induces the analgesic effect by activating the PAG-rostral ventromedial medulla (RVM) descending antinociceptive pathway, PAG neurons that project to RVM (PAG-RVM) were identified by microinjecting DiI(C18), a retrograde tracing dye, into the rat RVM. Subsequently, fluorescently labeled PAG-RVM projection neurons were acutely dissociated and selected for whole cell patch-clamp recordings. During current-clamp recordings, neurotensin depolarized retrogradely labeled PAG-RVM neurons and evoked action potentials. Voltage-clamp recordings indicated that neurotensin excited PAG-RVM neurons by opening the voltage-insensitive and nonselective cation channels. Both SR 48692, a selective NTR-1 antagonist, and SR 142948A, a nonselective antagonist of NTR-1 and NTR-2, failed to prevent neurotensin from exciting PAG-RVM neurons. Neurotensin failed to evoke cationic currents after internally perfusing PAG-RVM projection neurons with GDP-beta-S or anti-G(alpha q/11) antiserum. Cellular Ca(2+) fluorescence measurement using fura-2 indicated that neurotensin rapidly induced Ca(2+) release from intracellular stores of PAG-RVM neurons. Neurotensin-evoked cationic currents were blocked by heparin, an IP(3) receptor antagonist, and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a fast chelator of Ca(2+). These results suggest that by activating a novel subtype of neurotensin receptors, neurotensin depolarizes and excites PAG-RVM projection neurons through enhancing Ca(2+)-dependent nonselective cationic conductance. The coupling mechanism via G(alpha q/11) proteins is likely to involve the production of IP(3), and subsequent IP(3)-evoked Ca(2+) release leads to the opening of nonselective cation channels.

Neurotensin-induced Cl(-) current in guinea-pig dorsal root ganglion cells

In guinea-pig dorsal root ganglion cells held under voltage-clamp at -80 mV, neurotensin elicited an inward current (I(NT)) whose amplitude increased with increasing neurotensin concentration (40-4000 nM). The effect was blocked by a nonpeptide neurotensin antagonist. I(NT) occurred in the absence of the extracellular Na(+), but not in the absence of the intracellular Cl(-), and it was outward directed by reversing the driving force for Cl(-). I(NT), like the gamma-amino-butyric acid (GABA)-induced Cl(-) current (I(GABA)), remained little changed after virtual elimination of cytosolic free-ionized Ca(2+) or after treatment with a Ca(2+)-activated Cl(-) channel blocker, but, in contrast to I(GABA) it was resistant to the I(GABA) blocker picrotoxin, slower in time course and more easily desensitized when repeatedly elicited. I(NT) and I(GABA) were additive to each other. AG-protein inhibitor markedly reduced I(NT), and a G-protein activator produced an inward current during which no current could be elicited by neurotensin. These results show that neurotensin exerts an effect to activate Ca(2+)-insensitive Cl(-) channels distinct from those activated by GABA in guinea-pig dorsal root ganglion cells, and the effect may arise through a G-protein-dependent mechanism.

Neurotensin depolarizes cholinergic and a subset of non-cholinergic septal/diagonal band neurons by stimulating neurotensin-1 receptors

Identified cholinergic and a subtype of non-cholinergic, fast-firing neurons were recorded intracellularly in vitro from slices of guinea-pig brain. Recorded neurons were within the boundaries of the medial septum and vertical limb of the diagonal band of the forebrain. The effects of superfused neurotensin and neurotensin receptor antagonists were measured under single-electrode current clamp. Neurotensin consistently caused a dose-dependent, slow depolarization of cholinergic neurons that was accompanied by an increase in membrane resistance and a block of the long-duration (1-10 s) post-spike afterhyperpolarization when present. Neurotensin also blocked a shorter duration, slow afterhyperpolarization, but only in a minority of cholinergic neurons. When present, inhibition of the slow afterhyperpolarization changed the spike pattern from single spikes to short bursts. Inhibition of post-spike afterhyperpolarizations by neurotensin reversed more slowly than did other effects of neurotensin. Tetrodotoxin did not prevent the depolarizing effect of neurotensin. The non-selective neurotensin receptor antagonist, SR142948A, blocked the depolarizing effect of neurotensin but the low-affinity receptor antagonist, levocabastine, did not. A subgroup of noncholinergic, fast-firing neurons (23%) was also depolarized by neurotensin, an effect antagonized by SR142948A but not levocabastine. Neurotensin did not effect post-spike voltage transients or change the firing pattern of non-cholinergic neurons. These data suggest that neurotensin causes a slow depolarization and increased excitability of cholinergic and some noncholinergic neurons in an area of the brain that projects to the hippocampus. Neurotensin type 1 receptors appear to mediate these effects. Neurotensin may modulate hippocampal-dependent learning and memory processes through its effects on septohippocampal neurons.

The role of N-, Q- and R-type Ca2+ channels in feedback inhibition of ACh release from rat basal forebrain neurones

1. The Ca2+ channel subtypes controlling ACh release from basal forebrain neurones and the ionic basis underlying muscarinic receptor-mediated autoinhibition were studied using skeletal myoballs to detect ACh release from individual rat basal forebrain neurones in culture. 2. Somatic Ca2+ currents evoked using a simulated action potential waveform revealed that Ca2+ entry was primarily through N-, Q- and to a lesser extent R-, T- and L-type Ca2+ channels. 3. Muscarine (10 microM) inhibited N- and Q- but not R-, T- or L-type somatic Ca2+ channels. Agonist inhibition was totally blocked by pre-treatment with pertussis toxin (500 ng ml-1). 4. ACh release from discrete sites along basal forebrain neurites (1. 2 mM extracellular Ca2+) could be largely abolished by blocking Ca2+ entry through either N-type or Q-type Ca2+ channels. Inhibition of Ca2+ entry through L- or T-type channels had no effect upon release. Following inhibition of either N- or Q-type Ca2+ channels, release could be restored to near control levels by raising [Ca2+]o. After selectively blocking N-, Q-, L- and T-type channels, low levels of release could still be evoked as a result of Ca2+ entry through R-type Ca2+ channels. 5. Muscarinic receptor activation reversibly inhibited ACh release due to Ca2+ entry through N-, Q- and R-type Ca2+ channels. In contrast, inhibition of inwardly rectifying K+ channels using Ba2+ (3-10 microM) or substance P (0.03-0.1 microM), or block of SK or BK Ca2+-activated K+ channels with apamin (100 nM) or charbydotoxin (100 nM) respectively, had no effect upon either ACh release or its modulation by muscarinic agonists. 6. These results show that ACh release from individual release sites on basal forebrain neurones is controlled by multiple Ca2+ channel subtypes with overlapping Ca2+ microdomains and that autoinhibition of release results from M2 muscarinic receptor-mediated inhibition of these presynaptic Ca2+ channels rather than as a consequence of K+ channel activation.

Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

Ionic mechanisms of action of neurotensin in acutely dissociated neurons from the diagonal band of Broca of the rat

Whole cell recordings were performed on acutely dissociated neurons from the horizontal limb of the diagonal band of Broca (hDBB) from rats to elucidate the ionic mechanisms of action of neurotensin. Neurotensin caused a decrease in whole cell voltage-activated outward currents and failed to elicit a response when Ca2+ influx was blocked by changing the external solution to the one containing 0 mM Ca2+ and 50 microM Cd2+, suggesting the involvement of Ca2+-dependent conductances. Charybdotoxin, a specific blocker of voltage-sensitive calcium-activated K+ channels (IC), caused a decrease in outward currents comparable with that caused by blocking calcium influx and occluded the neurotensin-induced decrease in outward currents. Similarly, 50 microM tetraethylammonium ions also blocked the neurotensin response. Also neurotensin reduced whole cell barium currents (IBa) and calcium currents (ICa). Amiloride and omega-conotoxin GVIA, but not nimodipine, were able to eliminate the neurotensin-induced decrease in IBa. Thus T- and N- but not L-type calcium channels are subject to modulation by neurotensin, and this may account for its effects on IC. The predicted changes in action potential as a result of the blockade of currents through calcium channels culminating into changes in IC were confirmed in the bridge current-clamp recordings. Specifically, neurotensin application led to depolarization of the resting membrane potential, broadening of spike and a decrease in afterhyperpolarization and accommodation. These alterations in action potential characteristics that resulted in increased firing rate and excitability of the hDBB neurons also were produced by application of charybdotoxin. Neurotensin effects on these properties were occluded by 2 - [(1 - 7 - chloro - 4 - quinolinyl) - 5 - (2, 6 - di - methoxyphenyl) pyrazol-3-yl) carbonylamino] tricyclo (3.3.1.1.)decan-2-carboxylic acid, a nonpeptide high-affinity neurotensin receptor antagonist. Neurotensin blockade of IC, possibly through ICa, is a potential physiological mechanism whereby this peptide may evoke alterations in the cortical arousal, sleep-wake cycle, and theta rhythm.

Ionic mechanism of the slow afterdepolarization induced by muscarinic receptor activation in rat prefrontal cortex

The mammalian prefrontal cortex receives a dense cholinergic innervation from subcortical regions. We previously have shown that cholinergic stimulation of layer V pyramidal neurons of the rat prefrontal cortex results in a depolarization and the appearance of a slow afterdepolarization (sADP). In the current report we examine the mechanism underlying the sADP with the use of sharp microelectrode and whole cell recording techniques in in vitro brain slices. The ability of acetylcholine (ACh) and carbachol to induce the appearance of an sADP in pyramidal cells of layer V of prefrontal cortex is antagonized in a surmountable manner by atropine and is mimicked by application of muscarine or oxotremorine. These results indicate that ACh acts on muscarinic receptors to induce the sADP. In many cell types afterpotentials are triggered by calcium influx into the cell. Therefore we examined the possibility that calcium influx might be the trigger for the generation of the sADP. Consistent with this possibility, buffering intracellular calcium reduced or abolished the sADP but had little effect on the direct muscarinic receptor-induced depolarization also seen in these cells. These results, coupled to the previous observation that calcium channel blockers inhibit the sADP, indicated that the sADP results from a rise in intracellular calcium secondary to calcium influx into the cell. The ionic basis for the current underlying the sADP (IsADP) was examined with the use of ion substitution experiments. The amplitude of IsADP was found to be reduced in a graded fashion by replacement of extracellular sodium with N-methyl-D-glucamine (NMDG). In contrast no clear evidence for the involvement of potassium or chloride channels in the generation of the sADP or IsADP could be found. This result indicated that IsADP is carried by sodium ions flowing into the cell. However, the dependence of IsADP on extracellular sodium was less pronounced than expected for a pure sodium current. We interpret these results to indicate that the sADP is most likely mediated by nonselective cation channels. Examination of the current underlying the sADP at different voltages indicated that this current was also voltage dependent, turning off with hyperpolarization. We conclude that the sADP elicited by muscarinic receptor activation in rat cortex is mediated predominantly by a calcium- and voltage-sensitive nonselective cation current. This current could represent an important mechanism through which ACh can regulate neuronal excitability in prefrontal cortex.

Mechanisms of regulation of neurotensin receptors

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

Protein kinase C disrupts cannabinoid actions by phosphorylation of the CB1 cannabinoid receptor

We have found that phosphorylation of a G-protein-coupled receptor by protein kinase C (PKC) disrupts modulation of ion channels by the receptor. In AtT-20 cells transfected with rat cannabinoid receptor (CB1), the activation of an inwardly rectifying potassium current (Kir current) and depression of P/Q-type calcium channels by cannabinoids were prevented by stimulation of protein kinase C by 100 nM phorbol 12-myristate 13-acetate (PMA). In contrast, activation of Kir current by somatostatin was unaffected, and inhibition of calcium channels was only modestly attenuated. The possibility that PKC acted by phosphorylating CB1 receptors was confirmed by demonstrating that PKC phosphorylated a single serine (S317) of a fusion protein incorporating the third intracellular loop of CB1. Mutating this serine to alanine did not affect the ability of CB1 to modulate currents, but it eliminated disruption by PMA, demonstrating that PKC can disrupt ion channel modulation by receptor phosphorylation.

Protein kinase C disrupts cannabinoid actions by phosphorylation of the CB1 cannabinoid receptor

We have found that phosphorylation of a G-protein-coupled receptor by protein kinase C (PKC) disrupts modulation of ion channels by the receptor. In AtT-20 cells transfected with rat cannabinoid receptor (CB1), the activation of an inwardly rectifying potassium current (Kir current) and depression of P/Q-type calcium channels by cannabinoids were prevented by stimulation of protein kinase C by 100 nM phorbol 12-myristate 13-acetate (PMA). In contrast, activation of Kir current by somatostatin was unaffected, and inhibition of calcium channels was only modestly attenuated. The possibility that PKC acted by phosphorylating CB1 receptors was confirmed by demonstrating that PKC phosphorylated a single serine (S317) of a fusion protein incorporating the third intracellular loop of CB1. Mutating this serine to alanine did not affect the ability of CB1 to modulate currents, but it eliminated disruption by PMA, demonstrating that PKC can disrupt ion channel modulation by receptor phosphorylation.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

An immunocytochemical study on the distribution of two G-protein-gated inward rectifier potassium channels (GIRK2 and GIRK4) in the adult rat brain

G-protein-gated inward rectifier potassium channels mediate the synaptic actions of numerous neurotransmitters in the mammalian brain, and were recently shown to be candidates for genetic mutations leading to neuronal cell death. This report describes the localization of G-protein-gated inward rectifier potassium channel-2 and G-protein-gated inward rectifier potassium channel-4 proteins in the rat brain, as assessed by immunocytochemistry. G-protein-gated inward rectifier potassium channel-2 immunoreactivity was widely distributed throughout the brain, with the strongest staining seen in the hippocampus, septum, granule cell layer of the cerebellum, amygdala and substantia nigra pars compacta. In contrast, G-protein-gated inward rectifier potassium channel-4 immunoreactivity was restricted to some neuronal populations, such as Purkinje cells and neurons of the globus pallidus and the ventral pallidum. The presence of G-protein-gated inward rectifier potassium channel-2 immunoreactivity in substantia nigra pars compacta dopaminergic neurons was confirmed by showing its co-localization with tyrosine hydroxylase by double immunocytochemistry, and also by selectively lesioning dopaminergic neurons with the neurotoxin 6-hydroxydopamine. At the cellular level both proteins were localized in neuronal cell bodies and dendrites, but clear differences were seen in the degree of dendritic staining among neuronal groups. For some neuronal groups the staining of distal dendrites (notably dendritic spines) was strong, while for others the cell body and proximal dendrites were preferentially labelled. In addition, some of the results suggest that G-protein-gated inward rectifier potassium channel-2 protein could be localized in distal axonal terminal fields. A knowledge of the distribution of G-protein-gated inward rectifier potassium channel proteins in the brain could help to elucidate their physiological roles and to evaluate their potential involvement in neurodegenerative processes in animal models and human diseases.

Neurotensin and substance P inhibit low- and high-voltage-activated Ca2+ channels in cultured newborn rat nucleus basalis neurons

Inhibition of Ca2+ currents by the excitatory neurotransmitters neurotensin and substance P was investigated in cultured nucleus basalis neurons with the use of the whole cell patch-clamp technique. The whole cell Ca2+ current, elicited from a holding potential of -80 mV by a step pulse to 0 mV and measured at 100 ms, was inhibited 67.9% by neurotensin and 57.6% by substance P. Low-voltage-activated (LVA) Ca2+ current, elicited by a step pulse to -40 mV from a holding potential of -90 mV, was inhibited by both neurotensin (26.2%) and substance P (24.1%). High-voltage-activated Ca2+ currents were separated with the use of the Ca2+ channel antagonists. Nimodipine (3 microM) inhibited 24.2% of the whole cell Ca2+ current elicited by a step to 0 or +10 mV and measured at 100 ms. Under the same conditions, omega-conotoxin (omega-CgTx)-GVIA (0.5 microM) inhibited 46.4%, omega-CgTx-GVIA + nimodipine 58.7%, and omega-CgTx-MVIIC (5 microM) + nimodipine 75.7% of the current. Omega-Agatoxin (omega-Aga)-IVA (100 nM) did not produce any effect. Neurotensin inhibition of the whole cell Ca2+ current was attenuated by each of these treatments except for the omega-Aga-IVA treatment, which did not change the neurotensin effect. In contrast, neither the omega-Aga-IVA nor the nimodipine treatment had any effect on the substance-P-induced inhibition; the rest of the treatments attenuated the substance-P-induced response. Thus the data indicate that nucleus basalis neurons express LVA as well as L-, N-, and Q-type, but not the P-type, Ca2+ currents. N- and Q-type HVA Ca2+ currents, as well as LVA Ca2+ currents, are inhibited by both neurotensin and substance P. In contrast, L-type current is inhibited by neurotensin but not by substance P. In addition, a fraction of the total whole cell current was resistant to all Ca2+ channel antagonists and thus may correspond to the R-type Ca2+ current. This residual current was inhibited by both neurotensin and substance P. The inhibition of the whole cell Ca2+ current produced by both neurotransmitters was voltage independent, because a large depolarization (+70 mV) was not able to relieve either effect. In cells loaded with 0.1 mM guanosine 5'-[gamma-thio]triphosphate, response to both neurotensin and substance P became irreversible, indicating that the effects of both neurotransmitters were mediated through G proteins. However, pertussis toxin did not affect either the neurotensin or the substance P response.

Neurotensin inhibition of the hyperpolarization-activated cation current (Ih) in the rat substantia nigra pars compacta implicates the protein kinase C pathway

1. Whole-cell patch-clamp recording was performed from principal neurones of the substantia nigra pars compacta (SNc). In 66% of these neurones, neurotensin (NT) induced, at -60 mV, an inward current associated with an increase in conductance. 2. Principal neurones displayed, in response to hyperpolarizing voltage steps, the voltage-dependent inward cationic current, Ih. This current activated at potentials more negative than -65 mV and reached a maximum at -106 +/- 4 mV, with a half-activation potential of -86 +/- 3 mV. Its estimated reversal potential was -43 +/- 7 mV and its activation curve was fitted with two exponentials. 3. In 41% of neurones showing the inward current, NT (0.5 microM) also reversibly reduced the amplitude of Ih. The diminution was 48.5 +/- 12% when voltage steps were made from -60 to -95 mV. The decrease in Ih resulted from a reduction in the maximal current with no change in the voltage dependence of activation. 4. Forskolin (10 microM), an activator of adenylate cyclase, increased Ih by shifting its activation range to more positive potentials, but it did not alter the NT inhibition of Ih. 5. The effect of NT was blocked by staurosporine (0.5 microM) and by PKC-(19-31) (0.5 microM), a specific protein kinase C (PKC) inhibitor, but was unaffected by Walsh's peptide (100 microM), a specific inhibitor of protein kinase A. The reduction of Ih was mimicked by 1-oleoyl-2-acetyl-sn-glycerol (0.5-10 microM), an analogue of diacylglycerol, an endogenous PKC activator. 6. These results suggest that the inhibition of Ih by NT involves a phosphorylation mechanism that implies activation of PKC.

Neurotensin and dopamine D2 activation oppositely regulate the same K+ conductance in rat midbrain dopaminergic neurons

Midbrain dopaminergic neurons are excited by neurotensin (NT) and inhibited by dopamine. Interactions between these neurotransmitters have been reported, but no interaction has yet been identified at the level of ionic and signal transduction mechanisms. Using the whole-cell clamp technique, we examined the interaction of NT and quinpirole (QUIN) (a dopamine D2 agonist) on midbrain ventral tegmental area neurons cultured from the rat. We found that NT could inhibit the K+ conductance induced by QUIN. By interrupting normal signal transduction with the non-hydrolyzable GTP analogue GTPgammaS, we found that this interaction occurred downstream of the membrane neurotransmitter receptors. Similar interactions were observed between QUIN and tachykinin or metabotropic glutamate agonists.

Neurotensin and neuroendocrine regulation

More than two decades of research indicate that the peptide neurotensin (NT) and its cognate receptors participate to a remarkable extent in the regulation of mammalian neuroendocrine systems, potentially at multiple levels in a given system. NT-synthesizing neurons appear to exert a direct or indirect stimulatory influence on neurosecretory cells that synthesize gonadotropin-releasing hormone, dopamine (DA), somatostatin, and corticotropin-releasing hormone (CRH). In addition, context-specific synthesis of NT occurs in hypothalamic neurosecretory cells located in the arcuate nucleus and parvocellular paraventricular nucleus, including distinct subsets of cells which release DA, CRH, or growth hormone-releasing hormone into the hypophysial portal circulation. At the level of the anterior pituitary, NT stimulates secretion of prolactin and occurs in subsets of gonadotropes and thyrotropes. Moreover, circulating hormones influence NT synthesis in the hypothalamus and anterior pituitary, raising the possibility that NT mediates certain feedback effects of the hormones on neuroendocrine cells. Gonadal steroids alter NT levels in the preoptic area, arcuate nucleus, and anterior pituitary; adrenal steroids alter NT levels in the hypothalamic periventricular nucleus and arcuate nucleus; and thyroid hormones alter NT levels in the hypothalamus and anterior pituitary. Finally, clarification of the specific neuroendocrine roles subserved by NT should be greatly facilitated by the use of newly developed agonists and antagonists of the peptide.

Neuromedin C decreases potassium conductance and increases a non-specific conductance in rat suprachiasmatic neurones in brain slices in vitro

Whole-cell recordings were made, in both current and voltage clamp, from suprachiasmatic neurones maintained in coronal rat brain slices. In current clamp doses of 10 and 100 nM neuromedin C (NMC) were shown to increase basal firing rate in 9 out of 32 neurones. The excitatory responses to 100 nM NMC were accompanied by small increases in neuronal input resistance (25.0 +/- 9.9% in 4 out of 7 neurones tested) and depolarisations of membrane potential (9.8 +/- 3.4 mV in 4 out of 7 neurones tested). However, 10 nM NMC caused no changes in either neuronal input resistance or membrane potential despite the clear increases in neuronal firing rate. When voltage-clamped at -60 mV, 100 nM NMC induced an inward current of 14.8 +/- 1.2 pA in 46 of 210 neurones. The NMC-induced inward current was shown to be unaffected by perfusion with 1 microM tetrodotoxin (TTX). The inward current recorded at -60 mV was typically associated with a decrease in membrane conductance. Construction of current-voltage relationships in the absence and presence of 100 nM NMC showed that with the majority of the NMC-sensitive neurones the inward current either reversed polarity close to the potassium reversal potential or decreased at hyperpolarised potentials. This reversal potential was shifted to more depolarised potentials when the extracellular concentration of potassium was increased. The NMC-induced inward current was unaffected by reduction of the extracellular concentration of sodium or by addition of 0.2 mM cadmium. In potassium-free conditions, in both the dialysing pipette solution and perfusing saline, NMC was still able to induce an inward current. The additional reduction of the extracellular concentration of sodium, whilst recording in potassium-free conditions, was also unable to abolish the inward current. Recordings made with an electrode containing the non-hydrolysable guanosine triphosphate analogue, guanosine 5'-thio-triphosphate, resulted in NMC-induced inward currents which failed to recover to baseline. It is concluded that NMC excites a subpopulation of suprachiasmatic neurones by decreasing a resting potassium conductance and increasing a non-specific conductance, via a G-protein link.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

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.

Cellular distribution of neurotensin receptors in rat brain: immunohistochemical study using an antipeptide antibody against the cloned high affinity receptor

Receptors for the neuropeptide, neurotensin, were localized by immunohistochemistry in the rat brain by using an antibody raised against a sequence of the third intracellular loop of the cloned high affinity receptor. Selective receptor immunostaining was observed throughout the brain and brainstem. This immunostaining was totally prevented by preadsorbing the antibody with the immunogenic peptide. The regional distribution of the immunoreactivity conformed for the most part to that of [3H]- or [125I]-neurotensin binding sites previously identified by autoradiography. Thus, the highest levels of immunostaining were observed in the islands of Calleja, diagonal band of Broca, magnocellular preoptic nucleus, pre- and parasubiculum, suprachiasmatic nucleus, anterodorsal nucleus of the thalamus, substantia nigra, ventral tegmental area, pontine nuclei and dorsal motor nucleus of the vagus, all of which had previously been documented to contain high densities of neurotensin binding sites. There were, however, a number of regions reportedly endowed with neurotensin binding sites, including the central amygdaloid nucleus, periaqueductal gray, outer layer of the superior colliculus and dorsal tegmental nucleus, which showed no or divergent patterns of immunostaining, suggesting that they might be expressing a molecularly distinct form of the receptor. At the cellular level, neurotensin receptor immunoreactivity was predominantly associated with perikarya and dendrites in some regions (e.g., in the basal forebrain, ventral midbrain, pons and rostral medulla) and with axons and axon terminals in others (e.g., in the lateral septum, bed nucleus of the stria terminalis, neostriatum, paraventricular nucleus of the thalamus and nucleus of the solitary tract). These data indicate that neurotensin may act both post- and presynaptically in the central nervous system and confirm that some of its effects are exerted on projection neurons. There were also areas, such as the cerebral cortex, nucleus accumbens and para- and periventricular nucleus of the hypothalamus, which contained both immunoreactive perikarya/dendrites and axon terminals, consistent with either a joint association of the receptor with afferent and efferent elements or its presence on interneurons. Taken together, these results also suggest that the neurotensin high affinity receptor protein is associated with a neuronal population that is more extensive than originally surmised from in situ hybridization studies.

Positive feedback by a potassium-selective inward rectifier enhances tuning in vertebrate hair cells

Electrical resonance in vertebrate hair cells shapes receptor potentials and tunes each cell to a narrow band of frequencies. We have investigated the contribution of a potassium-selective inward rectifier (IR) to electrical resonance, isolating outward current carried by IR from other ionic currents active in the physiological voltage range (-75 to -30 mV) using a combination of potassium and calcium channel antagonists. IR expression is tightly regulated in the turtle's auditory epithelium, as revealed by the observation that its size declines systematically with resonant frequency. A critical feature of IR is the rapid inhibition produced by depolarization, which results in a negative slope in the steady-state current-voltage relation in the vicinity of the resting potential (-50 mV). The increasing block of outward current produced by depolarization is functionally equivalent to activating an inward current, suggesting that IR provides positive feedback and, in hair cells, serves an electrical function ordinarily reserved for voltage-dependent sodium and calcium currents. Additional support for this idea comes from the observation that superfusion with cesium selectively reduces IR and eliminates resonance in cells tuned to low frequencies and degrades resonant quality in cells tuned to more than 50 Hz.

Effects of neurotensin on visual neurons in the superficial laminae of the hamster's superior colliculus

Autoradiography with 125I-neurotensin in normal and enucleated hamsters was used to define the distribution of receptors for this peptide in the superficial layers of the superior colliculus (SC). Neurotensin binding sites were densely distributed in the stratum griseum superficiale (SGS), and results from the enucleated animals indicated that they were not located on retinal axons. The effects of neurotensin on individual superficial layer cells were tested in single-unit recording experiments. Neurotensin was delivered via micropressure ejection during visual stimulation (n = 75 cells), or during electrical stimulation of either the optic chiasm (OX; n = 47 cells) or visual cortex (CTX; n = 29 cells). In comparison with control values, application of neurotensin decreased visual responses of all SC cells tested to 54.1 +/- 34.9% (mean +/- standard deviation; range of decrement 7.5 to 100%; nine cells showed no effect or an increase in visual activity, which for four of these was > or = 30%). Neurotensin application also reduced responses to electrical stimulation of either OX or CTX, respectively, to 65.8 +/- 36.5% of control values (range of decrement 2.6 to 97.4%; 12 neurons showed a weak increment < or = 30%) and 68.0 +/- 38.5% (range of decrement 3.3 to 100%; five cells showed no effect or an increment, in one case > or = 30%). Of the 25 neurons tested with both OX and CTX stimulation, the correlation of evoked response suppression by neurotensin was highly significant (r = 0.70; P < 0.001). This suggests that the suppressive effects of neurotensin were common to both pathways. To test whether the inhibitory effects of neurotensin were presynaptic or postsynaptic, Mg2+ ions were ejected iontophoretically to abolish synaptic responses, and the neurons (n = 16) were activated by iontophoresis of glutamate and then tested with neurotensin. Neurotensin reduced the glutamate-evoked responses to an average 59.3 +/- 37.9% of control values (range 2.3 to 92.5%; one cell showed an increment > 30%). This result suggests that the site of action of neurotensin is most likely postsynaptic.

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

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

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.

Effects of neurotensin on rat supraoptic nucleus neurones in vitro

1. The electrophysiological actions of neurotensin on magnocellular neurosecretory cells (MNCs) were examined during intracellular recording from seventy-three supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Application of neurotensin tridecapeptide (NT(1-13); 1 nM to 3 microM) caused a membrane depolarization and reversibly attenuated the after-hyperpolarization (AHP) which followed current-evoked spike trains. This effect was accompanied by increased firing frequency during depolarizing current pulses evoked from a fixed potential. 3. The effects of neurotensin could be mimicked by the C-terminal fragment, NT(8-13), but not by the N-terminal fragment, NT(1-8). 4. Depolarizing responses to NT(1-13) or NT(8-13), retained during K+ channel blockade with internal Cs+, were accompanied by increased membrane conductance. Current- and voltage-clamp analyses revealed that neurotensin-evoked depolarizations result partly from the activation of a non-selective cationic conductance reversing near -34 mV. 5. Depolarizing responses to neurotensin were retained in the presence of TTX or in Ca(2+)-free solutions, indicating the involvement of receptors located on the plasma membrane of MNCs themselves. 6. Through these effects endogenously released neurotensin may modulate excitability, activity patterns and secretion from the hypothalamo-neurohypophysial axis.