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

Diverse actions of the modulatory peptide neurotensin on central synaptic transmission

Neurotensin (NT) is a 13 amino acid neuropeptide that is expressed throughout the central nervous system and is implicated in the etiology of multiple diseases and disorders. Many primary investigations of NT-induced modulation of neuronal excitability at the level of the synapse have been conducted, but they have not been summarized in review form in nearly 30 years. Therefore, the goal of this review is to discuss the many actions of NT on neuronal excitability across brain regions as well as NT circuit architecture. In the basal ganglia as well as other brain nuclei, NT can act through diverse intracellular signaling cascades to enhance or depress neuronal activity by modulating activity of ion channels, ionotropic and metabotropic neurotransmitter receptors, and presynaptic release of neurotransmitters. Further, NT can produce indirect effects by evoking endocannabinoid release, and recently has itself been identified as a putative retrograde messenger. In the basal ganglia, the diverse actions and circuit architecture of NT signaling allow for input-specific control of reward-related behaviors.

A Key Role for Neurotensin in Chronic-Stress-Induced Anxiety-Like Behavior in Rats

Chronic stress is a major cause of anxiety disorders that can be reliably modeled preclinically, providing insight into alternative therapeutic targets for this mental health illness. Neuropeptides have been targeted in the past to no avail possibly due to our lack of understanding of their role in pathological models. In this study we use a rat model of chronic stress-induced anxiety-like behaviors and hypothesized that neuropeptidergic modulation of synaptic transmission would be altered in the bed nucleus of the stria terminalis (BNST), a brain region suspected to contribute to anxiety disorders. We use brain slice neurophysiology and behavioral pharmacology to compare the role of locally released endogenous neuropeptides on synaptic transmission in the oval (ov) BNST of non-stressed (NS) or chronic unpredictably stressed (CUS) rats. We found that in NS rats, post-synaptic depolarization induced the release of vesicular neurotensin (NT) and corticotropin-releasing factor (CRF) that co-acted to increase ovBNST inhibitory synaptic transmission in 59% of recorded neurons. CUS bolstered this potentiation (100% of recorded neurons) through an enhanced contribution of NT over CRF. In contrast, locally released opioid neuropeptides decreased ovBNST excitatory synaptic transmission in all recorded neurons, regardless of stress. Consistent with CUS-induced enhanced modulatory effects of NT, blockade of ovBNST NT receptors completely abolished stress-induced anxiety-like behaviors in the elevated plus maze paradigm. The role of NT has been largely unexplored in stress and our findings highlight its potential contribution to an important behavioral consequence of chronic stress, that is, exaggerated avoidance of open space in rats.

Aβ selectively impairs mGluR7 modulation of NMDA signaling in basal forebrain cholinergic neurons: implication in Alzheimer's disease

Degeneration of basal forebrain (BF) cholinergic neurons is one of the early pathological events in Alzheimer's disease (AD) and is thought to be responsible for the cholinergic and cognitive deficits in AD. The functions of this group of neurons are highly influenced by glutamatergic inputs from neocortex. We found that activation of metabotropic glutamate receptor 7 (mGluR7) decreased NMDAR-mediated currents and NR1 surface expression in rodent BF neurons via a mechanism involving cofilin-regulated actin dynamics. In BF cholinergic neurons, β-amyloid (Aβ) selectively impaired mGluR7 regulation of NMDARs by increasing p21-activated kinase activity and decreasing cofilin-mediated actin depolymerization through a p75(NTR)-dependent mechanism. Cell viability assays showed that activation of mGluR7 protected BF neurons from NMDA-induced excitotoxicity, which was selectively impaired by Aβ in BF cholinergic neurons. It provides a potential basis for the Aβ-induced disruption of calcium homeostasis that might contribute to the selective degeneration of BF cholinergic neurons in the early stage of AD.

Aβ inhibition of ionic conductance in mouse basal forebrain neurons is dependent upon the cellular prion protein PrPC

Current therapies for Alzheimer's disease (AD) address a loss of cholinergic neurons, while accumulation of neurotoxic amyloid β (Aβ) peptide assemblies is thought central to molecular pathogenesis. Overlaps may exist between prionopathies and AD wherein Aβ oligomers bind to the cellular prion protein PrP(C) and inhibit synaptic plasticity in the hippocampus (Laurén et al., 2009). Here we applied oligomeric Aβ to neurons with different PrP (Prnp) gene dosage. Whole-cell recordings were obtained from dissociated neurons of the diagonal band of Broca (DBB), a cholinergic basal forebrain nucleus. In wild-type (wt) mice, Aβ₁₋₄₂ evoked a concentration-dependent reduction of whole-cell outward currents in a voltage range between -30 and +30 mV; reduction occurred through a combined modulation of a suite of potassium conductances including the delayed rectifier (I(K)), the transient outward (I(A)), and the iberiotoxin-sensitive (calcium-activated potassium, I(C)) currents. Inhibition was not seen with Aβ₄₂₋₁ peptide, while Aβ₁₋₄₂-induced responses were reduced by application of anti-PrP antibody, attenuated in cells from Prnp⁰/⁺ hemizygotes, and absent in Prnp⁰/⁰ homozygotes. Similarly, amyloidogenic amylin peptide depressed DBB whole-cell currents in DBB cells from wt mice, but not Prnp⁰/⁰ homozygotes. While prior studies give broad support for a neuroprotective function for PrP(C), our data define a latent pro-pathogenic role in the presence of amyloid assemblies.

Neurotensin Enhances GABAergic Activity in Rat Hippocampus CA1 Region by Modulating L-Type Calcium Channels

Neurotensin (NT) is a tridecapeptide that interacts with three NT receptors; NTS1, NTS2, and NTS3. Although NT has been reported to modulate GABAergic activity in the brain, the underlying cellular and molecular mechanisms of NT are elusive. Here, we examined the effects of NT on GABAergic transmission and the involved cellular and signaling mechanisms of NT in the hippocampus. Application of NT dose-dependently increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded from CA1 pyramidal neurons with no effects on the amplitude of sIPSCs. NT did not change either the frequency or the amplitude of miniature (m)IPSCs recorded in the presence of tetrodotoxin. Triple immunofluorescent staining of recorded interneurons demonstrated the expression of NTS1 on GABAergic interneurons. NT increased the action potential firing rate but decreased the afterhyperpolarization (AHP) amplitude in identified CA1 interneurons. Application of L-type calcium channel blockers (nimodipine and nifedipine) abolished NT-induced increases in action potential firing rate and sIPSC frequency and reduction in AHP amplitude, suggesting that the effects of NT are mediated by interaction with L-type Ca2+channels. NT-induced increase in sIPSC frequency was blocked by application of the specific NTS1 antagonist SR48692, the phospholipase C (PLC) inhibitor U73122, the IP3receptor antagonist 2-APB, and the protein kinase C inhibitor GF109203X, suggesting that NT increases γ-aminobutyric acid release via a PLC pathway. Our results provide a cellular mechanism by which NT controls GABAergic neuronal activity in hippocampus.

Proteolytic processing of SDF-1alpha reveals a change in receptor specificity mediating HIV-associated neurodegeneration

Proteolytic cleavage of constitutively expressed proteins can generate peptides with novel bioactive properties. Matrix metalloproteinase (MMP)-2 cleaves the 4 amino-terminal residues of the chemokine, stromal cell-derived factor (SDF)-1alpha, yielding a highly neurotoxic molecule, SDF(5-67), which fails to bind to its cognate receptor, CXCR4. Herein, we detected SDF(5-67) in brain monocytoid cells of HIV-infected persons, particularly in those with HIV-associated dementia. SDF(5-67) activated cell type-specific expression of proinflammatory genes including IL-1beta, TNFalpha, indoleamine 2',3'-dioxygenase (IDO), and IL-10 in both astrocytic and monocytoid cells (P < 0.05). Unlike SDF-1alpha, SDF(5-67) caused neuronal membrane perturbations with ensuing neurotoxicity and apoptosis (P < 0.05) through engagement of an inducible receptor. CXCR3 antagonists and siRNA-mediated knockdown of CXCR3 inhibited SDF(5-67)-stimulated neurophysiological changes, neuronal death, and neuroimmune activation (P < 0.05). Moreover SDF(5-67) bound directly to CXCR3 in a competitive manner, mediated by its amino terminus. In vivo neuroinflammation, neuronal loss, and neurobehavioral abnormalities caused by SDF(5-67) (P < 0.05) were prevented by a CXCR3 antagonist. These studies reveal additive neuropathogenic properties exerted by a proteolytically cleaved chemokine as consequences of a change in receptor specificity, culminating in neurodegeneration.

Neurotensin effects on N-type calcium currents among rat pallidal neurons: an electrophysiological and immunohistochemical study

The tridecapeptide neurotensin (NT) is involved in the modulation of dopamine (DA)-mediated functions in the nigrostriatal and mesocorticolimbic pathways. Its relevance in mammalian globus pallidus (GP) is questioned. A recent electrophysiological study on GP slices described NT-mediated robust membrane depolarization, depending upon the suppression of potassium conductance and/or the activation of cation current. Here, we have studied whether NT also affected high-voltage-activated calcium (Ca(2+)) currents, by means of whole-cell recordings on isolated GP neurons. In our hands, the full peptide and the segment NT8-13 reversibly inhibited N-like Ca(2+) current in about 60% of the recorded dissociated neurons, irrespective of their capacitance. The NT-mediated modulation showed no desensitization and was antagonized by the NT1 antagonists SR48692 and SR142948. These results imply an abundant expression of NTS(1) on GP cell somata. Then, we performed a light and immunofluorescence-confocal microscopy study of NTS(1) localization among GP neurons. We found that NTS(1) is localized in about 56% of GP neurons in both subpopulations of neurons, namely parvalbumin positive and negative. We conclude that NT, likely released from the striatal terminals in GP, acts through the postsynaptic NTS(1) preferentially localized in the lateral aspects of the GP. These data suggest a new implication (neither merely presynaptic nor simply "excitatory") for NT in the modulation of GP firing pattern. In addition, NT might have a role in affecting the interplay among the endogenous release of GABA/glutamate and DA. This hypothesis might have implications on both sensori-motor and associative functions of the GP and should be tested in DA-denervated disease models.

Interleukin-1beta regulation of N-type Ca2+ channels in cortical neurons

Interleukin-1beta (IL-1beta) has been found to play an important role in various diseases in the central nervous system (CNS) and exhibit neuroprotective effects in some conditions. The transmitter release in brain is controlled by voltage-gated Ca(2+) channels (VGCCs), predominantly N-type Ca(2+) channels (NCCs). Although both IL-1beta and NCCs are implicated regulating excitotoxicity and Ca(2+) homeostasis, it is not known whether IL-1beta modulates NCCs directly. In present study, we examined the effects of IL-1beta treatment (10 ng/ml, 24 h) on NCCs in cultured cortical neurons using patch-clamp recording and immunoblot assay. Our results showed that IL-1beta decreased NCC currents by approximately 50%, which made up 40% of the whole-cell Ca(2+) current demonstrated by omega-conotoxin-GVIA, and also significantly downregulated the expression of NCC protein. The residual Ca(2+) currents except L-type Ca(2+) channel currents and NCC currents were not affected by IL-1beta. Our finding, IL-1beta inhibits the activity of NCC via suppressing NCC protein expression provides new insight into the neuroprotective role of IL-1beta in CNS.

Interleukin-1beta downregulates the L-type Ca2+ channel activity by depressing the expression of channel protein in cortical neurons

Interleukin-1beta (IL-1beta), a proinflammatory cytokine, has been involved in various diseases of the central nervous system (CNS). Due to the diverse, "contradictory" effects of IL-1beta on neurons during insults to the brain, the mechanisms underlying these effects have not been elucidated. Calcium influx through the L-type Ca2+ channels (LCCs) is believed to play a critical role in the cascade of biochemical events leading to neuron death in these pathophysiological conditions. So far, the mechanism of the interaction of IL-1beta and LCCs in the initiation and progression of these diseases is unclear. In this study, we investigate systemically the effects of IL-1beta on the LCCs current, which are believed to be implicated in the cascade of biochemical events leading to neuron death in neuropathological conditions. Using patch clamp, we observe that IL-1beta treatment (10 ng/ml, 24 h) suppresses LCC currents by approximately 38%, which made up half of the whole-cell Ca2+ current determined by nifedipine. IL-1beta does not alter the characteristics of single LCC including current amplitude, open probability, and conductance, but decreases the number of the functioning channel by 40%. Moreover, immunoblot assay exhibits that IL-1beta reduces the expression of LCC proteins by 38 approximately 42% in both whole neuron and plasma membrane fraction, and demonstrates that IL-1beta downregulates the LCC activity via the reduction of LCC density. According to early research pretreatments longer than 12 h may play a crucial role in the neuroprotective effects of IL-1beta, our findings may establish an explanation for the protective effects of this interleukin on neurons in the late stage of injury, and could raise a new issue to clinical treatment for insults to brain.

Single transmembrane domain insulin-like growth factor-II/mannose-6-phosphate receptor regulates central cholinergic function by activating a G-protein-sensitive, protein kinase C-dependent pathway

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a single-pass transmembrane glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. However, its role in signal transduction after IGF-II binding remains unclear. In the present study, we report that IGF-II/M6P receptor in the rat brain is coupled to a G-protein and that its activation by Leu27IGF-II, an analog that binds rather selectively to the IGF-II/M6P receptor, potentiates endogenous acetylcholine release from the rat hippocampal formation. This effect is mediated by a pertussis toxin (PTX)-sensitive GTP-binding protein and is dependent on protein kinase Calpha (PKCalpha)-induced phosphorylation of downstream substrates, myristoylated alanine-rich C kinase substrate, and growth associated protein-43. Additionally, treatment with Leu27IGF-II causes a reduction in whole-cell currents and depolarization of cholinergic basal forebrain neurons. This effect, which is blocked by an antibody against the IGF-II/M6P receptor, is also sensitive to PTX and is mediated via activation of a PKC-dependent pathway. These results together revealed for the first time that the single transmembrane domain IGF-II/M6P receptor expressed in the brain is G-protein coupled and is involved in the regulation of central cholinergic function via the activation of specific intracellular signaling cascades.

Bidirectional modulation of GABAergic transmission by cholecystokinin in hippocampal dentate gyrus granule cells of juvenile rats

Cholecystokinin (CCK) interacts with two types of G protein-coupled receptors in the brain: CCK-A and CCK-B receptors. Both CCK and CCK-B receptors are widely distributed in the hippocampal formation, but the functions of CCK there have been poorly understood. In the present study, we initially examined the effects of CCK on GABA(A) receptor-mediated synaptic transmission in the hippocampal formation and then explored the underlying cellular mechanisms by focusing on the dentate gyrus region, where the highest levels of CCK-binding sites have been detected. Our results indicate that activation of CCK-B receptors initially and transiently increased spontaneous IPSC (sIPSC) frequency, followed by a persistent reduction. The effects of CCK were more evident in juvenile rats, suggesting that they are developmentally regulated. Cholecystokinin failed to modulate the miniature IPSCs recorded in the presence of TTX and the amplitude of the evoked IPSCs, but produced a transient increase followed by a reduction in action potential firing frequency recorded from GABAergic interneurons, suggesting that CCK acts by modulating the excitability of the interneurons to regulate GABA release. Cholecystokinin reduced the amplitude of the after-hyperpolarization of the action potentials, and application of paxilline or charybdotoxin considerably reduced CCK-mediated modulation of sIPSC frequency, suggesting that the effects of CCK are related to the inhibition of Ca(2+)-activated K(+) currents (I(K(Ca))). The effects of CCK were independent of the functions of phospholipase C, intracellular Ca(2+) release, protein kinase C or phospholipase A(2), suggesting a direct coupling between the G proteins of CCK-B receptors and I(K(Ca)). Our results provide a novel mechanism underlying CCK-mediated modulation of GABA release.

Fucoidan inhibits cellular and neurotoxic effects of β-amyloid (Aβ) in rat cholinergic basal forebrain neurons

The deposition of beta-amyloid protein (A beta), a 39-43 amino acid peptide, in the brain and a loss of cholinergic neurons in the basal forebrain are pathological hallmarks of Alzheimer's disease (AD). Seaweeds consumed in Asia contain Fucoidan, a sulfated polysaccharide. Fucoidan has been known to exhibit various biological actions, such as an anti-inflammatory and antioxidant action. In this study, using whole-cell patch clamp recordings we examined the effects of Fucoidan on A beta-induced whole-cell currents in acutely dissociated rat basal forebrain neurons. We further investigated whether Fucoidan is capable of blocking A beta neurotoxicity in primary neuronal cultures. In dissociated cells, bath application of A beta(25-35) (1 microM) caused a reduction of the whole-cell currents by 16%. Fucoidan, in a dose-dependent manner, blocks the A beta(25-35) reduction of whole-cell currents. Exposure of A beta(25-35) (20 microM) or A beta(1-42) (20 microM) to rat cholinergic basal forebrain cultures for 48 h resulted in 40-60% neuronal death, which was significantly decreased by pretreatment of cultures with Fucoidan (0.1-1.0 microM). Fucoidan also attenuated A beta-induced down-regulation of phosphorylated protein kinase C. A beta(1-42)-induced generation of reactive oxygen species was blocked by prior exposure of cultures to Fucoidan. Furthermore, A beta activation of caspases 9 and 3, which are signaling pathways implicated in apoptotic cell death, is blocked by pretreatment of cultures with Fucoidan. These results show that Fucoidan is able to block A beta-induced reduction in whole-cell currents in basal forebrain neurons and has neuroprotective effects against A beta-induced neurotoxicity in basal forebrain neuronal cultures.

Beta-amyloid peptide activates non-alpha7 nicotinic acetylcholine receptors in rat basal forebrain neurons

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by profound deficits in memory and cognitive function. Neuropathological hallmarks of the disease include a loss of basal forebrain cholinergic neurons and the deposition of beta-amyloid peptide (Abeta) in neuritic plaques. At a cellular level, considerable attention has focused on a study of Abeta interactions with the neuronal nicotinic acetylcholine receptor (nAChR) subtypes. In this study, using cell-attached and outside-out single channel recordings from acutely dissociated rat basal forebrain neurons, we report that Abeta and nicotine activate nAChRs with two distinct levels of single-channel conductance. Whole cell recordings from these neurons reveal Abeta and nicotine, in a concentration-dependent and reversible manner, evoke brisk depolarizing responses and an inward current. The effects of Abeta on both single channel and whole cell are blocked by the noncompetitive nAChR antagonist mecamylamine and competitive nAChR antagonist dihydro-beta-erythroidine, but not the specific alpha7-selective nAChR antagonist methyllycaconitine, indicating that Abeta activated non-alpha7 nAChRs on basal forebrain neurons. In addition, the non-alpha7 nAChR agonists UB-165, epibatidine, and cytisine, but not the selective alpha7 agonist AR-R17779, induced similar responses as Abeta and nicotine. Thus non-alpha7 nAChRs may also represent a novel target in mediating the effects of Abeta in AD.

Human amylin actions on rat cholinergic basal forebrain neurons: antagonism of beta-amyloid effects

Human amylin (hAmylin), a 37-amino acid pancreatic peptide, and amyloid beta protein (A beta), a 39-43 amino acid peptide, abundantly deposited in the brains of Alzheimer's patients, induce neurotoxicity in hippocampal and cortical cultures. Although the mechanism of this neurotoxicity is unknown, both peptides are capable of modulating ion channel function that may result in a disruption of cellular homeostasis. In this study, we examined the effects of hAmylin on whole cell currents in chemically identified neurons from the rat basal forebrain and the interactions of hAmylin-induced responses with those of A beta. Whole cell patch-clamp recordings were performed on enzymatically dissociated neurons of the diagonal band of Broca (DBB), a cholinergic basal forebrain nucleus. Bath application of hAmylin (1 nM to 5 microM) resulted in a dose-dependent reduction in whole cell currents in a voltage range between -30 and +30 mV. Single-cell RT-PCR analysis reveal that all DBB neurons responding to hAmylin or A beta were cholinergic. Using specific ion channel blockers, we identified hAmylin and A beta effects on whole cell currents to be mediated, in part, by calcium-dependent conductances. Human amylin also depressed the transient outward (IA) and the delayed rectifier (IK) potassium currents. The hAmylin effects on whole cell currents could be occluded by A beta and vice versa. Human amylin and A beta responses could be blocked with AC187 (50 nM to 1 microM), a specific antagonist for the amylin receptor. The present study indicates that hAmylin, like A beta, is capable of modulating ion channel function in cholinergic basal forebrain neurons. Furthermore, the two peptides may share a common mechanism of action. The ability of an amylin antagonist to block the responses evoked by hAmylin and A beta may provide a novel therapeutic approach for Alzheimer's disease.

Neurotensin modulates the amplitude and frequency of voltage-activated Ca2+ currents in frog pituitary melanotrophs: implication of the inositol triphosphate/protein kinase C pathway

Many excitatory neurotransmitters and neuropeptides regulate the activity of neuronal and endocrine cells by modulating voltage-operated Ca2+ channels. Paradoxically, however, excitatory neuromediators that provoke mobilization of intracellular calcium from inositol trisphosphate (IP3)-sensitive stores usually inhibit voltage-gated Ca2+ currents. We have recently demonstrated that neurotensin (NT) stimulates the electrical and secretory activities of frog pituitary melanotrophs, and increases intracellular calcium concentration in these cells. In the present study, we have investigated the effects of NT on Ca2+ currents in cultured frog melanotrophs by using the perforated patch-clamp technique. Frog neurotensin (f NT) reduced the amplitude and facilitated the inactivation of both L- and N-type Ca2+ currents. Application of the membrane-permeant Ca2+ chelator BAPTA-AM, the sarcoendoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, or the IP3 receptor antagonist 2-APB suppressed the reduction of Ca2+ currents induced by f NT. Incubation of melanotrophs with the diacylglycerol analogue PMA, which causes desensitization of protein kinase C (PKC), or with the PKC inhibitors chelerythrine and calphostin C, reduced the inhibitory effect of f NT. The NT-induced action potential waveforms, applied as voltage-clamp commands, decreased the amplitude of Ca2+ currents, and enhanced Ca2+ influx by increasing the Ca2+ spike frequency. Altogether, these data indicate that the inhibitory effect of f NT on Ca2+ currents results from activation of the IP3/PKC pathway. The observation that NT controls Ca2+ signalling through both amplitude and frequency modulations of Ca2+ currents suggests that NT might induce spacial and temporal changes of intracellular Ca2+ concentration leading to stimulation of exocytosis.

Nociceptin/orphanin FQ modulation of ionic conductances in rat basal forebrain neurons

Nociceptin/orphanin FQ (N/OFQ) is an endogenous opioid-like heptadecapeptide that plays an important role in a variety of physiological functions. N/OFQ and its receptor opioid receptor-like orphan receptor-1 are abundant in the diagonal band of Broca (DBB), a basal forebrain nucleus where the loss of cholinergic neurons is linked to memory and spatial learning deficits. In the whole animal, central injections of N/OFQ have been shown to disrupt spatial learning. In this study, we investigated the basis for these behavioral observations by examining the cellular effects of N/OFQ on chemically identified DBB neurons. Whole cell patch-clamp recordings were performed on enzymatically dissociated DBB neurons. Under voltage-clamp conditions, bath application of N/OFQ (10 pM-1 microM) resulted in a dose-dependent depression of whole cell currents. Single cell reverse transcription-polymerase chain reaction analysis identified cholinergic and fewer GABAergic cells to be N/OFQ-responsive. [Nphe(1)]nociceptin-(1-13)-NH(2) and CompB (J-113397) antagonized the N/OFQ response, but both compounds also displayed partial agonist activity. Using a combination of channel blockers we determined that the effects of N/OFQ were mediated via a suite of Ca(2+) (N- and L-type) and Ca(2+)-dependent K(+) (iberiotoxin-sensitive) conductances. In addition, biophysical analysis of voltage subtraction protocols revealed that N/OFQ reduces transient outward and the delayed rectifier K(+) currents. Because N-type and L-type Ca(2+) channels are important in the context of neurotransmitter release, our observations indicate that N/OFQ inhibition of Ca(2+)-dependent conductances in cholinergic neurons would be expected to result in depression of acetylcholine release, which may explain the behavioral actions of N/OFQ in the brain.

Novel excitatory actions of galanin on rat cholinergic basal forebrain neurons: implications for its role in Alzheimer's disease

Galanin, a 29-amino-acid neuropeptide, is generally viewed as an inhibitory neuromodulator in a variety of central systems. Galanin expression is upregulated in the cholinergic basal forebrain nuclei in Alzheimer's disease (AD) and is postulated to play an important role in memory and cognitive function. In this study, application of galanin to acutely dissociated rat neurons from the basal forebrain nucleus diagonal band of Broca (DBB), caused a decrease in whole cell voltage-activated currents in a majority of cells. Galanin reduces a suite of potassium currents, including calcium-activated potassium (I(C)), the delayed rectifier (I(K)), and transient outward potassium (I(A)) conductances, but not calcium or sodium currents. Under current-clamp conditions, application of galanin evoked an increase in excitability and a loss of accommodation in cholinergic DBB neurons. Using single-cell RT-PCR technique, we determined that galanin actions were specific to cholinergic, but not GABAergic DBB neurons The notion that galanin plays a deleterious role in AD is based, in part, on galanin hyperinnervation of cholinergic cells in the basal forebrain of AD patients, its ability to depress acetylcholine release and its inhibitory actions at other CNS sites. However, our results suggest that by virtue of its excitatory actions on cholinergic neurons, galanin may in fact play a compensatory role by augmenting the release of acetylcholine from remaining cholinergic basal forebrain neurons. This action might serve to delay the progression of AD pathology linked to a reduction in central cholinergic tone.

Cellular mechanisms for amyloid beta-protein activation of rat cholinergic basal forebrain neurons

The deposition of amyloid beta-protein (Abeta) in the brain and the loss of cholinergic neurons in the basal forebrain are two pathological hallmarks of Alzheimer's disease (AD). Although the mechanism of Abeta neurotoxicity is unknown, these cholinergic neurons display a selective vulnerability when exposed to this peptide. In this study, application of Abeta(25-35) or Abeta(1-40) to acutely dissociated rat neurons from the basal forebrain nucleus diagonal band of Broca (DBB), caused a decrease in whole cell voltage-activated currents in a majority of cells. This reduction in whole cell currents occurs through a modulation of a suite of potassium conductances including calcium-activated potassium (I(C)), the delayed rectifier (I(K)), and transient outward potassium (I(A)) conductances, but not calcium or sodium currents. Under current-clamp conditions, Abeta evoked an increase in excitability and a loss of accommodation in cholinergic DBB neurons. Using single-cell RT-PCR technique, we determined that Abeta actions were specific to cholinergic, but not GABAergic DBB neurons. Abeta effects on whole cell currents were occluded in the presence of membrane-permeable protein tyrosine kinase inhibitors, genistein and tyrphostin B-44. Our data indicate that the Abeta actions on specific potassium conductances are modulated through a protein tyrosine kinase pathway and that these effects are selective to cholinergic but not GABAergic cells. These observations provide a cellular basis for the selectivity of Abeta neurotoxicity toward cholinergic basal forebrain neurons that are at the epicenter of AD pathology.

Effects of neurotensin on discharge rates of rat suprachiasmatic nucleus neurons in vitro

The neuropeptide neurotensin and two classes of its receptors, the neurotensin receptor-1 and 2, are present in the suprachiasmatic nucleus of the mammalian hypothalamus. The suprachiasmatic nucleus houses the mammalian central circadian pacemaker, but the effects of neurotensin on cellular activity in this circadian pacemaker are unknown. In this study, we examined the effects of neurotensin on the spontaneous discharge rate of rat SCN cells in an in vitro slice preparation. Neurotensin (1-10 microM) increased cell firing rate in approximately 50% of cells tested, while approximately 10% of suprachiasmatic cells tested showed a decrease in firing rate in response to neurotensin. These effects of neurotensin were not altered by the GABA receptor antagonist bicuculline (20 microM) or the glutamate receptor antagonists, D-aminophosphopentanoic acid (50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). The neurotensin receptor selective antagonists SR48692 and SR142948a (10 microM) failed to antagonise neurotensin responses in the majority of cells examined. Compounds that function as agonists selective for the neurotensin-receptor subtypes 1 and 2, JMV-510 and JMV-431 respectively, elicited neurotensin-like responses in approximately 90% of cells tested. Six out of seven cells tested responded to both JMV-510 and JMV-431. Neuropeptide Y (100nM) treatment of suprachiasmatic nucleus slices was found to elicit profound suppression of neuronal firing rate. Co-application of neurotensin with neuropeptide Y significantly (P<0.05) reduced the duration of the response, as compared to that elicited with neuropeptide Y alone. Together, these results demonstrate for the first time the actions of neurotensin in the suprachiasmatic nucleus and raise the possibility that this neuropeptide may play a role in modulating circadian pacemaker function.

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.

Zinc modulation of ionic currents in the horizontal limb of the diagonal band of Broca

We examined modulation of ionic currents by Zn2+ in acutely dissociated neurons from the rat's horizontal limb of the diagonal band of Broca using the whole-cell patch-clamp technique. Application of 50 microM Zn2+ increased the peak amplitude of the transiently activated potassium current, I(A) (at + 30 mV), from 2.20+/-0.08 to 2.57+/-0.11 nA (n = 27). This response was reversible and could be repeated in 0 Ca2+/1 microM tetrodotoxin (n = 15). Zn2+ shifted the inactivation curve to the right, resulting in a shift in the half-inactivation voltage from 76.4+/-2.2 to -53.4+/-2.0 mV (n = 11), with no effect on the voltage dependence of activation gating (n = 15). There was no significant difference in the time to peak under control conditions (7.43+/-0.35 ms, n = 14) and in the presence of Zn2+ (8.20+/-0.57 ms, n = 14). Similarly, the time constant of decay of I(A) (tau(d)) at + 30 mV showed no difference (control: 38.68+/-3.68 ms, n = 15; Zn2+: 38.48+/-2.85 ms, n = 15). I(A) was blocked by 0.5-1 mM 4-aminopyridine. In contrast to its effects on I(A), Zn2+ reduced the amplitude of the delayed rectifier potassium current (I(K)). The reduction of outward K+ currents was reproducible when cells were perfused with 1 microM tetrodotoxin in a 0 Ca2+ external solution. The amplitude of the steady-state outward currents at +30 mV under these conditions was reduced from 6.40+/-0.23 (control) to 5.76+/-0.18 nA in the presence of Zn2+ (n = 16). The amplitudes of peak sodium currents (INa) were not significantly influenced (n = 10), whereas barium currents (I(Ba)) passing through calcium channels were potently modulated. Zn2+ reversibly reduced I(Ba) at -10 mV by approximately 85% from -2.06+/-0.14 nA under control conditions to -0.30+/-0.10 nA in the presence of Zn2+ (n = 14). Further analyses of Zn2+ effects on specific calcium channels reveals that it suppresses all types of high-voltage-activated Ca2+ currents. Under current-clamp conditions, application of Zn2+ resulted in an increase in excitability and loss of accommodation (n = 13), which appears to be mediated through its effects on Ca2+-dependent conductances.