Homocysteine potentiates amyloid ß -induced death receptor 4- and 5-mediated cerebral endothelial cell apoptosis, blood brain barrier dysfunction and angiogenic impairment.

Cerebrovascular dysfunction has been implicated as a major contributor to Alzheimer's Disease (AD) pathology, with cerebral endothelial cell (cEC) stress promoting ischemia, cerebral-blood flow impairments and blood-brain barrier (BBB) permeability. Recent evidence suggests that cardiovascular (CV)/cerebrovascular risk factors, including hyperhomocysteinemia (Hhcy), exacerbate AD pathology and risk. Yet, the underlying molecular mechanisms for this interaction remain unclear. Our lab has demonstrated that amyloid beta 40 (Aß40) species, and particularly Aß40-E22Q (AßQ22; vasculotropic Dutch mutant), promote death receptor 4 and 5 (DR4/DR5)-mediated apoptosis in human cECs, barrier permeability, and angiogenic impairment. Previous studies show that Hhcy also induces EC dysfunction, but it remains unknown whether Aß and homocysteine function through common molecular mechanisms. We tested the hypotheses that Hhcy exacerbates Aß-induced cEC DR4/5-mediated apoptosis, barrier dysfunction, and angiogenesis defects. This study was the first to demonstrate that Hhcy specifically potentiates AßQ22-mediated activation of the DR4/5-mediated extrinsic apoptotic pathway in cECs, including DR4/5 expression, caspase 8/9/3 activation, cytochrome-c release and DNA fragmentation. Additionally, we revealed that Hhcy intensifies the deregulation of the same cEC junction proteins mediated by Aß, precipitating BBB permeability. Furthermore, Hhcy and AßQ22, impairing VEGF-A/VEGFR2 signaling and VEGFR2 endosomal trafficking, additively decrease cEC angiogenic capabilities. Overall, these results show that the presence of the CV risk factor Hhcy exacerbates Aß-induced cEC apoptosis, barrier dysfunction, and angiogenic impairment. This study reveals specific mechanisms through which amyloidosis and Hhcy jointly operate to produce brain EC dysfunction and death, highlighting new potential molecular targets against vascular pathology in comorbid AD/CAA and Hhcy conditions.

Amyloid ß induces cardiac dysfunction and neuro-signaling impairment in the heart of an Alzheimer's disease model.

Aims: Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by cerebral amyloid ß (Aß) deposition and tau pathology. The AD-mediated degeneration of the brain neuro-signaling pathways, together with a potential peripheral amyloid accumulation, may also result in the derangement of the peripheral nervous system, culminating in detrimental effects on other organs, including the heart. However, whether and how AD pathology modulates cardiac function, neurotrophins, innervation, and amyloidosis is still unknown. Here, we report for the first time that cardiac remodeling, amyloid deposition, and neuro-signaling dysregulation occur in the heart of Tg2576 mice, a widely used model of AD and cerebral amyloidosis. Methods ad Results: Echocardiographic analysis showed significant deterioration of left ventricle function, evidenced by a decline of both ejection fraction and fraction shortening percentage in 12-month-old Tg2576 mice compared to age-matched WT littermates. Tg2576 mice hearts exhibited an accumulation of amyloid aggregates, including Aß, an increase in interstitial fibrosis and severe cardiac nervous system dysfunction. The transgenic mice also showed a significant decrease in cardiac nerve fiber density, including both adrenergic and regenerating nerve endings. This myocardial denervation was accompanied by a robust reduction in NGF and BDNF protein expression as well as GAP-43 expression (regenerating fibers) in both the brain and heart of Tg2576 mice. Accordingly, cardiomyocytes and neuronal cells challenged with Aß oligomers showed significant downregulation of BDNF and GAP-43, indicating a causal effect of Aß on the loss of cardiac neurotrophic function. Conclusions: Overall, this study uncovers possible harmful effects of AD on the heart, revealing cardiac degeneration induced by Aß through fibrosis and neuro-signaling pathway deregulation for the first time in Tg2576 mice. Our data suggest that AD pathology can cause deleterious effects on the heart, and the peripheral neurotrophic pathway may represent a potential therapeutic target to limit these effects.

FDA-approved carbonic anhydrase inhibitors reduce amyloid ß pathology and improve cognition, by ameliorating cerebrovascular health and glial fitness.

INTRODUCTION: Cerebrovascular pathology is an early and causal hallmark of Alzheimer's disease (AD), in need of effective therapies. METHODS: Based on the success of our previous in vitro studies, we tested for the first time in a model of AD and cerebral amyloid angiopathy (CAA), the carbonic anhydrase inhibitors (CAIs) methazolamide and acetazolamide, Food and Drug Administration-approved against glaucoma and high-altitude sickness. RESULTS: Both CAIs reduced cerebral, vascular, and glial amyloid beta (Aß) accumulation and caspase activation, diminished gliosis, and ameliorated cognition in TgSwDI mice. The CAIs also improved microvascular fitness and induced protective glial pro-clearance pathways, resulting in the reduction of Aß deposition. Notably, we unveiled that the mitochondrial carbonic anhydrase-VB (CA-VB) is upregulated in TgSwDI brains, CAA and AD+CAA human subjects, and in endothelial cells upon Aß treatment. Strikingly, CA-VB silencing specifically reduces Aß-mediated endothelial apoptosis. DISCUSSION: This work substantiates the potential application of CAIs in clinical trials for AD and CAA.

Locus coeruleus activation during environmental novelty gates cocaine-induced long-term hyperactivity of dopamine neurons.

A key feature of the brain is the ability to handle novelty. Anything that is new will stimulate curiosity and trigger exploration. Novelty preference has been proposed to predict increased sensitivity to cocaine. Different brain circuits are activated by novelty, but three specific brain regions are critical for exploring a novel environment: the noradrenergic neurons originating from the locus coeruleus (LC), the dopaminergic neurons from the ventral tegmental area (VTA), and the hippocampus. However, how exploring a novel environment can interfere with the reward system and control cocaine impact on VTA dopamine neuron plasticity is unclear. Here, we first investigated the effects of exposure to a novel environment on the tonic electrophysiological properties of VTA dopamine neurons. Then, we explored how exposure to a novel environment controls cocaine-evoked plasticity in dopamine neurons. Our findings indicate that LC controls VTA dopamine neurons under physiological conditions but also after cocaine.

Ih blockade reduces cocaine-induced firing patterns of putative dopaminergic neurons of the ventral tegmental area in the anesthetized rat.

The hyperpolarization-activated cation current (Ih) is a determinant of intrinsic excitability in various cells, including dopaminergic neurons (DA) of the ventral tegmental area (VTA). In contrast to other cellular conductances, Ih is activated by hyperpolarization negative to -55 mV and activating Ih produces a time-dependent depolarizing current. Our laboratory demonstrated that cocaine sensitization, a chronic cocaine behavioral model, significantly reduces Ih amplitude in VTA DA neurons. Despite this reduction in Ih, the spontaneous firing of VTA DA cells after cocaine sensitization remained similar to control groups. Although the role of Ih in controlling VTA DA excitability is still poorly understood, our hypothesis is that Ih reduction could play a role of a homeostatic controller compensating for cocaine-induced change in excitability. Using in vivo single-unit extracellular electrophysiology in isoflurane anesthetized rats, we explored the contribution of Ih on spontaneous firing patterns of VTA DA neurons. A key feature of spontaneous excitability is bursting activity; bursting is defined as trains of two or more spikes occurring within a short interval and followed by a prolonged period of inactivity. Burst activity increases the reliability of information transfer. To elucidate the contribution of Ih to spontaneous firing patterns of VTA DA neurons, we locally infused an Ih blocker (ZD 7288, 8.3 µM) and evaluated its effect. Ih blockade significantly reduced firing rate, bursting frequency, and percent of spikes within a burst. In addition, Ih blockade significantly reduced acute cocaine-induced spontaneous firing rate, bursting frequency, and percent of spikes within a burst. Using whole-cell patch-clamp, we determine the progressive reduction of Ih after acute and chronic cocaine administration (15 mg/k.g intraperitoneally). Our data show a significant reduction (~25%) in Ih amplitude after 24 but not 2 h of acute cocaine administration. These results suggest that a progressive reduction of Ih could serve as a homeostatic regulator of cocaine-induced spontaneous firing patterns related to VTA DA excitability.

Alpha-1 Adrenergic Receptors Modulate Glutamate and GABA Neurotransmission onto Ventral Tegmental Dopamine Neurons during Cocaine Sensitization.

The ventral tegmental area (VTA) plays an important role in the reward and motivational processes that facilitate the development of drug addiction. Presynaptic α1-AR activation modulates glutamate and Gamma-aminobutyric acid (GABA) release. This work elucidates the role of VTA presynaptic α1-ARs and their modulation on glutamatergic and GABAergic neurotransmission during cocaine sensitization. Excitatory and inhibitory currents (EPSCs and IPSCs) measured by a whole cell voltage clamp show that α1-ARs activation increases EPSCs amplitude after 1 day of cocaine treatment but not after 5 days of cocaine injections. The absence of a pharmacological response to an α1-ARs agonist highlights the desensitization of the receptor after repeated cocaine administration. The desensitization of α1-ARs persists after a 7-day withdrawal period. In contrast, the modulation of α1-ARs on GABA neurotransmission, shown by decreases in IPSCs' amplitude, is not affected by acute or chronic cocaine injections. Taken together, these data suggest that α1-ARs may enhance DA neuronal excitability after repeated cocaine administration through the reduction of GABA inhibition onto VTA dopamine (DA) neurons even in the absence of α1-ARs' function on glutamate release and protein kinase C (PKC) activation. α1-AR modulatory changes in cocaine sensitization increase our knowledge of the role of the noradrenergic system in cocaine addiction and may provide possible avenues for therapeutics.

Protein and surface expression of HCN2 and HCN4 subunits in mesocorticolimbic areas after cocaine sensitization.

The Ih is a mixed depolarizing current present in neurons which, upon activation by hyperpolarization, modulates neuronal excitability in the mesocorticolimbic (MCL) system, an area which regulates emotions such as pleasure, reward, and motivation. Its biophysical properties are determined by HCN protein expression profiles, specifically HCN subunits 1-4. Previously, we reported that cocaine-induced behavioral sensitization increases HCN2 protein expression in all MCL areas with the Ventral Tegmental Area (VTA) showing the most significant increase. Recent evidence suggests that HCN4 also has an important expression in the MCL system. Although there is a significant expression of HCN channels in the MCL system their role in addictive processes is largely unknown. Thus, in this study we aim to compare HCN2 and HCN4 expression profiles and their cellular compartmental distribution in the MCL system, before and after cocaine sensitization. Surface/intracellular (S/I) ratio analysis indicates that VTA HCN2 subunits are mostly expressed in the cell surface in contrast to other areas tested. Our findings demonstrate that after cocaine sensitization, the HCN2 S/I ratio in the VTA was decreased whereas in the Prefrontal Cortex it was increased. In addition, HCN4 total expression in the VTA was decreased after cocaine sensitization, although the S/I ratio was not altered. Together, these results demonstrate differential cocaine effects on HCN2 and HCN4 protein expression profiles and therefore suggest a diverse Ih modulation of cellular activity during cocaine addictive processes.

aPKC-Mediated Persistent Increase in AMPA/NMDA Ratio in the VTA Participates in the Neuroadaptive Signal Necessary to Induce NAc Synaptic Plasticity After Cocaine Administration.

Chronic cocaine exposure produces enduring neuroadaptations in the brain's reward system. Persistence of early cocaine-evoked neuroadaptations in the ventral tegmental area (VTA) is necessary for later synaptic alterations in the nucleus accumbens (NAc), suggesting a temporal sequence of neuroplastic changes between these two areas. However, the molecular nature of the signal that mediates this sequential event is unknown. Here we used the behavioral sensitization model and the aPKC inhibitor of late-phase LTP maintenance, ZIP, to investigate if a persistent increase in AMPA/NMDA ratio plays a role in the molecular mechanism that allows VTA neuroadaptations to induce changes in the NAc. Results showed that intra-VTA ZIP microinfusion successfully blocked cocaine-evoked synaptic enhancement in the VTA and the expected AMPA/NMDA ratio decrease in the NAc following cocaine sensitization. ZIP microinfusions also blocked the expected AMPA/NMDA ratio increase in the NAc following cocaine withdrawal. These results suggest that a persistent increase in AMPA/NMDA ratio, mediated by aPKCs, could be the molecular signal that enables the VTA to elicit synaptic alterations in the NAc following cocaine administration.

Cocaine sensitization increases subthreshold activity in dopamine neurons from the ventral tegmental area.

The progressive escalation of psychomotor responses that results from repeated cocaine administration is termed sensitization. This phenomenon alters the intrinsic properties of dopamine (DA) neurons from the ventral tegmental area (VTA), leading to enhanced dopaminergic transmission in the mesocorticolimbic network. The mechanisms underlying this augmented excitation are nonetheless poorly understood. DA neurons display the hyperpolarization-activated, nonselective cation current, dubbed Ih We recently demonstrated that Ih and membrane capacitance are substantially reduced in VTA DA cells from cocaine-sensitized rats. The present study shows that 7 days of cocaine withdrawal did not normalize Ih and capacitance. In cells from cocaine-sensitized animals, the amplitude of excitatory synaptic potentials, at -70 mV, was ∼39% larger in contrast to controls. Raise and decay phases of the synaptic signal were faster under cocaine, a result associated with a reduced membrane time constant. Synaptic summation was paradoxically elevated by cocaine exposure, as it consisted of a significantly reduced summation indexed but a considerably increased depolarization. These effects are at least a consequence of the reduced capacitance. Ih attenuation is unlikely to explain such observations, since at -70 mV, no statistical differences exist in Ih or input resistance. The neuronal shrinkage associated with a diminished capacitance may help to understand two fundamental elements of drug addiction: incentive sensitization and negative emotional states. A reduced cell size may lead to substantial enhancement of cue-triggered bursting, which underlies drug craving and reward anticipation, whereas it could also result in DA depletion, as smaller neurons might express low levels of tyrosine hydroxylase. NEW & NOTEWORTHY: This work uses a new approach that directly extracts important biophysical parameters from alpha function-evoked synaptic potentials. Two of these parameters are the cell membrane capacitance (Cm) and rate at any time point of the synaptic waveform. The use of such methodology shows that cocaine sensitization reduces Cm and increases the speed of synaptic signaling. Paradoxically, although synaptic potentials show a faster decay under cocaine their temporal summation is substantially elevated.

Alpha-1 adrenoreceptors modulate GABA release onto ventral tegmental area dopamine neurons.

The ventral tegmental area (VTA) plays an important role in reward and motivational processes involved in drug addiction. Previous studies have shown that alpha1-adrenoreceptors (α1-AR) are primarily found pre-synaptically at this area. We hypothesized that GABA released onto VTA-dopamine (DA) cells is modulated by pre-synaptic α1-AR. Recordings were obtained from putative VTA-DA cells of male Sprague-Dawley rats (28-50 days postnatal) using whole-cell voltage clamp technique. Phenylephrine (10 µM; α1-AR agonist) decreased the amplitude of GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked by electrical stimulation of afferent fibers (n = 7; p < 0.05). Prazosin (1 µM, α1-AR antagonist), blocked this effect. Paired-pulse ratios were increased by phenylephrine application (n = 13; p < 0.05) indicating a presynaptic site of action. Spontaneous IPSCs frequency but not amplitude, were decreased in the presence of phenylephrine (n = 7; p < 0.05). However, frequency or amplitude of miniature IPSCs were not changed (n = 9; p > 0.05). Phenylephrine in low Ca(2+) (1 mM) medium decreased IPSC amplitude (n = 7; p < 0.05). Chelerythrine (a protein kinase C inhibitor) blocked the α1-AR action on IPSC amplitude (n = 6; p < 0.05). Phenylephrine failed to decrease IPSCs amplitude in the presence of paxilline, a BK channel blocker (n = 7; p < 0.05). Taken together, these results demonstrate that α1-ARs at presynaptic terminals can modulate GABA release onto VTA-DA cells. Drug-induced changes in α1-AR could contribute to the modifications occurring in the VTA during the addiction process.

New method to visualize neurons with DAT in slices of rat VTA using fluorescent substrate for DAT, ASP+

The ventral tegmental area (VTA), and in particular dopamine (DA) neurons in this region of midbrain, has been shown to play an important role in motivation (goal-directed behavior), reward, and drug addiction. Most evidence that implicates VTA DA neurons in these functions are based on widely accepted but indirect electrophysiological characterization, including the hyperpolarization activated non-specific cation current (Ih), spike frequency, and inhibition by D2 receptor agonists. In this study, we used a known neuronal dopamine transporter (DAT) fluorescent substrate [4-(4- (dimethylamino) styryl)-N-methylpyridinium iodide] (ASP+) to visualize DAT-containing cell bodies of DA neurons in VTA region in rat brain slices. Uptake of 100 nM of ASP+ in brain slices of rat VTA region marked 38% of visible neurons, while other neurons from this region and 100% neurons from hippocampus slices were not fluorescent. Using patch-clamp techniques, we have found that pronounced Ih current was present in all fluorescent neurons from VTA area, also spike frequency was similar to the widely accepted values for DA neurons. Furthermore, additional study has shown that there are 84% coincidence of ASP+ fluorescence in neuronal cell bodies and Falck-Hillarp labeling of DA cells. Electrophysiological recordings during ASP+ application have confirmed that low concentrations (100 nM) of ASP+ have no visible effect on neuronal activity during 1-2 hours after staining. Thus, uptake of fluorescent monoamine analog ASP+ by DAT can be an additional criterion for identification of DAT-containing neurons in slices.

Cocaine sensitization increases I h current channel subunit 2 (HCN2) protein expression in structures of the mesocorticolimbic system.

Alteration of the biological activity among neuronal components of the mesocorticolimbic (MCL) system has been implicated in the pathophysiology of drug abuse. Changes in the electrophysiological properties of neurons involved in the reward circuit seem to be of utmost importance in addiction. The hyperpolarization-activated cyclic nucleotide current, I h, is a prominent mixed cation current present in neurons. The biophysical properties of the I h and its potential modulatory role in cell excitability depend on the expression profile of the hyperpolarization-activated cyclic nucleotide gated channel (HCN) subunits. We investigated whether cocaine-induced behavioral sensitization, an animal model of drug addiction, elicits region-specific changes in the expression of the HCN2 channel's subunit in the MCL system. Tissue samples from the ventral tegmental area, prefrontal cortex, nucleus accumbens, and hippocampus were analyzed using Western blot. Our findings demonstrate that cocaine treatment induced a significant increase in the expression profile of the HCN2 subunit in both its glycosylated and non-glycosylated protein isoforms in all areas tested. The increase in the glycosylated isoform was only observed in the ventral tegmental area. Together, these data suggest that the observed changes in MCL excitability during cocaine addiction might be associated with alterations in the subunit composition of their HCN channels.

Inhibition of Protein kinase Mzeta (PKMζ) in the mesolimbic system alters cocaine sensitization in rats.

Chronic cocaine use produces long-lasting changes in reward circuits that may underlie the transition from casual to compulsive patterns of drug use. Although strong neuroadaptations within the mesocorticolimbic system are known to occur, the specific role of these drug-induced plasticities on sensitization remains to be elucidated. Here we investigate whether PKMζ, a protein involved in maintaining long-term potentiation (LTP), plays a role in these cocaine-induced changes in synaptic strengthening. We performed whole-cell voltage clamp recordings of putative ventral tegmental area (VTA) dopamine (DA) cells 24 hours after five days of 15 mg/kg i.p. cocaine or isovolumetric saline injections. We observed that superfusion of 5µM ZIP (PKMζ inhibitory peptide) decreased AMPA currents and AMPA/NMDA ratios only in cocaine sensitized rats. In vivo ZIP microinfusions (10 nmol) into the VTA after cocaine sensitization decreased locomotor activity on a subsequent cocaine challenge only if given ZIP is given before the withdrawal period. On the other hand, ZIP microinfusions into the nucleus accumbens (NAc) core after a seven days withdrawal period disrupt the expression of locomotor sensitization. The present data provide a potentially relevant region, and time-specific PKMζ-dependent brain mechanism that enables sensitization. Our results support the vision that addiction involves a pathological learning process. They imply that if this synaptic strengthening is reversed, changes in the behavioral response may also be overturned.

Presynaptic inhibition of glutamate transmission by α2 receptors in the VTA.

The ventral tegmental area (VTA) forms part of the mesocorticolimbic system and plays a pivotal role in reward and reinforcing actions of drugs of abuse. Glutamate transmission within the VTA controls important aspects of goal-directed behavior and motivation. Noradrenergic receptors also present in the VTA have important functions in the modulation of neuronal activity. Here we studied the effects of α2 noradrenergic receptor activation in the alteration of glutamate neurotransmission in VTA dopaminergic neurons from male Sprague-Dawley rats. We used whole-cell patch-clamp recordings from putative VTA dopaminergic neurons and measured excitatory postsynaptic currents. Clonidine (40 µm) and UK 14,304 (40 µm), both α2 receptor agonists, reduced (approximately 40%) the amplitude of glutamate-induced excitatory postsynaptic currents. After clonidine administration, there was a dose-dependent reduction over the concentration range of 15-40 µm. Using yohimbine (20 µm) and two other α2 adrenergic receptor antagonists, idaxozan (40 µm) and atipemazole (20 µm), we demonstrated that the inhibitory action is specifically mediated by α2 receptors. Moreover, by inhibiting protein kinases with H-7 (75 µm), Rp-adenosine 3',5'-cyclic (11 µm) and chelerythrine (1 µm) it was shown that the clonidine-induced inhibition seems to involve a selective activation of the protein kinase C intracellular pathway. Increased paired-pulse ratios and changes in spontaneous and miniature excitatory postsynaptic current frequencies but not amplitudes indicated that the effect of the α2 agonist was presynaptically mediated. It is suggested that the suppression of glutamate excitatory inputs onto VTA dopaminergic neurons might be relevant in the regulation of reward and drug-seeking behaviors.

Alpha-noradrenergic receptors modulate the development and expression of cocaine sensitization.

The increased activity and stereotyped behaviors that result from repeated administration of cocaine is called cocaine sensitization. This sensitized response has been postulated as one of the basic pathophysiological mechanisms in drug addiction. Recent evidence indicates that noradrenergic neurotransmission might be implicated in some of the behavioral effects of cocaine. The present article examined the role of alpha-adrenergic receptor agonists and antagonists in the development and expression of cocaine sensitization. Rats were injected once per day, for 7 consecutive days, with the alpha-1 receptor antagonist prazosin (0.5 mg/kg, i.p.) 15 min before cocaine administration (15 mg/kg, i.p.). After 8 days, animals received a cocaine challenge (15 mg/kg, i.p.) and were tested for locomotion. Following a 7-day withdrawal period rats received a second cocaine challenge. One day after the last challenge, rats were reinstated to the initial protocol for 1 day. In another set of experiments, rats were injected twice per day with the alpha-2 receptor antagonists yohimbine (5 mg/kg, i.p.), idazoxan (0.25 mg/kg, i.p.), or with the alpha-2 agonist clonidine (0.025 mg/kg, i.p.), followed by cocaine injections (15 mg/kg, i.p.), for 7 consecutive days. Thereafter, the protocol was similar to that following prazosin administration. The results demonstrated that the alpha-1 receptor antagonist prazosin blocked the development and expression of cocaine sensitization. On the other hand, both alpha-2 antagonists failed to inhibit the development or the expression of cocaine sensitization. Instead, they produced an increase in locomotor activity during the first day of experimentation. The alpha-2 agonist clonidine attenuated the acute response to cocaine on day 1 and retarded the increased locomotor activity on the following 2 days. There was a dramatic increase in the level of sensitization after the first cocaine challenge. However, it inhibited the expression of cocaine sensitization during the reinstatement protocol. These results suggest that alpha adrenoreceptors play an important role in modulating different stages of cocaine sensitization and probably cocaine addiction.