Gamma-secretase modulators: a promising route for the treatment of Alzheimer's disease

Recent clinical data with three therapeutic anti-Aβ antibodies have demonstrated that removal of Aβ-amyloid plaques in early Alzheimer's disease (AD) can attenuate disease progression. This ground-breaking progress in AD medicine has validated both the amyloid cascade hypothesis and Aβ-amyloid as therapeutic targets. These results also strongly support therapeutic approaches that aim to reduce the production of amyloidogenic Aβ to prevent the formation of Aβ-pathology. One such strategy, so-called gamma-secretase modulators (GSM), has been thoroughly explored in preclinical settings but has yet to be fully tested in clinical trials. Recent scientific progress has shed new light on the role of Aβ in Alzheimer's disease and suggests that GSMs exhibit specific pharmacological features that hold great promise for the prevention and treatment of Alzheimer's disease. In this short review, we discuss the data that support why it is important to continue to progress in this class of compounds.

Antidepressant effects of novel positive allosteric modulators of Trk-receptor mediated signaling - a potential therapeutic concept?

BACKGROUND

Major depressive disorder (MDD) is defined as a complex mental disorder which is characterized by a pervasive low mood and aversion to activity. Several types of neurotransmitter systems e.g. serotonergic, glutamatergic and noradrenergic systems have been suggested to play an important role in the origination of depression, but neurotrophins such as brain derived neurotrophic factor (BDNF) have also been implicated in the disease process.

OBJECTIVES

The purpose of this study was to examine the effects of a newly developed class of molecules, characterized as positive allosteric modulators of neurotrophin/Trk receptor mediated signaling (Trk-PAM), on neurotransmitter release and depression-like behavior in vivo.

METHODS

The effect of and possible interaction of neurotrophin/Trk signaling pathways with serotonergic and glutamatergic systems in the modulation of depression-related responses was studied using newly developed Trk-PAM compounds (ACD855, ACD856 and AC26845), as well as ketamine and fluoxetine in the forced swim test (FST) in rodents. Moreover, in vivo microdialysis in freely moving rats was used to assess changes in neurotransmitter levels in the rat.

RESULTS

The results from the study show that several different compounds, which all potentiate Trk-receptor mediated signaling, display antidepressant-like activity in the FST. Moreover, the data also indicate that the effects of both fluoxetine and ketamine in the FST, both used in clinical practice, are mediated via BDNF/TrkB signaling, which could have implications for novel therapies in MDD.

CONCLUSIONS

Trk-PAMs could provide an interesting avenue for the development of novel therapeutics in this area.

Neuroprotective and Disease-Modifying Effects of the Triazinetrione ACD856, a Positive Allosteric Modulator of Trk-Receptors for the Treatment of Cognitive Dysfunction in Alzheimer's Disease

The introduction of anti-amyloid monoclonal antibodies against Alzheimer's disease (AD) is of high importance. However, even though treated patients show very little amyloid pathology, there is only a modest effect on the rate of cognitive decline. Although this effect can possibly increase over time, there is still a need for alternative treatments that will improve cognitive function in patients with AD. Therefore, the purpose of this study was to characterize the triazinetrione ACD856, a novel pan-Trk positive allosteric modulator, in multiple models to address its neuroprotective and potential disease-modifying effects. The pharmacological effect of ACD856 was tested in recombinant cell lines, primary cortical neurons, or animals. We demonstrate that ACD856 enhanced NGF-induced neurite outgrowth, increased the levels of the pre-synaptic protein SNAP25 in PC12 cells, and increased the degree of phosphorylated TrkB in SH-SY5Y cells. In primary cortical neurons, ACD856 led to increased levels of phospho-ERK1/2, showed a neuroprotective effect against amyloid-beta or energy-deprivation-induced neurotoxicity, and increased the levels of brain-derived neurotrophic factor (BDNF). Consequently, administration of ACD856 resulted in a significant increase in BDNF in the brains of 21 months old mice. Furthermore, repeated administration of ACD856 resulted in a sustained anti-depressant effect, which lasted up to seven days, suggesting effects that go beyond merely symptomatic effects. In conclusion, the results confirm ACD856 as a cognitive enhancer, but more importantly, they provide substantial in vitro and in vivo evidence of neuroprotective and long-term effects that contribute to neurotrophic support and increased neuroplasticity. Presumably, the described effects of ACD856 may improve cognition, increase resilience, and promote neurorestorative processes, thereby leading to a healthier brain in patients with AD.

Safety, Tolerability, Pharmacokinetics and Quantitative Electroencephalography Assessment of ACD856, a Novel Positive Allosteric Modulator of Trk-Receptors Following Multiple Doses in Healthy Subjects

BACKGROUND

ACD856 is a positive allosteric modulator of tropomyosin receptor kinase (Trk) receptors which has shown to have pro-cognitive and anti-depressant-like effects in various animal models. It is currently in clinical development for the treatment of Alzheimer's disease and other disorders where cognition is impaired and is also considered for indications such as depression or other neuropsychiatric diseases. ACD856 has a novel mechanism of action modulating the activity of the Trk-receptors, resulting in increased stimulation of the neurotrophin signaling pathways. Previous studies applying single intravenous and oral doses of ACD856 indicate that ACD856 is safe and well-tolerated by healthy volunteer subjects, and that it has suitable safety and pharmacokinetic properties for further clinical development.

OBJECTIVES

To investigate the safety and tolerability of 7 days of treatment with multiple ascending oral doses of ACD856 in healthy subjects, and to characterize its pharmacokinetic (PK) properties. In addition, pharmacodynamic effects of ACD856 using quantitative electroencephalography (qEEG) as an indicator for central target engagement were assessed.

DESIGN

This was a prospective, phase I, double-blind, parallel-group, placebo-controlled, randomized study of the safety, tolerability, PK and pharmacodynamics of multiple ascending oral doses of ACD856 in healthy subjects. ACD856 or placebo were administered in 3 ascending dose cohorts of 8 subjects. Within each cohort, subjects were randomized to receive either ACD856 (n=6) or placebo (n=2).

SETTING

The study was conducted at a First-in-Human unit in Sweden.

PARTICIPANTS

Twenty-four healthy male and female subjects.

INTERVENTION

The study medication was administered as an oral solution, with ACD856 or the same contents without the active ingredient (placebo). The dose levels ranged from 10 mg to 90 mg. ACD856 was administered once daily for 7 days, targeting steady state.

MEASUREMENTS

Safety and tolerability assessments included adverse events, laboratory, vital signs, 12-lead electrocardiogram (ECG), physical examination, assessment of stool frequency and questionnaires to assess symptoms of anxiety, depression, as well as suicidal ideation and behavior. In addition, cardiodynamic ECGs were extracted to evaluate cardiac safety. PK parameters were calculated based on measured concentrations of ACD856 in plasma, urine, and cerebrospinal fluid (CSF) samples. Metabolite profiling, characterization and analysis was performed based on and urine samples. qEEG was recorded for patients in the two highest dose cohorts (30 and 90 mg/day) as a pharmacodynamic assessment to explore central target engagement.

RESULTS

Treatment with ACD856 was well tolerated with no serious adverse events. No treatment emergent or dose related trends were observed for any of the safety assessments. ACD856 was rapidly absorbed and reached maximum plasma exposure at 30 to 45 minutes after administration. Steady state was reached before Day 6, with an elimination half-life at steady state of approximately 20 hours. At steady state, ACD856 exhibited accumulation ratios for Cmax and AUC of approximately 1.6 and 1.9 respectively. The exposure, Cmax and AUC0-24, increased proportionally with the dose. There was no unchanged ACD856 detected in urine. The metabolic pattern in urine and plasma was similar, and in alignment with the metabolites observed in preclinical toxicology studies. The level of ACD856 measured in CSF at steady state increased with dose, indicating Central Nervous System (CNS) exposure at relevant levels for pharmacodynamic effects. ACD856 demonstrated significant dose-dependent treatment-associated changes on qEEG parameters. Specifically, increase of the relative theta power and decrease of the fast alpha and beta power was observed, leading to an acceleration of the delta+theta centroid and an increase in the theta/beta ratio.

CONCLUSIONS

ACD856 was well tolerated at the tested dose levels (10-90 mg/daily for 7 days) in healthy subjects. The compound has a robust pharmacokinetic profile, with rapid absorption and dose-dependent exposure. ACD856 was shown to pass the blood-brain-barrier, reach relevant exposure in the CNS and to induce dose-dependent treatment-related changes on qEEG parameters, indicating central target engagement.

Active immunotherapy reduces NOTCH3 deposition in brain capillaries in a CADASIL mouse model

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is the most common monogenic form of familial small vessel disease; no preventive or curative therapy is available. CADASIL is caused by mutations in the NOTCH3 gene, resulting in a mutated NOTCH3 receptor, with aggregation of the NOTCH3 extracellular domain (ECD) around vascular smooth muscle cells. In this study, we have developed a novel active immunization therapy specifically targeting CADASIL-like aggregated NOTCH3 ECD. Immunizing CADASIL TgN3R182C¹⁵⁰ mice with aggregates composed of CADASIL-R133C mutated and wild-type EGF_1-5 repeats for a total of 4 months resulted in a marked reduction (38-48%) in NOTCH3 deposition around brain capillaries, increased microglia activation and lowered serum levels of NOTCH3 ECD. Active immunization did not impact body weight, general behavior, the number and integrity of vascular smooth muscle cells in the retina, neuronal survival, or inflammation or the renal system, suggesting that the therapy is tolerable. This is the first therapeutic study reporting a successful reduction of NOTCH3 accumulation in a CADASIL mouse model supporting further development towards clinical application for the benefit of CADASIL patients.

Neurotrophin-targeted therapeutics: A gateway to cognition and more?

Neurotrophins, such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), are small proteins expressed in the brain and peripheral tissues, which regulate several key aspects of neuronal function, including neurogenesis, synaptic plasticity and neuroprotection, but also programmed cell death. This broad range of effects is a result of a complex downstream signaling pathway, with differential spatial and temporal activation patterns further diversifying their physiological effects. Alterations in neurotrophin levels, or known polymorphisms in neurotrophin genes, have been linked to a variety of disorders, including depression and Alzheimer's disease (AD). Historically, their therapeutic potential in these disorders has been hampered by the lack of suitable tool molecules for clinical studies. However, recent advancements have led to the development of new therapeutic candidates, which are now in clinical testing.

Identification of Novel Positive Allosteric Modulators of Neurotrophin Receptors for the Treatment of Cognitive Dysfunction

Alzheimer's disease (AD) is the most common neurodegenerative disorder and results in severe neurodegeneration and progressive cognitive decline. Neurotrophins are growth factors involved in the development and survival of neurons, but also in underlying mechanisms for memory formation such as hippocampal long-term potentiation. Our aim was to identify small molecules with stimulatory effects on the signaling of two neurotrophins, the nerve growth factor (NGF) and the brain derived neurotrophic factor (BDNF). To identify molecules that could potentiate neurotrophin signaling, 25,000 molecules were screened, which led to the identification of the triazinetrione derivatives ACD855 (Ponazuril) and later on ACD856, as positive allosteric modulators of tropomyosin related kinase (Trk) receptors. ACD855 or ACD856 potentiated the cellular signaling of the neurotrophin receptors with EC50 values of 1.9 and 3.2 or 0.38 and 0.30 µM, respectively, for TrkA or TrkB. ACD855 increased acetylcholine levels in the hippocampus by 40% and facilitated long term potentiation in rat brain slices. The compounds acted as cognitive enhancers in a TrkB-dependent manner in several different behavioral models. Finally, the age-induced cognitive dysfunction in 18-month-old mice could be restored to the same level as found in 2-month-old mice after a single treatment of ACD856. We have identified a novel mechanism to modulate the activity of the Trk-receptors. The identification of the positive allosteric modulators of the Trk-receptors might have implications for the treatment of Alzheimer's diseases and other diseases characterized by cognitive impairment.

Agreement between frontal and occipital regional cerebral oxygen saturation in infants during surgery and general anesthesia an observational study

BACKGROUND

Advances in perioperative pediatric care have resulted in an increased number of procedures requiring anesthesia. During anesthesia and surgery, the patient is subjected to factors that affect the circulatory homeostasis, which can influence oxygenation of the brain. Near-infrared spectroscopy (NIRS) is an easy applicable noninvasive method for monitoring of regional tissue oxygenation (rScO₂%). Alternate placements for NIRS have been investigated; however, no alternative cranial placements have been explored.

AIM

To evaluate the agreement between frontal and occipital recordings of rScO₂% in infants using INVOS^TM during surgery and general anesthesia.

METHOD

A standard frontal monitoring of rScO₂% with NIRS was compared with occipital rScO₂% measurements in fifteen children at an age <1 year, ASA 1-2, undergoing cleft lip and/or palate surgery during general anesthesia with sevoflurane. An agreement analysis was performed according to Bland and Altman.

RESULTS

Mean values of frontal and occipital rScO₂% at baseline were largely similar (70.7 ± 4.77% and 69.40 ± 5.04%, respectively). In the majority of the patients, the frontal and occipital recordings of rScO₂ changed in parallel. There was a moderate positive correlation between frontal and occipital rScO₂% INVOS™ readings (rho[ρ]: 0.513, P < .01). The difference between frontal and occipital rScO₂ ranged from -31 to 28 with a mean difference (bias) of -0.15%. The 95% limit of agreement was -18.04%-17.74%. The error between frontal and occipital rScO₂ recordings was 23%.

CONCLUSION

The agreement between frontal and occipital recordings of brain rScO₂% in infants using INVOS^TM during surgery and general anesthesia was acceptable. In surgical procedures where the frontal region of the head is not available for monitoring, occipital recordings of rScO₂% could be an option for monitoring.

Nociceptin and the NOP receptor in aversive learning in mice

The endogenous neuropeptide nociceptin (N/OFQ), which mediates its actions via the nociceptin receptor (NOP), is implicated in multiple behavioural and physiological functions. This study examined the effects of the NOP agonists N/OFQ and the synthetic agonist Ro 64-6198, the antagonists NNN and NalBzoH, as well as deletion of the Pronociceptin gene on emotional memory in mice. The animals were tested in the passive avoidance (PA) task, dependent on hippocampal and amygdala functions. N/OFQ injected intraventricularly (i.c.v.) prior to training produced a biphasic effect on PA retention; facilitation at a low dose and impairment at higher doses. Ro 64-6198 also displayed a biphasic effect with memory facilitation at lower doses and impairment at a high dose. None of the agonists influenced PA training latencies. NNN did not significantly modulate retention in the PA task but antagonized the inhibitory effects of N/OFQ. NalBzoH facilitated memory retention in a dose-dependent manner and blocked the impairing effects of N/OFQ. However, neither NNN nor NalBzoH blocked the memory-impairing effects of Ro 64-6198. Finally, the Pnoc knockout mice exhibited enhanced PA retention latencies compared to the wild type mice. The biphasic effect of the natural ligand and Ro 64-6198 and the failure of the antagonists to block the action of Ro 64-6198 indicate complexity in ligand-receptor interaction. These results indicate that brain nociceptin and its NOP has a subtle role in regulation of mechanisms of relevance for treatment of disorders with processing disturbances of aversive events e.g. Alzheimer's disease, anxiety, depression and PTSD.

Broad analgesic activity of a novel, selective M1 agonist

Although the muscarinic receptor family has long been a source of potentially compelling targets for small molecule drug discovery, it was difficult to achieve agonist selectivity within the family. A new class of M1 muscarinic agonists has emerged, and these compounds have been characterized as agonists that activate the receptor at an allosteric site. Members of this class of M1 agonists have been shown to be selective across the muscarinic receptors. However, upon introduction of a novel pharmacologic mechanism, it is prudent to ensure that no new off-target activities have arisen, particularly within the context of in vivo experiments. Reported here, is the in vitro and in vivo characterization of a novel M1 agonist tool compound, PPBI, and demonstrations that the primary biological effects of PPBI are mediated through M1. PPBI reverses d-amphetamine locomotor activity, but fails to do so in transgenic mice that do not express M1. PPBI also reverses a natural deficit in a rat cognition model at a level of exposure which also activates cortical circuitry. Most notably, PPBI is analgesic in a variety of rat and mouse models and the analgesic effect of PPBI is reversed by an M1-preferring antagonist and an M1-selective toxin. Finally, the pharmacokinetic/pharmacodynamic measures of PPBI are compared across multiple endpoints which highlights that activity in models of psychosis and pain require higher exposures than that required in the cognition model.

The battle of Alzheimer's Disease - the beginning of the future Unleashing the potential of academic discoveries

Alzheimer's Disease (AD) is the most common form of dementia, affecting approximately 36 million people worldwide. To date there is no preventive or curative treatment available for AD, and in absence of major progress in therapeutic development, AD manifests a concrete socioeconomic threat. The awareness of the growing problem of AD is increasing, exemplified by the recent G8 Dementia Summit, a meeting held in order to set the stage and steer the compass for the future. Simultaneously, and paradoxically, we have seen key players in the pharmaceutical industry that have recently closed or significantly decreased their R&D spending on AD and other CNS disorders. Given the pressing need for new treatments in this area, other actors need to step-in and enter this drug discovery arena complementing the industrial efforts, in order to turn biological and technological progress into novel therapeutics. In this article, we present an example of a novel drug discovery initiative that in a non-profit setting, aims to integrate with both preclinical and clinical academic groups and pharmaceutical industry to explore the therapeutic potential of new concepts in patients, using novel biology, state of the art technologies and rapid concept testing.

AZD1080, a novel GSK3 inhibitor, rescues synaptic plasticity deficits in rodent brain and exhibits peripheral target engagement in humans

Abnormal tau phosphorylation resulting in detachment of tau from microtubules and aggregation are critical events in neuronal dysfunction, degeneration, and neurofibrillary pathology seen in Alzheimer's disease. Glycogen synthase kinase-3β (GSK3β) is a key target for drug discovery in the treatment of Alzheimer's disease and related tauopathies because of its potential to abnormally phosphorylate proteins and contribute to synaptic degeneration. We report the discovery of AZD1080, a potent and selective GSK3 inhibitor that demonstrates peripheral target engagement in Phase 1 clinical studies. AZD1080 inhibits tau phosphorylation in cells expressing human tau and in intact rat brain. Interestingly, subchronic but not acute administration with AZD1080 reverses MK-801-induced deficits, measured by long-term potentiation in hippocampal slices and in a cognitive test in mice, suggesting that reversal of synaptic plasticity deficits in dysfunctional systems requires longer term modifications of proteins downstream of GSK3β signaling. The inhibitory pattern on tau phosphorylation reveals a prolonged pharmacodynamic effect predicting less frequent dosing in humans. Consistent with the preclinical data, in multiple ascending dose studies in healthy volunteers, a prolonged suppression of glycogen synthase activity was observed in blood mononuclear cells providing evidence of peripheral target engagement with a selective GSK3 inhibitor in humans.

Decreased axonal transport rates in the Tg2576 APP transgenic mouse: improvement with the gamma-secretase inhibitor MRK-560 as detected by manganese-enhanced MRI

Neuropil deposition of beta-amyloid (Aβ) peptides is believed to be a key event in the neurodegenerative process of Alzheimer's disease (AD). An early and consistent clinical finding in AD is olfactory dysfunction with associated pathology. Interestingly, transgenic amyloid precursor protein (Tg2576) mice also show early amyloid pathology in olfactory regions. Moreover, a recent study indicates that axonal transport is compromised in the olfactory system of Tg2576 mice, as measured by manganese-enhanced magnetic resonance imaging (MEMRI). Here we tested whether the putative axonal transport deficit in the Tg2576 mouse model improves in response to a selective gamma-secretase inhibitor, N-[cis-4-[(4-chlorophenyl)-sulfonyl]-4-(2,5-difluorophenyl)cyclohexyl]-1,1,1-trifluoromethanesulfonamide (MRK-560). Tg2576 mice or wild-type (WT) littermates were treated daily with MRK-560 (30 μmol/kg) or vehicle for 4 (acute) or 29 days (chronic). The subsequent MEMRI analysis revealed a distinct axonal transport dysfunction in the Tg2576 mice compared with its littermate controls. Interestingly, the impairment of axonal transport could be fully reversed by chronic administration of MRK-560, in line with the significantly lowered levels of both soluble and insoluble forms of Aβ found in the brain and olfactory bulbs (OBs) following treatment. However, no improvement of axonal transport was observed after acute treatment with MRK-560, where soluble but not insoluble forms of Aβ were reduced in the brain and OBs. The present results show that axonal transport is impaired in Tg2576 mice compared with WT controls, as measured by MEMRI. Chronic treatment in vivo with a gamma-secretase inhibitor, MRK-560, significantly reduces soluble and insoluble forms of Aβ, and fully reverses the axonal transport dysfunction.

Cholinesterase inhibitors improve both memory and complex learning in aged beagle dogs

Similar to patients with Alzheimer's disease (AD), dogs exhibit age-dependent cognitive decline, amyloid-β (Aβ) pathology, and evidence of cholinergic hypofunction. The present study sought to further investigate the role of cholinergic hypofunction in the canine model by examining the effect of the cholinesterase inhibitors phenserine and donepezil on performance of two tasks, a delayed non-matching-to-position task (DNMP) designed to assess working memory, and an oddity discrimination learning task designed to assess complex learning, in aged dogs. Phenserine (0.5 mg/kg; PO) significantly improved performance on the DNMP at the longest delay compared to wash-out and partially attenuated scopolamine-induced deficits (15 μg/kg; SC). Phenserine also improved learning on a difficult version of an oddity discrimination task compared to placebo, but had no effect on an easier version. We also examined the effects of three doses of donepezil (0.75, 1.5, and 6 mg/kg; PO) on performance of the DNMP. Similar to the results with phenserine, 1.5 mg/kg of donepezil improved performance at the longest delay compared to baseline and wash-out, indicative of memory enhancement. These results further extend the findings of cholinergic hypofunction in aged dogs and provide pharmacological validation of the canine model with a cholinesterase inhibitor approved for use in AD. Collectively, these studies support utilizing the aged dog in future screening of therapeutics for AD, as well as for investigating the links among cholinergic function, Aβ pathology, and cognitive decline.

Oral pregabalin reverses cold allodynia in two distinct models of peripheral neuropathic pain

A major symptom of persistent neuropathic pain, which may develop after peripheral nerve injury, is hypersensitivity (allodynia) to normally innocuous cold stimuli. Although the anticonvulsant pregabalin has been demonstrated to relieve neuropathic pain, both in preclinical models and clinically, the analgesic effect of the drug in animals has not been profiled for cold hypersensitivity. Therefore, we examined the effect of pregabalin (single oral dosing: 30, 100, 300 micromol/kg) on cold allodynia in two models of chronic neuropathic pain, the spared nerve injury (SNI) and the spinal nerve ligation (SNL) models. A significant antiallodynic effect was observed with pregabalin at all doses tested with a maximal effect of 71% (SNI) and 60% (SNL), respectively compared to vehicle. For comparison, only the highest dose tested of pregabalin (300 micromol/kg), significantly decreased pain responses in phase 2 of the rat formalin test (approximately 67% pain inhibition). However, pregabalin at this high dose also affected other centrally mediated behavioural functions, such as motor activity and anxiolytic behaviour in naïve animals, which could potentially interfere with the pain readout. The present study demonstrates that oral administration of pregabalin significantly reduces both cold allodynia induced in the SNI and the SNL models of neuropathic pain as well as formalin-induced nociception, albeit with different sensitivity and potency.

Faster forgetting contributes to impaired spatial memory in the PDAPP mouse: deficit in memory retrieval associated with increased sensitivity to interference?

Two experiments were conducted to investigate the possibility of faster forgetting by PDAPP mice (a well-established model of Alzheimer's disease as reported by Games and colleagues in an earlier paper). Experiment 1, using mice aged 13-16 mo, confirmed the presence of a deficit in a spatial reference memory task in the water maze by hemizygous PDAPP mice relative to littermate controls. However, after overtraining to a criterion of equivalent navigational performance, a series of memory retention tests revealed faster forgetting in the PDAPP group. Very limited retraining was sufficient to reinstate good memory in both groups, indicating that their faster forgetting may be due to retrieval failure rather than trace decay. In Experiment 2, 6-mo-old PDAPP and controls were required to learn each of a series of spatial locations to criterion with their memory assessed 10 min after learning each location. No memory deficit was apparent in the PDAPP mice initially, but a deficit built up through the series of locations suggestive of increased sensitivity to interference. Faster forgetting and increased interference may each reflect a difficulty in accessing memory traces. This interpretation of one aspect of the cognitive deficit in human mutant APP mice has parallels to deficits observed in patients with Alzheimer's disease, further supporting the validity of transgenic models of the disease.

Dopaminergic modulation of the persistence of one-trial hippocampus-dependent memory

The persistence of new memory traces in the hippocampus, encoded following appropriate activation of glutamatergic receptors and the induction of synaptic plasticity, can be influenced by heterosynaptic activation of neuromodulatory brain systems. We therefore investigated the effects of a hippocampus-specific blockade of dopamine D1/D5 receptors on the persistence of spatial memory encoded in one trial using a delayed matching-to-place (DMP) task in a watermaze in which rats learn a new escape location each day. A within-subjects design was used such that both short (20 min) and long (6 h) retention intervals, and both drug (SCH23390, a D1/D5 receptor antagonist) and vehicle (aCSF) infusions were tested on different days in the same animals. Bilateral intrahippocampal infusion of SCH23390 (5 microg in 1 microL per side) prior to trial 1 (encoding) caused a differential impairment as a function of memory delay-with no effect during trial 2 (memory retrieval) after a 20-min interval, but a block of memory at 6 h. Further experiments revealed that infusion of SCH23390 immediately after trial 1 had no effect on retention 6 h later, and the poor memory seen at long retention intervals when the drug was present at encoding was not due to a state-dependent failure of retrieval. These results suggest that activation of D1/D5 receptors during memory encoding is necessary for the formation of a persistent memory trace in the hippocampus. The complementary effects of D1/D5 receptor blockade on the persistence of LTP and the duration of memory are consistent with the idea that changes in synaptic strength underlie memory.

N-methyl-d-aspartate receptors in the medial septal area have a role in spatial and emotional learning in the rat

Cholinergic and GABAergic neurons in the medial septal/vertical limb of the diagonal band of Broca (MS/vDB) area project to the hippocampus and constitute the septohippocampal pathway, which has been implicated in learning and memory. There is also evidence for extrinsic and intrinsic glutamatergic neurons in the MS/vDB, which by regulating septohippocampal neurons can influence hippocampal functions. The potential role of glutamatergic N-methyl-D-aspartate (NMDA) receptors within the MS/vDB for spatial and emotional learning was studied using the water maze and step-through passive avoidance (PA) tasks, which are both hippocampal-dependent. Blockade of septal NMDA receptors by infusion of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.3-5 microg/rat), infused 15 min prior to training, impaired spatial learning and memory at the 5 microg dose of D-AP5, while doses of 0.3 and 1 microg per rat had no effect. The impairment in spatial learning appears not to be caused by sensorimotor or motivational disturbances, or anxiogenic-like behavior. Thus, d-AP5-treated rats were not impaired in swim performance or visuospatial abilities and spent more time in the open arms of the elevated plus-maze. In the PA task, intraseptal D-AP5 infused 15 min before training impaired retention as examined 24 h after training. This impairment was observed already at the 0.3 microg dose, suggesting that NMDA receptors within the MS/vDB may be more important for emotional than spatial memory. In summary, the present data indicate that changes in septal glutamate transmission and NMDA receptor activity can influence activity-dependent synaptic plasticity in the hippocampus and thereby learning and memory.

Reappearance of hippocampal CA1 neurons after ischemia is associated with recovery of learning and memory

The pyramidal neurons of the hippocampal CA1 region are essential for cognitive functions such as spatial learning and memory, and are selectively destroyed after cerebral ischemia. To analyze whether degenerated CA1 neurons are replaced by new neurons and whether such regeneration is associated with amelioration in learning and memory deficits, we have used a rat global ischemia model that provides an almost complete disappearance (to approximately 3% of control) of CA1 neurons associated with a robust impairment in spatial learning and memory at two weeks after ischemia. We found that transient cerebral ischemia can evoke a massive formation of new neurons in the CA1 region, reaching approximately 40% of the original number of neurons at 90 days after ischemia (DAI). Co-localization of the mature neuronal marker neuronal nuclei with 5-bromo-2'-deoxyuridine in CA1 confirmed that neurogenesis indeed had occurred after the ischemic insult. Furthermore, we found increased numbers of cells expressing the immature neuron marker polysialic acid neuronal cell adhesion molecule in the adjacent lateral periventricular region, suggesting that the newly formed neurons derive from this region. The reappearance of CA1 neurons was associated with a recovery of ischemia-induced impairments in spatial learning and memory at 90 DAI, suggesting that the newly formed CA1 neurons restore hippocampal CA1 function. In conclusion, these results show that the brain has an endogenous capacity to form new nerve cells after injury, which correlates with a restoration of cognitive functions of the brain.

5-Hydroxytryptamine 1A receptor blockade facilitates aversive learning in mice: interactions with cholinergic and glutamatergic mechanisms

The effects of 5-hydroxytryptamine 1A (5-HT(1A)) receptor ligands on aversive learning were examined in the passive avoidance (PA) task in mice. Anxiety and autonomic functions were investigated using the elevated plus-maze and heart rate measurements. The main findings from this study are as follows. 1) Pretraining administration of the 5-HT(1A) receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide] facilitated PA retention at low doses (0.01 and 0.03 mg/kg) but impaired PA retention at higher doses (0.1-1.0 mg/kg), consistent with previous findings in the rat. 2) Similar to the acetylcholinesterase inhibitor physostigmine, pretraining administration of the 5-HT(1A) receptor antagonists [(R)-3-N,N-dicyclobutylamino-8 fluoro-3,4-dihydro-3H-1-benzopyran-5-carboxamide hydrogen(2R,3R)-tartrate monohydrate] NAD-299 (0.1-2 mg/kg) and [N-2-4-(2-methoxyphenyl)-1-piperazinylethyl-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride] WAY-100635 (0.3-3 mg/kg) enhanced PA retention. 3) The impairment (1 mg/kg) but not the facilitation (0.03 mg/kg) induced by 8-OH-DPAT was fully blocked by NAD-299 (0.3 mg/kg). 4) 5-HT(1A) receptor ligands given immediate post-training failed to alter PA retention. 5) NAD-299 (0.3-1 mg/kg) blocked the impairment of PA retention caused by a) the nonselective muscarinic receptor antagonist scopolamine and b) the non-competitive N-methyl-D-aspartate receptor antagonist MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate]. 6) A subthreshold dose of scopolamine completely blocked the facilitatory effect of NAD-299 on PA retention. 7) Anxiety-related behaviors and autonomic function were unchanged by NAD-299. 8) In situ hybridization showed that septal neurons expressing 5-HT(1A) receptor mRNA were codistributed with markers for cholinergic, GABAergic, and glutamatergic neurons. These results indicate that systemic administration of 5-HT(1A) receptor antagonists can facilitate cognitive performance, most likely by enhancing hippocampal/cortical cholinergic and glutamatergic neurotransmissions. Selective 5-HT(1A) receptor antagonists may be useful in the treatment of cognitive deficits such as Alzheimer's disease.

Profound but transient deficits in learning and memory after global ischemia using a novel water maze test

The pyramidal CA1 neurons of the hippocampus are critically involved in spatial learning and memory. These neurons are especially vulnerable to cerebral ischemia, but in spite of this, it has been consistently difficult to show any learning and memory deficits in two-vessel occlusion models of global ischemia. Transient global ischemia was induced in adult male rats under general anaesthesia administered by artificial respiration to prevent respiratory arrest. Systemic blood pressure was reduced to below 50 mmHg by instant adjustments of the halothane concentration, before and during bilateral occlusion of the carotid arteries. Cerebral blood flow was monitored by laser-Doppler flowmetry. Dying neurons were detected by TUNEL at 14 days after ischemia and surviving neurons by NeuN at 14 and 125 days after ischemia. Learning and memory was assessed in a novel water maze with three successive left-right choices. Transient global ischemia produced a profound and selective degeneration of CA1 neurons at 14 days after ischemia. This degeneration was associated with severe impairments in learning at 13 days after ischemia and in memory, as tested 24 h afterwards. At 125 days after ischemia, there was no significant learning and memory impairment, whereas the number of CA1 neurons was increased. These results show that transient global ischemia induced by two-vessel occlusion may lead to severe, but transient, impairments in learning and memory using a novel water maze, and that restored learning and memory is associated with an increased number of CA1 neurons.

Intraseptal muscarinic ligands and galanin: influence on hippocampal acetylcholine and cognition

The cholinergic neurons in the septohippocampal projection are implicated in hippocampal functions such as spatial learning and memory. The aim of this study was to examine how septohippocampal cholinergic transmission is modulated by muscarinic inputs and by the neuropeptide galanin, co-localized with acetylcholine (ACh) in septohippocampal cholinergic neurons, and how spatial learning assessed by the Morris water maze test is affected. Muscarinic inputs to the septal area are assumed to be excitatory, whereas galanin is hypothesized to inhibit septohippocampal cholinergic function. To test these hypotheses, compounds were microinjected into the medial septum and hippocampal ACh release was assessed by microdialysis probes in the ventral hippocampus of the rat. Blockade of septal muscarinic transmission by intraseptal scopolamine increased hippocampal ACh release suggesting that septal cholinergic neurons are under tonic inhibition. Stimulation of septal muscarinic receptors by carbachol also increased hippocampal ACh release. Despite this increase, both scopolamine and carbachol tended to impair hippocampus-dependent spatial learning. This finding also suggests a revision of the simplistic notion that an increase in hippocampal ACh may be facilitatory for learning and memory. Galanin infused into the medial septum enhanced hippocampal ACh release and facilitated spatial learning, suggesting that septal galanin, contrary to earlier claims, does not inhibit but excites septohippocampal cholinergic neurons. Galanin receptor stimulation combined with muscarinic blockade in the septal area resulted in an excessive increase of hippocampal ACh release combined with an impairment of spatial learning. This finding suggests that the level of muscarinic activity within the septal area may determine the effects of galanin on hippocampal cognitive functions. In summary, a limited range of cholinergic muscarinic transmission may contribute to optimal hippocampal function, a finding that has important implications for therapeutic approaches in the treatment of disorders of memory function.

Nociceptin/orphanin FQ modulates spatial learning via ORL-1 receptors in the dorsal hippocampus of the rat

The endogenous peptide nociceptin (orphanin FQ) plays a role in several important physiological functions in the CNS such as pain, anxiety and locomotion. It has previously been found that injection of 10 nmol nociceptin into the CA3 region of the hippocampus markedly impairs spatial learning and memory in the rat. The present study examined the effects of lower doses of nociceptin (3.3, 1, 0.33 and 0.1 nmol/rat) on spatial learning. The 3.3 nmol dose impaired spatial learning over the 5 days of training although the effect was not as strong as with 10 nmol. In contrast, the two lower doses, 1 and 0.33 nmol/rat, improved spatial learning whereas the lowest dose, 0.1 nmol/rat, had no significant effect. Both the impairing and facilitating effect of nociceptin could be blocked by an ORL-1 receptor antagonist, [Phe1Psi(CH(2)-NH)Gly2]NC(1-13)NH2 (10 nmol/rat), indicating that both effects are ORL-1 receptor-mediated. The 3.3 nmol dose of nociceptin did not impair the performance in the visual platform task and did not alter swim speed or motor activity indicating no effects on motivation or motor performance. Taken together, these results show that nociceptin has a biphasic dose-effect curve and provide further evidence for a role of this neuropeptide in cognitive processes in the hippocampus.

Evidence in locomotion test for the functional heterogeneity of ORL-1 receptors

1. The ORL1 agonists nociceptin and Ro 64-6198 were compared in their ability to modify spontaneous locomotor activity in male NMRI mice not habituated to the test environment. 2. Higher doses of nociceptin (>5 nmol i.c.v.) reduced whereas lower doses (<1 nmol i.c.v.) stimulated locomotor activity. Both effects were blocked by the putative ORL1 antagonists [NPhe1]nociceptin(1-13)NH2 (10 nmol i.c.v.) and UFP101 (10 nmol, i.c.v.). The effects were also blocked by naloxone benzoylhydrazone (1 mg x kg(-1) s.c.), but not by the nonselective opioid antagonist naloxone (1 mg x kg(-1) s.c.). 3 In contrast to nociceptin, the synthetic ORL1 agonist Ro 64-6198 (0.01-1.0 mg x kg(-1) i.p.) produced monophasic inhibition of locomotor activity, which was insensitive to the treatment with [NPhe1]nociceptin(1-13)NH2 or naloxone benzoylhydrazone. Treatment with UFP101 abolished the locomotor inhibition induced by Ro 64-6198 (1.0 mg x kg(-1)), whereas naloxone (1.0 mg x kg(-1), s.c.) further increased the locomotor-inhibitory effects. 4. Naloxone benzoylhydrazone (0.3; 1.0 and 3.0 mg x kg(-1) s.c.) increased locomotor activity, although the effect was statistically significant only with the highest dose used. 5. Pretreatment with the tyrosine hydroxylase inhibitor H44-68 totally eliminated the motor-stimulatory effects of low doses of nociceptin, probably via dopamine depletion. 6. The results suggest that nociceptin stimulates locomotor activity at low doses if dopamine activity is intact. High doses of nociceptin and all the tested doses of Ro 64-6198 seem to interact with a functionally different subset of ORL1 receptors. In addition, the effects of Ro 64-6198 are modulated by tonic opioid receptor activity.

Analysis of the role of the 5-HT1B receptor in spatial and aversive learning in the rat

The present study examined the role of the 5-HT1B receptor in learning and memory. The ability of the 5-HT1B receptor agonist anpirtoline and the selective 5-HT1B receptor antagonist NAS-181 to affect spatial learning in the water maze (WM) and aversive learning in the passive avoidance (PA) task were examined in the rat. Anpirtoline (0.1-1.0 mg/kg, s.c.) caused a dose-dependent impairment of learning and memory in both the WM and PA tasks. NAS-181 (1.0-10 mg/kg, s.c.) failed to alter performance of the WM task, but produced a dose-dependent (0.1-20 mg/kg) facilitation of PA retention. Furthermore, treatment with NAS-181 (10 mg/kg) fully blocked the impairment of the WM and PA performance caused by anpirtoline (1.0 mg/kg). In contrast, NAS-181 (3.0-10 mg/kg) did not attenuate the spatial learning deficit and the impairment of PA retention caused by scopolamine (0.1 mg/kg in WM task, 0.3 mg/kg in PA task, s.c.), a nonselective muscarinic antagonist. Moreover, a subthreshold dose of scopolamine (0.1 mg/kg) blocked the facilitation of PA retention induced by NAS-181 (1.0-10 mg/kg). In addition, the behavioral disturbances (eg thigmotaxic swimming and platform deflections) induced by anpirtoline and scopolamine were analyzed in the WM task and correlated with WM performance. These results indicate that: (1) 5-HT1B receptor stimulation and blockade result in opposite effects in two types of cognitive tasks in the rat, and that (2) the 5-HT1B antagonist NAS-181 can facilitate some aspects of cognitive function, most likely via an increase of cholinergic transmission. These results suggest that 5-HT1B receptor antagonists may have a potential in the treatment of cognitive deficits resulting from loss of cholinergic transmission.

Elements of a neurobiological theory of the hippocampus: the role of activity-dependent synaptic plasticity in memory

The hypothesis that synaptic plasticity is a critical component of the neural mechanisms underlying learning and memory is now widely accepted. In this article, we begin by outlining four criteria for evaluating the 'synaptic plasticity and memory (SPM)' hypothesis. We then attempt to lay the foundations for a specific neurobiological theory of hippocampal (HPC) function in which activity-dependent synaptic plasticity, such as long-term potentiation (LTP), plays a key part in the forms of memory mediated by this brain structure. HPC memory can, like other forms of memory, be divided into four processes: encoding, storage, consolidation and retrieval. We argue that synaptic plasticity is critical for the encoding and intermediate storage of memory traces that are automatically recorded in the hippocampus. These traces decay, but are sometimes retained by a process of cellular consolidation. However, we also argue that HPC synaptic plasticity is not involved in memory retrieval, and is unlikely to be involved in systems-level consolidation that depends on HPC-neocortical interactions, although neocortical synaptic plasticity does play a part. The information that has emerged from the worldwide focus on the mechanisms of induction and expression of plasticity at individual synapses has been very valuable in functional studies. Progress towards a comprehensive understanding of memory processing will also depend on the analysis of these synaptic changes within the context of a wider range of systems-level and cellular mechanisms of neuronal transmission and plasticity.

Acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in mice: effects of opioid receptor-like 1 receptor agonists and naloxone

The ability of the two opioid receptor-like receptor 1 (ORL1) agonists nociceptin (5 nmol i.c.v.) and synthetic (1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one hydrochloride (Ro 64-6198; 0.1, 0.3, and 1.0 mg/kg i.p.) and the opioid antagonist naloxone (0.1, 1.0, and 10.0 mg/kg s.c.) to modify ethanol-induced conditioned place preference was examined in NMRI male mice. The ORL1 agonists were found to significantly reduce the acquisition, expression, and ethanol-induced reinstatement of conditioned place preference. Unlike the ORL1 agonists, naloxone at the doses relevant for opioid receptor blockade failed to significantly influence the acquisition of ethanol-induced conditioned place preference. However, naloxone at 1.0 but not 0.1 mg/kg s.c. potently blocked the expression of ethanol-induced conditioned place preference and significantly inhibited ethanol-induced reinstatement of the conditioned place preference after extinction. Separate experiments indicated that nociceptin and Ro 64-6198 are both devoid of reinforcing or aversive properties. Naloxone, however, at 1.0 and 10.0 mg/kg, produced conditioned place aversion, indicating motivational properties of its own. Both nociceptin and Ro 64-6198 reduced locomotor activity after acute administration. However, tolerance developed very quickly to this effect and already after three i.c.v. (or i.p.) injections, there was no significant reduction of locomotor activity. It is concluded that ORL1 agonists can modulate the acquisition, expression, and reinstatement of the conditioned reinforcing effects of ethanol with no reinforcing or aversive properties of their own. This property might be a potential advantage in the treatment of alcoholism compared with nonselective opioid antagonist naltrexone.

Dose- and time-dependent bimodal effects of kappa-opioid agonists on locomotor activity in mice

The kappa-opioid agonists U50488H, bremazocine, and BRL52537, and the mu-opioid agonist morphine were compared in their ability to modify spontaneous motor activity in male NMRI mice. Higher, analgesic doses of the kappa-agonists reduced rearing, motility, and locomotion in nonhabituated mice. These effects, as well as the analgesic action of U50488H, were blocked by the selective kappa-opioid antagonists nor-binaltorphimine and DIPPA. In contrast, lower, subanalgesic doses (1.25 and 2.5 mg/kg for U50488H; 0.15 and 0.075 mg/kg for bremazocine, and 0.1 mg/kg for BRL52537) time dependently increased motor activity. The stimulatory effects of U50488H and bremazocine were not observed in habituated animals and were reduced by dopamine depletion. Surprisingly, the stimulatory effects of U50488H and bremazocine were not blocked by nor-binaltorphimine and DIPPA but they were completely eliminated by naloxone (0.1 mg/kg). The effects of morphine were dose-dependent; an initial limited suppression was followed by increased motility and locomotion (but not rearing) with a peak effect at 20 mg/kg both in habituated and nonhabituated mice. The selective mu-opioid antagonist beta-funaltrexamine blocked morphine-induced motor stimulation and analgesia but failed to affect the analgesic and motor stimulatory effects of U50488H. The results indicate that kappa-opioid agonists interact with different functional subtypes of opioid receptors. A stimulatory, naloxone-sensitive but nor-binaltorphimine- and DIPPA-insensitive subtype of opioid receptor appears to operate only when the dopamine system is tonically active in nonhabituated animals. At higher doses, kappa-agonists produce analgesia and motor suppression, effects mediated by a "classic" (inhibitory) kappa-opioid receptor.

Endomorphin-2 but not Leu-enkephalin modulates spatial learning when microinjected in the CA3 region of the rat hippocampus

The recently discovered endogenous mu-selective opioid peptide, endomorphin-2, and the endogenous delta-selective opioid peptide, Leu-enkephalin, were tested for their ability to affect spatial learning in the Morris water task. It was found that microinjection of 10 nmol endomorphin-2 into the CA3 region of the rat hippocampus significantly impaired spatial learning. However, the two lower doses tested, 3.3 and 1 nmol, had no effect in this test. Leu-enkephalin did not have any effect on spatial learning at the two doses tested (10 and 3.3 nmol). Neither peptide had any effect on motor performance as measured by swim speed. The results indicate that mu-receptors in the CA3 region of the rat hippocampus are more relevant than delta-receptors for spatial learning.

Nociceptin/orphanin FQ metabolism and bioactive metabolites

The endogenous ligand for the orphan NOR receptor (earlier named ORL1) was recently discovered. This ligand, nociceptin/orphanin FQ is involved in a number of pharmacological actions in the CNS, including modulation of pain and cognition. However, its specific physiological role remains to be determined. Two major pathways of metabolism have been identified; the action of aminopeptidase(s) that prominently occurs in plasma, and endopeptidase activity that successively generates the N-terminal 1-13 and 1-9 fragments. Both pathways result in fragments that are inactive at the NOR receptor. However, short N-terminal fragments appear to be active in blocking the release of substance P from primary afferent C-fiber terminals in the dorsal spinal cord. The same endopeptidase(s) may also be involved in the fragmentation of dynorphin A since the inhibitor profile is similar. Enzyme activity is upregulated by morphine using either peptide as substrate that may lead to pharmacological interactions.

Evidence that N-terminal fragments of nociceptin modulate nociceptin-induced scratching, biting and licking in mice

The intrathecal (i.t.) injection of 3.0 fmol nociceptin (orphanin FQ) elicited scratching, biting and licking responses in mice. N-terminal fragments of nociceptin, nociceptin (1-7), nociceptin (1-9) and nociceptin (1-13), induced no characteristic behavioral response. When these N-terminal fragments of nociceptin were injected simultaneously with nociceptin, the behavioral response induced by nociceptin was reduced dose-dependently. Nociceptin (1-13) was much more potent than nociceptin (1-7) and nociceptin (1-9) and antagonized nociceptin-induced response at equimolar doses. No significant effects of the N-terminal fragments were observed against the scratching, biting and licking response elicited by i.t. administration of substance P or N-methyl-D-aspartate. These results suggest that N-terminal fragments formed endogenously in the spinal cord may have an antagonistic effect on nociceptin-induced behavioral responses.

In vivo metabolism of nociceptin/orphanin FQ in rat hippocampus

The in vivo metabolism of the newly identified endogenous ligand for the ORL1 receptor, the opioid-like peptide nociceptin (orphanin FQ) in rat hippocampus was studied using size-exclusion chromatography linked to electrospray ionization mass spectrometry. The results show that nociceptin is metabolized step-wise in vivo into fragments (1-13) and (14-17) as well as (1-9) and (10-13), respectively. Interestingly, the (1-13) and (1-9) fragments have the same C-terminus, Arg-Ala-Lys, suggesting that this is a motif recognized by an enzyme which fragments the peptide in two consecutive steps. Injection of the (1-13) fragment into rat hippocampus had no effect on spatial learning or motor function under conditions where nociceptin is active, showing that this metabolic conversion reduces affinity for the ORL-1-receptor.

Modulation of acetylcholine and serotonin transmission by galanin. Relationship to spatial and aversive learning

This paper presents evidence that galanin is a potent in vivo modulator of basal acetylcholine release in the rat brain with qualitatively and quantitatively differential effects in the dorsal and ventral hippocampus. Galanin perfused through the microdialysis probe decreased basal acetylcholine release in the ventral hippocampus, while it enhanced acetylcholine release in the dorsal hippocampus. Galanin (3 nmol/rat) infused into the ventral hippocampus impaired spatial learning acquisition, while it tended to facilitate acquisition when injected into the dorsal hippocampus. These effects appear to be related to activation of GAL-R1 (ventral hippocampus) and GAL-R2 (dorsal hippocampus) receptors, respectively. However, the effects of galanin on acetylcholine release and on spatial learning appear not to be directly related to cholinergic mechanisms, but they may also involve interactions with noradrenaline and/or glutamate transmission. Galanin administered into the lateral ventricle failed to affect acetylcholine release, while this route of administration produced a long-lasting reduction in 5-HT release in the ventral hippocampus, indicating that galanin is a potent inhibitor of mesencephalic 5-HT neurotransmission in vivo. Subsequent studies supported this hypothesis, showing that the effects on 5-HT release in vivo are most likely mediated by a galanin receptor in the dorsal raphe. The implications of these findings are discussed in relation to the role of acetylcholine in cognitive functions in the forebrain and the role of the raphe 5-HT neurons in affective disorders.

Hippocampal dynorphin B injections impair spatial learning in rats: a kappa-opioid receptor-mediated effect

The hippocampus plays a central role in the acquisition and storage of information. Long-term potentiation in the mossy fibre pathway to the CA3 region in the hippocampus, an animal model of memory acquisition, is modulated by dynorphin peptides. This study investigated the possible role of hippocampal dynorphin in spatial learning. Male rats were trained in the Morris Water Task after microinjection with different doses of dynorphin B (1, 3.3 or 10 nmol/rat) or artificial cerebrospinal fluid (as control) into the CA3 region of the dorsal hippocampus. Dynorphin B was found to impair spatial learning at all tested doses. The synthetic kappa1-selective opiate receptor antagonist nor-binaltorphimine (2 nmol) also given into the hippocampus fully blocked the acquisition impairment caused by dynorphin B (10 nmol), while nor-binaltorphimine alone did not affect learning performance. These findings suggest that dynorphin peptides could play a modulatory role in hippocampal plasticity by acting on hippocampal kappa-receptors and thereby impair spatial learning.

Metabolism of beta-endorphin in plasma studied by liquid chromatography-electrospray ionization mass spectrometry

Degradation of synthetic human beta-endorphin by a human plasma proteinase was studied with high-performance liquid chromatography in combination with mass spectrometry. The peptide was metabolized at a rate of 25 pmol/min to the major fragments beta-endorphin (1-19) and (20-31), the latter reported as a potent inhibitor of morphine- and beta-endorphin-induced analgesia in mice. The proteinase responsible for this process was classified as a metal-dependent serine proteinase and was effectively inactivated by phenylmethylsulfonyl fluoride and ethylenediaminetetraacetic acid. Identification of the products formed during the enzymatic reaction was performed by liquid chromatography on-line with electrospray mass spectrometry, using a reversed-phase or a novel size-exclusion column capable of separating molecules between 0.1-7 kilodaltons. Peptide sequences were verified by tandem mass spectrometry experiments. The conversion of beta-endorphin may have physiological implications in the mechanism of pain. The obtained data suggest that several precautions should be considered during recovery and measurement of beta-endorphin in plasma by immunological techniques. The applied strategy may also be useful for studying metabolism of various peptidergic compounds with potential pharmacological significance.

Nociceptin/orphanin FQ microinjected into hippocampus impairs spatial learning in rats

The newly discovered peptide nociceptin/orphanin FQ has been found to increase reactivity to pain and to influence locomotor activity after intracerebroventricular administration. This study investigated the possible role of hippocampal nociceptin/orphanin FQ in spatial learning and in spontaneous locomotion. Male rats were trained in the Morris water task after microinjection of 10 nmol nociceptin/orphanin FQ or artificial cerebrospinal fluid (as control) into the CA3 region of the dorsal hippocampus. Nociceptin/orphanin FQ was found to severely impair spatial learning without interfering with swimming performance. Intrahippocampal injection of nociceptin/ orphanin FQ markedly decreased exploratory locomotor activity including vertical movements (rearing). The data suggest that nociceptin/orphanin FQ is a potent modulator of synaptic plasticity within the hippocampus.

Differential metabolism of dynorphins in substantia nigra, striatum, and hippocampus

To map the proteolytic enzymes metabolizing dynorphins in brain structures, size-exclusion chromatography linked to electrospray ionization mass spectrometry was used. Enzymes extracted from rat hippocampus, striatum, and substantia nigra were tested for their capability of converting dynorphin-related peptides. Dynorphin A was the most resistant to proteolytic conversion, whereas Big dynorphin and dynorphin B-29 were slowly converted to dynorphin A and dynorphins A and B, respectively. Dynorphin B and alpha-neoendorphin were the least resistant. Dynorphin B was rapidly converted to Leu-enkephalin in the striatum and hippocampus but to Leu-enkephalin-Arg6 in the substantia nigra. alpha-Neoendorphin was converted to Leu-enkephalin in all tissues investigated.