Alzheimer amyloid beta-peptide A-beta25-35 blocks adenylate cyclase-mediated forms of hippocampal long-term potentiation

Serotonin type 6 receptor antagonist attenuates the impairment of long-term potentiation and memory induced by Abeta

Alzheimer's disease (AD) is the most common cause of dementia, characterized by memory impairment and synaptic loss. Long-term potentiation (LTP), a type of synaptic plasticity, is impaired during AD. Serotonin type 6 receptor (5-HT6R) inactivation is proposed as a therapeutic target for AD. This study examined the effects of chronic administration of the 5-HT6R antagonist, SB-258585, on cognitive, memory, and hippocampal plasticity in a rat model of AD. Abeta neurotoxicity was induced in rats using Aβ (1.35 pmol intracerebroventricular [ICV] injection). The following groups were formed: control sustained surgery and saline-treated, Aβ+saline (1 μL ICV for 30 days), and Aβ+SB-258585 (0.024 mg/kg, ICV for 30 days). The learning and memory were tested using the novel object recognition and passive avoidance tests. Next, anesthetized rats were placed in a stereotaxic apparatus. The population spike (PS) amplitude and the slope of the excitatory postsynaptic potentials (fEPSPs) of the LTP were measured following high-frequency stimulation in the dentate gyrus. The Aβ injection reduced step-through latency in the passive avoidance test and decreased the discrimination index in the novel object test. Aβ diminished both the amplitude of hippocampal neuron population spikes and the slope of excitatory postsynaptic potentials, compared to the control group. The administration of SB-258585 in rats receiving Aβ attenuated the Aβ-induced deficits in cognition, memory, and LTP in comparison with the Aβ group. It can be concluded that chronic treatment with SB-258585 antagonist can prevent Aβ-related deficiencies in learning and memory performance by improving neuronal plasticity. SB-258585 can prevent the progression of AD.

Amyloid-β decreases nitric oxide production in cultured retinal neurons: a possible mechanism for synaptic dysfunction in Alzheimer's disease?

The neurotoxicity of the amyloid-β peptide (Aβ) appears to be, at least in part, related to pathological activation of glutamate receptors by Aβ aggregates. However, the downstream signaling pathways leading to neurodegeneration are still incompletely understood. Hyperactivation of nitric oxide synthase (NOS) and increased nitric oxide (NO) production have been implicated in excitotoxic neuronal damage caused by overactivation of glutamate receptors, and it has been suggested that increased NO levels might also play a role in neurotoxicity in Alzheimer's disease. We have examined the effect of blockade of NO production on the neurotoxicity instigated by Aβ₄₂ and by elevated concentrations of glutamate in chick embryo retinal neurons in culture. Results showed that L-nitroarginine methyl ester, a potent inhibitor of all NOS isoforms, had no protective effect against neuronal death induced by either Aβ₄₂ (20 μM) or glutamate (1 mM). Surprisingly, at short incubation times both Aβ and glutamate decreased NO production in retinal neuronal cultures in the absence of neuronal death. Thus, excitotoxic insults induced by Aβ and glutamate cause inhibition rather than activation of NO synthase in retinal neurons, suggesting that cell death induced by Aβ or glutamate is not related to increased NO production. On the other hand, considering the role of NO in long term potentiation and synaptic plasticity, the decrease in NO levels instigated by Aβ and glutamate suggests a possible mechanism leading to synaptic failure in AD.

Capturing adenylyl cyclases as potential drug targets

Cyclic AMP (cAMP) is an important intracellular signalling mediator. It is generated in mammals by nine membrane-bound and one soluble adenylyl cyclases (ACs), each with distinct regulation and expression patterns. Although many drugs inhibit or stimulate AC activity through the respective upstream G-protein coupled receptors (for example, opioid or beta-adrenergic receptors), ACs themselves have not been major drug targets. Over the past decade studies on the physiological functions of the different mammalian AC isoforms as well as advances in the development of isoform-selective AC inhibitors and activators suggest that ACs could be useful drug targets. Here we discuss the therapeutic potential of isoform-selective compounds in various clinical settings, including neuropathic pain, neurodegenerative disorders, congestive heart failure, asthma and male contraception.