Eva Bollen

Object recognition testing: methodological considerations on exploration and discrimination measures

The object recognition task (ORT) is a popular one-trial learning test for animals. In the current study, we investigated several methodological issues concerning the task. Data was pooled from 28 ORT studies, containing 731 male Wistar rats. We investigated the relationship between 3 common absolute- and relative discrimination measures, as well as their relation to exploratory activity. In this context, the effects of pre-experimental habituation, object familiarity, trial duration, retention interval and the amnesic drugs MK-801 and scopolamine were investigated. Our analyses showed that the ORT is very sensitive, capable of detecting subtle differences in memory (discrimination) and exploratory performance. As a consequence, it is susceptible to potential biases due to (injection) stress and side effects of drugs. Our data indicated that a minimum amount of exploration is required in the sample and test trial for stable significant discrimination performance. However, there was no relationship between the level of exploration in the sample trial and discrimination performance. In addition, the level of exploration in the test trial was positively related to the absolute discrimination measure, whereas this was not the case for relative discrimination measures, which correct for exploratory differences, making them more resistant to exploration biases. Animals appeared to remember object information over multiple test sessions. Therefore, when animals have encountered both objects in prior test sessions, the object preference observed in the test trial of 1h retention intervals is probably due to a relative difference in familiarity between the objects in the test trial, rather than true novelty per se. Taken together, our findings suggest to take into consideration pre-experimental exposure (familiarization) to objects, habituation to treatment procedures, and the use of relative discrimination measures when using the ORT.

GEBR-7b, a novel PDE4D selective inhibitor that improves memory in rodents at non-emetic doses.

BACKGROUND AND PURPOSE Strategies designed to enhance cerebral cAMP have been proposed as symptomatic treatments to counteract cognitive deficits. However, pharmacological therapies aimed at reducing PDE4, the main class of cAMP catabolizing enzymes in the brain, produce severe emetic side effects. We have recently synthesized a 3‐cyclopentyloxy‐4‐methoxybenzaldehyde derivative, structurally related to rolipram, and endowed with selective PDE4D inhibitory activity. The aim of the present study was to investigate the effect of the new drug, namely GEBR‐7b, on memory performance, nausea, hippocampal cAMP and amyloid‐β (Aβ) levels.

Phosphodiesterases in neurodegenerative disorders

Cyclic nucleotide phosphodiesterases (PDEs) are responsible for the breakdown of cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). As such, they are crucial regulators of levels of cyclic nucleotide‐mediated signaling. cAMP signaling and cGMP signaling have been associated with neuroplasticity and protection, and influencing their levels in the cell by inhibition of PDEs has become a much studied target for treatment in a wide array of disorders, including neurodegenerative disorders. In this review, we will focus on the involvement of PDEs in neurodegenerative disorders. In comparison with preclinical work, data on human patients are scarce. Alzheimers disease is associated with changes in PDE4, PDE7, and PDE8 expression in the brain. Altered functioning of PDE4 as well as PDE11 is associated with major depressive disorder. In multiple sclerosis, there are indications of alterations in expression of several PDE subtypes in the central nervous system; however, evidence is indirect. In Huntingtons disease and Parkinsons disease, most research has focused on PDE1B and PDE10, because of their abundant presence in striatal neurons. In another rare, neurodegenerative striatal motor disorder, that is, autosomal‐dominant striatal degeneration, genetic defects in PDE8B gene are thought to underlie the neurodegenerative processes. Although the latter disorder has showed a causative dysfunction of PDEs, this does not hold for the neurodegenerative disorders discussed above, in which changes in PDE levels seemingly rather represent secondary changes and compensation to prior existing dysfunction. However, normalizing cyclic nucleotide signaling via PDE inhibition remains interesting for the treatment of neurodegenerative disorders.

Inhibition of phoshodiesterase type 2 or type 10 reverses object memory deficits induced by scopolamine or MK-801

The objective of this study was to assess the effects of phosphodiesterase type 2 (PDE2) and type 10 (PDE10) inhibition on memory function in the object recognition task using the scopolamine- and MK-801-induced memory deficit model. The effects of the PDE2 inhibitor BAY 60-7550 and the PDE10 inhibitor PQ-10 on object recognition performance were investigated in the scopolamine (0.1mg/kg, i.p.) or MK-801 (0.125 mg/kg, i.p.) model. BAY 60-7550 was tested at a dose of 0.3-3mg/kg (p.o.) in both models; PQ-10 was tested at doses of 0.1-1mg/kg (p.o.) in the scopolamine model and 0.3-3mg/kg in the MK-801 model. All compounds were injected 30 min before the learning trial. Both BAY 60-7550 (1mg/kg) and PQ-10 (0.3mg/kg) attenuated the scopolamine-induced memory deficit. The MK-801-induced memory deficit was reversed after treatment with each PDE inhibitor at a dose of 1mg/kg or higher. PQ10 was highly brain penetrant, whereas 60-7550 levels in the brain were very low after oral treatment. We concluded that since BAY 60-7550 and PQ10 reversed both scopolamine- and MK-801-induced memory deficits, this supports the notion that dual substrate PDE inhibitors might be suitable candidates for cognition enhancement.

Improved Long-Term Memory via Enhancing cGMP-PKG Signaling Requires cAMP-PKA Signaling

Memory consolidation is defined by the stabilization of a memory trace after acquisition, and consists of numerous molecular cascades that mediate synaptic plasticity. Commonly, a distinction is made between an early and a late consolidation phase, in which early refers to the first hours in which labile synaptic changes occur, whereas late consolidation relates to stable and long-lasting synaptic changes induced by de novo protein synthesis. How these phases are linked at a molecular level is not yet clear. Here we studied the interaction of the cyclic nucleotide-mediated pathways during the different phases of memory consolidation in rodents. In addition, the same pathways were studied in a model of neuronal plasticity, long-term potentiation (LTP). We demonstrated that cGMP/protein kinase G (PKG) signaling mediates early memory consolidation as well as early-phase LTP, whereas cAMP/protein kinase A (PKA) signaling mediates late consolidation and late-phase-like LTP. In addition, we show for the first time that early-phase cGMP/PKG signaling requires late-phase cAMP/PKA-signaling in both LTP and long-term memory formation.

7,8-Dihydroxyflavone improves memory consolidation processes in rats and mice.

Brain-derived neurotrophic factor (BDNF) is a crucial regulator of neuronal survival and neuroplasticity in the central nervous system (CNS). As a result, there has been a growing interest in the role of BDNF in neuropsychiatric disorders associated with neurodegeneration, including depression and dementia. However, until now, BDNF-targeting therapies have yielded disappointing results. BDNF is thought to exert its beneficial effects on synaptic and neuronal plasticity mainly through binding to the tyrosine kinase B (TrkB) receptor. Recently, 7,8-dihydroxyflavone (7,8-DHF) was identified as the first selective TrkB agonist. In the present study the effect of 7,8-DHF on memory consolidation processes was evaluated. In healthy rats, 7,8-DHF improved object memory formation in the object recognition task when administered both immediately and 3h after learning. In a transgenic mouse model of Alzheimers disease, i.e. APPswe/PS1dE9 mice, spatial memory as measured in the object location task was improved after administration of 7,8-DHF. A similar memory improvement was found when their wild-type littermates were treated with 7,8-DHF. The acute beneficial effects in healthy mice suggest that effects might be symptomatic rather than curing. Nevertheless, this study suggests that 7,8-DHF might be a promising therapeutic target for dementia.

Effects of protein kinase A and G inhibitors on hippocampal cholinergic markers expressions in rolipram- and sildenafil-induced spatial memory improvement.

Although there are number of studies showing that phosphodiesterase (PDE) 4 and 5 inhibitors affect different kinds of memory, their effects on spatial memory consolidation in conjunction with the cholinergic activity in the hippocampus have not been studied before. In the present study firstly, rats were evaluated for the effects of different doses of the PDE4 inhibitor rolipram and the PDE5 inhibitor sildenafil on spatial memory consolidation in the water maze task. Rolipram or sildenafil was daily administered intraperitoneally 3 or 0 h after the last trial of training, respectively. Then in a separate related experiment the effect of the most efficient doses of rolipram or sildenafil accompanied by an intrahippocampally injected protein kinase A (PKA) or protein kinase G (PKG) inhibitor, respectively, was examined. Finally for determination of the hippocampal cholinergic activity the protein expression of hippocampal vesicular acetylcholine transporter (VAChT) and cholineacetyltransferase (ChAT) was measured. Rolipram at 0.03 mg/kg as well as sildenafil at 3 mg/kg increased spatial memory and their enhancing effect was completely blocked following inhibition of PKA and PKG, respectively. Furthermore, none of the treatments had a significant effect on the hippocampal ChAT and VAChT levels. Our data showed that rolipram and sildenafil enhanced spatial memory consolidation in an inverted U-shaped dose-response curve. This effect is dependent on the activity of cAMP/PKA- and cGMP/PKG-mediated pathways, respectively in the hippocampus. However, we did not find evidence for a chronic increase of cholinergic activity in the observed PDE inhibitor-induced memory improvement.

Phosphodiesterase inhibition and regulation of dopaminergic frontal and striatal functioning: clinical implications

Background: The fronto-striatal circuits are the common neurobiological basis for neuropsychiatric disorders, including schizophrenia, Parkinson’s disease, Huntington’s disease, attention deficit hyperactivity disorder, obsessive-compulsive disorder, and Tourette’s syndrome. Fronto-striatal circuits consist of motor circuits, associative circuits, and limbic circuits. All circuits share 2 common features. First, all fronto-striatal circuits consist of hyper direct, direct, and indirect pathways. Second, all fronto-striatal circuits are modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cyclic adenosine monophosphate/protein kinase A signaling cascade with an additional role for the cyclic guanosine monophosphate/protein kinase G pathway, both of which can be regulated by phosphodiesterases. Phosphodiesterases are thus a potential target for pharmacological intervention in neuropsychiatric disorders related to dopaminergic regulation of fronto-striatal circuits. Methods: Clinical studies of the effects of different phosphodiesterase inhibitors on cognition, affect, and motor function in relation to the fronto-striatal circuits are reviewed. Results: Several selective phosphodiesterase inhibitors have positive effects on cognition, affect, and motor function in relation to the fronto-striatal circuits. Conclusion: Increased understanding of the subcellular localization and unraveling of the signalosome concept of phosphodiesterases including its function and dysfunction in the fronto-striatal circuits will contribute to the design of new specific inhibitors and enhance the potential of phosphodiesterase inhibitors as therapeutics in fronto-striatal circuits.

Phosphodiesterase inhibition and modulation of corticostriatal and hippocampal circuits: Clinical overview and translational considerations

HighlightsCorticostriatal and hippocampal circuits constitute the neurobiological basis for many neuropsychiatric disorders.PDE1, 4 and 10 inhibitors seem interesting for corticostriatal disorders due to their modulation of DAergic system.PDE3, 4 and 9 inhibitors are currently most promising for hippocampal disorders.So far, the wealth of positive preclinical data has not been able to be translated to convincing clinical efficacy.The local net amount of cAMP and/or cGMP will eventually determine the cellular and functional fate. &NA; The corticostriatal and hippocampal circuits contribute to the neurobiological underpinnings of several neuropsychiatric disorders, including Alzheimers disease, Parkinsons disease and schizophrenia. Based on biological function, these circuits can be clustered into motor circuits, associative/cognitive circuits and limbic circuits. Together, dysfunctions in these circuits produce the wide range of symptoms observed in related neuropsychiatric disorders. Intracellular signaling in these circuits is largely mediated through the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway with an additional role for the cyclic guanosine monophosphate (cGMP)/ protein kinase G (PKG) pathway, both of which can be regulated by phosphodiesterase inhibitors (PDE inhibitors). Through their effects on cAMP response element‐binding protein (CREB) and Dopamine‐ and cAMP‐Regulated PhosphoProtein MR 32 kDa (DARPP‐32), cyclic nucleotide pathways are involved in synaptic transmission, neuron excitability, neuroplasticity and neuroprotection. In this clinical review, we provide an overview of the current clinical status, discuss the general mechanism of action of PDE inhibitors in relation to the corticostriatal and hippocampal circuits and consider several translational challenges.