The mediating role of phosphodiesterase type 4 in the dopaminergic modulation of motor impulsivity.

The current study investigated the mediating role of phosphodiesterase type 4 (PDE4) regulated cAMP in the dopaminergic modulation of premature responding (action restraint) in rats. Response inhibition, which includes action restraint, finds its neurobiological origin in cortico-striatal-thalamic circuitry and can be modulated by dopamine. Intracellularly, the effect of dopamine is largely mediated through the cAMP/PKA signaling cascade. Areas in the prefrontal cortex are very sensitive to their neurochemical environment, including catecholamine levels. As a result, we investigated the effects of intracellular modulation of the dopamine cascade by means of PDE4 inhibition by roflumilast on premature responding in a hypo, normal and hyper dopaminergic state of the brain. As a hypo dopaminergic model we induced a 6-OHDA lesion in the (rat) prefrontal cortex, more specifically the infralimbic cortex. For the hyper dopaminergic state we also turned to a well-established model of impaired action restraint, namely the systemic administration of d-amphetamine. In line with the notion of a U-shaped relation between dopamine and impulsive responding, we found that both increasing and decreasing dopamine levels resulted in an increase in premature responding in the choice serial reaction time task (CSRTT). The PDE4 inhibitor roflumilast increased premature responses in combination with d-amphetamine, whereas a decrease in premature responding after roflumilast treatment was found in the 6-OHDA lesioned animals. As a result, it would be interesting to test the effects of PDE4 inhibition in disorders affected by disrupted impulse control related to cortico-striatal-thalamic hypodopaminergia including attention deficit hyperactivity disorder (ADHD).

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

The corticostriatal and hippocampal circuits contribute to the neurobiological underpinnings of several neuropsychiatric disorders, including Alzheimer's disease, Parkinson's 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.

Phosphodiesterase 4 inhibition affects both the direct and indirect pathway: an electrophysiological study examining the tri-phasic response in the substantia nigra pars reticulata.

Fronto-striatal circuits constitute the neurobiological basis of many neuropsychiatric disorders. Part of the intracellular signaling within these circuits, including its dopaminergic modulation, is regulated by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling cascade. Based on the overall expression in human fronto-striatal circuitry, we tested the effects of a cAMP selective phosphodiesterase 4 (PDE4) inhibitor on the tri-phasic response in the dorsomedial substantia nigra pars reticulata (SNr) upon stimulation of the infralimbic cortex in rats. Our results show for the first time that stimulation of the cognitive infralimbic cortex leads to a tri-phasic response in SNr neurons. In addition and in line with previous biochemical and behavioral studies, PDE4 inhibition by roflumilast affects the direct pathway as well as the indirect pathway of which the latter appears more sensitive than the former.

Acute administration of roflumilast enhances immediate recall of verbal word memory in healthy young adults.

The need for new and effective treatments for dementia remains indisputably high. Phosphodiesterase inhibitors (PDE-Is) have proven efficacy as cognitive enhancers based on their positive effects in numerous preclinical studies. Especially the PDE4 subfamily is of interest due to its expression in the hippocampus, the key structure for memory formation. The current study investigates the memory enhancing effects of the clinically approved PDE4-I roflumilast in a test battery including the Verbal Learning Task (VLT) combined with electroencephalography (EEG) recording. This acute study was conducted according to a double-blind, randomized, placebo-controlled, 4-way crossover design. Three capsulated dosages of roflumilast HCl (Daxas) and a placebo were administered in four study periods. Administration occurred 1 h before testing to reach maximal plasma concentrations. Memory performance was assessed using a 30 word Verbal Learning Task. The number of words recalled both immediately and after 45 min and 24 h were included as outcome measures. EEG was recorded during the cognitive tasks on the first day. Different event-related potentials (ERPs) were considered with special emphasis on P600, as this peak has been related to word learning. Memory performance was significantly improved after acute administration of 100 µg roflumilast. Specifically, immediate recall performance on the VLT increased 2-3 words, accompanied by an enhanced P600 peak during word presentation at the third learning trial. No side effects typical for PDE4-Is were reported for the lowest and effective dose of 100 µg roflumilast. The current proof-of-concept study shows for the first time the potential of low-dose roflumilast administration as a memory enhancer in humans.

Phosphodiesterase inhibitors as a target for cognition enhancement in aging and Alzheimer's disease: a translational overview.

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Since both cAMP and cGMP signaling are essential in a variety of cellular functions, including neuroplasticity and neuroprotection, PDE-Is are receiving increased attention as possible targets for treatment of age-related cognitive decline as well as Alzheimer's disease (AD). In this review we will give a translational overview of the preclinical and clinical data on PDE-Is and cognition enhancement focusing on aging and AD. PDE2, 4 and 5 inhibitors improved memory performance in both aged animals and models of AD. Treatment with a PDE3-I or PDE7-I has not been tested in aged animals yet, but in mouse models of AD both PDE-Is improved memory performance. Unfortunately, there are no peer-reviewed studies on the effects of PDE-I treatment in aged human subjects except the possible positive effect on memory impairment of the PDE1-I vinpocetine. Three other types of PDE-Is have been tested on cognition in mild to moderate AD patients: the PDE3-I cilostazol is being tested as a co-treatment to the acetylcholinesterase inhibitor donepezil, but with inconsistent results; the PDE4-I MK-0952 has been tested, although the outcome has not been disclosed yet; and the PDE9-I PF- 04447943 was reported to have no effects on cognition. Obviously, the demonstration of clinical proof of concept for cognition enhancing effects of PDE-Is and the generation of isoform selective PDE-Is are the final hurdles to overcome in developing safe and efficacious novel PDE-Is for the treatment of age-associated cognitive decline or AD.

PDE and cognitive processing: beyond the memory domain.

Phosphodiesterase inhibitors (PDE-Is) enhance cAMP and/or cGMP signaling via reducing the degradation of these cyclic nucleotides. Both cAMP and cGMP signaling are essential for a variety of cellular functions and exert their effects both pre- and post-synaptically. Either of these second messengers relays and amplifies incoming signals at receptors on the cell surface making them important elements in signal transduction cascades and essential in cellular signaling in a variety of cell functions including neurotransmitter release and neuroprotection. Consequently, these processes can be influenced by PDE-Is as they increase cAMP and/or cGMP concentrations. PDE-Is have been considered as possible therapeutic agents to treat impaired memory function linked to several brain disorders, including depression, schizophrenia and Alzheimer's disease (AD). This review will, however, focus on the possible role of phosphodiesterases (PDEs) in cognitive decline beyond the memory domain. Here we will discuss the involvement of PDEs on three related domains: attention, information filtering (sensory- and sensorimotor gating) and response inhibition (drug-induced hyperlocomotion). Currently, these are emerging cognitive domains in the field of PDE research. Here we discuss experimental studies and the potential beneficial effects of PDE-I drugs on these cognitive domains, as effects of PDE-Is on these domains could potentially influence effects on memory performance. Overall, PDE4 seems to be the most promising target for all domains discussed in this review.