1. Neuroscience Drug Discovery DK, H. Lundbeck A/S, Valby, Denmark
The 5-HT6 receptor antagonist Lu AE58054 potentiates the effects of the acetylcholinesterase inhibitor donepezil on hippocampal acetylcholine efflux and oscillatory activity
Kjartan Frisch Herrik, Inge E.M. de Jong, Jan Torleif Pedersen, Jørn Arnt, Arne Mørk
Neuroscience Drug Discovery DK, H. Lundbeck A/S, Valby, Denmark
Introduction
Lu AE58054 potentiated donepezil-induced ACh efflux in the hippocampus
Summary and Conclusions
The 5-HT6 receptor is primarily expressed in the brain, particularly in cortical and limbic regions. Localization on GABA-ergic interneurons has been reported.
5-HT6 receptor antagonists have pro-cognitive effects in rodents and clinical studies suggest beneficial effects on cognition in Alzheimer’s disease (AD).
Lu AE58054 is a selective compound with high affinity for the human 5-HT receptor (Ki 0.83 nM).
We investigated the effects of Lu AE58054, alone or in combination with the acetylcholinesterase inhibitor donepezil, on neurotransmitter efflux in the prefrontal cortex and hippocampus, and on hippocampal oscillatory activity.
Lu AE58054 administered hours prior to Donepezil
Lu AE58054 potentiated the effects of the acetylcholinesterase inhibitor donepezil on extracellular levels of ACh in the hippocampus.
Lu AE58054 prolonged the effects of donepezil on brain-stem induced gamma oscillations in the hippocampus.
These mechanisms are likely to contribute to the pro-cognitive effects of Lu AE58054.
Activation of the 5-HT6 receptor by agonist WAY increased extra-cellular levels of GABA whilst reducing DA and 5-HT levels in the frontal cortex. Pretreatment with Lu AE58054 prevented the changes in transmitter efflux.
Based on the current data, the proposed mode of action is that Lu AE58054 increases neurotransmission by relieving tonic GABA-ergic inhibition, through HT6 receptors located on GABA- ergic interneurons.
Methods
Male Sprague Dawley rats were used.
The 5-HT6 receptor was inhibited by Lu AE58054 (10 mg/kg, p.o.) and stimulated by 5-HT6 receptor agonist WAY (30 mg/kg, s.c.). Acetylcholinesterase was inhibited by donepezil (1.3 mg/kg, s.c.).
Dialysates were sampled in the prefrontal cortex and dorsal hippocampus and analysed for γ-aminobutyric acid (GABA), dopamine (DA), serotonin (5-HT) and acetylcholine (ACh) using HPLC with electrochemical or tandem MS detection.
Local field potentials were recorded in the dorsal hippocampal fissure and fast-Fourier transformed. Electrical stimulation was applied to the reticular formation (6s pulses, 250 Hz, 100–250 µA every 100s) and power of oscillatory activity was compared between treatment with Lu AE (2 mg/kg, i.v.) and/or donepezil (1 mg/kg, i.v.).
Two-way RM ANOVA p<0.05: #P<0.05: vehicle + donepezil (1.3) vs. vehicle + vehicle, *P<0.05: Lu AE donepezil (1.3) vs. vehicle + donepezil (1.3). Student-Newman-Keuls post hoc.
Lu AE58054 (10 mg/kg, p.o.) alone did not modulate extracellular levels of ACh in the dorsal hippocampus.
The 5-HT6 receptor agonist WAY (30 mg/kg, p.o.) did not affect extracellular levels of ACh in the hippocampus (data not shown), supporting previous findings that the 5-HT6 receptor may not be located directly on cholinergic neurons.
These findings suggest that Lu AE58054 has a unique mode of action and may be beneficial as add-on to acetylcholinesterase inhibitors for the treatment of cognitive deficits in AD.
5-HT6R activation increased cortical levels of GABA, whilst reducing DA and HT. These effects were blocked by pretreatment with Lu AE58054
Lu AE58054 prolonged the effect of donepezil on brainstem-induced gamma oscillations in the hippocampus
5-HT6R agonist WAY
(30 mg/kg, s.c.)
Lu AE58054 (10 mg/kg, p.o.) 2 hrs prior to WAY (30 mg/kg, s.c.)
GABA
5-HT DA
injection
Lu AE58054 (10 mg/kg, p.o.) alone did not affect GABA, 5- HT, or DA levels in the prefrontal cortex (data not shown), suggesting that the tonus on the 5-HT6 receptor is low under basal conditions.
Two-way RM ANOVA p<0.05:
*P<0.05, **P<0.01, ***P<0.001 vehicle vs. WAY Student-Newman-Keuls post hoc.
Acknowledgements: The current study and presentation are supported by H. Lundbeck A/S and Otsuka Pharmaceutical Development & Commercialization, Inc.
Two-way RM ANOVA p=0.01: *P<0.05, **P<0.01, ***P<0.001 Lu AE donepezil vs. vehicle + donepezil. Holm-Sidak post hoc.
Lu AE58054 (2 mg/kg, i.v.) alone did not affect reticular-elicited gamma oscillations in the dorsal hippocampus, but prolonged the effects of donepezil (1 mg/kg, i.v.).
Lu AE58054 (2 mg/kg, i.v.) did not modulate reticular-elicited theta oscillations, either alone or in combination with donepezil. We are currently testing lower doses of donepezil to increase the window for potentiation.
Presented at Brain Prize Meeting 2013, Hindsgavl Castle, Denmark
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