Orexin receptors regulate hippocampal sharp wave-ripple complexes in ex vivo slices.

Orexin is a neuromodulatory peptide produced by lateral hypothalamic orexin neurons and binds to G-protein-coupled orexin-1 receptor and orexin-2 receptors. Whether orexin modulates learning and memory is not fully understood. Orexin has biphasic effects on learning and memory: promoting learning and memory at homeostatic levels and inhibiting at supra- and sub-homeostatic levels. Hippocampal sharp wave-ripples encode memory information and are essential for memory consolidation and retrieval. The role of orexin on sharp wave-ripples in hippocampal CA1 remains unknown. Here, we used multi-electrode array recordings in acute ex vivo hippocampal slices to determine the effects of orexin receptor antagonists on sharp wave-ripples. Bath-application of either the orexin-1 receptor antagonist N-(2-Methyl-6-benzoxazolyl)-N'-1,5-naphthyridin-4-yl urea (SB-334867) or the orexin-2 receptor antagonist N-Ethyl-2-[(6-methoxy-3-pyridinyl)[(2-methylphenyl)sulfonyl]amino]-N-(3-pyridinylmethyl)-acetamide (EMPA) reduced sharp wave and ripple incidence, sharp wave amplitude, and sharp wave duration. SB-334867 and EMPA effects on sharp wave amplitude and duration were equivalent, whereas EMPA exhibited a greater reduction of sharp wave and ripple incidence. EMPA also increased ripple duration, whereas SB-334867 had no effect. Inhibition of both orexin receptors with a dual orexin receptor antagonist N-[1,1'-Biphenyl]-2-yl-1-[2-[(1-methyl-1H-benzimidazol-2-yl)thio]acetyl-2-pyrrolidinedicarboxamide (TCS-1102) had effects similar to EMPA, however, sharp wave amplitude and duration were unaffected. Region-specific expression of orexin receptors suggests orexin may regulate sharp wave generation in CA3, dentate gyrus-mediated sharp wave modification, sharp wave propagation to CA1, and local ripple emergence in CA1. Our study indicates an orexin contribution to hippocampal sharp wave-ripple complexes and suggests a mechanism by which sub-homeostatic concentrations of orexin may inhibit learning and memory function.

Carbamazepine Reduces Sharp Wave-Ripple Complexes and Exerts Synapse-Specific Inhibition of Neurotransmission in Ex Vivo Hippocampal Slices.

Higher therapeutic concentrations of the antiseizure medication carbamazepine (CBZ) are associated with cognitive side effects. Hippocampal sharp wave-ripple complexes (SPW-Rs) are proposed to participate in memory consolidation during periods of quiet and slow-wave sleep. SPW-Rs are generated in the CA3 region and are regulated by multiple synaptic inputs. Here, we used a multi-electrode array to determine the effects of CBZ on SPW-Rs and synaptic transmission at multiple hippocampal synapses. Our results demonstrate that CBZ reduced SPW-Rs at therapeutically relevant concentrations (IC50 = 37 µM) and altered the core characteristics of ripples, important for information processing and consolidation. Moreover, CBZ inhibited neurotransmission in a synapse-specific manner. CBZ inhibition was most potent at the medial-perforant-path-to-CA3 and mossy-fiber-to-CA3 synapses (IC50s ~ 30 and 60 µM, respectively) and least potent at medial-perforant-path-to-dentate granule cell synapses (IC50 ~ 120 µM). These results suggest that the synapse-specific CBZ inhibition of neurotransmission reduces SPW-Rs and that the CBZ inhibition of SPW-Rs may underlie the cognitive impairments observed with therapeutic doses of CBZ.