Enhanced Feedback Inhibition Due to Increased Recruitment of Somatostatin-Expressing Interneurons and Enhanced Cortical Recurrent Excitation in a Genetic Mouse Model of Migraine

Migraine is a complex brain disorder, characterized by attacks of unilateral headache and global dysfunction in multisensory information processing, whose underlying cellular and circuit mechanisms remain unknown. The finding of enhanced excitatory, but unaltered inhibitory, neurotransmission at cortical synapses between pyramidal cells (PCs) and fast-spiking interneurons (FS INs) in mouse models of familial hemiplegic migraine (FHM) suggested the hypothesis that dysregulation of the excitatory-inhibitory (E/I) balance in specific circuits is a key pathogenic mechanism. Here, we investigated the cortical layer 2/3 (L2/3) feedback inhibition microcircuit involving somatostatin-expressing (SOM) INs in FHM1 mice of both sexes carrying a gain-of-function mutation in Ca(V)2.1. Unitary inhibitory neurotransmission at SOM IN-PC synapses was unaltered while excitatory neurotransmission at both PC-SOM IN and PC-PC synapses was enhanced, because of increased probability of glutamate release, in FHM1 mice. Short-term synaptic depression was enhanced at PC-PC synapses while short-term synaptic facilitation was unaltered at PC-SOM IN synapses during 25-Hz repetitive activity. The frequency-dependent disynaptic inhibition (FDDI) mediated by SOM INs was enhanced, lasted longer and required shorter high-frequency bursts to be initiated in FHM1 mice. These findings, together with previous evidence of enhanced disynaptic feedforward inhibition by FS INs, suggest that the increased inhibition may effectively counteract the increased recurrent excitation in FHM1 mice and may even prevail in certain conditions. Considering the involvement of SOM INs in γ oscillations, surround suppression and context-dependent sensory perception, the facilitated recruitment of SOM INs, together with the enhanced recurrent excitation, may contribute to dysfunctional sensory processing in FHM1 and possibly migraine. SIGNIFICANCE STATEMENT Migraine is a complex brain disorder, characterized by attacks of unilateral headache and global dysfunction in multisensory information processing, whose underlying cellular and circuit mechanisms remain unknown, although dysregulation of the excitatory-inhibitory (E/I) balance in specific circuits could be a key pathogenic mechanism. Here, we provide insights into these mechanisms by investigating the cortical feedback inhibition microcircuit involving somatostatin-expressing interneurons (SOM INs) in a mouse model of a rare monogenic migraine. Despite unaltered inhibitory synaptic transmission, the disynaptic feedback inhibition mediated by SOM INs was enhanced in the migraine model because of enhanced recruitment of the INs. Recurrent cortical excitation was also enhanced. These alterations may contribute to context-dependent sensory processing dysfunctions in migraine.