A neural circuit from the dorsal CA3 to the dorsomedial hypothalamus mediates balance between risk exploration and defense.

An appropriate balance between explorative and defensive behavior is essential for the survival and reproduction of prey animals in risky environments. However, the neural circuit and mechanism that allow for such a balance remains poorly understood. Here, we use a semi-naturalistic predator threat test (PTT) to observe and quantify the defense-exploration balance, especially risk exploration behavior in mice. During the PTT, the activity of the putative dorsal CA3 glutamatergic neurons (dCA3Glu) is suppressed by predatory threat and risk exploration, whereas the neurons are activated during contextual exploration. Moreover, optogenetic excitation of these neurons induces a significant increase in risk exploration. A circuit, comprising the dorsal CA3, dorsal lateral septal, and dorsomedial hypothalamic (dCA3Glu-dLSGABA-DMH) areas, may be involved. Moreover, activation of the dCA3Glu-dLSGABA-DMH circuit promotes the switch from defense to risk exploration and suppresses threat-induced increase in arousal.

Flexible and stretchable polymer optical fibers for chronic brain and vagus nerve optogenetic stimulations in free-behaving animals.

BACKGROUND: Although electrical stimulation of the peripheral and central nervous systems has attracted much attention owing to its potential therapeutic effects on neuropsychiatric diseases, its non-cell-type-specific activation characteristics may hinder its wide clinical application. Unlike electrical methodologies, optogenetics has more recently been applied as a cell-specific approach for precise modulation of neural functions in vivo, for instance on the vagus nerve. The commonly used implantable optical waveguides are silica optical fibers, which for brain optogenetic stimulation (BOS) are usually fixed on the skull bone. However, due to the huge mismatch of mechanical properties between the stiff optical implants and deformable vagal tissues, vagus nerve optogenetic stimulation (VNOS) in free-behaving animals continues to be a great challenge. RESULTS: To resolve this issue, we developed a simplified method for the fabrication of flexible and stretchable polymer optical fibers (POFs), which show significantly improved characteristics for in vivo optogenetic applications, specifically a low Young's modulus, high stretchability, improved biocompatibility, and long-term stability. We implanted the POFs into the primary motor cortex of C57 mice after the expression of CaMKIIα-ChR2-mCherry detected frequency-dependent neuronal activity and the behavioral changes during light delivery. The viability of POFs as implantable waveguides for VNOS was verified by the increased firing rate of the fast-spiking GABAergic interneurons recorded in the left vagus nerve of VGAT-ChR2 transgenic mice. Furthermore, VNOS was carried out in free-moving rodents via chronically implanted POFs, and an inhibitory influence on the cardiac system and an anxiolytic effect on behaviors was shown. CONCLUSION: Our results demonstrate the feasibility and advantages of the use of POFs in chronic optogenetic modulations in both of the central and peripheral nervous systems, providing new information for the development of novel therapeutic strategies for the treatment of neuropsychiatric disorders.