Cannabinoid CB1 receptors in distinct circuits of the extended amygdala determine fear responsiveness to unpredictable threat.

The brain circuits underlying behavioral fear have been extensively studied over the last decades. Although the vast majority of experimental studies assess fear as a transient state of apprehension in response to a discrete threat, such phasic states of fear can shift to a sustained anxious apprehension, particularly in face of diffuse cues with unpredictable environmental contingencies. Unpredictability, in turn, is considered an important variable contributing to anxiety disorders. The networks of the extended amygdala have been suggested keys to the control of phasic and sustained states of fear, although the underlying synaptic pathways and mechanisms remain poorly understood. Here, we show that the endocannabinoid system acting in synaptic circuits of the extended amygdala can explain the fear response profile during exposure to unpredictable threat. Using fear training with predictable or unpredictable cues in mice, combined with local and cell-type-specific deficiency and rescue of cannabinoid type 1 (CB1) receptors, we found that presynaptic CB1 receptors on distinct amygdala projections to bed nucleus of the stria terminalis (BNST) are both necessary and sufficient for the shift from phasic to sustained fear in response to an unpredictable threat. These results thereby identify the causal role of a defined protein in a distinct brain pathway for the temporal development of a sustained state of anxious apprehension during unpredictability of environmental influences, reminiscent of anxiety symptoms in humans.

Electrical properties of morphologically characterized neurons in the intergeniculate leaflet of the rat thalamus.

The intergeniculate leaflet (IGL) is a flat thalamic nucleus that responds to retinal illumination, but also to non-photic input from many brain areas. Its only known function is to modulate the circadian rhythm generated by the suprachiasmatic nucleus. Previously, the firing behavior of cells in IGL has been investigated with extra-cellular recordings, but intracellular recordings from morphologically identified mammalian IGL neurons are lacking. We recorded from and labeled IGL cells in rat brain slices to characterize their basic membrane properties and cell morphology. A high fraction of neurons (82.5%) were spontaneously active. The silent cells were identified as neurons by electrophysiological techniques. The spontaneous activity was due to intrinsic membrane properties, and not driven by rhythmic synaptic input. Most spontaneously active cells had a very regular firing pattern with a coefficient of variation of the spike intervals <0.12 in more than 50% of the cells. Rebound depolarization after a hyperpolarizing pulse, usually with one fast action potential on top, was observed in 80% of the cells. The silent neurons had a range of resting membrane potentials and spike thresholds overlapping with the active ones. This suggests that spontaneous activity was controlled by several, yet undetermined factors in addition to membrane potential. Within the IGL we found a broad range of morphologies without apparent categories and no significant correlation with activity. However, the spontaneous, usually regular, spiking and the rebound depolarization of IGL cells is typical a feature that distinguish them from neurons in ventral and from interneurons in the dorsal lateral geniculate nuclei.