NATUre NeUrOSCieNCesignal monosynaptically and effectively to the LA and to a newly identified GABAergic amygdala population. By driving a complex intra-amygdala activity pattern, Calr + LT cells play an instrumental role in the establishment of CS+US signals and the formation of fear memories in the LA. Furthermore, these thalamic cells alter their activity during memory trace formation and can discriminate between US-paired and unpaired signals during retrieval. Together, our study indicates that the LT pathway to the amygdala, composed of Calr + LT neurons, provides associated and plastic signals for the acquisition of cue-related fear behavior. resultsCalr + LT cells project to the LA and are activated during fear learning. First, we identified which thalamic populations are connected to the LA. Thalamic cells retrogradely labeled from the LA with cholera toxin B subunit (CTB; Fig. 1a-c) were mainly located around the auditory thalamus (the MGN) in the PIL and the SG regions (Fig. 1b; Supplementary Fig. 1a-c). Since Calr + cells were specifically abundant in these lateral thalamic regions (Fig. 1b; Supplementary Fig. 1), we analyzed the Calr content of the CTBlabeled cells. The majority of them expressed Calr (Fig. 1c,d), which indicates that the thalamo-LA pathway is primarily formed by Calr + PIL and SG populations, hereafter collectively referred to as Calr + LT neurons (Supplementary Information).Next, we used an activity-dependent immediate early gene assay to investigate the involvement of the Calr + LT neurons in transferring CS and/or US signals during fear learning (Fig. 1e; Supplementary Fig. 2). Expression of the immediate early gene c-Fos was analyzed in four groups of mice that received either a conditioning tone (7.5 kHz, 30 s; CS+) or a US (foot shock, 1 mA, 1 s; US), or a US-associated tone (CS+US). Naive mice were used as controls. While CS+ markedly increased c-Fos expression in the PIL and SG areas, US and CS+US further increased the number of activated neurons in the PIL region relative to what was observed in control mice (Fig. 1f-j; Supplementary Table 1). Since the majority of c-Foslabeled cells were also Calr + (Fig. 1j; Supplementary Table 2), it was concluded that the LA can receive all relevant sensory information necessary for associative learning from Calr + LT cells.Calr + LT cells transfer short-latency signals related to fear learning. The expression of c-Fos provided spatially precise data about the cellular origin of CS, US and CS+US information (Fig. 1) for the LA. However, it did not reveal whether these neurons can convey and integrate a CS and a US within a short time window 19 and supply the LA with a salient cue within 20 ms 7 . Thus, we performed extracellular in vivo recordings from Calr + LT cells to investigate the time course of activation to short tone and footshock stimuli, as well as their possible potentiation to an associated footshock-coupled tone. Calb2-Cre mice (Calb2 encodes Calr; Supplementary Fig. 1k-n) were injected with conditional Cre-dependent recomb...

Sleep cycles consist of rapid alterations between arousal states including transient perturbation of sleep rhythms, microarousals and full-blown awake states. Here we demonstrate that the calretinin containing (CR+) neurons in the dorsal medial thalamus (DMT) constitute a key diencephalic node that mediates distinct levels of forebrain arousal. Cell-type-specific activation of DMT/CR+ cells could elicit active locomotion lasting for minutes, stereotyped microarousals or transient disruption of sleep rhythms depending on the parameters of the stimulation. State transitions could be induced in both slow-wave and REM sleep. The DMT/CR+ cells displayed elevated activity prior to arousal, received selective subcortical inputs and innervated several forebrain sites via highly branched axons. Together, these features enable DMT/CR+ cells to summate subcortical arousal information and effectively transfer it as a rapid, synchronous signal to several forebrain regions to modulate the level of arousal.