Spatial and developmental heterogeneity of calcium signaling in olfactory ensheathing cells

Olfactory ensheathing cells (OECs) are specialized glial cells in the mammalian olfactory system supporting growth of axons from the olfactory epithelium into the olfactory bulb. OECs in the olfactory bulb can be subdivided into OECs of the outer nerve layer and the inner nerve layer according to the expression of marker proteins and their location in the nerve layer. In the present study, we have used confocal calcium imaging of OECs in acute mouse brain slices and olfactory bulbs in toto to investigate physiological differences between OEC subpopulations. OECs in the outer nerve layer, but not the inner nerve layer, responded to glutamate, ATP, serotonin, dopamine, carbachol, and phenylephrine with increases in the cytosolic calcium concentration. The calcium responses consisted of a transient and a tonic component, the latter being mediated by store-operated calcium entry. Calcium measurements in OECs during the first three postnatal weeks revealed a downregulation of mGluR(1) and P2Y(1) receptor-mediated calcium signaling within the first 2 weeks, suggesting that the expression of these receptors is developmentally controlled. In addition, electrical stimulation of sensory axons evoked calcium signaling via mGluR(1) and P2Y(1) only in outer nerve layer OECs. Downregulation of the receptor-mediated calcium responses in postnatal animals is reflected by a decrease in amplitude of stimulation-evoked calcium transients in OECs from postnatal days 3 to 21. In summary, the results presented reveal striking differences in receptor responses during development and in axon-OEC communication between the two subpopulations of OECs in the olfactory bulb

Increased GABAergic Efficacy of Central Amygdala Projections to Neuropeptide S Neurons in the Brainstem During Fear Memory Retrieval

The canonical view on the central amygdala has evolved from a simple output station towards a highly organized microcircuitry, in which types of GABAergic neurons in centrolateral (CeL) and centromedial (CeM) subnuclei regulate fear expression and generalization. How these specific neuronal populations are connected to extra-amygdaloid target regions remains largely unknown. Here we show in mice that a subpopulation of GABAergic CeL and CeM neurons projects monosynaptically to brainstem neurons expressing neuropeptide S (NPS). The CeL neurons are PKCδ-negative and are activated during conditioned fear. During fear memory retrieval, the efficacy of this GABAergic influence on NPS neurons is enhanced. Moreover, a large proportion of these neurons (~50%) contain prodynorphin and somatostatin, two neuropeptides inhibiting NPS neurons. We conclude that CeL and CeM neurons inhibit NPS neurons in the brainstem by GABA release and that efficacy of this connection is strengthened upon fear memory retrieval. Thereby, this pathway provides a possible feedback mechanism between amygdala and brainstem routes involved in fear and stress coping

Short-Term Adaptation of Conditioned Fear Responses Through Endocannabinoid Signaling in the Central Amygdala

The cannabinoid receptor type 1 (CB1) and the central nucleus of the amygdala (CeA) are both known to have crucial roles in the processing of fear and anxiety, whereby they appear to be especially involved in the control of fear states. However, in contrast to many other brain regions including the cortical subregions of the amygdala, the existence of CB1 in the CeA remains enigmatic. In this study we show that CB1 is expressed in the CeA of mice and that CB1 in the CeA mediates short-term synaptic plasticity, namely depolarization-induced suppression of excitation (DSE) and inhibition (DSI). Moreover, the CB1 antagonist AM251 increased both excitatory and inhibitory postsynaptic responses in CeA neurons. Local application of AM251 in the CeA in vivo resulted in an acutely increased fear response in an auditory fear conditioning paradigm. Upon application of AM251 in the basolateral nucleus of the amygdala (BLA) in an otherwise identical protocol, no such acute behavioral effects were detected, but CB1 blockade resulted in increased fear responses during tone exposures on the subsequent days. Moreover, we observed that the efficacy of DSE and DSI in the CeA was increased on the day following fear conditioning, indicating that a single tone-shock pairing resulted in changes in endocannabinoid signaling in the CeA. Taken together, our data show the existence of CB1 proteins in the CeA, and their critical role for ensuring short-term adaptation of responses to fearful events, thereby suggesting a potential therapeutic target to accompany habituation-based therapies of post-traumatic symptoms