The noradrenergic nucleus locus coeruleus (LC) is critically involved in the stress response and receives afferent input from a number of corticotropin releasing factor (CRF) containing structures. Several in vivo and in vitro studies in rat have shown that CRF robustly increases the firing rate of LC neurons in a dose-dependent manner. While it is known that these increases are dependent on CRF receptor subtype 1 and mediated by effects of cAMP intracellular signaling cascades on potassium conductance, the impact of CRF on synaptic transmission within LC has not been clarified. In the present study, we used whole-cell patch clamp electrophysiology to assess how varying concentrations of bath-applied CRF affect AMPA-receptor dependent spontaneous excitatory post-synaptic currents (sEPSCs). Compared to vehicle, 10, 25, and 100 nm CRF had no significant effects on any sEPSC parameters. Fifty nanomolar CRF, however, significantly increased sEPSC amplitude, half-width, and charge transfer, while these measures were significantly decreased by 200 nm CRF. These observations suggest that stress may differentially affect ongoing excitatory synaptic transmission in LC depending on how much CRF is released from presynaptic terminals. Combined with the well-documented effects of CRF on membrane properties and spontaneous LC discharge, these observations may help explain how stress and CRF release are able to modulate the signal to noise ratio of LC neurons. These findings have implications for how stress affects the fidelity of signal transmission and information flow through LC and how it might impact norepinephrine release in the CNS.

Serotonin (5-HT)-containing neurons in the dorsal raphe (DR) nucleus project throughout the forebrain and are implicated in many physiological processes and neuropsychiatric disorders. Diversity among these neurons has been characterized in terms of their neurochemistry and anatomical organization, but a clear sense of whether these attributes align with specific brain functions or terminal fields is lacking. DR 5-HT neurons can co-express additional neuroactive substances, increasing the potential for individualized regulation of target circuits. The goal of this study was to link DR neurons to a specific functional role by characterizing cells according to both their neurotransmitter expression and efferent connectivity; specifically, cells projecting to the medial prefrontal cortex (mPFC), a region implicated in cognition, emotion, and responses to stress. Following retrograde tracer injection, brainstem sections from Sprague-Dawley rats were immunohistochemically stained for markers of serotonin, glutamate, GABA, and nitric oxide (NO). 98% of the mPFC-projecting serotonergic neurons co-expressed the marker for glutamate, while the markers for NO and GABA were observed in 60% and less than 1% of those neurons, respectively. To identify potential target-specific differences in co-transmitter expression, we also characterized DR neurons projecting to a visual sensory structure, the lateral geniculate nucleus (LGN). The proportion of serotonergic neurons co-expressing NO was greater amongst cells targeting the mPFC vs LGN (60% vs 22%). The established role of 5-HT in affective disorders and the emerging role of NO in stress signaling suggest that the impact of 5-HT/NO co-localization in DR neurons that regulate mPFC circuit function may be clinically relevant.

An understanding of the dynamics of drug-target interactions is important in the drug discovery process. Information related to the binding kinetics of a drug toward its target or off-target aids in determining the efficacy or toxicity of a drug. Biophysical techniques such as surface plasmon resonance (SPR) have been available for over 20 years, but have been predominantly utilized to characterize protein-protein interactions. With improvements in instrument sensitivity and data analysis software, interactions between proteins (such as kinases) and small molecules have been successfully evaluated. More recently, the LanthaScreen Eu kinase binding assay for characterizing kinase inhibitors has been described. This assay monitors displacement of an Alexa Fluor 647-labeled tracer from the ATP-binding site of an epitope-tagged kinase by a test compound. Such behavior results in a decrease in time-resolved fluorescence energy transfer signal. In this report, a side-by-side comparison of the LanthaScreen Eu kinase binding assay and the SPR method was performed using inhibitors of focal adhesion kinase. The two methods yielded comparable results and identified compounds with time-dependent inhibition and relatively slow dissociation.

Glucocorticoid hormones and serotonin (5‐HT) are strongly associated with the development and treatment of depression, respectively. Glucocorticoids regulate the function of serotonergic neurons in the dorsal raphe nucleus (DR), which are the major source of 5‐HT to the forebrain. DR 5‐HT neurons are electrophysiologically heterogeneous, though whether this phenotypic variation aligns with specific brain functions or neuropsychiatric disease states is largely unknown. The goal of this work was to determine if chronic exogenous glucocorticoid administration differentially affects the electrophysiological profile of DR neurons implicated in the regulation of emotion versus visual sensation by comparing properties of cells projecting to medial prefrontal cortex (mPFC) versus lateral geniculate nucleus (LGN). Following retrograde tracer injection into mPFC or LGN, male Sprague‐Dawley rats received daily injections of corticosterone (CORT) for 21 days, after which whole‐cell patch clamp recordings were made from retrogradely labeled DR neurons. CORT‐treatment significantly increased the action potential half‐width of LGN‐projecting DR neurons, but did not significantly affect the firing frequency or excitatory postsynaptic currents of these cells. CORT‐treatment significantly reduced the input resistance, evoked firing frequency, and spontaneous excitatory postsynaptic current frequency of mPFC‐projecting DR neurons, indicating a concurrent reduction of both intrinsic excitability and excitatory drive. Our results suggest that the serotonergic regulation of cognitive and emotional networks in the mPFC may be more sensitive to the effects of glucocorticoid excess than visual sensory circuits in the LGN and that reduced 5‐HT transmission in the mPFC may underlie the association between glucocorticoid excess and depression.

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