Probes for INS

Acetylcholine is a potent neuromodulator capable of modifying patterns of acoustic information flow. In auditory cortex, cholinergic systems have been shown to increase salience/gain while suppressing extraneous information. However, the mechanism by which cholinergic circuits shape signal processing in the auditory thalamus (medial geniculate body, MGB) is poorly understood. The present study, in male Fischer Brown Norway rats, seeks to determine the location and function of presynaptic neuronal nicotinic acetylcholine receptors (nAChRs) at the major inputs to MGB and characterize how nAChRs change during aging. In vitro electrophysiological/optogenetic methods were used to examine responses of MGB neurons following activation of nAChRs during a paired-pulse paradigm. Presynaptic nAChR activation increased responses evoked by stimulation of excitatory corticothalamic and inhibitory tectothalamic terminals. Conversely, nAChR activation appeared to have little effect on evoked responses from inhibitory thalamic reticular nucleus and excitatory tectothalamic terminals. In situ hybridization data showed nAChR subunit transcripts in GABAergic inferior colliculus neurons and glutamatergic auditory cortical neurons supporting the present slice findings. Responses to nAChR activation at excitatory corticothalamic and inhibitory tectothalamic inputs were diminished by aging. These findings suggest that cholinergic input to the MGB increases the strength of tectothalamic inhibitory projections, potentially improving signal-to-noise ratio and signal detection while increasing corticothalamic gain, which may facilitate top-down identification of stimulus identity. These mechanisms appear negatively affected by aging, potentially diminishing speech perception in noisy environments. Cholinergic inputs to the MGB appear to maximize sensory processing by adjusting both top-down and bottom-up mechanisms in conditions of attention and arousal.Significance StatementThe pedunculopontine tegmental nucleus (PPTg) is the source of cholinergic innervation for sensory thalamus and is a critical part of an ascending arousal system which controls the firing mode of thalamic cells based on attentional demand. The present study describes the location and impact of aging on presynaptic neuronal nicotinic receptors (nAChRs) within the circuitry of the auditory thalamus (medial geniculate body; MGB). We show that nAChRs are located on ascending inhibitory and descending excitatory presynaptic inputs onto MGB neurons, likely selectively increasing gain and improving temporal clarity. In addition, we show that aging has a deleterious effect on nAChR efficacy. Cholinergic dysfunction at the level of MGB may negatively impact speech understanding in the elderly population.

Central leptin action is sufficient to restore euglycemia in insulinopenic type 1 diabetes (T1D); however, the underlying mechanism remains poorly understood. To examine the role of intracellular STAT3 pathways, we used LepRs/s mice with disrupted leptin-pSTAT3 signaling to test the effect of central leptin on euglycemia restoration. These mice developed STZ-induced T1D, which was surprisingly not associated with hyperglucagonemia, a typical manifestation in T1D. Further, leptin action on euglycemia restoration was abrogated in these mice, which was associated with refractory hypercorticosteronemia. To examine the role of fast-acting neurotransmitters glutamate and γ-aminobutyric acid (GABA), two major neurotransmitters in the brain, from LepR neurons, we used mice with disrupted release of glutamate, GABA or both from LepR neurons. Surprisingly, all mice responded normally to leptin-mediated euglycemia restoration, which was associated with expected correction from hyperglucagonemia and hyperphagia. In contrast, mice with loss of glutamate and GABA appeared to develop an additive obesity effect over those with loss of single neurotransmitter release. Thus, our study reveals that STAT3 signaling, but not fast-acting neurotransmitter release, is required for leptin action on euglycemia restoration, and that hyperglucagonemia is not required for T1D.

Stimuli that elicit itch are detected by sensory neurons that innervate the skin. This information is processed by the spinal cord; however, the way in which this occurs is still poorly understood. Here we investigated the neuronal pathways for itch neurotransmission, particularly the contribution of the neuropeptide somatostatin. We find that in the periphery, somatostatin is exclusively expressed in Nppb+ neurons, and we demonstrate that Nppb+somatostatin+ cells function as pruriceptors. Employing chemogenetics, pharmacology and cell-specific ablation methods, we demonstrate that somatostatin potentiates itch by inhibiting inhibitory dynorphin neurons, which results in disinhibition of GRPR+neurons. Furthermore, elimination of somatostatin from primary afferents and/or from spinal interneurons demonstrates differential involvement of the peptide released from these sources in itch and pain. Our results define the neural circuit underlying somatostatin-induced itch and characterize a contrasting antinociceptive role for the peptide.

Cellular interactions between delta and mu opioid receptors (DORs and MORs), including heteromerization, are thought to regulate opioid analgesia. However, the identity of the nociceptive neurons in which such interactions could occur in vivo remains elusive. Here we show that DOR-MOR co-expression is limited to small populations of excitatory interneurons and projection neurons in the spinal cord dorsal horn and unexpectedly predominates in ventral horn motor circuits. Similarly, DOR-MOR co-expression is rare in parabrachial, amygdalar, and cortical brain regions processing nociceptive information. We further demonstrate that in the discrete DOR-MOR co-expressing nociceptive neurons, the two receptors internalize and function independently. Finally, conditional knockout experiments revealed that DORs selectively regulate mechanical pain by controlling the excitability of somatostatin-positive dorsal horn interneurons. Collectively, our results illuminate the functional organization of DORs and MORs in CNS pain circuits and reappraise the importance of DOR-MOR cellular interactions for developing novel opioid analgesics.