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The nucleus tractus solitarius (NTS) receives dense terminations from cranial visceral afferents

The hypothalamus coordinates autonomic responses in part through arginine vasopressin (AVP) released in medial nucleus tractus solitarius (NTS).

Although the central terminals of cranial visceral afferents express vanilloid receptor 1 (VR1), little is known about their functional properties at this first synapse within the nucleus tractus solitarius (NTS). Here, we examined whether VR1 modulates afferent synaptic transmission. In horizontal brainstem slices, solitary tract (ST) activation evoked EPSCs. Monosynaptic EPSCs had low synaptic jitter (SD of latency to successive shocks) averaging 84.03 +/- 3.74 microsec (n = 72) and were completely blocked by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX). Sustained exposure to the VR1 agonist capsaicin (CAP; 100 nm) blocked ST EPSCs (CAP-sensitive) in some neurons but not others (CAP-resistant). CAP-sensitive EPSCs had longer latencies than CAP-resistant EPSCs (4.65 +/- 0.27 msec, n = 48 vs 3.53 +/- 0.28 msec, n = 24, respectively; p = 0.011), but they had similar jitter. CAP evoked two transient responses in CAP-sensitive neurons: a rapidly developing inward current (I(cap)) (108.1 +/- 22.9 pA; n = 21) and an increase in spontaneous synaptic activity. After 3-5 min in CAP, I(cap) subsided and ST EPSCs disappeared. NBQX completely blocked I(cap). The VR1 antagonist capsazepine (10-20 microm) attenuated CAP responses. Anatomically, second-order NTS neurons were identified by 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide transported from the cervical aortic depressor nerve (ADN) to stain central terminals. Neurons with fluorescent ADN contacts had CAP-sensitive EPSCs (n = 5) with latencies and jitter similar to those of unlabeled monosynaptic neurons. Thus, consistent with presynaptic VR1 localization, CAP selectively activates a subset of ST axons to release glutamate that acts on non-NMDA receptors. Because the CAP sensitivity of cranial afferents is exclusively associated with unmyelinated axons, VR1 identifies C-fiber afferent pathways within the brainstem.

. Cranial visceral afferents enter the brain at the solitary tract nucleus (NTS). GABAergic neurons are scattered throughout the NTS, but their relation to solitary tract (ST) afferent pathways is imprecisely known. We hypothesized that most GABAergic NTS neurons would be connected only indirectly to the ST. We identified GABAergic neurons in brain stem horizontal slices using transgenic mice in which enhanced green fluorescent protein (EGFP) expression was linked to glutamic acid decarboxylase expression (GAD ϩ ). Finely graded electrical shocks to ST recruit STsynchronized synaptic events with all-or-none thresholds and individual waveforms did not change with greater suprathreshold intensities-evidence consistent with initiation by single afferent axons. Most (ϳ70%) GAD ϩ neurons received ST-evoked excitatory postsynaptic currents (EPSCs) that had minimally variant latencies (jitter, SD of latency Ͻ200 s) and waveforms consistent with single, direct ST connections (i.e., monosynaptic). Increasing stimulus intensity evoked additional ST-synchronized synaptic responses with jitters Ͼ200 s including inhibitory postsynaptic currents (IPSCs), indicating indirect connections (polysynaptic). Shocks of suprathreshold intensity delivered adjacent (50 -300 m) to the ST failed to excite non-ST inputs to second-order neurons, suggesting a paucity of axons passing near to ST that connected to these neurons. Despite expectations, we found similar ST synaptic patterns in GAD ϩ and unlabeled neurons. Generally, ST information that arrived indirectly had small amplitudes (EPSCs and IPSCs) and frequency-dependent failures that reached Ͼ50% for IPSCs to bursts of stimuli. This ST afferent pathway organization is strongly use-dependent-a property that may tune signal propagation within and beyond NTS.

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The balance between excitation and inhibition dictates central integration. Glutamatergic and GABAergic neurotransmission dominate this process. Cranial primary afferents enter the brainstem to release glutamate (Glu) onto second-order neurons within the caudal nucleus tractus solitarius (NTS) to initiate autonomic reflexes. The simplest pathways for these reflexes contain as few as two central neurons, but display robust frequency-dependent behavior. Within NTS, multiple metabotropic Glu receptors (mGluRs) are present, but their roles are poorly understood. Using synaptically discriminated second-order NTS neurons in brainstem slices and mechanically dissociated NTS neurons with intact boutons, we show that Glu differentially controls GABA release via distinct presynaptic mGluRs. In second-order NTS neurons recorded in slices, activation of primary afferents at frequencies as low as 10 shocks per second released sufficient Glu to alter rates of spontaneous IPSCs (sIPSCs). In both approaches, group I mGluRs increased GABA release in some neurons, but, on different neurons, group II and group III mGluRs decreased the sIPSC rate. mGluR actions were remarkably rapid, with onset and reversal beginning within 100 msec. In all cases, mGluR actions were exclusively presynaptic, and mGluRs did not alter postsynaptic properties in second-order neurons in either slices or isolated neurons. Tests with capsaicin and ␣␤-methylene ATP suggest that myelinated and unmyelinated afferent pathways engage both mGluR-GABA mechanisms. Afferent Glu spillover provides heterosynaptic cross talk with GABAergic inhibition in NTS. This process may critically shape the dynamic character and use dependence for cranial afferent transmission at the first stage of autonomic reflexes.

Fisetin is a natural flavonoid from fruits and vegetables that exhibits antioxidant, neurotrophic, anti-inflammatory, and anti-cancer effects in various disease models. Up-regulation of heme oxygenase-1 (HO-1) expression protects against oxidative stress-induced cell death, and therefore, plays a crucial role in cytoprotection in a variety of pathological models. In the present study, we investigated the effect of fisetin on the up-regulation of HO-1 in human umbilical vein endothelial cells (HUVECs). Small interfering RNA and pharmacological inhibitors of PKC-δ and p38 MAPK attenuated HO-1 induction in fisetin-stimulated HUVECs. Fisetin treatment resulted in significantly increased NF-E2-related factor 2 (Nrf2) nuclear translocation, and antioxidant response element (ARE)-luciferase activity, leading to up-regulation of HO-1 expression. In addition, fisetin pretreatment reduced hydrogen peroxide (H(2)O(2))-induced cell death, and this effect was reversed by ZnPP, an inhibitor of HO-1. In summary, these findings suggest that induction of HO-1 expression via Nrf2 activation may contribute to the cytoprotection exerted by fisetin against H(2)O(2) -induced oxidative stress in HUVECs.