Central synapses spontaneously release neurotransmitter at low rates. In the brainstem, cranial visceral afferent terminals in caudal solitary tract nucleus (NTS) display pronounced, activity-dependent, asynchronous release of glutamate and this extra release depends on TRPV1 receptors (TRPV1ϩ). Asynchronous release is absent for afferents lacking TRPV1 (TRPV1Ϫ) and resting EPSC frequency was greater in TRPV1ϩ. Here, we studied this basal activity difference by assessing thermal sensitivity of spontaneous and miniature synaptic events in TRPV1ϩ and TRPV1Ϫ second-order NTS neurons. The spontaneous EPSC rate decreased when temperature was decreased, increased steeply between 30 and 42°C only in TRPV1ϩ neurons, and was calcium-dependent. TRPV1-specific antagonist SB366791, but not TTX, strongly attenuated thermal responses. Temperature changes failed to alter EPSC frequency in TRPV1Ϫ neurons. EPSC amplitudes and decay kinetics changed little with temperature. IPSCs in these second-order NTS neurons were unaltered by temperature. Such results suggest that activated, presynaptic TRPV1ϩ receptors trigger continuous resting release of glutamate vesicles at physiological temperatures only in capsaicin-responsive terminals. In mechanically isolated individual neurons harvested from medial NTS, increases in temperature increased the rate of glutamate release only in TRPV1ϩ neurons, whereas IPSC rates were unaffected. Cadmium failed to block thermal increases in glutamate release, suggesting that calcium entry through TRPV1 channels may trigger glutamate release independently of voltage-activated calcium channels. Together, our findings indicate a new form of afferent signaling in which TRPV1 channels within central terminals of peripheral afferents tonically generate glutamate release in NTS at 37°C in the absence of afferent action potentials.

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By searching the expressed sequence tag (EST) database, we identified partial cDNA sequences encoding a polypeptide with significant sequence identity to the human CC chemokine macrophage-inflammatory protein-1 alpha (MIP-1 alpha)/LD78 alpha. We determined the complete cDNA sequence that contained a reading frame of 89 amino acids with 61% identity to human MIP-1 alpha/LD78 alpha. The mRNA was expressed constitutively at high levels in human lung and at low levels in some lymphoid tissues. Furthermore, the mRNA was strongly induced in several human cell lines, including monocytic U937 cells, by PMA. From these results, we designated this novel CC chemokine as PARC from pulmonary and activation-regulated chemokine. In situ hybridization analyses showed that alveolar macrophages, follicular dendritic cells in the germinal centers of regional lymph nodes, and peripheral blood monocytes stimulated with LPS express PARC mRNA. Using the human CC chemokine yeast artificial chromosome contig that we constructed recently, we mapped the PARC gene (SCYA18) within one of the two subregions of the CC chemokine gene cluster at chromosome 17q11.2. To investigate its biologic activity, the PARC protein was expressed in insect cells. PARC was chemotactic for both activated (CD3+) T cells and nonactivated (CD14-) lymphocytes, but not for monocytes or granulocytes. Binding analysis using PARC fused with alkaline phosphatase-(His)6 showed the presence of a single class of receptors for PARC on lymphocytes with a Kd of 1.9 nM and 590 sites/cell. Thus, PARC is a novel CC chemokine with a close phylogenic relationship with MIP-1 alpha/LD78 alpha, but with a highly selective activity on lymphocytes.

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The effects of Ca2+, Sr2+, and Ba2+ on spontaneous and evoked glycinergic inhibitory postsynaptic currents (mIPSCs and eIPSCs) were studied using the "synaptic bouton" preparation of rat spinal neurons and conventional whole cell recording under voltage-clamp conditions. In response to application of Ca2+-free solution, the frequency of mIPSC initially rapidly decreased to 40 approximately 50% of control followed by a gradual further decline in mIPSC frequency to approximately 30% of control. Once mIPSC frequency had significantly decreased in Ca2+-free solution, application of Ca2+, Sr2+, or Ba2+ increased mIPSC frequency. The rank order of effect in restoring mIPSCs was Ba2+>>Ca2+>Sr2+. Moreover, the application of excess external [K+]o solution (30 mM) containing Sr2+ or Ba2+ after 2 h in Ca2+-free solution also increased mIPSC frequency in the order Sr2+>or==Ba2+>Ca2+. The mean mIPSC amplitude was not affected at all. In contrast, eIPSCs produced by focal stimulation of single boutons were completely abolished in Ca2+-free solution or when Ca2+ was replaced by Sr2+ or Ba2+ (2 mM each). However, eIPSCs were restored in increased concentrations of Sr2+ or Ba2+ (5 mM each). The results show that these divalent cations affect mIPSC and eIPSCs differently and indicate that the mechanisms underlying transmitter release that generates eIPSCs and mIPSC in presynaptic nerve terminals are different. The different mechanisms might be explained by the different sensitivity of synaptotagmin isoforms to Ca2+, Sr2+, and Ba2+.

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