Inhibition of [Ca2+]i transients in rat adrenal chromaffin cells by neuropeptide Y: role for a cGMP-dependent protein kinase-activated K+ conductance

Neuropeptide Y is expressed by osteocytes and can inhibit osteoblastic activity

Osteocytes are the most abundant osteoblast lineage cells within the bone matrix. They respond to mechanical stimulation and can participate in the release of regulatory proteins that can modulate the activity of other bone cells. We hypothesize that neuropeptide Y (NPY), a neurotransmitter with regulatory functions in bone formation, is produced by osteocytes and can affect osteoblast activity. To study the expression of NPY by the osteoblast lineage cells, we utilized transgenic mouse models in which we can identify and isolate populations of osteoblasts and osteocytes. The Col2.3GFP transgene is active in osteoblasts and osteocytes, while the DMP1 promoter drives green fluorescent protein (GFP) expression in osteocytes. Real-time PCR analysis of RNA from the isolated populations of cells derived from neonatal calvaria showed higher NPY mRNA in the preosteocytes/osteocytes fraction compared to osteoblasts. NPY immunostaining confirmed the strong expression of NPY in osteocytes (DMP1GFP(+)), and lower levels in osteoblasts. In addition, the presence of NPY receptor Y1 mRNA was detected in cavaria and long bone, as well as in primary calvarial osteoblast cultures, whereas Y2 mRNA was restricted to the brain. Furthermore, NPY expression was reduced by 30-40% in primary calvarial cultures when subjected to fluid shear stress. In addition, treatment of mouse calvarial osteoblasts with exogenous NPY showed a reduction in the levels of intracellular cAMP and markers of osteoblast differentiation (osteocalcin, BSP, and DMP1). These results highlight the potential regulation of osteoblast lineage differentiation by local NPY signaling.

cGMP-induced presynaptic depression and postsynaptic facilitation at glutamatergic synapses in visual cortex

The mechanisms by which the intracellular messenger cGMP can modulate synaptic efficacy remain poorly understood. Here we report that cGMP, acting through cGMP-dependent protein kinase (PKG), has multiple rapid and reversible effects on synaptic transmission in slices and cultures of rodent visual cortex. Extracellular application of the membrane permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and the PKG specific activator beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate sp-isomer (Sp-8-Br-PET-cGMPS) reduced stimulus-evoked EPSPs in slices. In cortical cultures, both analogs reduced the frequency of spontaneous EPSCs, but not their amplitude. In both slices and cultures, intracellular perfusion of the postsynaptic neurons with a pseudosubstrate inhibitory peptide specific for PKG had no effect on the reduction in EPSPs and EPSCs, indicating that the inhibition occurred at presynaptic sites. Whole-cell calcium currents in cultured cortical neurons were also reduced by both analogs, which may account for the effect on synaptic release. To determine whether cGMP was also acting at postsynaptic sites, we applied exogenous kainate/AMPA and NMDA to the recorded cells directly. cGMP and its analogs showed little effect on the postsynaptic kainate/AMPA responses but produced a dramatic enhancement of NMDA responses. cGMP-induced NMDA potentiation was prevented by the specific PKG inhibitory peptide infused into the postsynaptic cell. In summary, cGMP, acting through PKG, had depressive presynaptic and facilitatory postsynaptic actions at excitatory synapses in the visual cortex. We suggest that these opposing actions may be useful for altering the balance of synaptic inputs to cortical neurons in ways that enhance signals important for synaptic facilitation and neuronal plasticity.

Neuropeptide Y receptors differentially modulate G-protein-activated inwardly rectifying K+ channels and high-voltage-activated Ca2+ channels in rat thalamic neurons

1. Using whole-cell patch-clamp recordings, infrared videomicroscopy and fast focal solution exchange methods, the actions of neuropeptide Y (NPY) were examined in thalamic slices of postnatal (10-16 days) rats. 2. NPY activated a K+-selective current in neurons of the thalamic reticular nucleus (RT; 20/29 neurons) and ventral basal complex (VB; 19/25 neurons). The currents in both nuclei had activation and deactivation kinetics that were very similar to those of GABAB receptor-induced currents, were totally blocked by 0.1 mM Ba2+ and showed voltage-dependent relaxation. These properties indicate that the NPY-sensitive K+ current is mediated by G-protein-activated, inwardly rectifying K+ (GIRK) channels. 3. In RT neurons, NPY application reversibly reduced high-voltage-activated (HVA) currents to 33 +/- 5 % (n = 40) of the control level but did not affect the T-type currents. Inhibition of Ca2+ currents was voltage independent and was largely mediated by effects on N- and P/Q-type channels. 4. NPY activation of GIRK channels was mediated via NPY1 receptors, whereas inhibition of N- and P/Q-type Ca2+ channels was mediated by NPY2 receptors. 5. These results show that neuropeptide Y activates K+ channels and simultaneously inhibits HVA Ca2+ channels via different receptor subtypes.

Pre- and post-synaptic muscarinic receptors in thin slices of rat adrenal gland

The effects of activation of muscarinic receptors on chromaffin cells and splanchnic nerve terminals were studied in a rat adrenal slice preparation. In chromaffin cells, muscarine induced a transient hyperpolarization followed by a depolarization associated with cell spiking. The hyperpolarization was blocked by charybdotoxin (1 microM) and tetraethylammonium chloride (TEA, 1 mM), but was not affected by 200 microM Cd2+ or removal of external Ca2+, consistent with activation of BK channels. This would follow internal Ca2+ mobilization, as shown by Ca2+ imaging with fura-2 on isolated chromaffin cells in culture. Under voltage-clamp, outward BK currents were insensitive to MT3 toxin, a specific muscarinic m4 receptor antagonist. In contrast, muscarine-induced depolarization was due to a m4 receptor-mediated inward current blocked by MT3 toxin. This current was permeable to cations and was associated with Ca2+ entry and subsequently, Ca2+-induced Ca2+ release. Finally, both muscarine (25 microM) and oxotremorine (10 microM) decreased the amplitude and frequency of KCI-evoked excitatory postsynaptic currents, without affecting quantal size, consistent with a presynaptic inhibitory effect. Taken together, our data suggest that activation of m4 and probably m3 muscarinic receptors results in a strong, long-lasting excitation of chromaffin cells, as well as an uncoupling of synaptic inputs onto these cells.

Vasoactive intestinal peptide potentiates and directly stimulates catecholamine secretion from rat adrenal chromaffin cells

The actions of vasoactive intestinal polypeptide (VIP) on catecholamine secretion and changes in [Ca2+]i in single rat chromaffin cells were studied using amperometry and Indo-1. Application of VIP prior to acetylcholine (ACh) or co-application of VIP and ACh enhanced secretion by 94% and 153% respectively, compared to ACh alone. [Ca2+]i was increased by 17% when VIP was preapplied and by 73% upon co-application. Exposure to VIP before stimulation with 60 mM K+ enhanced secretion by 68%, but not [Ca2+]i. VIP application prior to DMPP and nicotine had no effect on [Ca2+]i, but increased [Ca2+]i signals to muscarine by 18%. VIP co-application potentiated only [Ca2+]i responses to muscarine, by 28%. The effect of VIP on muscarine-induced [Ca2+]i signals was mimicked by 8-Br-cAMP, and both were blocked by H-89, a protein kinase A inhibitor. Long-lasting increases in secretion accompanied by a sustained rise in [Ca2+]i to VIP alone were seen in 55% of cells. Removal of Ca2+ or addition of La3+ inhibited both responses, while L-, N- and P-type Ca2+ channel blockers were ineffective. SK&F 96365 inhibited VIP-induced secretion completely and rises in [Ca2+]i by 75%. Neither 8-Br-cAMP nor 8-Br-cGMP evoked responses similar to VIP alone. Thus in rat chromaffin cells, VIP acts both directly as a neurotransmitter in provoking sustained catecholamine secretion in a cAMP-independent manner, and also by enhancing ACh-induced secretion, via a cAMP-dependent action involving muscarinic receptors.