Quantitative differences between kinetic properties of Na(+) currents in postganglionic sympathetic neurones projecting to muscular and cutaneous effectors

p75 and TrkA signaling regulates sympathetic neuronal firing patterns via differential modulation of voltage-gated currents

Neurotrophins such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) act through the tropomyosin-related receptor tyrosine kinases (Trk) and the pan-neurotrophin receptor (p75) to regulate complex developmental and functional properties of neurons. While NGF activates both receptor types in sympathetic neurons, differential signaling through TrkA and p75 can result in widely divergent functional outputs for neuronal survival, growth, and synaptic function. Here we show that TrkA and p75 signaling pathways have opposing effects on the firing properties of sympathetic neurons, and define a mechanism whereby the relative level of signaling through these two receptors sets firing patterns via coordinate regulation of a set of ionic currents. We show that signaling through the p75 pathway causes sympathetic neurons to fire in a phasic pattern showing marked accommodation. Signaling through the NGF-specific TrkA, on the other hand, causes cells to fire tonically. Neurons switch rapidly between firing patterns, on the order of minutes to hours. We show that changes in firing patterns are caused by neurotrophin-dependent regulation of at least four voltage-gated currents: the sodium current and the M-type, delayed rectifier, and calcium-dependent potassium currents. Neurotrophin release, and thus receptor activation, varies among somatic tissues and physiological state. Thus, these data suggest that target-derived neurotrophins may be an important determinant of the characteristic electrical properties of sympathetic neurons and therefore regulate the functional output of the sympathetic nervous system.

Early diabetic neuropathy: Triggers and mechanisms

Peripheral neuropathy, and specifically distal peripheral neuropathy (DPN), is one of the most frequent and troublesome complications of diabetes mellitus. It is the major reason for morbidity and mortality among diabetic patients. It is also frequently associated with debilitating pain. Unfortunately, our knowledge of the natural history and pathogenesis of this disease remains limited. For a long time hyperglycemia was viewed as a major, if not the sole factor, responsible for all symptomatic presentations of DPN. Multiple clinical observations and animal studies supported this view. The control of blood glucose as an obligatory step of therapy to delay or reverse DPN is no longer an arguable issue. However, while supporting evidence for the glycemic hypothesis has accumulated, multiple controversies accumulated as well. It is obvious now that DPN cannot be fully understood without considering factors besides hyperglycemia. Some symptoms of DPN may develop with little, if any, correlation with the glycemic status of a patient. It is also clear that identification of these putative non-glycemic mechanisms of DPN is of utmost importance for our understanding of failures with existing treatments and for the development of new approaches for diagnosis and therapy of DPN. In this work we will review the strengths and weaknesses of the glycemic hypothesis, focusing on clinical and animal data and on the pathogenesis of early stages and triggers of DPN other than hyperglycemia.

Opioid mu receptor activation inhibits sodium currents in prefrontal cortical neurons via a protein kinase A- and C-dependent mechanism

Opioid transmission in the medial prefrontal cortex is involved in mood regulation and is altered by drug dependency. However, the mechanism by which ionic channels in cortical neurons are controlled by mu opioid receptors has not been elucidated. In this study, the effect of mu opioid receptor activation on voltage-dependent Na(+) currents was assessed in medial prefrontal cortical neurons. In 66 out of 98 nonpyramidal neurons, the application of 1 microM of DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-OL]-enkephalin), a specific mu receptor agonist, caused a decrease in the Na(+) current amplitude to approximately 79% of that observed in controls (half blocking concentration = 0.094 microM). Moreover, DAMGO decreased the maximum current activation rate, prolonged its time-dependent inactivation, and shifted the half inactivation voltage from -63.4 mV to -71.5 mV. DAMGO prolonged the time constant of recovery from inactivation from 5.4 ms to 7.4 ms. The DAMGO-evoked inhibition of Na(+) current was attenuated when GDP-beta-S (0.4 mM, Guanosine 5-[beta-thio]diphosphate trilithium salt) was included in the intracellular solution. Inhibitors of kinase A and C greatly attenuated the DAMGO-induced inhibition, while adenylyl cyclase and kinase C activators mirrored the DAMGO inhibitory effect. Na(+) currents in pyramidal neurons were insensitive to DAMGO. We conclude that the activation of mu opioid receptors inhibits the voltage-dependent Na(+) currents expressed in nonpyramidal neurons of the medial prefrontal cortex, and that kinases A and C are involved in this inhibitory pathway.

Effects of ATP and GTP on voltage-gated K+ currents in glandular and muscular sympathetic neurons

This study assesses the effects of ATP and GTP on the kinetic properties of voltage-gated K+ currents in anatomically identified postganglionic sympathetic neurons innervating the submandibular gland and the masseter muscle in rats. Three types of K+ currents were isolated: the I(Af) steady-state inactivating at more hyperpolarized potentials, I(As) steady-state inactivating at less hyperpolarized potentials than I(Af) and the I(K) current independent of membrane potential. The kinetic properties of these currents were tested in neurons with ATP (4 mM) and GTP (0.5 mM) or without ATP and GTP in the intracellular solution. In glandular and muscular neurons in the absence of ATP and GTP in the intracellular solution, the current density of I(Af) was significantly larger (142 pA/pF and 166 pA/pF, respectively) comparing to cells with ATP and GTP (96 pA/pF and 100 pA/pF, respectively). The I(As) was larger only in glandular neurons (52 pA/pF vs. 37 pA/pF).Conversely, I(K) current density was smaller in glandular and muscular neurons without ATP and GTP (17 pA/pF and 31 pA/pF, respectively) comparing to cells with ATP and GTP (57 pA/pF and 58 pA/pF, respectively). In glandular (15.5 nA/ms vs. 6.9 nA/ms) and muscular (10.9 nA/ms vs. 7.5 nA/ms) neurons, the I(Af) activated faster in the absence of ATP and GTP. Half inactivation voltage of I(Af) in glandular (-110.0 mV vs. -119.7 mV) and muscular (-108.4 vs. -117.3 mV) neurons was shifted towards depolarization in the absence of ATP and GTP. We suggest that the kinetic properties of K+ currents in glandular and muscular sympathetic neurons change markedly in the absence of ATP and GTP in the cytoplasm. Effectiveness of steady-state inactivated currents (I(Af) and I(AS)) increased, while effectiveness of steady-state noninactivated currents decreased in the absence of ATP and GTP. The effects were more pronounced in glandular than in muscular neurons.

Kinetic properties of voltage-gated Na+ currents in rat muscular sympathetic neurons with and without adenosine triphosphate and guanosine triphosphate in intracellular solution

Voltage-gated Na+ currents were recorded from anatomically identified postganglionic muscular sympathetic neurons without and with ATP and GTP in the intracellular solution. The main findings of the study were that cells without ATP and GTP in the intracellular solution express a higher amplitude and greater density of voltage-gated Na+ current, and their Na+ current activates faster and also inactivates faster time dependently. The current is also steady-state inactivated to a lesser degree and recovers from inactivation more slowly in cells without added ATP and GTP. These findings suggest that the presence of ATP and GTP, substrates for channel phosphorylation, changes the kinetic properties of Na+ currents.

Postdecentralization plasticity of voltage-gated K+ currents in glandular sympathetic neurons in rats

This paper presents the kinetic and pharmacological properties of voltage-gated K(+) currents in anatomically identified glandular postganglionic sympathetic neurons isolated from the superior cervical ganglia in rats. The neurons were labelled by injecting the fluorescent tracer Fast Blue into the submandibular gland. The first group of neurons remained intact, i.e. innervated by the preganglionic axons until the day of current recordings (control neurons). The second group of neurons was denervated by severing the superior cervical trunk 4-6 weeks prior to current recordings (decentralized neurons). In every control and decentralized neuron three categories of voltage-dependent K(+) currents were found. (i) The I(Af) K(+) current, steady state, inactivated at hyperpolarized membrane potentials. This current was fast activated and fast time-dependently inactivated, insensitive to TEA and partially depressed by 4-AP. (ii) The I(As) K(+) current, which was steady-state inactivated at less hyperpolarized membrane potentials than I(Af). The current activation and time-dependent inactivation kinetics were slower than those of I(Af). I(As) was blocked by TEA and partially inhibited by 4-AP. (iii) The IK K(+) current did not undergo steady-state inactivation. In decentralized compared to control neurons the maximum I(Af) K(+) current density (at +50 mV) increased from 116.9 +/- 8.2 to 189.0 +/- 11.5 pA/pF, the 10-90% current rise time decreased from 2.3 to 0.7 ms and the recovery from inactivation was faster. Similarly, in decentralized compared to control neurons the maximum I(As) K(+) current density (at +50 mV) increased from 49.9 +/- 3.5 to 74.3 +/- 5.0 pA/pF, the 10-90% current rise time shortened from 29 to 16 ms and the recovery from inactivation of the current was also faster. The maximum density (at +50 mV) of I(K) in decentralized compared to control neurons decreased from 76.6 +/- 3.9 to 60.7 +/- 6.3 pA/pF. We suggest that the upregulation of voltage-gated time-dependently-inactivated K(+) currents and their faster recovery from inactivation serve to restrain the activity of glandular sympathetic neurons after decentralization.

Postdecentralization plasticity of voltage-gated Na+ currents in rat glandular sympathetic neurons

The kinetic properties of voltage-gated Na(+) currents in two groups of glandular postganglionic sympathetic neurons were assessed. The first group of neurons remained innervated by preganglionic axons until the day of current recordings, while the second--decentralized 4 weeks prior to recordings. An increase of maximum current amplitude and density was noted in decentralized neurons. Na(+) currents activated and time-dependently inactivated more slowly in decentralized than in control neurons. Furthermore, after decentralization the currents steady-state inactivated at less hyperpolarized potentials as well as reactivated faster from inactivation. We conclude that the Na(+) currents in decentralized postganglionic glandular sympathetic neurons undergo up-regulation.

Kinetic and pharmacological properties of Ca(2+) currents in postganglionic sympathetic neurones projecting to muscular and cutaneous effectors

Voltage-gated Ca(2+) channels are expressed in neurones and greatly influence neuronal activity by activating Ca(2+)-dependent K(+) channels. The whole cell patch-clamp technique was used to compare the kinetic and pharmacological properties of voltage-dependent Ca(2+) currents in two groups of sympathetic neurones identified by the fluorescent tracer Fast Blue: putative muscular sympathetic neurones (MSN) and putative cutaneous sympathetic neurones (CSN). The tracer was injected into the muscular part of the diaphragm (to mark MSN) and into the skin of the ear (to mark CSN). The capacitance of MSN (23.0 pF) was larger than the capacitance of CSN (12.6 pF). The maximum current in MSN (1.3 nA) was also larger than in CSN (0.93 nA). However, the current density was larger in CSN (77. 3 pA/pF) than in MSN (57.7 pA/pF) and the current activation rate was faster in CSN (0.27 nA/ms) than in MSN (0.19 nA/ms). V(1/2) and slope factors of activation and inactivation were not significantly different for MSN and CSN. The majority of Ca(2+) current was available for activation in both categories of neurones at resting membrane potential. Ca(2+) currents in MSN and CSN were blocked by nifedipine (7.0 and 3.6%, respectively), omega-Agatoxin-IVA (23.0 and 25.6%, respectively) and omega-conotoxin-GVIA (67.0 and 65.1%, respectively). We found that CSN are twice as small, have higher Ca(2+) current density and their Ca(2+) activation rate is faster in comparison to MSN. Such properties may lead to faster rise of Ca(2+) concentration in the cytoplasm of the CSN comparing to MSN and more effectively dampen their activity due to more effective activation of Ca(2+)-dependent K(+) current. Both kinds of neurones express high proportion of N and P/Q Ca(2+) current.