Effect of calcium channel blockers on norepinephrine release and modulation by prejunctional D2 dopamine receptors

Mechanisms of neuromodulatory volume transmission

A wealth of neuromodulatory transmitters regulate synaptic circuits in the brain. Their mode of signaling, often called volume transmission, differs from classical synaptic transmission in important ways. In synaptic transmission, vesicles rapidly fuse in response to action potentials and release their transmitter content. The transmitters are then sensed by nearby receptors on select target cells with minimal delay. Signal transmission is restricted to synaptic contacts and typically occurs within ~1 ms. Volume transmission doesn't rely on synaptic contact sites and is the main mode of monoamines and neuropeptides, important neuromodulators in the brain. It is less precise than synaptic transmission, and the underlying molecular mechanisms and spatiotemporal scales are often not well understood. Here, we review literature on mechanisms of volume transmission and raise scientific questions that should be addressed in the years ahead. We define five domains by which volume transmission systems can differ from synaptic transmission and from one another. These domains are (1) innervation patterns and firing properties, (2) transmitter synthesis and loading into different types of vesicles, (3) architecture and distribution of release sites, (4) transmitter diffusion, degradation, and reuptake, and (5) receptor types and their positioning on target cells. We discuss these five domains for dopamine, a well-studied monoamine, and then compare the literature on dopamine with that on norepinephrine and serotonin. We include assessments of neuropeptide signaling and of central acetylcholine transmission. Through this review, we provide a molecular and cellular framework for volume transmission. This mechanistic knowledge is essential to define how neuromodulatory systems control behavior in health and disease and to understand how they are modulated by medical treatments and by drugs of abuse.

Differential involvement of conotoxin-sensitive mechanisms in neurogenic vasodilatation responses: effects of age

During aging there is a decline in sensory nerve function that is associated with reduced neurogenic inflammation and poor wound repair. The cellular mechanism(s) responsible for this decline in function with age is not well understood. We previously reported that sensory nerves in aged rats release sensory neuropeptides preferentially in response to low-frequency (5 Hz) as compared with higher-frequency (15 Hz) antidromic electrical stimulation, and that low-frequency transcutaneous electrical nerve stimulation accelerates wound healing. The present study investigates possible mechanisms for this preferential response. Using laser Doppler techniques, we have measured changes in blood flow in the base of vacuum-induced blisters induced in the rat hind footpad of young and old animals in response to low-frequency (5 Hz) or high-frequency (15 Hz) electrical stimulation (20 V, 2 ms for 1 minute) of the sciatic nerve. The relative contributions of the sensory neuropeptides, substance P and calcitonin gene-related peptide (CGRP), and of N-type voltage-gated calcium channels to the vascular responses were assessed by using the specific receptor antagonists RP67580, which is 2-(1-imino-2-(2 methoxy phyenyl) ethyl)-7,7 diphenyl-4 perhydroisoindolone-(3aR, 7aR); CGRP(8-37); and omega-conotoxin GVIA (Conus geographus), respectively. The results showed a greater involvement of substance P at high-frequency electrical stimulation and of CGRP at low-frequency stimulation. Our finding that omega-conotoxin-sensitive N-type calcium channel function was preserved with age and was only involved in the vascular response to low-frequency electrical stimulation could explain our previous report demonstrating beneficial effects of low-frequency transcutaneous electrical nerve stimulation to wound repair in aged animals. The current results have important practical implications for improving tissue repair in the aged.

Role of the D2 dopamine receptor in molecular adaptation to chronic hypoxia in PC12 cells

We have previously shown that pheochromocytoma (PC12) cells rapidly depolarize and undergo Ca2+ influx through voltage-dependent Ca2+ channels in response to moderate hypoxia and that intracellular free Ca2+ is modulated by activation of dopamine D2 receptors in this cell type. The present study shows that D2 (quinpirole-mediated) inhibition of a voltage-dependent Ca2+ current (ICa) in PC12 cells is dramatically attenuated after chronic exposure to moderate hypoxia (24 h at 10% O2). Pretreatment of cells with pertussis toxin abolished D2-mediated inhibition of ICa. The D2-induced inhibition of ICa did not depend on protein kinase A (PKA), as it persisted both in the presence of a specific PKA inhibitor (PKI) and in PKA-deficient PC12 cells. Prolonged exposure to hypoxia (24 h) significantly reduced the level of Gi/o alpha immunoreactivity, but did not alter G beta levels. Furthermore, dialysis of recombinant G(o) alpha protein through the patch pipette restored the inhibitory effect of quinpirole in cells chronically exposed to hypoxia. We conclude that the attenuation of the D2-mediated inhibition of ICa by chronic hypoxia is caused by impaired receptor-G protein coupling, due to reduced levels of G(o) alpha protein. This attenuated feedback modulation of ICa by dopamine may allow for a more sustained Ca2+ influx and enhanced cellular excitation during prolonged hypoxia.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Acute vasorelaxant effects of metformin and attenuation by stimulation of sympathetic agonist release

We recently discovered 1) that intravenous injection of the antidiabetic drug metformin in the rat rapidly reduces arterial pressure elevations maintained by the alpha-adrenoceptor agonist phenylephrine (PE) and 2) that direct administration of metformin to isolated rat tail arterial tissue rings rapidly relaxes PE-induced contractions. To further characterize this potential direct vasodilator action, we examined effects of metformin on contractions induced not only by PE but also by norepinephrine (NE) and by nonadrenergic agonists (5-hydroxytryptamine, 5HT; arginine vasopressin, AVP). Also, because the rat tail artery contains abundant adrenergic nerve endings we conducted these tests not only in arterial rings with nerve endings intact but in rings in which they had been removed by pretreatment with 6-hydroxydopamine. In intact rings, metformin at levels from approximately 0.2 to 20 mmol/L rapidly relaxed half-maximal contractions induced by PE and NE similarly and to a markedly greater degree than contractions induced by 5-HT (p<0.05). Metformin did not relax AVP-induced contractions. In addition, removal of adrenergic nerve endings facilitated metformin's relaxant effects (p<0.05). Thus, the acute vasodilator action of metformin appears 1) to be selectively more powerful on arterial smooth muscle contractions induced adrenergically versus nonadrenergically and 2) to be buffered by a possible metformin-induced release of endogenous NE from adrenergic nerve endings. Such results were not seen during relaxation produced by either the calcium channel inhibitor nifedipine or the nitrovasodilator nitroprusside suggesting that metformin's effects are mediated by other mechanisms.

Rat tail artery norepinephrine release: age and effect of mitochondrial blockade

An age-related increase in stimulation-evoked fractional norepinephrine release has been demonstrated in tail artery of Fischer-344 rats from 6 to 20 months of age. Previous studies have ruled out alterations in function of uptake mechanisms, metabolism, or feedback via prejunctional alpha-2 adrenoceptors, suggesting that an age-related decline in calcium homeostasis may be responsible. To test this possibility, stimulation-evoked fractional norepinephrine release from perivascular adrenergic nerves of rat tail arteries in the presence of a blocker of mitochondrial calcium uptake, dinitrophenol (100 microM), was measured using HPLC with electrochemical detection. When the mitochondrial proton gradient was dissipated by dinitrophenol, tail arteries from 20-month-old animals showed a marked increase in stimulation-evoked norepinephrine release whereas arteries from 6-month-old animals were not affected. One hypothesis is that the inhibition of mitochondrial calcium uptake in the adrenergic nerves from older animals resulted in an elevated intracellular calcium level. In agreement with this idea, when extracellular calcium was raised from 1.6 to 5 mM, stimulation-evoked norepinephrine release was increased in the 6-month tail arteries in the presence of dinitrophenol. These data suggest that there may be a decline in calcium regulation in older nerves leading to increased norepinephrine release with advancing age.