Modulation of ACh-induced currents in rat adrenal chromaffin cells by ligands of alpha2 adrenergic and imidazoline receptors

Alpha2-adrenoceptors in adrenomedullary chromaffin cells: functional role and pathophysiological implications

Chromaffin cells from the adrenal medulla participate in stress responses by releasing catecholamines into the bloodstream. Main control of adrenal catecholamine secretion is exerted both neurally (by the splanchnic nerve fibers) and humorally (by corticosteroids, circulating noradrenaline, etc.). It should be noted, however, that secretory products themselves (catecholamines, ATP, opioids, ascorbic acid, chromogranins) could also influence the secretory response in an autocrine/paracrine manner. This form of control is activity-dependent and can be either inhibitory or excitatory. Among the inhibitory influences, it stands out the one mediated by α₂-adrenergic autoreceptors activated by released catecholamines. α₂-adrenoceptors are G protein-coupled receptors capable to inhibit exocytotic secretion through a direct interaction of Gβγ subunits with voltage-gated Ca²⁺ channels. Interestingly, upon intense and/or prolonged stimulation, α₂-adrenergic receptors become desensitized by the intervention of G protein-coupled receptor kinase 2 (GRK2). In several experimental models of heart failure, there has been reported the up-regulation of GRK2 and the loss of functioning of inhibitory α₂-adrenoceptors resulting in enhanced release of adrenomedullary catecholamines. Given the importance of circulating catecholamines in the pathophysiology of heart failure, the recovery of α₂-adrenergic modulation of the secretory response from chromaffin cells appears as a novel strategy for a better control of the patients with this cardiac disease.

Activation of GABA(B) receptors potentiates inward rectifying potassium currents in satellite glial cells from rat trigeminal ganglia: in vivo patch-clamp analysis

In a previous study, we demonstrated that inflammation suppressed inward rectifying K(+) (Kir) currents in satellite glial cells (SGCs) from the trigeminal ganglia (TRGs) and that this impairment of glial potassium homeostasis in the trigeminal ganglion (TRG) contributed to trigeminal pain. The aim of the present study was to investigate whether activation of GABAB receptors modulates the Kir current in SGCs using in vivo patch-clamp and immunohistochemical techniques. Immunohistochemically, we found that immunoreactivity for glial-specific Kir channel subunit Kir4.1 and the GABAB receptor was co-expressed in SGCs from the TRGs. In vivo whole-cell recordings were made using SGCs from the TRGs of urethane-anesthetized rats. Application of baclofen, a GABAB receptor agonist, significantly increased the mean peak amplitude of Kir currents in a concentration-dependent and reversible manner. Baclofen-induced potentiation of the Kir current was abolished by co-application of 3-amino-2-(4-chlorophenyl)-2-hydroxyprophylsulfonic acid (saclofen). In addition, baclofen significantly potentiated the density of the Ba(2+)-sensitive Kir current, and resulted in hyperpolarization of the mean membrane potential. These results suggest that activation of GABAB receptors potentiates the Kir current in SGCs and that GABA released from the TRG neuronal soma could contribute to buffering of extracellular K(+) concentrations following excitation of TRG neurons during the processing of sensory information, including the transmission of noxious stimuli.

Peripheral inflammation suppresses inward rectifying potassium currents of satellite glial cells in the trigeminal ganglia

Previous studies indicate that silencing Kir4.1, a specific inward rectifying K(+) (Kir) channel subunit, in sensory ganglionic satellite glial cells (SGCs) induces behavioral hyperalgesia. However, the function of Kir4.1 channels in SGCs in vivo under pathophysiological conditions remains to be determined. The aim of the present study was to examine whether peripheral inflammation in anesthetized rats alters the SGC Kir4.1 current using in vivo patch clamp and immunohistochemical techniques. Inflammation was induced by injection of complete Freund's adjuvant into the whisker pad. The threshold of escape from mechanical stimulation applied to the orofacial area in inflamed rats was significantly lower than in naïve rats. The mean percentage of small/medium diameter trigeminal ganglion (TRG) neurons encircled by Kir4.1-immunoreactive SGCs in inflamed rats was also significantly lower than in naïve rats. In vivo whole-cell recordings were made using SGCs in the trigeminal ganglia (TRGs). Increasing extracellular K(+) concentrations resulted in significantly smaller potentiation of the mean peak amplitude of the Kir current in inflamed compared with naïve rats. In addition, the density of the Ba(2+)-sensitive Kir current associated with small-diameter TRG neurons was significantly lower in inflamed rats compared with naïve rats. Mean membrane potential in inflamed rats was more depolarized than in naïve rats. These results suggest that inflammation could suppress Kir4.1 currents of SGCs in the TRGs and that this impairment of glial potassium homeostasis in the TRGs contributes to trigeminal pain. Therefore, the Kir4.1 channel in SGCs may be a new molecular target for the treatment of trigeminal inflammatory pain.

Dexmedetomidine and clonidine inhibit the function of Na(v)1.7 independent of α(2)-adrenoceptor in adrenal chromaffin cells

PURPOSE

Besides being administered systemically for sedation and analgesia, α(2)-agonists such as dexmedetomidine and clonidine have been administered with intrathecal, epidural, or perineural injections, leading to an antinociceptive effect at the spinal cord or peripheral nerve level. However, the mechanism for this remains unclear. In the present study, we examined whether dexmedetomidine and clonidine could inhibit the function of tetrodotoxin-sensitive Na(+) channels, which play important roles in the generation of pain.

METHODS

Cultured bovine adrenal chromaffin cells expressing the tetrodotoxin-sensitive Na(v)1.7 Na(+) channel isoform were incubated in KRP buffer containing 2 μCi (22)NaCl for 5 min without or with dexmedetomidine or clonidine in the absence or presence of veratridine, α-scorpion venom, β-scorpion venom, Ptychodiscus brevis toxin-3 or ouabain. Cells were then washed and counted radioactively.

RESULTS

Dexmedetomidine and clonidine reduced veratridine-induced (22)Na(+) influx via Na(v)1.7 in a concentration-dependent manner (EC(50) = 50 μM and 530 μM), even in the presence of ouabain, an inhibitor of Na(+), K(+)-ATPase. Dexmedetomidine and clonidine shifted the concentration-response curve of veratridine for (22)Na(+) influx downward without altering the EC(50) of veratridine. Atipamezole and yohimbine, α(2)-antagonists, did not prevent the inhibition of veratridine-induced (22)Na(+) influx by dexmedetomidine. Dexmedetomidine and clonidine combined with lidocaine induced more inhibition of veratridine-induced (22)Na(+) influx than each drug did individually. Atipamezole and yohimbine did not prevent the lidocaine-enhancing effect of dexmedetomidine and clonidine.

CONCLUSION

Dexmedetomidine and clonidine inhibit the function of Na(v)1.7 independent of α(2)-adrenoceptor. These results may lead to a deeper understanding of the peripheral antinociceptive effects of α (2)-agonists.

Alteration of primary afferent activity following inferior alveolar nerve transection in rats

Background

In order to evaluate the neural mechanisms underlying the abnormal facial pain that may develop following regeneration of the injured inferior alveolar nerve (IAN), the properties of the IAN innervated in the mental region were analyzed.

Results

Fluorogold (FG) injection into the mental region 14 days after IAN transection showed massive labeling of trigeminal ganglion (TG). The escape threshold to mechanical stimulation of the mental skin was significantly lower (i.e. mechanical allodynia) at 11-14 days after IAN transection than before surgery. The background activity, mechanically evoked responses and afterdischarges of IAN Aδ-fibers were significantly higher in IAN-transected rats than naive. The small/medium diameter TG neurons showed an increase in both tetrodotoxin (TTX)-resistant (TTX-R) and -sensitive (TTX-S) sodium currents (I_Na) and decrease in total potassium current, transient current (I_A) and sustained current (I_K) in IAN-transected rats. The amplitude, overshoot amplitude and number of action potentials evoked by the depolarizing pulses after 1 μM TTX administration in TG neurons were significantly higher, whereas the threshold current to elicit spikes was smaller in IAN-transected rats than naive. Resting membrane potential was significantly smaller in IAN-transected rats than that of naive.

Conclusions

These data suggest that the increase in both TTX-S I_Na and TTX-R I_Na and the decrease in I_A and I_k in small/medium TG neurons in IAN-transected rats are involved in the activation of spike generation, resulting in hyperexcitability of Aδ-IAN fibers innervating the mental region after IAN transection.

Adrenal adrenoceptors in heart failure: fine-tuning cardiac stimulation

Chronic heart failure (HF) is characterized by sympathetic hyperactivity reflected by increased circulating catecholamines (CAs), which contributes significantly to its morbidity and mortality. Therefore, sympatholytic treatments, that is, treatments that reduce sympathetic hyperactivity, are being pursued currently for the treatment of HF. Secretion of CAs from the adrenal gland, which is a major source of CAs, is regulated by alpha(2)-adrenoceptors (alpha(2)ARs), which inhibit, and by beta-adrenoceptors (betaARs), which enhance CA secretion. All ARs are G-protein-coupled receptors (GPCRs), whose signaling and function are regulated tightly by the family of GPCR kinases (GRKs). Despite the enormous potential of adrenal ARs for the regulation of sympathetic outflow, elucidation of their properties has only begun recently. Here, recent advances regarding the roles of adrenal ARs in the regulation of sympathetic outflow in HF and the regulatory properties of ARs are discussed, along with the potential benefits and challenges of harnessing their function for HF therapy.

Inhibitory Effect of Clonidine on Cholinergic Transmission

Equilibrium geometry and electronic structure of three local anesthetic molecules and clonidine were computed. It was shown that clonidine molecule could incorporate into local anesthetic binding centers in potential-operated Na+ channels and in ionic channels coupled to nicotinic cholinergic receptors. The modulatory effect of clonidine on contractile responses of skeletal muscle showed that blockade of Na+ channels and nicotinic cholinergic receptors is a part of its analgesic action.

Alteration of the second branch of the trigeminal nerve activity following inferior alveolar nerve transection in rats

After transection of the inferior alveolar nerve (IAN), the whisker pad area, which is innervated by the infraorbital nerve (ION) that was not injured, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 4.0 g, indicating mechanical allodynia. A total of 68 single fiber discharges were recorded from ION fibers at 3 days after IAN transection. The responses of C- and A-fibers were classified according to their conduction velocity. The C-fiber activities were not affected by IAN transection, whereas A-fiber activities were significantly enhanced by IAN transection as indicated by an increase in background activity and mechanically evoked response. Since the A-fiber responses were significantly affected by IAN transection, patch clamp recording was performed from middle to large diameter retrogradely labeled and acutely dissociated trigeminal ganglion (TRG) neurons. The I(K) (sustained) and I(A) (transient) currents were significantly smaller and hyperpolarization-activated current (I(h)) was significantly larger in TRG neurons of rats with IAN transection as compared to those of naive rats. Furthermore, current injection into TRG neurons induced high frequency spike discharges in rats with IAN transection. These data suggest that changes in K(+) current and I(h) observed in the uninjured TRG neurons reflect an increase in excitability of TRG neurons innervated by the ION after IAN transection, resulting in the development of mechano-allodynia in the area adjacent to the injured IAN innervated region.

Activaton of GABAB receptor inhibits the excitability of rat small diameter trigeminal root ganglion neurons

A selective GABA(B) receptor agonist, baclofen, is known to suppress neuropathic pain. In the present study, we investigated the effect of baclofen on the excitability of trigeminal root ganglion (TRG) neurons by using the whole cell and perforated patch-clamp recording techniques. Under voltage-clamp (V(h)=-60 mV), voltage-dependent K(+) currents were recorded in the small diameter TRG neurons (<30 microm) and isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement. Separation of the K(+) current components was achieved by the response to variation in the conditioning voltage. Two distinct K(+) current components, a transient (I(A)) and a sustained (I(k)), were identified. Baclofen significantly increased I(A) by 74.8% (50 microM) and in a dose-dependent manner (1-50 microM). Similarly, I(K) was also enhanced by baclofen administration (41.8%: 50 microM). The relative amplitude of potentiation of I(A) was significantly higher than that of I(K) (P<0.05). Baclofen-sensitive I(A) and I(K) currents were antagonized by K(+) channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The augmentation of K(+) currents was antagonized by 3-amino-2-(4-chlorophenyl)-2-hydroxypropylsulfonic acid (saclofen; GABA(B) antagonist). In the current clamp mode, the resting membrane potential was -62+/-1.6 mV (n=24). Hyperpolarization of the membrane potential was elicited by baclofen (10-50 microM), and the response was associated with a decrease in the input resistance. Baclofen induced-hyperpolarization was blocked by saclofen (100 microM). In the presence of both 4-AP and TEA, no significant changes in membrane potential induced by baclofen application were observed. In the presence of BaCl(2), baclofen-evoked hyperpolarization with decreased resistance was observed. During application of baclofen, the firing rate of the action potentials by depolarizing step pulses was decreased. Application of baclofen reduced action potential duration evoked by a depolarization current pulse.These results indicated that activation of GABA(B) receptors inhibits the excitability of rat small diameter TRG neurons and this inhibitory action is mediated by potentiation of voltage-dependent K(+) currents. We therefore concluded that modification of nociceptive transmission in the trigeminal system by activation of GABA(B) receptors occurs at the level of small TRG neuron cell bodies and/or their primary afferent terminals, which are potential targets of analgesia by baclofen.

Changes of the excitability of rat trigeminal root ganglion neurons evoked by alpha(2)-adrenoreceptors

The aim of the study was to examine the effects of alpha(2)-adrenoreceptor agonists on the excitability of trigeminal root ganglion (TRG) neurons using the perforated patch-clamp technique, and to determine whether these neurons express mRNA for alpha(2)-adrenoreceptors. In current-clamp mode, the resting membrane potential was -57.4+/-1.2 mV (n=26). Most neurons (71%) were hyperpolarized by clonidine (5-50 microM) in a concentration-dependent manner. The response was associated with an increase of cell input resistance. In addition, clonidine reduced the repetitive firing evoked by depolarizing current pulses. An alpha(2)-adrenergic agonist, UK14,304, (10-20 microM) also hyperpolarized TRG neurons. The clonidine- and UK14,304-induced hyperpolarization was blocked by idazoxan (alpha(2)-adrenoreceptor antagonist). In voltage-clamp, clonidine (1-50 microM) reversibly reduced the hyperpolarization- and time-dependent cationic current. The effect was mimicked by UK14,304 (10-20 microM), and antagonized by idazoxan. Hyperpolarization-activated cationic current was blocked by extracellular Cs(+) (2 mM) or a specific blocker, ZD7288 (20 microM). Analysis of tail currents revealed that a reversal potential of the clonidine-sensitive component of hyperpolarization-activated cationic current was -46 mV. Single-cell reverse transcription-polymerase chain reaction analysis demonstrated the expression of mRNA for alpha(2A)- and alpha(2C)-adrenoreceptors. These results demonstrate that activation of alpha(2)-adrenoreceptors can hyperpolarize TRG neurons, and that the inhibitory effect is associated with inhibition of hyperpolarization-activated cationic current. Our results suggest that activation of alpha(2)-adrenoreceptors in the absence of nerve injury may have an inhibitory effect on nociceptive transmission in the trigeminal system at the level of both TRG neuronal cell bodies and primary afferent terminals.

Effects of vasopressin on isolated rat adrenal chromaffin cells

It has been demonstrated that arginine vasopressin (AVP) is synthesized not only in specific hypothalamic nuclei, but also in the adrenal medulla where it is thought to regulate adrenal functions by autocrine and paracrine mechanisms. In order to further characterise the effects of AVP on rat adrenal chromaffin cells, we examined: (a) the mRNA expression for V(1a) and V(1b) AVP receptors in these cells; (b) the effects of AVP on the membrane potential and membrane currents measured with the whole-cell patch-clamp technique; and (c) effect of AVP on catecholamine release from single adrenal chromaffin cells measured with carbon fibre microelectrodes. Reverse transcription-polymerase chain reaction (RT-PCR) on tissue punch samples obtained from the adrenal medulla demonstrated message for both the V(1a) and V(1b) receptors, while material obtained from the adrenal cortex showed expression of the V(1a) receptor only. Single-cell RT-PCR conducted on acutely isolated chromaffin cells showed message for the V(1a) receptor in 84% of cells, while 38% of cells also contained message for the V(1b) receptor (n=45). Under current-clamp recording, responses to AVP application (4-40 microM) were variable; 22/34 (65%) tested cells were depolarised, 29% hyperpolarised, and the remaining cells showed a biphasic response. Changes in membrane potential of either direction were dose-dependent and accompanied by a decrease in cell membrane resistance. Under voltage-clamp (V(hold)=-60 mV), AVP evoked inward current in 27/52 (52%) and outward current in 16/52 (31%) chromaffin cells. Both types of AVP-evoked responses were blocked by co-application of a nonselective V(1a)/V(1b) antagonist. Application of AVP evoked prolonged bursts of amperometric currents (indicative of catecholamine release) in 4/9 tested cells, but reduced the currents evoked by ACh application in all tested cells (n=7). These findings demonstrate a complex action of AVP on adrenal chromaffin cells, with individual adrenal chromaffin cells responding with either excitation or inhibition. This response pattern may be related to the expression of V(1) receptor subtypes.