The presence of beta2-adrenoceptors sensitizes alpha2A-adrenoceptors to desensitization after chronic epinephrine treatment

Background

In addition to the regulation of blood pressure, α₂- and β-adrenoceptor (AR) subtypes play an important role in the modulation of noradrenergic neurotransmission in the human CNS and PNS. Several studies suggest that the α₂-AR responsiveness in cells and tissues after chronic epinephrine (EPI) or norepinephrine (NE) exposure may vary, depending on the β-AR activity present there. Recently, we reported that in BE(2)-C human neuroblastoma cells (endogenously expressing α_2A- and β₂-AR), chronic EPI treatment (300 nM) produced a dramatic β-adrenoceptor-dependent desensitization of the α_2A-AR response. The aim of this study is to determine if stable addition of a β₂-AR to a second neuroblastoma cell line (SH-SY5Y), that normally expresses only α_2A-ARs that are not sensitive to 300 nM EPI exposure, would suddenly render α_2A-ARs in that cell line sensitive to treatment with the same EPI concentration.

Methods

These studies employed RT-PCR, receptor binding and inhibition of cAMP accumulation to confirm α₂-AR subtype expression. Stable clones of SH-SY5Y cells transfected to stably express functional β₂-ARs (SHβ₂AR4) were selected to compare sensitivity of α₂-AR to EPI in the presence or absence of β₂-ARs.

Results

A series of molecular, biochemical and pharmacological studies indicated that the difference between the cell lines could not be attributed to α₂-AR heterogeneity. We now report that after transfection of functional β₂-AR into SH-SY5Y cells (SHβ₂AR4), chronic treatment with modest levels of EPI desensitizes the α_2A-AR. This effect results from a β₂-AR dependent down-regulation of native α_2A-ARs by EPI accompanied by enhanced translocation of GRK2 and GRK3 to the membrane (required for GRK-mediated phosphorylation of agonist-occupied receptors).

Conclusion

This study further supports the hypothesis that the presence of the β-AR renders the α_2A-AR more susceptible to desensitization with physiological levels of EPI.

Desensitization of alpha 2A-adrenoceptor signalling by modest levels of adrenaline is facilitated by beta 2-adrenoceptor-dependent GRK3 up-regulation

(1) Adrenaline (ADR) and noradrenaline (NA) can simultaneously activate inhibitory alpha(2)- and stimulatory beta-adrenoceptors (AR). However, ADR and NA differ significantly in that ADR is a potent beta(2)-AR agonist while NA is not. Only recently has the interaction resulting from the simultaneous activation of alpha(2)- and beta(2)-AR been examined at the cellular level to determine the mechanisms of alpha(2)-AR regulation following concomitant activation of both alpha(2)- and beta(2)-ARs by chronic ADR. (2) This study evaluates beta(2)-AR regulation of alpha(2A)-AR signalling following chronic ADR (300 nM) and NA (1 and 30 micro M) treatments of BE(2)-C human neuroblastoma cells that natively express both beta(2)- and alpha(2A)-ARs. (3) Chronic (24 h) treatment with ADR (300 nM) desensitized the response to the alpha(2A)-AR agonist, brimonidine, in BE(2)-C cells. Addition of the beta-AR antagonist, propranolol, blocked the ADR-induced alpha(2A)-AR desensitization. Unlike ADR, chronic NA (1 micro M) treatment had no effect on the alpha(2A)-AR response. However if NA was increased to 30 micro M for 24 h, alpha(2A)-AR desensitization was observed; this desensitization was partially reversed by propranolol. (4) Chronic ADR (300 nM) treatment reduced alpha(2A)-AR binding levels, contributing to the alpha(2A)-AR desensitization. This decrease was prevented by addition of propranolol during ADR treatment. Chronic NA (30 micro M), like ADR, treatment lowered specific binding, whereas 1 micro M NA treatment was without effect. (5) Chronic ADR treatment produced a significant increase in GRK3 levels and this was blocked by propranolol or GRK2/3 antisense DNA treatment. This antisense DNA, common to both GRK2 and GRK3, also blocked chronic ADR-induced alpha(2A)-AR desensitization and down-regulation. (6) Acute (1 h) ADR (300 nM) or NA treatment (1 micro M) produced alpha(2A)-AR desensitization. The desensitization produced by acute treatment was beta-AR independent, as it was not blocked by propranolol. (7) We conclude that chronic treatment with modest levels of ADR produces alpha(2A)-AR desensitization by mechanisms that involve up-regulation of GRK3 and down-regulation of alpha(2A)-AR levels through interactions with the beta(2)-AR.

Orphanin FQ/nociceptin and naloxone benzoylhydrazone activate distinct receptors in BE(2)-C human neuroblastoma cells

kappa(3) opioid receptors have a unique binding and analgesic profile, as originally defined by naloxone benzoylhydrazone (NalBzoH). Although antisense studies demonstrated the close relationship between kappa(3) opioid and Orphan opioid receptor-like receptor (ORL1) and implied they were generated from the same gene, these studies also revealed differences in the sensitivity profiles of NalBzoH and orphanin FQ/nociceptin (OFQ/N), indicating that they were not identical. To help define the relationship between kappa(3) and ORL1 receptors, we utilized BE(2)-C human neuroblastoma cells that natively express functional ORL1 and kappa(3) opioid receptors. (125)I-[Tyr(14)]OFQ/N binds to a single population of receptors in BE(2)-C cells. Competition binding and adenylyl cyclase studies clearly illustrated marked selectivity differences between the ORL1 and the kappa(3) sites. Furthermore, antisense DNA targeting ORL1 blocked the inhibition of cAMP by OFQ/N, but not by NalBzoH. Thus, the receptor mechanisms mediating the activity of OFQ/N and NalBzoH in BE(2)-C cells are distinct.

beta-Funaltrexamine inactivates ORL1 receptors in BE(2)-C human neuroblastoma cells

The potential interactions of natively expressed mu-opioid and opioid receptor-like (ORL1) receptors were studied by exposing intact BE(2)-C cells to agonists or antagonists for 1 h. Pretreatment with the mu-opioid receptor agonist, [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), or the ORL1 receptor agonist, orphanin FQ/nociceptin desensitized both mu-opioid and ORL1 receptor responses. beta-Funaltrexamine (beta-FNA) pretreatment also blocked both mu-opioid and ORL1 receptor responses, but only mu-opioid receptor binding was reduced. Moreover, beta-FNA (1 microM) failed to inhibit specific ORL1 receptor binding.

Nitric oxide is required for the maintenance but not initiation of ganglionic long-term potentiation

The role of nitric oxide in long-term potentiation of the nicotinic pathway of synaptic transmission in the isolated superior cervical ganglia of rat was studied. Long-term potentiation was induced by a brief tetanizing pulse (tetanus, 20 Hz/20 s) to the preganglionic nerve. The amplitude of the extracellularly recorded postganglionic compound action potential was used as an index of synaptic transmission. Pretreatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (10 microM) or L-N(G)-nitro-arginine (10 microM) 30 min before tetanus, inhibited long-term potentiation. The inactive enantiomer of the nitric oxide synthase inhibitor, N(G)-nitro-D-arginine methyl ester (10 microM), failed to inhibit the long-term potentiation when given 30 min before the tetanus. Washout of L-N(G)-nitro-arginine, but not N(G)-nitro-L-arginine methyl ester, resulted in complete recovery of long-term potentiation. The nitric oxide synthase inhibitor had no significant effect on the basal ganglionic neurotransmission or post-tetanic potentiation. Furthermore, established long-term potentiation was blocked by superfusion of ganglia with N(G)-nitro-L-arginine methyl ester 1 h after a tetanus. Pretreatment of ganglia with the nitric oxide donor, sodium nitroprusside (100 microM), or the nitric oxide synthase substrate, L-arginine (1 mM), completely prevented the inhibitory effects of N(G)-nitro-L-arginine methyl ester on the tetanus-induced long-term potentiation. These findings present evidence for a requirement of nitric oxide for the maintenance but not induction of long-term potentiation in rat isolated superior cervical ganglia.

Nitric oxide mediates long-term potentiation in rat superior cervical ganglion

Long-term potentiation (LTP) of the nicotinic pathway of the superior cervical ganglion (SCG) is remarkably similar to that of the hippocampus which has been shown to involve nitric oxide (NO). We investigated a similar possible involvement of NO in the LTP of the SCG of the rat. Treatment of ganglia with the NO-synthase inhibitor N(G)-nitro-L-arginine (L-NOARG, 10 microM) blocked LTP at the maintenance phase.