Dopamine acts as a partial agonist for α2A adrenoceptor in melanin-concentrating hormone neurons

Abstract

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that promotes positive energy balance and anxiety. Since dopamine (DA) is also closely implicated in these functions, the present study investigated the effect of DA on MCH neurons. Using whole-cell patch-clamp recordings in rat brain slices, we found that DA hyperpolarizes MCH neurons by activating G-protein-activated inwardly rectifying K(+) (GIRK) channels. Pharmacological study indicated that the effect was mediated by α2A adrenoceptors, not DA receptors. DA-induced outward current was also observed in the presence of tetrodotoxin or the dopamine β-hydroxylase inhibitor fusaric acid, suggesting that DA directly binds to α2A receptors on MCH neurons, rather than acting presynaptically or being transformed into norepinephrine (NE) in the slice preparation. The effects of NE and DA were concentration-dependent with EC(50) of 5.9 and 23.7 μm, respectively, and a maximal effect of 106.6 and 57.2 pA, respectively, suggesting that DA functions as a partial agonist. Prolonged (5 min) activation of α2A receptors by either DA or NE attenuated the subsequent response to DA or NE, while 5 s applications were not sufficient to induce desensitization. Therefore, a history of α2A receptor activation by DA or NE can have a lasting inhibitory effect on the catecholaminergic transmission to MCH neurons. Our study suggests that α2A receptors expressed by MCH neurons may be one of the pathways by which DA and NE can interact and modulate mood and energy homeostasis, and this cross talk may have functional implications in mood disorders and obesity.

Figure 1.

Dopamine inhibits MCH neurons via GIRK channel activation. A, Representative experiment showing a DA-induced reversible hyperpolarization. Vertical lines denote responses to negative and positive current injections. B, Same experiment as in A, except traces from the time points indicated in A are expanded to show the detailed characteristics of MCH neurons, such as a lack of I_h, rebound depolarization, and spontaneous firing. DA hyperpolarizes MCH cell and abolishes action potentials. C, Changes in resting membrane potential induced by DA. Each dot represents a cell and horizontal lines indicate the average value. *p < 0.05, paired t test versus baseline. D, Representative voltage-clamp trace showing a DA-induced current. E, Left, Current–voltage relationship in the presence and absence of DA. Inset, Voltage protocol used. Right, Subtracted trace showing the DA-induced current. F, Tertiapin-Q (TQ) attenuates DA current. **p < 0.01, unpaired t test. Numbers denote the numbers of cells tested.

Figure 2.

Dopamine acts as a partial agonist of α2A receptors. A, Summary of DA-induced currents in the presence of DA or α2 antagonist. DA effect is blocked by α2 (yohimbin) and α2A antagonist (BLR44405), but not by DA antagonists (LE300 and/or sulpiride). Ineffectiveness of D1- and D2-like agonists is also shown (SKF81297 + quinpirole). DA effect persists in the dopamine-β-hydroxylase inhibitor fusaric acid. **p < 0.01 versus DA 100 μm, ANOVA with Dunnett's posttest. ns, Not significant. B, NE response is also inhibited by BLR44408. ***p < 0.0005, unpaired t test. C, Concentration–response curve for DA (n = 3–8) and NE (n = 5–6), demonstrating that DA is a partial agonist. Numbers in bar graphs indicate cell numbers included.

Figure 3.

Cross-desensitization of α2A receptors by DA and NE. A–D, Representative traces from MCH neurons showing desensitizing response to repeated applications of NE and/or DA. E, F, Summary of DA (E) or NE (F) currents following a previous DA or NE application. G, Recovery rate of DA currents from desensitization induced by DA. Second peak current amplitude is normalized to the first one. H, I, α2A receptors do not desensitize by short agonist applications. A 5 s NE application does not desensitize the subsequent DA response (H), and vice versa (I). For both DA and NE, peak responses to 5 s or 5 min application are comparable. Numbers in bar graphs denote the numbers of cells tested. For comparison, DA 100 μm and NE 100 μm data from Figures 1 and 2 are shown. *p < 0.05, **p < 0.01. ns, Not significant, ANOVA with Dunnett's posttest.