Alpha 1‐adrenoceptors in rat dorsal raphe neurons: regulation of two potassium conductances.

Abstract

1. a -Adrenoceptor activation caused two separate effects in rat dorsal raphe neurons: a depolarization and an increase in the duration of the after-hyperpolarization following the action potential. The depolarization often resulted in repetitive action potentials.The al-adrenoceptor antagonists prazosin and WB 4101 blocked the depolarization induced by phenylephrine. The concentration-response curve to phenylephrine was shifted to the right by VWB 4101.2. Under voltage clamp, a,-adrenoceptor agonists caused an inward current at -60 mV, which often became smaller at negative potentials but rarely reversed polarity even at strongly negative potentials. Using whole-cell recording, the inward current reversed polarity at the equilibrium potential for potassium in the majority of cells. IntracellularCs' decreased or abolished the a1-mediated inward current. The inward current was dependent on external calcium, but not on the degree of internal calcium buffering. Removal of external calcium or addition of MgCl2, CoCl2 or CdCl2 reduced or blocked the effects of a1-adrenoceptor agonists. Barium and strontium supported and even augmented the inward current induced by ax-adrenoceptor agonists, whereas nifedipine and wo-conous toxin had no effect. In contrast, internal dialysis with the calcium chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid (BAPTA) did not inhibit the inward current.3. The a,-induced depolarization was blocked (or occluded) by the inclusion of GTP-y-S (100 /SM) in the recording pipette. The phorbol-ester 4-phorbol 12,13-dibutyrate (PDBu) had no action on the membrane potential and depressed the phenylephrine-induced depolarization. This depression was reversed by the non-selective protein kinase inhibitor staurosporin. 4. Phenylephrine and noradrenaline increased a late component of the afterhyperpolarization (late-AHP) that followed a single action potential. The a,-sensitive late-AHP was blocked by apamine suggesting that it is a calcium-dependent potassium conductance. 5. Thapsigargin reduced the duration of the late-AHP and blocked the phenylephrinemediated prolongation. Caffeine also augmented the late-AHP and ryanodine blocked the augmentation induced by caffeine. The augmentation induced by phenylephrine was not occluded by caffeine and was still present after the caffeine-induced augmentation was blocked by ryanodine. 6. In slices pretreated with manoalide the depolarization induced by al-agonists was not changed; however, the late-AHP was reduced in duration and the a1-receptor-mediated augmentation of the late-AHP was decreased.7. The results suggest that the depolarization of dorsal raphe neurons by ac-adrenoceptor activation is through a decrease in potassium conductance that is independent of the activation of the phospholipase C pathway. The augmentation of the late-AHP is mediated by release of calcium from intracellular stores and may serve to regulate activity during the depolarization induced by a,-adrenoceptor activation.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

“…The role of the AHP in neurons is to control their excitability. In DRN neurons the AHP has an early and late component, the latter of which is mediated, as in many other neurons of the brain, by a calcium-dependent potassium current (Crunelli et al, 1983;Pan et al, 1994). The early component of the AHP may contribute to the AHP tau described here.…”

“…The early component of the AHP may contribute to the AHP tau described here. This early-AHP is insensitive to apamin, a neurotoxin that blocks most calcium-dependent potassium currents (Pan et al, 1994). In the future, it will be necessary to use selective channel blockers to characterize the specific ionic mechanisms responsible for this membrane characteristic.…”

“…Although 5-CT is a mixed 5-HT 1/7 agonist, the hyperpolarization response examined here has been shown to be mediated by the 5-HT 1A receptor (Williams et al, 1988;Corradetti et al, 1996). α 1 adrenergic receptors were stimulated by bath application of the selective agonist phenylephrine (PE, 3 μM; Vandermaelen and Aghajanian, 1983;Pan et al, 1994).…”

“…Although the response to activation of 5-HT 1A receptors is often cited as a defining feature of 5-HT DRN cells, activation of this receptor by 5-CT hyperpolarized both 5-HT (N=14) and non-5-HT-containing cells (N=15) and the magnitude of that hyperpolarization was comparable (Table 2; To explore this phenomenon more fully, the adrenergic agonist PE was administered in the perfusion bath of silent cells. PE elicits firing in 5-HT cells by stimulating α 1 receptors, a major excitatory input to 5-HT DRN cells in vivo that is severed in the in vitro slice preparation (Vandermaelen and Aghajanian, 1983;Pan et al, 1994). PE (3 μM) elicited firing in both 5-HT (N=5) and non-5-HT cells (N=7).…”

“…5A To explore this phenomenon more fully, the adrenergic agonist PE was administered in the perfusion bath of silent cells. PE elicits firing in 5-HT cells by stimulating α 1 receptors, a major excitatory input to 5-HT DRN cells in vivo that is severed in the in vitro slice preparation (Vandermaelen and Aghajanian, 1983;Pan et al, 1994). PE (3 μM) elicited firing in both 5-HT (N=5) and non-5-HT cells (N=7).…”