Presynaptic α4β2 Nicotinic Acetylcholine Receptors Increase Glutamate Release and Serotonin Neuron Excitability in the Dorsal Raphe Nucleus

Abstract

Several behavioral effects of nicotine are mediated by changes in serotonin (5-HT) release in brain areas that receive serotonergic afferents from the dorsal raphe nucleus (DRN). In vitro experiments have demonstrated that nicotine increases the firing activity in the majority of DRN 5-HT neurons and that DRN contains nicotinic acetylcholine receptors (nAChRs) located at both somata and presynaptic elements. One of the most common presynaptic effects of nicotine is to increase glutamate release. Although DRN receives profuse glutamatergic afferents, the effect of nicotine on glutamate release in the DRN has not been studied in detail. Using whole-cell recording techniques, we investigated the effects of nicotine on the glutamatergic input to 5-HT DRN neurons in rat midbrain slices. Low nicotine concentrations, in the presence of bicuculline and tetrodotoxin (TTX), increased the frequency but did not change the amplitude of glutamate-induced EPSCs, recorded from identified 5-HT neurons. Nicotine-induced increase of glutamatergic EPSC frequency persisted 10 -20 min after drug withdrawal. This nicotinic effect was mimicked by exogenous administration of acetylcholine (ACh) or inhibition of ACh metabolism. In addition, the nicotine-induced increase in EPSC frequency was abolished by blockade of ␣4␤2 nAChRs, voltagegated calcium channels, or intracellular calcium signaling but not by ␣7 nAChR antagonists. These data suggest that both nicotine and endogenous ACh can increase glutamate release through activation of presynaptic ␣4␤2 but not ␣7 nAChRs in the DRN. The effect involves long-term changes in synaptic function, and it is dependent on voltage-gated calcium channels and presynaptic calcium stores.

Contribution of α4β2 nAChR in nicotine-induced intracellular calcium response and excitability of MSDB neurons

“…Nicotine-induced calcium response was not affected by post-treatment of DhβE, which suggest that nicotine-induced calcium response may trigger the cascade of intracellular events which does not require a continued activation of nAChR. Although α4β2 nAChR is calcium permeable, which may contribute to nicotine-induced calcium responses in MSDB neurons (Karadsheh et al, 2004), other calcium sources such as voltage-gated calcium channels (Dickinson et al, 2008;Garduno et al, 2012) or Ca 2þ -induced Ca 2þ release from intracellular calcium stores (Arora et al, 2013;Dickinson et al, 2008;Garduno et al, 2012) may also contribute to nicotine-induced calcium response.…”

“…The reason is unknown. It may be related to endogenous release of ACh and/or increased expression or promoting assembly of α4β2 nAChR in the exposure of DhβE, a role which is independent from its receptor blockage (Bertrand, 2001;Garduno et al, 2012;Nashmi and Lester, 2007). Evidence showed that DhβE alone increased P20 amplitude and but decreased the nicotine-induced augmentation of P20 amplitude (Featherstone et al, 2012).…”

Nicotinic modulation of Ca2+ oscillations in rat cortical neurons in vitro

“…DH␤E pretreatment decreased the SCO frequency, indicative of the activation of ␣ 4 ␤ 2 -nAChRs by endogenously released ACh during SCO, which may be supported by the abundant expression of ␣ 4 ␤ 2 -nAChRs at glutamatergic and GABAergic presynaptic terminals that modulated glutamate and GABA release, critical for the [Ca 2ϩ ] i oscillations (8,9,33). Interestingly, DH␤E itself increased SCO amplitude, which may be explained by the finding that DH␤E enhanced the [Ca 2ϩ ] i response in MSDB neurons (31) and is in agreement with the evidence that DH␤E increased P20 amplitude on event-related ␥-activity (7).…”

Nicotinic modulation of serotonergic activity in the dorsal raphe nucleus

“…The DRN receives glutamatergic afferents from cortical and subcortical areas and contains a significant number of glutamater gic interneurons (SoizaReilly and . In a recent work, the effect of nicotine on glutamate release in midbrain slices containing the DRN was studied (Garduño et al, 2012 persisted in the presence of tetrodotoxin (TTX), which indicates that firing activity of glutamatergic neurons is not required for this effect. The nicotinic effect was mim icked by externally applied ACh or eserine, an inhibitor of acetylcholinesterase that increases the levels of endog enous ACh ( Figure 5A and B).…”