Mechanisms of facilitation of synaptic glutamate release by nicotinic agonists in the nucleus of the solitary tract

Abstract

is the principal integrating relay in the processing of visceral sensory information. Functional nicotinic acetylcholine receptors (nAChRs) have been found on presynaptic glutamatergic terminals in subsets of caudal NTS neurons. Activation of these receptors has been shown to enhance synaptic release of glutamate and thus may modulate autonomic sensory-motor integration and visceral reflexes. However, the mechanisms of nAChR-mediated facilitation of synaptic glutamate release in the caudal NTS remain elusive. This study uses rat horizontal brainstem slices, patch-clamp electrophysiology, and fluorescent Ca 2ϩ imaging to test the hypothesis that a direct Ca 2ϩ entrance into glutamatergic terminals through active presynaptic non-␣7-or ␣7-nAChR-mediated ion channels is sufficient to trigger synaptic glutamate release in subsets of caudal NTS neurons. The results of this study demonstrate that, in the continuous presence of 0.3 M tetrodotoxin, a selective blocker of voltage-activated Na ϩ ion channels, facilitation of synaptic glutamate release by activation of presynaptic nAChRs (detected as an increase in the frequency of miniature excitatory postsynaptic currents) requires external Ca 2ϩ but does not require activation of presynaptic Ca 2ϩ stores and presynaptic high-and low-threshold voltage-activated Ca 2ϩ ion channels. Expanding the knowledge of mechanisms and pharmacology of nAChRs in the caudal NTS should benefit therapeutic approaches aimed at restoring impaired autonomic homeostasis. brainstem; nicotinic acetylcholine receptors THE NUCLEUS OF THE SOLITARY TRACT (NTS) is the site of the first synapse for primary afferents to the central nervous system (CNS) from autonomic sensory receptors located in the gastrointestinal tract, heart, blood vessels, and other visceral tissues. The NTS plays a key role in the maintenance of autonomic homeostasis and projects within the brainstem [e.g., to the dorsal motor nucleus of the vagus (DMV) and the ventrolateral medulla] that support parasympathetic and sympathetic visceral reflexes (e.g., gastrointestinal and baroreflex), as well as to higher brain regions (e.g., the thalamus and the hypothalamus) that support behavioral and endocrine functions (2, 46). Therefore, modulation of neuronal activity and synaptic transmission in the NTS by biologically active compounds such as nicotinic agonists can have potent effects on basic autonomic functions and visceral reflexes.The NTS contains a highly heterogeneous population of neurons characterized by a broad spectrum of morphological, electrophysiological, and cytochemical properties and multiple projection targets (3,6,12,17,20,23,44,55). Expression of nicotinic and muscarinic acetylcholine receptors (nAChRs and mAChRs, respectively) also varies across the NTS region (44, 55). Electrophysiological recordings from the NTS in brainstem slices revealed that nAChRs and mAChRs are likely expressed by different NTS neurons (43, 55). Moreover, subsets of small (somal area, ϳ137 m 2 ), elongated (form factor, ϳ0.62) caudal NTS neur...

Responsiveness to nicotine of neurons of the caudal nucleus of the solitary tract correlates with the neuronal projection target

“…A picospritzer (Parker Hannifin Instrumentation, Cleveland, OH) was used for agonist applications via pipettes (4 -7 M⍀) identical to those used for patch-clamp recordings. This drug application technique does not generate substantial pressure puff-elicited current artifacts, as evidenced from our previously published reports (Kalappa et al 2011;Smith and Uteshev 2008;Uteshev and Smith 2006).…”

“…Genie Plus syringe pumps (Kent Scientific, Torrington, CT) were used to add TTX to ACSF just before it entered the recording chamber. The final drug concentration in the bath was calculated based on the known concentration of the stock solution and adjustable rates of pumps (Kalappa et al 2011). A picospritzer (Parker Hannifin Instrumentation, Cleveland, OH) was used for agonist applications via pipettes (4 -7 M⍀) identical to those used for patch-clamp recordings.…”

“…Glutamate is a key excitatory neurotransmitter in the brain stem, but many drugs can modulate glutamatergic neurotransmission in the dorsal-vagal complex via activation of presynaptic ligand-gated receptors (Bailey et al 2002(Bailey et al , 2006bBrowning et al 2006) such as nicotinic acetylcholine receptors (nAChRs) (Kalappa et al 2011;Smith and Uteshev 2008). nAChRs are expressed in most brain regions including the NTS and the dorsal motor nucleus of the vagus (DMV) (Ferreira et al 2002;Kalappa et al 2011;Smith and Uteshev 2008).…”

“…nAChRs are expressed in most brain regions including the NTS and the dorsal motor nucleus of the vagus (DMV) (Ferreira et al 2002;Kalappa et al 2011;Smith and Uteshev 2008). Accordingly, nicotinic agonists exhibit potency for modulation of autonomic functions (Beleslin and Krstic 1987;Ferreira et al 2000;Jo et al 2002;Laffan and Borison 1957;Nagata and Osumi 1990).…”

“…In addition, it remains unclear whether the behavioral effects of nicotinic agonists reflect activation of nAChRs within a specific category of autonomic pathways and thus will affect some autonomic functions more than others. For example, increased fidelity of nicotine-sensitive glutamatergic neurotransmission in the caudal NTS (Kalappa et al 2011;Smith and Uteshev 2008) may enhance autonomic sensory-motor and visceral-central integration and alter the impact of centrally originating versus viscerosensoryoriginating inputs.…”