The nucleus of the solitary tract (NTS) is the principal integrating relay in the processing of visceral sensory and gustatory information. In the present study, patch-clamp electrophysiological experiments were conducted using rat horizontal brainstem sections. Pre-synaptic and somatic/dendritic nicotinic acetylcholine receptors (nAChRs) expressed in neurons of the caudal NTS (cNTS) were found to be randomly distributed between pre-synaptic and somatic/dendritic sites (χ2 = 0.72, df = 3, p > 0.87, n = 200). Pre-synaptic nAChRs were detected by their facilitating effects on glutamatergic neurotransmission of a sub-population of cNTS neurons (categorized as "effect-positive") upon brief picospritzer applications of 0.1-0.5 mM nicotine. These effects were resistant to inhibition by 20 nM methyllycaconitine (MLA) and 4 μM dihydro-β-erythroidine (DHβE), and were replicated by brief picospritzer applications of 0.2-1 mM cytisine. Picospritzer applications of 0.2 mM RJR-2403, a potent agonist of α4β2 nAChRs, did not facilitate synaptic release of glutamate in effect-positive cNTS neurons. The population of somatic/dendritic nAChRs has been found to be heterogeneous and included nAChRs that were activated by RJR-2403 and/or cytisine, or insensitive to cytisine, or inhibited by MLA. The presented results are consistent with the expression of β4-containing (i.e., β4*) nAChRs, likely α3β4*, in pre-synaptic terminals of effect-positive cNTS neurons. Somatic/dendritic nAChRs appear to involve both α7 and non-α7 subunits. Heterogeneity in the subunit composition of pre-synaptic and somatic/dendritic nAChRs may underlie diverse roles that these receptors play in regulation of behavioral and visceral reflexes, and may reflect specific targeting by endogenous nicotinic agents and nicotine.
- Solitary tract