Neurokinin-1 receptors in cholinergic neurons of the rat ventral pallidum have a predominantly dendritic distribution that is affected by apomorphine when combined with startle-evoking auditory stimulation

Cholinergic neurons of the basal forebrain are implicated in startle reflex inhibition by a prior weak stimulus often referred to as prepulse inhibition (PPI) and used as an index of sensorimotor gating deficits in schizophrenia. Gating deficits can be produced in rodent models by acute systemic administration of apomorphine, a non-selective dopamine D1 and D2 receptor agonist that also affects trafficking of neurokinin-1 (NK1) receptors induced by startle evoking auditory stimulation (AS) in midbrain neurons. We used electron microscopic immunolabeling of NK1 receptors and the vesicular acetylcholine transporter (VAchT) to test the hypothesis that the subcellular distributions of these receptors in cholinergic neurons of the rat ventral pallidum are subject to a similar regulation. In vehicle controls, NK1 immunogold was often seen near cytoplasmic endomembranes in somata and large dendrites, but was more equally distributed in cytoplasmic and plasmalemmal compartments of medium dendrites, and principally located on the plasma membrane of small dendrites. These labeling patterns appeared to be largely independent of whether the NK1 receptor was co-expressed with VAchT, however only the medium and small VAchT-labeled dendrites showed significant treatment-specific differences in NK1 immunogold distributions. The NK1 receptor immunogold particle density on the plasma membrane of medium cholinergic dendrites was significantly enhanced by combined apomorphine and AS, while neither alone affected either the plasmalemmal density or the equality of the plasmalemmal and cytoplasmic distributions of NK1 receptors in these dendrites. Small cholinergic dendrites showed a significant AS-induced increase in both the plasmalemmal and cytoplasmic density of NK1 gold particles, and an apomorphine-induced disruption of the preferential plasmalemmal targeting of the NK1 receptors. These results provide ultrastructural evidence that NK1 receptors in cholinergic neurons of the ventral pallidum have subcellular locations and plasticity conducive to active involvement in dopamine-dependent sensorimotor processing.