Urotensin II acts as a modulator of mesopontine cholinergic neurons

Abstract

Urotensin II (UII) is a vasomodulatory peptide that was not predicted to elicit CNS activity. However, because we have recently shown that the urotensin II receptor (UII-R) is selectively expressed in rat mesopontine cholinergic (MPCh) neurons, we hypothesize that UII may have a central function. The present study demonstrates that the UII system is able to modulate MPCh neuron activity. Brain slice experiments demonstrate that UII excites MPCh neurons of the mouse laterodorsal tegmentum (LDTg) by activating a slow inward current. Furthermore, microinfusion of UII into the ventral tegmental area produces a sustained increase in dopamine efflux in the nucleus accumbens, as measured by in vivo chronoamperometry. In agreement with UII activation of MPCh neurons, intracerebroventricular injections of UII significantly modulate ambulatory movements in both rats and mice but do not significantly affect startle habituation or prepulse inhibition. The present study establishes that UII is a neuromodulator that may be exploited to target disorders involving MPCh dysfunction.

Introduction

Urotensin II (UII) is a vasomodulatory peptide that has been recently shown to be the natural ligand of a G protein-coupled receptor formerly called GPR14 [1], [25], [28], [30], now known as the urotensin II receptor (UII-R). Although UII is well known for its vascular effects (for review see [9]), several reports have indicated that it may also have effects in the central nervous system [7], [11], [17], [25], [27]. In particular, we have shown that the UII-R is selectively expressed in mesopontine cholinergic (MPCh) neurons of the rat brain [7], inferring that UII can modulate the activity of MPCh neurons. In addition, the fact that UII-R activation elicits increases in cytoplasmic calcium ion concentrations in vitro [1], [24], [30] indicates that UII could promote cell depolarization. Studies have measured the effects of UII after injections into the brain [17], [23], [25], [27], and our recent data indicate UII promotes REM sleep by exciting MPCh neurons of the pedunculopontine tegmental nucleus (PPTg; [18]). Here, we have extended this analysis by examining UII actions on an additional group of MPCh neurons in the laterodorsal tegmental (LDTg) nucleus and on other functions associated with MPCh neurons.

Activation of the PPTg and LDTg is thought to contribute to a number of behavioral and physiological phenomena. For example, Garcia-Rill et al. [15] demonstrated that electrical stimulation of these regions elicited stepping behavior in the rat. In addition, lesions or electrical inhibition of the mesopontine tegmentum in non-human primates can induce ataxia [22], [29]. Together, these studies have suggested that the brainstem area encompassing MPCh neurons profoundly influences motor function in mammals [38]. MPCh neurons have also been implicated in the modulation of the sensorimotor reactivity. Garcia-Rill et al. [16] predicted that activation of neurons in the mesopontine would reduce habituation of the startle reflex to a loud auditory stimulus. MPCh neurons are also intimately involved in prepulse inhibition (PPI), a phenomenon in which the amplitude of the startle reflex is reduced by the presence of a preceding muted warning stimulus (for review see [10], [20]). Elucidating the role of MPCh neurons in PPI is of great interest because PPI deficits are a hallmark and diagnostic symptom in many neuropsychiatric disorders (for review see [20]). It is believed that the MPCh input from the PPTg to the caudal pontine nucleus inhibits the startle reflex [21], as destruction of the PPTg impairs PPI [21], [34]. Therefore, we predict that agents such as UII, which potentially modulate MPCh activity, should influence both the startle reflex and PPI.

Several diverse techniques were employed to establish UII as a modulator of MPCh function. First, the response of MPCh neurons to UII stimulation was determined in vitro. Then, to test whether UII activates neurons in vivo, chronoamperometric recordings of dopamine efflux in the nucleus accumbens (NAc) were carried out after UII microinjection into the ventral tegmental area (VTA). Electrical stimulation of MPCh neurons located in the region of the PPTg and LDTg has been shown to induce increases in basal dopamine efflux in the striatum and NAc [12], [13]. Furthermore, the MPCh neurons constitute the only known cholinergic excitatory input to the VTA dopaminergic cells [31]. Given that our previous study showed that the VTA contains binding sites for radiolabeled UII [7], it was expected that UII injected into the VTA would produce changes in NAc dopamine efflux. Lastly, behavioral paradigms associated with MPCh activity were used to extend the anatomical and neurochemical findings.