The Role of Cholinergic Midbrain Neurons in Startle and Prepulse Inhibition

Abstract

One of the two major cholinergic centers of the mammalian brain is located in the midbrain, i.e., the pedunculopontine tegmentum (PPTg) and the adjacent laterodorsal tegmentum. These cholinergic neurons have been shown to be important for e.g., arousal, reward associations, and sleep. They also have been suggested to mediate sensorimotor gating, measured as prepulse inhibition of startle (PPI). PPI disruptions are a hallmark of schizophrenia and are observed in various other psychiatric disorders, where they are associated with, and often predictive of, other cognitive symptoms. PPI has been proposed to be mediated by a short midbrain circuitry including inhibitory cholinergic projections from PPTg to the startle pathway. Although the data indicating the involvement of the PPTg is very robust, some more recent evidence challenges that there is a cholinergic contribution to PPI. We here use transient optogenetic activation of specifically the cholinergic PPTg neurons in male and female rats to address their role in startle modulation in general, and in PPI specifically. Although we could confirm the crucial role of PPTg cholinergic neurons in associative reward learning, validating our experimental approach, we found that activation of cholinergic PPTg neurons did not inhibit startle responses. In contrast, activation of cholinergic PPTg neurons enhanced startle, which is in accordance with their general role in arousal and indicate a potential involvement in sensitization of startle. We conclude that noncholinergic PPTg neurons mediate PPI in contrast to the longstanding hypothetical view that PPI is mediated by cholinergic PPTg neurons. Activation of cholinergic neurons in the midbrain has been assumed to mediate prepulse inhibition of startle (PPI), a common measure of sensorimotor gating that is disrupted in schizophrenia and other psychiatric disorders. We here revisit this long-standing hypothesis using optogenetic activation of these specific neurons combined with startle testing in rats. In contrast to the hypothetical role of these neurons in startle modulation, we show that their activation leads to an increase of baseline startle and to prepulse facilitation. This supports recent data by others that have started to cast some doubt on the cholinergic hypothesis of PPI, and calls for a revision of the theoretical construct of PPI mechanisms.

Prepulse Inhibition of the Auditory Startle Reflex Assessment as a Hallmark of Brainstem Sensorimotor Gating Mechanisms

“…The anatomical connection between the SC and the pedunculopontine tegmental nucleus (PPTg) activates a cholinergic projection to the PnC that mediates the PPI. In contrast to the longstanding hypothetical view that PPI is mediated by cholinergic PPTg neurons, recent studies applying optogenetics in rats [ 94 ] or using selective cholinergic lesion in PPTg [ 95 ] have demonstrated that the non-cholinergic PPTg neurons mediate PPI, whereas cholinergic neurons of PPTg enhance startle and lead to prepulse facilitation [ 94 ]. This controversy should be explored further to clarify the presumed role of PPTg cholinergic neurons in the sensory gating mechanisms of the ASR.…”

Cholinergic Projections From the Pedunculopontine Tegmental Nucleus Contact Excitatory and Inhibitory Neurons in the Inferior Colliculus

“…The PPT extends rostro-ventrally from this location almost as far as the substantia nigra in the rostral and ventral midbrain. Nearly half of the neurons in the PPT region respond to sound, and the cholinergic neurons have been implicated in acoustic startle and tone-specific plasticity (e.g., Reese et al, 1995a , b , c ; Xiong et al, 2009 ; Suga, 2012 ; Azzopardi et al, 2018 ).…”

Prepulse Inhibition of the Startle Reflex as a Predictor of Vulnerability to Develop Locomotor Sensitization to Cocaine

“…In light of this evidence, DA availability in the NAc seems to be critical in determining the level of PPI observed; i.e., a higher DA level will induce a PPI deficit, while a lower DA level will increase PPI (Arenas et al, 2017). The role of DA in determining PPI levels has been bolstered since activation of midbrain cholinergic neurons has been shown not to mediate PPI levels (Azzopardi et al, 2018). One of the main brain nuclei that modulate PPI is the pedunculopontine tegmentum (PPTg; Arenas et al, 2017), which is one of the major cholinergic centers of the brain (Gut and Winn, 2016).…”

“…One of the main brain nuclei that modulate PPI is the pedunculopontine tegmentum (PPTg; Arenas et al, 2017), which is one of the major cholinergic centers of the brain (Gut and Winn, 2016). However, it has recently been demonstrated that cholinergic PPTg neurons do not mediate PPI, in contrast to a longstanding hypothesis maintaining that they do (Azzopardi et al, 2018). Furthermore, the lack of motor and rewarding effects of cocaine observed in the mice with a low PPI level may reflect an altered DA system.…”