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Prepulse inhibition (PPI) is an operational measure of sensorimotor gating. Deficits of PPI are a hallmark of schizophrenia and associated with several other psychiatric illnesses such as e.g. autism spectrum disorder, yet the mechanisms underlying PPI are still not fully understood. There is growing evidence contradicting the long-standing hypothesis that PPI is mediated by a short feed-forward midbrain circuitry including inhibitory cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the startle pathway. Here, we employed a chemogenetic approach to explore the involvement of the PPTg in general, and cholinergic neurons specifically, in PPI. Activation of inhibitory DREADDs (designer receptors exclusively activated by designer drugs) in the PPTg by systemic administration of clozapine-N-oxide (CNO) disrupted PPI, confirming the involvement of the PPTg in PPI. In contrast, chemogenetic inhibition of specifically cholinergic PPTg neurons had no effect on PPI, but inhibited morphine-induced conditioned place preference (CPP) in the same animals, showing that the DREADDs were effective in modulating behavior. These findings support a functional role of the PPTg and/or neighboring structures in PPI in accordance with previous lesion studies, but also provide strong evidence against the hypothesis that specifically cholinergic PPTg neurons are involved in mediating PPI, implicating rather non-cholinergic midbrain neurons.

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.SIGNIFICANCE STATEMENTActivation 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.

The role of acetylcholine and specific nicotinic receptors in sensorimotor gating and higher cognitive function has been controversial. Here, we used a commercially available mouse with a null mutation in the Chrna7tm1Bay gene [α7-nicotinic acetylcholine receptor (nAChR) knockout (KO) mouse] in order to assess the role of the α7-nAChR in sensorimotor gating and spatial learning. We examined prepulse inhibition (PPI) of startle and nicotine-induced enhancement of PPI. We also tested short- and long-term habituation of the startle response as well as of locomotor behaviour in order to differentiate the role of this receptor in the habituation of evoked behaviour (startle) vs. motivated behaviour (locomotion). To address higher cognition, mice were also tested in a spatial learning task. Our results showed a mild but consistent PPI deficit in α7-nAChR KO mice. Furthermore, they did not show nicotine-induced enhancement of startle or PPI. Short- and long-term habituation was normal in KO mice for both types of behaviours, evoked or motivated, and they also showed normal learning and memory in the Barnes maze. Thorough analysis of the behavioural data indicated a slightly higher degree of anxiety in α7-nAChR KO mice; however, this could only be partially confirmed in an elevated plus maze test. In summary, our data suggest that α7-nAChRs play a minor role in PPI, but seem to mediate nicotine-induced PPI enhancement. We found no evidence to suggest that they are important for habituation or spatial learning.

Genetic variations in the large-conductance, voltage- and calcium activated potassium channels (BK channels) have been recently implicated in mental retardation, autism and schizophrenia which all come along with severe cognitive impairments. In the present study we investigate the effects of functional BK channel deletion on cognition using a genetic mouse model with a knock-out of the gene for the pore forming α-subunit of the channel. We tested the F1 generation of a hybrid SV129/C57BL6 mouse line in which the slo1 gene was deleted in both parent strains. We first evaluated hearing and motor function to establish the suitability of this model for cognitive testing. Auditory brain stem responses to click stimuli showed no threshold differences between knockout mice and their wild-type littermates. Despite of muscular tremor, reduced grip force, and impaired gait, knockout mice exhibited normal locomotion. These findings allowed for testing of sensorimotor gating using the acoustic startle reflex, as well as of working memory, spatial learning and memory in the Y-maze and the Morris water maze, respectively. Prepulse inhibition on the first day of testing was normal, but the knockout mice did not improve over the days of testing as their wild-type littermates did. Spontaneous alternation in the y-maze was normal as well, suggesting that the BK channel knock-out does not impair working memory. In the Morris water maze knock-out mice showed significantly slower acquisition of the task, but normal memory once the task was learned. Thus, we propose a crucial role of the BK channels in learning, but not in memory storage or recollection.

Background Aging is typically accompanied by memory decline and changes in hippocampal function. Among these changes is a decline in the activity of the dentate gyrus (DG) during behavior. Lasting memory, however, is thought to also require recapitulation of recent memory traces during subsequent rest – a phenomenon, termed memory trace reactivation, which is compromised in hippocampal CA1 with progressive age. This process has yet to be assessed in the aged DG, despite its prominent role in age-related memory impairment. Using zif268 transcription to measure granule cell recruitment, DG activity in adult and aged animals was assessed both during spatial exploration and as animals remained at rest in the home cage in order to detect potential memory-related replay. Results Consistent with the observation of memory trace reactivation in DG, the probability that an individual granule cell transcribes zif268 during rest in the animal’s home cage is increased by recent experience in a novel environment. Surprisingly, a comparable increase was observed in the probability of granule cells in the aged DG expressing zif268 during rest. Moreover, no significant age-related difference was observed in the number of granule cells expressing zif268 during rest. Thus, the number and pattern of granule cell expression of zif268 during rest is preserved in aged animals, despite a significant decline in exploration-related zif268 expression. Conclusions These data lead to the hypothesis that the input the aged DG receives from backprojections from CA3 (the region widely hypothesized to mediate reactivation) remains functionally intact despite loss of innervation from the perforant path.

Habituation is considered the most basic form of learning. It describes the decrease of a behavioural response to a repeated non-threatening sensory stimulus and therefore provides an important sensory filtering mechanism. While some neuronal pathways mediating habituation are well described, underlying cellular/molecular mechanisms are not yet fully understood. In general, there is an agreement that short-term and long-term habituation are based on different mechanisms. Historically, a distinction has also been made between habituation of motivated versus reflexive behaviour. In recent studies in invertebrates the large conductance voltage- and calcium-activated potassium (BK) channel has been implicated to be a key player in habituation by regulating synaptic transmission. Here, we tested mice deficient for the pore forming α-subunit of the BK channel for short-term and long-term habituation of the acoustic startle reflex (reflexive behaviour) and of the exploratory locomotor behaviour in the open field box (motivated behaviour). Short-term habituation of startle was completely abolished in the BK knock-out mice, whereas neither long-term habituation of startle nor habituation of motivated behaviour was affected by the BK deficiency. Our results support a highly preserved mechanism for short-term habituation of startle across species that is distinct from long-term habituation mechanisms. It also supports the notion that there are different mechanisms underlying habituation of motivated behaviour versus reflexive behaviour.

The neurotransmitter acetylcholine (ACh) plays a crucial role in both the central and peripheral nervous system. Central cholinergic transmission is important for cognitive functions and cholinergic disruptions have been associated with different neural disorders. We here tested the role of cholinergic transmission in basic cognitive functions, i.e. in prepulse inhibition (PPI) and short-term habituation (STH) as well as long-term habituation (LTH) of startle using mice with a 65% knockdown (KD) of the vesicular ACh transporter (VAChT). These mice are slow in refilling cholinergic synaptic transmitter vesicles, leading to a reduced cholinergic tone. Prepulse inhibition has been assumed to be mediated by cholinergic projections from the midbrain to the reticular formation. Surprisingly, PPI and STH were normal in these mice, whereas LTH was disrupted. This disruption could be rescued by pre-testing injections of the ACh esterase inhibitor galantamine, but not by post-testing injections. The lack of a PPI deficit might be because of the fact that VAChT KD mice show disruptions mainly in prolonged cholinergic activity, therefore the transient activation by prepulse processing might not be sufficient to deplete synaptic vesicles. The disruption of LTH indicates that the latter depends on a tonic cholinergic inhibition. Future experiments will address which cholinergic cell group is responsible for this effect.