The Ascending Mesolimbic Cholinergic System—A Specific Division of the Reticular Activating System Involved in the Initiation of Negative Emotional States

From emotional arousal to executive action. Role of the prefrontal cortex

Emotional arousal is caused by the activity of two parallel ascending systems targeting mostly the subcortical limbic regions and the prefrontal cortex. The aversive, negative arousal system is initiated by the activity of the mesolimbic cholinergic system and the hedonic, appetitive, arousal is initiated by the activity of the mesolimbic dopaminergic system. Both ascending projections have a diffused nature and arise from the rostral, tegmental part of the brain reticular activating system. The mesolimbic cholinergic system originates in the laterodorsal tegmental nucleus and the mesolimbic dopaminergic system in the ventral tegmental area. Cholinergic and dopaminergic arousal systems have converging input to the medial prefrontal cortex. The arousal system can modulate cortical EEG with alpha rhythms, which enhance synaptic strength as shown by an increase in long-term potentiation (LTP), whereas delta frequencies are associated with decreased arousal and a decrease in synaptic strength as shown by an increase in long-term depotentiation (LTD). It is postulated that the medial prefrontal cortex is an adaptable node with decision making capability and may control the switch between positive and negative affect and is responsible for modifying or changing emotional state and its expression.

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Structural and Functional Neuroanatomy of Core Consciousness: A Primer for Disorders of Consciousness Clinicians

Understanding the structural and functional neuroanatomy of core consciousness (ie, wakefulness and awareness) is an asset to clinicians caring for persons with disorders of consciousness. This article provides a primer on the structural and functional neuroanatomy of wakefulness and awareness. The neuroanatomical structures supporting these elements of core consciousness functions are reviewed first, after which brief description of the clinically evaluable relationships between disruption of these structures and disorders of consciousness (ie, brain-behavior relationships) are outlined. Consideration of neuroanatomy at the mesoscale (ie, the mesocircuit hypothesis) as well as in relation to several large-scale neural networks is offered.

Whole-Body Vibration Prevents Neuronal, Neurochemical, and Behavioral Effects of Morphine Withdrawal in a Rat Model

Peripheral mechanoreceptor-based treatments such as acupuncture and chiropractic manipulation have shown success in modulating the mesolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) of the midbrain and projecting to the nucleus accumbens (NAc) of the striatum. We have previously shown that mechanoreceptor activation via whole-body vibration (WBV) ameliorates neuronal and behavioral effects of chronic ethanol exposure. In this study, we employ a similar paradigm to assess the efficacy of WBV as a preventative measure of neuronal and behavioral effects of morphine withdrawal in a Wistar rat model. We demonstrate that concurrent administration of WBV at 80 Hz with morphine over a 5-day period significantly reduced adaptations in VTA GABA neuronal activity and NAc DA release and modulated expression of δ-opioid receptors (DORs) on NAc cholinergic interneurons (CINs) during withdrawal. We also observed a reduction in behavior typically associated with opioid withdrawal. WBV represents a promising adjunct to current intervention for opioid use disorder (OUD) and should be examined translationally in humans.

The bidirectional association of 24-h activity rhythms and sleep with depressive symptoms in middle-aged and elderly persons

BACKGROUND

In older populations disturbed 24-h activity rhythms, poor sleep, and depressive symptoms are often lingering and co-morbid, making treatment difficult. To improve insights into these commonly co-occurring problems, we assessed the bidirectional association of sleep and 24-h activity rhythms with depressive symptoms in middle-aged and elderly persons.

METHODS

In 1734 participants (mean age: 62.3 ± 9.3 years, 55% women) from the prospective Rotterdam Study, 24-h activity rhythms and sleep were estimated with actigraphy (mean duration: 146 ± 19.6 h), sleep quality with the Pittsburgh Sleep Quality Index, and depressive symptoms with the Center for Epidemiological Studies Depression scale. Repeated measures were available for 947 participants (54%) over a median follow-up of 6 years (interquartile range = 5.6-6.3). Linear-mixed models were used to assess temporal associations of 24-h activity rhythms and sleep with depressive symptoms in both directions.

RESULTS

High 24-h activity rhythm fragmentation (IV) (B = 1.002, 95% confidence interval (CI) = 0.641-1.363), long time in bed (TIB) (B = 0.111, 95% CI = 0.053-0.169), low sleep efficiency (SE) (B = -0.015, 95% CI = -0.020 to -0.009), long sleep onset latency (SOL) (B = 0.009, 95% CI = 0.006-0.012), and low self-rated sleep quality (B = 0.112, 95% CI = 0.0992-0.124) at baseline were associated with increasing depressive symptoms over time. Conversely, more depressive symptoms at baseline were associated with an increasing 24-h activity rhythm fragmentation (B = 0.002, 95% CI = 0.001-0.003) and TIB (B = 0.009, 95% CI = 0.004-0.015), and a decreasing SE (B = -0.140, 95% CI = -0.196 to -0.084), SOL (B = 0.013, 95% CI = 0.008-0.018), and self-rated sleep quality (B = 0.193, 95% CI = 0.171-0.215) over time.

CONCLUSION

This study demonstrates a bidirectional association of 24-h activity rhythms, actigraphy-estimated sleep, and self-rated sleep quality with depressive symptoms over a time frame of multiple years in middle-aged and elderly persons.

Water Drinking Behavior Associated with Aversive Arousal in Rats: An Integrative Approach

Cholinergic muscarinic stimulation of vast areas of the limbic brain induced a well-documented polydipsia in laboratory rats. This excessive water-drinking behavior has not received any convincing biological and physiological interpretation for the last 50 years. This review offers such an interpretation and suggests that cholinergically induced drinking response, mostly by carbachol, is associated with activation of the ascending mesolimbic cholinergic system that serves for initiation of emotional aversive arousal of the organism. The ascending cholinergic system originates from the laterodorsal tegmental nucleus, has a diffuse nature, and affects numerous subcortical limbic structures. It is proposed that the carbachol-induced drinking response is related to the state of anxiety and does not serve the regulation of thirst. Instead, the response is anxiety-induced polydipsia that might occur as a soothing procedure that decreases the aversiveness of the negative emotional state induced by carbachol. It is concluded that carbachol-induced water-drinking behavior is a rewarding process that contributes to alleviating the feeling of anxiety by bringing some relief from the cholinergically induced aversive state, and it is a homologue to anxiety-driven polydipsia in humans.

Temporal dynamics of affect in the brain: Evidence from human imaging and animal models

Emotions are time-varying internal states that promote survival in the face of dynamic environments and shifting homeostatic needs. Research in non-human organisms has recently afforded specific insights into the neural mechanisms that support the emergence, persistence, and decay of affective states. Concurrently, a separate affective neuroscience literature has begun to dissect the neural bases of affective dynamics in humans. However, the circuit-level mechanisms identified in animals lack a clear mapping to the human neuroscience literature. As a result, critical questions pertaining to the neural bases of affective dynamics in humans remain unanswered. To address these shortcomings, the present review integrates findings from humans and non-human organisms to highlight the neural mechanisms that govern the temporal features of emotional states. Using the theory of affective chronometry as an organizing framework, we describe the specific neural mechanisms and modulatory factors that arbitrate the rise-time, intensity, and duration of emotional states.

Characterizing the human APOE epsilon 4 knock-in transgene in female and male rats with multimodal magnetic resonance imaging

The APOE Ɛ4 genotype is the most prevalent genetic risk for Alzheimer's disease (AD). Women carriers of Ɛ4 have higher risk for an early onset of AD than men. Human imaging studies suggest apolipoprotein Ɛ4 may affect brain structures associated with cognitive decline in AD many years before disease onset. It was hypothesized that female APOE Ɛ4 carriers would present with decreased cognitive function and neuroradiological evidence of early changes in brain structure and function as compared to male carriers. Six-month old wild-type (WT) and human APOE Ɛ4 knock-in (TGRA8960), male and female Sprague Dawley rats were studied for changes in brain structure using voxel-based morphometry, alteration in white and gray matter microarchitecture using diffusion weighted imaging with indices of anisotropy, and functional coupling using resting state BOLD functional connectivity. Images from each modality were registered to, and analyzed, using a 3D MRI rat atlas providing site-specific data on over 168 different brain areas. Quantitative volumetric analysis revealed areas involved in memory and arousal were significantly different between Ɛ4 and wild-type (WT) females, with few differences between male genotypes. Diffusion weighted imaging showed few differences between WT and Ɛ4 females, while male genotypes showed significant different measures in fractional anisotropy and apparent diffusion coefficient. Resting state functional connectivity showed Ɛ4 females had greater connectivity between areas involved in cognition, emotion, and arousal compared to WT females, with male Ɛ4 showing few differences from controls. Interestingly, male Ɛ4 showed increased anxiety and decreased performance in spatial and episodic memory tasks compared to WT males, with female genotypes showing little difference across behavioral tests. The sex differences in behavior and diffusion weighted imaging suggest male carriers of the Ɛ4 allele may be more vulnerable to cognitive and emotional complications compared to female carriers early in life. Conversely, the data may also suggest that female carriers are more resilient to cognitive/emotional problems at this stage of life perhaps due to altered brain volumes and enhanced connectivity.

Deciphering midbrain mechanisms underlying prepulse inhibition of startle

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.

Brain-wide Mapping of Mono-synaptic Afferents to Different Cell Types in the Laterodorsal Tegmentum

The laterodorsal tegmentum (LDT) is a brain structure involved in distinct behaviors including arousal, reward, and innate fear. How environmental stimuli and top-down control from high-order sensory and limbic cortical areas converge and coordinate in this region to modulate diverse behavioral outputs remains unclear. Using a modified rabies virus, we applied monosynaptic retrograde tracing to the whole brain to examine the LDT cell type specific upstream nuclei. The LDT received very strong midbrain and hindbrain afferents and moderate cortical and hypothalamic innervation but weak connections to the thalamus. The main projection neurons from cortical areas were restricted to the limbic lobe, including the ventral orbital cortex (VO), prelimbic, and cingulate cortices. Although different cell populations received qualitatively similar inputs, primarily via afferents from the periaqueductal gray area, superior colliculus, and the LDT itself, parvalbumin-positive (PV⁺) GABAergic cells received preferential projections from local LDT neurons. With regard to the different subtypes of GABAergic cells, a considerable number of nuclei, including those of the ventral tegmental area, central amygdaloid nucleus, and VO, made significantly greater inputs to somatostatin-positive cells than to PV⁺ cells. Diverse inputs to the LDT on a system-wide level were revealed.

The antagonistic relationship between aversive and appetitive emotional states in rats as studied by pharmacologically-induced ultrasonic vocalization from the nucleus accumbens and lateral septum

Rats can emit 22-kHz or 50-kHz ultrasonic vocalizations (USVs) in negative, as well as positive contexts which index their emotional state. 22-kHz USVs are emitted during aversive contexts and can be initiated by activation of the ascending cholinergic pathways originating from the laterodorsal tegmental nucleus or initiated pharmacologically by injection of cholinergic agonists into target areas of these pathways (medial cholinoceptive vocalization strip). Conversely, 50-kHz USVs are emitted during positive pro-social contexts and can be initiated by stimulation of ascending dopaminergic pathways originating from the ventral tegmental area or by injection of dopamine agonists into target areas of these pathways (nucleus accumbens shell). Recently, we have shown an inhibitory effect a positive emotional state has on the emission of carbachol-induced 22-kHz USVs from the anterior hypothalamic/medial preoptic area. However, this structure is a fragment of that cholinoceptive vocalization strip. We wanted to examine if we could observe similar effect when the aversive state is induced from the lateral septum, the most rostral division of the cholinoceptive vocalization strip. The results supported previous findings. First, microinjection of the dopamine agonist R-(-)-apomorphine into the nucleus accumbens shell resulted in increased emission of frequency modulated (FM) 50-kHz USVs that are regarded as signals expressing a positive emotional state in rats. Second, FM 50-kHz USVs and not flat (F) 50-kHz USVs were able to decrease 22-kHz USVs induced by microinjections of carbachol into the lateral septum. This research provides further support to the hypothesis that the initiation of a positive emotional state functionally antagonizes initiation of a negative emotional state in rats.

Emission of 22 kHz vocalizations in rats as an evolutionary equivalent of human crying: Relationship to depression

There is no clear relationship between crying and depression based on human neuropsychiatric observations. This situation originates from lack of suitable animal models of human crying. In the present article, an attempt will be made to answer the question whether emission of rat aversive vocalizations (22 kHz calls) may be regarded as an evolutionary equivalent of adult human crying. Using this comparison, the symptom of crying in depressed human patients will be reanalyzed. Numerous features and characteristics of rat 22 kHz aversive vocalizations and human crying vocalizations are equivalent. Comparing evolutionary, biological, physiological, neurophysiological, social, pharmacological, and pathological aspects have shown vast majority of common features. It is concluded that emission of rat 22 kHz vocalizations may be treated as an evolutionary vocal homolog of human crying, although emission of 22 kHz calls is not exactly the same phenomenon because of significant differences in cognitive processes between these species. It is further concluded that rat 22 kHz vocalizations and human crying vocalizations are both expressing anxiety and not depression. Analysis of the relationship between anxiety and depression reported in clinical studies supports this conclusion regardless of the nature and extent of comorbidity between these pathological states.

The Innate Alarm System and Subliminal Threat Presentation in Posttraumatic Stress Disorder: Neuroimaging of the Midbrain and Cerebellum

BACKGROUND

The innate alarm system, a network of interconnected midbrain, other brainstem, and thalamic structures, serves to rapidly detect stimuli in the environment prior to the onset of conscious awareness. This system is sensitive to threatening stimuli and has evolved to process these stimuli subliminally for hastened responding. Despite the conscious unawareness, the presentation of subliminal threat stimuli generates increased activation of limbic structures, including the amygdala and insula, as well as emotionally evaluative structures, including the cerebellum and orbitofrontal cortex. Posttraumatic stress disorder (PTSD) is associated with an increased startle response and decreased extinction learning to conditioned threat. The role of the innate alarm system in the clinical presentation of PTSD, however, remains poorly understood.

METHODS

Here, we compare midbrain, brainstem, and cerebellar activation in persons with PTSD (n = 26) and matched controls (n = 20) during subliminal threat presentation. Subjects were presented with masked trauma-related and neutral stimuli below conscious threshold. Contrasts of subliminal brain activation for the presentation of neutral stimuli were subtracted from trauma-related brain activation. Group differences in activation, as well as correlations between clinical scores and PTSD activation, were examined. Imaging data were preprocessed utilizing the spatially unbiased infratentorial template toolbox within SPM12.

RESULTS

Analyses revealed increased midbrain activation in PTSD as compared to controls in the superior colliculus, periaqueductal gray, and midbrain reticular formation during subliminal threat as compared to neutral stimulus presentation. Controls showed increased activation in the right cerebellar lobule V during subliminal threat presentation as compared to PTSD. Finally, a negative correlation emerged between PTSD patient scores on the Multiscale Dissociation Inventory for the Depersonalization/Derealization subscale and activation in the right lobule V of the cerebellum during the presentation of subliminal threat as compared to neutral stimuli.

CONCLUSION

We interpret these findings as evidence of innate alarm system overactivation in PTSD and of the prominent role of the cerebellum in the undermodulation of emotion observed in PTSD.

Rat 50-kHz ultrasonic vocalizations as a tool in studying neurochemical mechanisms that regulate positive emotional states

BACKGROUND

Adolescent and adult rats emit 50-kHz ultrasonic vocalizations (USVs) to communicate the appetitive arousal and the presence of positive emotional states to conspecifics.

NEW METHOD

Based on its communicative function, emission of 50-kHz USVs is increasingly being evaluated in preclinical studies of affective behavior, motivation and social behavior.

RESULTS

Emission of 50-kHz USVs is initiated by the activation of dopamine receptors in the shell subregion of the nucleus accumbens. However, several lines of evidence show that non-dopaminergic receptors may influence the numbers of 50-kHz USVs that are emitted, as well as the acoustic parameters of calls.

COMPARISON WITH EXISTING METHODS

Emission of 50-kHz USVs is a non-invasive method that may be used to study reward and motivation without the need for extensive training and complex animal manipulations. Moreover, emission of 50-kHz USVs can be used alone or combined with other well-standardized behavioral paradigms (e.g., conditioned place preference, self-administration).

CONCLUSIONS

This review summarizes the current evidence concerning molecular mechanisms that regulate the emission of 50-kHz USVs. Moreover, the review discusses the usefulness of 50-kHz USVs as an experimental tool to investigate how different neurotransmitter systems regulate the manifestations of positive emotional states, and also use of this tool in preclinical modeling of psychiatric diseases.

The Brainstem in Emotion: A Review

Emotions depend upon the integrated activity of neural networks that modulate arousal, autonomic function, motor control, and somatosensation. Brainstem nodes play critical roles in each of these networks, but prior studies of the neuroanatomic basis of emotion, particularly in the human neuropsychological literature, have mostly focused on the contributions of cortical rather than subcortical structures. Given the size and complexity of brainstem circuits, elucidating their structural and functional properties involves technical challenges. However, recent advances in neuroimaging have begun to accelerate research into the brainstem’s role in emotion. In this review, we provide a conceptual framework for neuroscience, psychology and behavioral science researchers to study brainstem involvement in human emotions. The “emotional brainstem” is comprised of three major networks – Ascending, Descending and Modulatory. The Ascending network is composed chiefly of the spinothalamic tracts and their projections to brainstem nuclei, which transmit sensory information from the body to rostral structures. The Descending motor network is subdivided into medial projections from the reticular formation that modulate the gain of inputs impacting emotional salience, and lateral projections from the periaqueductal gray, hypothalamus and amygdala that activate characteristic emotional behaviors. Finally, the brainstem is home to a group of modulatory neurotransmitter pathways, such as those arising from the raphe nuclei (serotonergic), ventral tegmental area (dopaminergic) and locus coeruleus (noradrenergic), which form a Modulatory network that coordinates interactions between the Ascending and Descending networks. Integration of signaling within these three networks occurs at all levels of the brainstem, with progressively more complex forms of integration occurring in the hypothalamus and thalamus. These intermediary structures, in turn, provide input for the most complex integrations, which occur in the frontal, insular, cingulate and other regions of the cerebral cortex. Phylogenetically older brainstem networks inform the functioning of evolutionarily newer rostral regions, which in turn regulate and modulate the older structures. Via these bidirectional interactions, the human brainstem contributes to the evaluation of sensory information and triggers fixed-action pattern responses that together constitute the finely differentiated spectrum of possible emotions.

Negative Affect-Associated USV Acoustic Characteristics Predict Future Excessive Alcohol Drinking and Alcohol Avoidance in Male P and NP Rats

BACKGROUND

Negative emotional status and adverse emotional events increase vulnerability to alcohol abuse. Ultrasonic vocalizations (USVs) emitted by rats are a well-established model of emotional status that can reflect positive or negative affective responses in real time. Most USV studies assess counts, yet each USV is a multidimensional data point characterized by several acoustic characteristics that may provide insight into the neurocircuitry underlying emotional response.

METHODS

USVs emitted from selectively bred alcohol-naïve and alcohol-experienced alcohol-preferring and nonpreferring rats (P and NP rats) were recorded during 4-hour sessions on alternating days over 4 weeks. Linear mixed modeling (LMM) and linear discriminant analysis (LDA) were applied to USV acoustic characteristics (e.g., frequency, duration, power, and bandwidth) of negative affect (22 to 28 kilohertz [kHz])- and positive (50 to 55 kHz) affect-related USVs.

RESULTS

Hundred percent separation between alcohol-naïve P and NP rats was achieved through a linear combination (produced by LDA) of USV acoustic characteristics of 22- to 28-kHz USVs, whereas poor separation (36.5%) was observed for 50- to 55-kHz USVs. 22- to 28-kHz LDA separation was high (87%) between alcohol-experienced P and NP rats, but was poor for 50- to 55-kHz USVs (57.3%). USV mean frequency and duration were the highest weighted characteristics in both the naïve and experienced 22- to 28-kHz LDA representations suggesting that alcohol experience does not alter the representations. LMM analyses of 22- to 28-kHz USV acoustic characteristics matched the LDA results. Poor LDA separation was observed between alcohol-naïve and alcohol-experienced P rats for both 22- to 28-kHz and 50- to 55-kHz USVs.

CONCLUSIONS

Advanced statistical analysis of negative affect-associated USV data predicts future behaviors of excessive alcohol drinking and alcohol avoidance in selectively bred rats. USV characteristics across rat lines reveal affect-related motivation to consume alcohol and may predict neural pathways mediating emotional response. Further characterization of these differences could delineate particular neurocircuitry and methods to ameliorate dysregulated emotional states often observed in human alcohol abusers.

Dementia praecox redux: a systematic review of the nicotinic receptor as a target for cognitive symptoms of schizophrenia

Most individuals with schizophrenia suffer some cognitive dysfunction: such deficits are predictive of longer-term functioning; and current dopamine-blocking antipsychotics have made little impact on this domain. There is a pressing need to develop novel pharmacological agents to tackle this insidious but most disabling of problems. The acetylcholinergic system is involved in cognitive and attentional processing, and its metabotropic and nicotinic receptors are widespread throughout the brain. Deficits in acetylcholinergic functioning occur in schizophrenia, and high rates of tobacco smoking have been posited to represent a form of self-medication. The nicotinic acetylcholine receptor (nAChR) has emerged as a putative target to improve cognitive deficits in schizophrenia, and this study systematically reviewed the emerging data. Nineteen studies were identified, covering three compound classes: agonists at the α7 and α 4β2 nAChRs, and positive allosteric modulators. Overall data are underwhelming: some studies showed significant improvements in cognition but as many studies had negative findings. It remains unclear if this represents drug limitations or nascent study methodology problems. The literature is particularly hindered by variability in inclusion of smokers, generally small sample sizes, and a lack of consensus on cognitive test batteries. Future work should evaluate longer-term outcomes, and, particularly, the effects of concomitant cognitive training.

Pharmacology of Ultrasonic Vocalizations in adult Rats: Significance, Call Classification and Neural Substrate

Pharmacological studies of emotional arousal and initiation of emotional states in rats measured by their ultrasonic vocalizations are reviewed. It is postulated that emission of vocalizations is an inseparable feature of emotional states and it evolved from mother-infant interaction. Positive emotional states are associated with emission of 50 kHz vocalizations that could be induced by rewarding situations and dopaminergic activation of the nucleus accumbens and are mediated by D1, D2, and partially D3 dopamine receptors. Three biologically significant subtypes of 50 kHz vocalizations have been identified, all expressing positive emotional states: (1) flat calls without frequency modulation that serve as contact calls during social interactions; (2) frequencymodulated calls without trills that signal rewarding and significantly motivated situation; and (3) frequency-modulated calls with trills or trills themselves that are emitted in highly emotional situations associated with intensive affective state. Negative emotional states are associated with emission of 22 kHz vocalizations that could be induced by aversive situations, muscarinic cholinergic activation of limbic areas of medial diencephalon and forebrain, and are mediated by M2 muscarinic receptors. Two biologically significant subtypes of 22 kHz vocalizations have been identified, both expressing negative emotional sates: (1) long calls that serve as alarm calls and signal external danger; and (2) short calls that express a state of discomfort without external danger. The positive and negative states with emission of vocalizations are initiated by two ascending reticular activating subsystems: the mesolimbic dopaminergic subsystem as a specific positive arousal system, and the mesolimbic cholinergic subsystem as a specific negative arousal system.

Assessment of sensorimotor gating following selective lesions of cholinergic pedunculopontine neurons

Sensorimotor gating is the state-dependent transfer of sensory information into a motor system. When this occurs at an early stage of the processing stream it enables stimuli to be filtered out or partially ignored, thereby reducing the demands placed on advanced systems. Prepulse inhibition (PPI) of the acoustic startle reflex (ASR) is the standard measure of sensorimotor gating. A brainstem-midbrain circuitry is widely viewed as mediating both PPI and ASR. In this circuitry, the pedunculopontine tegmental nucleus (PPTg) integrates sensory input and cortico-basal ganglia output and, via presumed cholinergic signaling, inhibits ASR-generating neurons within the reticular formation. Non-selective damage to all neuronal types within PPTg reduces PPI. We assessed whether this effect originates in the loss of cholinergic signaling by examining ASR and PPI in rats bearing non-selective (excitotoxic) or selective cholinergic (Dtx-UII) lesions of PPTg. Excitotoxic lesions had no effect on ASR but reduced PPI at all prepulse levels tested. In contrast, selective depletion of cholinergic neurons reduced ASR to the extent that PPI was not measurable with standard (10-20 s) inter-trial intervals. Subsequent testing revealed appreciable ASRs could be generated when the inter-trial interval was increased (180 s). Under these conditions, PPI was assessed and no deficits were found after lesions of cholinergic PPTg neurons. These results show that cholinergic output from PPTg is essential for rapidly regenerating the ASR, but has no influence on PPI. Results are discussed in terms of sensorimotor integration circuitry and psychiatric disorders that feature disrupted ASR and PPI.