Basal Forebrain Cholinergic Neurons Selectively Drive Coordinated Motor Learning in Mice

The basal forebrain cholinergic system as target for cell replacement therapy in Parkinson's disease

In recent years there has been a renewed interest in the basal forebrain cholinergic system as a target for the treatment of cognitive impairments in patients with Parkinson's disease, due in part to the need to explore novel approaches to treat the cognitive symptoms of the disease and in part to the development of more refined imaging tools that have made it possible to monitor the progressive changes in the structure and function of the basal forebrain system as they evolve over time. In parallel, emerging technologies allowing the derivation of authentic basal forebrain cholinergic neurons from human pluripotent stem cells are providing new powerful tools for the exploration of cholinergic neuron replacement in animal models of Parkinson's disease-like cognitive decline. In this review, we discuss the rationale for cholinergic cell replacement as a potential therapeutic strategy in Parkinson's disease and how this approach can be explored in rodent models of Parkinson's disease-like cognitive decline, building on insights gained from the extensive animal experimental work that was performed in rodent and primate models in the 1980s and 90s. Although therapies targeting the cholinergic system have so far been focused mainly on patients with Alzheimer's disease, Parkinson's disease with dementia may be a more relevant condition. In Parkinson's disease with dementia, the basal forebrain system undergoes progressive degeneration and the magnitude of cholinergic cell loss has been shown to correlate with the level of cognitive impairment. Thus, cell therapy aimed to replace the lost basal forebrain cholinergic neurons represents an interesting strategy to combat some of the major cognitive impairments in patients with Parkinson's disease dementia.

Neuroprotection of Cholinergic Neurons with a Tau Aggregation Inhibitor and Rivastigmine in an Alzheimer's-like Tauopathy Mouse Model

Basal forebrain cholinergic dysfunction, most likely linked with tau protein aggregation, is a characteristic feature of Alzheimer's disease (AD). Recent evidence suggests that tau protein is a putative target for the treatment of dementia, and the tau aggregation inhibitor, hydromethylthionine mesylate (HMTM), has emerged as a potential disease-modifying treatment. However, its efficacy was diminished in patients already receiving approved acetylcholinesterase inhibitors. In this study, we ask whether this negative interaction can also be mimicked in experimental tau models of AD and whether the underlying mechanism can be understood. From a previous age profiling study, 6-month-old line 1 (L1) tau transgenic mice were characterized by a severe reduction in several cholinergic markers. We therefore assessed whether long-term pre-exposure with the acetylcholinesterase inhibitor rivastigmine alone and in conjunction with the tau aggregation inhibitor HMTM can reverse cholinergic deficits in L1. Rivastigmine and HMTM, and combinations of the two compounds were administered orally for 11 weeks to both L1 and wild-type mice. The brains were sectioned with a focus on the basal forebrain, motor cortex and hippocampus. Immunohistochemical staining and quantification of choline acetyltransferase (ChAT), tyrosine kinase A (TrkA)-positive neurons and relative optical intensity (ROI) for vesicular acetylcholine transporter (VAChT), and acetylcholinesterase (AChE) reactivity confirmed reversal of the diminished cholinergic phenotype of interneurons (nucleus accumbens, striatum) and projection neurons (medial septum, nucleus basalis magnocellularis) by HMTM, to a greater extent than by rivastigmine alone in L1 mice. Combined administration did not yield additivity but, in most proxies, led to antagonistic effects in which rivastigmine decreased the benefits shown with HMTM alone. Local markers (VAChT and AChE) in target structures of the basal forebrain, motor cortex and hippocampal CA3 seemed to be normalized by HMTM, but not by rivastigmine or the combination of both drugs. HMTM, which was developed as a tau aggregation inhibitor, strongly decreased the tau load in L1 mice, however, not in combination with rivastigmine. Taken together, these data confirm a cholinergic phenotype in L1 tau transgenic mice that resembles the deficits observed in AD patients. This phenotype is reversible by HMTM, but at the same time appears to be subject to a homeostatic regulation induced by chronic pre-treatment with an acetylcholinesterase inhibitor, which interferes with the efficacy of HMTM. The strongest phenotypic reversal coincided with a normalization of the tau load in the cortex and hippocampus of L1, suggesting that tau accumulation underpins the loss of cholinergic markers in the basal forebrain and its projection targets.

Abnormally decreased functional connectivity of the right nucleus basalis of Meynert in Alzheimer's disease patients with depression symptoms

Dysfunction of the basal forebrain is the main pathological feature in patients with Alzheimer's disease (AD). The aim of this study was to explore whether depressive symptoms cause changes in the functional network of the basal forebrain in AD patients. We collected MRI data from depressed AD patients (n = 24), nondepressed AD patients (n = 14) and healthy controls (n = 20). Resting-state functional magnetic resonance imaging data and functional connectivity analysis were used to study the characteristics of the basal forebrain functional network of the three groups of participants. The functional connectivity differences among the three groups were compared using ANCOVA and post hoc analyses. Compared to healthy controls, depressed AD patients showed reduced functional connectivity between the right nucleus basalis of Meynert and the left supramarginal gyrus and the supplementary motor area. These results increase our understanding of the neural mechanism of depressive symptoms in AD patients.

Role of Basal Forebrain Neurons in Adrenomyeloneuropathy in Mice and Humans

OBJECTIVE

X-linked adrenoleukodystrophy is caused by mutations in the peroxisomal half-transporter ABCD1. The most common manifestation is adrenomyeloneuropathy, a hereditary spastic paraplegia of adulthood. The present study set out to understand the role of neuronal ABCD1 in mice and humans with adrenomyeloneuropathy.

METHODS

Neuronal expression of ABCD1 during development was assessed in mice and humans. ABCD1-deficient mice and human brain tissues were examined for corresponding pathology. Next, we silenced ABCD1 in cholinergic Sh-sy5y neurons to investigate its impact on neuronal function. Finally, we tested adeno-associated virus vector-mediated ABCD1 delivery to the brain in mice with adrenomyeloneuropathy.

RESULTS

ABCD1 is highly expressed in neurons located in the periaqueductal gray matter, basal forebrain and hypothalamus. In ABCD1-deficient mice (Abcd1-/y), these structures showed mild accumulations of α-synuclein. Similarly, healthy human controls had high expression of ABCD1 in deep gray nuclei, whereas X-ALD patients showed increased levels of phosphorylated tau, gliosis, and complement activation in those same regions, albeit not to the degree seen in neurodegenerative tauopathies. Silencing ABCD1 in Sh-sy5y neurons impaired expression of functional proteins and decreased acetylcholine levels, similar to observations in plasma of Abcd1-/y mice. Notably, hind limb clasping in Abcd1-/y mice was corrected through transduction of ABCD1 in basal forebrain neurons following intracerebroventricular gene delivery.

INTERPRETATION

Our study suggests that the basal forebrain-cortical cholinergic pathway may contribute to dysfunction in adrenomyeloneuropathy. Rescuing peroxisomal transport activity in basal forebrain neurons and supporting glial cells might represent a viable therapeutic strategy. ANN NEUROL 2024;95:442-458.

A head-to-head comparison of two DREADD agonists for suppressing operant behavior in rats via VTA dopamine neuron inhibition

RATIONALE

Designer receptors exclusively activated by designer drugs (DREADDs) are a tool for "remote control" of defined neuronal populations during behavior. These receptors are inert unless bound by an experimenter-administered designer drug, commonly clozapine-n-oxide (CNO). However, questions have emerged about the suitability of CNO as a systemically administered DREADD agonist.

OBJECTIVES

Second-generation agonists such as JHU37160 (J60) have been developed, which may have more favorable properties than CNO. Here we sought to directly compare effects of CNO (0, 1, 5, & 10 mg/kg, i.p.) and J60 (0, 0.03, 0.3, & 3 mg/kg, i.p.) on operant food pursuit.

METHODS

Male and female TH:Cre + rats and their wildtype (WT) littermates received cre-dependent hM4Di-mCherry vector injections into ventral tegmental area (VTA), causing inhibitory DREADD expression in VTA dopamine neurons of TH:Cre + rats. All rats were trained to stably lever press for palatable food on a fixed ratio 10 schedule, and doses of both agonists were tested on separate days in counterbalanced order.

RESULTS

All three CNO doses reduced operant rewards earned in rats with DREADDs, and no CNO dose had behavioral effects in WT controls. The highest J60 dose tested significantly reduced responding in DREADD rats, but this dose also increased responding in WTs, indicating non-specific effects. The magnitude of CNO and J60 effects in TH:Cre + rats were correlated and were present in both sexes.

CONCLUSIONS

Findings demonstrate the usefulness of directly comparing DREADD agonists when optimizing behavioral chemogenetics, and highlight the importance of proper controls, regardless of the DREADD agonist employed.

Peptidergic modulation of motor neuron output via CART signaling at C bouton synapses

The intensity of muscle contraction, and therefore movement vigor, needs to be adaptable to enable complex motor behaviors. This can be achieved by adjusting the properties of motor neurons, which form the final common pathway for all motor output from the central nervous system. Here, we identify roles for a neuropeptide, cocaine- and amphetamine-regulated transcript (CART), in the control of movement vigor. We reveal distinct but parallel mechanisms by which CART and acetylcholine, both released at C bouton synapses on motor neurons, selectively amplify the output of subtypes of motor neurons that are recruited during intense movement. We find that mice with broad genetic deletion of CART or selective elimination of acetylcholine from C boutons exhibit deficits in behavioral tasks that require higher levels of motor output. Overall, these data uncover spinal modulatory mechanisms that control movement vigor to support movements that require a high degree of muscle force.

Chemogenetic activation of the ventral subiculum-BNST pathway reduces context fear expression

An inability to reduce fear in nonthreatening environments characterizes many anxiety disorders. The pathway from the ventral subiculum (vSUB) to the bed nucleus of the stria terminalis (BNST) is more active in safe contexts than in aversive ones, as indexed by FOS expression. Here, we used chemogenetic techniques to specifically activate the vSUB-BNST pathway during both context and cued fear expression by expressing a Cre-dependent hM3D(Gq) receptor in BNST-projecting vSUB neurons. Activation of the vSUB-BNST pathway reduced context but not cued fear expression. These data suggest that the vSUB-BNST pathway contributes to behavioral responses to nonaversive contexts.

A head-to-head comparison of two DREADD agonists for suppressing operant behavior in rats via VTA dopamine neuron inhibition

Rationale

Designer receptors exclusively activated by designer drugs (DREADDs) are a tool for "remote control" of defined neuronal populations during behavior. These receptors are inert unless bound by an experimenter-administered designer drug, most commonly clozapine-n-oxide (CNO). However, questions have emerged about the suitability of CNO as a systemically administered DREADD agonist.

Objectives

Second-generation agonists such as JHU37160 (J60) have been developed, which may have more favorable properties than CNO. Here we sought to directly compare effects of CNO (0, 1, 5, & 10 mg/kg, i.p.) and J60 (0, 0.03, 0.3, & 3 mg/kg, i.p.) on operant food pursuit.

Methods

Male and female TH:Cre+ rats and their wildtype (WT) littermates received cre-dependent hM4Di-mCherry vector injections into ventral tegmental area (VTA), causing inhibitory DREADD expression in VTA dopamine neurons in TH:Cre+ rats. Rats were trained to stably lever press for palatable food on a fixed ratio 10 schedule, and doses of both agonists were tested on separate days in a counterbalanced order.

Results

All three CNO doses reduced operant food seeking in rats with DREADDs, and no CNO dose had behavioral effects in WT controls. The highest tested J60 dose significantly reduced responding in DREADD rats, but this dose also increased responding in WTs, indicating non-specific effects. The magnitude of CNO and J60 effects in TH:Cre+ rats were correlated and were present in both sexes.

Conclusions

Findings demonstrate the usefulness of directly comparing DREADD agonists when optimizing behavioral chemogenetics, and highlight the importance of proper controls, regardless of the DREADD agonist employed.