The ascending basal forebrain cholinergic system

The anterior perforated substance (APS) revisited: Commented anatomical and imagenological views

INTRODUCTION

Since 2002, when we published our article about the anterior perforated substance (APS), the knowledge about the region has grown enormously.

OBJECTIVE

To make a better description of the anatomy of the zone with new dissection material added to the previous, to sustain the anatomical analysis of the MRI employing the SPACE sequence, interacting with our imagenology colleagues. Especially, we aim to identify and topographically localize by MRI the principal structures in APS-substantia innominata (SI).

METHOD

The presentation follows various steps: (1) location and boundaries of the zone and its neighboring areas; (2) schematic description of the region with simple outlines; (3) cursory revision of the SI and its three systems; (4) serial images of the dissections of the zone and its vessels, illustrated and completed when possible, by MRI images of a voluntary experimental subject (ES).

RESULTS

With this method, we could expose most of the structures of the region anatomically and imagenologically.

DISCUSSION

The zone can be approached for dissection with magnification and the habitual microsurgical instruments with satisfactory results. We think that fibers in this region should be followed by other anatomical methods in addition to tractography. The principal structures of ventral striopallidum and extended amygdala (EA) can be identified with the SPACE sequence. The amygdala and the basal ganglion of Meynert (BGM) are easily confused because of their similar signal. Anatomical clues can orient the clinician about the different clusters of the BGM in MRI.

CONCLUSIONS

The dissection requires a previous knowledge of the zone and a good amount of patience. The APS is a little space where concentrate essential vessels for the telencephalon, "en passage" or perforating, and neural structures of relevant functional import. From anatomical and MRI points of view, both neural and vascular structures follow a harmonious and topographically describable plan. The SPACE MRI sequence has proved to be a useful tool for identifying different structures in this area as the striatopallidal and EA. Anatomical knowledge of the fibers helps in the search of clusters of the basal ganglion.

Effect of Estradiol on Neurotrophin Receptors in Basal Forebrain Cholinergic Neurons: Relevance for Alzheimer's Disease

The basal forebrain is home to the largest population of cholinergic neurons in the brain. These neurons are involved in a number of cognitive functions including attention, learning and memory. Basal forebrain cholinergic neurons (BFCNs) are particularly vulnerable in a number of neurological diseases with the most notable being Alzheimer's disease, with evidence for a link between decreasing cholinergic markers and the degree of cognitive impairment. The neurotrophin growth factor system is present on these BFCNs and has been shown to promote survival and differentiation on these neurons. Clinical and animal model studies have demonstrated the neuroprotective effects of 17β-estradiol (E2) on neurodegeneration in BFCNs. It is believed that E2 interacts with neurotrophin signaling on cholinergic neurons to mediate these beneficial effects. Evidence presented in our recent study confirms that altering the levels of circulating E2 levels via ovariectomy and E2 replacement significantly affects the expression of the neurotrophin receptors on BFCN. However, we also showed that E2 differentially regulates neurotrophin receptor expression on BFCNs with effects depending on neurotrophin receptor type and neuroanatomical location. In this review, we aim to survey the current literature to understand the influence of E2 on the neurotrophin system, and the receptors and signaling pathways it mediates on BFCN. In addition, we summarize the physiological and pathophysiological significance of E2 actions on the neurotrophin system in BFCN, especially focusing on changes related to Alzheimer's disease.

Estradiol modulation of neurotrophin receptor expression in female mouse basal forebrain cholinergic neurons in vivo

The neuroprotective effect of estradiol (E2) on basal forebrain cholinergic neurons (BFCNs) has been suggested to occur as a result of E2 modulation of the neurotrophin system on these neurons. The present study provides a comprehensive examination of the relationship between E2 and neurotrophin signaling on BFCNs by investigating the effect of E2 deficiency on the expression levels of neurotrophin receptors (NRs), TrkA, TrkB, and p75 on BFCNs. The number of TrkA receptor-expressing choline acetyltransferase-positive neurons was significantly reduced in the medial septum (MS) in the absence of E2. A significant reduction in TrkB-expressing choline acetyltransferase-positive cells was also observed in ovariectomized mice in the MS and nucleus basalis magnocellularis (NBM). p75 receptor expression was reduced in the NBM and striatum but not in the MS. We also showed that estrogen receptor (ER)-α was expressed by a small percentage of TrkA- and TrkB-positive neurons in the MS (12%) and NBM (19%) and by a high percentage of TrkB-positive neurons in the striatum (69%). Similarly, ERα was expressed at low levels by p75 neurons in the MS (6%) and NBM (9%) but was not expressed on striatal neurons. Finally, ERα knockout using neuron-specific estrogen receptor-α knockout transgenic mice abolished all E2-mediated changes in the NR expression on BFCNs. These results indicate that E2 differentially regulates NR expression on BFCNs, with effects depending on the NR type and neuroanatomical location, and also provide some evidence that alterations in the NR expression are, at least in part, mediated via ERα.

Vascular dementia and the cholinergic pathways

Vascular cognitive impairment/vascular dementia have been the subject of a large number of studies, due to their high prevalence and broad preventive and compensatory therapeutic potential. The knowledge of the cerebral anatomy correlated to the vascular territories of irrigation enables understanding of clinical manifestations, as well as classification into the several types of syndromic presentations. The central cholinergic system exercises important neuromodulatory functions on cerebral circuits related to cognitive and behavioral integration, as well as on vasomotor control related to cerebral blood flow adjustments. The acquisition of data on the anatomy of the cholinergic pathways, including the localization of the nuclei of the basal prosencephalon and the routes of their projections, established an important milestone. The knowledge of the vascular distribution and of the trajectories of the cholinergic pathways allows identification of the strategic points where a vascular lesion can cause interruption. The ensuing denervation leads to cholinergic hypofunction in the involved territories. This information proves important to better evaluate the sites of vascular lesions, emphasizing their strategic localizations in relation to the cholinergic pathways, and offering more robust foundations for treatment aiming at enhancing cholinergic activity.

Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Alzheimer's disease (AD) is the most common form of degenerative dementia and is characterized by progressive impairment in cognitive function during mid- to late-adult life. Brains from AD patients show several distinct neuropathological features, including extracellular beta-amyloid-containing plaques, intracellular neurofibrillary tangles composed of abnormally phosphorylated tau, and degeneration of cholinergic neurons of the basal forebrain. In this review, we will present evidence implicating involvement of the basal forebrain cholinergic system in AD pathogenesis and its accompanying cognitive deficits. We will initially discuss recent results indicating a link between cholinergic mechanisms and the pathogenic events that characterize AD, notably amyloid-beta peptides. Following this, animal models of dementia will be discussed in light of the relationship between basal forebrain cholinergic hypofunction and cognitive impairments in AD. Finally, past, present, and future treatment strategies aimed at alleviating the cognitive symptomatology of AD by improving basal forebrain cholinergic function will be addressed.

Electrophysiological properties of cholinergic and noncholinergic neurons in the ventral pallidal region of the nucleus basalis in rat brain slices

The ventral pallidum is a major source of output for ventral corticobasal ganglia circuits that function in translating motivationally relevant stimuli into adaptive behavioral responses. In this study, whole cell patch-clamp recordings were made from ventral pallidal neurons in brain slices from 6- to 18-day-old rats. Intracellular filling with biocytin was used to correlate the electrophysiological and morphological properties of cholinergic and noncholinergic neurons identified by choline acetyltransferase immunohistochemistry. Most cholinergic neurons had a large whole cell conductance and exhibited marked fast (i.e., anomalous) inward rectification. These cells typically did not fire spontaneously, had a hyperpolarized resting membrane potential, and also exhibited a prominent spike afterhyperpolarization (AHP) and strong spike accommodation. Noncholinergic neurons had a smaller whole cell conductance, and the majority of these cells exhibited marked time-dependent inward rectification that was due to an h-current. This current activated slowly over several hundred milliseconds at potentials more negative than -80 mV. Noncholinergic neurons fired tonically in regular or intermittent patterns, and two-thirds of the cells fired spontaneously. Depolarizing current injection in current clamp did not cause spike accommodation but markedly increased the firing frequency and in some cells also altered the pattern of firing. Spontaneous tetrodotoxin-sensitive GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) were frequently recorded in noncholinergic neurons. These results show that cholinergic pallidal neurons have similar properties to magnocellular cholinergic neurons in other parts of the forebrain, except that they exhibit strong spike accommodation. Noncholinergic ventral pallidal neurons have large h-currents that could have a physiological role in determining the rate or pattern of firing of these cells.

Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network

Distinct motor patterns are selected from a multifunctional neuronal network by activation of different modulatory projection neurons. Subsets of these projection neurons can contain the same neuromodulator(s), yet little is known about the relative influence of such neurons on network activity. We have addressed this issue in the stomatogastric nervous system of the crab Cancer borealis. Within this system, there is a neuronal network in the stomatogastric ganglion (STG) that produces many versions of the pyloric and gastric mill rhythms. These different rhythms result from activation of different projection neurons that innervate the STG from neighboring ganglia and modulate STG network activity. Three pairs of these projection neurons contain the neuropeptide proctolin. These include the previously identified modulatory proctolin neuron and modulatory commissural neuron 1 (MCN1) and the newly identified modulatory commissural neuron 7 (MCN7). We document here that each of these neurons contains a unique complement of cotransmitters and that each of these neurons elicits a distinct version of the pyloric motor pattern. Moreover, only one of them (MCN1) also elicits a gastric mill rhythm. The MCN7-elicited pyloric rhythm includes a pivotal switch by one STG network neuron from playing a minor to a major role in motor pattern generation. Therefore, modulatory neurons that share a peptide transmitter can elicit distinct motor patterns from a common target network.