Neuropeptides in Alzheimer's disease: a review and morphological results

Aging, but not tau pathology, impacts olfactory performances and somatostatin systems in THY-Tau22 mice

Somatostatin (SOM) cortical levels decline in Alzheimer's disease (AD) in correlation with cognitive impairment severity, the latter being closely related to the presence of neurofibrillary tangles. Impaired olfaction is another hallmark of AD tightly related to tau pathology in the olfactory pathways. Recent studies showed that SOM modulates olfactory processing, suggesting that alterations in SOM levels participate to olfactory deficits in AD. Herein, we first observed that human olfactory peduncle and cortex are enriched in SOM cells and fibers, in aged postmortem brains. Then, the possible link between SOM alterations and olfactory deficits was evaluated by exploring the impact of age and tau hyperphosphorylation on olfactory SOM networks and behavioral performances in THY-Tau22 mice, a tauopathy transgenic model. Distinct molecular repertoires of SOM peptide and receptors were associated to sensory or cortical olfactory processing structures. Aging mainly affected SOM neurotransmission in piriform and entorhinal cortex in wild-type mice, although olfactory performances decreased. However, no further olfactory impairment was evidenced in THY-Tau22 mice until 12 months although tau pathology early affected olfactory cortical structures. Thus, tau hyperphosphorylation per se has a limited impact on olfactory performances in THY-Tau22 mice.

Postnatal development of somatostatin- and neuropeptide Y-immunoreactive neurons in rat cerebral cortex: a double-labeling immunohistochemical study

The postnatal development of somatostatin (SOM)- and neuropeptide Y (NPY)-immunoreactive (ir) neurons was examined in rat cerebral cortex, while considering their coexistence in cortical neurons. Using double immunohistochemical staining for SOM and NPY with diaminobenzidine and benzidine dihydrochloride as chromogens, we subdivided immunoreactive cells into double-labeled SOM/NPY-, SOM only-, and NPY only-ir neurons. SOM/NPY- and SOM only-ir neurons were detectable even at the day of birth, in contrast on NPY only-ir cells which first appeared in most cortices from week two. The morphological features of double-labeled SOM/NPY neurons differed with those of SOM only- and NPY only-ir neurons. No apparent changes in the shape and size of single-labeled neurons occurred with age; throughout their postnatal life they were round and ovoid, had a thin rim of perinuclear cytoplasm, and short processes. However, the features of SOM/NPY-ir neurons were not consistent according to postnatal age; by day P7, these neurons showed immature features and they began to show more advanced neuronal characteristics by week P2, when they had a larger and more intensely-stain cytoplasm. In addition, their processes were longer, thicker and more complex than at earlier ages. At this age, SOM/NPY-ir somata were close to their near maximum size. From week P4, they became smaller and were lightly labeled. SOM/NPY-ir somata were larger than SOM only- and NYP only-ir somata at and after two weeks of age. The present results, showing different postnatal maturation patterns such as time of appearance and morphological features, raise the possibilities that double-labeled SOM/NPY and single-labeled immunoreactive neurons may be different populations regulated by different mechanisms in their development, and with different functional properties during development.

Widespread deficits in somatostatin but not neuropeptide Y concentrations in Alzheimer's disease cerebral cortex

Somatostatin-like immunoreactivity (SLI) and neuropeptide Y-like immunoreactivity (NPYLI) were measured in the cerebral cortex of 49 patients with Alzheimer's disease (AD), and 9 elderly controls. Concentrations of SLI were lower in AD patients relative to controls in 9 of 10 cortical regions. In contrast, no significant differences in NPYLI concentrations between the two groups were observed in any of 10 regions. These studies suggest a dissociation between SLI deficits and NPYLI concentrations in the postmortem cerebral cortex of AD patients. The apparent sparing of NPYLI-containing neurons suggests that neuropeptide Y may be located within a separate group of neurons compared to somatostatin.

The role of galanin in cholinergically-mediated memory processes

1. Galanin, a 29 amino-acid neuroactive peptide, has been shown to affect diverse processes throughout the nervous system and to coexist with several "classical" neurotransmitters, including norepinephrine, serotonin, and acetylcholine. 2. Galanin coexists with acetylcholine in neurons of the medial septum, diagonal band, and nucleus basalis of Meynert, cells which degenerate during the course of Alzheimer's disease. 3. In the ventral hippocampus, galanin inhibits the release of acetylcholine and inhibits carbachol stimulated phosphatidyl inositol hydrolysis. 4. Galanin impairs choice accuracy in learning and memory paradigms in rats, and is therefore hypothesized to be a contributory factor in the memory and cognitive disabilities found in Alzheimer's patients. 5. Newly developed galanin antagonists, by eliminating putative inhibitory effects of endogenous galanin on cholinergic function, may serve as useful therapies for memory disorders.

Effects of chronic codergocrine mesylate administration on the brain somatostatinergic system in aged rats

Codergocrine mesylate (dihydroergotoxine; DHET), which is an ergot derivative, has been reported to counteract some age-induced impairments in brain function, but the mechanism of these effects is not known. We examined the effect of chronic DHET administration on the somatostatinergic system in the brains of aged rats. Intraperitoneal injections of DHET (1 mg/kg per day) or of vehicle were given to aged rats for 14 days, and resulted in a significant increase in somatostatin (SOM) receptor binding in all six brain regions examined except the hindbrain. DHET had no effect on SOM receptor binding in the brains of young-adult rats. However, the SOM concentration in aged rats was nearly identical to that in young-adult rats and the SOM concentration in different brain areas did not change after chronic administration of DHET. Thus, the present results suggest that chronic administration of DHET can ameliorate at least one of the age-induced impairments of brain somatostatinergic function.

Growth hormone secretion in Alzheimer's disease: 24-hour profile of basal levels and response to stimulation and suppression studies

The 24-h growth hormone secretory pattern and GH response to growth hormone releasing hormone, the alpha 2-adrenoceptor agonist clonidine and the somatostatin-analogue SMS 201-995 were evaluated in 9 patients with Alzheimer's disease and 9 age- and body body-matched control subjects. The secretory profile did not differentiate between patients and controls. Both secreted the largest amount of GH during the early nighthours between 22.00-02.00, whereas the majority of daytime GH levels were below the assay's detection limit (0.4 ng/ml). No difference was found in GH response to GHRH between patients and controls. All subjects showed significantly enhanced GH secretion after GHRH. Dividing the patients into two groups according to age-of-onset (less than 60 years greater than), there was a trend toward larger GH responses to GHRH for the early-onset group. No other parameter differentiated the groups. GH levels after clonidine were blunted in all subjects but one AD patient, probably due to an age-dependent attenuation frequently observed in subjects over 45 years of age. Finally, the administration of the somatostatin-analogue did not render conclusive results, since spontaneous decline of GH concentration was already beginning 2 hours before the drug was given and continued steadily throughout the observation period. In conclusion, patients with only mild to moderate degree of Alzheimer's disease have no prominent changes in GH regulation.

Somatostatin hyperpolarizes neurons and inhibits spontaneous activity in the rat dorsolateral septal nucleus

Intracellular recordings were made from rat brain neurons in a submerged slice preparation containing the dorsolateral septal nucleus (DLSN). Somatostatin-14 (SS-14) was applied to these neurons by superfusing solutions containing known concentrations of the peptide or by pressure ejection from micropipettes. With either method of treatment, SS-14 produced membrane hyperpolarization and decreased membrane resistance in a concentration-dependent manner. The hyperpolarizing response to SS-14 occurred in virtually all neurons tested and appeared to result from a direct action on DLSN neurons mediated by an increased permeability to potassium ions. The SS-14-induced membrane hyperpolarization was not blocked by naloxone, bicuculline, tetrodotoxin, or calcium-free, high-magnesium superfusion media. In a small number of neurons, SS-14 application produced a membrane depolarization which did not exhibit clear concentration-dependence and was blocked by superfusion of calcium-free, high-magnesium media indicating an indirect action. These findings reveal that SS-14 is a potent inhibitor of DLSN neurons in vitro and provide the first evidence that receptors for this putative neurotransmitter are located on postsynaptic neurons in this nucleus. Synaptically released SS-14 may play an important role in the modulation of septohippocampal function.

Direct synaptic contacts of catecholamine fibers on neuropeptide Y-containing neurons of the rat cerebral cortex

A direct synapse between catecholamine (CA) fibers and neuropeptide Y (NPY)-containing neurons was demonstrated in rat cerebral cortex by using an immunohistochemical double-staining method under electron microscopy.

Co-existence of galanin and acetylcholine: is galanin involved in memory processes and dementia?

Galanin-like immunoreactivity co-exists with choline acetyltransferase-like immunoreactivity in neurons of the septal-hippocampal and nucleus basalis of Meynert-neocortical pathways. These structures mediate some forms of cognition, and characteristically degenerate in Alzheimer's disease. Biochemical, neurophysiological and behavioral studies indicate that galanin acts as an inhibitory modulator of cholinergic function. In this article, we consider the possibility of a role for galanin in memory processes and dementia.

Intracerebral drug delivery in rats with lesion-induced memory deficits

Pharmacological treatments directed at increasing cortical acetylcholine activity in patients with Alzheimer's disease have largely been disappointing, perhaps because denervated areas of brain may not be exposed to adequate amounts of drug. A new method has been developed to enable localized intracerebral delivery of neurotransmitter substances using a polymeric drug delivery system. Microspheres of a polyanhydride sebacic acid copolymer were impregnated with bethanechol, an acetylcholinesterase-resistant cholinomimetic. Twenty rats received bilateral fimbria-fornix lesions, producing cholinergic denervation of the hippocampus and marked impairment in spatial memory. The animals were trained for 2 weeks to run after which they received bilateral intrahippocampal implants of saline (five rats), blank polymer (five rats), or bethanechol-impregnated polymer (10 rats). Following implantation, spatial memory was assessed by radial-maze performance testing for 40 days. Untreated lesioned rats showed persistently poor spatial memory, entering maze arms with near random frequency. Similarly, animals treated with saline and blank polymer did not improve after implantation. Rats treated with bethanechol-impregnated microspheres, however displayed significant improvement within 10 days after implantation; this improvement persisted for the duration of the experiment (p less than 0.05, Student's t-test). Histological analysis of regional acetylcholinesterase staining showed widespread loss of activity throughout the hippocampus bilaterally in all animals. The microsphere implants were visible within the hippocampus, with minimal reactive changes in surrounding brain. It is concluded that intracerebral polymeric drug delivery successfully reversed lesion-induced memory deficits, and has potential as a neurosurgical treatment method for Alzheimer's disease and other neurodegenerative disorders.

Subpopulations of somatostatin 28-immunoreactive neurons display different vulnerability in senile dementia of the Alzheimer type

We tested whether the vulnerability of somatostatin (SST) neurons in senile dementia of the Alzheimer type (SDAT) depended upon their co-localization with neuropeptide Y (NPY). Density estimates of SST28- and NPY-immunoreactive neurons and percentage of double-labeled SST-NPY neurons were obtained in the cortex (areas 9 and 25) and the bed nucleus of stria terminalis (BST), in 6 SDAT and 5 control cases. Counts of senile plaques (SP) and neurofibrillary tangles (NFT) were done on thioflavin S stains. In both cortical areas, a decrease in the density of SST28-IR neurons was found in SDAT cases (-60% in area 25 and -80% in area 9), whereas density of NPY-IR neurons was unchanged. Accordingly, the proportion of single-labeled SST neurons decreased; this decrease was significantly correlated with SP (r = -0.89, P less than 0.001). We conclude that single SST-IR neurons, in cortical layers II-III, and V, are preferentially lost relative to co-localized SST-NPY neurons. In the BST, no significant reduction of SST-IR, NPY-IR neurons nor of the percentage of single labeled SST neurons was found, despite the presence of SP. Thus one subpopulation of SST neurons, defined by associated neurochemical characters (not co-localized with NPY nor with NADPH diaphorase) and by topography (cortical layers III and V) appears to be particularly vulnerable in SDAT. The potential importance of their position in neural circuitry is emphasized.

Neuron numbers and dendritic extent in normal aging and Alzheimer's disease

Factors which limit the interpretation of studies of aging brain include: secular trends, species and strain differences, effects of tissue processing, and bias which may be introduced at many levels of an experimental design. With these limitations considered, evidence is reviewed regarding neuron numbers and dendritic extent in normally aging rodent, monkey and human brain and in Alzheimer's disease. It is concluded that neuron loss and change in dendritic extent in normal aging are regionally specific, and that corresponding brain regions do not always change in similar ways in rodents and primates. It is suggested that such differences may, in part, be due to inconsistent definitions of 'aged' among species. In Alzheimer's disease there is excess neuron loss and dendritic regression in some, but not all, brain regions. Measures of the morphological substrates of brain function show appreciable overlap between AD and control groups. It is hypothesized that the static, post-mortem status of brain morphology may not adequately reflect the functional capabilities of the dynamic morphology of the living brain.

Qualitative and quantitative changes in normal aging and Alzheimer's disease

Qualitative changes in normal aging and Alzheimer's disease (AD) are as important as quantitative changes. Although qualitative changes do not c-vary with quantitative differences in every respect, quantitative changes, nevertheless, often accurately reflect qualitative changes. Moreover, the complementary use of each provides a more detailed characterization of brain changes with aging and AD. The quantitative and qualitative changes of choline acetyltransferase, as well as somatostatin-containing neurons and dendrites, in human cortex are described as examples of this.