Age-related change of neuropeptide Y-immunoreactive neurons in the cerebral cortex of aged rats

Neuropeptides: The Evergreen Jack-of-All-Trades in Neuronal Circuit Development and Regulation

Neuropeptides are key modulators of adult neurocircuits, balancing their sensitivity to both excitation and inhibition, and fine-tuning fast neurotransmitter action under physiological conditions. Here, we reason that transient increases in neuropeptide availability and action exist during brain development for synapse maturation, selection, and maintenance. We discuss fundamental concepts of neuropeptide signaling at G protein-coupled receptors (GPCRs), with a particular focus on how signaling at neuropeptide GPCRs could underpin neuronal morphogenesis. We use galanin, a 29/30 amino acid-long neuropeptide, as an example for its retrograde release from the dendrites of thalamic neurons to impact the selection and wiring of sensory afferents originating at the trigeminal nucleus through galanin receptor 1 (GalR1) engagement. Thus, we suggest novel roles for neuropeptides, expressed transiently or permanently during both pre- and postnatal neuronal circuit development, with potentially life-long effects on circuit layout and ensuing behavioral operations.

Age-related changes in neurochemical components and retinal projections of rat intergeniculate leaflet

Aging leads to several anatomical and functional deficits in circadian timing system. In previous works, we observed morphological alterations with age in hypothalamic suprachiasmatic nuclei, one central component of this system. However, there are few data regarding aging effects on other central components of this system, such as thalamic intergeniculate leaflet (IGL). In this context, we studied possible age-related alterations in neurochemical components and retinal projections of rat IGL. For this goal, young (3 months), adult (13 months), and aged (23 months) Wistar rats were submitted to an intraocular injection of neural tracer, cholera toxin subunit b (CTb), 5 days before a tissue fixation process by paraformaldehyde perfusion. Optical density measurements and cell count were performed at digital pictures of brain tissue slices processed by immunostaining for glutamic acid decarboxylase (GAD), enkephalin (ENK), neuropeptide Y (NPY) and CTb, characteristic markers of IGL and its retinal terminals. We found a significant age-related loss in NPY immunoreactive neurons, but not in immunoreactivity to GAD and ENK. We also found a decline of retinal projections to IGL with age. We conclude aging impairs both a photic environmental clue afferent to IGL and a neurochemical expression which has an important modulatory circadian function, providing strong anatomical correlates to functional deficits of the aged biological clock.

Old-onset caloric restriction effects on neuropeptide Y- and somatostatin-containing neurons and on cholinergic varicosities in the rat hippocampal formation

Caloric restriction is able to delay age-related neurodegenerative diseases and cognitive impairment. In this study, we analyzed the effects of old-onset caloric restriction that started at 18 months of age, in the number of neuropeptide Y (NPY)- and somatostatin (SS)-containing neurons of the hippocampal formation. Knowing that these neuropeptidergic systems seem to be dependent of the cholinergic system, we also analyzed the number of cholinergic varicosities. Animals with 6 months of age (adult controls) and with 18 months of age were used. The animals aged 18 months were randomly assigned to controls or to caloric-restricted groups. Adult and old control rats were maintained in the ad libitum regimen during 6 months. Caloric-restricted rats were fed, during 6 months, with 60 % of the amount of food consumed by controls. We found that aging induced a reduction of the total number of NPY- and SS-positive neurons in the hippocampal formation accompanied by a decrease of the cholinergic varicosities. Conversely, the 24-month-old-onset caloric-restricted animals maintained the number of those peptidergic neurons and the density of the cholinergic varicosities similar to the 12-month control rats. These results suggest that the aging-associated reduction of these neuropeptide-expressing neurons is not due to neuronal loss and may be dependent of the cholinergic system. More importantly, caloric restriction has beneficial effects in the NPY- and SS-expressing neurons and in the cholinergic system, even when applied in old age.

Trajectory of the main GABAergic interneuron populations from early development to old age in the rat primary auditory cortex

In both humans and rodents, decline in cognitive function is a hallmark of the aging process; the basis for this decrease has yet to be fully characterized. However, using aged rodent models, deficits in auditory processing have been associated with significant decreases in inhibitory signaling attributed to a loss of GABAergic interneurons. Not only are these interneurons crucial for pattern detection and other large-scale population dynamics, but they have also been linked to mechanisms mediating plasticity and learning, making them a prime candidate for study and modeling of modifications to cortical communication pathways in neurodegenerative diseases. Using the rat primary auditory cortex (A1) as a model, we probed the known markers of GABAergic interneurons with immunohistological methods, using antibodies against gamma aminobutyric acid (GABA), parvalbumin (PV), somatostatin (SOM), calretinin (CR), vasoactive intestinal peptide (VIP), choline acetyltransferase (ChAT), neuropeptide Y (NPY), and cholecystokinin (CCK) to document the changes observed in interneuron populations across the rat's lifespan. This analysis provided strong evidence that several but not all GABAergic neurons were affected by the aging process, showing most dramatic changes in expression of parvalbumin (PV) and somatostatin (SOM) expression. With this evidence, we show how understanding these trajectories of cell counts may be factored into a simple model to quantify changes in inhibitory signaling across the course of life, which may be applied as a framework for creating more advanced simulations of interneuronal implication in normal cerebral processing, normal aging, or pathological processes.

Reduced density of neuropeptide Y neurons in the somatosensory cortex of old male and female rats: relation to cholinergic depletion and recovery after nerve growth factor treatment

Synthesis of neuropeptide Y in the neocortex and activity of the basalocortical cholinergic system are both reduced in the aging brain. We hypothesized that, by stimulating the activity of the basal forebrain cholinergic neurons, nerve growth factor might also be capable of restoring the synthesis of neuropeptide Y in cortical neurons. Old male and female rats were intraventricularly infused with nerve growth factor for 14 days and their brains were analyzed in order to quantify the densities of neuropeptide Y-immunoreactive neurons and of fiber varicosities stained for vesicular acetylcholine transporter protein in layers II/III, V and VI of the primary somatosensory barrel-field cortex. The areal densities of neuropeptide Y neurons and of vesicular acetylcholine transporter protein varicosities in all cortical laminae were found to be dramatically decreased in old rats when compared with young rats. However, infusions of nerve growth factor, known to exert a powerful trophic effect upon cortically projecting cholinergic neurons, have led to considerable recovery of vesicular acetylcholine transporter protein-positive terminal fields, which was paralleled by complete restoration of function in neuropeptide Y-producing neurons. With respect to the gender differences, although the density of cortical neuropeptide Y neurons was found to be significantly higher in young females than in young males and the opposite was true for vesicular acetylcholine transporter protein-positive varicosities, the general pattern of age- and treatment-related changes in these neurochemical markers was similar in both sexes. Overall, the age- and treatment-related variations in the density of cortical neuropeptide Y cells were found to correlate with those observed in the density of vesicular acetylcholine transporter protein varicosities. These results lend support to the idea that there is a causal relationship between age-related changes in cortical cholinergic and neuropeptide Y-ergic neurotransmitter systems.

Aging and sex influence the anatomy of the rat anterior cingulate cortex

Cognitive processes supported by the prefrontal cortex undergo an age-related decline. Until very recently, nonhuman animal models of aging have relied on the exclusive use of male subjects. This study was designed to investigate the influence of age, sex, and ovarian hormonal state on anatomy of the rat medial prefrontal cortex (anterior cingulate cortex). Dendritic tree extent and spine density were examined in young adult (3-5 mos.) and aged (20-24 mos.) male and female rats. Young adult females were examined either at proestrus or estrus, and aged females were examined in one of two reproductively senescent (estropausal) phases, persistent estrus or persistent diestrus. Neither the estrous cycle nor state of estropause influenced spine density or dendritic tree extent. However, the anatomy of the anterior cingulate cortex of young adult rats was sexually dimorphic, with males having greater dendritic spine density as well as arborization. While there was a reduction in density and tree extent with age for both sexes, this reduction was more pronounced for males, resulting in a disappearance of most sex differences with age. Thus the results of this study suggest that aging of the rodent cerebral cortex may follow a sexually dimorphic pattern.

Age-related change of neuropeptide Y-immunoreactive neurons in the rat main olfactory bulb

The change of neuropeptide Y (NPY)-immunoreactive (IR) neurons in the rat main olfactory bulb as a result of aging was investigated at several aging stages over a two-year period; postnatal 1-24 months (P 1-P 24). From P 1 to P 12, the number of NPY-IR neurons and fibers increased with highest number in P 12, and the type of NPY-IR neurons had changed from bipolar neurons with short processes to bipolar/multipolar neurons with long processes. At P 24 the population of NPY-IR neurons and fibers had significantly decreased. Furthermore, the morphology of NPY-IR neurons showed a tendency to decrease in size and processes. It is suggested that the decrease of the number and size of NPY-IR neurons and fibers may underlie the age-related changes in the olfactory processes.

Multiple receptors for neuropeptide Y in the hippocampus: putative roles in seizures and cognition

Neuropeptide Y (NPY) is widely distributed throughout the central nervous system (CNS) and is one of the most conserved peptides in evolution, suggesting an important role in the regulation of basic physiological functions, including learning and memory. In addition, experimental studies have suggested that NPY, together with its receptors, may have a direct implication in several pathological disorders, including epilepsy/seizure. NPY-like immunoreactivity and NPY receptors have been shown to be present throughout the brain, but is concentrated in the hippocampus. The hippocampal formation has been repeatedly implicated in the modulation of cognition, as well as the pathogenesis of seizure. This review will concentrate on the hippocampal distribution of NPY, its receptors and the putative role played by this peptide in seizure, together with the regulation of cognitive function associated with learning and memory.

Peptide immunoreactivity in aged rat cortex and hippocampus as a function of memory and BDNF infusion

Brain-derived neurotrophic factor (BDNF) modulates neuropeptide levels in hippocampus and cortex of young adult rats. Neuropeptide levels are altered in some age-related disorders, such as Alzheimer's and Parkinson's Disease. BDNF may be able to rectify peptide abnormalities but, because plasticity decreases with age, BDNF may not alter peptide levels as readily in aged animals. To determine if BDNF would regulate peptide levels in aged rats, young, aged memory-impaired, and unimpaired rats were infused with BDNF or vehicle into hippocampus and cortex. Cell profile counts, cell profile areas, fiber counts, and/or fiber terminal densities were measured for sections immunostained for neuropeptide Y (NPY), somatostatin (SOM), cholecystokinin-8 (CCK), and dynorphin A(1-8) (DYN). Results showed that BDNF upregulated cortical NPY-immunoreactivity (ir) and SOM-ir, upregulated hippocampal NPY-ir, and downregulated hippocampal DYN-ir in both aged and young rats. In addition, BDNF significantly and selectively normalized the areas of atrophied deep cortical CCK-ir cell profiles in aged-impaired rats. Finally, decreased CCK-ir fiber density was found in the hippocampal formation of aged memory-impaired rats.

Prenatal ethanol effects on NGF level, NPY and ChAT immunoreactivity in mouse entorhinal cortex: a preliminary study

It has been reported that maternal ethanol consumption leads to deficits in the limbic areas involved in cognitive functions and interferes with synthesis and utilization of neurotrophins. In the present study, it was hypothesized that prenatal alcohol intake might induce neuroanatomical alterations in the entorhinal cortex (EC). We also investigated the possible EC involvement of brain nerve growth factor (NGF), the first neurotrophin to be isolated, during such pathological events. To test this hypothesis, we used pregnant mice exposed to ethanol during EC neurogenesis (starting about gestational day 8). Our data show that prenatal alcohol intake in male mice alters the EC neuronal growth and differentiation. These morphological alterations are accompanied by an altered NGF level in the EC of prenatal alcohol-treated mice. We also found a decrease in choline acetyltransferase- and neuropeptide Y-immunopositive neurons in the EC of alcohol-exposed mice. However, the relationship between neuronal damage induced in the EC by ethanol, low presence of NGF, and the possible functional and behavioral consequences remains to be elucidated.

Age and species-dependent differences in the neurokinin B system in rat and human brain

Neurokinin B and its cognate neurokinin-3 receptor are expressed more in the forebrain than in brain stem structures but little is known about the primary function of this peptide system in the central processing of information. In general, few studies have specifically addressed age-related changes of tachykinins, notably the changes in number and/or distribution of the neurokinin B-expressing and neurokinin-3 receptor-bearing neurons. Data on functions and changes of neurokinins in physiological aging are limited and apply mainly to the substance P/neurokinin-1 receptor system. In the present study, we analyzed neurokinin B/neurokinin-3 receptor system in young (5 months) versus middle aged (15 months) and old rats (23-25 months) and also in aging human brains. For the majority of the immunohistochemically examined regions of the rat brain, there was no statistically significant change in neuronal number and size of the neurokinin B and neurokinin-3 receptor staining. In the adult human brain, there was no age-associated change of the number or size of neurokinin-B-positive neurons. However, we found a major decline in number of neurokinin-3 receptor-expressing neurons between young/middle aged (30 years to 69 years) versus old (70 years and older) adults. Interestingly, numbers of neurokinin-3 receptor-positive microglia increased whereas the neurokinin-3 receptor-positive astrocytes remained unchanged in both aging rat and human brains. Finally, in addition to assessing the morphological and quantitative changes of the neurokinin B/neurokinin-3 receptor system in the rat and human brain, we discuss functional implications of the observed interspecies differences.

Cholinergic muscarinic mechanisms regulate neuropeptide Y gene expression via protein kinase C in human neuroblastoma cells

Neuropeptide Y (NPY) participates in the control of several functions in the nervous system. NPYergic neurons present in brain areas involved in cognitive processes are linked to ascending projections of the cholinergic system, a finding that suggests a role for acetylcholine in the control of these cells. In the present study, the effect of the activation of cholinergic muscarinic receptors on the expression of the human NPY gene was assessed. The SH-SY5Y neuroblastoma cell line was used as an in vitro model of human neurons; NPY mRNA levels were evaluated by Northern blot analysis. The results indicate that: (a) the expression of the human NPY gene in SH-SY5Y cells is stimulated by the cholinergic muscarinic agonist, carbachol; (b) this effect is mediated by the M3 muscarinic receptor subtype, as indicated by the inhibitory effect of the M3 antagonist 4-DAMP; (c) protein kinase C (PKC) activation plays an important role in the induction of NPY gene expression in this system, as suggested by experiments with the PKC activator, TPA, and the PKC inhibitor, GF 109203X; (d) the stimulation of NPY mRNA levels by TPA and by carbachol in SH-SY5Y cells requires de novo synthesis of RNA and protein. In conclusion, the present study shows that the activation of PKC-coupled muscarinic receptors as the M3 subtype positively modulates the expression of the human NPY gene in SH-SY5Y neuroblastoma cells, via PKC-related mechanisms.