Rat basal forebrain cholinergic lesion affects neuronal nitric oxide synthase activity in hippocampal and neocortical target regions

The interaction between hippocampal cholinergic and nitrergic neurotransmission coordinates NMDA-dependent behavior and autonomic changes induced by contextual fear retrieval

RATIONALE

Re-exposing an animal to an environment previously paired with an aversive stimulus evokes large alterations in behavioral and cardiovascular parameters. Dorsal hippocampus (dHC) receives important cholinergic inputs from the basal forebrain, and respective acetylcholine (ACh) levels are described to influence defensive behavior. Activation of muscarinic M1 and M3 receptors facilitates autonomic and behavioral responses along threats. Evidence show activation of cholinergic receptors promoting formation of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) in dHC. Altogether, the action of ACh and NO on conditioned responses appears to converge within dHC.

OBJECTIVES

As answer about how ACh and NO interact to modulate defensive responses has so far been barely addressed, we aimed to shed additional light on this topic.

METHODS

Male Wistar rats had guide cannula implanted into the dHC before being submitted to the contextual fear conditioning (3footshocks/085 mA/2 s). A catheter was implanted in the femoral artery the next day for cardiovascular recordings. Drugs were delivered into dHC 10 min before contextual re-exposure, which occurred 48 h after the conditioning procedure.

RESULTS

Neostigmine (Neo) amplified the retrieval of conditioned responses. Neo effects (1 nmol) were prevented by the prior infusion of a M1-M3 antagonist (fumarate), a neuronal nitric oxide synthase inhibitor (NPLA), a NO scavenger (cPTIO), a guanylyl cyclase inhibitor (ODQ), and a NMDA antagonist (AP-7). Pretreatment with a selective M1 antagonist (pirenzepine) only prevented the increase in autonomic responses induced by Neo.

CONCLUSION

The results show that modulation in the retrieval of contextual fear responses involves coordination of the dHC M1-M3/NO/cGMP/NMDA pathway.

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

Aging is a complex biological process that increases the risk of age-related cognitive degenerative diseases such as dementia, including Alzheimer’s disease (AD), Lewy Body Dementia (LBD), and mild cognitive impairment (MCI). Even non-pathological aging of the brain can involve chronic oxidative and inflammatory stress, which disrupts the communication and balance between the brain and the immune system. There has been an increasingly strong connection found between chronic neuroinflammation and impaired memory, especially in AD. While microglia and astrocytes, the resident immune cells of the central nervous system (CNS), exerting beneficial effects during the acute inflammatory phase, during chronic neuroinflammation they can become more detrimental. Central cholinergic circuits are involved in maintaining normal cognitive function and regulating signaling within the entire cerebral cortex. While neuronal-glial cholinergic signaling is anti-inflammatory and anti-oxidative, central cholinergic neuronal degeneration is implicated in impaired learning, memory sleep regulation, and attention. Although there is evidence of cholinergic involvement in memory, fewer studies have linked the cholinergic anti-inflammatory and anti-oxidant pathways to memory processes during development, normal aging, and disease states. This review will summarize the current knowledge of cholinergic effects on microglia and astroglia, and their role in both anti-inflammatory and anti-oxidant mechanisms, concerning normal aging and chronic neuroinflammation. We provided details on how stimulation of α7 nicotinic acetylcholine (α7nACh) receptors can be neuroprotective by increasing amyloid-β phagocytosis, decreasing inflammation and reducing oxidative stress by promoting the nuclear factor erythroid 2-related factor 2 (Nrf2) pathways and decreasing the release of pro-inflammatory cytokines. There is also evidence for astroglial α7nACh receptor stimulation mediating anti-inflammatory and antioxidant effects by inhibiting the nuclear factor-κB (NF-κB) pathway and activating the Nrf2 pathway respectively. We conclude that targeting cholinergic glial interactions between neurons and glial cells via α7nACh receptors could regulate neuroinflammation and oxidative stress, relevant to the treatment of several neurodegenerative diseases.

Nitric oxide donor molsidomine promotes retrieval of object recognition memory in a model of cognitive deficit induced by 192 IgG-saporin

Nitric oxide (NO) plays a leading role in learning and memory processes. Previously, we showed its ability to modify the deleterious effect of immunotoxin 192 IgG-saporin (192-IgG-SAP) in the cholinergic system. The aim of this study was to analyze the potential of a NO donor (molsidomine, MOLS) to prevent the recognition memory deficits resulting from the septal cholinergic denervation by 192 IgG-SAP in rats. Quantification of neuronal and endothelial nitric oxide synthase (nNOS and eNOS, respectively) expression was evaluated in striatum, prefrontal cortex, and hippocampus. In addition, a choline acetyltransferase immunohistochemical analysis was performed in medial septum and assessed the effect of MOLS treatment on the spatial working memory of rats through a recognition memory test. Results showed that 192-IgG-SAP reduced the immunoreactivity of cholinergic septal neurons (41%), compared with PBS-receiving control rats (p < 0.05). Treatment with MOLS alone failed to antagonize the septal neuron population loss but prevented the progressive abnormal morphological changes of neurons. Those animals exposed to 192-IgG-SAP immunotoxin exhibited a reduction of cortical nNOS expression against the control group, whereas expression was enhanced in the 192-IgG-SAP + MOLS group. The most relevant finding was the recovering of the discrimination index exhibited by the 192-IgG-SAP + MOLS group. When compared with the rats exposed to the 192-IgG-SAP immunotoxin, they reached values similar to those observed in the PBS group. Our results show that although MOLS failed to block the cholinergic neurons loss induced by 192-IgG-SAP, it avoided the neuronal damage progression.

Effects of the dimeric PSD-95 inhibitor UCCB01-144 on functional recovery after fimbria-fornix transection in rats

Pharmacological inhibition of PSD-95 is a promising therapeutic strategy in the treatment of stroke, and positive effects of monomeric and dimeric PSD-95 inhibitors have been reported in numerous studies. However, whether therapeutic effects will generalize to other types of acute brain injury such as traumatic brain injury (TBI), which has pathophysiological mechanisms in common with stroke, is currently uncertain. We have previously found a lack of neuroprotective effects of dimeric PSD-95 inhibitors in the controlled cortical impact model of TBI in rats. However, as no single animal model is currently able to mimic the complex and heterogeneous pathophysiology of TBI, it is necessary to assess treatment effects across a range of models. In this preliminary study we investigated the neuroprotective abilities of the dimeric PSD-95 inhibitor UCCB01-144 after fimbria-fornix (FF) transection in rats. UCCB01-144 or saline was injected into the lateral tail vein of rats immediately after sham surgery or FF-transection, and effects on spatial delayed alternation in a T-maze were assessed over a 28-day period. Task acquisition was significantly impaired in FF-transected animals, but there were no significant effects of UCCB01-144 on spatial delayed alternation after FF-transection or sham surgery, although decelerated learning curves were seen after treatment with UCCB01-144 in FF-transected animals. The results of the present study are consistent with previous research showing a lack of neuroprotective effects of PSD-95 inhibition in experimental models of TBI.

NADPH-diaphorase Histochemical Labeling Patterns in the Hippocampal Neuropil and Visual Cortical Neurons in Weaned Rats Reared during Lactation on Different Litter Sizes

Tissue distribution of nitric oxide-synthases was investigated in the rat hippocampus and visual cortex under nutritional changes induced by modification of the litter size. Young (30-45-days-old) rats, suckled in litters formed by 3,6 or 12 pups (called small, medium and large litters, respectively), were studied by using nicotine-adenine-dinucleotide phosphate-diaphorase histochemistry (shortly, diaphorase), a simple and robust procedure to characterize tissue distribution of nitric oxide-synthases. We assessed morphometric features of the diaphorase-positive cells in visual cortex, and the neuropil histochemical activity in hippocampal CA1 and dentate gyrus using densitometry analysis. In the large-litter group, the labeled-cell density in white matter of area 17 was higher, as compared to the small-litter group. There was a clear trend, in the large-litter group, to lower values of soma area, dendritic field and branches per neuron, but the differences were not significant. Densitometry analysis of hippocampus revealed a significant increase in the relative neuropil histochemical activity of the dentate gyrus molecular layer in the larger litters, which may be associated to increased compensatory blood flow in the hippocampus. The pathophysiological mechanisms of the observed changes remain to be investigated.

Methodological aspects of functional transcranial Doppler sonography and recommendations for simultaneous EEG recording

The neurovascular coupling describes a vasoregulative principle of the brain that adapts local cerebral blood flow in accordance with the underlying neuronal activity. It is the basis of modern indirect brain imaging techniques. Because of its wide availability and high tolerability the functional transcranial Doppler has been often used to assess brain function in clinical conditions. In the present paper we will give an overview of the current understanding of the coupling, explain basic principles of the Doppler technique and summarize relevant findings of functional Doppler tests in the different vascular territories of the brain. Finally, the concept of a combined functional electroencephalogram and transcranial Doppler technique will be outlined, which allows simultaneous investigation of the neuronal and vascular responses of neurovascular coupling.

Neurovascular coupling in Alzheimer patients: effect of acetylcholine-esterase inhibitors

Dualistic effects of acetylcholine-esterase inhibitors on neuronal as well as vasoregulative function have been debated. This study investigated for the first time effects of medication on both components. Visually evoked potentials and resultant hemodynamic responses were assessed in Alzheimer patients (n=31) without vascular lesions in a MRI scan and compared to controls (n=20). After baseline recordings (AD0) tests were repeated under 2x1.5 to 2x3mg (AD1) and 2x4.5 to 2x6mg (AD2) rivastigmine/d. Long-term effects were investigated under 6 months of medication (AD2L). The ADAS, MMSE and DEMTECT were used to assess cognitive function at AD0, AD2 and AD2L. Improvement in vasoregulative function was independent from changes in evoked potentials. Acetylcholine-esterase inhibitors demonstrate substantial vascular effects in humans, which are independent from changes in neuronal function.

Nitric oxide-induced cGMP synthesis in the cholinergic system during the development and aging of the rat brain

cGMP synthesis in cholinergic neurons of the basal forebrain, the caudate putamen, and the tegmento-pedunculopontine nucleus of the rat was studied during development after birth at P1, P4, P10, and P21, in the adult, and during aging. NO-mediated cGMP synthesis in these neurons was studied using the approach of in vitro incubation of brain slices in combination with cGMP-immunocytochemistry. The percentage of NO-responsive, cGMP-synthesizing cholinergic cells in the septum and diagonal band of Broca decreased from 75% to 6% in adult animals and to 2% in aged ones. In the caudate putamen, this decrease was from 81% to 21% in adult and 11% in aged animals. Cholinergic cells of the tegmento-pedunculopontine nucleus were unresponsive to NO and never showed cGMP-immunoreactivity. In addition, it was observed that the amount of NO-responsive, cGMP-synthesizing cholinergic fibers in the hippocampus declined in parallel with the maturation of the septal-hippocampal cholinergic pathway, whereas in the caudate putamen, this colocalization became complete 2 weeks after birth. It is concluded that the property of NO-mediated cGMP synthesis in the cholinergic nuclei of the forebrain is developmentally regulated after birth and that NO-cGMP signal transduction has a role in establishing cholinergic neuronal connections in the hippocampus and caudate putamen.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

The effect of walking on regional blood flow and acetylcholine in the hippocampus in conscious rats

Recent studies in our laboratory have demonstrated that stimulation of the septal complex (i.e., the medial septal nucleus and the nucleus of the diagonal band) increases extracellular acetylcholine (ACh) release and, consequently, results in an increase in regional cerebral blood flow in the hippocampus (Hpc CBF) via activation of the nicotinic ACh receptors (nAChRs) [Neurosci. Lett. 107 (1989) 135; Neurosci. Lett. 112 (1990a) 263]. The present study aimed to examine the effects of walking on Hpc CBF, measured by laser Doppler flowmetry, in conscious rats. Walking at a moderate speed (4 cm/s) on a treadmill for 30 s produced increases in Hpc CBF and mean arterial pressure (MAP), reaching 107 +/- 1% and 105 +/- 1% of the prewalking control values, respectively. Walking for 3 min produced an increase in ACh release in the extracellular space of the hippocampus. The increase in Hpc CBF during walking was attenuated by mecamylamine (abbreviated as MEC here; 2 mg/kg, i.v.), a nAChR antagonist permeable to the blood-brain barrier (BBB), but not by hexamethonium (denoted as C6 here; 20 mg/kg, i.v.), a nAChR antagonist impermeable to the BBB, while the walking-induced increase in MAP was abolished by either agent. The response of Hpc CBF and MAP were not altered by atropine (abbreviated as ATR here; 0.5 mg/kg, i.v.), a muscarinic AChR antagonist permeable to the BBB. The increase in Hpc CBF during walking was attenuated by N(omega)-nitro-L-arginine methyl ester (L-NAME, 3 and 30 mg/kg, i.v.), a nitric oxide synthase (NOS) inhibitor, and the reduced responses were reversed following the intravenous (i.v.) administration of a physiological precursor of NO, L-arginine (600 mg/kg). The results suggest that the increase in Hpc CBF during walking is independent of MAP and attributable at least to activation of the nAChRs by the cholinergic vasodilator nerves projecting to the hippocampus and to production of NO in the hippocampus.

Fibrillary beta-amyloid deposits are closely associated with atrophic nitric oxide synthase (NOS)-expressing neurons but do not upregulate the inducible NOS in transgenic Tg2576 mouse brain with Alzheimer pathology

Transgenic mice (Tg2576) that express the Swedish double mutation of human amyloid precursor protein and develop Alzheimer-like beta-amyloid deposits in the aged brain, were used to study the effect of beta-amyloid deposition on expression of both neuronal (nNOS) and inducible nitric oxide synthase (iNOS) in cells surrounding beta-amyloid plaques. Nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and double immunofluorescent labeling revealed that most of the fibrillary, thioflavine-S-positive cortical beta-amyloid deposits in 13-, 17-, and 21-month-old transgenic animals were closely associated with dystrophic nNOS-positive neurons, while nNOS-bearing neurons located more distal to plaques appeared to be unaffected. There was no significant expression of iNOS in transgenic mouse brain. The data suggest enhanced vulnerability of nNOS-containing neocortical neurons to beta-amyloid toxicity. Alternatively, expression of nNOS may also be a response to plaque-mediated damage of neurons, consistent with a neuroprotective role of nitric oxide.