Cholinergic Denervation Research Articles

Extensive reinnervation of the hippocampus by embryonic basal forebrain cholinergic neurons grafted into the septum of neonatal rats with selective cholinergic lesions

Reconstruction of the septohippocampal pathways by axons extending from embryonic cholinergic neuroblasts grafted into the neuron-depleted septum has been explored in the neonatal rat by using a novel lesioning and grafting protocol. Neonatal ablation of the basal forebrain cholinergic projection neurons, accompanied by extensive bilateral cholinergic denervation of the hippocampus and neocortex, was produced at postnatal day (PD) 4 by 192 immunoglobulin (IgG)-saporin intraventricularly. Four days later, cholinergic neuroblasts (from embryonic day 14 rats) were implanted bilaterally into the neuron-depleted septum by using a microtransplantation approach. The results show that homotopically implanted septal neurons survive and integrate well into the developing septal area, extending axons caudally along the myelinated fimbria-fornix and supracallosal pathways that are able to reach the appropriate targets in the denervated hippocampus and cingulate cortex as early as 4 weeks postgrafting. Moreover, the laminar innervation patterns established by the graft-derived axons closely resembled the normal ones and remained essentially unchanged up to at least 6 months, which was the longest postoperative time studied. The reinnervating fibers restored tissue choline acetyltransferase activity (up to 50% of normal) in the dorsal hippocampus and the parietooccipital cortex. Retrograde labeling with Fluoro-Gold from the host hippocampus combined with immunocytochemistry confirmed that most of the projecting neurons, indeed, were cholinergic. The results suggest that the graft-host interactions that are necessary for target-directed axon growth are present in the septohippocampal system during early postnatal maturation. Thus, the present approach may contribute to overcome the functional limitations inherent in the use of ectopically placed intrahippocampal transplants. © 1996 Wiley-Liss, Inc.

Cholinergic deafferentation enhances rat hippocampal pyramidal neuron responsiveness to locally applied nicotine

We tested whether cholinergic denervation of the hippocampas of young rats would result in an enhancement of CAI pyramidal cell responsiveness to nicotine. Electrolytic lesions of the medial septal area were performed in young male Fisher 344 rats, One month later the rats were anesthetized with pentobarbital and nicotine was locally applied to CAI pyramidal neurons using pressure microejection. The dose of nicotine required to excite the pyramidal neurons was significantly lower for cells recorded from rats with septal lesions. However, no changes in hippocampal cytisine or α-bungarotoxin binding were found.

Cholinergic deafferentation enhances rat hippocampal pyramidal neuron responsiveness to locally applied nicotine

We tested whether cholinergic denervation of the hippocampas of young rats would result in an enhancement of CAI pyramidal cell responsiveness to nicotine. Electrolytic lesions of the medial septal area were performed in young male Fisher 344 rats, One month later the rats were anesthetized with pentobarbital and nicotine was locally applied to CAI pyramidal neurons using pressure microejection. The dose of nicotine required to excite the pyramidal neurons was significantly lower for cells recorded from rats with septal lesions. However, no changes in hippocampal cytisine or α-bungarotoxin binding were found.

Increased β-amyloid precursor protein mRNA in the rat cerebral cortex and hippocampus after chronic systemic atropine treatment

Rats were treated with once-daily subcutaneous injections of atropine or normal saline for 10 days. Cryostat sections of fresh-frozen brain were subjected to quantitative muscarinic receptor ([3H]quinuclidinylbenzilate (QNB)) binding autoradiography, and quantitative in-situ hybridization autoradiography for β-amyloid precursor protein (β-APP) mRNA using an oligonucleotide probe recognizing all major isoforms. QNB binding in the atropine-treated group was increased 6–7% in the areas measured (dentate gyrus, CA1, and cerebral cortex), confirming that the treatment was effective in inducing muscarinic receptor upregulation. Hybridization signal for β-APP mRNA was increased 15–20% in the atropine-treated group in the same regions. As chronic atropine treatment models the muscarinic effects of cholinergic denervation, these results suggest that age-related cholinergic neuron loss may result in upregulation of β-APP.

Muscarinic receptors mediating depression and long-term potentiation in rat hippocampus.

1. Two concentration-dependent effects of the muscarinic agonist carbachol (CCh) were characterized in submerged slices of rat hippocampus using extracellular recordings of excitatory postsynaptic potentials (EPSPs): muscarinic long-term potentiation (LTP(m)) and depression. 2. LTP(m) of the EPSP slope was seen following long exposure (20 min) of the slice to low concentrations of CCh (0.2-0.5 microM). This LTP(m) was not accompanied by a change in the size of the afferent fibre volley or by a change in paired-pulse potentiation, consistent with a postsynaptic locus of CCh action. 3. Intracellular recordings from voltage-clamped neurons of inward current evoked by iontophoretically applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) revealed that, while cellular responses to NMDA rose transiently upon superfusion with 0.5 microM CCh, responses to AMPA increased gradually and remained potentiated after washout of CCh. 4. LTP(m) is mediated by an M2 muscarinic receptor. Two M2 muscarinic receptor antagonists, methoctramine and AFDX-116, blocked LTP(m). The M2 agonist oxotremorine induced LTP(m) at low agonist concentrations. None of the M1 and M3 receptor agonists and antagonists tested affected LTP(m). 5. Muscarinic fast onset depression of the EPSP was seen in response to higher concentrations of CCh (2-5 mu M). This depression was accompanied by an increase in paired-pulse potentiation, indicating a possible presynaptic locus of action. The M3 muscarinic receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) blocked the muscarinic depression of the EPSP slope. M1, M2 and M4 muscarinic antagonists did not block this response. 6. Blockade of the muscarinic depression by 4-DAMP did not uncover a suppressed LTP(m). However, addition of picrotoxin facilitated the expression of LTP(m) induced by high concentrations of CCh, indicating an involvement of interneurons in regulation of LTP(m). 7. Cholinergic denervation produced by fimbria-fornix transection resulted in supersensitivity of both M2- and M3-mediated effects, indicating that the receptors mediating these effects are not located on presynaptic cholinergic fibres. In the presence of 4-DAMP and picrotoxin the dose-response curve for CCh-induced effects in slices from lesioned animals was shifted to the left relative to that of normal animals, indicating a supersensitivity of both receptor types.

The cerebral cortex of the rat and visual attentional function: dissociable effects of mediofrontal, cingulate, anterior dorsolateral, and parietal cortex lesions on a five-choice serial reaction time task.

Dissociable effects of bilateral excitotoxic lesions of different regions of the rat neocortex, including medial prefrontal and anterior cingulate cortices, were investigated in a five-choice serial reaction time task that provides several indices of the accuracy and speed of attentional function. Whereas medial prefrontal cortical lesions impaired performance of the task as revealed by a reduction in choice accuracy, an increase in the latency to respond correctly to the visual target and enhanced perseverative responding, lesions of the anterior cingulate cortex specifically increased premature responding. By contrast, lateral frontal cortical lesions did not significantly disrupt baseline performance of the task, but rather increased the latency to respond correctly to the visual target during various behavioral manipulations, for example, when the length of the intertrial interval was varied unpredictably and during interpolation of distracting bursts of white noise. Lesions of the parietal cortex failed to disrupt any aspect of task performance investigated. These behavioral effects in the five-choice task were compared with the effect of these same lesions on acquisition and retention of a one-trial passive avoidance task. The main finding from this paradigm was that lesions of the lateral frontal cortex produced a significant disruption to the retention of passive avoidance, which stands in marked contrast to the successful retention observed by animals of the other lesion groups. In addition, this pattern of results reveals that the 'disinhibitory' effect of cingulate cortex lesions are relatively specific to the five-choice attentional task. Finally, the results of the present study are compared with the findings of previous experiments using the five-choice task, which have examined the effect of selective manipulations of the ascending noradrenergic, cholinergic, dopaminergic, and serotonergic projections. In particular, the deficits in attentional function observed following cholinergic lesions of the nucleus basalis magnocellularis appear to be attributable to cholinergic denervation of the medial frontal cortex. These results are discussed in terms of the role of parallel distributed neural systems within the neocortex that mediate continuous attentional performance in the rat.

Chapter 28 The systems-level organization of cholinergic innervation in the human cerebral cortex and its alterations in Alzheimer's disease

The cholinergic innervation of the cerebral cortex has now become one of the most dynamic areas of research and offers exciting opportunities for exploring the chemical neuroanatomy of cognition, age-related memory impairments, and Alzheimer's disease. The cholinergic pathway from the basal forebrain is the most massive of all extra-thalamic corticofugal projections of the cerebral cortex. Developments based on electron microscopy, immunocytochemistry, antibodies to recombinant receptor subtypes, in situ hybridization, single unit recordings, and selective immunotoxic lesioning have introduced a wealth of new information on the organization and function of this pathway in various animal species, including humans. The cholinergic pathway emanating from the basal forebrain constitutes one of the most important modulatory afferents of the mammalian cortex. The initial expectation that the cholinergic deficiency would provide a unifying pathophysiological basis for Alzheimer's disease and that cholinergic therapies would cure the dementia are clearly too optimistic. Nonetheless, the cortical cholinergic denervation remains one of the earliest, most severe, and most consistent transmitter changes in this disease. The cholinergic depletion may provide an important substrate for the neuropsychological features of Alzheimer's disease and may eventually yield important clues to its pathogenesis.

Systematic regional variations in the loss of cortical cholinergic fibers in Alzheimer's disease.

The loss of cortical cholinergic fibers in Alzheimer's disease was investigated using choline acetyltransferase immunohistochemistry and acetylcholinesterase histochemistry. Within both the normal and Alzheimer's cerebral cortex, the two methods revealed an identical pattern of fiber staining. In the normal brain, cholinergic fiber density was highest in limbic and paralimbic cortical zones, intermediate in most sensory-motor and association zones, and lowest within the primary visual and visual association areas of the occipital lobe. In general, supragranular cortical layers contained a higher density of cholinergic fibers, and most of these were oriented vertically. In Alzheimer's disease, an overall 55% loss of cortical cholinergic fibers was detected. There was, however, marked regional variations in the extent of this loss in different cortical areas. Cortical areas within the temporal lobe, particularly the temporal association areas, displayed a dramatic loss of cholinergic fibers. By contrast, the anterior cingulate cortex, primary visual, primary somatosensory, and primary motor cortex displayed a relative preservation of cholinergic fibers. As a whole, greater loss of cholinergic fibers was detected in supragranular layers and in fibers oriented vertical to the cortical surface. These results indicate that cholinomimetic therapies are likely to have different effects on cholinergic transmission in various cortical areas. The precise mechanisms that lead to the regional variations in cortical cholinergic denervation in Alzheimer's disease remain to be elucidated.

The effect of hippocampal sympathetic ingrowth and cholinergic denervation on hippocampal M2 cholinergic receptors.

After cholinergic denervation of the hippocampus, via medial septal (MS) lesions, peripheral sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus. In this study, we sought to determine the effect of hippocampal sympathetic ingrowth (HSI) on the M2 subtype of muscarinic cholinergic receptors, by examining the membrane binding of [3H]AF-DX 384 in hippocampal tissue from control rats, rats with HSI and rats with MS lesions + concurrent ganglionectomy (CD group). In dorsal hippocampus, Kd was found to be increased while Bmax was decreased in the CD group as compared with both the HSI and control group which did not differ from one another. In ventral hippocampus, Kd was found to be increased while Bmax was decreased in the CD group when compared only with the control group. These results suggest that sympathetic ingrowth, which has its greatest concentration in dorsal hippocampus, can 'normalize' the M2 receptor in hippocampus.

Hippocampal sympathetic ingrowth and cholinergic denervation uniquely alter muscarinic receptor subtypes in the hippocampus

Following cholinergic denervation of the hippocampus by medial septal lesions, an unusual neoronal reorganization occurs, in which peripheral sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus. Previously, we have found that both hippocampal sympathetic ingrowth (HSI) and cholinergic denervation (CD), alone, altered the total number and affinity of muscarinic cholinergic receptors (mAChR). In this study, we utilized the muscarinic antagonist [ 3H]Pirenzepine, in combination with membrane radioligand binding techniques, to determine the effects of HSI and CD on hippocampal M 1 and M 1 + M 3 mAChR subtypes, 4 weeks after MS lesions. In both the dorsal and ventral hippocampus, HSI was found to markedly diminish the number of M 1 AChRs, while CD was found to increase the number of M 1 AChRs. Neither treatment affected the affinity of the M 1 AChR. However, when M 1 + M 3 binding was assessed, CD was found to decrease the affinity in both hippocampal regions, without altering the number of receptors. Neither affinity nor number of M 1 + M 3 receptors was altered by HSI. The results of this study suggest that both cholinergic denervation and hippocampal sympathetic ingrowth uniquely affect hippocampal muscarinic receptors.

The effect of cholinergic denervation and hippocampal sympathetic ingrowth on the internalization of muscarinic receptors in rat hippocampus

Following cholinergic denervation of the hippocampus by medial septal (MS) lesions, an unusual neuronal reorganization occurs in which peripheral sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus (hippocampal sympathetic ingrowth; HSI). Previously, we have found that with MS lesions, animals with (the HSI (+) group) and without (HSI (−) group) ingrowth differed in carbachol stimulated PI hydrolysis, in PKC activity, and in muscarinic cholinergic receptors (mAChR). In this study, performed in hippocampal slices obtained four weeks after MS lesions, we utilized the hydrophilic muscarinic antagonist [ 3H] N-methylscopolamine ([ 3H]NMS) and hydrophobic muscarinic antagonist [ 3H]quinuclidinyl benzilate ([ 3H]QNB) in the presence of either 4-α-phorbol or phorbol 12,13-dibutyrate (PDBu) to determine the effect of MS lesions with and without ingrowth on PKC-mediated mAChR internalization. In the presence of PDBu, a group effect was observed in [ 3H]NMS binding, with control groups > HSI (+) group > HSI (−) group. However, [ 3H]QNB binding was similar across groups. These results suggest that the cholinergic denervation of the hippocampus enhances the internalization of mAChRs, which is modified in the presence of HSI.

Selective Immunolesion of the Basal Forebrain Cholinergic Neurons: Effects on Hippocampal Activity During Sleep and Wakefulness in the Rat

Intracerebroventricular injection of the toxin 192 IgG-saporin (4μg) kills the cholinergic neurons of the basal forebrain bearing the low affinity NGF receptor (NGFr). The effect of this cholinergic denervation on the hippocampal and cortical electrical activity (EEG) was studied during sleep and wakefulness. EEG was recorded under freely-moving conditions in lesioned (n=10) and control (n=6) rats (8–16 days post-injection). In lesioned rats, active (AW) and quiet (QW) wakefulness episode durations were similar to those of controls whereas the REM sleep duration was reduced, 8 days post-lesion (P<0.01). Bouts of REM sleep were more numerous but shorter. The hippocampal theta activity was still present in lesioned-rats during AW (type 1 theta), QW (type 2 theta) and REM sleep. The frequency was unchanged but the amplitude of the three types of theta was significantly reduced (P<0.01). Type 2 theta occurred with shorter and less regular bouts (P<0.05). Abnormal slow waves (2–4 Hz) were observed during wakefulness. Histology showed a dramatic loss of NGFr-positive neurons in the basal forebrain and a decline in hippocampal and cortical acetylcholinesterase activity. These results suggest that the cholinergic septohippocampal input is not the primary pacemaker for the hippocampal theta rhythm.

930-102 Coronary Denervation Attenuates Vasospasm Induced by Pericoronary Application of Methacholine in Pigs

Autonomic innervation of the coronary arteries is thought to intervene in the genesis of coronary vasospasm, We assessed the potential effect of selective coronary denervation in the inhibition of vasospasm. Chronic sympathetic and parasympathetic denervation was provoked by topical application of phenol to the proximal LAD in 20 pigs through a lateral thoracotomy. Two weeks later, we performed an midsternal thoracotomy. Heart rate, aortic and ventricular (LV) pressure, LV dP/dt, coronary blood flow (CBF) at the LAD, and 32–channel epicardial ST segment mapping were monitored during every MCH application. Phenol-induced coronary adrenergic and cholinergic denervation was confirmed by histofluorescence and cholinesterase staining, Isolated LAD rings of 5 phenol-denervated pigs were studied to confirm smooth muscle integrity, Application of MCH to control pigs (n = 10) induced an statistically significant fall in CBF (75 ± 19%) accompanied by electrical and mechanical signs of ischemia: mean ST segment elevation on the epicardial electrograms of 9,7 ± 4.7 mV, a drop in systolic LV pressure (-20 ± 8.4 mmHg) and a decrease in LV dP/dt (-404 ± 203 mmHg/s). There were no changes in heart rate. The parameters returned to normal values after 5 to 8 min. As compared to controls, phenol treated pigs showed a significant attenuation ofthe fall in CBF (25 ± 17%), lesser ST segment elevation (2.4 ± 2.1 mV), a lower drop in LV pressure (-8 ± 2.3 mmHg), and a smaller decrease in LV dP/dt (-167 ± 104 mmHg/s). Coronary angiography performed in 6 additional pigs evidenced that the coronary spasm occurred at the site of MCH exposure. Isolated LAD rings of phenol treated pigs constrict after incubation with endothelin-1 indicating preserved vascular smooth muscle function. In 5 control pigs, a second application of MCH after muscarinic receptor blockade with atropine (0.4 mg/kg iv.) resulted in complete abolition of the vasospasm confirming that vasospasm was mediated by the muscarinic receptor stimulation. In conclusion, pericoronary autonomic denervation protects against coronary constriction induced by direct muscarinic receptor stimulation with methacholine in situ the pig heart.

Nerve growth factor immunoreactivity and sympathetic sprouting in the rat hippocampal formation

Several lines of evidence support a role for nerve growth factor (NGF) in the sympathetic sprouting response that occurs following septal cholinergic denervation of the rat hippocampal formation. The present study was undertaken to compare the distribution of NGF-like immunoreactivity and the topography of sympathetic sprouting in rats receiving medial septal lesions. Comparisons were made using adjacent sections of the hippocampal formation stained either for NGF-like immunoreactivity or for NGF receptor-immunoreactivity (p75, to visualize sympathetic fibers). p75-immunoreactive sympathetic axons were localized within the same regions exhibiting NGF-like staining, i.e., the hilus of the dentate gyrus and stratum lucidum in the CA3 area. Furthermore, the sympathetic fibers that invaded the hippocampal formation exhibited NGF-like immunostaining. These results provide additional evidence in support of NGF's role in this collateral sprouting response in the mature rat CNS.

Differential changes in cholinergic markers from selected brain regions after specific immunolesion of the rat cholinergic basal forebrain system

The aim of this study was to characterize the effects of cortical cholinergic denervation on cholinergic parameters in the cerebral cortex and basal forebrain using a novel immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor armed with cytotoxin saporin) to efficiently and selectively lesion cholinergic neurons in rat basal forebrain. Seven days following an intracerebroventricular injection of the cholinergic immunotoxin 192IgG-saporin the binding levels of nicotinic and M1- and M2-muscarinic acetylcholine receptors (mAChR), high-affinity choline uptake sites, as well as the m1-m4 mAChR mRNA were determined in coronal brain sections by both receptor autoradiography and in situ hybridization, and quantified by image analysis. Hemicholinium-3 binding to high-affinity choline uptake sites was decreased by up to 45% in all cortical regions and in the hippocampus after a single injection of the immunotoxin compared to controls. In contrast, M1-mAChR sites were increased over the corresponding control value in the anterior parts of cingulate, frontal, and piriform cortex by about 20%, in the hindlimb/forelimb areas (18%), in the parietal cortex (35%), in the occipital cortex area 2 (17%), as well as in the temporal cortex (25%) following immunolesion. M2-mAChR levels were found to be significantly increased in the posterior part of the parietal cortex area 1 (by about 22%) and in the occipital cortex area 2 (20%) only. With respect to laminar cortical localization, M2-mAChRs and choline uptake sites were altered in all cortical layers, whereas M1-mAChRs were preferentially affected in the upper cortical layers by the immunolesion. The increase in M1-mAChR binding in the temporal and occipital cortex as a consequence of the immunolesion was complemented by an increase in the amount of m1 and m3 mAChR mRNA of about 20% in these regions. The elevated levels of M2-mAChR sites in the occipital and temporal cortex following immunolesion were accompanied by an increase in the m4 (by 25%) but not m2 mAChR mRNA. There was no effect of the immunolesion on the m1-m4 mAChR mRNA in frontal cortical regions. in the basal forebrain, however, immunolesioning caused about a 40% decrease in the level of m2 mAChR mRNA in the medial and lateral septum as well as in the vertical and horizontal limb of the diagonal band, whereas M1- and M2-mAChR binding and the levels of m1, m3, and m4 mAChR mRNA were not affected by the immunolesion in any of the basal forebrain nuclei studied.(ABSTRACT TRUNCATED AT 400 WORDS)

Denervation induced abnormal phosphorylation in hippocampal neurons

This study demonstrates that combined dopaminergic and cholinergic denervation of the hippocampus results in the appearance of morphologically altered, Tau reactive, apical dendrites of granule cells in the rat dentate gyrus. The denervated granule cells and their apical dendrites also display immunoreactivity to a mitogen-activated protein kinase, ERK-1, and also evidence of abnormal phosphorylation of these dendrites as revealed by SMI-31 immunoreactivity. Dopaminergic denervation alone also causes mitogen activated protein kinase reactivity without the Tau-reactive apical dendrites. These results suggest an analogy to synaptophysin loss and the appearance of dendritic threads described in Alzheimer's disease (AD), as an early stage in the formation of neurofibrillary tangles (NFT). This is the first animal model in which abnormal phosphorylation of Tau has been shown to be produced experimentally in vivo.

Nerve Growth Factor Influences the Distribution of Sympathetic Sprouting into the Hippocampal Formation by Implanted Superior Cervical Ganglia

Following cholinergic denervation of the hippocampal formation, peripheral sympathetic fibers from the superior cervical ganglion (SCG) sprout into hippocampal tissue. The molecular mechanism controlling this process is unknown, although a role by trophic factors seems likely. In the present study, neonatal SCG were used as biological probes to investigate the association between NGF-immunoreactive regions in the hippocampal formation and areas innervated by regrowing sympathetic fibers. Cholinergic deafferentation of the hippocampal formation was achieved by a fimbriafornix transection and neonatal SCG were placed into the lesion cavity, abutting the rostral pole of the hippocampus. At 16 days following ganglia transplantation, NGF immunoreactivity within the hippocampal formation appeared indistinguishable from unlesioned controls and was localized within the mossy fiber region of the dentate gyrus and CA3 and CA2 hippocampal subfields. Sympathetic innervation, revealed in adjacent sections by dopamine β-hydroxylase or p75LNGFR immunoreactivity, was also restricted primarily to the mossy fiber region. Ablation of the entorhinal cortex at the time of transplantation resulted in the appearance of an additional discrete band of NGF immunoreactivity within the outer molecular layer of the dentate gyrus. In animals receiving an entorhinal lesion concurrent with the SCG transplant, sympathetic fibers were observed not only within the mossy fiber region but also within the outer molecular layer of the dentate gyrus where the lesion-induced NGF immunoreactivity appeared. These results support the hypothesis that topographic distributions of NGF may be used in the hippocampal formation to define terminal fields for sprouting NGF-sensitive neuronal populations.

Cholinergic denervation of the rat hippocampus by 192-IgG-saporin: electrophysiological evidence.

The consequences of intracerebroventricular injection of the toxin 192-IgG-saporin on the electrophysiological properties of CA1 pyramidal cells were investigated using intracellular recordings in the in vitro hippocampal slice preparation. We present the first electrophysiological evidence of a dysfunction of hippocampal cholinergic afferents following injection of 192-IgG-saporin. The synaptic events mediated by acetylcholine were altered in such animals: the slow cholinergic excitatory postsynaptic potentials as well as the cholinergic activation of GABAergic interneurones were dramatically depressed or even absent; the amplitude and duration of the afterhyperpolarization following a burst of spikes were increased, while other neuronal properties were not modified. These specific alterations suggest that the toxin 192-IgG-saporin is a specific tool for the experimental study of cholinergic denervation in the hippocampus.

Facilitatory but nonessential role of the muscarinic cholinergic system in the generation of long-term potentiation of population spikes in the dentate gyrus in vivo

The role of the muscarinic cholinergic system in the generation of LTP in the medial perforant path-dentate granule cell synapses in vivo was investigated using anesthetized rats. Cholinergic denervation with AF64A, a cholinergic toxin, did not significantly affect LTP induced by a strong tetanus (100 pulses at 100 Hz), but attenuated the LTP induced by a weak tetanus (30 pulses at 60 Hz). The i.c.v. injection of scopolamine (1.5–50 nmol) did not significantly affect the LTP induced by the strong tetanus but attenuated the magnitude of LTP produced by the weak tetanus in a concentration-dependent manner. These results suggest that the cholinergic system is not essential for induction of LTP by strong stimuli but plays a role in facilitating the generation of LTP by weak stimuli. Furthermore, the induction of LTP by a weak tetanus was blocked by pirenzepine but affected by neither AF-DX116 nor 4-diphenylacetoxy- N-methylpiperidine. The LTP-facilitatory action of the cholinergic system is probably mediated by muscarinic M 1 receptors.

Neuropeptide Y and somatostatin in the neocortex of young and aging rats: Response to nucleus basalis lesions

Lesions of the nucleus basalis of Meynert (NBM) have been used to mimic, in part, cholinergic deficits occurring in age-related neurodegenerative disorders, i.e., Alzheimer's disease. In our study, the effect of a persistent cholinergic denervation of the frontoparietal cortex on neuropeptide Y (NPY) and somatostatin (SOM) was examined in young adult (3 months old) and aging (> 18 months old) rats, 1, 3 and 6 months after bilateral stereotaxic NBM lesions with quisqualic acid. In aging, non-lesioned rats a significant decrease in radioimmunologically and immunohistochemically detectable NPY and SOM was found with no further changes after lesions. Morphological markers for these peptidergic populations (cell size and number, NADPH-diaphorase histochemistry, electron microscopy) demonstrated no signs of alterations in both age groups after lesion. Densitometric analysis of peptide fibre networks displayed a heterogeneous response with a significant rarefication in young rats 1 month after the lesion, followed by restoration and a tendency towards increase 6 months post lesioning in individual animals. These findings were confirmed by radioimmunological measurements. Examination of synaptic and cytoskeletal markers, i.e., synaptophysin, GAP-43, MAP-2, Tau-1 and amyloid precursor protein, did not reveal any signs for neuronal reorganization or sprouting. These data are discussed in the context of plasticity and pathology in age-related neurodegenerative disorders with cholinergic impairment.