The effect of central cholinergic and noradrenergic denervation on hippocampal sympathetic ingrowth and apoptosis-like reactivity in the rat

Noradrenergic sympathetic sprouting and cholinergic reinnervation maintains non-amyloidogenic processing of AβPP

Alzheimer's disease (AD) is characterized by amyloid-β (Aβ) plaques, hyperphosphorylated tau neurofibrillary tangles, and cholinergic dysfunction. Cholinergic degeneration can be mimicked in rats by lesioning medial septum cholinergic neurons. Hippocampal cholinergic denervation disrupts retrograde nerve growth factor (NGF) transport, leading to its accumulation, which subsequently triggers sprouting of noradrenergic sympathetic fibers from the superior cervical ganglia into hippocampus. Previously we reported that coincident with noradrenergic sprouting is the partial reinnervation of hippocampus with cholinergic fibers and the maintenance of a M1 muscarinic acetylcholine receptor (M1 mAChR) dependent long-term depression at CA3-CA1 synapses that is lost in the absence of sprouting. These findings suggest that sympathetic sprouting and the accompanying cholinergic reinnervation maintains M1 mAChR function. Importantly, noradrenergic sympathetic and cholinergic sprouting have been demonstrated in human postmortem AD hippocampus. Furthermore, M1 mAChRs are a recent focus as a therapeutic target for AD given their role in cognition and non-amyloidogenic processing of amyloid-β protein precursor (AβPP). Here we tested the hypotheses that noradrenergic sympathetic sprouting is triggered by NGF, that sprouting maintains non-amyloidogenic AβPP processing, and that sprouting is prevented by intrahippocampal Aβ42 infusion. We found that NGF stimulates sprouting, that sprouting maintains non-amyloidogenic AβPP processing, and that Aβ42 is not only toxic to central cholinergic fibers innervating hippocampus but it prevents and reverses noradrenergic sympathetic sprouting and the accompanying cholinergic reinnervation. These findings reiterate the clinical implications of sprouting as an innate compensatory mechanism and emphasize the importance of M1 mAChRs as an AD therapeutic target.

Hypertrophy and neuron loss: structural changes in sheep SCG induced by unilateral sympathectomy

Recently, superior cervical ganglionectomy has been performed to investigate a variety of scientific topics from regulation of intraocular pressure to suppression of lingual tumour growth. Despite these recent advances in our understanding of the functional mechanisms underlying superior cervical ganglion (SCG) growth and development after surgical ablation, there still exists a need for information concerning the quantitative nature of the relationships between the removed SCG and its remaining contralateral ganglion and between the remaining SCG and its modified innervation territory. To this end, using design-based stereological methods, we have investigated the structural changes induced by unilateral ganglionectomy in sheep at three distinct timepoints (2, 7 and 12 weeks) after surgery. The effects of time, and lateral (left-right) differences, were examined by two-way analyses of variance and paired t-tests. Following removal of the left SCG, the main findings were: (i) the remaining right SCG was bigger at shorter survival times, i.e. 74% at 2 weeks, 55% at 7 weeks and no increase by 12 weeks, (ii) by 7 weeks after surgery, the right SCG contained fewer neurons (no decrease at 2 weeks, 6% fewer by 7 weeks and 17% fewer by 12 weeks) and (iii) by 7 weeks, right SCG neurons were also larger and the magnitude of this increase grew substantially with time (no rise at 2 weeks, 77% by 7 weeks and 215% by 12 weeks). Interaction effects between time and ganglionectomy-induced changes were significant for SCG volume and mean perikaryal volume. These findings show that unilateral superior cervical ganglionectomy has profound effects on the contralateral ganglion. For future investigations, it would be interesting to examine the interaction between SCGs and their innervation targets after ganglionectomy. Is the ganglionectomy-induced imbalance between the sizes of innervation territories the milieu in which morphoquantitative changes, particularly changes in perikaryal volume and neuron number, occur? Mechanistically, how would those changes arise? Are there any grounds for believing in a ganglionectomy-triggered SCG cross-innervation and neuroplasticity?

Viral risk factor for seizures: Pathobiology of dynorphin in herpes simplex viral (HSV-1) seizures in an animal model

Up to 89% of patients with herpes simplex virus type-1 (HSV-1) encephalitis can have seizures. Possibly, viruses are environmental triggers for seizures in genetically vulnerable individuals. Inherited dynorphin promoter polymorphisms are associated with temporal lobe epilepsy and febrile seizures in man. In animals, the dynorphin system in the hippocampus regulates excitability. The hypothesis that reduced dynorphin expression in dentate gyrus of hippocampus due to HSV-1 infection leads to epileptic responses was tested in a rat model of HSV-1 encephalitis using EEG recording, histopathological and neuropharmacologic probes. HSV-1 infection causes loss of dynorphin A-like immunoreactivity in hippocampus, an effect independent of direct viral interference and cell loss. A kappa opioid receptor agonist U50488 effectively blocks ictal activity, linking absence of dynorphin to propensity for epileptic activity. These findings show a vulnerability of hippocampal dynorphin during infection, suggesting a neurochemical basis for seizures that may be generalizable to other encephalitic viruses.

Sympathetic sprouting drives hippocampal cholinergic reinnervation that prevents loss of a muscarinic receptor-dependent long-term depression at CA3-CA1 synapses

Degeneration of septohippocampal cholinergic neurons results in memory deficits attributable to loss of cholinergic modulation of hippocampal synaptic circuits. A remarkable consequence of cholinergic degeneration is the sprouting of noradrenergic sympathetic fibers from the superior cervical ganglia into hippocampus. The functional impact of sympathetic ingrowth on synaptic physiology has never been investigated. Here, we report that, at CA3-CA1 synapses, a Hebbian form of long-term depression (LTD) induced by muscarinic M1 receptor activation (mLTD) is lost after medial septal lesion. Unexpectedly, expression of mLTD is rescued by sympathetic sprouting. These effects are specific because LTP and other forms of LTD are unaffected. The rescue of mLTD expression is coupled temporally with the reappearance of cholinergic fibers in hippocampus, as assessed by the immunostaining of fibers for VAChT (vesicular acetylcholine transporter). Both the cholinergic reinnervation and mLTD rescue are prevented by bilateral superior cervical ganglionectomy, which also prevents the noradrenergic sympathetic sprouting. The new cholinergic fibers likely originate from the superior cervical ganglia because unilateral ganglionectomy, performed when cholinergic reinnervation is well established, removes the reinnervation on the ipsilateral side. Thus, the temporal coupling of the cholinergic reinnervation with mLTD rescue, together with the absence of reinnervation and mLTD expression after ganglionectomy, demonstrate that the autonomic-driven cholinergic reinnervation is essential for maintaining mLTD after central cholinergic cell death. We have discovered a novel phenomenon whereby the autonomic and central nervous systems experience structural rearrangement to replace lost cholinergic innervation in hippocampus, with the consequence of preserving a form of LTD that would otherwise be lost as a result of cholinergic degeneration.

Pro- and anti-apoptotic evidence for cholinergic denervation and hippocampal sympathetic ingrowth in rat dorsal hippocampus

In rat, injection of the specific cholinotoxin, 192 IgG-saporin, into the medial septum results not only in a selective cholinergic denervation of hippocampus, but in an ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglion, into the hippocampus. A similar process, in which peripheral noradrenergic axons invade hippocampus, may also occur in Alzheimer's disease. Since apoptotic cell death has been demonstrated in the selective neuronal loss found in Alzheimer's disease, the aim of this study was to measure apoptotic protein expression and DNA fragmentation in hippocampal sympathetic ingrowth and cholinergic denervation. Western blot, TdT-mediated dUTP nick end labeling, and oligo ligation techniques were used. Choline acetyltransferase activity and norepinephrine concentrations were also measured. As seen in our previous results, an increase in apoptotic markers was induced by cholinergic denervation alone (medial septum lesion + ganglionectomy), while hippocampal sympathetic ingrowth (medial septum + sham ganglionectomy) reduced or normalized apoptotic effects to control group levels. A decrease in choline acetyltransferase activity was also found in the dorsal hippocampus of hippocampal sympathetic ingrowth and cholinergic denervation groups. An increase in norepinephrine concentration was found in hippocampal sympathetic ingrowth but not in cholinergic denervation group. Results of this study suggest that cholinergic denervation is responsible for most of the proapoptotic responses, while hippocampal sympathetic ingrowth produces a protective effect in the process of programmed cell death in rat dorsal hippocampus. This effect may be a secondary to an altered relationship between norepinephrine-acetylcholine.