Gastrin-Releasing Peptide Signaling Plays a Limited and Subtle Role in Amygdala Physiology and Aversive Memory

Links between synaptic plasticity in the lateral amygdala (LA) and Pavlovian fear learning are well established. Neuropeptides including gastrin-releasing peptide (GRP) can modulate LA function. GRP increases inhibition in the LA and mice lacking the GRP receptor (GRPR KO) show more pronounced and persistent fear after single-trial associative learning. Here, we confirmed these initial findings and examined whether they extrapolate to more aspects of amygdala physiology and to other forms of aversive associative learning. GRP application in brain slices from wildtype but not GRPR KO mice increased spontaneous inhibitory activity in LA pyramidal neurons. In amygdala slices from GRPR KO mice, GRP did not increase inhibitory activity. In comparison to wildtype, short- but not long-term plasticity was increased in the cortico-lateral amygdala (LA) pathway of GRPR KO amygdala slices, whereas no changes were detected in the thalamo-LA pathway. In addition, GRPR KO mice showed enhanced fear evoked by single-trial conditioning and reduced spontaneous firing of neurons in the central nucleus of the amygdala (CeA). Altogether, these results are consistent with a potentially important modulatory role of GRP/GRPR signaling in the amygdala. However, administration of GRP or the GRPR antagonist (D-Phe6, Leu-NHEt13, des-Met14)-Bombesin (6–14) did not affect amygdala LTP in brain slices, nor did they affect the expression of conditioned fear following intra-amygdala administration. GRPR KO mice also failed to show differences in fear expression and extinction after multiple-trial fear conditioning, and there were no differences in conditioned taste aversion or gustatory neophobia. Collectively, our data indicate that GRP/GRPR signaling modulates amygdala physiology in a paradigm-specific fashion that likely is insufficient to generate therapeutic effects across amygdala-dependent disorders

Potent amyloidogenicity and pathogenicity of Aβ43.

The amyloid-β peptide Aβ42 is known to be a primary amyloidogenic and pathogenic agent in Alzheimer\u27s disease. However, the role of Aβ43, which is found just as frequently in the brains of affected individuals, remains unresolved. We generated knock-in mice containing a pathogenic presenilin-1 R278I mutation that causes overproduction of Aβ43. Homozygosity was embryonic lethal, indicating that the mutation involves a loss of function. Crossing amyloid precursor protein transgenic mice with heterozygous mutant mice resulted in elevated Aβ43, impairment of short-term memory and acceleration of amyloid-β pathology, which accompanied pronounced accumulation of Aβ43 in plaque cores similar in biochemical composition to those observed in the brains of affected individuals. Consistently, Aβ43 showed a higher propensity to aggregate and was more neurotoxic than Aβ42. Other pathogenic presenilin mutations also caused overproduction of Aβ43 in a manner correlating with Aβ42 and with the age of disease onset. These findings indicate that Aβ43, an overlooked species, is potently amyloidogenic, neurotoxic and abundant in vivo

The evolution of Aβ peptide burden in the APP23 transgenic mice: implications for Aβ deposition in Alzheimer disease

Background: High levels of Aβ in the cerebral cortex distinguish demented Alzheimer’s disease (AD) from nondemented elderly individuals, suggesting that decreased amyloid-beta (Aβ) peptide clearance from the brain is a key precipitating factor in AD.Materials and Methods: The levels of Aβ in brain and plasma as well as apolipoprotein E (ApoE) in brain were investigated by enzyme-linked immunosorbent assay (ELISA) and Western blotting at various times during the life span of the APP23 transgenic (Tg) and control mice. Histochemistry and immunocytochemistry were used to assess the morphologic characteristics of the brain parenchymal and cerebrovascular amyloid deposits and the intracellular amyloid precursor protein (APP) deposits in the APP23 Tg mice.Results: No significant differences were found in the plasma levels of Aβ between the APP23 Tg and control mice from 2–20 months of age. In contrast, soluble Aβ levels in the brain were continually elevated, increasing 4-fold at 2 months and 33-fold in the APP23 Tg mice at 20 months of age when compared to the control mice. Soluble Aβ42 was about 60% higher than Aβ40. In the APP23 Tg mice, insoluble Aβ40 remained at basal levels in the brain until 9 months and then rose to 680 µg/g cortex by 20 months. Insoluble A?40 was negligible in non-Tg mice at all ages. Insoluble Aβ42 in APP23 Tg mice rose to 60 µg/g cortex at 20 months, representing 24 times the control Aβ42 levels. Elevated levels of ApoE in the brain were observed in the APP23 Tg mice at 2 months of age, becoming substantially higher by 20 months. ApoE colocalized with Aβ in the plaques. Beta-amyloid precursor protein (βAPP) deposits were detected within the neuronal cytoplasm from 4 months of age onward. Amyloid angiopathy in the APP23 Tg mice increased markedly with age, being by far more severe than in the Tg2576 mice.Conclusions: We suggest that the APP23 Tg mouse may develop an earlier blockage in Aβ clearance than the Tg2576 mice, resulting in a more severe accumulation of Aβ in the perivascular drainage pathways and in the brain. Both Tg mice reflect decreased Aβ elimination and as models for the amyloid cascade they are useful to study AD pathophysiology and therapy

Dietary Cu stabilizes brain superoxide dismutase 1 activity and reduces amyloid Abeta production in APP23 transgenic mice

The Cu-binding beta-amyloid precursor protein (APP), and the amyloid Abeta peptide have been proposed to play a role in physiological metal regulation. There is accumulating evidence of an unbalanced Cu homeostasis with a causative or diagnostic link to Alzheimer's disease. Whereas elevated Cu levels are observed in APP knockout mice, APP overexpression results in reduced Cu in transgenic mouse brain. Moreover, Cu induces a decrease in Abeta levels in APP-transfected cells in vitro. To investigate the influence of bioavailable Cu, transgenic APP23 mice received an oral treatment with Cu-supplemented sucrose-sweetened drinking water (1). Chronic APP overexpression per se reduced superoxide dismutase 1 activity in transgenic mouse brain, which could be restored to normal levels after Cu treatment (2). A significant increase of brain Cu indicated its bioavailability on Cu treatment in APP23 mice, whereas Cu levels remained unaffected in littermate controls (3). Cu treatment lowered endogenous CNS Abeta before a detectable reduction of amyloid plaques. Thus, APP23 mice reveal APP-induced alterations linked to Cu homeostasis, which can be reversed by addition of dietary Cu

Exogenous GRP or GRPR antagonist did not affect expression of conditioned fear.

<p><b>A</b>) 600 ng GRP or 3000 ng GRPR antagonist (D-Phe⁶,Leu-NHEt¹³,des-Met¹⁴)-Bombesin(6–14) was infused into the amygdala of C57BL/6 mice, that were conditioned with 6 CS-US pairings as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034963#pone-0034963-g002" target="_blank">Fig. 2C</a>, 10 min prior to testing freezing in the conditioning context 24 h later. <b>B</b>) Effect of intra-amygdala infusion of 600 ng GRP 10 min prior to testing freezing in response to the CS. <b>C</b>) Location of the bilateral injection sites determined from post-hoc histological analysis.</p

GRPR KO animals showed no differences in conditioned tast aversion (CTA) or neophobia.

<p><b>A</b>) CTA was evoked by pairing a novel taste, saccharin, with a LiCl injection to induce illness the day before testing (LiCl; n = 12 mice per group). Control animals were offered the novel taste saccharin but injected with NaCl (NaCl groups; n = 11 mice per group) or given only water to drink and injected with NaCl the previous day (saccharin naive groups; n = 12 mice per group). <b>B</b>) Attenuation of neophobia and neophobia were assessed by comparing the aversion to saccharin on first exposure (saccharin naive) with the aversion shown by mice that were exposed to saccharin the previous day (NaCl). On successive days the neophobia was attenuated by repeatedly being given the chance to drink saccharin flavored water.</p