Peripherally administered choleocystokinin-8 increases neurotrophins in the brain

Abstract

The aim of this thesis was to test the hypothesis that the gastric hormone Cholecystokinin-8 (CCK-8) can modulate the production of Nerve Growth Factor (NGF) in the brain.

NGF is the first discovered and the best characterized neurotrophic factor which plays a crucial role in the survival and development of basal forebrain cholinergic neurons (BFCN). BFCN depend on NGF also during adult life and thus NGF also is involved in the maintenance of BFNC-related functions, such as memory and learning. Since CCK-8 has been shown to stimulate the synthesis of neurotransmitters and neuropeptides in the brain and to improve memory and learning when peripherally injected, we decided to study whether NGF could be a possible brain factor mediating the central effects of CCK-8.

We found that intraperitoneal injection with CCK-8 increased NGF synthesis in the brains of adult mice and rats. The CCK-8 effects are dose- and time-dependent and the strongest stimulatory effects on NGF are obtained with 8nmol/kg. The HPLC analysis and the evidence obtained in vivo confirm that CCK-8 induces an increase in biologically active form of NGF by an up-regulation of brain NGF biosynthesis. Throught NGF, CCK-8 stimulates the expression of cholinergic markers in the forebrain and counteracts the cholinergic cell degeneration in fimbria-fornix lesioned mice.

The NGF response to CCK administration is abolished following pre-treatment with atropine, while pre-treatment with CCKA and CCKB receptor antagonists differentially affect the NGF levels in the hippocampus and hypothalamus. Indeed, the CCKB antagonist blocks the CCK-induced NGF synthesis in the hippocampus, while the CCKA antagonist selectively inhibits the CCK effects on hypothalamic NGF.

Analysing the NGF rnRNA expression using a newly developed RT-PCR ELISA method we observed that hypothalamic but not hippocampal NGF synthesis is affected by electrical vagal stimulation. Since CCK-8 is released during vagal stimulation, this data led us to hypothesise that the vagal nerve mediates the stimulatory effects of CCK-8 on hypothalamic NGF production. A non-vagal mediated mechanism - most probably due to the activation of CCKB receptors - may be inferred for the CCK-induced NGF increase in the hippocampus.

Moreover, this thesis shows that glutamate when injected intravenously in a subtoxic dose increases NGF concentrations in the hippocampus and hypothalamus. We also observed that pre-treatment with a selective NMDA receptor antagonist blocked the CCK-induced NGF synthesis in the hippocampus. These results suggest that CCK and the vagal nerve may regulate the NGF synthesis by modulating the release of excitatory amino acids.

In conclusion, this thesis provides evidences that the neuroprotective action of CCK on forebrain neurons is mediated by the synthesis and release of NGF. Moreover, the data suggests that CCK exerts a stimulatory effect on neurotrophin production in brain acting via two different pathways. One pathway involves the vagal nerve and it is mainly mediated by CCKA receptors, while the other pathway is mediated through the activation of CCB receptors which are not of vagal origin.