Institutionen för fysiologi och farmakologi / Department of Physiology and Pharmacology
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