Abstract
Galanin is believed to be co-released with acetylcholine by neurones projecting from
the medial septum and nucleus ofMeynert to the hippocampus in rodents. Galanin
inhibits acetylcholine and glutamate release, thereby depressing excess neuronal
excitability in the brain. Although this effect established galanin as an endogenous
neuroprotective substance, released only during high frequency neuronal firing, it
may also explain why it impairs memory and cognition in vivo. The sustained
increase in glutamatergic synaptic strength following high frequency stimulation of
hippocampal neurones, a phenomenon termed long-term potentiation (LTP), has
been widely recognised as a model ofthe synaptic changes that may underlie
learning and memory in vertebrates. It may thus be predicted that the physiological
action ofgalanin at the cellular level would be to depress LTP, thereby causing an
impairment in mnemonic processes mediated by the hippocampus. Experiments were
designed to address aspects ofthis hypothesis, namely: (1) in vitro characterisation of
the effect ofgalanin agonists and antagonists on synaptic transmission and plasticity
in the CA1 area of rodent hippocampus and (2) investigation of glutamatergic
synaptic plasticity in galanin knockout mice and their wild-type littermates.
Exogenous galanin induced a dose-dependent increase in the slope of baseline
fEPSPs, which appeared to be dependent on the pathways from CA3 to CA1 being
intact, but it did not have any effect on paired-pulse facilitation ratios (PPF) in low
concentration. However, in higher concentration, galanin induced a significant
decrease in PPF in intact slices. In CA3-hemisected hippocampal slices the
aforementioned effects did not occur. The effect of galanin on LTP and long-term
depression (LTD) of glutamate mediated synaptic transmission in apical and basal
dendrites of CA1 pyramidal neurones were investigated using both intracellular and
extracellular recording techniques in vitro. LTP induced in either apical or basal
dendrites of CA1 pyramidal neurones by different paradigms was significantly
inhibited by galanin. Galanin also inhibited LTP in hippocampal slices prepared from
wild-type mice. This effect was reversible by the known galanin antagonist, galantide
(Ml5). Galanin did not affect isolated pure NMDA receptor-mediated postsynaptic
potentials or the loss of spike frequency adaptation and increase in input resistance evoked by metabotropic glutamate receptor activation, indicating that its inhibition of
LTP was downstream ofthese receptors. Galanin applied had no effect the
expression of LTP indicating that galanin may inhibit LTP by interfering with kinase
activity necessary for the induction of LTP, e.g. protein kinase C. Galanin did not
affect the induction of LTD. Subsequent studies in the galanin-null transgenic mice
yielded no effect on synaptic strength or paired pulse facilitation ratios. Galanin gene
deletion caused a significant impairment of LTP, which was only observed in basal
dendrites, the magnitude ofwhich increased with age. The underlying molecular
mechanism for this impairment might be a significantly faster saturation of synaptic
plasticity in the mutant mice in vivo, compared to wild-type mice. No effects of
galanin were noted in mutant mice. This could suggest a developmental loss of
galanin-responsive cells concomitant with global galanin gene deletion.
In summary, galanin seems to have a modulatory effect on excitatory
neurotransmitters in the hippocampus, such as glutamate, thereby delaying the
neurodegenerative effect of age. The research described in this thesis is deemed of
importance in biomedical research of drug therapy for protection against
neurodegenerative disease.
