Central neuropeptide Y (NPH) expression and function : role in stress, experimental anxiety, and cognition

Abstract

Neuropeptide Y (NPY), a 36 amino acid peptide abundantly expressed throughout the mammalian nervous system, has been implicated in experimental anxiety and stress related responses, feeding, and learning and memory. These functions are mediated via different receptor subtype populations (Y1-Y6), all belonging to the G-protein coupled receptor superfamily. The Y1 -subtype has been shown to mediate the anxiolytic effects of NPY, while the Y2 subtype is involved in regulation of circadian rhythms and neuronal excitability in the hippocampus, and may, thus, be the receptor subtype involved in the peptide's effects on memory function. Stimulation of food-intake by NPY has been proposed to be mediated by Y5 and/or Y1-receptors within the hypothalamus. In this thesis, regulation of central NPY-expression has been examined in relation to stress, anxiety, and cognitive function in the rat.

First, we examined the effects of acute or repeated restraint stress on NPY expression. A single restraint stress has been shown to be anxiogenic on the elevated plus-maze, an effect mediated through the central nucleus of the amygdala, while NPY is anxiolytic. A single restraint stress was shown to significantly decrease NPY mRNA and peptide expression in the amygdala, while repeating the restraint once per day for 10 consecutive days lead to an increase in amygdala NPY expression (mRNA and peptide), and to an endocrine and behavioral adaptation, i.e. an habituation, to the stressor. On the basis of these findings, we have proposed that NPY might be part of the mechanism regulating the behavioral responses to stress. A powerful way to examine the function of a gene and its product is to overexpress the gene in vivo. We used lipid-mediated gene transfer as a method for introducing genetic material, here cDNA for NPY, or, for optimization purposes, the enzyme chloramphenicol-acetyl-transferase (CAT) into the adult rat CNS. We were able to demonstrate the feasibility of in vivo gene transfer in the adult rat brain, and the production of functional protein measured as enzymatic activity. However, expression did not reach a sufficient level and / or distribution to be useful for our purposes.

Instead, a rat overexpressing NPY was used to study the effects of NPY on experimental anxiety and response to restraint stress, cognitive function, locomotion, voluntary ethanol consumption and feeding. No effect of the genotype was seen on locomotion, voluntary ethanol consumption, food-intake, or anxiety related behavior on the elevated plus-maze under baseline (unstressed) conditions. However, the transgenic subjects displayed an insensitivity to the anxiogenic effects of a restraint stress when the stress preceeded behavioral testing on the elevated plus-maze. There was a marked release of punished responding in the punished drinking test in transgenic subjects compared to the wildtype litter-mate controls. Also, a highly significant impairment of spatial learning in transgenic subjects was seen in the acquisition-phase of testing on the Morris water-maze. Control experiments and previously published data indicate that anti-stress and memory impairment effects of NPY are independent. The work presented here supplies further evidence for the involvement of endogenous NPY in stress-related behaviors, experimental anxiety, and cognitive function.