The effects of intrahypothalamic injection of neurogastrointestinal peptides on the multiple-unit-activity in selected areas of the pig brain

Abstract

Many peptides that are known to exist in the gastrointestinal tract and have established physiological functions in the process of digestion have also been localized in the brain. Several of these peptides are known to affect animal behavior and for some, synaptic turnover mechanisms and receptors have been identified. We wanted to determine whether the injection of six of these peptides into the ventromedial nucleus of the hypothalamus (VMN) would produce changes in its electrical activity which would concomitantly be reflected in electrical changes in five hypothalamic and limbic areas that are anatomically associated with the VMN and have roles in feeding behavior. A stereotaxic atlas of the brain of young, commercial cross-bred pigs was created to facilitate accurate cannulae and electrode placements. Microelectrodes were stereotactically placed in the amygdala (A), substantia nigra (SN), globus pallidus (GP), lateral hypothalamus (LH), and nucleus accumbens (NA) in each of ten pigs, and a cannula-electrode assembly placed in the VMN. Six peptides, cholecystokinin-octapeptide (CCK-OP), bombesin (B), somatostatin (S), vasoactive intestinal polypeptide (VIP), gastrin (G), and neurotensin (N) were injected into the VMN of the six pigs in a random block trial design with carrier (.9% NaCl) as a control. Multiple-Unit-Activity (MUA) and field potentials (slow wave activity) were recorded before and after injection of each peptide and the control. The pre and post injection MUA was quantified by cumulative integration. The directional change occuring in the VMN was statistically compared to the direction of change at the other five recording sights. A significant direct relationship in the change in the MUA of the VMN compared to the change in activity at the other five structures was noted. The relative spectral power and its geometric descriptors revealed statistically significant increases in the delta and theta frequency bands of the field potentials of most structures with the injection of several of the peptides. The results of these studies demonstrate direct correlative changes in the MUA and the VMN with the other recorded areas upon injection of the putative peptide transmitters. The MUA was more sensitive at disclosing the effects of the peptide injections than was the field potential analysis. Based on these results there appears to be a need for additional studies that would investigate specific peptide effects within the individual sites and attempt to correlate the changes in the MUA with changes in the slow wave activity.