EVIDENCE FOR AND AGAINST THE NAPLES HIGH-EXCITABILITY AND LOW-EXCITABILITY RATS AS GENETIC MODEL TO STUDY HIPPOCAMPAL FUNCTIONS

The Naples high- (NHE) and low-excitability (NLE) are two rat lines, s electively bred for high and low activity levels in a Lat-maze, respec tively. Because the activity level in a novel environment depends main ly on the integrity of the hippocampal formation, and NLE and NHE rats differ with a similar background of emotionality, arterial blood pres sure, and learning ability, they have been proposed as animal model to study hippocampal functions. Our aim is to prove evidence in favor an d against this hypothesis. The evidence in favor indicates that NLE/NH E rats have a defective spatial processing, and pertains to (a) Differ ential activity in a spatial novelty situation (selection trait), prop ortional to the stimulus complexity rats are exposed to (NHE are hyper - and NLE-rats hypoactive); and (b) Impaired working memory in a six-a rm non-reinforced tunnel maze in both lines compared to random-bred ra ts, that was reversed by the introduction of a reinforcer. In addition , multiple evidence of (i) lower intra- + infrapyramidal mossy fiber t erminals in both NLE/NHE vs. controls; (ii) increased sensitivity of h ippocampal elements to microinjections of vasopressin (but not oxytoci n) and of ''delta'' (but not ''mu'') opioids; (iii) lower number of hi gh-affinity glucocorticoid receptors; (iv) lower number of alpha-but n ot beta-adrenergic receptors in the hippocampus and hypothalamus of NH E rats only; and (v) the genotype-dependent behavior of a DNA fraction with fast turnover, suggest that both NHE/NLE are ''disintegrated'' a t the hippocampal interface. Further, neurobehavioral covariations amo ng individual differences reveal nonlinear, complex relationships, an evidence apparently against the hypothesis. The data altogether sugges t that NLE/NHE rats may be a useful model to test the role of the hipp ocampus in the integration of spatial and emotional information.