Chemical variation in the ventral hippocampus and other brain sites during conditioned avoidance

Abstract

No differences between learning and nonlearning rats were found in neurochemical measures in the ventral hippocampus but differences were present in the medial ventral cortex.In previous experiments (Gaito, Mottin, & Davison, in press), chemical measures of the medial ventral cortex (MV) differentiated between learning and noniearning animals. The separation of ventral cortex from underlying white matter is difficult, and ventral cortices tend to contain some subcortical tissue. Therefore, the significant results in MV might be due to the sole contribution of portions of the hippocampus. Since the hippocampus had been implicated in learning events (e.g., see Adey et al, 1960;Gaito, 1966), it is reasonable to hypothesize that the significant differences in MV were due to hippocampal function. Two replicated experiments were conducted to evaluate this hypothesis. METHOD In the two experiments, the task was one-way active avoidance conditioning with the shock level set at 1.5 mA. Eight pairs of littermates were used in each experiment; their ages varied from 69 to 80 days; all but two litters were males (Wistar strain).In both experiments there were two groups of rats: Shock Avoidance, E (given 15 trials in 15 min), and Controls, C (in the shock chamber for 15 min without any treatment). All rats were allowed a IS-min adaptation period in the shock chamber.At the end of the 30-min period in the apparatus each rat was sacrificed by immersion in liquid nitrogen for 10 sec. The brain was rapidly removed and sectioned into the ventral hippocampus (VH) and 10 sites as in previous experiments (Gaito, Mottin, & Davison, in press): anterior ventral cortex (AV), medial ventral cortex (MV), posterior ventral cortex (PV), anterior dorsal cortex (AD), medial dorsal cortex (MD), posterior dorsal cortex (PD), cerebral hemispheres (CH), cerebellum (CB), upper brain stem (UBS), and lower brain stem (LBS) (sectioned just below the inferior colliculi). RNA, DNA, and proteins were extracted by a modified SchmidtTannhauser procedure and the amounts of each were determined by spectrophotometric analysis.One half hour before being placed in the apparatus each rat was injected intracranially with 5 pc of valine-C ' 4 (specific activity-II? mc/mM) to determine incorporation rates into protein.The dependent variables in Experiments I and 2 were: RNA, DNA, and protein (all per gam of tissue, wet weight), RNA/DNA, prot/DNA, prot/RNA, and relative specific activity of the protein fraction (R.S.A. Prot), i.e., specific activity of the protein fraction (S.A. Prot)/specific activity of the cell pool fraction (S.A. Cell Pool). Littermates by Conditions analyses of variance were conducted with the rejection region set at a p of .10 so as to decrease the p of Type H errors. Because Type I errors increase as a result of this aspect, only tissues which showed significant results in two or more chemical measures and/or experiments were considered real ones.RESULTS AND DISCUSSION In both experiments E's showed six or more avoidance Psyc;hon. Sci., 196...