Hippocampal testosterone relates to reference memory performance and synaptic plasticity in male rats

Abstract

Steroids are important neuromodulators influencing cognitive performance and synaptic plasticity. While the majority of literature concerns adrenal- and gonadectomized animals, very little is known about the "natural" endogenous release of hormones during learning. Therefore, we measured blood and brain (hippocampus, prefrontal cortex) testosterone, estradiol, and corticosterone concentrations of intact male rats undergoing a spatial learning paradigm which is known to reinforce hippocampal plasticity. We found significant modulations of all investigated hormones over the training course. Corticosterone and testosterone were correlated manifold with behavior, while estradiol expressed fewer correlations. In the recall session, testosterone was tightly coupled to reference memory (RM) performance, which is crucial for reinforcement of synaptic plasticity in the dentate gyrus. Intriguingly, prefrontal cortex and hippocampal levels related differentially to RM performance. Correlations of testosterone and corticosterone switched from unspecific activity to specific cognitive functions over training. Correspondingly, exogenous application of testosterone revealed different effects on synaptic and neuronal plasticity in trained versus untrained animals. While hippocampal long-term potentiation (LTP) of the field excitatory postsynaptic potential (fEPSP) was prolonged in untrained rats, both the fEPSP- and the population spike amplitude (PSA)-LTP was impaired in trained rats. Behavioral performance was unaffected, but correlations of hippocampal field potentials with behavior were decoupled in treated rats. The data provide important evidence that besides adrenal, also gonadal steroids play a mechanistic role in linking synaptic plasticity to cognitive performance.

Keywords: LTP; dentate gyrus; endogenous; gonadal steroids; learning; stress.

Figure 1

Training protocol, learning performance, and effects of food deprivation on hormone household. (A) The same five of 36 holes of the holeboard were baited (depicted in black). The right panel illustrates behavioral parameters. Breaking light beams of the holes was classified as either inspection of the hole (upper beam) or visit (middle beam). Areas surrounding the holes were termed “cells” or “special cells” in case of baited holes. In (B) the protocol and the training groups for hormone sampling are depicted. The full training protocol consisted of three sessions (abbreviated with S). Three training groups (G1–3) were sacrificed 15 min after the session and two trained control groups (tcG2 and tcG3) were sacrificed 24 h after the session. (C) Averaged performance as measured in latency to find all pellets and reference memory errors (RME). Animals performed significantly better at the end of session 2 as compared to the beginning. (D) Food deprivation (fd) caused significantly elevated corticosterone serum levels as well as reduced testosterone levels (F) in blood as well as in brain tissue samples. (E) Estradiol levels were not significantly affected by food deprivation.

Figure 2

The profile of endogenous hormone titers measured in serum (left panel) and brain tissue probes (right panel). Given are mean hormone concentrations for (A) corticosterone serum, (B) corticosterone brain tissue, (C) estradiol serum, (D) estradiol brain tissue, (E) testosterone serum, and (F) testosterone brain tissue probes. Time of sacrifice was dependent on session length and is indicated at the bottom. Chamber controls (untrained animals) are depicted in gray and abbreviated with ccG. Trained controls (tcG) were sacrificed 24 h after their last training at the same time point as the corresponding training group and controlled for long-term effects of the training on the hormone household.

Figure 3

Correlation analysis of hormones and behavioral data. To illustrate the relevancy of hormones over the training course, in (A) the number of significant correlations per session (acute groups G1, G2, and G3 only) are depicted color coded for each tissue type (S = serum, P = PFC and H = hippocampus). Testosterone expressed the highest number of significant correlations in the last session. In (B) the same data was arranged with parameters classified into three behavioral categories: working memory (WM), reference memory (RM) or unspecific activity (agility) related. (C) Outcome of the multiple regression analysis grouped according to the behavioral classification. (D) Illustrates the number of correlations occurring between a behavioral parameter and tissue types: if the parameter correlated significantly with one tissue only, it is termed “single,” “double” for two tissues and triple if correlation occurred between the parameter and all three samples. The gray insets illustrate the tissue pairs present in the double correlation bar.

Figure 4

Role of testosterone during recall. (A) The r-value of significant correlations of testosterone with behavior in session 3 are depicted color coded for each tissue type (S, serum; P, PFC; and H, hippocampus) and parameter. spec, special cell entries; INS, inspections, VIS, visits; doub-VIS, number of double visit holes; hol-INS, number holes inspected. (B) While hippocampal testosterone correlated with the task-promoting parameter special cell entries (spec) prefrontal testosterone correlated with RME-INS. (C) There was an inversion of correlation direction with RME-INS respective RME-VIS from early to late trials for PFC and serum testosterone as well as PFC corticosterone, but not for hippocampal testosterone. (S = serum, P = PFC and H = hippocampus).

Figure 5

Effect of testosterone application on field potential measurements. (A,B) Performance of a holeboard recall session (hb) after LTP induction (tet) led to LTP reinforcement in control animals for both, the PSA and the fEPSP. In contrast, intracerebroventricular application of testosterone (app) 30 min prior to LTP induction impaired the behavioral reinforcement of LTP. (C,D) In untrained animals, testosterone application reduced the PSA-baseline transmission while leaving the fEPSP intact. (E,F) PSA values of testosterone treated, untrained animals undergoing an LTP induction protocol were reduced at 24 h after induction, while fEPSP potentiation was prolonged for 8 h. (G,H) Brain tissue testosterone and corticosterone levels of chamber controls (cc, n = 16), vehicle applied (vc, n = 9), and animals receiving 1 μg testosterone (T, n = 9). (I) Example traces of PSA recordings in the paired pulse protocol.