Neurosteroids: deficient cognitive performance in aged rats depends on low pregnenolone sulfate levels in the hippocampus

Abstract

Pregnenolone sulfate (PREG S) is synthesized in the nervous system and is a major neurosteroid in the rat brain. Its concentrations were measured in the hippocampus and other brain areas of single adult and aged (22-24 month-old) male Sprague-Dawley rats. Significantly lower levels were found in aged rats, although the values were widely scattered and reached, in about half the animals, the same range as those of young ones. The spatial memory performances of aged rats were investigated in two different spatial memory tasks, the Morris water maze and Y-maze. Performances in both tests were significantly correlated and, accompanied by appropriate controls, likely evaluated genuine memory function. Importantly, individual hippocampal PREG S and distance to reach the platform in the water maze were linked by a significant correlation, i.e., those rats with lower memory deficit had the highest PREG S levels, whereas no relationship was found with the PREG S content in other brain areas (amygdala, prefrontal cortex, parietal cortex, striatum). Moreover, the memory deficit of cognitively impaired aged rats was transiently corrected after either intraperitoneal or bilateral intrahippocampal injection of PREG S. PREG S is both a gamma-aminobutyric acid antagonist and a positive allosteric modulator at the N-methyl-D-aspartate receptor, and may reinforce neurotransmitter system(s) that decline with age. Indeed, intracerebroventricular injection of PREG S was shown to stimulate acetylcholine release in the adult rat hippocampus. In conclusion, it is proposed that the hippocampal content of PREG S plays a physiological role in preserving and/or enhancing cognitive abilities in old animals, possibly via an interaction with central cholinergic systems. Thus, neurosteroids should be further studied in the context of prevention and/or treatment of age-related memory disorders.

Figure 1

Individual PREG S levels (log ng/g tissue) in the hippocampus of young adult (3 month old, n = 12) and aged (24 month old, n = 28) rats. Horizontal lines represent the median of each distribution. A marked variability was observed in the aged rats and their median score was below the lowest young adult score.

Figure 2

Learning performance of aged rats in the water-maze: correlation with PREG S levels in the hippocampus. PREG S concentrations were expressed in log (ng/g). The performance was expressed as the mean distance to reach the hidden platform during the last four days of the test. Low PREG S levels were linked with longer distances, i.e., worse performances [y = (−7.01 ± 2.18)x + 12.61].

Figure 3

Effects of intraperitoneal PREG S injection on the performance of impaired aged rats in the Y-maze. (A) Male rats, 23 months old (n = 30) were tested for their memory performance in the Y-maze (ITI = 6 h). The parameter measured was the number of visits in novel arm (%). The horizontal bar represents the median of the distribution. Rats were divided in two groups: impaired rats () (n = 15), whose score was below the median value; unimpaired rats (○) (n = 15), whose score was above median. The mean value of the impaired group did not differ from chance level (t = 1.0, df = 14, ns), while the mean value of the unimpaired group differed from chance level (t = 6.0, df = 14, ns). (B) The impaired animals (n = 15) were split into two subgroups. Vehicle injection had no effect on the performances of impaired rats (n = 8) in the Y-maze (ITI = 6 h). In contrast, the subgroup treated with PREG S (n = 7) performed significantly above chance level 7 h postinjection (t = 5.5, df = 6; ∗, P < 0.02), but returned to chance level 10 days later (t = 2.0, df = 6, ns).

Figure 4

Effects of PREG S bilateral infusion into the hippocampus on the performance of impaired aged rats in the Y-maze. Impaired male rats, 22 months old (n = 12) were split into two subgroups and received infusion of PREG S or vehicle. Vehicle injection had no effect on the performances of impaired rats (n = 6) in the Y-maze (ITI = 6 h). In contrast, the subgroup treated with PREG S (n = 6) performed significantly above chance level 6 h postinjection (t = 4.1, df = 5, ∗∗, P < 0.01), but returned to chance level 7 days later (t = 1.1, df = 5, ns).

Figure 5

Effects of the i.c.v. injection of PREG S (20 μg) on extracellular concentrations of ACh in the hippocampus. The output of ACh, expressed as the percentage of baseline, is increased 20, 30, and 40 min postinjection in PREG S group compared with vehicle group (∗∗, P < 0.01, ∗∗∗, P < 0.001).