Growth hormone secretagogue receptor (GHS-R1a) knockout mice exhibit improved spatial memory and deficits in contextual memory

Abstract

Although the hormone ghrelin is best known for its stimulatory effect on appetite and regulation of growth hormone release, it is also reported to have beneficial effects on learning and memory formation in mice. Nevertheless, controversy exists about whether endogenous ghrelin acts on its receptors in extra-hypothalamic areas of the brain. The ghrelin receptor (GHS-R1a) is co-expressed in neurons that express dopamine receptor type-1 (DRD1a) and type-2 (DRD2), and we have shown that a subset of GHS-R1a, which are not occupied by the agonist (apo-GHSR1a), heterodimerize with these two receptors to regulate dopamine signaling in vitro and in vivo. To determine the consequences of ghsr ablation on brain function, congenic ghsr -/- mice on the C57BL6/J background were subjected to a battery of behavioral tests. We show that the ghsr -/- mice exhibit normal balance, movement, coordination, and pain sensation, outperform ghsr +/+ mice in the Morris water maze, but show deficits in contextual fear conditioning.

Figure 1

Ghsr −/− and ghsr +/+ mice display normal pain sensation and motor coordination. A) Mice were placed on a hot plate set to 55C and removed when signs of discomfort were shown. The mean latency to nociceptive response is shown for each group (p>0.05). B) Mice were placed on a Rota-rod accelerating from 4 to 40 rpm over 5 minutes. Four trials were averaged for each mouse. The mean latency to fall from the rotating rod is shown for each group (p>0.05).

Figure 2

Ghsr −/− mice habituated faster to novel environments. Both ghsr −/− and ghsr +/+ mice showed reduction in total locomotor activity over time in the open field, ghsr −/−showed a greater reduction compared to ghsr +/+ at time points 2 and 3 (t-test *p < 0.05; **p<0.01).

Figure 3

Ghsr −/− mice show anxiety-like behavior. A) Percentage of time spent in the center of the arena during 0–5 min in open field. B) Ghsr−/− mice translated from center to sides of the arena in significantly less time than ghsr +/+ (*p<0.05 compared to ghsr +/+).

Figure 4

Ghsr −/− and ghsr +/+ mice performed similarly during acquisition in the Morris water maze. A) Both genotypes showed similar decline in latency time to find the hidden platform. B) Both genotypes swam similar distances towards platform.

Figure 5

Ghsr −/− performed better than ghsr +/+ in the Morris water maze 7 days after acquisition. A) Both groups spent similar amount of time in the target quadrant 24 hours after acquisiton (probe trial 1), (p>0.05). B) Ghsr −/− mice spent significantly more time in the target quadrant than ghsr +/+ mice (*p<0.05 compared to ghsr +/+).

Figure 6

Ghsr −/− mice displayed a deficit in `lasting' contextual fear-conditioning. In both figures, 0–5 minutes represent the total percent freezing during the test. Additional bars depict the data in 1-minute intervals. A) The percent freezing response was similar for both genotypes on the first test performed 24 hours after training. B) Ghsr −/− mice percent freezing was significantly lower during minutes 2 (*p<0.05) and 3 (*p<0.05) of the second test, performed 30 days after training.

Figure 7

Ghsr −/− mice weigh less than ghsr +/+ mice (*p<0.05).