Secretin receptor-deficient mice exhibit impaired synaptic plasticity and social behavior

Abstract

Secretin is a peptide hormone released from the duodenum to stimulate the secretion of digestive juice by the pancreas. Secretin also functions as a neuropeptide hormone in the brain, and exogenous administration has been reported to alleviate symptoms in some patients with autism. We have generated secretin receptor-deficient mice to explore the relationship between secretin signaling in the brain and behavioral phenotypes. Secretin receptor-deficient mice are overtly normal and fertile; however, synaptic plasticity in the hippocampus is impaired and there are slightly fewer dendritic spines in the CA1 hippocampal pyramidal cells. Furthermore, secretin receptor-deficient mice show abnormal social and cognitive behaviors. These findings suggest that the secretin receptor system has an important role in the central nervous system relating to social behavior.

Figure 1

Generation of secretin receptor-deficient mice. (A) The targeting vector (Sctr-TV1) was designed to replace exon 1 of the secretin receptor with the lacZ reporter and a PGKneobpA selection marker. (B) Southern blot analysis distinguishes the sec^m1Brd and wild-type alleles. The BamHI restriction fragment length of the wild-type allele is 8.5 kb and m1 allele is 7.2 kb. (C) RT-PCR analysis of expression of secretin receptor in different tissues from secretin receptor-deficient mice and their wild-type littermates.

Figure 2

Neuronal and glial marker analysis in adult brain of secretin receptor-deficient mice. Similar expression patterns are observed in both secretin receptor-deficient and wild-type littermates detected by immunohistochemistry using antibodies to (A and B) GFAP in the hippocampal region; (C and D) MAP2 in the hippocampal region; (E and F) tryptophan hydroxylase in the raphe nucleus; (G and H) tyrosine hydroxylase in the substantia nigra. (A, C, E and G) Wild-type littermates and (B, D, F and H) secretin receptor-deficient mice.

Figure 3

Secretin receptor expression in adult brain revealed by staining of the lacZ reporter. The expression pattern was not different between secretin receptor m1/+ and m1/m1 mice. Expressions in (A) anterior olfactory neuron, (B) hippocampal CA1 region and (C) cerebellum. (D) Enlargement of box area in (C). (E) Enlargement of box area in (B).

Figure 4

Impaired synaptic transmission and LTP of hippocampal CA1 slices. (A) Synaptic transmission was determined from pEPSPs of field recordings from the hippocampal CA1 synapses. Loss of the secretin receptor appears to have a deleterious effect on overall synaptic transmission (F_[2,16] = 4.095, P = 0.0366, m1/m1, n = 21; wild-type, n = 11). (B) There were no significant changes in the short-term synaptic plasticity by testing the paired-pulse facilitation (PPF). (C) Deficiency of hippocampal LTP induction and maintenance phase in secretin receptor m1/m1 was observed (two-way ANOVA: genotype: F_[1,481] = 83.97, P < 0.0001; time: F_[25,481] = 1.90, P < 0.0001, m1/m1, n = 13; +/+, n = 11).

Figure 5

Reduced dendritic spine number in secretin receptor-deficient mice. The average number of apical (A) and basilar (B) dendritic branches of CA1 pyramidal neurons in mutant mice was similar to the number in wild-type mice. (C) Higher magnification images of apical dendrites in the CA1 pyramidal neurons revealed decreased spine density in the mutant (right) versus wild-type littermates (left). (D) The average number of dendritic spines on 100 μm of the first order apical dendritic branches in mutant mice was significantly reduced when compared with the average number of dendritic spines at similar sites in wild-type littermates (P < 0.0001).

Figure 6

Reversal water maze behavior abnormalities in secretin receptor mutant mice. (A) Hidden platform test (latency). Both secretin receptor-deficient mice (m1/m1) and wild-type littermates (+/+) showed significant reductions in latency (search time) over the eight blocks of training. There was no significant difference between m1/m1 and +/+ (P >0.05). (B) Reversal hidden platform test (latency). Secretin receptor-deficient mice (m1/m1) did not show a decrease in latency over the four blocks of the test when compared with the wild-type mice (+/+) (two-way ANOVA: genotype: F_[1,43] = 8.656, P < 0.005; time: F_[3,129] = 3.828, P < 0.02).

Figure 7

Secretin receptor-deficient mice showed abnormal social behavior. (A) Tube test of social dominance. Secretin receptor-deficient mice won more matches in the tube test against wild-type littermates than expected by chance (P < 0.001). (B) Partition test of social recognition. Time near the partition during the familiar partner baseline test was not significantly different between secretin receptor-deficient and wild-type mice. (C) Secretin receptor-deficient mice displayed a significantly lower recognition ratio compared with the wild-type mice during the test with unfamiliar partner (P < 0.03). Data are expressed as the mean +/− SEM. (D) Locomotor activity of the secretin receptor-deficient mice. There was a significant difference in the movement speed when compared with wild-type littermates. (E) Secretin receptor-deficient mice performed as well as wild-type controls on the rotarod initially, but by the end of training, the secretin receptor-deficient mice were impaired (trial genotype interaction, P = 0.045; trial 1-6, P-values >0.05; trials 7-8, P-values <0.05). (F) Secretin receptor-deficient mice showed no significant difference in conditional fear during the test after a 24 h or 14-day delay.