Maudsley Nonreactive Research Articles

The genetics of neuroticism: Insights from the Maudsley rat model and human studies.

We examine some of the genetic features of neuroticism (N) taking as an animal model the Maudsley Reactive (MR) and Maudsley Nonreactive (MNR) rat strains which were selectively bred, respectively, for high and low open-field defecation (OFD) starting in the late 1950s. To draw analogies with human genetic studies, we explore the genetic correlation of N with irritable bowel syndrome (IBS). We review progress with the rat model and developments in the field of human complex trait genetics, including genetic association studies that relate to current understanding of the genetics of N. The widespread differences in the tone of the peripheral sympathetic nervous system that have been found between the Maudsley strains, particularly those observed in the colon, may underly the differences in OFD (MNR, higher sympathetic tone and zero defecation). In humans, a large genome-wide association study (GWAS) reported six genes contributing to IBS, four of which were implicated in mood and anxiety disorders or were expressed in the brain, with three of the four also expressed in the nerve fibers and ganglia of the gut. Heritability of N is estimated at around 50% in twin and family studies, and GWASs identified hundreds of loci, enabling estimation of genome-wide correlations (rg) with other traits. Significantly, the estimate for rg between risk of IBS, anxiety, N, and depression was >0.5 and suggested genetic pleiotropy without evidence for causal mechanisms. Findings on the adrenergic pharmacology of the colon, coupled with new understanding of the role of the locus ceruleus in modifying afferent information from this organ, generate hypotheses that challenge traditional cause/effect notions about the relationship of the central nervous system to peripheral events in response to stress, suggest specific targets for gene action in the Maudsley model and emphasize the value of reciprocal evaluation of genetic architecture underlying N in rodents and humans.

An electrophysiological evaluation of serotonergic dorsal raphe neurons in maudsley rats

Extracellular electrophysiological recording techniques were used to study serotonergic dorsal raphe (DRN) neurons in Maudsley Reactive (MR), Maudsley Non-Reactive (MNRA) and Sprague Dawley (SD; reference control strain) rats. No significant differences were observed in the average discharge rates of DRN neurons from SD, MR and MNRA rats. The sensitivity of DRN neuron somatodendritic 5-HT1 A autoreceptors to the inhibitory effects of i.v. 8-OH-DPAT or α 1-adrenoceptors to the excitatory effects of iontophoretic phenylephrine did not differ significantly among strains. These findings are discussed in light of the previously reported strain-dependent differences in anxiety-like behavior and noradrenergic locus coeruleus neurons.

Sex differences and the effects of tail pinch on ethanol drinking in maudsley rats

Voluntary consumption of 10% ethanol (EtOH) vs. water was compared in EtOH-naive, male and female Maudsley Reactive (MR) inbred rats that were exposed to either daily episodes of tail pinch (TP), or no-stress controls for 35 consecutive days. Females drank significantly more EtOH than males as measured by preference for 10% EtOH vs. water, and for intake of pure EtOH. Females exposed to TP had a higher preference for EtOH relative to controls after 21 days, but there was no difference over the entire 35 days. Males exposed to TP tended to drink more EtOH during the last week of testing relative to controls. Experiment 2 compared MR females with Maudsley Nonreactive (MNRA) females for 3 weeks in the conditions described above; TP exposure increased EtOH preference in MR, but not in MNRA, females. MR females also consumed significantly more EtOH than MNRA females. In both experiments female rats drank amounts of EtOH following a 6-week period of EtOH abstinence comparable to their preabstinence levels. The large degree of variability found in inbred MR rats has implications for developing a model for the investigation of environmental influences on the development of EtOH abuse in genetically susceptible individuals.

Central noradrenergic reactivity to stress in Maudsley rat strains

Maudsley reactive (MR) and Maudsley nonreactive (MNRA) rats were submitted to a single session of acute 5-min immobilization stress and immediately sacrificed by decapitation. Subsequent neurochemical analysis revealed an elevation of 3,4-dihydroxyphenylacetic acid levels in the locus coeruleus and in the ventrolateral medulla, but not in the dorsomedial medulla, of rats of the two strains compared with nonstressed controls. This response was greater in the MR than in the MNRA group, suggesting a strain difference in the reactivity of the central noradrenergic cells to acute stress.

Maudsley reactive and non-reactive rats differ in exploratory behavior but not in learning.

The Maudsley reactive (MR) and Maudsley non-reactive (MNR) inbred rat strains were created as an animal model of anxiety, based on open field behavior. We wished to determine whether previously described characteristic open field behavior and learning deficits in conditioned avoidance could be generalized to other paradigms of exploratory behavior (such as the staircase test) and learning (such as the T-maze swim test). As with other open field paradigms, the MR rats showed stable and large differences in the staircase test, compared with the MNR rats. In contrast, large and stable learning differences in conditioned avoidance and T-maze swim tests were not observed. We conclude that the MR rats exhibit exploratory behavior that is inhibited in a wide variety of paradigms, but learning deficits are not major characteristics of the phenotype. The inhibited exploratory behavior is a stable characteristic with large inter-strain differences, making measures of open field behavior suitable for quantitative trait loci analysis to determine the genes which explain these large and stable strain differences. Identification of these genes could provide clues to the genetic origins of some human anxiety disorders.

Saccharin Intake Predicts Ethanol Intake in Genetically Heterogeneous Rats as Well as Different Rat Strains

Saccharin and ethanol intakes were measured in seven strains of rats known to differ in their preferences for ethanol: The Fawn-Hooded (FH), alcohol-preferring (P) and Maudsley Reactive rats have been reported to drink ethanol voluntarily, whereas the alcohol-nonpreferring, Maudsley Nonreactive and Flinders Line (FSL and FRL) rats do not. Saccharin and ethanol intakes were highly correlated (r = +0.61) over all strains, with the FH rats drinking the most of both solutions. Correlation coefficients between pairs of drinking versus nondrinking rat strains were even higher. In a second experiment, genetically heterogeneous F2 progeny from cross-breeding the ethanol-preferring FH rats with the ethanol-nonpreferring Flinders Resistant Line (FRL) rats were studied. The results indicated a high positive correlation between saccharin and ethanol intakes (+0.65). These findings suggest that the association between saccharin and ethanol intakes previously reported in rat strains with different preferences for ethanol may have a similar genetic basis.

Analysis of relative mRNA levels and protein patterns in brains of rat strains bred for differing levels of emotionality.

mRNA and protein populations were studied in the brains of Maudsley reactive (MR) and Maudsley nonreactive (MNR) rat strains, which exhibit differing levels of emotionality. Translational analysis of forebrain mRNA indicated that the relative levels of two translation products (42 kDA, pI 5.0; 30 kDa, pI 5.8) were increased in the MR compared to the MNR strain. In addition, a charge-shift variant of a 36 kDa protein was present in the MR strain. Analysis of brain protein patterns indicated that a protein of 39 kDa, pI 5.0, was found to be more abundant in MR compared with MNR strains in both frontal cortex and hippocampus and the relative level of one protein (40 kDa, pI 5.8) was decreased in the frontal cortex.

Brain adenosine receptors in Maudsley reactive and non-reactive rats

Previous work in our laboratory has shown that the Maudsley reactive (MR) strain of rats cannot be differentiated from the Maudsley non-reactive (MNR) strain regarding the number or affinity of their brain benzodiazepine binding sites. In the present study we show that the number of cerebellar adenosine receptors (as studied using [ 3H]cyclohexyladenosine, [ 3H]CHA, as the ligand) are increased by 15–30% in the MR strain. This alteration was corroborated by quantitative autoradiographic analysis and found to be localized to the molecular layer of the cerebellum where adenosine receptors are believed to reside on parallel fibers of cerebellar granule cells.

“Peripheral” benzodiazepine binding sites in the maudsley reactive rat: Selective decrease confined to peripheral tissues

“Peripheral” benzodiazepine binding sites (PBS) were studied in both the CNS and peripheral tissues of Maudsley reactive (MR) and Maudsley non-reactive (MNR) rats using the PBS specific ligand [ 3H]Ro 5-4864. A statistically significant reduction in the density of PBS was found in heart and kidney of the MR compared to the MNR. Similar reductions in the density of PBS were not observed in a number of areas of the central nervous system (including cortex, hippocampus, and hypothalamus) or other peripheral tissues, such as lung and adrenal. This selective decrease in PBS in a strain selectively bred for a high degree of “fearfulness” may be related to previous findings of a reduction in the density of PBS in the same tissues in rats subjected to uncontrollable shock. These observations suggest that PBS in heart and kidney may be altered in response to fear or anxiety.

Genetic variation, chronic stress, and the central and peripheral noradrenergic systems.

Male rats from two inbred strains genetically selected for variation in response to stress were exposed for 9 consecutive days to 50 min of intermittent foot shock and killed 24 h after the last stress session. Maudsley Nonreactive (MNRA/Har) rats, genetically selected for decreased reactivity to stress, exhibited a significantly greater elevation (114% above control levels) in tyrosine hydroxylase activity in the locus ceruleus after chronic stress than Maudsley Reactive (MR/Har) animals (63% above control levels), genetically selected for heightened reactivity to stress. In the peripheral noradrenergic system, chronic stress produced an increase in norepinephrine (NE) content of small intestine and submaxillary gland of MNRA/Har rats, whereas chronic stress had no effect on NE content of these same tissues in MR/Har animals. These results are consistent with the proposition that genetic selection for variation in susceptibility to stress has altered the capacity of the central and peripheral noradrenergic system to adapt to chronic stress.

Open-field behavior and the peripheral sympathetic nervous system in the MR/N and MNR/N rat strains.

The Maudsley Reactive (MR) and Maudsley Non-Reactive (MNR) strains of rats, respectively selected by P. L. Broadhurst for high and low open-field defecation (OFD), were acquired by the National Institutes of Health in 1963 at the 21st generation of inbreeding and have been inbred as the MR/N and MNR/N strains at that location for more than 40 generations. The present experiment shows that, despite the lack of deliberate selection for more than 40 generations, the strains still differ predictably in OFD. Strain differences in open-field activity, originally found to be inversely associated with those in OFD, have also been preserved. The existence of an association between the peripheral sympathetic system and OFD originally established in the Har derivation of the Maudsley strains was also confirmed in the MR/N and MNR/N strains: MR/N male rats, with the relatively higher levels of OFD, exhibited lower concentrations of norepinephrine (NE) in spleen and descending colon than MNR/N rats. The pattern of biochemical and morphological findings obtained in this study and previous data indicate that the MNR/N and the MNR/Har strains are both derived from the British stocks of this strain carrying the agouti allele (AA). A distinction should be made between these strains and the MNRA/Har strain, which carries the nonagouti allele (aa) and which has been genetically isolated from the Nonreactive strains bearing the agouti allele since early in the selection experiment.

Central serotonergic function and behavior in the Maudsley reactive and nonreactive strains: a reevaluation.

Confirming earlier research, Maudsley Reactive (MR) male rats had higher concentrations of serotonin (5-hydroxytryptamine) in brain stem than did Maudsley Nonreactive (MNR) rats. Comparisons of MR with a second MNR substrain (MNRA) revealed no strain differences in brain stem serotonin. The two MNR substrains exhibit similar levels of open-field defecation, and both differ substantially from MR animals. The present results suggest that, contrary to previous claims, brain stem serotonin is not consistently related to open-field defecation in the Maudsley strains.

Central and peripheral norepinephrine metabolism in rat strains selectively bred for differences in response to stress

Rats of the Maudsley nonreactive (MNRA) strain were found to contain higher levels of norepinephrine in heart, spleen, and hypothalamus than animals of the Maudsley reactive (MR) strain. Total adrenal catecholamines were also greater in nonreactive animals. There was a trend toward higher endogenous norepinephrine concentration in MR rats in brainstem and telencephalon, but this was not statistically significant. Turnover measurements calculated from the fall of norepinephrine at 1 and 4 hours after a single dose of levo-α-methylparatyrosine showed no significant strain differences in telencephalon or brainstem, but MNRA animals had a faster rate of norepinephrine decline in heart than had MR rats. Possibly indicative of a higher rate of norepinephrine metabolism, the percentage of 3H-non-catechol metabolites relative to total counts was higher in brainstem of MNRA rats 90 min after intraventricular injection of 3H-norepinephrine. However, the disagreement between this estimate of norepinephrine metabolism and that provided by the α-methyl-paratyrosine technique prevents a conclusive statement about norepinephrine metabolism in the two strains. The results are discussed in the light of the established differences in behavior between the strains as well as other work exploring relationships between catecholamine metabolism and emotionality.

Serum prolactin and hypothalamic dopamine in rat strains selectively bred for differences in susceptibility to stress.

Rat strains selectively bred for behavioral differences in susceptibility to mild stress exhibited differences in serum prolactin (Prl) and hypothalamic dopamine (DA). Male rats from the Maudsley Reactive (MR) strain had higher levels of serum Prl and hypothalamic DA than rats from the Maudsley Non-Reactive (MNRA) strain. MR rats also showed a greater percentage increase in serum Prl 1 h after blockade of catecholamine synthesis by administration of alpha-methylparatyrosine (alphaMpT). The correlation of basal serum Prl levels with individual susceptibility to stress constitutes further evidence that Prl may play a functional role in the organism's response to stress. Strain differences in hypothalamic DA function may play a role in the etiology of strain differences in serum Prl.