Genetic correlations among ethanol-related behaviors and neurotensin receptors in long sleep (LS) x short sleep (SS) recombinant inbred strains of mice

Gut-brain peptides in corticostriatal-limbic circuitry and alcohol use disorders

Peptides synthesized in endocrine cells in the gastrointestinal tract and neurons are traditionally considered regulators of metabolism, energy intake, and appetite. However, recent work has demonstrated that many of these peptides act on corticostriatal-limbic circuitry and, in turn, regulate addictive behaviors. Given that alcohol is a source of energy and an addictive substance, it is not surprising that increasing evidence supports a role for gut-brain peptides specifically in alcohol use disorders (AUD). In this review, we discuss the effects of several gut-brain peptides on alcohol-related behaviors and the potential mechanisms by which these gut-brain peptides may interfere with alcohol-induced changes in corticostriatal-limbic circuitry. This review provides a summary of current knowledge on gut-brain peptides focusing on five peptides: neurotensin, glucagon-like peptide 1, ghrelin, substance P, and neuropeptide Y. Our review will be helpful to develop novel therapeutic targets for AUD.

Confirmation of quantitative trait loci for ethanol sensitivity and neurotensin receptor density in crosses derived from the inbred high and low alcohol sensitive selectively bred rat lines

RATIONALE

Genetically influenced alcohol sensitivity is thought to be an important risk factor for the development of alcoholism. An effective first step for identifying genes that mediate variation in alcohol sensitivity is through quantitative trait loci (QTL) mapping in model organisms.

OBJECTIVE

Fourteen provisional QTLs related to alcohol sensitivity were previously mapped in an F2 derived from the IHAS1 and ILAS1 rat lines. The objective of the current study was to confirm those QTLs in an independently derived F2 and in congenics that were bred for two of the loci.

MATERIALS AND METHODS

IHAS1 X ILAS1 F2 (n=450) were tested for alcohol-induced loss of righting reflex (LORR), blood ethanol concentration at regain of righting reflex (BECRR), sensitivity and acute tolerance on the Rotarod, and neurotensin receptor density (NTR1). Rats were genotyped at the 14 candidate loci and QTL mapping was conducted. Reciprocal congenic strains were bred for loci on chromosomes 2 and 5 and tested for LORR and BECRR.

RESULTS

Four LORR QTLs were mapped at the suggestive or significant level (chromosomes 2, 5, 12, and 13). BECRR was mapped to chromosomes 5, 12, and 13 either in the original or current experiment. Results of the congenic experiment also support QTLs for LORR and BECRR on chromosomes 2 and 5. QTLs for NTR1 density and behavior on the Rotarod were not confirmed.

CONCLUSIONS

QTL mapping in crosses derived from the IHAS1 and ILAS1 has successfully identified loci related to alcohol sensitivity. Recombinant congenics are now being bred to more finely map the confirmed QTLs.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Rapid ethanol tolerance mediated by adaptations in acute tolerance in inbred mouse strains

It has been postulated that decreased acute sensitivity to ethanol is an important genetically-mediated risk factor for the development of alcoholism. Previous work in mice and rats has indicated that ethanol sensitivity can be reduced in a genotype-dependent manner by a single dose of ethanol 24 h prior to testing, so-called 'rapid' tolerance. The current studies were undertaken to determine if the observed rapid tolerance was mediated by alterations in initial sensitivity or acute functional tolerance (AFT), the two primary components of acute sensitivity. Separate groups of C57BL/6, DBA/2, ILS, and ISS inbred mouse strains were administered a single pretreatment dose of saline or ethanol (5 g/kg). The original and modified versions of the loss of righting reflex test, ethanol-induced hypothermia, and ataxia on a stationary dowel rod were tested 24 h later. Dependent on the test and strain, varying degrees of rapid tolerance were observed; a pronounced sensitization was detected in one case. There was a concomitant increase in the rate and/or magnitude of AFT with little change in initial sensitivity suggesting that rapid tolerance was mediated primarily by alterations in AFT. This conclusion may have implications for the contribution of acute sensitivity to human alcoholism.

Sensitivity to ketamine, alone or in combination with ethanol, is altered in mice selectively bred for sensitivity to ethanol's locomotor effects

BACKGROUND

Sensitivity to erthanol's locomotor activating and reinforcing effects may be influenced by some common neural mechanisms. Mice selectively bred in replicate for increased (FAST-1 and FAST-2) and decreased (SLOW-1 and SLOW-2) sensitivity to ethanol's locomotor stimulant effects are useful for investigating the neural substrates of ethanol's effects. Previous studies have suggested that differences in N-methyl-d-aspartate (NMDA) receptors may underlie differences in ethanol-induced locomotion in these mice. This study examined the responses of FAST and SLOW mice to ketamine, a fast-acting NMDA antagonist. In addition, reverse-selected lines (r-FAST-1, r-FAST-2, r-SLOW-1, and r-SLOW-2) were tested as a means of verifying correlations detected in the forward-selected lines. Two initial studies characterized ketamine-induced locomotion in DBA/2J (D2) mice, an inbred strain chosen for its high sensitivity to ethanol-induced locomotion.

METHODS

After a 2- to 3-day period of habituation to test procedures, mice were given intraperitoneal injections of ketamine alone (0, 5, 10, 20, 30, and 60 mg/kg) or in combination with 1 or 2 g/kg ethanol. Locomotor activity was measured for 20 to 30 min in automated activity monitors.

RESULTS

When administered alone, ketamine dose-dependently stimulated the locomotor activity of D2 mice and also reduced the amount of ethanol-induced stimulation. Ketamine stimulated locomotion more in FAST mice than in SLOW mice. Reverse selection abolished these differences, because r-FAST and r-SLOW mice did not differ in their responses to ketamine. Ketamine potentiated ethanol's locomotor effects within FAST mice and potentiated ethanol's locomotor depressant effect within one replicate of SLOW mice.

CONCLUSIONS

We propose that sensitivities to ethanol- and ketamine-induced locomotion are genetically correlated and that the combined effects of ethanol and ketamine in FAST mice reflect a leftward shift in ethanol's biphasic dose-response curve.

Sex differences in ethanol-related behaviors in genetically defined murine stocks

Over the past 30 years, there have been a number of important developments in our understanding of the etiology and consequences of excessive drinking among humans. Probably one of the most important findings to date is that there are large individual differences among humans in appetite for alcohol and age of onset of problem drinking. We recognize this finding in at least two different alcoholic types, each with its own estimate of genetic influence. We have also come to realize that there are important differences between men and women, both in etiology of problem drinking and in the consequences of chronic alcohol use. In this chapter, the advantages and limitations of applying genetically defined animal models, primarily, selected lines and inbred strains of mice, are evaluated with examples from the literature.

Quantitative genetics and mouse behavior

Quantitative differences are observed for most complex behavioral and pharmacological traits within any population. Both environmental and genetic influences regulate such individual differences. The mouse has proven to be a superb model in which to investigate the genetic basis for quantitative differences in complex behaviors. Genetically defined populations of mice, including inbred strains, heterogeneous stocks, and selected lines, have been used effectively to document these genetic differences. Recently, quantitative trait loci methods have been applied to map the chromosomal regions that regulate variation with the goal of eventually identifying the gene polymorphisms that reside in these regions.

A genetic basis for neuroendocrine-immune interactions

Psychoneuroimmunology is an exciting, complex field that elucidates interactions among the nervous, endocrine, and immune systems. The contribution of psychosocial factors and behavioral processes to these interactions has been the focus of numerous studies designed to investigate the intricate pathways that are involved in the "mind-body connection." In addition, the effects of this connection on the development and progression of various disease conditions are of considerable interest. Although efforts have been made to identify the cellular and molecular mechanisms underlying these relationships, the impact of genetic makeup on the communication among these systems has yet to be fully realized. The development of sophisticated genetic analytical methods and gene mapping techniques now provide the "tools" to determine the influence of genetics on behavior-neuroendocrine-immune interactions--an area of study that may represent the next frontier in psychoneuroimmunology.

Neurotensin levels and receptors in HAS and LAS rat brains: effects of ethanol

Previous studies of neurotensin (NT) levels and NT receptor densities in specific brain regions of mice selectively bred for differences in sensitivity to ethanol have shown that NTergic processes may mediate some actions of ethanol. In the present study, we have determined the levels of NT and NT receptor densities in specific brain regions of HAS and LAS rats that have been selectively bred for differences in sensitivity to ethanol-induced loss of righting response. Regional differences in NT levels were observed in brains from both HAS and LAS rats and values in hypothalamus, ventral midbrain, and nucleus accumbens from female rats were 25 to 75% higher than levels in corresponding regions from male rats. However, there were no significant line differences in NT-ir levels in corresponding regions from HAS and LAS animals. High-affinity binding (NTH Bmax values), measured by Scatchard analyses, were higher in ventral midbrain from HAS males than from LAS males. NTH receptor densities were higher in HAS males than in HAS females; sex differences were not observed in the LAS line. There were no significant line or sex differences between HAS and LAS in low-affinity (NTL) Bmax values in any brain region. In HAS females, subhypnotic doses of ethanol produced a decrease in NT levels in nucleus accumbens, whereas, hypnotic doses caused an increase in NT levels. Likewise, hypnotic doses elicited increases in NT levels in hypothalamus of female HAS and LAS, but not in ventral midbrain or caudate putamen. These results are consistent with low dose activation of mesolimbic and nigrostriatal dopaminergic neurons in which NT is colocalized with dopamine and with high dose inhibition of these pathways.

Classical and neoclassical approaches to the genetic analysis of alcohol-related phenotypes

Theoretical descriptions are given for two breeding methods in animal genetics that might be of use in alcohol research. These methods are marker-based selection and marker-based development of congenic strains, both using DNA markers such as polymerase chain reaction-detectable polymorphisms as the criteria for breeding. Such designs would utilize these markers as indicators of adjacent Quantitative Trait Loci (QTL) that are influential on alcohol-related phenotypes. Issues in the logic and implementation of these methods, such as proximity of the markers and the QTL allele, are explored. A third method, development of congenic strains with phenotypic screening, is also described. This method is currently being used to create two sets of congenic lines on a C57BL/6 inbred mouse background. The criterion phenotype is locomotor activation to 1.5 g/kg (i.p.) ethanol. Data are reported on the success of transferring the activation phenotype from two strains, DBA/2Abg and MOLD/Rk-Abg, onto the nonactivated C57BL/6Abg background. The value of these methods in alcohol research is outlined with regard to both identification of relevant genes and for their use as tools in basic research on mechanism of alcohol action.

Cross-tolerance between ethanol and neurotensin in mice selectively bred for ethanol sensitivity

Neurotensin (NT), a tridecapeptide that satisfies criteria as a neurotransmitter, mimics many actions of ethanol, and evidence indicates that some of the acute effects of ethanol are mediated in part by NT. Recent studies have shown that chronic ethanol treatment produced a downregulation of NT receptors in mesolimbic brain regions of long sleep (LS) mice and that reduced NT binding capacity was associated with acquisition and decay of tolerance to ethanol-induced locomotor inhibition and hypothermia in these mice. The present study was undertaken to determine whether cross-tolerance develops between NT and ethanol and whether chronic NT infusion produces NT receptor downregulation. Animals chronically treated with ethanol were tolerant to NT-mediated locomotor inhibition at a dose of 1.8 pmol NT, ICV, and were tolerant to NT-induced hypothermia at 1.8 and 6.0 pmol NT. Following repeated injections or continuous infusion of NT ICV, LS mice showed tolerance to both NT- and ethanol-induced hypothermia and locomotor inhibition. Indeed, ethanol doses that are hypnotic in control mice (2.8 g/kg) were not effective in abolishing locomotor activity following chronic NT administration. Results with chronic saline infusion ICV indicate that stress alters sensitivity to ethanol-induced hypothermia. Chronic infusion of NT ICV produced a region-specific downregulation of high-affinity NT receptors in the striatum. The results demonstrate that cross-tolerance develops between NT and ethanol, and further support a role for neurotensinergic systems in the actions of ethanol.

Effects of acute and chronic ethanol administration on neurotensinergic systems

A summary of pharmacogenetic studies designed to test the hypothesis that NT receptors might mediate or regulate some of the actions of ethanol is presented. Indeed, there are significant genetic correlations between ethanol-induced locomotor activation and high-affinity NT receptor densities in the FC. The results suggest that further studies are needed to determine the role of NT receptors in the FC and MPFC in regulating locomotor activity. In other studies, chronic ethanol treatment, under conditions that produced tolerance to ethanol and caused NT receptor downregulation in the NA and VMB, caused tolerance to locomotor inhibitory effects of centrally administered NT and blunted the effects of intra-VTA NT on dopamine metabolism in the NA and CP. The results show a relationship between NT receptor densities and pharmacological effects of ethanol.

Quantitative trait loci (QTL) applications to substances of abuse: physical dependence studies with nitrous oxide and ethanol in BXD mice

Recombinant inbred (RI) mouse strains were developed primarily as a tool to detect and provisionally map major gene loci--those with effects large enough to cause a bimodal distribution in the trait of interest. This implied that progress toward gene mapping was possible only for gene loci accounting for at least half of the genetic variance. More recently, QTL (quantitative trait loci) approaches have been advanced that do not require bimodal distributions and are thus applicable to a much wider range of phenotypes. They offer the prospect of meaningful progress toward detecting and mapping minor as well as major gene loci affecting any trait of interest, provided there is a significant degree of genetic determination among the RI strains. This paper presents a review of RI gene mapping efforts concerning phenotypes related to drug abuse and presents new data for studies now in progress for nitrous oxide and acute ethanol withdrawal intensity. These two studies exemplify several strengths and limitations of the RI QTL approach.