Serotonin and genetic differences in sensitivity and tolerance to ethanol hypothermia

Combined and sequential effects of alcohol and methamphetamine in animal models

Comorbid drug use, often alcohol with other drugs, poses significant health and societal concerns. Methamphetamine is among the illicit drugs most often co-used with alcohol. The current review examines the animal literature for impacts of comorbid alcohol and methamphetamine exposure. We found evidence for additive or synergistic effects of combined or sequential exposure on behavior and physiology. Dopaminergic, serotonergic, and glutamatergic systems are all impacted by combined exposure to alcohol and methamphetamine and cyclooxygenase-2 activity plays an important role in their combined neurotoxic effects. Adverse consequences of comorbid exposure include altered brain development with prenatal exposure, impaired learning and memory, motor deficits, gastrotoxicity, hepatotoxicity, and augmented intake under some conditions. Given high susceptibility to drug experimentation in adolescence, studies of co-exposure during the adolescent period and of how adolescent exposure to one drug impacts later use or sensitivity to the other drug should be a priority. Further, to gain traction on prevention and treatment, additional research to identify motivational and neurobiological drivers and consequences of comorbid use is needed.

Serotonin transporter gene variation is associated with alcohol sensitivity in rhesus macaques exposed to early-life stress

BACKGROUND

Decreased sensitivity to alcohol has been demonstrated to be a predictor of alcoholism in humans, and variation in the gene-linked polymorphic region of the serotonin transporter (5-HTTLPR) is associated with the response to the motor-impairing effects of alcohol. In a nonhuman primate model of excessive alcohol intake, we have shown that decreased serotonin turnover is associated with both lower initial sensitivity to alcohol and higher prospective alcohol consumption using rhesus macaques. In addition, we have demonstrated that macaques separated from their mothers and reared in peer-only groups are more likely to consume alcohol as adults.

METHOD

To examine the relationship between serotonin transporter genotype, early rearing experience, and initial sensitivity to alcohol, peer- and mother-reared, adolescent, alcohol-naive rhesus macaques (n = 123) were rated for intoxication after intravenous administration of ethanol (2.2 g/kg and 2.0 g/kg for males and females, respectively) during two testing periods. Serotonin transporter (rh5-HTTLPR) genotype was determined using polymerase chain reaction followed by gel electrophoresis, and data were analyzed using ANOVA and the Mann-Whitney U test.

RESULTS

Our analyses demonstrate an effect of serotonin transporter gene variation on ethanol sensitivity, such that animals homozygous for the l allele exhibited decreased sensitivity to the ataxic and sedating effects of alcohol. This effect remained after correction for blood ethanol concentrations and birth cohort. When animals were segregated according to rearing condition, serotonin transporter gene variation predicted intoxication scores among peer-reared animals.

CONCLUSIONS

As in some human reports, this study demonstrates a diminution in the response to alcohol in animals homozygous for the l rh5-HTTLPR allele. The phenotypic expression of this genotype in l/s animals, however, is environmentally dependent.

Genetic basis of anxiety-like behaviour: a critical review

The way genetic and/or environmental factors influence psychiatric disorders is an enduring question in the field of human psychiatric diseases. Anxiety-related disorders provide a relevant example of how such an interaction is involved in the aetiology of a psychiatric disease. In this paper we review the literature on that subject, reporting data derived from human and rodent studies. We present in a critical way the animal models used in the studies aimed at investigating the genetic basis of anxiety, including inbred mice, selected lines, multiple marker strains, or knockout mice and review data reporting environmental components influencing anxiety-related behaviours. We conclude that anxiety is a complex behaviour, underlined not only by genetic or environmental factors but also by multiple interactions between these two factors.

Sensitivity and tolerance to ethanol in mouse lines selected for ethanol-induced hypothermia

Within-family selective breeding techniques have been used to create two lines of mice to be insensitive (HOT) and two lines to be sensitive (COLD) to the hypothermic effects of an acute 3.0-g/kg ethanol (EtOH) injection. Previous studies have found HOT mice to be relatively resistant to the development of tolerance to this effect, whereas COLD mice readily develop tolerance. The breeding program is currently in selected Generation 52, and the HOT and COLD mice differ by about 10 degrees C (average of both replicates) in their selected hypothermic response. Starting with selection Generation 20, separate lines of mice were inbred from the HOT-2 and COLD-2 selected lines, while selection continued for the original two replicate lines of HOT and COLD mice. To assess whether different dose treatments would produce differential tolerance development in the HOT and COLD selected lines, we administered different dose regimens across 5 days to HOT and COLD mice. The COLD mice developed tolerance while the HOT mice did not, regardless of total EtOH administered. In a separate study, we administered EtOH (3.0 g/kg) to mice for 3 days to assess a shorter tolerance paradigm. We also present here responses to the selection dose of 3.0-g/kg EtOH in the inbred HOT (IHOT-2) and COLD (ICOLD-2) mice tested after 41 generations of brother-sister mating. In addition, we report recent attempts to find doses of EtOH that would produce an equivalent initial hypothermic response in each of the six lines (HOT-1, COLD-1, HOT-2, COLD-2, ICOLD-2, and IHOT-2). When doses were selected to produce similar initial hypothermic sensitivity, tolerance was tested by giving three daily doses and examining the attenuation of the hypothermic response on the third day. All three COLD lines developed significant tolerance, while the HOT lines did not. The HOT and COLD mice provide a genetic model to study mechanisms mediating acute EtOH-induced hypothermia as well as tolerance development.

High-dose methamphetamine treatment alters presynaptic GABA and glutamate immunoreactivity

The goal of this study was to determine if high-dose methamphetamine treatment altered presynaptic immunoreactivity for the amino acid neurotransmitters GABA and glutamate within the basal ganglia. Methamphetamine (15 mg/kg every 6 h, four doses) treatment in rats resulted in severe hyperthermia and a long-lasting (four weeks) depletion of striatal dopamine content (>80%). Severe dopamine loss correlated with a decrease in the density of presynaptic immunolabeling for GABA one week post-drug, and an increase after four weeks. Although no changes were seen in presynaptic striatal glutamate immunoreactivity, there was a significant increase in the percentage of glutamate-immuno-positive terminals associated with perforated postsynaptic densities. Rats given the same dose of methamphetamine but prevented from becoming hyperthermic showed less severe dopamine depletions and a lack of ultrastructural or immunocytochemical changes. In addition, induction of hyperthermia in the absence of drug decreased immunolabeling within mitochondria, but had no effect on dopamine content, morphology or nerve terminal immunoreactivity. Altered presynaptic GABA immunolabeling and terminal size were found in both the striatum and globus pallidus, suggesting that dynamic changes occur in the striatopallidal pathway following methamphetamine-induced dopamine loss. In addition, ultrastructural changes in glutamate-positive synapses which have been correlated with increased synaptic activity were found. These results are similar to changes in GABA and glutamate synapses that follow nigrostriatal dopamine loss in 6-hydroxydopamine-lesioned animals and in Parkinson's disease, and provide the first direct evidence that methamphetamine-induced dopamine loss alters the GABAergic striatopallidal pathway. Exposure to either methamphetamine or prolonged hyperpyrexia decreased mitochondrial Immunoreactivity, indicating that hyperthermia may contribute to methamphetamine toxicity by affecting energy stores.

Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion

The goal of this study was to investigate changes in glutamatergic synapses in the striatum of rats at two different time-points following a unilateral injection of 6-hydroxydopamine into the medial forebrain bundle. One month following this lesion of the nigrostriatal pathway, there was an increase (70%) in the mean percentage of asymmetrical synapses within the dorsolateral striatum containing a discontinuous, or perforated, postsynaptic density, possibly suggesting an increase in glutamatergic activity. This was correlated, in the same brain region, with a decrease (44%) in the density of glutamate immunoreactivity within nerve terminals associated with all asymmetrical synapses and also with those terminals associated with a perforated postsynaptic density. These morphological changes were consistent with an increase (>two-fold) in the basal extracellular level of striatal glutamate, as measured by in vivo microdialysis. The density of GABA immunolabeling within symmetrical nerve terminals was increased (25%) at this one month time-period. Dopamine levels within the lesioned striatum were >99% depleted. However, at three months, while an increase in the mean percentage of striatal perforated synapses was maintained, a significant increase (50%) in the density of striatal nerve terminal glutamate immunolabeling within all asymmetrical synapses and those associated with a perforated postsynaptic density was observed. This was correlated with a small, but significant, decrease (32%) in the basal extracellular level of striatal glutamate. The density of GABA immunolabeling within nerve terminals associated with a symmetrical contact remained elevated at this three month time-period, while striatal dopamine levels remained depleted. While the density of nerve terminal GABA immunolabeling remained elevated at both the one and three month time-periods, there appeared to be a differential effect on glutamatergic synapses. The in vivo microdialysis data suggest that glutamate synapses were more active at a basal level at one month and become less active compared to the control group at the three month time-period. These data suggest that there are compensatory changes in glutamatergic synapses within the striatum following a 6-hydroxydopamine lesion that appear to be independent of the level of striatal dopamine or GABA. We propose that changes in the activity of the thalamo-cortico-striatal pathway may help to explain the differential time-course change in striatal glutamatergic synaptic activity.

Methamphetamine alters presynaptic glutamate immunoreactivity in the caudate nucleus and motor cortex

It has been suggested that methamphetamine (METH)-induced neurotoxicity requires the activation of both dopamine (DA) and glutamate (GLU) systems. To investigate the possibility that METH-induced increases in extracellular GLU, as measured by in vivo microdialysis [Nash and Yamamoto (1992) Brain Res., 581:237-243], arise from neuronal stores, postembedding immunogold electron microscopy was used to measure the density of presynaptic GLU immunoreactivity within the striatum, the shell of the nucleus accumbens, and the motor cortex. Rats were treated with METH (5 mg/kg), or an equivalent volume of saline (SAL), every 2 h for a total of four injections. No ultrastructural evidence of terminal degeneration was observed. Significant decreases in the density of nerve terminal GLU immunolabeling occurred 12 h following METH administration within the primary motor cortex and the ventrolateral caudate/putamen, and a trend towards depletion was seen within the dorsolateral caudate/putamen. Although GLU immunolabeling within the shell of the nucleus accumbens was unaffected, DA content was decreased in all regions examined 1 week following METH treatment. The lack of degeneration, coupled with a partial recovery of DA levels, suggests that moderate doses of METH may inhibit DA biosynthesis without widespread terminal loss. Furthermore, METH administration results in a decrease in presynaptic GLU that correlates both temporally and anatomically with delayed GLU overflow, suggesting that neuronally derived GLU may play a role in METH-induced neurotoxicity. However, there does appear to be a dissociation between DA loss and altered GLU immunocytochemistry within the nucleus accumbens.

Serotonin and alcohol intake, abuse, and dependence: findings of animal studies

Despite a relatively large body of literature on the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake, the functional significance of serotonergic neurotransmission and its relationship to alcohol intake, abuse, and dependence remains to be fully elucidated. In part two of this review, the experimental (animal) data is summarized along two lines: the effects of serotonergic manipulations on the intake of alcohol, and the effects of acute and chronic alcohol intake, as well as the withdrawal of chronic alcohol, on the serotonergic system. It is concluded that serotonin mediates ethanol intake as a part of its larger role in behavior modulation, such that increases in serotonergic functioning decrease ethanol intake, and decreased serotonergic functioning increases ethanol intake. Ethanol produces transient increases in serotonergic functioning that activate the mesolimbic dopaminergic reward system. The results are discussed in light of recent theories describing the regulatory role of serotonin in general behavior.

Invited commentary: cryopreservation as a tool for the study of selectively bred lines in rodent behavioral genetics

Cryopreserved embryos should survive centuries. Upon rederivation, in vitro survival is high. Upon reimplantation, in vivo survival is substantial. Genetic stability of the embryos at -196 degrees C is theoretically absolute for several decades and likely high for centuries. Cryopreservation and rederivation services are available at a reasonable cost from commercial vendors. Preservation of a sufficient number of embryos can serve as an insurance policy against loss of the lines from several causes. In addition to the protective utility of a single banked sample, repeated preservation during the course of selection offers several advantages. Correlated responses to selection which appear late in selection may be verified by use of a cross-sectional experimental design to take a "snapshot" of the gradually developing response. After attainment of the selection limit, a generation may be chosen retrospectively which represents the best compromise between maximizing response and minimizing inbreeding irrelevant to the selected trait. Estimates of changes in gene frequencies could be derived from molecular biological analyses. An example of such an experiment in progress employing embryos from the COLD and HOT selected mouse lines is discussed.