Neurohypophyseal peptide influences on ethanol tolerance and acute effects of ethanol

Current state of knowledge about the mechanisms of alcohol tolerance

Far from being a simple homeostatic response to the presence of ethanol in the brain, tolerance is now recognized as a complex process which can develop within various time frames (acute, rapid, chronic) and in which the drug interacts with various environmental and cognitive factors, including associative and operant learning. A major question is whether the acute form is an innate adaptive response, which is converted into the rapid and chronic forms by the action of these external influences. So far, all behavioral and neuropharmacological manipulations that alter chronic tolerance also modify rapid and acute tolerance in similar ways. These include lesions of serotonergic forebrain projections, blockade of NMDA-type glutamate receptors and calcium "L" channels, central or peripheral injection of vasopressin and blockade of vasopressin V1 receptors. Cytochemical and immunofluorescence studies, combined with use of retrograde tracers, indicate the existence of a septohippocampal circuit which may mediate the interactions of these diverse elements in the production and maintenance of tolerance. There is limited evidence that development of tolerance leads to increased consumption of ethanol in experimental animals, but the clinical significance of these findings remains to be proven.

Differential increase in Fos immunoreactivity in hypothalamic and septal nuclei by arginine8-vasopressin and desglycinamide9-arginine8-vasopressin

Subcutaneous or intracerebroventricular injection of either arginine8-vasopressin or desglycinamide9-arginine8-vasopressin has been shown to facilitate memory, reduce or reverse the effects of amnesic drugs, and maintain tolerance to some effects of ethanol. These actions of vasopressin (and, by inference, of desglycinamide9-arginine8-vasopressin) are mediated by vasopressin V1 receptors in brain, via a c-fos-dependent mechanism, but the receptors at which the desglycinamide analog acts have not been identified. The precise central sites are also not known, but evidence of several types suggested the anterior hypothalamus and septum as probable loci of vasopressin action. In the present work, this question was studied by immunocytochemistry, using antibodies against Fos and Fos-like proteins. The numbers of Fos-immunoreactive nuclei were counted in several related brain regions and structures, after administration of arginine8-vasopressin, des-Gly9-[Arg8]-vasopressin or saline. A subcutaneous injection of vasopressin, but not of saline, enhanced Fos expression in the paraventricular, supraoptic and suprachiasmatic nuclei of the hypothalamus, but the desglycinamide analog stimulated Fos expression only in the suprachiasmatic nucleus. Vasopressin injection significantly increased the number of Fos-immunoreactive cells in the intermediate lateral septum, medial septum, and dorsal and ventral divisions of the lateral septum. In contrast, the desglycinamide analog increased the numbers of Fos-immunoreactive cells in the dorsal and intermediate portions of the lateral septum, but caused no change in the medial septum, and a decrease in the ventral portion of the lateral septum. Increased Fos expression was also found in the subfornical organ after subcutaneous injection of either vasopressin or the desglycinamide analog. Double labeling with antibodies against Fos protein and against vasopressin revealed that most of the vasopressin-induced Fos-immunoreactive cells in the supraoptic, paraventricular and suprachiasmatic hypothalamic nuclei are also vasopressin immunoreactive, i.e. they are vasopressin-producing neurons. These findings suggest that a circuit involving V1 receptors in the subfornical organ, connecting fibres to the suprachiasmatic nucleus, and vasopressinergic projections from the suprachiasmatic nucleus to the lateral septum, may play a central role in mediating the actions of both vasopressin and its desglycinamide analog in the maintenance of ethanol tolerance.

Arginine-8-vasopressin potentiates acute ethanol intoxication

AVP maintains ethanol (EtOH) tolerance after cessation of chronic EtOH treatment. However, the acute interaction of AVP and EtOH has not been well characterized. Rats were trained on a moving belt and the EtOH dose-response relationship (range 1.0-2.0 g/kg) was determined after pretreatment with saline, AVP (2.5-40 micrograms SC or 10 ng ICV), the AVP-V1 receptor antagonist [Des-Gly9,d(CH2)5(1),O-Et-Tyr2, Val4,Arg8]-vasopressin (10 ng ICV), or AVP in combination with the V1 antagonist. AVP produced a 16% decrease in the EtOH ED50 when given either SC or ICV; this decrease, which appears to represent true potentiation rather than additivity, was prevented by the preadministration of the V1 antagonist. Other rats were made EtOH-tolerant by 7 daily injections of either EtOH alone (1.8 g/kg IP) or EtOH (1.5 g/kg IP) + AVP (10 micrograms SC), followed by a practice session on the moving belt. In both sets of tolerant animals, AVP potentiation of acute EtOH effects was still seen on day 6. The mechanism of AVP potentiation of EtOH-induced impairment is unknown, but the failure of the V1 antagonist alone to alter the effect of EtOH suggests that endogenous AVP is not involved directly in modulating EtOH intoxication.

Oxytocin attenuates tolerance not only to the hypothermic but also to the myorelaxant and akinesic effects of ethanol in mice

Inhibition of ethanol tolerance by oxytocin has been demonstrated previously using the hypothermic effect only. The purpose of the present experiment was to investigate the effect of oxytocin on the development of tolerance to ethanol-induced hypothermia, myorelaxation and akinesia in mice. Four groups of mice received daily intraperitoneal injections of saline or oxytocin (0.005 mg) plus saline or ethanol (2 g/kg). The peptide was administered 2 hours before ethanol. For five consecutive days, temperature measurements were performed 20 minutes before and after ethanol injection. Myorelaxation and akinesia were evaluated following the second temperature measure. Oxytocin pretreatment, which had no intrinsic effects, resulted in a robust selective attenuation of tolerance to ethanol-induced hypothermia, myorelaxation and akinesia. These results suggest that the mechanisms for peptide modulation are common to these three typical effects of ethanol.

Arginine8-vasopressin potentiates the motor incoordinating effects of pentobarbital

Arginine8-vasopressin (AVP, 10 micrograms), injected s.c., potentiated the motor-impairing effects of pentobarbital (10-20 mg/kg) injected i.p. in rats 1 h after AVP. Motor incoordination was assessed on the moving belt task. However, AVP (0.1-100 nM) failed to enhance pentobarbital potentiation of GABA-mediated 36Cl- uptake in rat cerebral cortical or cerebellar microsacs. There was also no effect of a 10 micrograms s.c. injection of AVP 1 h before killing, on pentobarbital potentiation of GABA-mediated chloride flux in either cerebral cortical or cerebellar microsacs.

Vasopressin produces long-lasting increase in transmitter release

Arginine-vasopressin has a widespread distribution in the nervous system, and has been implicated in cellular and behavioral functions. Its effect on the neuromuscular synapse reveals that it produces long-lasting augmentation in synaptic transmission due to an increase in spontaneous and nerve stimulation evoked quantal transmitter release. No significant postsynaptic effect was detected.

Cyclo(His-Pro) and the development of tolerance to the hypothermic effect of ethanol

Acute intraperitoneal administration of ethanol to rats causes a dose-dependent transient hypothermia. On repeated exposure, however, rats develop tolerance to hypothermic effects of ethanol. Cyclo(His-Pro), an endogenous brain peptide, modifies both acute and chronic themomodulatory effects of alcohol. For example, a) acute pretreatment of rats with increasing amounts of cyclo(His-Pro) produces a progressive decrease in ethanol hypothermia, and b) chronic cyclo(His-Pro) administration augments the development of tolerance to hypothermic effects of alcohol. While the mechanism of cyclo(His-Pro) action is not clear, these data are interpreted to suggest that this peptide may play important roles in ethanol intoxication, preference, tolerance, and/or addiction.

The role of arginine vasopressin in alcohol dependence and withdrawal

The development and maintenance of tolerance to the physiological and behavioral effects of repeated exposure to ethanol can be altered markedly by the presence of arginine vasopressin (AVP). In addition, AVP has been implicated in the etiology of convulsions, including those induced by exposure to high ambient temperatures. In light of these findings, experiments were conducted to determine the role, if any, that AVP might play in the pathogenesis of alcohol-withdrawal convulsions. Thirty-two male Long Evans (LE) rats and 32 age-matched male homozygous Brattleboro (DI) rats (genetically deficient in AVP) were exposed to ethanol vapor concentrations adjusted to maintain blood alcohol levels of each rat at 150-350 mg/dl. Following at least 5 days of ethanol exposure, the animals were withdrawn. From 3-24 hr after cessation of ethanol administration, withdrawal severity was assessed by observing the response of each animal to a 60-120 sec period of auditory stimulation. No significant differences were observed in either latency to onset or severity of the convulsions in LE and DI rats upon ethanol withdrawal. Thus, alcohol-withdrawal convulsions, unlike hyperthermia-induced convulsions, may be mediated by a neurochemical substrate other than AVP.

Biochemical basis of alcoholism: statements and hypotheses of present research

Experimental results and theoretical considerations on the biology of alcoholism are devoted to the following topics: genetically determined differences in metabolic tolerance; participation of the alternative alcohol metabolizing systems in chronic alcohol intake; genetically determined differences in functional tolerance of the CNS to the hypnotic effect of alcohol; cross tolerance between alcohol and centrally active drugs; dissociation of tolerance and cross tolerance from physical dependence; permanent effect of uncontrolled drinking behavior induced by alkaloid metabolites in the CNS; genetically determined alterations in the function of opiate receptors; and genetic predisposition to addiction due to innate endorphin deficiency. For the purpose of introducing the most important research teams and their main work, statements from selected publications of individual groups have been classified as to subject matter and summarized. Although the number for summary-quotations had to be restricted, the criterion for selection was the relevance to the etiology of alcoholism rather than consequences of alcohol drinking.

The effects of neurohypophyseal hormones on tolerance to the hypothermic effect of ethanol

Mice were made tolerant to the hypothermic effect of ethanol by repeated administration of ethanol (4 g/kg, 25% v/v, IP) on three consecutive days. The colonic temperature was measured in individually-housed animals immediately before and 45 min after ethanol treatment. Peptide treatments with various schedules were made SC 2 hr before the first ethanol challenge. The decrease in hypothermic response was accepted as a tolerance phenomenon, which developed in control animals by day 2. A single injection of oxytocin (OXT) or lysine vasopressin (LVP [0.1 or 1 IU peptide] animal) before the first ethanol injection did not change the initial sensitivity to ethanol. This absence of acute interactions is also reflected in the sleep onset and sleep duration after 5 g/kg ethanol (IP). In contrast, both OXT and LVP affected the development of tolerance. Repeated treatments with graded doses of OXT (0.5-2 IU) or LVP (0.25-1 IU) every day for 3 days blocked the development of tolerance. 0.002 IU LVP facilitated the development of hypothermic tolerance. The remaining doses of the peptides were ineffective. A high dose of LVP (1 IU) facilitated hypothermic tolerance if the peptide was injected when tolerance to ethanol had developed fully without previous peptide treatment. OXT, on the other hand, was ineffective in this particular experimental model. The data suggest that both neurohypophyseal hormones (LVP and OXT) block the early developmental phase of tolerance to ethanol. On the other hand, LVP facilitated the expression of tolerance if the peptide was given to mice with fully developed tolerance.

Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents: II

This survey begins a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1978, but data from many earlier papers are also tabulated.

Genetic differences in the effect of ethanol on plasma corticosterone and nonesterified fatty acid in rats

The influence of genotype on the responsiveness to ethanol in rodents was examined using biochemical measures. Nine different strains of rats were treated with saline or various doses of ethanol, and the changes in plasma corticosterone and of nonesterified free fatty acids were measured 1 hr later. It was found that the BUF/N strain of rats had the lowest response on both measures and also had the lowest blood ethanol levels. The most sensitive strains to ethanol were the ACI/N with respect to corticosterone, and WKY/N and M520/N with respect to nonesterified free fatty acids. The highest blood ethanol levels were seen in the MR/N and the WKY/N strains of rats.

A major metabolite of arginine vasopressin in the brain is a highly potent neuropeptide

A peptide that accumulated as the major product during the proteolysis of arginine vasopressin by rat brain synaptic membranes was isolated and its structure was shown to be the hexapeptide pGlu-Asn-Cys(Cys)-Pro-Arg-Gly-NH2. When administered intracerebroventricularly in extremely low doses, this vasopressin fragment and its desglycinamide derivative facilitated memory consolidation in a passive avoidance situation. These vasopressin metabolites, which are devoid of pressor activity, constitute highly potent neuropeptides with selective effects on memory and related processes; they are activated via proteolytic processing of vasopressin by brain peptidases.

Centrally mediated effects of neurohypophyseal hormones

The neurohypophyseal hormones oxytocin and vasopressin cause a variety of biological effects in animals which are mediated by central nervous system mechanisms. Among the best studied of these effects is the modulation of both memory processes and the development of drug tolerance and dependence. Neurohypophyseal hormones have also been shown to alter various physiological parameters such as heart rate and body temperature following central administration. In addition, these peptides can profoundly alter spontaneous, unlearned behavior in several rodent species. Many of the centrally mediated effects of neurohypophyseal hormones have been shown to be elicited at sites within the brain stem and the limbic system where vasopressin and oxytocin occur in cell bodies, axons and nerve terminals, suggesting a physiological role for these peptide effects. The various central effects of neurohypophyseal hormones involve different mechanisms which can be distinguished from one another on the basis of required dose, time-course of action, and structure-activity relationships. Thus, alterations of spontaneous behavior are mediated by putative receptors closely related to vasopressin receptors in blood vessels responsible for the peripheral pressor response while the effects on memory processes are mediated by a mechanism which is not closely related to those involved in the peripheral hormonal effects of the peptides. The influence of neurohypophyseal hormones on memory and attention may be useful clinically. A potential role for these peptides in mental disorders is discussed.

Vasopressin maintenance of ethanol tolerance requires intact brain noradrenergic systems

We have demonstrated that the mammalian antidiuretic hormone, arginine vasopressin (AVP), will maintain functional tolerance to the hypnotic effect of ethanol in mice, beyond the time in which such tolerance normally dissipates. However, when mice are made tolerant to ethanol and then injected intraventricularly with 6-hydroxydopamine (6-OHDA), AVP is no longer effective in maintaining tolerance. The action of AVP was attenuated by a dose of 6-OHDA which significantly lowered brain norepinephrine, but not dopamine levels, suggesting that the maintenance of ethanol tolerance by AVP may require the presence of intact noradrenergic pathways in brain.

Structural requirements for neurohypophyseal peptide maintenance of ethanol tolerance

Tolerance to the hypnotic effect of ethanol in mice is prolonged by daily subcutaneous administration of arginine vasopressin and certain analogs of this hormone. The major structural requirement for maintenance of ethanol tolerance by these peptides appears to be the N-terminal "ring" structure of vasopressin containing two amino acid residues with aromatic side chains. Peptides structurally related to the C-terminal portion of the neurohypophyseal hormones are less active in maintaining tolerance than the intact hormones. The structure-activity pattern observed for the effects of peptides on ethanol tolerance is similar to that described for neurohypophyseal peptide inhibition of extinction of an active avoidance response, an action thought to reflect peptide effects on memory consolidation. The results are in line with our hypothesis that similar CNS recognition sites may mediate neurohypophyseal peptide effects on ethanol tolerance and certain memory processes. The neurohypophyseal hormones and analogs did not affect the hypnotic or hypothermic response to an acute injection of ethanol, indicating that the determination of tolerance was not influenced by a direct peptide-ethanol interaction. The hormones themselves, however, did cause a drop in body temperature in the mice, which could be a result of either central or peripheral hormonal actions.

Arginine vasopressin deficient Brattleboro rats fail to develop tolerance to the hypothermic effects of ethanol

We have tested the hypothesis that animals with reduced levels of arginine vasopressin (AVP) would show reduced tolerance to ethanol. Brattleboro rats either heterozygous or homozygous for the diabetes insipidus (DI) trait and normal Sprague-Dawley rats were exposed to ethanol vapor for 21 days. Two days later, tolerance was evaluated by monitoring body temperature reductions after intraperitoneal injection of 2 g/kg (20% w/v) ethanol. Under the same conditions of chronic ethanol exposure, Sprague-Dawley rats, but not Brattleboro rats, displayed tolerance to the hypothermic effects of intraperitoneal ethanol. This phenomenon did not appear to be related to differences in ethanol metabolism or blood alcohol levels in Brattleboro rats. These data support a possible role for AVP in the development or maintenance of tolerance.