Modification of morphine-induced changes in striatal (3H)-spiroperidol binding and stereotype-behavior by cyclo(Leu-Gly)

A protein kinase inhibitor, H-7, blocks naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The influence of an inhibitor of cAMP-dependent protein kinase and protein kinase C, H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine], on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in levels of dopamine (DA) and its metabolites in morphine- or butorphanol-dependent rats was investigated. Animals were infused continuously with morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) for 3 days. Naloxone precipitated withdrawal syndrome and decreased the levels of DA in the cortex, striatum, and midbrain; 3, 4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain; and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Concomitant infusion of H-7 and opioid blocked both the expression of withdrawal signs and the decreases in DA, DOPAC, and HVA levels in a dose-dependent manner. These results suggest that the enhancement of cAMP-dependent protein kinase and/or protein kinase C activity accompanying the increase of DA neuron activity during continuous infusion of opioids leads to an abrupt reduction in levels of DA and its metabolites precipitated by naloxone, which is intimately involved in the expression of physical dependence on opioids.

Modification of naloxone-induced withdrawal signs by dextromethorphan in morphine-dependent mice

In the present study the effect of dextromethorphan on naloxone-induced withdrawal signs in morphine-dependent mice was examined. In addition, the modulatory role of dopaminergic mechanisms upon the effect of dextromethorphan was investigated. Mice were rendered dependent on morphine by subcutaneous (s.c.) injections of morphine sulfate three times a day for 3 days, and withdrawal signs were induced by intraperitoneal (i.p.) administration of naloxone 2 h after the 10th injection of morphine sulfate on day 4. Dextromethorphan (20-50 mg/kg, i.p.) caused a significant decrease in withdrawal jumping, paw-shakes, grooming, burrows, writhing and diarrhea in morphine-dependent mice. The mixed dopamine D1/D2 receptor agonist apomorphine (0.5 and 1 mg/kg, s.c.) reduced the response induced by dextromethorphan. The effect of apomorphine was blocked by the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7- ol maleate) (0.5 and 1 mg/kg, i.p.) but not by the dopamine D2 receptor antagonist sulpiride (25 and 50 mg/kg, s.c.) nor the peripheral dopamine receptor antagonist domperidone (5 and 10 mg/kg, s.c.). These results suggest that the dopaminergic system(s) may in part mediate the suppressive action of the NMDA receptor antagonist dextromethorphan on naloxone-induced withdrawal signs in morphine-dependent mice.

Increased toxicity of methamphetamine in morphine-dependent mice

1. The effect of methamphetamine on morphine-dependent mice was investigated by calculating the LD50 (i.p.), measuring motor activity, anorectic actions, and body temperature. 2. Methamphetamine was more toxic in morphine-dependent mice (LD50 = 20.6 mg/kg) than in normal mice (LD50 = 43.2 mg/kg). 3. Methamphetamine-induced locomotor activity was greater in morphinized than in nonmorphinized mice at doses of 2.5 and 5 mg/kg i.p. 4. Methamphetamine also increased the body temperature of morphinized mice more than that of normal mice (P < 0.05). 5. These findings suggest that methamphetamine is more toxic in morphine-dependent than in nondependent mice.

Naloxone-precipitated changes in biogenic amines and their metabolites in various brain regions of butorphanol-dependent rats

Influence of a naloxone (an opioid receptor antagonist) challenge (5 mg/kg, IP) on levels of biogenic amines and their metabolites in various brain regions of rats infused continuously with butorphanol (a mu/delta/kappa mixed opioid receptor agonist; 26 nmol/microliter/h) or morphine (a mu-opioid receptor agonist; 26 nmol/microliter/h) was investigated using high-performance liquid chromatography with electrochemical detection (HPLC-ED). Naloxone precipitated a withdrawal syndrome and decreased the levels of: dopamine (DA) in the cortex and striatum, 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum, homovanilic acid (HVA) in the striatum, limbic, midbrain, and pons/medulla regions in butorphanol-dependent rats. However, the levels of norepinephrine (NE), serotonin (5-hydroxytryptamine; 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the regions studied were not affected by naloxone-precipitated withdrawal. In addition, naloxone increased the HVA/DA ratio in the cortex, while this ratio was reduced in the limbic, midbrain, and pons/medulla. The reduction of 5-HIAA/5-HT ratio was also detected in the limbic area. In the animals rendered dependent on morphine, the results obtained were similar to those of butorphanol-dependent rats except for changes of 5-HIAA levels in some brain regions. These results suggest that an alteration of dopaminergic neuron activity following a reduction of DA and its metabolites in specific brain regions (e.g., striatum, limbic, midbrain, and pons/medulla) play an important role in the expression of the opioid withdrawal syndrome.

In vivo treatment with mu and delta, but not kappa-selective opioid agonists reduces [3H]spiperone binding to the guinea-pig striatum: autoradiographic evidence

In guinea-pigs, acute treatment with mu and delta receptor opioid agonists induces sedation and immobility [1,5], and attenuates the behavioural activation produced by the dopamine D2 agonist quinpirole [5]. In contrast, kappa-selective opioid agonists induce dystonic-like movements [4,5,8]. This has led us to investigate the possibility of an interaction between acute opioid treatment and the dopamine D2 system. The effect of acute treatment with mu, delta and kappa opioid agonists on [3H]spiperone binding sites (dopamine D2) in guinea-pig brain was studied using receptor autoradiography. The mu preferring agonist morphine (15 mg/kg subcutaneously, SC) given for 2 h, and the delta receptor selective agonist DPDPE (Tyr-D-Pen-Gly-Phe-D-Pen) (20 nM, intracerebroventricularly, ICV) given for 0.5 h, both decreased the density of specific (butaclamol displaceable) [3H]spiperone binding in the caudate putamen by 23.8 +/- 1.7% and 24.2 +/- 2.7% respectively, and in nucleus accumbens by 26.1 +/- 2.7% and 21.9 +/- 4.6% respectively compared to saline treated animals. There were no significant changes in the level of [3H]spiperone binding to other brain regions examined including frontal cortex, hippocampus, substantia nigra, ventral tegmental area, amygdala, hypothalamic nuclei and cerebellum. In other experiments, incubation of coronal slices from various brain regions with [3H]spiperone, in the presence of a high concentration of morphine (20 microM) or DPDPE (10 microM) did not affect the level of binding, thus precluding effects due to residual tissue levels of drugs after in vivo treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention and reversal of dopamine receptor supersensitivity by cyclo(leucyl-glycyl) (CLG): biphasic dose-response curves

Chronic administration (21 days) of haloperidol (HAL) (IP, 1.0 mg/kg/day) induced a behavioral supersensitivity (stereotypic sniffing) to dopamine (DA) agonists (apomorphine) and upregulation (increased Bmax for sulpiride-inhibitable [3H]spiroperidol binding) of striatal and limbic D2 DA receptors (DAr). Coadministration of cyclo(leucyl-glycyl) (CLG; 8mg/kg, SC; every third day, every other day, but not every day) with HAL attenuated the behavioral supersensitivity. D2-DAr binding assays showed 1) that CLG-induced changes in Bmax parallel these behavioral changes and 2) that the biphasic CLG dose-response curve may involve CLG failure at high cumulative doses to lower Bmax. CLG also reversed an already established D2 DAr supersensitivity/upregulation (i.e., when CLG was injected daily for four days after the withdrawal of HAL). CLG alone did not alter behavior or binding. CLG's ability to both prevent and reverse D2 DAr upregulation/supersensitivity in animal models suggests that CLG may be useful, within a therapeutic window, in clinical disorders that are thought to involve upregulated DAr (e.g., tardive dyskinesia, L-DOPA-induced dyskinesias, and schizophrenia).

Evidence for the behavioral supersensitivity of dopamine D2 receptors without receptor up-regulation in morphine-abstinent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of dopamine D2 receptors in brain regions and spinal cord was determined in the rat. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with 6 morphine pellets for a 7-day period, each containing 75 mg of morphine free base. Rats implanted with placebo pellets served as controls. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to a challenge dose of morphine. Similarly, the development of physical dependence was evidenced by a decrease in body weight and colonic temperature after morphine pellet removal (withdrawal). The binding characteristics (Bmax and Kd values) of [3H]spiroperidol to dopamine D2 receptors were determined in the tissues of morphine-tolerant and morphine-abstinent rats. In the tolerant rats, the pellets were left intact at the time of sacrificing, whereas, in the abstinent rats the pellets were removed 18 h prior to sacrificing. The binding of [3H]spiroperidol was determined in membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups. [3H]Spiroperidol bound to brain regions and spinal cord at a single high affinity site. The Bmax or the Kd values in brain regions and spinal cord of morphine-tolerant and -abstinent rats did not differ from their respective placebo controls. The behavioral responses to a selective dopamine D2 receptor agonist, 2-bromo-alpha-ergocryptine were also determine in the morphine-abstinent rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Modifications of striatal D2 dopaminergic postsynaptic sensitivity during development of morphine tolerance-dependence in mice

Alterations in the activity of striatal dopaminergic neurons have been implicated in the development of morphine tolerance-dependence in rodents. To further explore this possibility, we examined the activity of these neurons in mice exposed to morphine during 4 days (addiction group) and subsequently treated with naloxone (withdrawal group). The efficiency of opiate treatment was assessed behaviorally. Striatal dopaminergic activity was evaluated by measuring: a) the ratio between the amounts of L-3,4-dihydroxyphenylacetic acid (DOPAC), the main intraneuronal metabolite of dopamine (DA), and the neurotransmitter itself, as an index of presynaptic activity; and b) the number and affinity of D1 and D2 dopaminergic receptors, as well as the amount of their coupled second messenger, cyclic adenosine monophosphate (cAMP), as postsynaptic parameters. Spontaneous motor activity was decreased in chronically morphine-exposed mice. In these animals, the number of striatal D2 receptors also decreased, with no changes in their affinity, whereas the number and affinity of D1 receptors remained unchanged. This hyposensitivity of D2 receptors was paralleled by an increase in the amount of cAMP with a good statistical correlation between both parameters. Treatment with naloxone of morphine-exposed mice resulted in the typical jumping behavior indicative of opiate withdrawal. The differences in D2 receptors between placebo- and morphine-exposed mice disappeared after naloxone-induced opiate withdrawal, although this effect was due more to the inhibitory effect of naloxone on the density of these receptors in placebo-exposed mice rather than to a stimulatory effect in morphine-addicted mice. The morphine-induced increase in cAMP content also disappeared after naloxone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphine, D-Pen2, D-Pen5 enkephalin and U50,488H differentially affect the locomotor activity and behaviours induced by quinpirole in guinea-pigs

The effects of morphine D-Pen2, D-Pen5 enkephalin (DPDPE) and U50,488H on the behavioural syndrome elicited by the dopamine (DA) D-2 agonist quinpirole, were investigated. Morphine (1, 5 and 15 mg/kg SC) and morphine administered intracerebroventricularly (ICV) (2 x 5 microliters, 10(-3) M; total dose = 10 nmol) produced piloerection and sedation. DPDPE-ICV (2 x 5 microliters and 2 x 10 microliters, 10(-3) M; total doses = 10 and 20 nmol) produced piloerection and sedation similar to morphine. U50,488H (1 mg/kg SC) induced locomotor activity and some stereotyped behaviour, whereas U50,488H (5 and 10 mg/kg SC) induced muscle rigidity and dystonic-like movements. The locomotor and behavioural response elicited by quinpirole (3 mg/kg IP) was attenuated in guinea-pigs pretreated with morphine (1, 5 and 15 mg/kg SC), morphine-ICV (2 x 5 microliters, 10(-3) M), and DPDPE-ICV (2 x 5 microliters and 2 x 10 microliters, 10(-3) M). These effects were reversed by naloxone (15 mg/kg SC). U50,488H (1 mg/kg SC) increased the quinpirole-induced locomotor activity, whereas U50,488H (5 and 10 mg/kg SC) decreased the locomotor activity and stereotyped behaviours produced by quinpirole. These results indicate that the gross behavioural effects of mu, delta and kappa opioids differ in guinea-pigs compared to other rodent species, and suggest differential involvement of these opioid receptor subtypes with DA D-2 receptor-mediated activity.

The absolute density of neurotransmitter receptors in the brain. Example for dopamine receptors

Since the absolute density of dopamine receptors can vary in disease, it is essential to establish the normal values for the absolute densities of D1 and D2 receptors in the brain. Absolute densities are most conveniently reported in units of picomoles per gram of original tissue, readily permitting their comparison to data obtained by positron emission tomography in patients. The density of D1 receptors is approximately 120 pmol/g in the rat striatum and 19 pmol/g in the human striatum. The density of D2 receptors is about 32 pmol/g in the rat striatum and 14 pmol/g in the human striatum, these values being determined by Teflon-glass homogenization and the centrifugation method. The customary Polytron-homogenization procedure results in a loss of about 9% of the D2 receptors in rat tissue and about 28% in human tissues; filtration results in a further loss of about 12%. There is general agreement between the in vitro and in vivo densities, but only if the receptors are measured by the amount of radioisotope specifically displaced.

Further evidence that a single dose of an opiate can increase dopamine receptor sensitivity in mice

Previous findings in our laboratory indicated that a single administration of morphine or levorphanol to mice could induce the development of supersensitive dopamine receptors. To further study this phenomenon, the ability of haloperidol to inhibit dopamine agonist-induced climbing was determined in mice 3 h following morphine (10 mg/kg i.p.) or levorphanol (2.0 mg/kg i.p.) pretreatment. The dose of haloperidol required to inhibit climbing behavior induced by 2.4 mg/kg (i.p.) of apomorphine or (-) N-n-propylnorapomorphine was increased significantly in the opiate-pretreated mice. Morphine pretreatment did not significantly affect the apparent pA2 of apomorphinehaloperidol for climbing behavior suggesting that the affinity of the dopamine receptor was not altered. This observation was supported by a lack of difference in the Kd of haloperidol binding sites between saline- and morphine-pretreated mice. There was, however, a significant increase in the Bmax in the morphine-pretreated animals. This increase was blocked when 5 mEq/kg of lithium (i.p.) or 5 mg/kg naloxone (i.p., administered twice) was administered concurrently with the morphine. Concurrent lithium or naloxone administration also attenuated the morphine-induced increase in dose of haloperidol required to inhibit dopamine agonist-induced climbing behavior. These results suggest that a single administration of an opiate can cause the development of dopamine receptor supersensitivity which is due to an increase in dopamine receptor density.

Radio-frequency analysis of the effect of haloperidol and cyclo (leucyl-glycyl) on apomorphine-induced stereotypy

Our previous studies indicated that the peptide cyclo(leucyl-glycyl) (cLG) prevents the development of supersensitivity to dopamine in several animal models at both biochemical and behavioral levels. We therefore tested cLG in a paradigm more commonly used to model tardive dyskinesia, namely chronic haloperidol administration to rats. We found that cLG administered subcutaneously at a dose of 8 mg/kg, blocked about 50% of the supersensitizing effects of of haloperidol on apomorphine-induced stereotypic behaviors. Further, we used a novel method, radio-frequency analysis, that quantifies sniffing and other stereotypic movements. Unlike methods that rely on visual observation of stereotypy and utilize an ordinal scale, these measurements are rated by an automatic motility monitor and utilize a ratio scale. Unlike other automated motility monitors, this device can distinguish between various forms of stereotypic behaviors. Since parametric statistics can be used, there is a significant improvement in the efficiency of the task of rating and comparing stereotypy scores.

Chronically administered morphine increases dopamine receptor sensitivity in mice

Chronic morphine treatment has been suggested to cause the development of supersensitive dopamine receptors. This increase in sensitivity was detected as a hypersensitivity in direct-acting dopamine agonists and as an increase in the affinity of dopamine receptors. However, these binding studies were performed in animals which had been withdrawn from morphine for a period of 24-48 h prior to killing. In the present study mice were implanted with pellets containing 75 mg of morphine free base. The pellets were left in situ in all experiments. One group of mice exhibited an increased sensitivity to apomorphine 72 h following pellet implantation as evidenced by a decrease in the ED50 of apomorphine for inducing cage climbing behavior. A second matched group of mice was found to have a significant increase in whole brain [3H]spiroperidol binding sites. These results suggest that chronic morphine treatment can cause the development of central supersensitive dopamine receptors. Lithium administered concurrently with the morphine attenuated the increased sensitivity to apomorphine and the increase in the number of [3H]spiroperidol binding sites. Concurrent lithium treatment also facilitated the degree of analgesic tolerance, and naloxone-induced withdrawal hypothermia. The ability of lithium to enhance analgesic tolerance while simultaneously attenuating the increase in dopamine receptors suggests that alterations in dopamine receptors might modify the degree of analgesic tolerance which develops to chronic morphine administration, or might modify the animal's response to thermal stimuli. The mechanism by which lithium enhanced naloxone-induced hypothermia is presently unknown.

Morphine dependence and protracted abstinence: regional alterations in CNS radioligand binding

Rats (Fisher F-344) were given free access to a 10% sucrose solution containing 0.5 mg/ml morphine sulfate (controls received the sucrose vehicle only) as their sole source of fluid. Daily morphine intake averaged 119 +/- 21 mg/kg, an amount sufficient to induce physical dependence. After 18 days on this regimen, the control and dependent subjects were sacrificed. A protracted abstinence group was weaned from morphine by reducing its concentration in the vehicle by 20% over the next 5 days, followed by a 5-week drug-free period before sacrifice concurrent with the other groups. These subjects showed no signs of an abstinence syndrome. Binding assays for alpha-2 adrenergic sites (3H-clonidine), beta-1/beta-2 adrenergic sites (3H-dihydroalprenolol), and dopaminergic (D2)/serotonergic (5-HT2) sites (3H-spiroperidol) were performed on tissue from frontal cortex, hippocampus, striatum, and brainstem. No alterations in 3H-clonidine or 3H-dihydroalprenolol binding were observed in dependence or protracted abstinence, suggesting that noradrenergic systems are well-regulated both during dependence and in protracted abstinence. 3H-spiroperidol binding was significantly elevated in the striatum (D2 sites) and hippocampus (5-HT2 sites) during dependence. Hippocampal 3H-spiroperidol binding returned to control levels in protracted abstinence, reflecting a morphine-induced change in 5-HT2 binding sites which had normalized by 5 weeks post-drug. Striatal 3H-spiroperidol binding was significantly decreased below control levels after withdrawal, suggesting that alterations of D2 sites in this structure may play a role in protracted abstinence.

Neuropeptides differentially effect various forms of morphine tolerance

Two different forms of tolerance to morphine were shown to develop. One form, environment dependent (ED), was associated with cues paired with the arrival of the drug. The second form of tolerance, environment independent (EI), was not dependent on any cues. ED tolerance was induced using a multiple injection model in which a cue (orange scent) preceeded each morphine injection (ip). After 12 days the animals were tolerant to the i.p. injection. However if morphine was injected i.c.v. tolerance was no longer evident. EI tolerance was induced by a pellet implant method. These animals were tolerant to morphine regardless of the route of administration. Peptides arginine vasopressin (AVP) and cyclo (Leu-Gly) (cLG) were tested for their ability to alter tolerance. AVP was found to facilitate the development of ED tolerance but had no effect on EI tolerance. On the other hand cLG blocked the development of EI tolerance without affecting ED tolerance. The determination of brain morphine levels indicated that the ED tolerance produced by this method is dispositional while EI tolerance is functional.

Enhanced striatal 3H-spiroperidol binding induced by chronic haloperidol treatment inhibited by peptides administered during the withdrawal phase

Chronic intragastric administration of haloperidol (1.5 mg/kg/day) for 21 days followed by a 3-day withdrawal period resulted in the development of enhanced locomotor activity response to apomorphine, and an increase in the number of binding sites for 3H-spiroperidol in the striatal membranes of the rat brain. Subcutaneous administration of Pro-Leu-Gly-NH2 or cyclo(Leu-Gly) in doses of 2 mg/kg/day given for 3-days after termination of haloperidol treatment inhibited the enhanced response to apomorphine, as well as the increases in the number of 3H-spiroperidol binding sites in the striatum. If indeed, the supersensitivity of striatal dopamine receptors is one of the mechanisms in the development of tardive dyskinesia symptoms, the present results suggest that the above peptides may be helpful in ameliorating some of the symptoms of tardive dyskinesia induced by neuroleptic drugs.

Cyclo(Leu-Gly) has opposite effects on D-2 dopamine receptors in different brain areas

Cyclo(Leu-Gly) (cLG), a diketopiperazine analog of Pro-Leu-Gly-NH2 (MIF), affects a number of physiological and behavioral responses to the endogenous neurotransmitter, dopamine (DA). In the present series of experiments, the effect of in vivo administration of cLG (8 mg/kg) was investigated five days following subcutaneous administration. It was found that cLG administration of cLG (8 mg/kg) was investigated five days following subcutaneous administration. It was found that cLG administration caused a supersensitive behavioral response, measured by increased stereotypic sniffing, to the DA agonist, apomorphine (APO). At the same time, an increase was found in the affinity for dopamine (DA), as measured by dopamine inhibition of 3H-spiroperidol binding to D-2 DA receptors in striatum (nigro-striatal DA tract). In contrast, the same peptide treatment caused a subsensitive physiological response to APO-induced hypothermia, concomitant with a decrease in affinity for dopamine, as measured by DA inhibition of 3H-spiroperidol binding to D-2 DA receptors in hypothalamus (incerto-hypothalamic DA tract). These results suggest that a single neuromodulatory agent, the peptide cLG, can elicit diametrically opposite effects on D-2 DA receptors and on the corresponding physiological endpoints in two different brain areas.

Retardation of cerebral dopamine turnover after morphine withdrawal and its enhanced acceleration by acute morphine administration in rats

To clarify the effects of withdrawal from chronic morphine treatment on cerebral dopamine (DA) turnover, we have measured the alpha-methyl-p-tyrosine (alpha MT)-induced depletion of DA in five brain areas of male Wistar rats given morphine twice daily for 40 or 60 days. After the last morphine dose (50 or 70 mg/kg) the rats were withdrawn for 1, 2 or 4 days. In order to study the development of tolerance some of the rats were challenged with 10 mg/kg of morphine. Withdrawal of morphine retarded the alpha MT-induced DA depletion in the limbic forebrain and after long enough chronic treatment in the striatum, too. The challenge dose of morphine accelerated the cerebral DA depletion slightly less in rats withdrawn for 1 day from 60-day chronic morphine treatment than in rats treated chronically with saline, but it enhanced the DA depletion more in rats withdrawn from morphine for 2 and 4 days than in chronic saline rats. This enhancement was clearest in rats withdrawn for 4 days from 60-day treatment. Thus withdrawal from morphine seems to sensitize the rats to the DA depletion accelerating effect of morphine. Our results show that repeated administration of morphine creates no marked tolerance to the DA depletion accelerating effect of morphine. In contrast, the dopaminergic neurones of the chronically treated rats seem to depend on continuous morphine administration for their normal functioning. Furthermore, the retarded DA turnover after discontinuation of morphine treatment seems to sensitize the dopaminergic neurones to the DA depletion accelerating effect of morphine.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Peptide inhibition of morphine-induced dopaminergic supersensitivity

Injection of the peptide cyclo(Leu-Gly) into rats prior to chronic exposure to morphine, inhibits: 1) the development of analgesic tolerance; 2) some signs of physical dependence; and, 3) morphine-induced increases in behavioral responses to dopamine agonists. Although there was no change in the total number of high affinity striatal dopamine receptors, chronic morphine treatment did increase the affinity of the ligand at the receptor. The peptide blocked not only the affinity change, but the increased behavioral response to apomorphine as well. These behavioral changes correlate significantly with the neurochemical changes in dopamine receptors following chronic morphine treatment. Therefore, some of the pharmacological efforts of morphine may be mediated by changes in CNS dopamine receptors and that the peptides may act by inhibiting these neurochemical changes.