Potentiation of the aphrodisiac effect of N-n-propyl-norapomorphine by naloxone

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.

Effects of morphine or naloxone on cocaine-induced genital reflexes in paradoxical sleep-deprived rats

The involvement of opioidergic neurotransmission in the modulation of genital reflexes induced by paradoxical sleep deprivation (PSD) and cocaine in rats was the aim of the present study. Morphine (0, 1, 5 and 10 mg/kg) and naloxone (0, 0.3, 3 and 30 mg/kg) were administered prior to saline or cocaine to rats that had been deprived of sleep and the incidence of penile erections (PE) and ejaculations (EJ) was measured. PSD alone induced PE in 50% and EJ in 20% of the rats, but these behaviors were not influenced by morphine or naloxone. Cocaine potentiated the incidence of genital reflexes in PSD rats to 90% (PE) and 70% (EJ). Morphine and not naloxone significantly reduced the percentage of rats displaying this response at the highest doses. Morphine also significantly reduced PE and EJ frequencies at 10 mg/kg. Furthermore, this inhibitory effect of morphine on genital reflexes was prevented by the prior injection of naloxone. Although a number of factors are involved in such a complex phenomenon as PE and EJ, our data show that activation of the opioidergic systems by the agonist morphine reduces genital reflexes-induced by cocaine in PSD males while the antagonist, naloxone, did not have any significant effect. The findings suggest that the stimulating effects of cocaine in potentiating genital reflexes in PSD rats can be unidirectionally modified by opioidergic systems.

Metachlorophenylpiperazine (m-CPP) induced intracavernous pressure responses in anaesthetized rats

Here we have recorded the effects of metachlorophenylpiperazine (m-CPP) on intracavernous pressure (ICP) in anesthetized rats pretreated with various pharmacological agents in an attempt to determine the mechanism and relevance of the m-CPP induced ICP response to other models of erection. m-CPP elicited consistent and significantly greater increases in ICP (71.5+/-6.6 mmHg) compared with the mixed 5-HT(2a/2c) agonists trifluoromethylphenylpiperazine (3.4+/-1.3 mmHg) and quipazine (10.9+/-1.8 mmHg). Blockade of 5-HT(2a) receptors with ketanserin failed to unmask any stimulatory effect of quipazine (7.2+/-1.0 mmHg). m-CPP induced ICP responses (71+/-7.0 mmHg) were unaffected in the presence of mianserin (63+/-5 mmHg) and ketanserin (51+/-12 mmHg). Spiperone significantly reduced the m-CPP induced increase in ICP (8.0+/-1.0 mmHg). Naloxone, yohimbine and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) failed to elicit increases in ICP on their own. All three drugs significantly reduced the latency to the first m-CPP induced ICP response compared to saline. Yohimbine increased the duration of m-CPP induced ICP responses whereas 8-OHDPAT increased the mean number of m-CPP induced ICP responses compared to saline. The effects of m-CPP on ICP in anesthetized rats may not be mediated by 5-HT(2c) receptors and appears to be similar to erection in copula, but not erection elicited by other drugs or penile sheath retraction.

The effect of naloxone on spontaneous and evoked dopamine release in the central and peripheral nervous systems

A number of studies have reported that the opiate antagonist naloxone (NX) inhibits behaviors dependent upon central dopamine (DA) release. However, equally compelling evidence from other studies suggests that NX excites a facilitatory effect. The present review was undertaken to resolve the issue by critically evaluating the effects of NX on DA release; the substrate subserving these behaviors. Included are studies reporting an effect of NX on spontaneous as well as drug altered DA release in various central regions. In the preponderant majority of these studies, NX was found to significantly enhance DA release in the virtually every major DA pathway, irrespective of whether DA release was initially stimulated or inhibited by various agents. It is concluded that NX most probably enhances behaviors induced by DA release, especially when administered in low, specific doses. Studies finding an inhibitory effect of NX on such behaviors may inadvertently produce conditions which mask the stimulatory effects of NX on DA release-dependent behaviors.

Pharmacological analysis of male rat sexual behavior

Pharmacological influences on male rat sexual behavior are reviewed in an attempt to identify neurotransmitters and their respective receptor types that regulate various factors comprising the behavioral pattern. Evidence is presented that: (1) serotonergic influence is generally inhibitory to sexual behavior, although two receptor subtypes may lower ejaculation threshold; (2) dopaminergic agonists facilitate several aspects of copulatory behavior and ex copula genital responses; (3) noradrenergic activity appears to increase sexual arousal; (4) cholinergic agonists facilitate ejaculation, or in some cases, delay or prevent initiation of copulation; (5) GABA agonists inhibit sexual responses both in and ex copula; (6) opiate agonists appear to inhibit copulation and penile reflexes, although antagonists have mixed effects; (7) ACTH and MSH peptides promote copulatory behavior and genital responses; (8) oxytocin facilitates ex copula penile responses, but may contribute to postejaculatory refractoriness; and (9) long-term exposure to prolactin inhibits sexual behavior and penile responses. Although some progress has been made in identifying neurotransmitter-receptor effects on behavioral components, copulatory behavior is complex and no drug has been found to affect only a single component. Furthermore, drug specificity is only relative.

Antagonism of drug-induced yawning and penile erections in rats

A number of centrally active drugs were tested for antagonism of physostigmine- or apomorphine-induced yawning and for apomorphine-induced penile erections. The alpha 2-adrenoceptor antagonists piperoxan and idazoxan inhibited the yawning response without affecting the penile erections. The 5HT agonist quipazine and the histamine antagonist dexchlorpheniramine inhibited the yawning response more effectively than the penile erections. Dexchlorpheniramine even enhanced the apomorphine-induced penile erections and induced penile erections in physostigmine-treated rats. The 5HT antagonists metergoline and methysergide blocked the apomorphine-induced penile erections without affecting the yawning response. The alpha 2-adrenoceptor agonist clonidine, the dopamine antagonist sulpiride, the antihistaminic mepyramine and the benzodiazepine chlordiazepoxide inhibited both yawning and penile erections at the same dose level. The alpha 1-adrenoceptor antagonists prazosin and phenoxybenzamine were inactive. It is concluded that yawning and penile erections can be differentially affected by drug treatments. Also, while concomitant yawning and penile erections can be selectively induced by a class of dopamine receptor agonists, the same selectivity does not apply to antagonism of these induced behaviours.

Sexual excitement and stretching and yawning induced by B-HT 920

The azepine derivative B-HT 920, a putative agonist at dopamine (DA) autoreceptors, injected IP in adult male rats, induced numerous penile erections (PE) and stretching and yawning (SY), considered typical signs of central DA receptor stimulation, without eliciting stereotyped behaviour (SB). Both signs induced by B-HT 920 were dose related and significantly enhanced with respect to controls from 10 to 1,000/micrograms/kg. Pretreatment with the neuroleptics haloperidol (0.025, 0.5 and 1 mg/Kg IP), sulpiride (20 and 40 mg/Kg) and alpha 2-antagonist yohimbine (1 and 3 mg/Kg IP) antagonized the behavioural effect of B-HT 920 whereas the alpha 1-antagonist prazosin (1 mg/Kg IP) had no effect on the response. The impressive activity of B-HT 920 in producing SY and PE, along with its inability to evoke SB, supports the role of DA autoreceptors in the regulation of SY and sexual behaviour.

Influence of cimetidine, ranitidine and imidazole on the behavioral effects of (+/-) N-n-propylnorapomorphine in male rats

Cimetidine injected IP 15 min before (+/-) N-n-propylnorapomorphine (NPA) antagonized in dose-dependent fashion the penile erections (PE) and stretching and yawning (SY) induced by this typical dopaminergic agonist in male rats. Ranitidine, which acts on H2 histamine receptors in much the same way as cimetidine despite its lack of an imidazole ring, failed to produce the same effect. On the other hand, imidazole itself was similar to cimetidine in antagonizing PE and SY induced by (+/-) NPA, whether injected IP or ICV. Neither imidazole nor cimetidine antagonized the stereotyped behaviour (SB) induced by (+/-) NPA. Indeed, imidazole reduced the latency of this response. A mechanism which may underly these effects is discussed, as well as the possible preclinical use of this test in animals.

Narcotic antagonist potentiation of apomorphine drug effect: a stereospecific, centrally mediated drug action

Systemic pretreatment with naloxone but not its (+)-enantiomer significantly potentiated apomorphine-induced stereotypic climbing activity in mice. Systemic or central pretreatment with naltrexone also significantly potentiated the apomorphine drug effect. The N-methyl derivative of naltrexone had no influence when administered systemically but significantly potentiated the apomorphine drug effect when administered centrally. These findings suggest that narcotic antagonist potentiation of apomorphine-induced stereotypic climbing in mice is a stereospecific phenomenon that originates in the central nervous system.

Possible noradrenergic involvement in naloxone potentiation of apomorphine-induced stereotypic climbing in mice

Apomorphine-induced stereotypic climbing behavior in mice was significantly potentiated by pretreatment with the opiate receptor blocker naloxone. In animals additionally pretreated with alpha-methyl-p-tyrosine, brain levels of norepinephrine (NE) and dopamine (DA) were markedly reduced and naloxone potentiation of apomorphine-induced stereotypic climbing was blocked. In mice pretreated with diethyldithiocarbamic acid, brain NE was slightly reduced, brain DA was slightly elevated but naloxone potentiation of apomorphine-induced stereotypic climbing was unaltered. In animals pretreated with reserpine, both brain NE and DA were significantly reduced by naloxone potentiation of apomorphine-induced stereotypic climbing was not affected. In other experiments, pretreatment with low doses of the alpha-adrenergic receptor blocker BE-2254 failed to suppress climbing activity induced by apomorphine alone but did successfully prevent naloxone potentiation of apomorphine-induced stereotypic climbing. These findings suggest the possibility that NE and alpha-adrenergic receptors might play a role in the potentiating influence of naloxone upon apomorphine-induced stereotypic climbing activity in mice.

Enhancement of sexual motivation in male rats by yohimbine

Yohimbine hydrochloride, an alpha 2-adrenoceptor antagonist, increased sexual motivation in male rats as evidenced by increased mounting performance in mating tests conducted after genital anesthetization, increased percentage of male rats ejaculating in their first heterosexual encounter, and induction of copulatory behavior in sexually inactive male rats. These observations lead to the suggestion that alpha-adrenoceptors are important modulators of sexual arousal in intact male rats. These results indicate that pharmacological treatment of sexual (libido) dysfunction may be useful.

The yawning-penile erection syndrome as a model for putative dopamine autoreceptor activity

The efficacy of several drugs to elicit yawning and penile erections were determined in rats. The dopamine agonists, N-propylnorapomorphine, apomorphine, pergolide, (+/-)-3-PPP, TL-99 and N,N-dipropylamino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene (N,N-dipropyl A-5,6-DTN) all elicited yawning accompanied by an increase in spontaneous penile erections. The potencies of these drugs in causing yawning closely resemble published data concerning their actions in biochemical tests reputedly indicative of autoreceptor activity. In contrast, SK&F 38393, A-5,6-DTN and clonidine produced no yawning and few or no penile erections. Although physostigmine also caused yawning, the effect was not accompanied by penile erections. Studies with the optical isomers of 3-PPP showed that (+)-3-PPP was considerably more potent than (-)-3-PPP. Haloperidol antagonised dopamine agonist-induced yawning and penile erections. Apomorphine-induced yawning and penile erections were also antagonised by sulpiride and atropine but not by domperidone. The suitability of elicitation of the combined syndrome of yawning plus penile erections as useful behavioural model for dopamine autoreceptor agonists is discussed.

Sleep pattern alterations by naloxone. Partial prevention by haloperidol

A continuous intravenous infusion of the opiate receptor antagonist naloxone, at doses ranging from 0.7 to 1.0 micrograms/kg/min (total dose 8.8-13.2 mg), was performed in eight normal subjects during the first 240 min of night sleep. A significant reduction in stage REM and in stage 4 percent duration was observed. Pretreatment with the dopamine receptor blocking agent haloperidol (2 mg intramuscularly before sleep onset) partially prevented the naloxone effect, suggesting that it is due, at least in part, to the stimulation of dopamine receptors.

Potentiation by naloxone of the anti-oxotremorine effect of L-DOPA

Pretreatment with the catecholamine precursor L-DOPA but not the narcotic antagonist naloxone suppressed the tremorigenic effect of oxotremorine in mice. However, when animals were pretreated with both L-DOPA and several different doses of naloxone, there was a dose-related potentiation of the antitremor effect of L-DOPA. Naloxone also produced dose-dependent potentiation of the antitremor activity of lower doses of L-DOPA in the presence of the peripheral decarboxylase inhibitor carbidopa. These findings suggest a possible therapeutic application for naloxone in treatment of dopamine dysfunction disorders.

Potentiation of apomorphine-induced stereotypies by naloxone and L-prolyl-L-leucyl-glycinamide

We compared the influences of pretreatment with the narcotic antagonist drug naloxone and the neuropeptide L-prolyl-L-leucyl-glycinamide (PLG) upon apomorphine-induced stereotypic climbing activity in mice and apomorphine-induced contralateral rotational behavior in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. Naloxone produced dose-related potentiation in the mouse climbing model, while PLG was without effect. On the other hand, PLG produced dose-related potentiation in the rat rotational paradigm, while naloxone was without appreciable influence. These findings show an asymmetrical potentiation of apomorphine by naloxone and PLG in these two standard experimental models of striatal dopaminergic activity.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.