Microdialysis study of striatal dopamine in MPTP-hemilesioned rats challenged with apomorphine and amphetamine

Motor impairments of Parkinson`s disease (PD) appear only after the loss of more than 70% of the DAergic neurons of the substantia nigra pars compacta (SNc). An earlier phase of this disease can be modeled in rats that received a unilateral infusion of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine (MPTP) into the SNc. Though these animals do not present gross motor impairments, they rotate towards the lesioned side when challenged with DAergic drugs, like amphetamine and apomorphine. The present study aimed to test whether these effects occur because the drugs disrupt compensatory mechanisms that keep extracellular levels of dopamine in the striatum (DA(E)) unchanged. This hypothesis was tested by an in vivo microdialysis study in awake rats with two probes implanted in the right and left striatum. Undrugged rats did not present turning behaviour and their basal DA(E) did not differ between the lesioned and sham-lesioned sides. However, after apomorphine treatment, DA(E) decreased in both sides, but to a larger extent in the lesioned side at the time the animals started ipsiversive turning behaviour. After amphetamine challenge, DA(E) increased in both sides, becoming significantly higher in the non-lesioned side at the time the animals started ipsiversive turning behaviour. These results are in agreement with the hypothesis that absence of gross motor impairments in this rat model of early phase PD depends on maintenance of extracellular DA by mechanisms that may be disrupted by events demanding its alteration to higher or lower levels. (C) 2010 Elsevier B.V. All rights reserved.

Effects of microinjections of apomorphine and haloperidol into the inferior colliculus on the latent inhibition of the conditioned emotional response

Electrical or chemical stimulation of the inferior colliculus (IC) induces fear-like behaviors. More recently, consistent evidence has shown that electrical stimulation of the central nucleus of the IC supports Pavlovian conditioning and latent inhibition (Li). LI is characterized by retardation in conditioning and also by an impaired ability to ignore irrelevant stimuli, after a non-reinforced pre-exposure to the conditioned stimulus. LI has been proposed as a behavioral model of cognitive abnormalities seen in schizophrenia. The aim of the present study was to determine whether dopaminergic mechanisms in the IC are involved in LI of the conditioned emotional response (CER). To induce LI, a group of rats was pre-exposed (PE) to six tones in two sessions, while rats that were not pre-exposed (NPE) had two sessions without tone presentations. The conditioning consisted of two tone presentations to the animal, followed immediately by a foot shock. PE and NPE rats received IC microinjections of physiological saline, the dopaminergic agonist apomorphine (9.0 mu g/0.5 mu L/side), or the dopaminergic antagonist haloperidol (0.5 mu g/0.5 mu L/side) before both pre-exposure and conditioning. During the test, the PE rats that received saline or haloperidol had a lower suppression of the licking response compared to NPE rats that received vehicle or haloperidol, indicating that latent inhibition was induced. There was no significant difference in the suppression ratio in rats that received apomorphine injections into the IC, indicating reduced latent inhibition. These results suggest that dopamine-mediated mechanisms of the IC are involved in the development of LI. (C) 2008 Elsevier Inc. All rights reserved.

New insights into the oxidation pathways of apomorphine

A detailed study of the oxidative behaviour of apomorphine in aqueous media is reported. Resorting to the synthesis of apomorphine derivatives it was possible to identify all the anodic oxidation peaks of apomorphine, which are related to the oxidation of the catechol and tertiary amine groups. These findings were revealed to be important since they could lead to a better understanding of the biological interactions of apomorphine and gain insight into its metabolic pathways. During the voltammetric studies, it was also found that apomorphine forms a complex with borate through the catechol group leading to an increase of its oxidation potential. This property could be very useful with regard to the stabilization of apomorphine solutions since it could drastically reduce its autoxidation.

Oxidative behaviour of apomorphine and its metabolites

The metabolism of apomorphine is quite complex due to interactions with proteins and other tissue components that affect its pharmacokinetic profile. The electrochemical oxidation mechanism of apomorphine and of some synthesised apomorphine derivatives was studied. It was found to be related to the reaction of o-diphenol and tertiary amine groups and strongly dependent on pH.

Flow injection electrochemical determination of apomorphine

Few analytical methods are currently available for determination of apomorphine, the active substance of a new oral formulation used in the treatment of erectile dysfunction. In this way a flow injection electrochemical method (FIA-EC) was developed for its quantification and applied to pharmaceutical dosage forms. Based in previous findings regarding the stability of apomorphine in borate buffer and after optimization of several analytical parameters a single channel flow injection manifold was set up that enables the determination of this drug over the concentration range of 3 to 16 µmol L-1 with a detection limit of 0.5 µmol L-1 at a sampling rateof 90 h-1. The simplicity and rapidity of the FIA-EC method used, its reproducibility and sensitivity make it suitable for quality control of pharmaceutical preparations of apomorphine intended for clinical use and research.

Genotoxic, neurotoxic and neuroprotective activities of apomorphine and its oxidized derivative 8-oxo-apomorphine

Apomorphine is a dopamine receptor agonist proposed to be a neuroprotective agent in the treatment of patients with Parkinson's disease. Both in vivo and in vitro studies have shown that apomorphine displays both antioxidant and pro-oxidant actions, and might have either neuroprotective or neurotoxic effects on the central nervous system. Some of the neurotoxic effects of apomorphine are mediated by its oxidation derivatives. In the present review, we discuss recent studies from our laboratory in which the molecular, cellular and neurobehavioral effects of apomorphine and its oxidized derivative, 8-oxo-apomorphine-semiquinone (8-OASQ), were evaluated in different experimental models, i.e., in vitro genotoxicity in Salmonella/microsome assay and WP2 Mutoxitest, sensitivity assay in Saccharomyces cerevisiae, neurobehavioral procedures (inhibition avoidance task, open field behavior, and habituation) in rats, stereotyped behavior in mice, and Comet assay and oxidative stress analyses in mouse brain. Our results show that apomorphine and 8-OASQ induce differential mutagenic, neurochemical and neurobehavioral effects. 8-OASQ displays cytotoxic effects and oxidative and frameshift mutagenic activities, while apomorphine shows antimutagenic and antioxidant effects in vitro. 8-OASQ induces a significant increase of DNA damage in mouse brain tissue. Both apomorphine and 8-OASQ impair memory for aversive training in rats, although the two drugs showed a different dose-response pattern. 8-OASQ fails to induce stereotyped behaviors in mice. The implications of these findings are discussed in the light of evidence from studies by other groups. We propose that the neuroprotective and neurotoxic effects of dopamine agonists might be mediated, in part, by their oxidized metabolites.

A comparative study on the effects of the benzodiazepine midazolam and the dopamine agents, apomorphine and sulpiride, on rat behavior in the two-way avoidance test

In recent years. studies in behavioral pharmacology have shown the involvement of dopaminergic mechanisms in avoidance behavior as assessed by the two-way active avoidance test (CAR). Changes in dopaminergic transmission also occur in response to particularly threatening challenges. However, studies on the effects of benzodiazepine (BZD) drugs ill this test are still unclear. Given the interplay of dopamine and other neurotransmitters in the neurobiology of anxiety and schizophrenia the aim of this work was to evaluate the effects of systemic administration of midazolam, the dopaminergic agonist apomorphine, and the D(2) receptor antagonist sulpiride using the CAR, a test that shows good sensitivity to typical neuroleptic drugs. Whereas midazolam did not alter the avoidance response. apomorphine increased and sulpiride reduced them in this test. Escape was not affected by any drug treatments. Heightened avoidance was not associated with the increased motor activity caused by apomorphine. In contrast with the benzodiazepine midazolam, activation of post-synaptic D(2) receptors with apomorphine facilitates, whereas the D(2) receptor antagonism with sulpiride inhibited the acquisition of the avoidance behavior. Together, these results bring additional evidence for a role of D(2) mechanisms in the acquisition of the active avoidance. (C) 2009 Elsevier Inc. All rights reserved.

Effects of apomorphine on mating behavior, flank marking and aggression in male hamsters

In male rats, the dopamine agonist apomorphine (APO) generally facilitates copulatory behavior. However, disruptive effects of high APO doses have been reported. These have been interpreted in diverse ways, as products of a dopaminergic system that inhibits sexual behavior or as consequences of APO's stimulation of competing responses. To test the generality of these effects, we observed APO's impact on copulatory behavior in male hamsters. Several effects were observed, all attributable to a relatively high dose and involving the disruption of male behavior. More unexpectedly, APO treatment caused males to attack estrous stimulus females in the course of these tests. To clarify these effects, we observed the effects of APO on flank marking, a type of scent marking closely allied to aggression and dominance in hamsters. Treatment reliably decreased the latency of marking. It also increased the rate of marking when appropriate measures were taken to prevent this effect from being obscured by drug-induced cheek pouching. Together, these results confirm and extend APO's well-known ability to increase aggression. Further, they suggest that APO-induced aggression can intrude into other contexts so as to disrupt, or possibly facilitate, other forms of social behavior.

The Effects of Apomorphine on Sexual Behavior and Aggression in Male Golden Hamsters

The general dopamine agonist apomorphine has been shown to have mostly facilitative effects on sexual behavior in rodents (Domingues & Hull, 2005; Bitran & Hull, 1987). A study looking at the effectsof apomorphine on sexual behavior in male golden hamsters observed that after systemic injections of apomorphine the males became aggressive towards the estrous females (Floody, unpublished). Studies on aggressive behavior have shown that apomorphine has facilitative effects on aggression in rodents (Nelson & Trainor, 2007; van Erp & Miczek, 2000; Ferrari, van Erp, Tornatzky, & Miczek, 2003). The studies presented here attempt to unravel the effects that apomorphine has on sexual and aggressive behavior in male golden hamsters. Studies 1, 2, 3, and 4 focused on the effects of apomorphine on aggression and Study 5 focused on the effects of apomorphine on sexual behavior. It was important for the purposes ofthis study to have separate, specific measures of aggression and sexual behavior that did not involve a social context that would involve multiple behaviors and motivations. The measure used to assessaggression was flank marking behavior. The measure used to assess sexual behavior was the number of vocalizations in response to sexual stimuli. The results from Studies 1, 2, and 3 suggested thatapomorphine increased aggressive motivation in a dose-dependent manner. In Studies 1 and 2 there was a high occurrence of stereotyped cheek pouching that interfered with the flank marking behavior. In Study 3 the procedure was modified to prevent cheek pouching and flank marking was observed uninhibited. Study 5 suggested a decrease in vocalizations after apomorphine treatment. However, this decrease may have been a result of the increase in stereotyped licking behavior. Results suggested that systemic apomorphine treatments increase aggressive motivation in hamsters. The increase in aggressive motivation may confuse the perception of the sensory signals that the males receive from the estrous females. They may haveperceived the estrous female as a nonestrous female which they would normally associate with an aggressive interaction (Lehman, Powers, & Winans, 1983).

Systemic apomorphine alters HPA axis responses to interleukin-1 beta adminstration but not sound stress

Apomorphine is a dopamine receptor agonist that was recently licensed for the treatment of erectile dysfunction. However, although sexual activity can be stressful, there has been little investigation into whether treatments for erectile dysfunction affect stress responses. We have examined whether a single dose of apomorphine, sufficient to produce penile erections (50 mug/kg, i.a.), can alter basal or stress-induced plasma ACTH levels, or activity of central pathways thought to control the hypothalamic-pituitary-adrenal axis in rats. An immune challenge (interleukin-1beta, 1 mug/kg, i.a.) was used as a physical stressor while sound stress (100 dB white noise, 30 min) was used as a psychological stressor. Intravascular administration of apomorphine had no effect on basal ACTH levels but did substantially increase the number of Fos-positive amygdala and nucleus tractus solitarius catecholamine cells. Administration of apomorphine prior to immune challenge augmented the normal ACTH response to this stressor at 90 min and there was a corresponding increase in the number of Fos-positive paraventricular nucleus corticotropin-releasing factor cells, paraventricular nucleus oxytocin cells and nucleus tractus solitarius catecholamine cells. However, apomorphine treatment did not alter ACTH or Fos responses to sound stress. These data suggest that erection-inducing levels of apomorphine interfere with hypothalamic-pituitary-adrenal axis inhibitory feedback mechanisms in response to a physical stressor, but have no effect on the response to a psychological stressor. Consequently, it is likely that apomorphine acts on a hypothalamic-pituitary-adrenal axis control pathway that is unique to physical stressors. A candidate for this site of action is the nucleus tractus solitarius catecholamine cell population and, in particular, A2 noradrenergic neurons. (C) 2003 Elsevier Science Ltd. All rights reserved.

Lesion of the subthalamic nucleus reverses motor deficits but not death of nigrostriatal dopaminergic neurons in a rat 6-hydroxydopamine-lesion model of Parkinson's disease

The objective of the present study was to determine whether lesion of the subthalamic nucleus (STN) promoted by N-methyl-D-aspartate (NMDA) would rescue nigrostriatal dopaminergic neurons after unilateral 6-hydroxydopamine (6-OHDA) injection into the medial forebrain bundle (MFB). Initially, 16 mg 6-OHDA (6-OHDA group) or vehicle (artificial cerebrospinal fluid - aCSF; Sham group) was infused into the right MFB of adult male Wistar rats. Fifteen days after surgery, the 6-OHDA and SHAM groups were randomly subdivided and received ipsilateral injection of either 60 mM NMDA or aCSF in the right STN. Additionally, a control group was not submitted to stereotaxic surgery. Five groups of rats were studied: 6-OHDA/NMDA, 6-OHDA/Sham, Sham/NMDA, Sham/Sham, and Control. Fourteen days after injection of 6-OHDA, rats were submitted to the rotational test induced by apomorphine (0.1 mg/kg, ip) and to the open-field test. The same tests were performed again 14 days after NMDA-induced lesion of the STN. The STN lesion reduced the contralateral turns induced by apomorphine and blocked the progression of motor impairment in the open-field test in 6-OHDA-treated rats. However, lesion of the STN did not prevent the reduction of striatal concentrations of dopamine and metabolites or the number of nigrostriatal dopaminergic neurons after 6-OHDA lesion. Therefore, STN lesion is able to reverse motor deficits after severe 6-OHDA-induced lesion of the nigrostriatal pathway, but does not protect or rescue dopaminergic neurons in the substantia nigra pars compacta.

The development of a rat model of erectile dysfunction after radical prostatectomy: preliminary findings

To develop a rat model of erectile dysfunction (ED) after cavernous nerve injury. Given the great similarity between the anatomical structure of the cavernous nerve in rats and humans, 24 rats underwent dissections and the cavernous nerves were identified with the aid of an operating microscope. Then the rats were randomized into two groups: sham-operated controls and a bilateral cavernous nerve section group. At 3 months after surgery, the rats were evaluated for their response to an apomorphine challenge. The erectile response after an apomorphine challenge was normal in all the control rats, while there were no erections in the bilateral injured group. The rat major autonomic ganglion and its cavernous nerve can be identified with the aid of a microscope. Rats are inexpensive and easy to handle, thus a good animal for developing an ED model of cavernous nerve injury. In the present study, the rats with cavernous nerve injury lost erectile capacity in a reliable and reproducible fashion. Because of the great similarity between the cavernous nerve of rats and humans, one may consider this technique as a reliable experimental model for studying ED after radical prostatectomy.