Fluoxetine prevents 8-OH-DPAT-induced hyperphagia in Fischer inbred rats.

Ovariectomized, Fischer rats were hormonally primed with 10 µg estradiol benzoate and 50 µg progesterone or were treated with the sesame seed oil vehicle. Food intake was measured 2 h and 24 h after treatment with 0.25 mg/kg of the 5-HT(1A) receptor agonist, (±)-8-hydroxy 2-(di-n-propylamino) tetralin (8-OH-DPAT), 5 mg/kg of the selective serotonin reuptake inhibitor, fluoxetine, or their combination. Consistent with prior studies, two hour food intake of rats given fluoxetine and 8-OH-DPAT did not differ from vehicle controls. 8-OH-DPAT-induced hyperphagia, evident at 2 h, was blocked by co-treatment with fluoxetine. However, in contrast to prior studies, 5 mg/kg fluoxetine, alone, had only modest effects on food intake. Differences in our experimental protocols and/or the strain of rat may account for the lower anorectic response to fluoxetine. Nevertheless, the absence of a significant response to fluoxetine, alone, coupled with the drug's attenuation of the hyperphagic effect of 8-OH-DPAT, leads to the suggestion that the behavioral response to the combined treatment is more complex than that of simple additivity. Consistent with this suggestion, 24 h food intake of rats given 8-OH-DPAT and fluoxetine was lower than that of vehicle or 8-OH-DPAT-treated rats.

Progesterone reduces the effect of the serotonin 1B/1D receptor antagonist, GR 127935, on lordosis behavior.

Ovariectomized rats were hormonally primed with 10 microg estradiol benzoate or with estradiol benzoate plus 500 microg progesterone. Rats received a bilateral infusion with 200 ng of the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1-1'-biphenyl-4-carboxamide hydrochloride (GR 127935), into the ventromedial nucleus of the hypothalamus (VMN), followed by a 5 min restraint or home cage experience. In estrogen-primed females that had experienced minimal handling between ovariectomy and use in the experiment, infusion with the water vehicle transiently inhibited lordosis behavior, and the 5-HT(1B/1D) receptor antagonist amplified this inhibition. There were no effects in rats hormonally primed with estrogen and progesterone. Handling for two days before the experiment reduced the effects of the infusions in estrogen-primed rats. However, when a 5 min restraint experience followed infusion with GR 127935, there was a significant decline in lordosis behavior that persisted for 10 to 15 min after the experience. Regardless of the prior experience or type of infusion, the addition of progesterone to the hormonal priming completely prevented the lordosis inhibition. These findings are consistent with prior evidence that progesterone protects against the inhibitory effects of a 5 min restraint experience on lordosis behavior. Moreover, these are the first experiments to demonstrate an inhibitory effect of a selective 5-HT(1B/1D) receptor antagonist in the VMN on lordosis behavior of estrogen primed, but not estrogen and progesterone primed, ovariectomized rats.

Modest effects of repeated fluoxetine on estrous cyclicity and sexual behavior in Sprague Dawley female rats.

In an earlier study, we reported that daily fluoxetine treatment (10 mg/kg/day) rapidly disrupted estrous cyclicity and sexual receptivity in adult, regularly cycling Fischer rats. The current study was designed to investigate if comparable fluoxetine treatment would similarly affect intact, regularly cycling Sprague Dawley rats. In the first experiment, fluoxetine was injected for 24 days. After 11-14 days of daily fluoxetine treatment, 40% of the rats showed a transient disturbance of the estrous cycle with elimination of sexual receptivity. In these affected rats, reduced sexual receptivity generally preceded disruption of vaginal cyclicity. In a second experiment, a shorter exposure was used to attempt to dissociate effects of fluoxetine on behavior and estrous cyclicity. Nine days of fluoxetine treatment eliminated sexual receptivity and proceptivity (hops/darts) in 40% and 46%, respectively, of rats without altering the estrous cycle. Female rats then received a 10th fluoxetine injection 30 min prior to assessment of sexual motivation (measured with the male preference paradigm). There was no effect of fluoxetine on male preference, but fluoxetine significantly reduced the number of crossings and seconds of grooming during preference testing. Therefore, effects of fluoxetine on estrous cyclicity and behavior of Sprague Dawley female rats were smaller and required longer to develop than previously reported in Fischer female rats. These findings reinforce a probable relationship between fluoxetine's effect on sexual activity and neuroendocrine disturbances and illustrate the importance of strain selection in attempting to model human disease.

Daily male exposure attenuates estrous cycle disruption by fluoxetine.

Fluoxetine (Prozac) produces sexual dysfunction in a substantial number of patients. In the few animal studies designed to address this sexual dysfunction in females, data have been inconsistent. Some investigators report that the drug disrupts sexual behavior without affecting the estrous cycle while we have reported robust effects of fluoxetine on the estrous cycle. The current studies were designed to initiate examination of procedural differences that may account for these contradictory outcomes. In the first experiment, intact, regularly cycling female rats were injected daily for 10 days with 10 mg/kg fluoxetine (intraperitoneally) or vehicle. Male-exposed, fluoxetine- or vehicle-treated rats were housed in a room with males and placed for 5 min/day into a male's cage. Other fluoxetine-treated females were housed in a room separate from males. In the second experiment, this protocol was repeated for 20 days and an additional group of females were exposed to male bedding for 5 min/day. Without male exposure, fluoxetine rapidly disrupted vaginal estrus and sexual receptivity so that approximately 50% of the rats failed to show vaginal estrus during the first 5 days; and the majority of the rats had a blocked cycle by 10 days of treatment. With male exposure, these reproductive effects were attenuated. The majority of rats cycled normally during the first 5 days of treatment and more than half cycled throughout the experiment. Loss of behavioral receptivity occurred even when normal estrous cyclicity was present. Although exposure to the male's bedding may have delayed the onset of estrous cycle disruption, five min daily exposure to a male's bedding did not prevent the disruptive effects of fluoxetine. These findings are consistent with evidence that fluoxetine's effect on female sexual dysfunction may result, in part, from the drugs' disruption of the hypothalamic-pituitary-gonadal axis. However, the data also evidence dissociation between the effects of fluoxetine on vaginal and behavioral estrus. These findings may also explain why different laboratories have reported the presence or absence of estrous cycle disturbances following daily treatment with fluoxetine.

The role of the selective serotonin reuptake inhibitor fluoxetine in temperature regulation in ovariectomized rat models.

Thermoregulation is an integrated network of neuroendocrine, autonomic and somatosensory responses. Thermoregulatory dysfunction occurs during fluctuations or decline of gonadal hormone levels and results in vasomotor symptoms such as hot flushes and/or night-time sweating. The neurotransmitter serotonin (5-HT), has been reported to play a role in thermoregulation via changes in extracellular 5-HT levels and/or activation of various 5-HT receptors. The purpose of this study was to evaluate the role of the selective 5-HT reuptake inhibitor (SSRI), fluoxetine (FLX), on temperature regulation using ovariectomized (OVX) rodent models of thermoregulation. Single, subcutaneous (s.c.) administration of FLX (3, 10, 30 and 60 mg/kg) dose-dependently reduced core body temperature (CBT). FLX at 3 and 10 mg/kg s.c. showed no statistically significant decrease on tail-skin temperature (TST), whereas at higher doses (30 and 60 mg/kg) a significant decrease in TST was noted in the telemetry model. To mimic chronic SSRI treatment, a 5-HT(1A) antagonist (WAY-100635; 0.3 mg/kg) was administered 20 min prior to FLX (10 mg/kg). This combination showed no significant improvement on temperature dysfunction compared to FLX alone. Similarly, in a morphine-dependent model of temperature dysfunction FLX, was inactive at 10 mg/kg whereas the 30 and 60 mg/kg s.c. dose abated the naloxone-induced increase in TST by 55 and 81%, respectively. In summary, FLX affected CBT at all doses, but alleviated thermoregulatory dysfunction only at higher doses that are non-selective for the 5-HT system.

Effects of the 5-HT2A antagonist mirtazapine in rat models of thermoregulation.

Thermoregulation is a complex intercommunicative function requiring coordination between core body temperature (CBT), the central nervous system, and peripheral vasculature. In menopausal women, dysregulation of thermoregulatory mechanisms leads to hot flushes and night sweats. A previous study in ovariectomized (OVX) rats has suggested that mirtazapine can alleviate thermoregulatory dysfunction by blocking 5-HT(2A) receptor signaling. This is in opposition to other work in which 5-HT(2A) receptor blockade appeared to exacerbate thermoregulatory dysfunction in OVX rats. Thus, the goals of the present study were to reexamine the effects of mirtazapine on temperature regulation in OVX rat models and explore further the role of 5-HT(2A) receptor blockade. Mirtazapine exhibited potent functional antagonism (EC(50)=0.62 nM) at the cloned human 5-HT(2A) receptor. In the morphine-dependent model of thermoregulatory dysfunction, mirtazapine (10 mg/kg, i.p.) induced an increase in tail-skin temperature (TST) prior to naloxone administration. In the telemetry model, mirtazapine (0.3-3 mg/kg, i.p.) caused an increase in TST. However, at the highest dose tested (10 mg/kg, i.p.), mirtazapine induced a small but significant decrease in TST followed by an increase in TST. To examine this finding further, mirtazapine's effect on CBT was determined. Administration of mirtazapine (1-3 mg/kg, i.p.) resulted in a slight decrease in CBT but at the 10 mg/kg dose a dramatic decrease (-3.6 degrees C) in CBT was observed. These data support the concept that 5-HT(2A) receptors play a role in temperature regulation but that functional blockade of these receptors by mirtazapine is not a likely mechanism for restoring thermoregulatory processes in OVX rats.

Caloric restriction increases neurotrophic factor levels and attenuates neurochemical and behavioral deficits in a primate model of Parkinson's disease.

We report that a low-calorie diet can lessen the severity of neurochemical deficits and motor dysfunction in a primate model of Parkinson's disease. Adult male rhesus monkeys were maintained for 6 months on a reduced-calorie diet [30% caloric restriction (CR)] or an ad libitum control diet after which they were subjected to treatment with a neurotoxin to produce a hemiparkinson condition. After neurotoxin treatment, CR monkeys exhibited significantly higher levels of locomotor activity compared with control monkeys as well as higher levels of dopamine (DA) and DA metabolites in the striatal region. Increased survival of DA neurons in the substantia nigra and improved manual dexterity were noted but did not reach statistical significance. Levels of glial cell line-derived neurotrophic factor, which is known to promote the survival of DA neurons, were increased significantly in the caudate nucleus of CR monkeys, suggesting a role for glial cell line-derived neurotrophic factor in the anti-Parkinson's disease effect of the low-calorie diet.

Effects of chronic intraputamenal infusion of glial cell line-derived neurotrophic factor (GDNF) in aged Rhesus monkeys.

In this study, 17-23 year old Rhesus monkeys were used as an early model of Parkinson's disease (PD). Four animals received chronic infusions of GDNF and four received vehicle infusions into the right putamen via programmable pumps for 8 weeks. Weekly videotaping was performed to record general motor performance and a monkey movement analysis panel (mMAP) was used to quantify fine and coarse upper limb motor performance. The GDNF-treated animals showed significant improvements in their overall motor performance in the last 3 weeks of the study compared to controls. Fine motor time of the upper limbs improved significantly in both the GDNF-treated and control animals. After 8 weeks of drug administration, the animals were euthanized and tissue punches were taken from the basal ganglia for measures of dopamine (DA) and DA metabolite levels. In the right putamen, GDNF infusion produced a 217% increase in homovanillic acid (HVA) levels. In addition, DA levels increased by 50% in the right caudate nucleus and there were 122 and 76% increases in 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the right and left caudate nucleus, respectively. HVA levels were also seen to be increased by 212% in the right caudate nucleus. Finally, changes were seen in the right globus pallidus, with 390 and 171% increases in DA and HVA levels, respectively. These data support the hypothesis that GDNF may be beneficial for the treatment of damaged or degenerating DA neurons in aged monkeys and possibly in aged humans.

Chronic, controlled GDNF infusion promotes structural and functional recovery in advanced parkinsonian monkeys.

The powerful trophic effects that glial cell line-derived neurotrophic factor (GDNF) exerts on midbrain dopamine neurones suggest its use in treating Parkinson's disease. However, some important questions remain about the possible therapeutic applications of GDNF. Here we demonstrate that the chronic infusion of 5 or 15 micro g/day GDNF into the lateral ventricle or the striatum, using programmable pumps, promotes restoration of the nigrostriatal dopaminergic system and significantly improves motor functions in rhesus monkeys with neural deficits modelling the terminal stages of Parkinson's disease. The functional improvements were associated with pronounced upregulation and regeneration of nigral dopamine neurones and their processes innervating the striatum. When compared with vehicle recipients, these functional improvements were associated with (i) >30% bilateral increase in nigral dopamine neurone cell size; (ii) >20% bilateral increase in the number of nigral cells expressing the dopamine marker tyrosine hydroxylase; (iii) >70 and >50% bilateral increase in dopamine metabolite levels in the striatum and the pallidum, respectively; (iv) 233 and 155% increase in dopamine levels in the periventricular striatal region and the globus pallidus, respectively, on the lesioned side; and (v) a five-fold increase in tyrosine hydroxylase-positive fibre density in the periventricular striatal region on the lesioned side. In addition, chronic GDNF treatment did not induce the side-effects generally associated with chronic administration of levodopa, the most widely used treatment for Parkinson's disease. Thus, the results suggest that the prolonged and controlled delivery of GDNF into the brain could be used to intervene in long-term neurodegenerative disease processes like Parkinson's disease. Additional studies are required to determine the potential differences between chronic, intraventricular and intraputamenal (or intranigral) delivery of GDNF to maximize the efficacy of infusion treatments.

How does the brain control lifespan?

There is generally a positive correlation between brain/body size ratio and lifespan, particularly among mammals, suggesting a role for the brain in determining lifespan. Recent studies in diverse organisms including nematodes, flies and rodents have provided evidence that, indeed the brain may control lifespan. Signaling pathways involved in both central nervous system and peripheral stress responses and regulation of energy metabolism may play important roles in lifespan determination. Indeed, genetic and environmental manipulations of these systems can greatly affect lifespan by changing levels of hormones that modulate energy metabolism, stress resistance and regenerative capacity of cells throughout the body. A signal transduction pathway in neurons involving receptors coupled to phosphatidylinositol-3-kinase, Akt and glycogen synthase kinase-3beta appears to play a key role in regulation of longevity by the brain. Mutations in genes that encode proteins in the insulin signaling pathway can increase lifespan in C. elegans and Drosophila, this signaling pathway in neurons in the brain may be particularly important in limiting lifespan. Dietary restriction results in the upregulation of brain-derived neurotrophic factor (BDNF) in the brain, which may increase the resistance of neurons to aging. Interestingly, BDNF signaling in the brain can increase peripheral insulin sensitivity, suggesting a mechanism whereby the brain can control lifespan. We speculate that during evolution the brain took on the task of monitoring and controlling peripheral energy metabolism, and thereby regulating lifespan in the context of food availability. Roles for other evolutionarily conserved brain signaling pathways in lifespan determination are likely to be discovered in the near future.