Castration reduces the effect of serotonin-1A receptor stimulation on prepulse inhibition in rats

This study examined the interaction between hormones and serotonin-1A (5-HT1A) receptor modulation of prepulse inhibition (PPI) of the acoustic startle response. Male and female rats were gonadectomized; some castrated rats received testosterone- or estrogen-filled implants. Rats were randomly injected with saline or 0.02 or 0.50 mg/kg 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist. All rats showed a dose-dependent disruption of PPI in response to 8-OH-DPAT. In untreated castrated rats, this disruption was significantly reduced (33% compared with 78% in sham-operated rats). Testosterone treatment reversed this reduction, but estrogen was less effective. Ovariectomized and sham-operated rats showed similar PPI in response to 8-OH-DPAT. These data suggest that the effect of 8-OH-DPAT on PPI in male rats depends on circulating hormone levels, particularly testosterone.

Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition

While an involvement of brain serotonin systems in schizophrenia has been suggested by many studies, the relative role of different serotonergic projections in the brain remains unclear. We therefore examined the effects of selective brain serotonin depletion on psychotropic drug-induced locomotor hyperactivity and prepulse inhibition, two animal models of aspects of schizophrenia. Pentobarbital-anesthetized (60 mg/kg, i.p.) male Sprague-Dawley rats were stereotaxically microinjected with 1 microl of a 5 microg/microl solution of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into either the dorsal or median raphe nucleus. At 2 weeks after the surgery, rats with dorsal raphe lesions did not show changes in psychotropic drug-induced locomotor hyperactivity, but displayed partial disruption of prepulse inhibition. In contrast, rats with median raphe lesions showed significant enhancement of phencyclidine-induced, but not amphetamine-induced locomotor hyperactivity and a marked disruption of prepulse inhibition. These results provide evidence for differential involvement of serotonergic projections in locomotor hyperactivity and prepulse inhibition. This study may help to explain the role of different serotonin projections in the brain in the pathophysiology of schizophrenia.

Impaired spatial reference memory in aromatase-deficient (ArKO) mice

Previous studies indicate an important role for estrogen in memory and learning. Aromatase-knockout (ArKO) mice are unable to produce estrogen because they lack a functional Cyp 19 gene that encodes for aromatase, the enzyme that converts testosterone into estrogen. Using a Y-maze test for short-term spatial reference memory, we found that both male and female ArKO mice performed significantly worse than wildtype controls. Gonadectomy reduced Y-maze responses in male and female wildtype controls, but had no effect in ArKO mice. After gonadectomy, there was no significant difference between wildtype and ArKO mice. For the first time using ArKO mice, our findings confirm the importance of estrogen in memory in both males and females.

Effect of paradoxical sleep deprivation and stress on passive avoidance behavior

Previous studies have shown that several types of stress can induce memory impairment. However, the memory effects of paradoxical sleep deprivation (PSD), a stressor in itself, are unclear. We therefore compared passive avoidance behavior of rats undergoing PSD and PSD stress yoked-control (PSC) using the "reversed flowerpot method." When rats were kept isolated on a PSC platform for 24 h immediately after criterion training, retention trials showed impaired aversive memory storage. When delayed for 24 h after criterion training, PSC stress did not disrupt retention performance. In rats subjected to PSD, either immediately or 24 h after criterion training, there was no disruption of aversive memory consolidation. These results suggest that, during stress, paradoxical sleep plays a role in erasing aversive memory traces, in line with the theory that we "dream in order to forget."

Reduced startle habituation and prepulse inhibition in mice lacking the adenosine A2A receptor

Adenosine and dopamine receptors interact in the CNS to modulate behaviour, including sensorimotor gating. Prepulse inhibition (PPI) has been suggested to be an operational measure of sensorimotor gating. PPI and startle habituation are disrupted in patients with schizophrenia. In experimental animals, both parameters are modulated by dopaminergic and adenosine receptor agonists and antagonists. In the present study, we measured PPI and startle habituation in mice that lack functional adenosine A(2A) receptors. Startle amplitudes, startle habituation and PPI were significantly reduced in mice homozygous null for the adenosine A(2A) receptor (A(2A)(-/-)). In addition, differential effects of amphetamine and MK-801 on startle amplitude, startle habituation and PPI were observed between A(2A)(-/-) and wildtype controls. These data support the involvement of adenosine A(2A) receptors in regulation of PPI and startle habituation.

Deficient prepulse inhibition of acoustic startle in Hooded-Wistar rats compared with Sprague-Dawley rats

1. Prepulse inhibition of acoustic startle has been suggested as a model of sensorimotor gating and central sensory information processing. Prepulse inhibition is impaired in patients with schizophrenia and responses can be restored by antipsychotic drug treatment. In the present study, startle and prepulse inhibition of startle were compared in different rat strains. 2. Sprague-Dawley rats showed robust inhibition of startle responses by increasing intensities of prepulse delivered just before the startle stimulus. In contrast, at both 4 and 10 weeks of age, rats of the Hooded-Wistar line had markedly reduced prepulse inhibition, although startle responses were not different. 3. Treatment with the dopamine receptor agonist apomorphine (0.1 mg/kg) or the N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 (0.1 mg/kg) caused disruption of prepulse inhibition in Sprague-Dawley rats. In Hooded-Wistar rats, apomorphine further reduced the already low level of prepulse inhibition, but MK-801 treatment had no significant effect. This suggests that the impaired prepulse inhibition in Hooded-Wistar rats could be caused by changes in glutamatergic activity and/or NMDA receptors in these rats. 4. In photocell cages, spontaneous exploratory activity and inner zone activity were significantly lower in Hooded-Wistar rats than in Sprague-Dawley rats. Similarly, on the elevated plus-maze, Hooded-Wistar rats showed a lower propensity to visit the open arms. In contrast, amphetamine (0.5 mg/kg)-induced locomotor hyperactivity, an animal model of psychosis, was enhanced in Hooded-Wistar rats. 5. These data suggest that the Hooded-Wistar line could be a useful genetic animal model to study the interaction of glutamatergic and dopaminergic mechanisms in anxiety and schizophrenia.

Acute effects of antipsychotic drugs on cardiovascular responses to stress

The effect of acute treatment with clozapine, risperidone and haloperidol on cardiovascular response to open field novelty stress was investigated in rats using radio-telemetry and video-tracking analysis. Pretreatment with clozapine dose-dependently inhibited the pressor response, tachycardia and increase in dP/dt and caused a marked reduction of exploratory locomotor activity. Similar effects were observed after risperidone treatment. Haloperidol treatment markedly reduced locomotor activity but its cardiovascular effects were limited to a more rapid return of heart rate towards baseline levels. These data suggest that particularly the atypical antipsychotic drugs, clozapine and risperidone, but not the typical antipsychotic, haloperidol, reduce cardiovascular stress responses, an effect that could reflect their anxiolytic action. Such anxiolytic effects could contribute to the beneficial clinical effects of atypical antipsychotic drugs in patients with schizophrenia.

Effect of atropine or atenolol on cardiovascular responses to novelty stress in freely-moving rats

Cardiac hemodynamic mechanisms involved in cardiovascular responses to stress were studied in conscious, freely-moving female spontaneously hypertensive rats exposed for 15 min to an open-field. When pretreated with saline, the rats displayed a rapid rise in blood pressure, heart rate, aortic dP/dt and locomotor activity. In rats pretreated with 0.5 mg/kg of methylatropine, the tachycardia was slightly, but significantly reduced. In rats pretreated with 1 mg/kg of atenolol, the tachycardis and rise in dP/dt were markedly reduced. These data suggest that the cardiac responses to stress include predominantly cardiac sympathetic activation and a minor component of vagal withdrawal.

Involvement of corticosterone in cardiovascular responses to an open-field novelty stressor in freely moving rats

The aim of the present study was to investigate the modulatory action of different concentrations of circulating corticosterone occupying either predominantly mineralocorticoid receptors (MR) or both MR and glucocorticoid receptors (GR) in control of cardiovascular responses to a novelty stressor. Six groups of rats were instrumented with radiotelemetry transmitters: sham-operated controls, adrenalectomised (ADX) controls, ADX with chronic implantation of a 20-mg corticosterone pellet, ADX with chronic implantation of a 100-mg corticosterone pellet, ADX receiving acute bolus injection of 0.25 mg/kg of corticosterone, and ADX with both implantation of a 20-mg corticosterone pellet and bolus treatment. Exposure to the novelty of an open field caused an increase in blood pressure, heart rate, body temperature and exploratory locomotor activity. The pressor response was dose-dependently increased in ADX rats implanted with a corticosterone pellet. Bolus injection of corticosterone at 10 min prior to novelty had no effect. The tachycardia was reduced in ADX rats compared to sham-operated controls, and this effect was restored by implantation of a 20-mg, but not 100-mg, corticosterone pellet. Bolus injection of corticosterone facilitated the return of heart rate towards baseline levels. The increase in body temperature was reduced in ADX rats, a deficit that was normalised by implantation of either corticosterone dose but not by acute bolus treatment. Locomotor activity was not different between the groups except for a slightly more rapid decline of locomotor activity in both groups treated with a bolus injection of corticosterone. These data show an important role of putative brain MR in maintaining adequate cardiovascular and behavioural responsiveness to a mild psychological stressor, while additional acute or chronic occupation of GR has further differential and sometimes opposing effects.