Joseph Rochford

DISSOCIATION BETWEEN BEHAVIORAL AND HORMONAL RESPONSES TO THE FORCED SWIM STRESS IN LACTATING RATS

Retention of immobility in the Porsolt forced swim test is believed to be dependent upon glucocorticoid secretion in male rats. Because lactating females exhibit increased basal glucocorticoid secretion and blunted stress responses, we tested the hypothesis that lactation‐induced changes in adrenal glucocorticoid and in circulating estrogen and progesterone levels would improve retention and/or acquisition of immobility. Immobility was recorded during 3 intervals of 5 min on day 1 (acquisition) and one 5 min interval 24 h later (retention). Blood samples were collected before the swim test and at various times after the onset of stress for plasma ACTH and corticosterone (B) determinations. Male rats (young = 200 g, old = 325 g) were compared to virgin females (V) and to lactating females in early (day 8‐10, EL) and late (day 17–19, LL) lactation. Adrenalectomy (ADX) and ovariectomy (OVX) were performed 5 and 10 days prior to testing, respectively.

Reactivity to novelty in cognitively-impaired and cognitively-unimpaired aged rats and young rats

Two distinct populations of aged, Long-Evans rats can be identified on the basis of performance in the Morris water maze task. Aged (24 month) unimpaired rats perform similarly to young (six month) animals. Aged, impaired rats display latencies to find the submerged platform greater than two standard deviations from the mean of the young animals. A hallmark of efficient cognitive processing is the ability to cope with environmental change. Consequently, the present studies were conducted to assess if aged, impaired animals display differential reactivity to repeated exposure to novel stimuli. Reactivity was assessed by examining the degree of (i) consumption of a novel gustatory/olfactory stimulus (sweetened milk), (ii) pain inhibition induced by exposure to a novel hot-plate (48.5 degrees C) apparatus and (iii) exploratory behaviour in an elevated plus maze and a novel open field. Aged, impaired rats exhibited lower milk consumption on day one and protracted reactivity (lower consumption over days two to eight) in comparison to aged, unimpaired and young animals. Aged, impaired rats were more reactive to novelty on the hot plate test (as indicated by longer paw lick latencies); this novelty-induced pain inhibition did not habituate in aged, impaired rats following repeated plate exposures. The degree of exploratory behaviour in both the plus maze and the open field was reduced in aged, impaired rats. This effect was not entirely a consequence of deficient affective mechanisms, as measures of anxiety (e.g., time in open arms, time in inner squares) were not different among aged impaired, aged unimpaired and young animals. These results are the first to demonstrate that behavioural deficits observed in aged, impaired animals extend beyond the impairments observed in the water maze. This behavioural profile is attributed, in part, to heightened anxiety. In addition, the impairments observed in aged, impaired animals may also reflect a reduced sensitivity to the positive incentive properties of novel stimuli.

NCAM-180 knockout mice display increased lateral ventricle size and reduced prepulse inhibition of startle.

NCAM-180 knockout mice, which have documented deficits in neural migration, were used to determine whether developmental abnormalities could lead to morphological changes and alterations in sensory motor gating mechanisms. Measurement of the lateral ventricle showed that NCAM-180−/− mice had marked increases in both the left and right anterior horns of the lateral ventricle. Furthermore, these mice also displayed a reduction of prepulse inhibition that was differentially affected by the dopamine agonist apomorphine. These results are discussed in light of the known increase in lateral ventricle size and reduction in prepulse inhibition that are seen in schizophrenia.

The interaction between prenatal stress and neonatal handling on nociceptive response latencies in male and female rats

Neonatal handling produces physiological and behavioral changes that persist into adulthood. These effects are opposite to those resulting from prenatal stress (PS). We examined the interaction between PS and handling on nociception in adult male and female rats. Randomly selected pregnant rats were subjected to restraint stress on days 13-17 of gestation for 25 min each day, or left undisturbed. At birth, selected stressed/nonstressed litters were assigned to be handled. handling consisted of 15 min of separation from the dam, once per day, from postnatal days 1-14. At 4 months of age, rats were placed on a 50 degrees C hot plate, and their latencies to paw lick were recorded. Prenatal stress and handling interacted to affect latencies in male rats. Handled (H)/PS rats had significantly lower paw lick latencies than nonhandled (NH)/PS rats (p < 0.05). However, handling had no effect on the male offspring of control dams. Handling elevated paw lick latencies in the female offspring of control dams, an effect that was most pronounced in diestrous vs. estrous rats. The NH/PS rats showed significantly elevated latencies compared to NH/NS rats (p < 0.05). These results suggest that handling effects on nociception are most apparent in rats subjected to PS; in males at least, these effects would otherwise not be present.

Cholinergic systems and long-term potentiation in memory-impaired apolipoprotein E-deficient mice

Impairments in cholinergic neurotransmitter systems of the basal forebrain are a hallmark of Alzheimers disease pathophysiology. The presence of the epsilon4 allele of apolipoprotein E was recently implicated as a major risk factor in both familial and sporadic Alzheimers disease. The present study examined the integrity of cholinergic and non-cholinergic systems in apolipoprotein E-deficient, memory-impaired mice. Choline acetyltransferase activity, hippocampal acetylcholine release, nicotinic and muscarinic (M1 and M2) receptor binding sites and acetylcholinesterase cell or terminal density showed no signs of alteration in either three-month or 9.5-month-old apolipoprotein E-deficient mice compared to controls. In contrast, long-term potentiation was found to be markedly reduced in these mice, but increases in the strength of stimulation induced the same level of long-term potentiation as that observed in controls. These alterations did not appear to be the consequence of modifications in the binding properties of glutamatergic receptors (N-methyl-D-aspartate and [RS]-alpha-amino-3-hydroxy-5-methylisoxazole propionic acid) but from defective regulation of the (RS)-alpha-amino-3-hydroxy-5-methylisoxazole propionic acid receptor by phospholipase A2 activity. These results support the notion that apolipoprotein E plays a fundamental role in neuronal plasticity, which could in turn affect cognitive performance through imbalances in extra- and intracellular lipid homeostasis.

Behavioral reactivity to aversive stimuli in a transgenic mouse model of impaired glucocorticoid (type II) receptor function: effects of diazepam and FG-7142

Abstract Transgenic mice with impaired type II-glucocorticoid receptor mediated feedback inhibition of hypthalamic-pituitary-adrenal activity were assessed in three different tests assessing behavioral reactivity to aversive stimuli, the elevated plus maze, the Thatcher-Britton novelty-conflict paradigm, and the startle paradigm. Transgenic mice more frequently entered and spent more time in the open arms of the elevated plus in comparison to B6C/3F1 mice. Transgenic mice took significantly longer to begin eating in the Thatcher-Britton novelty conflict paradigm, and displayed increased reactivity in the startle paradigm. Administration of 1 or 2 mg/kg diazepam reversed the behavioral effects observed in all three tests. Administration of the benzodiazepine receptor inverse agonist N-methyl-β-carboline-3 carboxamide (FG-7142, 10 mg/kg) reduced the ratio of open to total arm entries and the time spent in the open arms of the plus maze in transgenic, but not B6C/3F1, mice. This dose of FG-7142 did not influence performance of either strain in the Thatcher-Britton or startle paradigms. These results are discussed in terms of the hypothesis that the transgenic mice are more sensitive to the aversive properties of novel stimuli, and that they may have difficulty discriminating between signals of relative safety and danger.

Lactation-induced reduction in rats' acoustic startle is associated with changes in noradrenergic neurotransmission.

The acoustic startle response (ASR) with or without fear conditioning was compared between cycling (CYC) and lactating (LACT) female rats. ASR sensitivity to changes in endogenous noradrenergic (NA) release was examined using the alpha-2 NA receptor drugs yohimbine and clonidine. Groups of CYC and LACT females were also tested in the open field. ASR was reduced in all LACT, compared with that in CYC females. Both groups exhibited a robust response to fear conditioning and unpotentiated ASR subsequent to conditioning was increased in LACT females. The lowest dose of yohimbine significantly increased ASR in LACT females, but not in CYC females. Clonidine reduced ASR in both groups of females, with a greater potency in CYC females. In the open field, LACT females displayed a shorter latency to emerge, less freezing behavior, and more entries into the field than did CYC females. The authors concluded that (a) LACT females are less anxious in a novel environment and that decreased anxiety can be efficiently counteracted by fear conditioning, and (b) changes in NA neurotransmission contribute to lactation-induced modifications in ASR.

Apolipoprotein E knockout mice display procedural deficits in the Morris water maze: analysis of learning strategies in three versions of the task.

Apolipoprotein E knockout (apoEKO) mice have been shown to be impaired in the spatial Morris water maze (MWM). However, several groups failed to replicate this finding. One reason for this inconsistency may stem from variations in the experimental protocols and environment between laboratories. In the present study, we have tested if age and variations in protocol implementation that specifically affect salience of the visual extramaze cues influence performance and navigational strategies in the MWM. We tested three- and 12-month-old apoEKO and wild type mice in three versions of the MWM differing on the availability of visual extramaze cues: (1) salient cues, (2) diffuse cues, and (3) absence of cues. Our results show that the presence of salient cues enhances acquisition performance of wild type, but not apoEKO mice in the MWM. This effect was restricted to the acquisition phase since apoEKO mice reached a level of performance that was comparable to that of controls toward the end of the task. No significant differences were detected between apoEKO and controls in either the diffuse cues or absence of cues paradigms. Thigmotaxic tendencies were observed in apoEKO mice and correlated high latency scores. Thigmotaxis may have interfered with the initial ability to engage in a proficient navigational strategy. These findings suggest that, in contrast to what has been proposed in the past, apoEKO mice appear not to be impaired in spatial memory per se but are deficient in a procedural component of the MWM. Furthermore, the procedural deficit and corresponding thigmotaxic tendencies of apoEKO mice appeared to increase with age. Taken together, these findings confirm our hypothesis that age and variations in experimental protocols can influence MWM performances.

The cholesterol-lowering drug probucol increases apolipoprotein E production in the hippocampus of aged rats: implications for Alzheimer's disease.

Several recent epidemiological studies have proposed that cholesterol-lowering drug Statin may provide protection against Alzheimers disease (AD). Probucol is a non-Statin cholesterol-lowering drug and a potent inducer of apolipoprotein E (apoE) production in peripheral circulation. A recent clinical study using Probucol in elderly AD subjects revealed a concomitant stabilisation of cognitive symptoms and significant increases in apoE levels in the cerebral spinal fluid in these patients. To gain insight into the mechanisms underlying these effects, we treated a cohort of aged male rats (26-month-old) with oral dose of Probucol for 30 days. Specifically, we examined the effects of Probucol on apoE production and its receptors (low density lipoprotein receptor [LDLr] and low density lipoprotein receptor-related protein [LRP]), astroglial marker of cell damage (glial fibrillary acidic protein [GFAP]), markers of neuronal synaptic plasticity and integrity (synaptosomal associated protein of 25 kDa [SNAP-25] and synaptophysin) as well as cholesterol biosynthesis (3-hydroxy-3-methylglutaryl coenzyme A reductase [HMGCoAr]) in the hippocampus. We report that Probucol induces the production of apoE and one of its main receptors, LRP, increases HMGCoAr (rate-limiting enzyme in cholesterol synthesis), substantially attenuates age-related increases in glial activation, and induces production of synaptic marker SNAP-25, a molecule commonly associated with synaptogenesis and dendritic remodeling. These findings suggest that Probucol could promote neural and synaptic plasticity to counteract the synaptic deterioration associated with brain aging through an apoE/LRP-mediated system. Consistent with the beneficial effects of other cholesterol-lowering drugs such as the Statin, Probucol could also offers additional benefits based on apoE neurobiology.

Fluoxetine-induced increases in open-field habituation in the olfactory bulbectomized rat depend on test aversiveness but not on anxiety.

Little is known regarding the functional processes underlying the treatment efficacy of antidepressant drugs. Given the close association between stress, anxiety and depression, distinguishing the common and disparate features of these processes may contribute to our current understanding. Using the olfactory bulbectomized (OBX) rat, an animal model sensitive to a variety of antidepressant drugs, this study examined the effects of chronic fluoxetine administration on open-field behavior under different conditions of stressfulness (luminance) and compared the fluoxetine effects to those evoked by the anxiolytic lorazepam. Sham-operated and OBX rats received 21 daily injections of fluoxetine (10 mg/kg), one or seven injections of lorazepam (0.1 and 0.5 mg/kg) or vehicle prior to testing in the open field or plus maze. Time series data were collected and fit with exponential regression models to estimate behavioral reactivity, habituation and residual rate of responding. Relative to sham controls, OBX rats displayed increased locomotor activity in the high luminance open field but showed decreased activity in the lower luminance open field. Time series analysis revealed that while sham animals showed increased habituation in the high compared to lower luminance open field, OBX rats did not significantly modify their responding between the two conditions. Chronic fluoxetine treatment invoked rectifying effects in OBX animals only in the high luminance open field by increasing the rate of habituation. Both acute and subchronic administration of lorazepam also reduced OBX hyperactivity but did so only by decreasing the residual rate of responding. As expected, lorazepam administration significantly increased the ratio of open-to-total arm activity in the elevated plus maze. These findings suggest that OBX responding in the open field may be maladaptive, reflecting an inability to modify behavior appropriately in certain environmental contexts. Chronic antidepressant treatment enhances habituation of OBX animals only under more stressful or aversive conditions and appears to do so in a manner temporally distinct from anxiolytic treatment.