Neonatal programming of innate immune function.

The early life environment can be crucial in influencing the development of an animal's long-term physiology. There is now much evidence to suggest that perinatal challenges to an animal's immune system will result in changes in adult rat behavior, physiology, and molecular pathways following a single inflammatory event during development caused by the bacterial endotoxin lipopolysaccharide (LPS). In particular, it is now apparent that neonatal LPS administration can influence the adult neuroimmune response to a second LPS challenge through hypothalamic-pituitary-adrenal axis modifications, some of which are caused by alterations in peripheral prostaglandin synthesis. These pronounced changes are accompanied by a variety of alterations in a number of disparate aspects of endocrine physiology, with significant implications for the health and well-being of the adult animal. In this review, we discuss the newly elucidated mechanisms by which neonatal immune challenge can permanently alter an animal's endocrine and metabolic physiology and the implications this has for various disease states.

Viral-like brain inflammation during development causes increased seizure susceptibility in adult rats.

Viral infections of the CNS and their accompanying inflammation can cause long-term neurological effects, including increased risk for seizures. To examine the effects of CNS inflammation, we infused polyinosinic:polycytidylic acid, intracerebroventricularly to mimic a viral CNS infection in 14 day-old rats. This caused fever and an increase in the pro-inflammatory cytokine, interleukin (IL)-1beta in the brain. As young adults, these animals were more susceptible to lithium-pilocarpine and pentylenetetrazol-induced seizures and showed memory deficits in fear conditioning. Whereas there was no alteration in adult hippocampal cytokine levels, we found a marked increase in NMDA (NR2A and C) and AMPA (GluR1) glutamate receptor subunit mRNA expression. The increase in seizure susceptibility, glutamate receptor subunits, and hippocampal IL-1beta levels were suppressed by neonatal systemic minocycline. Thus, a novel model of viral CNS inflammation reveals pathophysiological relationships between brain cytokines, glutamate receptors, behaviour and seizures, which can be attenuated by anti-inflammatory agents like minocycline.

Profound hypothermia determines the anticonvulsant and neuroprotective effects of swim stress.

In contrast to most stressors that appear to be proconvulsant in nature, forced swimming (or swim stress) produces substantial anticonvulsant effects. Here we describe a series of experiments designed to identify the specific factors of swim stress (e.g., duration, swimming behavior, water temperature, and frequency of exposure) that are essential for the emergence of anticonvulsant effects in the rat. Our results revealed that the anticonvulsant effect of swim stress against lithium-pilocarpine convulsions occurred only when swim durations were at least 5 min in length and in water temperatures of 20 degrees C or less. Moreover, this anticonvulsant effect was not associated with habituation even after 10 days of repeated swimming. Treatment with lithium-pilocarpine coupled with 10 degrees C or 20 degrees C swim stress for 10 min caused pronounced hypothermia (10 to 15 degrees C reduction in body temperature) that required at least 12 h to return to baseline. One day after seizures were induced, swim stressed rats showed significantly fewer degenerating neurons in the hippocampus as revealed by Fluoro-Jade B staining. These results suggest that dramatically lowered body temperature could be the critical factor that produces the anticonvulsive and neuroprotective effects of swim stress.

Neonatal inflammation produces selective behavioural deficits and alters N-methyl-D-aspartate receptor subunit mRNA in the adult rat brain.

Peripheral inflammation causes production of central cytokines that alter transmission at the N-methyl-D-aspartate receptor (NR). During development, NRs are important for synaptic plasticity and network connectivity. We therefore asked if neonatal inflammation would alter expression of NRs in the brain and behavioural performance in adulthood. We gave lipopolysaccharide (LPS) (100 microg/kg, i.p.) or saline to male rats on postnatal day (P)5, P14, P30 or P77. Subsequently we assessed mRNA levels of the NR1, NR2A, B, C and D subunits in the hippocampus and cortex either acutely (2 h) or in adulthood using real-time reverse transcriptase-polymerase chain reaction. We explored learning and memory behaviours in adult rats using the Morris water maze and contextual fear conditioning paradigms. Hippocampal NR1 mRNA was acutely increased in the P5- and P77-treated rats but was reduced in adults treated with LPS at P5, P30 and P77. P14 LPS-treated rats showed few acute changes but showed pronounced increases in NR2A, B, C and D subunit mRNA later in adulthood. The cortex displayed relatively few acute changes in expression in the neonatal-treated rats; however, it showed robust changes in NR2B, C and D mRNA in all groups given LPS in adulthood. Behavioural deficits were observed specifically in the P5 and P30 LPS-treated groups in the water maze probe trial and fear conditioning tests, consistent with hippocampal NR1 mRNA down-regulation. Thus, a single bout of inflammation during development can programme specific and persistent differences in NR mRNA subunit expression in the hippocampus, which could be associated with behavioural and cognitive deficits in adulthood.

Lagged association between geomagnetic activity and diminished nocturnal pain thresholds in mice.

A wide variety of behaviors in several species has been statistically associated with the natural variations in geomagnetism. To examine whether changes in geomagnetic activity are associated with pain thresholds, adult mice were exposed to a hotplate paradigm once weekly for 52 weeks during the dark cycle. Planetary A index values from the previous 6 days of a given hotplate session were correlated with the mean response latency for subjects to the thermal stimulus. We found that hotplate latency was significantly (P < 0.05) and inversely correlated (rho = -0.25) with the daily geomagnetic intensity 3 days prior to testing. Therefore, if the geomagnetic activity was greater 3 days before a given hotplate trial, subjects tended to exhibit shorter response latencies, suggesting lower pain thresholds or less analgesia. These results are supported by related experimental findings and suggest that natural variations in geomagnetic intensity may influence nociceptive behaviors in mice.

Diverse physiological consequences of long-term sucrose consumption in female rats.

Although a considerable amount of work has addressed the short-term consequences of sucrose ingestion on a variety of morphological, physiological and hematological measures, relatively few studies have investigated these parameters following long-term sucrose intake. Results of the present experiments indicated that female rats given ad libitum access to a 10% sucrose solution for 9 months were conspicuously larger and had significantly heavier thyroid and spleen weights compared to rats offered either a restricted (20 min) daily sucrose option or control rats given only water. Rats given free access to sucrose also displayed abnormal serum levels of creatinine, sodium, phosphorus, alkaline phosphatase, alanine aminotransferase, uric acid and cholesterol which could indicate dysfunction in many organs including the kidney and liver. In the brain, however, chronic sucrose access was not associated with any discernable changes in neuronal or glial cell density within selected brain regions, but brain weight was found to be highly negatively correlated (r = - 0.97) with total sucrose intake. Finally, we report that high doses of estradiol can significantly attenuate the intake of sucrose in female rats. Together these findings demonstrate that free access to a sucrose solution for long periods can induce profound effects on rat physiology and may have important implications for the management of diet in humans.

Sucrose ingestion decreases seizure onset time in female rats treated with lithium and pilocarpine.

To extend previous work concerning diet and overt seizures in rats, we tested the hypothesis that ingestion of 10% sucrose-water could reduce seizure onset time (SOT) in rats given lithium and pilocarpine. We found that female but not male rats given free access to a 10% sucrose-water solution for 3 weeks exhibited shorter SOTs than age- and sex-matched control subjects. A separate experiment determined that SOT was significantly reduced whether female rats were provided 1, 2, 3, or 4 weeks of free access to sucrose. Moreover, the daily volume of sucrose ingested was significantly correlated (r=-0.42) with SOT regardless of the duration of sucrose treatment (in weeks). These findings suggest that a diet supplemented with sugar can facilitate the emergence of behavioral seizures in female rats given lithium and pilocarpine. We discuss the potential role of dopamine in mediating the sucrose-induced changes in SOT.

Alpha2-adrenergic inhibition prevents the accompanied anticonvulsant effect of swim stress on behavioral convulsions induced by lithium and pilocarpine.

There has been much debate regarding the potential influence of stress on epilepsy. Many studies have reported that stress can affect seizure susceptibility through eliciting either proconvulsant or anticonvulsant effects within the nervous system. In this study, we investigated the potential anticonvulsant effect of a 10-min swim stress on convulsions induced by a single systemic injection of lithium chloride followed 4 h later with pilocarpine. Rats pretreated with lithium chloride and exposed to a 10-min swim stressor prior to pilocarpine injection displayed a significant delay to seizure onset compared to unstressed rats or rats exposed to swim stress 10 min after lithium chloride, 2 h after lithium chloride, or immediately after pilocarpine injection. We then determined whether administration of a glucocorticoid antagonist (mifepristone; 10 or 50 mg/kg), an alpha(2)-adrenergic antagonist (yohimbine; 2 or 5 mg/kg), or a nonspecific opioid blocker (naloxone; 0.2 or 1 mg/kg) could prevent the anticonvulsant effect of swim stress. Only the high dose of yohimbine was capable of inhibiting the anticonvulsant effect of swim stress on lithium-pilocarpine seizures. Our findings highlight the importance of an endogenous noradrenergic-dependent anticonvulsant system in mediating the effects of swim stress on seizures. Further studies exploring the benefits of treatments with noradrenergic acting drugs in epilepsy is well warranted.

Geomagnetic activity during the previous day is correlated with increased consumption of sucrose during subsequent days: is increased geomagnetic activity aversive?

In five separate blocks over a period of several months for 33 female rats the amount of geomagnetic activity during the day before ad libitum access to 10% sucrose or water was positively correlated with the volume of sucrose consumed per 24-hr. period. The strength of the correlation (.62 to .77) declined over the subsequent 10 days from between .12 to -.18 and resembled an extinction curve. In a subsequent experiment four rats exposed to 5 nT to 8 nT, 0.5-Hz magnetic fields that ceased for 30 min. once every 4 hr. for 4 days consumed 11% more sucrose than the four rats exposed to no field. We suggest that the initial consumption of 10% sucrose may have been reinforced because it diminished the aversive physiological effects associated with the increased geomagnetic activity. However, over the subsequent days, as geomagnetic activity decreased or habituation occurred, negative reinforcement did not maintain this behavior.

Elevated nociceptive thresholds in rats with multifocal brain damage induced with single subcutaneous injections of lithium and pilocarpine.

To quantify the variability in thermal pain perception of rats with chemically induced brain injury following subcutaneous lithium and pilocarpine administration, 9 female Wistar rats were subjected to a nociceptive (hotplate) paradigm. At approximately 200 days of age, subjects were injected subcutaneously with 3 mEq/kg of lithium chloride followed 4 hr. later by 30 mg/kg of the cholinergic agonist pilocarpine to generate lesions that mimic human temporal lobe (limbic) epilepsy. Over 2 trials 4 of 9 subjects exhibited thermal latencies that exceeded 60 sec. while the remaining subjects obtained mean latencies of 13.40 sec. before demonstrating the criterion nociceptive response. These findings suggest that the multifocal neuronal necrosis subsequent to single peripheral injections of lithium and pilocarpine, followed by the neuroleptic, acepromazine, may significantly augment pain thresholds in certain rats within experimentally epileptic populations.

Chronic administration of the L-type calcium channel blocker nimodipine can facilitate the acquisition of sequence learning in a radial-arm maze.

Nimodipine, a dihydropyridine L-type voltage-gated calcium-channel blocker, was examined for its potential effect on the acquisition of a complex-arm sequence task in an automated radial maze. Young (60-day-old) male Wistar rats were injected with saline or nimodipine (5 mg/kg) 15 min prior to radial maze training, or immediately following the radial maze testing. The results of the learning task (over 7 days of testing) showed that rats injected with nimodipine each training session acquired the task more quickly and more efficiently compared to saline-treated animals. There were no significant differences for rats that were pre-/post-treated with nimodipine during the maze-learning task. The number of incorrect arm entries and number of additional lever presses in the same arm were found to be significantly lower in rats treated with nimodipine compared to saline-injected controls. The beneficial effect of nimodipine treatment occurred only in rats that were acquiring the task, and not in rats that had already learned the arm sequence paradigm. There were no potential non-specific influences on locomotor activity or appetite caused by chronic nimodipine treatments. These results strongly suggest that nimodipine can facilitate the acquisition of a complex learning task.

Learning and memory in agmatine-treated rats.

Agmatine, a noncompetitive N-methyl-D-aspartate (NMDA) antagonist, was examined for its role in water maze place learning, contextual and auditory-cued (discrete) fear learning and conditioned taste aversion learning, when administered systemically. Male Wistar rats were given saline or 1, 5, 10 or 50 mg/kg agmatine ip 20 min prior to or 30 min following daily training sessions in a hidden-platform (place learning) water maze task. Agmatine did not affect latencies to find the hidden platform or preference for the training quadrant during probe trials. When administered 20 min prior to contextual or auditory-cued fear-conditioning sessions, these doses of agmatine evoked a linear dose-dependent impairment in the magnitude of learned fear to the contextual stimuli when assessed during extinction trials 24 h later, but had no effect on the magnitude of learned fear to the auditory stimulus. Inferences of baseline motor activity and ability to respond to the presentation of footshock stimuli were not affected by the treatment. Injections of 50 mg/kg agmatine concurrently with a malaise-evoking agent following presentations to a novel sucrose solution abolished learned taste aversions; this agent did not evoke conditioned taste aversions alone. These studies indicate that systemically administered agmatine selectively impairs behavioral inferences of specific types of learning and memory.

Voluminous sucrose consumption in female rats: increased "nippiness" during periods of sucrose removal and possible oestrus periodicity.

In a two-bottle paradigm in which water and 10% sucrose water were always available, female rats drank about 200 cc of the sugar water (about 65 g of sucrose/kg) per day for 4 wk. There were no significant decreases in consumption over this time. In Exp. 2 female rats singly housed were given two bottles containing water for 1 wk. and then a bottle containing water and a bottle containing 15% sucrose for the next week for 6 wk. When sucrose was available, the rats ate 33% more rat chow. When sucrose was removed, the rats displayed more episodes of biting a stimulus when the food cubes were being removed for daily measurements. Some females exhibited a marked 4- to 5-day periodicity in sucrose (7.5%) consumption. The persistent and voluminous consumption of sucrose water and enhanced agonistic-like behavior during periods of withdrawal suggests the presence of a robust phenomenon with potential dinical applications to the challenge of addiction.