Sarah J. Spencer

Postnatal Inflammation Increases Seizure Susceptibility in Adult Rats

There are critical postnatal periods during which even subtle interventions can have long-lasting effects on adult physiology. We asked whether an immune challenge during early postnatal development can alter neuronal excitability and seizure susceptibility in adults. Postnatal day 14 (P14) male Sprague Dawley rats were injected with the bacterial endotoxin lipopolysaccharide (LPS), and control animals received sterile saline. Three weeks later, extracellular recordings from hippocampal slices revealed enhanced field EPSP slopes after Schaffer collateral stimulation and increased epileptiform burst-firing activity in CA1 after 4-aminopyridine application. Six to 8 weeks after postnatal LPS injection, seizure susceptibility was assessed in response to lithium–pilocarpine, kainic acid, and pentylenetetrazol. Rats treated with LPS showed significantly greater adult seizure susceptibility to all convulsants, as well as increased cytokine release and enhanced neuronal degeneration within the hippocampus after limbic seizures. These persistent increases in seizure susceptibility occurred only when LPS was given during a critical postnatal period (P7 and P14) and not before (P1) or after (P20). This early effect of LPS on adult seizures was blocked by concurrent intracerebroventricular administration of a tumor necrosis factor α (TNFα) antibody and mimicked by intracerebroventricular injection of rat recombinant TNFα. Postnatal LPS injection did not result in permanent changes in microglial (Iba1) activity or hippocampal cytokine [IL-1β (interleukin-1β) and TNFα] levels, but caused a slight increase in astrocyte (GFAP) numbers. These novel results indicate that a single LPS injection during a critical postnatal period causes a long-lasting increase in seizure susceptibility that is strongly dependent on TNFα.

Medial prefrontal cortex control of the paraventricular hypothalamic nucleus response to psychological stress: Possible role of the bed nucleus of the stria terminalis

The medial prefrontal cortex (mPFC) has been strongly implicated in control of the paraventricular nucleus of the hypothalamus (PVN) response to stress. Because of the paucity of direct projections from the mPFC to the PVN, we sought to investigate possible brain regions that might act as a relay between the two during psychological stress. Bilateral ibotenic acid lesions of the rat mPFC enhanced the number of Fos‐immunoreactive cells seen in the PVN after exposure to the psychological stressor, air puff. Altered neuronal recruitment was seen in only one of the candidate relay populations examined, the ventral bed nucleus of the stria terminalis (vBNST). Furthermore, bilateral ibotenic acid lesions of the BNST caused a significant attenuation of the PVN response to air puff. To better characterize the structural relationships between the mPFC and PVN, retrograde tracing studies were conducted examining Fos expression in cells retrogradely labeled with cholera toxin b subunit (CTb) from the PVN and the BNST. Results obtained were consistent with an important role for both the mPFC and BNST in the mpPVN CRF cell response to air puff. We suggest a set of connections whereby a direct PVN projection from the ipsilateral vBNST is involved in the mpPVN response to air puff and this may, in turn, be modulated by an indirect projection from the mPFC to the BNST. J. Comp. Neurol. 481:363–376, 2005.

Early life immune challenge- : effects on behavioural indices of adult rat fear and anxiety

Neonatal exposure to an immune challenge has been shown to alter many facets of adult physiology including fever responses to a similar infection. However, there is a paucity of information regarding its effects on adult behaviours. Male Sprague-Dawley rats were administered a single injection of the bacterial endotoxin lipopolysaccharide (LPS) at 14 days old and were compared, when they reached adulthood, with neonatally saline-treated controls in several behavioural tests of unconditioned fear and anxiety. There was no effect of the neonatal treatment on performance in either the elevated plus maze, modified Porsolts forced swim test or the open field test. However, neonatally LPS-treated rats did show significantly reduced exploration of novel objects introduced to the open field arena, indicating an effect of the neonatal immune challenge on behaviours relating to anxiety in the adult.

Early-life immune challenge: defining a critical window for effects on adult responses to immune challenge.

Many aspects of mammalian physiology are functionally immature at birth and continue to develop throughout at least the first few weeks of life. Animals are therefore vulnerable during this time to environmental influences such as stress and challenges to the immune system that may permanently affect adult function. The adult immune system is uniquely sensitive to immune challenges encountered during the neonatal period, but it is unknown where the critical window for this programming lies. We subjected male Sprague–Dawley rats at postnatal day (P)7, P14, P21, and P28 to either a saline or lipopolysaccharide (LPS) injection and examined them in adulthood for differences in responses to a further LPS injection. Adult febrile and cyclooxygenase-2 responses to LPS were attenuated in rats given LPS at P14 and P21, but not in those treated at P7 or P28, while P7-LPS rats displayed lower adult body weights than those treated at other times. P28-LPS rats also tended to display enhanced anxiety in the elevated plus maze. In further experiments, we examined maternal–pup interactions, looking at the mothers preference in two pup-retrieval tasks, and found no differences in maternal attention to LPS-treated pups. We therefore demonstrate a ‘critical window’ for the effects of a neonatal immune challenge on adult febrile responses to inflammation and suggest that there are other critical time points during development for the programming of adult physiology. We also show that the neonatal LPS effects on the adult immune system are not likely due to overt differences in maternal attention.

Neonatal immune challenge alters nociception in the adult rat

Abstract Intense pain or intense peripheral inflammation experienced during development can have pronounced effects upon adult pain sensation. However, little is known about the more commonly encountered mild systemic inflammation, such as that experienced with mild illness. Neonatal exposure to lipopolysaccharide (LPS), an established model of immune system activation, has been shown to affect febrile and cyclooxygenase‐2 (COX‐2) responses to a similar exposure in adulthood. Adult LPS also elicits a range of sickness behaviours, including enhanced responses to painful stimuli. We, therefore, hypothesized that adult sensation and pain responses could be affected by a neonatal LPS challenge. Male and female Sprague–Dawley rats were administered LPS at postnatal day 14 and were tested in adulthood for nociceptive responses to thermal and mechanical stimuli using, respectively, a plantar test apparatus and von Frey filaments, before and after adult LPS. Expression of dorsal root ganglion and lumbar spinal cord COX‐2 was also examined. Animals treated as neonates with saline showed the expected hypersensitivity to painful stimuli after adult LPS as well as enhanced spinal cord COX‐2. Neonatally LPS‐treated rats, however, showed a significantly different profile. They displayed enhanced baseline nociception and elevated basal spinal cord COX‐2 compared with neonatally saline‐treated rats. Also, rather than the expected hyperalgesia after adult LPS, no changes in nociceptive responses and a reduction in spinal cord COX‐2 expression were observed. These findings have important implications for the understanding of pain and its management and highlight the importance of the neonatal period in the development of pain pathways.

Early Life Activation of Toll-Like Receptor 4 Reprograms Neural Anti-Inflammatory Pathways

A single postnatal exposure to the bacterial endotoxin, lipopolysaccharide (LPS), reduces the neuroimmune response to a subsequent LPS exposure in the adult rat. The attenuated fever and proinflammatory response is caused by a paradoxical, amplified, early corticosterone response to LPS. Here we identify the mechanisms underlying the heightened corticosterone response to LPS in adults after early life exposure to LPS. In postnatal LPS-treated rats, hypothalamic corticotrophin-releasing hormone mRNA, pituitary proopiomelanocortin mRNA, and circulating adrenocorticotrophic hormone were all increased after adult exposure to LPS without significant modification to hippocampal or hypothalamic glucocorticoid receptor mRNA or protein or vagally mediated afferent signaling to the brain. Postnatal LPS administration did cause a persistent upregulation of the LPS Toll-like receptor-4 (TLR4) mRNA in liver and spleen, but not in brain, pituitary, or adrenal gland. In addition, cyclooxygenase-2 (COX-2), which is a prostaglandin biosynthetic enzyme and is normally undetectable in most peripheral tissue, was constitutively expressed in the liver. Adult immune activation of the upregulated TLR4 and COX-2 caused a rapid, amplified rise in circulating, but not brain, prostaglandin E2 that induced an early, enhanced activation of the hypothalamic-pituitary-adrenal (HPA) axis. Thus, postnatal LPS reprograms the neuroimmune axis by priming peripheral tissues to create a novel, prostaglandin-mediated activation of the HPA axis brought about by increased constitutive expression of TLR4 and COX-2.

Neonatal programming of innate immune function

The early life environment can be crucial in influencing the development of an animals long-term physiology. There is now much evidence to suggest that perinatal challenges to an animals 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 animals endocrine and metabolic physiology and the implications this has for various disease states.

Peripheral Inflammation Exacerbates Damage After Global Ischemia Independently of Temperature and Acute Brain Inflammation

Background and Purpose— Concomitant infection can exacerbate damage caused by cerebral ischemia. However, the interaction between and relative importance of the febrile and inflammatory components of the immune response is still unknown. Methods— Male Sprague-Dawley rats were subjected to a 2-vessel occlusion with hypotension, immediately followed by intraperitoneal injection of lipopolysaccharide or pyrogen-free saline. Results— Inflammation immediately after 2-vessel occlusion exacerbated hippocampal cell loss at 3 days and enhanced anxiety-related behaviors in the elevated plus maze and open field. These effects were not associated with differences in body temperature changes or with hippocampal pro-inflammatory cytokine production or hippocampal microglial activation. Conclusion— We show a previously undocumented dissociation between lipopolysaccharide-exacerbated damage after global ischemia in the rat and the temperature and acute brain immune response, indicating that the mechanism for enhanced lipopolysaccharide damage is hippocampal cytokine and temperature independent in this case.

Thalamic paraventricular nucleus lesions facilitate central amygdala neuronal responses to acute psychological stress

The thalamic paraventricular nucleus (PVT) is activated by stress and projects to forebrain structures directly implicated in processing stress-related information. Accordingly, it seems likely the PVT plays an important role in modulating stress responses. We examined effects of excitotoxic PVT lesions on forebrain Fos expression patterns normally elicited by an acute psychological stressor. PVT lesions significantly increased stress-induced Fos in a key stress-processing region, the central amygdala.

Rat neonatal immune challenge alters adult responses to cerebral ischaemia

Infection, inflammation, and hyperthermia associated with cerebral ischaemia are known to contribute to enhanced neuronal cell loss and more severe behavioural deficits. Because neonatal exposure to an immune challenge has been shown to alter the severity of inflammatory and febrile responses to a further immune challenge experienced in adulthood, we hypothesised that this could also alter temperature responses and neuronal survival after ischaemia. Thus, male Sprague–Dawley rats were treated at postnatal day 14 with a single injection of the bacterial endotoxin lipopolysaccharide (LPS) and were examined as adults for temperature changes, behavioural deficits, and neuronal cell loss associated with global cerebral ischaemia after a two-vessel occlusion (2 VO). Neonatally LPS-treated rats showed behavioural differences in a novel object exploration paradigm, as well as altered temperature responses to the 2 VO compared with neonatally salinetreated controls. Interestingly, these neonatally LPS-treated rats also showed increased cell loss in the central nucleus of the amygdala, a region that is important in the processing of emotional responses, but that is not usually examined in animal models of cerebral ischaemia. No differences were seen in the CA1, CA3, or dentate gyrus regions of the hippocampus. This work shows the importance of examining brain regions other than the hippocampus in association with global ischaemia. We also highlight the importance of the early period of development in programming an animals ability to deal with injury such as cerebral ischaemia in adulthood.