Highly activated c-fos expression in specific brain regions (ependyma, circumventricular organs, choroid plexus) of histidine decarboxylase deficient mice in response to formalin-induced acute pain

Identifying the In Vivo Cellular Correlates of Antipsychotic Drugs

GPCRs such as 5-HT2A and D2 are implicated in the therapeutic and the side effects of antipsychotics. However, the pattern of brain activity that leads to the behavioral effects of antipsychotics is poorly understood. To address this question, we used the transgenic 'FosTRAP' mice (Mus musculus), where a fluorescent reporter marks the cells responsive to the stimulus of interest. Here, the stimulus was an administration of various antipsychotic drugs. In case of typical antipsychotics such as Haloperidol, the c-fos active cells were predominantly found in the striatum, whereas in case of the atypical antipsychotics (Clozapine and Olanzapine), c-fos-induced cells were more numerous in the cortical regions, e.g., orbital cortex, piriform cortex. Curiously, we also observed ependymal cells to be a novel cellular target of atypical antipsychotics. 5-HT2A is considered to be a major target for atypical antipsychotics. Therefore, we bred 'FosTRAP' mice with 5-HT2A knock-out (KO) mice and tested their response to the prototype of atypical antipsychotics, Clozapine. Interestingly, the absence of 5-HT2A did not significantly affect the number of c-fos-induced cells in the cortical regions. However, the ependymal cells showed a dramatically reduced response to Clozapine in the absence of 5-HT2A. In summary, the TRAP system has allowed us to identify various region-specific activity induced by antipsychotics and novel cellular targets of the antipsychotics. These results serve as a "proof of principle" study that can be extended to explore the biochemical and physiological changes brought about by antipsychotics and specifically identify antipsychotic-responsive cells in the live tissue.

Brain histamine depletion enhances the behavioural sequences complexity of mice tested in the open-field: Partial reversal effect of the dopamine D2/D3 antagonist sulpiride

Markers of histaminergic dysregulation were found in several neuropsychiatric disorders characterized by repetitive behaviours, thoughts and stereotypies. We analysed the effect of acute histamine depletion by means of i. c.v. injections of alpha-fluoromethylhistidine, a blocker of histidine decarboxylase, on the temporal organization of motor sequences of CD1 mice behaviour in the open-field test. An ethogram encompassing 9 behavioural components was employed. Durations and frequencies were only slightly affected by treatments. However, as revealed by multivariate t-pattern analysis, histamine depletion was associated with a striking increase in the number of behavioural patterns. We found 42 patterns of different composition occurring, on average, 520.90 ± 50.23 times per mouse in the histamine depleted (HD) group, whereas controls showed 12 different patterns occurring on average 223.30 ± 20.64 times. Exploratory and grooming behaviours clustered separately, and the increased pattern complexity involved exclusively exploratory patterns. To test the hypothesis of a histamine-dopamine interplay on behavioural pattern phenotype, non-sedative doses of the D2/D3 antagonist sulpiride (12.5-25-50 mg/kg) were additionally administered to different groups of HD mice. Sulpiride counterbalanced the enhancement of exploratory patterns of different composition, but it did not affect the mean number of patterns at none of the doses used. Our results provide new insights on the role of histamine on repetitive behavioural sequences of freely moving mice. Histamine deficiency is correlated with a general enhancement of pattern complexity. This study supports a putative involvement of histamine in the pathophysiology of tics and related disorders.

Direct modulation of GFAP-expressing glia in the arcuate nucleus bi-directionally regulates feeding

Multiple hypothalamic neuronal populations that regulate energy balance have been identified. Although hypothalamic glia exist in abundance and form intimate structural connections with neurons, their roles in energy homeostasis are less known. Here we show that selective Ca²⁺ activation of glia in the mouse arcuate nucleus (ARC) reversibly induces increased food intake while disruption of Ca²⁺ signaling pathway in ARC glia reduces food intake. The specific activation of ARC glia enhances the activity of agouti-related protein/neuropeptide Y (AgRP/NPY)-expressing neurons but induces no net response in pro-opiomelanocortin (POMC)-expressing neurons. ARC glial activation non-specifically depolarizes both AgRP/NPY and POMC neurons but a strong inhibitory input to POMC neurons balances the excitation. When AgRP/NPY neurons are inactivated, ARC glial activation fails to evoke any significant changes in food intake. Collectively, these results reveal an important role of ARC glia in the regulation of energy homeostasis through its interaction with distinct neuronal subtype-specific pathways.

Histamine in migraine and brain

BACKGROUND

Histamine has been studied in both health and disease since the initial description a century ago. With its vasodilative effect, it was suggested early on to be involved in the pathophysiology of migraine. Over the past 25 years, much has been learned about histamine as a neurotransmitter in the central nervous system. The role of this neurotransmitter system in migraine has not been previously reviewed.

OBJECTIVE

Discuss a potential role of the brain histaminergic system in migraine.

METHODS

Unstructured literature search with a no specific hypothesis-driven approach.

RESULTS

There is substantial evidence that systemically given histamine may elicit, maintain, and aggravate headache. The mechanisms for this are not known, and histamines do not penetrate the blood-brain barrier (BBB). However, circulating histamine may influence hypothalamic activity via the circumventricular organs that lack BBB. In the rat, prolonged activation of meningeal nociceptors induced by dural mast cell degranulation has been observed. Subcutaneous injections of N-alpha-methyl histamine, a catabolite of histamine with high affinity to the histamine H3 receptor, probably have some migraine preventive effect. A negative feedback on histamine release from mast cells in proximity to C-fiber endings has been a postulated mechanism. Most antihistamines have shown to be ineffective as acute medication for migraine. Two centrally acting potent H1 receptor antagonists (cinnarizine and cyproheptadine) have been reported to be efficacious in preventing migraine. However, the proof for this is limited, and their efficacy has been ascribed other actions than the antihistaminergic. In general, lack of specificity and side effects limit the potential use of centrally acting H1 and H2 antagonists. Brain histamine is synthesized by neurons that are restricted to the posterior basal hypothalamus, more specific to the tuberomamillary nucleus (TMN), and that project practically to the whole central nervous system. The posterior hypothalamus is a suspected locus in quo in several primary headaches. Recently, a positron emission tomography study performed in the prodromal phase of migraine attacks supported the idea of initial involvement of this area. In another recent study, the thalamic nuclei receiving trigeminal output was also shown to have direct connections with the ventral TMN. The central histaminergic system plays an important role in the complex sleep-wake cycle, promoting cortical excitability during wakening and attention, and it consolidates the wake state. The period of the day, in the evenings and during the night, when there is reduced susceptibility for migraine attacks corresponds with less central histaminergic firing. Activation of both the H3 and the H4 receptor promotes inhibitory actions on neurons. The H3 receptor causes autoinhibition of the histaminergic neurons themselves, and centrally acting H3 receptor agonist prodrugs have shown to both inhibit neurogenic inflammation in dura, to induce sleep, and to produce antinociception. There are no registered ongoing studies on H3 and H4 receptor ligands in migraine.

CONCLUSION

The role of the central histaminergic system in migraine is largely unexplored, but findings from preclinical research may be linked to several aspects of the disorder. The histaminergic system of the brain may play an important role, especially in the initial phase of an attack, and histamine H3 and H4 receptor ligands may potentially have migraine prophylactic properties. However, the basis for this is still circumstantial, and the evidence is lacking.

N-(2-hydroxy phenyl) acetamide produces profound inhibition of c-Fos protein and mRNA expression in the brain of adjuvant-induced arthritic rats

Chronic pain and cognitive decline are characteristic symptoms of rheumatoid arthritis. One of the immediate early gene c-fos is overexpressed during peripheral and central noxious conditions and can be used as a marker for neuronal activity/excitability. In the adjuvant-induced arthritis Sprague-Dawley rat model, we examined the dynamics of c-Fos protein and mRNA expression in the amygdala, cortex, hippocampus, and thalamus and evaluated the effects of N-(2-hydroxy phenyl) acetamide (NA-2), a derivative of salicylic acid. The paw volume was assessed as an indicator of peripheral edema and the hyperalgesia associated with arthritis was monitored by gait analysis. The region of interests of the brain from arthritic and non-arthritic animals were used to isolate the RNA and were then reverse transcribed into cDNA. The PCR products were electrophoresed on 1% agarose gel and the gels were visualized in gel-doc system. The frozen brain sections were stained for c-Fos using immunohistochemistry. Negative control experiments were performed without the primary and secondary antibodies to rule out the nonspecific tissue binding of antibodies. We report a significant increase in the c-Fos expression in the arthritic control animals. In comparison to the control group, the treatment of NA-2 treatment was found to block the development of the arthritis-induced c-Fos protein and mRNA expression and peripheral edema. It also significantly reduces the gait deficits which were otherwise observed in the arthritic control group. Both the immunohistochemistry and PCR analysis revealed NA-2 to be more potent in comparison to member of non-steroidal anti-inflammatory drug.

Peripheral osmotic stimulation inhibits the brain's innate immune response to microdialysis of acidic perfusion fluid adjacent to supraoptic nucleus

During the brain's innate immune response microglia, astroglia and ependymal cells resolve/repair damaged tissue and control infection. Released interleukin-1beta (IL-1beta) reaching cerebroventricles stimulates circumventricular organs (CVOs; subfornical organ, SFO; organum vasculosum lamina terminalis, OVLT), the median preoptic nucleus (MePO), and magnocellular and parvocellular neurons in the supraoptic (SON) and paraventricular (PVN) nuclei. Hypertonic saline (HS) also activates these osmosensory CVOs and neuroendocrine systems, but, in contrast to IL-1beta, inhibits the peripheral immune response. To examine whether the brain's innate immune response is attenuated by osmotic stimulation, sterile acidic perfusion fluid was microdialyzed (2 microl/min) in the SON area of conscious rats for 6 h with sterile HS (1.5 M NaCl) injected subcutaneously (15 ml/kg) at 5 h. Immunohistochemistry identified cytokine sources (IL-1beta(+); OX-42(+) microglia) and targets (IL-1R(+); inducible cyclooxygenase, COX-2(+); c-Fos(+)) near the probe, in CVOs, MePO, ependymal cells, periventricular hypothalamus, SON, and PVN. Inserting the probe stimulated magnocellular neurons (c-Fos(+); SON; PVN) via the MePO (c-Fos(+)), a response enhanced by HS. Microdialysis activated microglia (OX-42(+); amoeboid/hypertrophied; IL-1beta(+)) in the adjacent SON and bilaterally in perivascular areas of the PVN, periventricular hypothalamus and ependyma, coincident with c-Fos expression in ependymal cells and COX-2 in the vasculature. These microglial responses were attenuated by HS, coincident with activating parvocellular and magnocellular neuroendocrine systems and elevating circulating IL-1beta, oxytocin, and vasopressin. Acidosis-induced cellular injury from microdialysis activated the brain's innate immune response by a mechanism inhibited by peripheral osmotic stimulation.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.