Decreased central histamine in the amygdaloid kindling rats

Histamine in the posterodorsal medial amygdala modulates cardiovascular reflex responses in awake rats

Centrally injected histamine (HA) affects heart rate (HR), arterial blood pressure (BP), and sympathetic activity in rats. The posterodorsal medial amygdala (MePD) has high levels of histidine decarboxylase, connections with brain areas involved with the modulation of cardiovascular responses, and is relevant for the pathogenesis of hypertension. However, there is no report demonstrating the role of the MePD histaminergic activity on the cardiovascular function in awake rats. The aims of the present work were: 1) to study the effects of two doses (10-100 nM) of HA microinjected in the MePD on basal cardiovascular recordings and on baroreflex- and chemoreflex-mediated responses; 2) to reveal whether cardiovascular reflex responses could be affected by MePD microinjections of (R)-alpha-methylhistamine (AH3), an agonist of the inhibitory autoreceptor H3; and, 3) to carry out a power spectral analysis to evaluate the contribution of the sympathetic and parasympathetic components in the variability of the HR and BP recordings. When compared with the control group (microinjected with saline, 0.3 microl), HA (10 nM) promoted an increase in the MAP50, i.e. the mean value of BP at half of the HR range evoked by the baroreflex response. Histamine (100 nM) did not affect the baroreflex activity, but significantly decreased the parasympathetic component of the HR variability, increased the sympathetic/parasympathetic balance at basal conditions (these two latter evaluated by the power spectral analysis), and promoted an impairment in the chemoreflex bradycardic response. Microinjection of AH3 (10 microM) led to mixed results, which resembled the effects of both doses of HA employed here. Present data suggest that cardiovascular changes induced by baroreceptors and chemoreceptors involve the histaminergic activity in the MePD. This neural regulation of reflex cardiovascular responses can have important implications for homeostatic and allostatic conditions and possibly for the behavioral displays modulated by the rat MePD.

Lesion of the tuberomammillary nucleus E2-region attenuates postictal seizure protection in rats

Postictal seizure protection (PSP) is an endogenous anticonvulsant phenomenon that follows an epileptic seizure and inhibits the induction of further seizures. The tuberomammillary nucleus (TM), located in the posterior hypothalamus, consists of five subregions and is the sole source of histaminergic neurons in the brain. To determine whether the TM is involved in PSP in rats, we tested the effects of bilateral electrolytic lesions of the TM E2-region on seizures induced by intermittent maximal electroshock (MES). The TM E2-region lesions significantly attenuated PSP during the intermittent MES procedure. Furthermore, intracerebroventricular injection of alpha-fluoromethylhistidine (100 microg), a selective and irreversible histidine decarboxylase inhibitor, mimicked the attenuation of PSP induced by the lesion of TM E2-region. In addition, neurochemical experiments revealed that the TM E2-region lesions markedly decreased basal histamine levels in the cortex, hippocampus, brainstem and hypothalamus, but had no significant effect on basal glutamate and GABA levels. Moreover, intermittent MES induced a persistent decrease of brain histamine levels in both sham-operated and lesioned rats. These results indicate that through its intrinsic histaminergic system, the TM may exert powerful inhibitory function during the intermittent MES procedure and actively participate in the mechanisms of PSP.

Influence of low dietary histamine on seizure development of chemical kindling induced by pentylenetetrazol in rats

AIM

To determine the role of dietary low histamine on the seizure development of pentylenetetrazol (PTZ)-induced kindling in rats.

METHODS

After 14 d of feeding on a low histamine diet (LH, containing 0.145 mumol/g of histamine), the rats were chemically kindled by repeated intraperitoneal injection of a subconvulsant dose of PTZ (35 mg/kg) once every 48 h, and seizure activity of kindling was recorded for 30 min. Histamine in brain samples was analyzed using a high performance liquid chromatography system with a fluorescence spectrofluorometer.

RESULTS

The LH diet induced an increase in seizure response (seizure susceptibility) to the first trial of PTZ, and resulted in facilitation of subsequent PTZ kindling process (seizure development). The histamine levels in the cortex, hippocampus, and hypothalamus of LH-treated rats decreased significantly and these changes correlated well with seizure behavior (r = 0.875, 0.651, and 0.796, respectively). In addition, chronic kindled seizures resulted in a significant increase of the histamine content in the cortex and hypothalamus in the LH-fed groups.

CONCLUSION

These findings indicate that the histamine in daily food could influence the brain histaminergic function, and play an important role in regulating seizure susceptibility.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Chemical kindling induced by pentylenetetrazol in histamine H(1) receptor gene knockout mice (H(1)KO), histidine decarboxylase-deficient mice (HDC(-/-)) and mast cell-deficient W/W(v) mice

The role of brain histamine on seizure development of pentylenetetrazol (PTZ)-induced kindling was examined in H(1)-receptor gene knockout (H(1)KO), histidine decarboxylase-deficient (HDC(-/-)) and mast cell-deficient (W/W(v)) mice. All H(1)KO, HDC(-/-) and W/W(v) mice had accelerated seizure development of PTZ-induced kindling when compared to their respective wild-type mice. The daily PTZ-kindling increased histamine content in the cortex and diencephalon of H(1)KO mice, whereas the histamine content in the diencephalon of W/W(v) mice was decreased. The present study indicates that histamine plays a suppressive role in seizure development through H(1)-receptors.

The role of central histaminergic neuron system as an anticonvulsive mechanism in developing brain

Experimental and epidemiological studies have indicated that central histaminergic neuron system plays an important role in inhibition of convulsive disorders through histamine H(1)-receptors, especially in developing period. Histamine H(1) antagonists increase the duration of electrically induced convulsions in 21-day-old mice, but not in 42-day-old mice. Epidemiological studies suggested that histamine H(1) antagonist may be one of the risk factors in febrile convulsions. In histidinemic patients who were considered to have high brain histamine content, the incidence of convulsions was lower than that of ordinary population.The centrally acting histamine H(1) antagonists including pyrilamine and ketotifen facilitate the development of amygdaloid kindling in rats, an experimental model of epileptogenic process. On the contrary, epinastine, a histamine H(1) antagonist which scarcely enters the brain, shows no facilitation. These findings suggest that the central histaminergic neuron system plays an inhibitory role on the seizure development through central histamine H(1)-receptors.Recently, three cases has been reported in which West syndrome developed 8-10 days after ketotifen or oxatomide administration. Considering experimental and clinical studies, histamine H(1) antagonists may be associated with West syndrome and may be hazardous to infants. Further careful experimental and clinical studies will be required to elucidate the relationships between West syndrome and central histaminergic neuron system.

[Studies on histamine with L-histidine decarboxylase, a histamine-forming enzyme, as a probe: from purification to gene knockout]

I have been studying the functions of the histaminergic neuron system in the brain, the location and distribution of which we elucidated with antibody raised against L-histidine decarboxylase (a histamine-forming enzyme) as a marker in 1984. For this purpose, we used two methods employing (1) pharmacological agents like alpha-fluoromethylhistidine, an HDC inhibitor, and agonists and antagonists of H1, H2 and H3 receptors and (2) knockout mice of the HDC- and H1- and H2-receptor genes. In some cases, we used positron emission tomography (PET) of H1 receptors in living human brains. It turned out that histamine neurons are involved in many brain functions, and particularly, histamine is one of the neuron systems to keep awakefulness. Histamine also plays important roles in bioprotection against various noxious or unfavorable stimuli (convulsion, nociception, drug sensitization, ischemic lesions, stress and so on). Finally, I briefly described interesting phenotypes found in peripheral tissues of HDC-KO mice; the most striking finding is that mast cells in HDC-KO mice are fewer in number, smaller in size and less dense in granule density than those of wild type mice, indicating that histamine is related to the proliferation and differentiation of mast cells. In conclusion, histamine is important not only in the central and peripheral systems as studied so far but also may be related to some new functions that are now under investigation in our laboratories.

Neuronal histamine release elicited by hyperthermia mediates tracheal dilation and pressor response

Whether brain histaminergic neurons contribute to the regulation of tracheal tone and peripheral vascular tone under hyperthermia was investigated in anesthetized rabbits. Histamine release from the rostral ventrolateral medulla (RVLM), the raphe nuclei, and the solitary nucleus of the medulla oblongata was significantly increased by hyperthermia. The increased histamine was significantly suppressed by 10−6 M tetrodotoxin microdialyzed in each area. Tracheal pressure and mean arterial pressure were significantly decreased and increased by hyperthermia, respectively. An H1-receptor antagonist, 5 × 10−6 M (+)-chlorpheniramine, bilaterally microdialyzed in the RVLM significantly enhanced histamine release in the RVLM as well as significantly suppressed tracheal dilation and pressor response caused by hyperthermia. These data indicate that histamine release in the medulla oblongata is enhanced by hyperthermia. The enhanced histamine is the neuronal origin and the cause of tracheal dilation and pressor response at least via H1 receptors in the RVLM. Brain histaminergic neurons play important roles in tracheal tone and peripheral vascular tone via H1 receptors in the RVLM and homeostasis on body temperature.

Modulation of audiogenic seizures by histamine and adenosine receptors in the inferior colliculus

Susceptibility to behaviorally similar audiogenic seizures (AGS) occurs genetically and is inducible during ethanol withdrawal (ETX). Comparisons between AGS mechanisms of genetically epilepsy-prone rats (GEPR-9s) and ethanol-withdrawn rats (ETX-Rs) are yielding information about general pathophysiological mechanisms of epileptogenesis. The inferior colliculus (IC) is the AGS initiation site. Excitatory amino acid (EAA) abnormalities in the IC are implicated in AGS, and histamine and adenosine receptor activation each reduce EAA release and inhibit several seizure types. Previous studies indicate that focal infusion of an adenosine receptor agonist into the IC blocked AGS in GEPR-9s, but the effects of adenosine receptor activation in the IC on AGS in ETX-Rs are unknown. The effects of histamine receptor activation on either form of AGS are also unexamined. The present study evaluated effects of histamine or a nonselective adenosine A(1) agonist, 2-chloroadenosine, on AGS by focal microinjection into the IC. Ethanol dependence and AGS susceptibility were induced in normal rats by intragastric ethanol. Histamine (40 or 60 nmol/side) significantly reduced AGS in GEPR-9s, but histamine in doses up to 120 nmol/side did not affect AGS in ETX-Rs. 2-Chloroadenosine (5 or 10 nmol/side) did not affect AGS in ETX-Rs, despite the effectiveness of lower doses of this agent in GEPR-9s reported previously. Thus, histamine and adenosine receptors in the IC modulate AGS of GEPR-9s, but do not modulate ETX-induced AGS. The reasons for this difference may involve the chronicity of AGS susceptibility in GEPR-9s, which may lead to more extensive neuromodulation as compensatory mechanisms to limit the seizures compared to the acute AGS of ETX-Rs.

Histamine H1 receptor binding capacities in the amygdalas of the amygdaloid kindled rat

The histamine H1 receptor binding capacity of the amygdalas of amygdaloid kindled rats was studied. In the kindled nonstimulated amygdala, significant decreases in K(D) and B(max) values compared with those of control amygdala were found 1 week after the last kindled seizure. One month after the last kindled seizure, the decreased K(D) value was sustained in the kindled nonstimulated amygdala. This decreased Bmax value 1 week after the last kindled seizure in nonstimulated amygdala may partly and transiently contribute to kindled seizure susceptibility. The decreased K(D) value in nonstimulated amygdala observed until 1 month after the last kindled seizure indicates the long-lasting increment of binding affinity of the pyrilamine binding site of the histamine H1 receptor in the steady state of kindled seizure susceptibility.