Differences in c-fos immunoreactivity due to age and mode of seizure induction

The aim of this study was to determine whether the regional distribition and time course of immunoreactivity to the c-fos protein varies with maturation and method of seizure induction. The effect of the two chemical convulsants, pentylenetetrazol (PTZ) and flurothyl, on the spatial and temporal pattern of c-fos-like immunoreactivity in immature (postnatal day (P) 10) was compared to that in adult rats. Patterns of c-fos-like immunoreactivity following O₂ deprivation were also evaluated at the 2 ages because hypoxia is acutely epileptogenic in immature animals but not adults. C-fos-like immunoreactivity was examined at 2, 4, and 6 h after onset of chemically induced seizures or O₂ deprivation at both ages. After PTZ or flurothyl seizures, both ages exhibited similar patterns of IR in amygdala, pyriform cortex, and hypothalamus. Age-dependent regional differences were most prominent in cortex: superficial layers of retrosplenial, cingulate, and neocortex stained in adults; staining was confined to deep layers of neocortex in P10 rats. Intense staining of dentate gyrus and hippocampus occurred with more prolonged seizures, but not brief seizures. PTZ administration resulted in staining at 2 h after seizure onset and was reduced by 4 h in adults, but immunoreactivity was not seen until 4 and 6 h after seizure onset in immature rats, indicating an age effect on the time course of IR. In immature rats, immunoreactivity patterns after hypoxia were markedly different from PTZ or flurothyl; staining was confined to layer VI of neocortex in these animals, and rarely involved limbic structures. These differences in the pattern of c-fos immunoreactivity suggest that the neuronal populations involved in epileptogenesis are influenced by age as well as seizure phenotype and intensity.     

Zif268 and Fos-like immunoreactivity in tetanus toxin-induced epilepsy: reciprocal changes in the epileptic focus and the surround

Altered gene expression for a number of molecules has been suggested as one of the underlying mechanisms of epileptogenesis. Changes in expression of the immediate early genes, zif268 and c-fos, were investigated in chronic focal epilepsy induced by tetanus toxin (TT, 20–35 ng) injected in the rat motor cortex. Most rats injected with TT and perfused on postoperative day 5, 7 or 14 had recurrent focal seizures after a latent period of 4–13 days, and showed enhanced Zif268 immunoreactivity in a cluster of neurons at the injection site, as well as reduced Zif268 immunoreactivity in a distinct cortical zone around this cluster. C-fos or Fos-related immunoreactivity was decreased over widespread areas of frontoparietal and piriform cortex in epileptic rats, except for a focus at the injection site which, in most cases, showed increases in Fos-like immunoreactivity. Some epileptic rats showed increased Zif268 immunoreactivity in neurons of the ipsilateral ventral lateral and central lateral thalamic nuclei and increased Zif268 and Fos-like immunoreactivity in the pontine nuclei. Rats perfused before onset of seizures, showed no overt changes other than a slight decrease in Zif268 and Fos-like immunoreactivity at the injection site. The reciprocal changes in Zif268 immunoreactive neurons in the epileptic focus and the immediate surround parallel changes in gene expression for a number of molecules important in epileptogenesis and suggest a state of functional disconnection of the epileptic focus from other cortical areas that may contribute to the development and maintenance of focal epilepsy.     

Effects of hypoxia-ischemia and seizures on neuronal and glial-like c-fos protein levels in the infant rat

Unilateral carotid ligation in immature rats, followed by 2 h of hypoxia led to ischemic cell change from 2 h after the insult, on the ligated side of the brain. There was a time-dependent induction of immunoreactive c-fos protein in neurones but not glia or ependyma on the non-ligated side of the brain. Induction only occurred in rats that had seizures post hypoxia-ischemia. In the ligated hemisphere c-fos protein was induced in glial-like cells in the corpus callosum, fornix/fimbria and internal capsule and in ependymal cells lining the lateral ventricle starting from 2 h after hypoxia but subsiding by 3 days. No neuronal c-fos induction was seen in areas showing neuronal damage. MK-801 or carbamazepine, which prevented hypoxia-ischemia-induced seizures, also prevented c-fos induction in the non-ligated hemisphere while MK-801 was associated with increased c-fos induction in hippocampal neurones from the ligated side, as well as in glial-like and ependymal cells.These results suggest several processes are involved following the hypoxic-ischemic insult. Firstly, severe hypoxia-ischemia is associated with a reduction in neuronal c-fos protein levels, probably as a result of neuronal failure and death. Secondly, post hypoxic seizures cause c-fos induction in surviving neurones. Thirdly, glial-like from regions in which there is neural loss also exhibit induction of c-fos, which may be important for their subsequent proliferation or for the production of growth factors.     

l-Dopa stimulates c-fos expression in dopamine denervated striatum by combined activation of D-1 and D-2 receptors

Administration of l-dopa to unilaterally 6-hydroxydopamine-lesioned rats, activates the early gene c-fos in the lesioned caudate-putamen. D-1 receptor blockade by SCH 23390, preventedl-dopa-induced Fos-like immunoreactivity in the whole caudate-putamen, while D-2 receptor blockade by raclopride reduced Fos-like immunoreactivity only in the dorso-lateral portion. The results suggest thatl-dopa induces c-fos primarily through an activation of D-1 receptors, while D-2 receptor stimulation plays a facilitatory influence on D-1 mediated c-fos expression.     

Effect of cortical cooling on interictal epileptiform activities

ObjectiveTo determine if applying chilled solution to exposed cerebral cortex can reduce interictal epileptiform activities in patients during surgery.MethodsElectrocorticography was used to record the epileptiform activity of 12 patients (ages 18–53) undergoing cortical mapping and resection surgery. Interictal spikes were counted at baseline and compared with spikes after applying room temperature and chilled Lactated Ringer's or normal saline solution.ResultsCortical irrigation with 150-cm³, chilled (4 °C) normal saline solution reduced the mean number of interictal spikes from 11.46 to 4.87 spikes per minute (p = 0.04). There was no significant reduction in the epileptic spike frequency when room temperature normal saline was used.ConclusionThe application of chilled solution directly to the cortex can reduce interictal epileptiform activities, suggesting that seizure potential can be suppressed to avoid evoked seizures during intraoperative surgery.     

Distinct mechanisms for expression of Fos-like immunoreactivity and synaptic potentiation in telencephalic hyperstriatum of the quail chick

In the intermediate and medial hyperstriatum ventrale (IMHV), a telencephalic region essentially involved in the initial processes of early learning tasks in poultry chicks, induction of an immediate early gene c-fos correlates significantly with the degree of learning (K.V. Anokhin, R. Mileusnic, I.Y. Shamakina, S.P.R. Rose, Effects of early experience on c-fos gene expression in the chick forebrain, Brain Res. 544 (1991) 101–107; B.J. McCabe, G. Horn, Learning-related changes in Fos-like immunoreactivity in the chick forebrain after imprinting, Proc. Natl. Acad. Sci. USA 91 (1994) 11417–11421). In slices of IMHV in vitro, on the other hand, tetanic stimulation at a low frequency induces a potentiation of synaptic responses (P.M. Bradley, B.D. Burns, A.C. Webb, Potentiation of synaptic responses in slices from the chick forebrain, Proc. R. Soc. Lond. B. 243 (1991) 19–24; T. Matsushima, K. Aoki, Potentiation and depotentiation of DNQX-sensitive fast excitatory synaptic transmission in telencephalon of the quail chick, Neurosci. Lett. 185 (1995) 179–182). In this study, we have examined a possible causal link between these two forms of activity-dependent processes, c-fos expression and synaptic potentiation. C-fos was visualized immunohistochemically using antibody raised against the Fos-protein, and potentiation was evaluated on the basis of field potential responses to local electrical stimulation. Tetanic stimulation (5 Hz×300 pulses) was required for potentiation, but not for c-fos expression. Conversely, a negative correlation appeared between them, and slices with relatively high density of Fos-like immunoreactive cells around the stimulation site failed to show potentiation. Furthermore, drugs similarly effective in blocking potentiation (such as AP5 (NMDA receptor antagonist) and bicuculline (GABA_A receptor antagonist)) had different effects on the c-fos induction. While AP5 had minor, if any, effects on c-fos expression, bicuculline enhanced it selectively around the site of stimulation. Our results suggest that these two processes are basically distinct, and could represent different aspects in the formation of memory traces in IMHV.     

Cycloheximide prevents kainate-induced neuronal death and c-fos expression in adult rat brain

The present study was directed at evaluating the possible involvement of protein synthesis in excitotoxin-induced neuronal damage and prolonged expression of the proto-oncogene, c-fos. Kainic acid-induced seizure activity elicited varying degrees of neuronal damage and cell loss in selectively vulnerable regions of the adult rat limbic system. Pretreatment with cycloheximide, a protein synthesis inhibitor, did not alter behavioral seizure characteristics, but markedly attenuated damage to susceptible neuronal populations. A prolonged increase in c-fos mRNA was observed byin situ hybridization up to 16 h after the onset of seizures in regions exhibiting neuronal death. Pretreatment with cycloheximide did not affect the transient induction of c-fos observed in numerous structures, but significantly reduced the prolonged expression of c-fos mRNA in kainatevulnerable regions. Despite producing massive seizure activity, systemic kainic acid administration during the early postnatal period did not induce any neuronal death, and did not result in prolonged c-fos expression in any brain structures. The developmental onset of selective neuronal vulnerability coincided with that of prolonged c-fos expression in susceptible neuronal populations. In adult rats, seizure activity induced by pentylenetetrazole did not produce neuronal damage nor did it produce prolonged c-fos expression. These results not only demonstrate that kainate-induced neurotoxicity and the prolonged expression of c-fos are both prevented by cycloheximide, but also strengthen the idea that prolonged c-fos expression is a marker of neuronal death.     

Epileptiform ictal discharges are prevented by periodic interictal spiking in the olfactory cortex

Interictal potentials are commonly observed between seizures in human epilepsies and in animal models of epilepsy. It is uncertain whether interictal spiking in partial epilepsies is causally related with the onset of an ictal discharge. To analyze the reciprocal correlation between interictal and ictal epileptiform events, we performed extracellular recordings in the limbic system of the in vitro isolated guinea pig brain preparation. Arterial perfusion of bicuculline (50 microM) in vitro consistently induced a focal ictal discharge in the hippocampal-entorhinal region that in one third of the experiments was associated with periodic interictal spikes in the piriform cortex. In the absence of active interictal spiking, the piriform cortex was secondarily invaded by the ictal discharge initiated in the hippocampal-entorhinal region, whereas no secondary ictal entrainment was observed in the presence of periodic piriform cortex spikes at circa 0.1 to 0.2 Hz. Similarly, ictal events never occurred when arterial perfusion of bicuculline was preceded by a local injection of the same drug in the piriform cortex, a procedure that induces a sustained interictal spiking. A reduced responsiveness to incoming paroxysmal discharges generated in the hippocampus was observed during the interval between two interictal spikes in the piriform cortex.     

Relationship between brain glucose metabolism positron emission tomography (PET) and electroencephalography (EEG) in children with continuous spike-and-wave activity during slow-wave sleep

We studied the relationship between brain glucose metabolism patterns and objectively measured interictal epileptiform abnormalities in six children with intractable epilepsy and continuous spike-and-wave activity during slow-wave sleep. Five of the six patients showed lateralized positron emission tomographic (PET) findings, with the hemisphere showing a relative increase in glucose metabolism concordant with the presumed origin of the generalized interictal spike activity delineated by quantitative electroencephalographic (EEG) analysis. One of these five patients achieved seizure freedom following cortical resection involving the areas of unilateral multifocal hypermetabolism, and another patient has been approved for cortical resection. The results in the present study add further support to the hypothesis that the generalized spike-waves in most cases of continuous spike-and-wave activity during slow-wave sleep are the result of secondary bilateral synchrony. Resective surgery can be effective in selected patients with uncontrolled seizures associated with continuous spike-and-wave activity during slow-wave sleep provided that there is concordance between focal abnormalities on PET and EEG.     

Pharmacologic evidence for abnormal thalamocortical functioning in GABA receptor beta3 subunit-deficient mice, a model of Angelman syndrome

PURPOSE: gamma-Aminobutyric acid receptor (GABA(A)r) subunit beta3-deficient mice model Angelman syndrome by displaying impaired learning, abnormal EEG with interictal spikes and slowing, myoclonus, and convulsions. The beta3-subunit deficiency causes a failure of intrathalamic reticular nucleus inhibition, leading to abnormally synchronized thalamocortical oscillations. We postulated that this pathophysiology underlies the abnormal cortical EEG and triggers interictal spikes and seizures, but extrathalamic regions also contribute to interictal spikes and seizures, so that the EEG slowing should reveal an absence-like response profile, whereas spikes and seizures have dual responsiveness to absence and partial-seizure drugs. METHODS: Recording electrodes were implanted over the parietal cortices of wild-type, heterozygotes, and homozygous null mice. In each experiment, EEG was recorded for 45 min, either drug or vehicle administered, and EEG recorded for another 3 h. Each EEG was scored for slow-wave activity, interictal spikes, and seizures by a reader blinded to treatments. RESULTS: Interictal spiking and percentage of time in EEG slowing in heterozygotes were increased by the proabsence drug baclofen (GABA(B)-receptor agonist), whereas CGP 35348 (GABA(B)-receptor antagonist) had the opposite effect. The antiabsence drug ethosuximide markedly suppressed EEG slowing and interictal spiking in heterozygote and null mice. Broad-spectrum clonazepam and valproate were more effective on interictal spiking than on EEG slowing, and fosphenytoin suppressed only interictal spiking. CONCLUSIONS: The results suggest that this model of Angelman syndrome, although not expressing typical absence seizures, is characterized by hypersynchronous thalamocortical oscillations that possess absence-like pharmacologic responsiveness and promote EEG slowing, interictal spikes, and convulsive seizures.     

Differential forebrain c-fos mRNA induction by ether inhalation and novelty: evidence for distinctive stress pathways

Previous studies indicate the existence of subtypes of stressors invoking distinct patterns of neuronal integration. Pathways activated by stress appear to depend on whether the perceived threat is processive/neurogenic or systemic in nature. To test this hypothesis, the present study compares magnitude and extent of c-fos mRNA induction in response to novelty (open field (OF), representing a processive stressor) or ether exposure (representing a systemic stressor). Exposure to the OF or ether fumes both produced increases in plasma corticosterone (CORT) levels; notably, peak levels of secretion were elevated in the ether group, suggestive of augmented HPA secretory activity to this stressor. In situ hybridization analysis of c-fos mRNA induction reveals common and divergent activational properties in the two stress groups. The extent of c-fos mRNA expression was similar in the paraventricular nucleus (PVN), despite stress-related differences in CORT secretion. Analysis of the dorsomedial hypothalamic nucleus, suprachiasmatic nucleus (SCN) and limbic sites, specifically, the lateral septum and medial amygdaloid nucleus, indicate greater c-fos mRNA induction in animals exposed to OF stress. The frontoparietal cortex was only forebrain region showing differential activation by ether. Differential c-fos induction was not observed in the medial preoptic area (ventrolateral quadrant), paraventricular thalamus, dorsolateral striatum or hippocampus. The results indicate that processive and systemic stressors differ not only in the patterning of neuronal activation in the CNS, but also in the extent to which selected stress-sensitive regions are induced.     

Correlation Between Interictal Regional Cerebral Blood Flow and Depth-Recorded Interictal Spiking in Temporal Lobe Epilepsy

Summary: Purpose: Single photon emission computed tomography (SPECT) is used as an adjunctive method in preoperative localization of epileptic foci. In temporal lobe epilepsy (TLE), interictal hypoperfusion is observed in 60–70% of cases. Correlation with ictal EEG changes is observed in ～50–60% of cases. Relationships with interictal EEG have been studied less. We compared interictal SPECT data obtained in 20 patients with their interictal intracerebral electrical activity recorded by depth electrodes to evaluate a potential relationship. Methods: We studied 20 sequential patients whose clinical, surface, and depth EEG data indicated seizure originating in the temporal lobe and who had interictal [^99mTc]hexamethyl-propylene amine oxime (HMPAO)-SPECT stereo-EEG (SEEG). Intracerebral electrodes were placed according to the patient's profile. The interictal extent of epileptiform activity allowed delineation of the irritative zone. Interictal spike frequency was also analyzed semiquantitatively. Visual and numerical SPECT analysis was performed blind to SEEG data. Results: Interictal hypoperfusion was observed in 16 patients, involving the epileptogenic temporal lobe in 14. Except for 1 patient who manifested lateral temporal hypoperfusion corresponding to a mass lesion, two distinctive patterns of hypoperfusion were noted: (a) mesial hypoperfusion (5 patients), and (b) global temporal hypoperfusion (8 patients). In 8 patients, hypoperfusion had also extended into the adjacent cortex. Temporal mesial hypoperfusion was associated with spiking limited to the mesial structures, whereas global temporal hypoperfusion or hypoperfusion extending beyond the temporal lobe was associated with a similar topographic pattern of spikes. Conclusions: Comparison between SPECT and SEEG data collected in the interictal phase indicated that the extent of the hypoperfused area correlated topographically with that of the underlying irritative zone.     

Fos induction in the medullary dorsal horn and C1 segment of the spinal cord by acute inflammation in aged rats

In order to elucidate the effect of aging on nociceptive neurons in the central nervous system, c-fos was used as a marker of excitability of neurons in the medullary dorsal horn (MDH) and the first spinal segment (C1) following noxious stimulation of the lateral face of young and aged rats. The distribution of c-fos-positive cells was dense in the superficial laminae and sparse in the deep laminae of the MDH and C1 in both young and aged animals following subcutaneous injection of formalin into the lateral face, whereas few c-fos-positive cells were labeled after saline injection. The distribution of c-fos-positive cells in the superficial laminae of the aged rats was found to be denser and more rostro-caudally expanded compared to that in the young rats. C-fos-positive cells were distributed more rostro-caudally in aged than in young rats. There was no difference between young and aged rats in the distribution of c-fos-positive cells in the deep laminae. Substance P (SP), 5-HT and calcitonin gene-related peptide-like immunoreactive (CGRP-LI) fibers and varicosities showed similar distribution density in the MDH and C1. Furthermore, many 5-HT-LI aberrant fibers and varicosities were observed in the MDH and C1 of the aged rats. The SP-LI and CGRP-LI cells in the trigeminal ganglion of aged rats were larger than those of young rats. These findings suggest that a deficit of the descending 5-HT inhibitory system produces the increment of c-fos-positive cells in the MDH and C1 of aged rats, resulting in the recruitment of a larger number of neurons in the superficial laminae of the MDH and C1 for conveying nociceptive sensory information to the central nervous system.     

Effect of locus coeruleus lesion on c-fos expression in the cerebral cortex caused by yohimbine injection or stress

The injection of the α-2 adrenoceptor antagonist, yohimbine, has been shown to increase c-fos immunoreactivity in the rat cerebral cortex. To determine the extent to which this response is mediated by the central noradrenergic system, the present studies examined it in rats previously given unilateral 6-OHDA lesions of the locus coeruleus. The lesions were found to produce a significant attenuation of the response. A similar effect on the c-fos immunoreactive response to restraint stress was found. It is concluded that the noradrenergic system plays a necessary role in the above c-fos responses in the cortex to yohimbine and to stress. The c-fos protein therefore appears to be involved in the effects of noradrenergic neurotransmission in the CNS.     

Sexual stimulation activates c-fos within estrogen-concentrating regions of the female rat forebrain

Regions of the brain that concentrate estrogen and progesterone are thought to regulate female sexual behavior by altering gene expression and neural sensitivity to afferent stimulation. We used immunocytochemistry and in situ hybridization to examine c-fos gene expression within estrogen-concentrating regions of the forebrain following various types of sexual stimulation with or without hormone treatment. Ovariectomized rats received injections of estradiol benzoate 48 h and progesterone 4 h before testing. Control rats that had been ovariectomized at least 5 months before testing did not receive hormone treatment. Rats were then either placed into bilevel testing chambers with sexually vigorous males, received manual stimulation of the flanks, received vaginocervical stimulation with a glass rod, or were left in their home cages. Copulation with intromission and ejaculation in hormone-treated rats, or stimulation of the vaginal cervix in both hormone-treated and control rats, produced a dramatic induction of c-fos mRNA and Fos-like immunoreactivity in estrogen-concentrating regions, such as the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, ventromedial hypothalamus, lateral habenula, and medial amygdala, in addition to regions that do not readily concentrate estrogen, such as the neocortex, thalamus, and striatum. Mechanical stimulation of the flanks produced a smaller induction of Fos in these rats, whereas hormone treatment alone had no effect. These data demonstrate that afferent sensory stimulation, but not estrogen or progesterone, regulates c-fos gene expression within different estrogen-concentrating and non-concentrating regions of the female rat forebrain.     

Expression of c-fos mRNA after cortical ablation in rat brain is modulated by basic fibroblast growth factor (bFGF) and the NMDA receptor is involved in c-fos expression

Expression of c-fos mRNA after cortical injury was studied using the in situ hybridization technique. Strong signals for c-fos mRNA were observed immediately after cortical ablation in neurons throughout the cortex ipsilateral to the injury. However, this c-fos mRNA expression was transient and disappeared within 6 h after the injury. When basic fibroblast growth factor (bFGF; 1 μg) was applied to the site of ablation, c-fos mRNA signals were observed for a much longer period. Even 24 h after injury, diffuse expression of c-fos mRNA was detected throughout the cortex, being mainly confined to non-neuronal cells. Intraperitoneal injection of MK-801 (3 mg/kg), a non-competitive NMDA receptor antagonist, suppressed the expression of c-fos mRNA after cortical ablation. It suppressed both the immediate and late expression induced by cortical ablation and bFGF. The immediate expression of c-fos in neurons is likely to be due to spreading depression, while neuronal-glial interactions would be involved in the mechanism of late c-fos expression by non-neuronal cells. Our results suggest that induction of c-fos after cortical injury can be modulated by topically applied bFGF and that the N-methyl-d-aspartate (NMDA) receptor is involved in c-fos expression not only caused by injury itself but also induced by injury and bFGF. As the immediate early genes regulate secondary gene responses, the induction of c-fos may contribute to neuronal plasticity and bFGF may enhance its effect.     

Association of c-fos mRNA expression and excitotoxicity in primary cultures of mouse neocortical and cerebellar neurons

The effect of excitatory amino acids (EAAs) on c-fos mRNA expression was studied in primary cultures of mouse cerebellar granule cells and in neocortical neurons after 2 and 7 days in vitro (div). In cultured granule cells at 2 and 7 div, and in cortical neurons at 2 div, exposure to low levels (≤10 μM) of a variety of EAAs (viz. glutamate [Glu], S-sulpho-L-cysteine [SC], N-methyl-D-aspartate [NMDA], α-amino-3-hydroxy-5-methyl-4-isoxazole [AMPA], and kainate [KA]) resulted in a transient increase in the level of c-fos mRNA which peaked at 30 min but returned to a basal level by 120 min. However, exposure of granule cells (7 div) to high levels (250 μM) of Glu, NMDA, KA, SC and of cortical neurons (7 div) to high levels (250 μM) of Glu, NMDA, KA, SC, or AMPA and to low levels (≤10 μM) of Glu and AMPA resulted in a delay in c-fos mRNA induction but a subsequent, progressive increase that was sustained for at least 240 min. Furthermore, this effect was accompanied by a dose-related increase in the release of the cytosolic enzyme, lactate dehydrogenase, used as an indicator of excitotoxicity. A ratio (Q^240/30) for the steady-state levels of c-fos mRNA after 30 min and 240 min of exposure to EAAs was determined which showed that Q^240/30 >2 correlated reproducibly with excitotoxic cell death, whereas a ratio of ≤1 correlated with a nonexcitotoxic event. In both cell types at 7 div, coadministration of the selective NMDA receptor antagonist, DL(±)-2-amino-5-phosphonopentanoic acid (APV) with cytotoxic levels of Glu 1) protected against EAA-induced neurotoxicity and 2) exhibited a transient c-fos mRNA expression (Q^240/30 values ≈1). In contrast, the AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), provided no protection against excitotoxicity and had no significant effect on the Glu-induced delay in c-fos mRNA expression. These results suggest that the Q^240/30 c-fos mRNA ratio may 1) be used as a predictive index for excitotoxic neuronal death, 2) provide information on the identity of the receptor subtype mediating excitotoxicity in different brain cell types, and 3) aid in establishing the role of excitotoxicity during the development of neurons in vitro. J. Neurosci. Res. 48:533–542, 1997. © 1997 Wiley-Liss Inc.     

Adenosine A2A receptor agonists increase Fos-like immunoreactivity in mesolimbic areas

Expression of the early-gene c-fos is an useful method for studying potential sites of action of drugs active in the CNS. Stimulation of adenosine A_2A receptors by CGS 21680 (5 mg/kg) induced an increase in Fos-like immunoreactivity in the rat nucleus accumbens shell, while in the rostral pole and core CGS 21680 induced Fos-like immunoreactivity only after a high dose. CGS 21680 (5 mg/kg) stimulated c-fos expression also in the lateral septal nucleus and dorso-medial striatum, but not in the dorso-lateral striatum. A similar pattern of Fos-like immunoreactivity was obtained after administration of the A_2A agonist HENECA (5 mg/kg) which displays higher selectivity for A_2A receptors than CGS 21680. Administration of the selective A_2A antagonist SCH 58261 counteracted CGS 21680-induced Fos-like immunoreactivity. Lesions of the dopaminergic mesostriatal projection by 6-hydroxydopamine and stimulation of dopamine D₂/D₃ receptors by quinpirole, prevented CGS 21680-induced Fos-like immunoreactivity in the nucleus accumbens shell. The present results show that stimulation of A_2A receptors induces a profile of c-fos expression similar to that of atypical neuroleptics. A_2A receptor stimulation has been reported to have dopamine antagonistic actions, it is therefore suggested that A_2A agonists might have antipsychotic activity without producing extrapyramidal side effects.