Noradrenergic regulation of immediate early gene expression in rat forebrain: differential effects of alpha 1- and alpha 2-adrenoceptor drugs

Interdependent adrenergic receptor regulation of Arc and Zif268 mRNA in cerebral cortex

Norepinephrine is a neurotransmitter that signals by stimulating the α1, α2 and β adrenergic receptor (AR). We determined the role of these receptors in regulating the immediate early genes, Activity Regulated Cytoskeleton Associated Protein (Arc) and Zif268 in the rat cerebral cortex. RX821002, an α2-AR antagonist, produced Arc and Zif268 elevations across cortical layers. Next we examined the effects of delivering RX821002 with an α1-AR antagonist, prazosin, and a β-AR antagonist, propranolol. RX821002 given with a prazosin and propranolol cocktail, or with each of these antagonists individually, decreased Arc and Zif268 to saline-treated control levels in most cortical layers. Arc and Zif268 levels were also similar to saline-treated control levels when rats were given a prazosin and propranolol cocktail alone, or when each of these antagonists were delivered individually. Taken together, these data reveal that α2-AR uniquely exert a tonic inibitory regulation of both Arc and Zif268 compared to α1 and β-AR. However, the ability of RX821002 to increase Arc and Zif268 is interdependent with α1 and β-AR signaling.

Changes in postnatal norepinephrine alter alpha-2 adrenergic receptor development

Alpha-2 adrenergic receptors (A2AR) regulate multiple brain functions and are enriched in developing brain. Studies demonstrate norepinephrine (NE) plays a role in regulating brain maturation, suggesting it is important in A2AR development. To investigate this we employed models of NE absence and excess during brain development. For decreases in NE we used N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), a specific noradrenergic neurotoxin. Increased noradrenergic terminal density was produced by methylazoxymethanol acetate (MAM) treatment. A2AR density was assayed with [(3)H]RX821002 autoradiography. DSP4 lesions on postnatal day (PND) 3 produce A2AR decreases in many regions by PND 5. A2AR recover to control levels by PND 15 and 25 and there is no further change in total receptor density. We also assayed A2AR in brains lesioned with DSP4 on PND 13, 23, 33 and 43 and harvested 22 days post-lesion. A2AR levels remain similar to control at each of these time points. We examined A2AR functionality and high affinity state with epinephrine-stimulated [(35)S]GTPγS and [(125)I]p-iodoclonidine autoradiography, respectively. On PND 25, control animals and animals lesioned with DSP4 on PND 3 have similar levels of [(35)S]GTPγS incorporation and no change in high affinity state. This is in contrast to increases in A2AR high affinity state produced by DSP4 lesions of mature brain. We next investigated A2AR response to increases in norepinephrine levels produced by MAM. In contrast to DSP4 lesions, increasing NE results in a large increase in A2AR. Animals treated with MAM on gestational day 14 had cortical [(3)H]RX821002 binding 100-200% greater than controls on PND 25, 35, 45, 55 and 65. These data indicate that NE regulation of A2AR differs in developing and mature brain and support the idea that NE regulates A2AR development and this has long term effects on A2AR function.

Swim stress excitation of nucleus incertus and rapid induction of relaxin-3 expression via CRF1 activation

Relaxin-3 (RLX3), a newly identified member of the relaxin peptide family, is distinguished by its enriched expression in GABA projection neurons of the pontine nucleus incertus (NI), which are postulated to participate in forebrain neural circuits involved in behavioural activation and stress responses. In this regard, corticotrophin-releasing factor-1 receptor (CRF(1)) is abundantly expressed by NI neurons; central CRF administration activates c-fos expression in NI; and various stressors have been reported to increase NI neuron activity. In studies to determine whether a specific neurogenic stressor would activate RLX3 expression, we assessed the effect of a repeated forced swim (RFS) on levels of RLX3 mRNA and heteronuclear (hn) RNA in rat NI by in situ hybridization histochemistry of exon- and intron-directed oligonucleotide probes, respectively. Exposure of rats to an RFS (10 min at 23 degrees C, 24 h apart), markedly increased RLX3 mRNA levels in NI at 30-60 min after the second swim, before a gradual return to basal levels over 2-4 h, while RLX3 hnRNA levels were significantly up-regulated at 60-120 min post-RFS, following a transient decrease at 30 min. Systemic treatment of rats with a CRF(1) antagonist, antalarmin (20 mg/kg, i.p.) 30 min prior to the second swim, blunted the stress-induced effects on RLX3 transcripts. Relative levels of RLX3-immunostaining in NI neurons appeared elevated at 3 h post-swim, but not at earlier time points (30-60 min). These results suggest that acute stress-induced CRF secretion can rapidly alter RLX3 gene transcription by activation of CRF(1) present on NI neurons. More generally, these studies support a role for RLX3 neural networks in the normal neural and physiological response to neurogenic stressors in the rat.

Differential effects of neonatal norepinephrine lesions on immediate early gene expression in developing and adult rat brain

Activity regulated cytoskeletal protein (Arc), c-fos and zif268 are immediate early genes (IEGs) important for adult brain plasticity. We examined developmental expression of these IEGs and the effect of neonatal noradrenergic lesion on their expression in developing and mature brain. N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4), a specific noradrenergic neurotoxin, was administered to rats on postnatal day (PND) 3 and in situ hybridization was used to assay Arc, c-fos and zif268 mRNA on PND 13, 25 and 60. In contrast to decreases in Arc, c-fos and zif268 expression produced by noradrenergic lesions of mature brain, lesions on PND 3 yield a strikingly different effect. Neonatal lesions produce increases in c-fos and zif268 expression in specific frontal cortical layers on PND 13, while Arc shows no change. These lesions lead to increases in zif268 expression in frontal cortical layers on PND 25, with no changes in c-fos or Arc expression, and on PND 60 they produce a significant increase in c-fos expression in hippocampus with no significant changes in Arc or zif268 expression. 2-[2-(2-Methoxy-1,4-benzodioxanyl)]imidazoline hydrochloride (RX821002), an alpha-2 adrenergic receptor (A2AR) antagonist, administered to control PND 60 animals produces elevations of Arc, zif268 and c-fos mRNAs. This response was eliminated in animals lesioned with DSP-4 on PND 3. These data indicate that norepinephrine regulation of IEG expression differs in developing and mature brain and that loss of developmental norepinephrine leads to abnormally high postnatal IEG expression. Previous studies have shown an important role for norepinephrine in brain development. Our data support the idea that norepinephrine plays an important role during CNS development and that changes in noradrenergic signaling during development may have long lasting effects, potentially on learning and memory.

Central alpha1-adrenergic system in behavioral activity and depression

Central alpha(1)-adrenoceptors are activated by norepinephrine (NE), epinephrine (EPI) and possibly dopamine (DA), and function in two fundamental and opposed types of behavior: (1) positively motivated exploratory and approach activities, and (2) stress reactions and behavioral inhibition. Brain microinjection studies have revealed that the positive-linked receptors are located in eight to nine brain regions spanning the neuraxis including the secondary motor cortex, piriform cortex, nucleus accumbens, preoptic area, lateral hypothalamic area, vermis cerebellum, locus coeruleus, dorsal raphe and possibly the C1 nucleus of the ventrolateral medulla, whereas the stress-linked receptors are present in at least three areas including the paraventricular nucleus of the hypothalamus, central nucleus of the amygdala and bed nucleus of the stria terminalis. Recent studies utilizing c-fos expression and mitogen-activated protein kinase activation have shown that various diverse models of depression in mice produce decreases in positive region-neural activity elicited by motivating stimuli along with increases in neural activity of stress areas. Both types of change are attenuated by various antidepressant agents. This has suggested that the balance of the two networks determines whether an animal displays depressive behavior. A central unresolved question concerns how the alpha(1)-receptors in the positive-activity and stress systems are differentially activated during the appropriate behavioral conditions and to what extent this is related to differences in endogenous ligands or receptor subtype distributions.

Role of CNS α1-adrenoceptor activity in central fos responses to novelty

The present study investigated, by use of fos immunohistochemistry, whether the functional activity of alpha(1)-adrenoceptors is elevated during heightened behavioral activity in brain regions shown earlier to contain motoric alpha(1)-receptors. In confirmation, marked c-fos responses that were blocked by an alpha(1)-antagonist (prazosin) were found in four of these brain regions (secondary motor, cingulate, piriform cortices, and nucleus accumbens) of animals exposed to a mildly novel environment (clean cage), which elicits a high degree of sustained exploratory activity. Experimental restriction of exploratory activity in the novel cage by a small enclosure did not reduce the fos responses in these areas, and in fact, enhanced gene expression when carried out in home-caged animals suggesting that the fos response may be more closely associated with the motivation to be active rather than activity itself. Experiments with locally administered alpha(1)-agonists and antagonists in the cortex by reverse dialysis showed that the above mentioned alpha(1)-dependent-fos responses were the result of activation of local alpha(1)-receptors in these brain regions. Unlike the aforementioned brain regions, the fos response of the locus coeruleus was not blocked by prazosin, and this nucleus also showed a marked fos increase to prazosin itself possibly as a compensatory response to the blockade of forebrain alpha(1)-receptors.

The role of norepinephrine in epilepsy: from the bench to the bedside

This article provides a brief review of the role of norepinephrine (NE) in epilepsy, starting from early studies reproducing the kindling model in NE-lesioned rats, through the use of specific ligands for adrenergic receptors in experimental models of epilepsy, up to recent advances obtained by using transgenic and knock-out mice for specific genes expressed in the NE system. Data obtained from multiple experimental models converge to demonstrate the antiepileptic role of endogenous NE. This effect predominantly consists in counteracting the development of an epileptic circuit (such as in the kindling model) rather than increasing the epileptic threshold. This suggests that NE activity is critical in modifying epilepsy-induced neuronal changes especially on the limbic system. These data encompass from experimental models to clinical applications as recently evidenced by the need of an intact NE innervation for the antiepileptic mechanisms of vagal nerve stimulation (VNS) in patients suffering from refractory epilepsy. Finally, recent data demonstrate that NE loss increases neuronal damage following focally induced limbic status epilepticus, confirming a protective effect of brain NE, which has already been shown in other neurological disorders.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Alpha-2 adrenergic receptor development in rat CNS: an autoradiographic study

During development norepinephrine plays a role in determining the morphologic organization of the CNS and the density and future responsiveness of adrenergic receptors. alpha-2 Adrenergic receptors, one of three adrenergic receptor types, regulate important adult CNS functions and may have a distinct role during development. We examined alpha-2 receptor distribution and density in the rat brain at postnatal days 1, 5, 10, 15, 21, 28 and in adults using the antagonist [(3)H]RX821002 for autoradiography. Binding kinetics and pharmacology for alpha-2 adrenergic receptors were the same in adults and neonates. There was an overall increase in alpha-2 receptor levels during postnatal development with great variability in pattern and timing of receptor density changes among brain regions. Three major patterns were apparent. First, in many regions receptor density increased during postnatal development, generally reaching adult levels around postnatal day 15. Within this group there was variability in timing between regions and there were several regions with receptor densities higher than adult levels during the postnatal period. Second, there were regions with very high levels of receptors at birth and little or no change in density during the postnatal period. Third, some regions demonstrated decreasing or transient expression of alpha-2 adrenergic receptors in the course of postnatal development, including white matter regions, cerebellum and many brainstem nuclei, suggesting specific roles for alpha-2 receptors during development. This study investigates the development of alpha-2 adrenergic receptors in the rat CNS. It demonstrates there is region-specific regulation of alpha-2 receptor development and identifies brain regions where these receptors may play a specific and critical role in the regulation normal development.

Protective effects of the alpha 2-adrenoceptor antagonist, dexefaroxan, against degeneration of the basalocortical cholinergic system induced by cortical devascularization in the adult rat

It has been hypothesized [Colpaert, F.C., 1994. In: Briley, M., Marien, M. (Eds.), Noradrenergic Mechanisms in Parkinson's Disease. CRC Press, Boca Raton, FL, pp. 225-254] that a deficiency in the noradrenergic system originating from the locus coeruleus is a decisive factor in the progression of central neurodegenerative disorders including Alzheimer's disease, and that treatments which boost noradrenergic transmission (e.g. via blockade of alpha(2)-adrenoceptors) could provide both symptomatic and trophic benefits against the disease. Studies in the rat in vivo demonstrating that the selective alpha(2)-adrenoceptor antagonist dexefaroxan increases acetylcholine release in the cortex, improves measures of cognitive performance and protects against excitotoxin lesions, support this concept. As a further test of the hypothesis, we investigated the effect of dexefaroxan in a rat model of unilateral cortical devascularization that induces a loss of the cortical cholinergic terminal network and a retrograde degeneration of the cholinergic projections that originate in the nucleus basalis magnocellularis. Lesioned and sham-operated rats received a 28-day subcutaneous infusion of dexefaroxan (0.63 mg/rat/day) or vehicle, delivered by osmotic minipumps implanted on the day of the cortical devascularization procedure. In lesioned rats, the dexefaroxan treatment was associated with a significantly higher number and size of vesicular acetylcholine transporter-immunoreactive boutons in comparison to the vehicle treatment; this effect was most marked within cortical layer V. Dexefaroxan also significantly reduced the atrophy of cholinergic neurons within the nucleus basalis magnocellularis. Dexefaroxan had no observable effect on any of these parameters in sham-operated cohorts. These results show that systemically administered dexefaroxan mitigates cholinergic neuronal degeneration in vivo, and provide further evidence for a therapeutic potential of the drug in neurodegenerative diseases such as Alzheimer's disease, where central cholinergic function is progressively compromised.

Differential modulatory effects of alpha- and beta-adrenoceptor agonists and antagonists on cortical immediate-early gene expression following focal cerebrocortical lesion-induced spreading depression

Unilateral, focal cerebrocortical lesion (FCL) and associated spreading depression (SD) increase immediate-early gene (IEG) expression throughout the ipsilateral hemisphere. Noradrenergic transmission is involved in the regulation of basal- and stimulation-induced expression of IEGs in cerebral cortex; and is modulated by both injury and SD. The present study further investigated the association between the noradrenergic system and cortical adaptive responses, by examining basal and FCL(SD)-induced cortical IEG expression following acute treatment with alpha(1)-, alpha(2)- and beta(1/2)-adrenoceptor (AR) agonists or antagonists. Activation of alpha(1)-ARs by NVI-085, or beta-ARs by salbutamol, increased cortical NGFI-A, c-jun and c-fos mRNA levels, whereas inhibition of alpha(1)-ARs by prazosin, or beta-ARs by propranolol, had no marked effect. The alpha(2)-AR agonists, clonidine and UK14304 also had no effect on basal IEG levels, while blockade of alpha(2)-ARs by methoxyidazoxan significantly increased NGFI-A and c-fos expression, but decreased c-jun mRNA levels. This latter effect confirms the complex and differential nature of IEG regulation in brain. In FCL(SD) rats, all AR agonists generally produced a supra-additive (synergistic) effect on expression of the examined IEGs, compared with drug-treatment or FCL alone. Prazosin reduced FCL(SD)-induced elevations of c-jun and c-fos, but not NGFI-A, mRNA. Methoxyidazoxan enhanced NGFI-A and c-fos mRNA expression after FCL(SD), but reduced c-jun. Propranolol enhanced all lesion-induced IEG levels. These results confirm that alpha(1)- and beta-ARs normally mediate a stimulatory, and alpha(2)-ARs a net inhibitory, influence on cortical cell activity (reflected by NGFI-A, c-fos expression); and demonstrate that alterations in noradrenergic tone modulate the level of cellular activation during and after SD, which is primarily elicited by K(+)/glutamate via NMDA receptors and Ca(2+)-associated mechanisms. In turn, noradrenergic transmission and interactions with excitatory systems are likely to be important in responses to brain injury, including regulation of IEGs and their downstream target genes.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.

The role of the locus coeruleus in the development of Parkinson's disease

In Parkinson's disease, together with the classic loss of dopamine neurons of the substantia nigra pars compacta, neuropathological studies and biochemical findings documented the occurrence of a concomitant significant cell death in the locus coeruleus. This review analyzes the latest data obtained from experimental parkinsonism indicating that, the loss of norepinephrine in Parkinson's disease might worsen the dopamine nigrostriatal damage. Within this latter context, basic research provided a new provocative hypothesis on the significance of locus coeruleus in conditioning the natural history of Parkinson's disease. In particular, the loss of a trophic influence of these neurons might be crucial in increasing the sensitivity of nigrostriatal dopamine axons to various neurotoxic insults. In line with this, recently, it has been shown that locus coeruleus activity plays a pivotal role in the expression of various immediate early genes and in inducing the phosphorilation of cyclic adenosine monophosphate response element-binding proteins, suggesting a role of the nucleus in sustaining a protective effect.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Differential spatiotemporal alterations in adrenoceptor mRNAs and binding sites in cerebral cortex following spreading depression: selective and prolonged up-regulation of alpha1B-adrenoceptors

Noradrenaline, an important transmitter in the CNS, is involved in cerebral plasticity and functional recovery after injury. Experimental brain injury, including KCl application onto the brain surface, induces a slow-moving cortical depolarization/depression wave called cortical spreading depression (CSD). Interestingly, CSD does not produce neuronal damage but can protect cortical neurons against subsequent neurotoxic insults, although the mechanisms involved are unknown. This study examined the status of alpha- and beta-adrenoceptors (ARs) in cerebral cortex following CSD. Anesthetized rats had unilateral CSD induced by a 10-min topical application of KCl to the frontoparietal cortex and were killed at various times thereafter. Levels of alpha1-, alpha2-, beta1-, and beta2-AR mRNA and binding were examined using in situ hybridization histochemistry and radioligand autoradiography. Levels of alpha1b-AR mRNA in the affected neocortex were significantly increased by 20-40% at 1, 2, and 7 days (P </= 0.01) compared with contralateral levels, but were not significantly above control values at 2 and 4 weeks after CSD induction. Cortical alpha1B-AR binding sites were also increased by 45-65% 1 and 2 weeks (P </= 0.01) after CSD in a similar, but delayed, profile to alpha1b-AR mRNA. CSD rapidly increased beta1-AR mRNA by 45% at 1 h (P </= 0.01) and produced a delayed decrease of 25% in alpha2a-AR mRNA at 2 days and 1 week (P </= 0.05), but had no effect on corresponding levels of binding sites. In contrast, CSD had no effect on the remaining AR-subtype mRNAs or binding levels in neocortex under identical conditions. These results reveal a long-term up-regulation of alpha1B-ARs induced by an acute cortical stimulation/depression. Subtype-selective responses of ARs to CSD reflect an important differential regulation of expression of each receptor in vivo and suggest that alpha1B-ARs are particularly likely to be involved in cortical adaptive responses to physical injury at both local and distant locations.

Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Noradrenergic drugs, acting on alpha adrenoceptors, have been found to play an important role in the initiation and modulation of locomotor pattern in adult cats after spinal cord transection. There are at least two subtypes of alpha adrenoceptors, alpha1 and alpha2 adrenoceptors. The aim of this study was to investigate the effects of selective alpha1 and alpha2 agonists in the initiation and modulation of locomotion in adult chronic cats in the early and late stages after complete transection at T13. Five cats, chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used in this study. Noradrenergic drugs including alpha2 agonists (clonidine, tizanidine, and oxymetazoline) and an antagonist, yohimbine, one alpha1 agonist (methoxamine), and a blocker, prazosin, as well as norepinephrine were injected intrathecally. EMG activity synchronized to video images of the hindlimbs were recorded before and after each drug injection. The results show differential effects of alpha1 and alpha2 agonists in the initiation of locomotion in early spinal cats (i.e., in the first week or so when there is no spontaneous locomotion) and in the modulation of locomotion and cutaneous reflexes in the late-spinal cats (i.e., when cats have recovered spontaneous locomotion). In early spinal cats, all three alpha2 agonists were found to initiate locomotion, although their action had a different time course. The alpha1 agonist methoxamine induced bouts of nice locomotor activity in three spinal cats some hours after injection but only induced sustained locomotion in one cat in which the effects were blocked by the alpha1 antagonist prazosin. In late spinal cats, although alpha2 agonists markedly increased the cycle duration and flexor muscle burst duration and decreased the weight support or extensor activity (effects blocked by an alpha2 antagonist, yohimbine), alpha1 agonist increased the weight support and primarily the extensor activity of the hindlimbs without markedly changing the timing of the step cycle. Although alpha2 agonists, especially clonidine, markedly reduced the cutaneous excitability and augmented the foot drag, the alpha1 agonist was found to increase the cutaneous reflex excitability. This is in line with previously reported differential effects of activation of the two receptors on motoneuron excitability and reflex transmission. Noradrenaline, the neurotransmitter itself, increased the cycle duration and at the same time retained the cutaneous excitability, thus exerting both alpha1 and alpha2 effects. This work therefore suggests that different subclasses of noradrenergic drugs could be used to more specifically target aspects of locomotor deficits in patients after spinal injury or diseases.

Alpha2-adrenoreceptor agonist, dexmedetomidine, alters acute gene expression after global ischemia in gerbils

The effect of dexmedetomidine, a selective alpha2-adrenoreceptor agonist and neuroprotective agent on the expression of immediate early genes and heat shock protein hsp70, was studied using quantitative in situ hybridization in a global ischemia model. At the dose previously shown to be neuroprotective dexmedetomidine inhibited the expression of c-fos and hsp70 mRNA, did not affect jun-B mRNA, and enhanced the induction of NGFI-A mRNA in the postischemic gerbil hippocampus. The reduced gene expression of c-fos and hsp70 was detected in the CA1 pyramidal cells which are prone to ischemic degeneration, whereas the increased gene expression of NGFI-A was measured from the CA3 and dentate gyrus, areas relatively resistant to ischemia. These alterations in early gene expression possibly reflect the mechanisms mediating the neuroprotective effects of alpha2-adrenoreceptor agonists.

Neuronal activation in the forebrain following electrical stimulation of the cuneiform nucleus in the rat: hypothalamic expression of c-fos and NGFI-A messenger RNA

Forebrain neuronal connections associated with the cardiovascular response to unilateral, low-intensity, electrical stimulation of the mesencephalic cuneiform nucleus were examined in the halothane-anesthetized and paralysed rat by in situ hybridization histochemistry using specific 35S-labelled oligonucleotides for detection of c-fos and nerve growth factor inducible-A gene (NGFI-A) messenger RNAs. Stimulation of the cuneiform nucleus led to increases in mean arterial pressure and heart rate, whereas no cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals [see concurrent mid- and hindbrain study [Lam W. et al. (1996) Neuroscience 71, 193-211]. Cuneiform nucleus stimulation was associated with increased c-fos and NGFI-A messenger RNA levels bilaterally in the ventromedial, dorsomedial and lateroanterior hypothalamic nuclei, lateral and anterior hypothalamic areas, and ipsilaterally in the medial amygdaloid nucleus, at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. C-fos, but not NGFI-A, messenger RNA expression was increased bilaterally in the piriform cortex and subparafascicular thalamic nucleus. These results are consistent with the existence of direct and indirect projections between the cuneiform nucleus and the aforementioned activated areas, the functions of which may include the control of reproduction and metabolism, as well as cardiovascular regulation. The ipsilateral nature of responses in certain brain areas may be explained by the absence of decussating pathways and/or the presence of multisynaptic connections which attenuate bilateral signal transmission. The existence of structures that are known to receive afferent projections from the cuneiform nucleus, but that were not activated, may be explained by synaptic depolarization not reaching the threshold for immediate early gene expression or by a net inhibitory effect on innervated neurons. Characterization of these activated forebrain regions using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the response to stimulation of the cuneiform nucleus.

Rapid and transient increases in cellular immediate early gene and neuropeptide mRNAs in cortical and limbic areas after amygdaloid kindling seizures in the rat

Changes in transcription factor and neuropeptide gene expression are likely to be involved in the cascade of genetic and molecular events leading to permanent changes in neuronal activity associated with kindling and epilepsy. Both acute-transient and delayed-sustained changes in transcription factor or immediate early gene (IEG) activity have previously been reported in response to different stimuli. In the present study in situ hybridization was used to investigate the possible time course (30 min-8 week) of IEG and neuropeptide mRNA induction in forebrain in a kindling model of epilepsy. Kindling was produced by daily unilateral stimulation of the amygdala. IEG mRNAs were detected using [35S]-labelled oligonucleotide probes specific for c-fos, c-jun, NGFI-A (PC1) and PC3 transcripts. Possible changes in the level of mRNAs encoding the neuropeptides somatostatin (SOM) and neuropeptide Y (NPY) were also studied. Stimulation-induced seizures produced dramatic bilateral increases in all IEG mRNAs in the dentate gyrus after 30 min to 1 h. Ipsilateral or bilateral increases in c-fos and PC3 mRNA were observed in the piriform cortex of individual animals at 30 min post-stimulation. While the distribution and apparent basal expression of the different IEGs varied (NGFI-A and c-jun > c-fos and PC3), the degree of induction in the dentate gyrus was similar for all IEGs studied (i.e. 200-300%). No long-term changes in IEG mRNA expression were detected beyond 2 h and up to 8 week after the last seizure. Increased levels of preproSOM and preproNPY mRNAs were consistently observed in hilar interneurons, but not in pyramidal or granule cells of the hippocampus, after 1-2 h. These increases were not maintained at later times. The short-term effects on IEG and neuropeptide mRNAs observed suggest that these changes are consequence of seizure activity with the development of kindling. In contrast, no evidence was found of any substantial, long-lasting effects on these parameters associated with the established kindled state.

Neuronal gene expression in the waking state: a role for the locus coeruleus

Several transcription factors are expressed at higher levels in the waking than in the sleeping brain. In experiments with rats, the locus coeruleus, a noradrenergic nucleus with diffuse projections, was found to regulate such expression. In brain regions depleted of noradrenergic innervation, amounts of c-Fos and nerve growth factor-induced A after waking were as low as after sleep. Phosphorylation of cyclic adenosine monophosphate response element-binding protein was also reduced. In contrast, electroencephalographic activity was unchanged. The reduced activity of locus coeruleus neurons may explain why the induction of certain transcription factors, with potential effects on plasticity and learning, does not occur during sleep.

Chromogranin mRNA levels in the brain as a marker for acute and chronic changes in neuronal activity: effect of treatments including seizures, osmotic stimulation and axotomy in the rat

Chromogranin/secretogranins are a family of acidic, soluble proteins with a widespread distribution in secretory vesicles of endocrine and nervous tissues. The effects of experimental stimuli of differing duration and intensity on chromogranin B and secretogranin II mRNA levels in relevant areas of the rat brain were examined by in situ hybridization histochemistry using 35S-labelled oligonucleotides. Effects of two 'chronic stimulation' paradigms were studied - the effect of 4 days of water or food deprivation on mRNA levels in the hypothalamus and the effect of unilateral cervical vagotomy on transcript levels in the dorsal vagal complex 1, 2 and 7 days after surgery. After 4 days of water deprivation secretogranin II mRNA was significantly increased in supraoptic nucleus (366 +/- 21% of control, P < 0.01), the magnocellular paraventricular nucleus (209 +/- 20% of control, P < 0.01) and the parvocellular paraventricular nucleus (147 +/- 6% of control, P < 0. 05) after 4 days of food deprivation. Seven days after unilateral cervical vagotomy, secretogranin II and chromogranin B mRNA levels were markedly decreased in the ipsilateral dorsal motor nucleus of the vagus (25 +/- 4 and 47 +/- 8% of contralateral values respectively, P < 0.01). Rapid changes in chromogranin mRNA were also detected following shorter duration 'acute stimulation' - in the hypothalamus after hypertonic saline injection, in the hippocampus after electrical stimulation-induced kindled seizures, and in the cerebral cortex after unilateral craniotomy. A large increase in secretogranin II mRNA was detected in the supraoptic nucleus (202 +/- 25% of control, P < 0.01) and the magnocellular paraventricular nucleus (168 +/- 29% of control, P < 0.05) 3 h after a single intraperitoneal injection of hypertonic (1.8 M) saline. Markedly increased levels of secretogranin II (125-160% of control) and chromogranin B (140-230% of control) mRNA were observed in granule cells of the dentate gyrus 0.5-2 h after amygdaloid stimulation-induced seizures. A moderate increase in secretogranin II mRNA (144 +/- 11% of contralateral side, P < 0.01) was found in the underlying cerebral cortex 2.5 h after unilateral craniotomy. These results indicate that measurement of changes in chromogranin mRNA, particularly secretogranin II, is a useful means of assessing both rapid and long-lasting increases and decreases in neuronal activity and, in contrast to immediate early gene mRNA levels, may better reflect specific changes in neuronal secretory activity associated with transmitter/peptide release.

Increased nerve growth factor inducible-A gene and c-fos messenger RNA levels in the rat midbrain and hindbrain associated with the cardiovascular response to electrical stimulation of the mesencephalic cuneiform nucleus

Functional neuronal connections associated with the cardiovascular response to unilateral low-intensity electrical stimulation of the mesencephalic cuneiform nucleus were examined in the halothane-anaesthetized and paralysed rat by in situ hybridization histochemistry using specific 35S-labelled oligonucleotides for detection of nerve growth factor inducible-A gene (NGFI-A) and c-fos messenger RNAs. Stimulation of the cuneiform nucleus increased mean arterial pressure and heart rate by 20 +/- 0.5 mmHg and 35 +/- 3 b.p.m., respectively, while no significant cardiovascular response was observed in animals stimulated in the inferior colliculus or in sham-operated animals. Cuneiform nucleus stimulation produced increased NGFI-A and c-fos messenger RNA levels in the Kölliker-Fuse and parabrachial nuclei ipsilaterally, and the cuneiform nucleus, dorsal periaqueductal gray and caudal ventrolateral medulla bilaterally at levels significantly greater than those in inferior colliculus-stimulated, sham-operated and naive, unoperated animals. NGFI-A, but not c-fos, messenger RNA expression was increased bilaterally in the caudal portion of the nucleus of the solitary tract and inferior olive. These results are consistent with previous neuroanatomical tract-tracing studies of afferent and efferent pathways from the cuneiform nucleus and indicate that these midbrain and hindbrain structures may be involved in the pressor and tachycardic responses associated with stimulation of the cuneiform nucleus. The ipsilateral nature of responses in certain brain areas may be explained by the absence of decussating pathways and/or the presence of multisynaptic connections which attenuate bilateral signal transmission. Characterization of these activated neuronal structures using other compatible labelling techniques should further elucidate the mechanisms by which these central nervous system structures are integrated in the cardiovascular responses to stimulation of the cuneiform nucleus.

c-Fos expression during wakefulness and sleep

We have recently demonstrated that c-fos expression is strongly induced by both spontaneous and forced wakefulness in many brain regions. c-Fos expression was considerably increased in regions involved in the regulation of arousal states, such as the locus coeruleus (noradrenergic neurons) and the medial preoptic area (non-GABAergic neurons). With c-fos antisense injection in the medial preoptic area, we demonstrated that c-fos expression in this region is causally involved in sleep regulation. c-Fos expression in other areas, such as the cerebral cortex and the hippocampus, may be related to the functional consequences of prolonged wakefulness and to the need of sleep. Further work should explore the mechanisms leading to changes in the expression of c-fos, and possibly of its target genes, during the sleep-wake cycle.