Enkephalin, substance P, and serotonin axonal input to c-fos-like immunoreactive neurons of the rat spinal cord

Prenatal exposure to dietary fat induces changes in the transcriptional factors, TEF and YAP, which may stimulate differentiation of peptide neurons in rat hypothalamus

Gestational exposure to a high-fat diet (HFD) stimulates the differentiation of orexigenic peptide-expressing neurons in the hypothalamus of offspring. To examine possible mechanisms that mediate this phenomenon, this study investigated the transcriptional factor, transcription enhancer factor-1 (TEF), and co-activator, Yes-associated protein (YAP), which when inactivated stimulate neuronal differentiation. In rat embryos and postnatal offspring prenatally exposed to a HFD compared to chow, changes in hypothalamic TEF and YAP and their relationship to the orexigenic peptide, enkephalin (ENK), were measured. The HFD offspring at postnatal day 15 (P15) exhibited in the hypothalamic paraventricular nucleus a significant reduction in YAP mRNA and protein, and increased levels of inactive and total TEF protein, with no change in mRNA. Similarly, HFD-exposed embryos at embryonic day 19 (E19) showed in whole hypothalamus significantly decreased levels of YAP mRNA and protein and TEF mRNA, and increased levels of inactive TEF protein, suggesting that HFD inactivates TEF and YAP. This was accompanied by increased density and fluorescence intensity of ENK neurons. A close relationship between TEF and ENK was suggested by the finding that TEF co-localizes with this peptide in hypothalamic neurons and HFD reduced the density of TEF/ENK co-labeled neurons, even while the number and fluorescence intensity of single-labeled TEF neurons were increased. Increased YAP inactivity by HFD was further evidenced by a decrease in number and fluorescence intensity of YAP-containing neurons, although the density of YAP/ENK co-labeled neurons was unaltered. Genetic knockdown of TEF or YAP stimulated ENK expression in hypothalamic neurons, supporting a close relationship between these transcription factors and neuropeptide. These findings suggest that prenatal HFD exposure inactivates both hypothalamic TEF and YAP, by either decreasing their levels or increasing their inactive form, and that this contributes to the stimulatory effect of HFD on ENK expression and possibly the differentiation of ENK-expressing neurons.

Tramadol and its enantiomers differentially suppress c-fos-like immunoreactivity in rat brain and spinal cord following acute noxious stimulus

Tramadol hydrochloride, (1RS,2RS)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)-cyclohexanol hydrochloride, is an orally-active, centrally-acting analgesic with a putative dual mechanism of action, including an opioid and non-opioid component. The analgesic properties of tramadol and the possible co-existence of dual mechanisms has been postulated to be due to complementary and interactive pharmacologies of its enantiomers. We examined the ability of tramadol, its enantiomers, and morphine as reference to suppress c-fos-like immunoreactivity (c-fos-ir) in rat spinal cord and brain regions following a noxious stimulus (i.p. administration of 3.5% acetic acid). c-fos-ir was measured by immunocytochemistry and the stained cells in each region were counted 2 h after the acetic-acid injection (2:25 h after tramadol or morphine). Equi-analgesic doses of s.c. morphine (10 mg/kg) or tramadol (30 mg/kg) significantly suppressed c-fos-ir in all areas examined, except dorsal central gray of the spinal cord. The enantiomers of tramadol had distinctive patterns of suppression, neither one suppressed c-fos-ir in all of the regions, and hence neither one alone accounted for the suppression produced by the racemate. These findings support differential and complementary effects of tramadol enantiomers in sub-populations of spinal and supraspinal nociceptive neurons, consistent with the proposed antinociceptive interaction between the enantiomers.

Locomotor-activated neurons of the cat. I. Serotonergic innervation and co-localization of 5-HT7, 5-HT2A, and 5-HT1A receptors in the thoraco-lumbar spinal cord

Monoamines are strong modulators and/or activators of spinal locomotor networks. Thus monoaminergic fibers likely contact neurons involved in generating locomotion. The aim of the present study was to investigate the serotonergic innervation of locomotor-activated neurons within the thoraco-lumbar spinal cord following induction of hindlimb locomotion. This was determined by immunohistochemical co-localization of serotonin (5-HT) fibers or 5-HT(7)/5-HT2A/5-HT1A receptors with cells expressing the activity-dependent marker c-fos. Experiments were performed on paralyzed, decerebrate cats in which locomotion was induced by electrical stimulation of the mesencephalic locomotor region. Abundant c-fos immunoreactive cells were observed in laminae VII and VIII throughout the thoraco-lumbar segments of locomotor animals. Control sections from the same segments showed significantly fewer labeled neurons, mostly within the dorsal horn. Multiple serotonergic boutons were found in close apposition to the majority (80-100%) of locomotor cells, which were most abundant in lumbar segments L3-7. 5-HT7 receptor immunoreactivity was observed on cells across the thoraco-lumbar segments (T7-L7), in a dorsoventral gradient. Most locomotor-activated cells co-localized with 5-HT7, 5-HT2A, and 5-HT1A receptors, with largest numbers in laminae VII and VIII. Co-localization of c-fos and 5-HT7 receptor was highest in the L5-L7 segments (>90%) and decreased rostrally (to approximately 50%) due to the absence of receptors on cells within the intermediolateral nucleus. In contrast, 60-80 and 35-80% of c-fos immunoreactive cells stained positive for 5-HT2A and 5-HT1A receptors, respectively, with no rostrocaudal gradient. These results indicate that serotonergic modulation of locomotion likely involves 5-HT(7)/5-HT2A/5-HT1A receptors located on the soma and proximal dendrites of serotonergic-innervated locomotor-activated neurons within laminae VII and VIII of thoraco-lumbar segments.

Electroacupuncture suppresses hyperalgesia and spinal Fos expression by activating the descending inhibitory system

Although electroacupuncture (EA) is widely used to treat pain, its mechanisms have not been completely understood. The present study investigated the descending inhibitory system involvement in EA action. Inflammatory pain was induced by injecting complete Freund's adjuvant subcutaneously into one hind paw of rats with dorsolateral funiculus lesions and sham-operated rats. EA treatment, 10 Hz at 3 mA, was given twice for 20 min each, once immediately post- and again 2 h post-Freund's adjuvant at GB 30, at the junction of the lateral 1/3 and medial 2/3 of the distance between the greater trochanter and sacral hiatus. For sham EA control, acupuncture needles were inserted bilaterally into GB 30 without electrical or manual stimulation. Paw withdrawal latency to a noxious thermal stimulus was measured at baseline and 20 min after EA treatment. Compared to sham EA, EA significantly (P<0.05, n=9) increased withdrawal latency of the inflamed hind paws in the sham-operated rats but not in those with dorsolateral funiculus lesions, indicating that lesioning blocked EA-produced anti-hyperalgesia. EA, compared to sham EA, also significantly inhibited Fos expression in laminae I-II of the spinal cord in the sham-operated rats (58.4+/-6.5 vs. 35.2+/-5.4 per section) but not in those with dorsolateral funiculus lesions. Further, EA activated serotonin- and catecholamine-containing neurons in the nucleus raphe magnus and locus coeruleus that project to the spinal cord. The results demonstrate that EA inhibits transmission of noxious messages and hyperalgesia by activating supraspinal neurons that project to the spinal cord.

Acupuncture at GV01 Relieves Somatic Pain Referred by Colitis in Rats

The present study aimed to expand our previous findings regarding the therapeutic effects and underlying mechanisms of acupuncture at GV01 in colitis. Our results showed that acupuncture at GV01 has antinociceptive effects on referred somatic pain induced by experimental colitis, and that endogenous opioid pathways may mediate these effects.

A semi-quantitative analysis of Fos expression by mustard oil

In the present study, a semi-quantitative analysis of Fos expression by mustard oil was performed. For this purpose, mustard oil was applied to the skin of the right hind foot of Wistar rats at various concentrations: 5%, 10%, 30%, 50%, 80% and 100% in liquid paraffin. The distribution and number of activated Fos-positive cells in the stimulated side (ipsilateral) and contralateral side of the spinal cord were investigated following the application. The ED50 of the response was also determined. The number of Fos-labelled cells gradually increased in a dose dependent manner in both sides of superficial layers (laminae I-II) of the spinal cord with increasing concentration of mustard oil. The increase between the doses was found significant in the ipsilateral superficial layers. The increase was significant in the contralateral superficial layers at concentrations above 50%. Very few Fos-labelled cells were observed around the central canal region in all concentrations. Higher doses of the mustard oil did not increase the number of activated cells in the deeper layers. However, the expression in the deeper layers (laminae III-X) does not show a consistent trend. Also, none of the concentrations used produced labelling in neurons of the deep ventral horn neurons or in motor neurons. Forty percent (40%) of mustard oil gave an approximately 1/2 maximum response i.e. an approximate ED 50. This may be important for studies using intrathecal application of antagonist following the mustard oil activation of skin nerve fibres.

Multiple neurotransmitter receptors contribute to the spinal Fos expression

The aim of this study is to identify the receptors which could potentially mediate the activation of c-Fos. Therefore, the effects of neurotransmitter receptor agonists in the activation of c-Fos in spinal neurons were studied by intrathecal injection of excitatory amino acid (EAA) receptor agonists: N-Methyl-D-Aspartate (NMDA), (S)-alpha-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic acid (AMPA), 2-Carboxyl-3-carboxymethyl-4-isopropenylpyMidine (Kainic acid, KA), (1S-3R)-1-Aminocyclopentane-1, 3-dicarboxylacid (ACPD), and substance-P receptor (neurokinin-1) agonist, [Sar9, Met (O2)11] SP (SarMet-SP). All drugs tested activated the production of c-Fos in spinal dorsal horn neurons. AMPA was found as the most potent agonist tested producing market production of c-Fos particularly in neurons of lamina II at doses of 10 pM per 10-microl injection. At this dose, other agonists were relatively ineffective. At higher doses, AMPA significantly increased the activated cells. NMDA significantly increased c-Fos production to a marked extent only at doses above 10 nM per 10-microl injection. KA and ACPD were least potent of the excitatory amino acid agonists. Injection of SarMet-SP at doses of 1 nM activated Fos selectively in neurons of lamina I. A dose-dependent increase in number of c-Fos-positive cells was observed for AMPA, KA, ACPD, and SarMet-SP, whereas NMDA gave a very strong expression after a high dose with no dose dependency. These finding suggest that multiple neurotransmitter receptors lead to c-Fos production in spinal neurons.

Electroacupuncture induces c-Fos expression in the rostral ventrolateral medulla and periaqueductal gray in cats: relation to opioid containing neurons

Our previous studies have shown that electroacupuncture (EA) at the Neiguan-Jianshi (P5-P6) acupoints inhibits sympathetic outflow and attenuates excitatory visceral cardiovascular reflexes through enkephalin- or beta-endorphin-related opioid receptors in the rostral ventrolateral medulla (rVLM). It is not known whether EA at these acupoints activates neurons containing enkephalin or beta-endorphin in the rVLM as well as in the periaqueductal gray (PAG) that are involved in EA-mediated central neural regulation of sympathetic activity. The present study evaluated activated neurons in the rVLM and PAG by detecting c-Fos immunoreactivity, and identified the relationship between c-Fos nuclei and neuronal structures containing enkephalin or beta-endorphin in these regions. To enhance the detection of cell bodies containing enkephalin or beta-endorphin, colchicine (90-100 microg/kg) was injected into the subarachnoid space in anesthetized cats 28-30 h prior to EA or the sham-operated control for EA. Following bilateral barodenervation and cervical vagotomy, EA (1-4 mA, 2 Hz, 0.5 ms) was performed at the P5-P6 acupoints (overlying median nerve; n=7) for 30 min. Identical procedures, with the exception of electrical stimulation, were carried out in five control animals. EA decreased blood pressure (BP) in four of seven cats (5-15 mm Hg) while the sham procedure for EA produced no responses. Perikarya containing enkephalin were found in the rVLM and rarely in the PAG, while no cell bodies labeled with beta-endorphin were identified in either region. Compared to animals in the control group, more c-Fos immunoreactivity, located principally in close proximity to fibers containing enkephalin or beta-endorphin, was observed in the rVLM and ventrolateral PAG (vlPAG) in EA-treated cats. Moreover, neurons double-labeled with c-Fos and enkephalin in the rVLM were significantly increased in cats following EA stimulation (P<0.05). These data indicate that EA at the P5-P6 acupoints activates neurons in the rVLM and vlPAG. These activated neurons contain enkephalin in the rVLM, and most likely interact with nerve fibers containing enkephalin or beta-endorphin in both the rVLM and vlPAG. The results from this study provide the first anatomical evidence showing that EA at the P5-P6 acupoints has the potential to influence neuronal structures (perikarya, axons and/or dendrites) containing enkephalin or beta-endorphin in specific regions of the brain stem. These neurons likely form the substrate for EA's influence on sympathoexcitatory cardiovascular reflexes.

The novel analgesic and high-efficacy 5-HT1A receptor agonist, F 13640 induces c-Fos protein expression in spinal cord dorsal horn neurons

The very-high-efficacy, selective 5-HT(1A) receptor agonist, F 13640 produces uniquely powerful analgesia in rat models of chronic pain by novel neuroadaptive mechanisms (inverse tolerance and co-operation with nociception) [Neuropharmacology 43 (2002) 945-958]. A signal transduction theory and evidence suggest that F 13640 initiates these mechanisms, paradoxically, by mimicking the central effects of nociceptive stimulation. We report that the i.p. injection of F 13640 induces c-Fos protein expression in the L3-L5 segments of the spinal cord. Some 65% of c-Fos protein immunoreactive (c-Fos-IR) nuclei occurred bilaterally in the dorsal horn laminae I-II and V-VI, spinal areas that contain neurons responsive to nociceptive stimulation. This pattern is not unlike that found earlier in arthritic rats, a model of somatotopically widespread nociception. Dose-response studies indicated that c-Fos protein expression was induced at doses (0.63 and 2.5 mg/kg, i.p.) at which previous studies had found F 13640 to produce hyperalgesia. Time-response studies found that c-Fos-IR nuclei appeared within 1-4 h after 0.63 mg/kg of F 13640, with a maximum at 2 h. This parallels literature evidence that c-Fos expression reaches peak late after, and outlasts, nociceptive stimulation. Similar to opioids counteracting noxiously induced c-Fos expression, 10 mg/kg (s.c.) of morphine reduced the number of c-Fos-IR nuclei induced by 0.63 mg/kg of F 13640 (by 45+/-5%; P<0.001). The induction by F 13640 of c-Fos protein expression may relate to the initial hyperalgesia which earlier data indicate the agent to produce early upon its administration.

Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes

The effect of N-[(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithiol]-1-oxopropyl]-L-phenylalanine benzyl ester (RB101), a dual inhibitor of the enkephalin-degrading enzymes, neutral endopeptidase and aminopeptidase N, was assessed in anaesthetised rats on the C-fibre reflex elicited by electrical stimulation within the sural nerve territory and recorded from the ipsilateral biceps femoris muscle. The temporal evolution of the pharmacological response was monitored by the repeated application of a constant stimulus intensity, namely three times threshold (3 T). In addition, recruitment curves were built by varying the stimulus intensity from 0 to 7 T. RB101 (7.5, 15 and 30 mg kg(-1), i.v.) induced a dose-dependent, naloxone-reversible depression of the reflex, which lasted around 60 min with the highest dose. The ED(50) was calculated as 16.9 mg kg(-1). Analyses of the recruitment curves revealed: (1) a significant increase of threshold; (2) a significant depression of the reflex in the ascending part of the curve; and (3) a lack of major depressive effects on the responses elicited by the strongest stimuli (corresponding to the plateau of the curve). The increase in the nociceptive threshold by enkephalin-degrading enzyme inhibitors, confirms previous data obtained from behavioural tests. In addition, the present study revealed an efficacy of these compounds over a wide range of stimulus intensities, albeit excluding the highest.

c-fos Expression and NADPH-d reactivity in spinal neurons after fatiguing stimulation of hindlimb muscles in the rat

The distribution of Fos-immunoreactive (Fos-ir) and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-reactive neurons in the rat lumbar spinal cord was examined following muscle fatigue caused by intermittent high-rate (100 s(-1)) electrical stimulation of the triceps surae muscle or the ventral root L5 (VRL5) for 30 min. Following both types of stimulation, the fatigue-related c-fos gene expression was more extensive in the L2-L5 segments on the stimulated side, and the majority of Fos-ir neurons were concentrated in the dorsal horn. After direct muscle stimulation, the highest number of Fos-ir neurons were detected in two regions: layer 5, and superficial layers (1 and 2(o)), although many labeled cells were also found in layers 3, 4, 6, and 7. In response to VRL5 stimulation, the maximal density of Fos-ir neurons was detected in the middle and lateral parts of layers 1 and 2(o), the zone of termination of high-threshold muscle afferents(.) Statistically significant prevalence of Fos-ir cell number was also found in layers 5 and 7 on the stimulated side. A few Fos-ir neurons were detected in the ventral horn (layer 8 and area 10) on both sides. The lamellar distribution of NADPH-d-reactive neurons was similar over all experimental groups of animals. In the L3-L6 segments, such reactive cells were arranged in two distinct regions: dorsal horn (layers 2(i), 3, and 5) and area 10; in the L1 and L2 segments, an additional cluster of NADPH-d positive cells was found in the intermediolateral cell column (IML). Double-labeled cells were not detected. We suggest that c-fos expression in response to muscle fatigue reveals activity of functionally different types of spinal neurons which could operate together with NOS-containing cells in pre-motoneuronal networks to modulate the motoneuron output.

Reciprocal circuits involved in nitroglycerin-induced neuronal activation of autonomic regions and pain pathways: a double immunolabeling and tract-tracing study

This study uses tract-tracing protocols to determine the circuitry of specific nuclei involved in nitroglycerin-induced activation. Combined retrograde and anterograde tracers were injected into nuclei which consistently demonstrate robust Fos expression following our systemic nitroglycerin injection paradigm. The nuclei, which conform to these criteria, that we have evaluated in this study are the locus coeruleus, parabrachial nucleus and paraventricular nucleus of the hypothalamus. Dual Fos/tracer immunocytochemistry in treated animals documented the existence of a subset of autonomic nuclei which are activated by nitroglycerin injection and have reciprocal connections. From the nature of this rich interconnection we suggest that nitroglycerin activates autonomic responses involved in cardiovascular pressor mechanisms. Nuclei which show strong Fos labeling following nitroglycerin administration, but not traced in this study, include the nucleus trigeminalis caudalis and the ventrolateral column of the periaqueductal gray, both of which mediate nociceptive modalities. These data confirm and expand on our previous findings and demonstrate that nitroglycerin activates a complex set of structures that are functionally and structurally interconnected to articulate an integrated response.

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.

Neurochemical mechanisms of nitroglycerin-induced neuronal activation in rat brain: a pharmacological investigation

Nitroglycerin is a nitric oxide donor which induces sustained expression of Fos protein, a marker of neuronal activation, in specific neuronal groups in the central nervous system. The mechanisms which underlie nitroglycerin-induced neuronal activation are elusive at this time, although a precise role has been suggested for the pool of neurons containing nitric oxide synthase as well as for catecholaminergic and peptidergic pathways. The aim of this study was to provide further details on the central effect of nitroglycerin by means of a pharmacological manipulation of nitroglycerin-induced neuronal activation with inhibitors of the nitric oxide synthase, modulators of the sympathetic drive and mediators of pain perception. Adult male Sprague-Dawley rats received L-NGnitro-arginine methyl ester, 7-nitro-indazole, ephedrine sulfate, indomethacin, capsaicin or vehicle before the subcutaneous injection of nitroglycerin (10 mg/kg b.w.). They were sacrificed 4 hr after nitroglycerin administration and brain sections were processed for immunocytochemical visualization of Fos. All the pharmacological treatments administered before injecting nitroglycerin selectively influenced Fos expression in the different brain nuclei. The data obtained suggest that nitroglycerin-induced neuronal activation is mediated by nociceptive and barosensitive mechanisms. Nitric oxide seems to represent the most important mediator of this phenomenon. The sympathetic system and prostaglandin synthesis are also likely to be involved.

Ultrastructure of Fos-labeled neurons relating to nociceptive primary afferent and substance P terminals in rat spinal superficial laminae

The presence of Fos-labeled neurons at ultrastructural level was confirmed in the spinal superficial laminae following an injection of formalin into rat hindpaw in the present study. The Fos-like immunoreactive products were found exclusively in regions associated with the euchromatin in the nuclei of Fos-labeled neurons. By the methods used-of anterograde transport of horseradish peroxidase conjugated to wheat-germ agglutinin and immunocytochemistry-it was observed that some Fos-labeled neuronal bodies received synaptic contacts from, or were apposed directly to, small diameter primary afferent terminals in the spinal superficial laminae. By means of double-labeled immunocytochemistry, a direct apposition was often observed and a synaptic relationship was occasionally found between Fos-labeled neuronal bodies and substance P-like immunoreactive terminals.

A contribution of neurokinin-1 receptor to formalin-induced c-fos expression in the rat spinal dorsal horn

Formalin applied to a hindpaw of the rat resulted in c-fos expression in the spinal dorsal horn neurons on the ipsilateral side of stimulation. The role of neurokinin-1 (NK-1) receptor in this response was examined using a selective NK-1 antagonist L-668169. L-668169 administered intrathecally (1 microg/10 microl, 10 microg/10 microl) 15 min before injection of 2% formalin into one hindpaw significantly decreased the number of Fos-labeled neurons in the dorsal horn. A smaller dose of L-668169 (0.1 microg/10 microl) was ineffective. It was also showed that a selective NK-1 agonist Sar-SP perfused intrathecally (10 microg/10 microl) did result in the production of c-fos mainly in laminae I and that SP-like immunoreactive varicosities apposed neurons with Fos-labeled nuclei at their perikaryal membrane in the superficial laminae. These results indicate that NK-1 receptors may contribute to the activation of c-fos in the spinal dorsal horn neurons.

Systemic nitroglycerin induces Fos immunoreactivity in brainstem and forebrain structures of the rat

Nitroglycerin is a vasodilator which induces vascular relaxation by releasing nitric oxide in the wall of blood vessels. It has been suggested that the cardiovascular inhibitory responses which are induced by this drug are mediated by central structures. In this study, we evaluated the distribution and intensity of Fos immunoreactivity in rat brain nuclei following the systemic administration of nitroglycerin. In the medulla, a significant number of Fos-immunoreactive neurons were observed in the nucleus tractus solitarius, ventrolateral medulla, area postrema and spinal trigeminal nucleus caudalis. A robust staining was seen in the parabrachial nucleus, locus coeruleus and ventrolateral periaqueductal grey. In the hypothalamus, Fos-positive cells were densely packed in the paraventricular and supraoptic nuclei. Other areas where significant staining was observed include the central nucleus of the amygdala and the subfornical organ. These findings demonstrate that the systemic administration of nitroglycerin is capable of activating a spectrum of functionally diverse brain regions. This spectrum includes areas involved in reflex adjustments to nitroglycerin-induced hypotension, areas involved in sensory nociceptive perception and areas associated with integrative regulation of autonomic, behavioral and neuroendocrine functions.

Effects of descending inhibitory systems on the c-Fos expression in the rat spinal cord during formalin-induced noxious stimulation

This study provides morphological evidence for the activation of the descending modulatory control by nociceptive afferent pathways. Fos-like immunoreactivity in the spinal dorsal horn is used as an indicator of efficacy of transmission of noxious inputs at this level. Wistar rats were anesthetized with nembutal and the spinal dorsolateral funiculus was transected unilaterally at the level of T11,12. Two days later, an equal volume (0.2 ml) of formalin (5% in saline) was injected into the plantar aspect of two hindpaws. After 1 h of injection, rats were deeply anesthetized and killed for the immunocytochemical examination of Fos-like protein product by using an immunocytochemical technique. The results show that the mean number of Fos protein-like immunoreactive neurons is significantly lower in the superficial laminae and in the neck of the dorsal horn on the side ipsilateral to the intact dorsolateral funiculus than that on the opposite side (i.e. 18.4 +/- 1.0 vs 30.0 +/- 1.3 and 33.9 +/- 0.2 vs 56.8 +/- 1.7, respectively). We conclude that the peripheral noxious inputs (which ascend via the ventral half of the spinal cord in this study) can activate the supraspinal descending inhibitory systems, which in turn suppress the synthesis of Fos-like protein in the related dorsal horn neurons.

Effects of Kelatorphan and morphine before and after noxious stimulation on immediate-early gene expression in rat spinal cord neurons

Expression of the immediate-early genes (IEG) c-FOS, NGF1-A and c-JUN was induced by noxious thermal stimulation in neurons of the rat spinal cord dorsal horn. Intravenous injection of Kelatorphan (5, 10 and 20 mg/kg), an inhibitor of multiple enkephalin-degrading enzymes, 20 min before noxious stimulation reduced the overall number of dorsal horn neurons expressing c-FOS and NGF1-A by up to 20-30%. While c-FOS expression was suppressed in superficial and deep laminae of the spinal cord, NGF1-A and c-JUN was only suppressed in superficial laminae. Morphine (5, 7.5 and 10 mg/kg) produced a dose-dependent reduction of c-FOS expression by up to 70% only when injected before noxious stimulation. Morphine injected 10 min after the noxious treatment was virtually ineffective. The depressant effect of Kelatorphan and morphine could be prevented by prior application of the opioid antagonist naloxone. Naloxone itself slightly increased the overall number of c-FOS-positive neurons in all laminae of the spinal cord. The present data support the existence of a tonic release of endogenous opioid peptides at the spinal level and show that inhibition of their peptidase-induced degradation modulates IEG expression in dorsal horn neurons of the rat. The finding that opioid agonists were ineffective when applied after stimulation underline the necessity of pre-emptive analgesia to prevent long-term activity-dependent changes in spinal cord neurons.

Application of morphine prior to noxious stimulation differentially modulates expression of Fos, Jun and Krox-24 proteins in rat spinal cord neurons

The expression of Fos, Jun and Krox-24 proteins was investigated in spinal cord neurons of the rat 2, 4 and 8 h following noxious thermal stimulation of one hind-paw and pre-treatment with morphine. The number of neurons expressing c-Fos, c-Jun, Jun B and Krox-24 were maximal after 2 h and thereafter declined. The number of Fos B and Jun D immunoreactive neurons increased constantly for up to 8 h with Jun D showing expression above baseline only after 4 h following stimulation. Intravenous application of morphine (5 and 10 mg/kg) 20 min before noxious heat stimulation decreased the expression of all six proteins at any time-point with a predilective effect on neurons of deeper laminae of the dorsal horn. The suppressive effects of morphine were more pronounced with the higher dose of morphine and completely reversed by intravenous naloxone (1 and 10 mg/kg). The temporospatial patterns of expression following morphine were similar to those seen without morphine, but in a much smaller number of neurons and with a shorter time-course. However, despite the high dose of morphine and continuous halothane anaesthesia during the whole experimental procedures, a considerable number of neurons expressing the various genes remained in all laminae of the spinal cord. At 2 h following noxious heat stimulation morphine had decreased the number of labelled neurons for c-Fos, Fos B, Krox-24, c-Jun and Jun B to 30-60% of control levels in laminae I-II and to 10-30% in laminae III-VII,X of the spinal cord. At 4 h the level of reduction had further increased while Jun D was only moderately reduced to 75% in all laminae of the spinal cord. Eight hours following noxious heat plus morphine application we did not detect noxious evoked immunoreactivity for c-Fos, Krox-24, c-Jun and Jun B, while there was residual labelling for Fos B in the superficial dorsal horn and for Jun D in laminae I-VII and X of the spinal cord. The different temporospatial pattern of immediate early gene expression in neurons of the spinal cord dorsal horn following noxious stimulation suggest that variable transcription complexes may interact with DNA regulatory sequences and could thus activate alternative secondary response genes, even under protection of a high dosage of morphine applied before noxious stimulation.

Neurochemical identification of fos-positive neurons using two-colour immunoperoxidase staining

The discovery of immediate early genes (IEG) has provided neuroscientists with a new functional mapping technique. Labelling of neural tissue for the protein product of IEG provides an activity map with single-cell resolution. When combined with labelling for the chemical identity of the neuron, this provides a powerful tool for the investigation of specific cell populations along a neuraxis. Here we describe in detail a method which allows simultaneous bright-field visualization of neurochemically identified cells displaying increased IEG expression. This technique is evaluated in tissue from rats subjected to stimuli known to induce the expression of the IEG c-fos in various medullary catecholaminergic and hypothalamic neurosecretory cell groups. A 2-colour immunoperoxidase technique was used to visualize Fos, the nuclear protein product of c-fos, and the cytoplasmic antigens tyrosine hydroxylase (TH), phenylethanolamine N-methyl transferase (PNMT), oxytocin (OT) and vasopressin (VP). This involved simultaneous application of primary antibodies raised in different species followed by sequential application of appropriate biotinylated secondary antibodies and the avidin-biotin-peroxidase technique. Fos was visualized with nickel-intensified diaminobenzidine (Ni-DAB) in the first sequence while TH, PNMT, OT or VP were visualized with DAB alone, resulting in readily distinguishable black and amber reaction products, respectively. This dual immunoperoxidase technique is time saving compared to techniques using sequential application of primary antibodies and avoids the disadvantages associated with fluorescence techniques.