Involvement of postsynaptic alpha 1- and alpha 2-adrenoceptors in the flexor reflex activity in the spinal rats

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Strategies for regeneration and repair in spinal cord traumatic injury

Spinal cord injury is frequently followed by the loss of supraspinal control of sensory, autonomic and motor functions at the sublesional level. In order to enhance recovery in spinal cord-injured patients, we have developed three fundamental strategies in experimental models. These strategies define in turn three chronological levels of postlesional intervention in the spinal cord. Neuroprotection soon after injury using pharmacological tools to reduce the progressive secondary injury processes that follow during the first week after the initial lesion. This strategy was conducted up to clinical trials, showing that a pharmacological therapy can reduce the permanent neurological deficit that usually follows an acute injury of the central nervous system (CNS). A second strategy, which is initiated not long after the lesion, aims at promoting axonal regeneration by acting on the main barrier to regeneration of lesioned axons: the glial scar. Finally a mid-term substitutive strategy is the management of the sublesional spinal cord by sensorimotor stimulation and/or supply of missing key afferents, such as monoaminergic systems. These three strategies are reviewed. Only a combination of these different approaches will be able to provide an optimal basis for potential therapeutic interventions directed to functional recovery after spinal cord injury.

Distribution of alpha2-adrenoceptor mRNAs in the rat lumbar spinal cord in normal and axotomized rats

Using riboprobe in situ hybridization we have studied the distribution of alpha2A-, alpha2B- and alpha2c-adrenoceptor (AR) mRNAs in the lumbar spinal cord in normal rats and after peripheral axotomy. A strong alpha2A- and alpha2C AR mRNA labelling was found in motoneurons and other cells in the ventral horns. In the dorsal horns strong alpha2A-AR mRNA labelling was found in all layers and in the lateral spinal nucleus, whereas alpha2C-AR mRNA was found in lower numbers of cells in various layers. The alpha2B-AR mRNA signal was only detected in some small cells superficially in the dorsal horn. With regard to axotomy only a marginal effect was observed for alpha2C-AR mRNA in the ventral horn. The results suggest that alpha2-ARs are involved both in sensory and motor processing.

Recovery of locomotion following transplantation of monoaminergic neurons in the spinal cord of paraplegic rats

Severe traumatic lesions of the spinal cord yield a permanent deficit of motricity in adult mammals and specifically a loss of locomotor activity of hindlimbs when the lesion is located at the lower thoracic level. To restore this function, we have developed a paradigm of transplantation in rats based on a transection model of the spinal cord and the subsequent injection at the sublesional level of a suspension of embryonic brainstem monoaminergic neurons which play a key role in the modulation of locomotion. A genuine locomotion was characterized in transplanted animals by electromyographic and electroneurographic recordings. This correlated with a specific reinnervation pattern of targets, where typical synapses were found, and with the normalization of biochemical parameters.

Effects of clonidine on a C-fibre reflex in the rat

A C-fibre reflex elicited by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in anaesthetized rats. The temporal evolution of the response was studied using a constant stimulus intensity (3 x threshold) and recruitment curves were built by varying stimulus intensity from 0 to 7 x threshold. The intravenous administration of 0.02-0.2 mg/kg clonidine resulted in a dose-dependent depression of the C-fibre reflex. The alpha 2-adrenoceptor antagonist idazoxan completely prevented this depressive effect of clonidine. The effects of clonidine on the C-fibre reflex elicited by a wide range of stimulus intensities were investigated using recruitment curves: following 0.16 mg/kg clonidine, a dramatic shift of the recruitment curve to the right was seen with both an increase in the threshold and a decrease in the slope. Clonidine also produced a dose-dependent increase in blood pressure, but this was not correlated with the depression of the nociceptive reflex.

Regional study of spinal alpha 2-adrenoceptor densities after intraspinal noradrenergic-rich implants on adult rats bearing complete spinal cord transection or selective chemical noradrenergic denervation

One of the challenges of restorative neuronal transplantation in the CNS of mammals is the appropriate integration of grafted cells in the host circuitry. One key parameter is the specific influence of grafted cells upon corresponding receptors. In order to test this issue on the lesioned spinal cord of adult rats, two models of spinal cord denervation were used: the first one consisted of a complete transection 1 week prior to an intraspinal transplantation of embryonic locus coeruleus (LC) primordia cell suspension; the second one was a chemical destruction of the spinal noradrenergic (NA) system 1 month prior to a similar transplantation. Five weeks after transplantation, spinal sections were processed for autoradiographic quantification of alpha 2-adrenoceptor binding sites densities. In most regions, alpha 2-adrenoceptor densities remained comparable or higher than before graft; interestingly, in lumbar dorsal horn, lumbar intermediate zone and sacral distal dorsal horn of transected-grafted rats, they returned to control level. Results are discussed in relation to the parallel study performed concerning alpha 1-adrenoceptors.

Normal distribution of alpha 2-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 2 (alpha 2)-adrenoceptors along cervical, thoracic, lumbar, and sacral segments of the spinal cord of normal rats has been studied by quantitative autoradiography using the specific alpha 2-antagonist [3H]rauwolscine as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 2-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 2-adrenoceptor densities revealed the presence of these receptors throughout the whole gray matter with a preferential location in the superficial dorsal horn. This pattern was the same at all rostro-caudal levels of the cord and appeared very well correlated with the distribution of NA terminals revealed by immunohistochemistry, particularly in the superficial layers of the dorsal horn. After total transection of the spinal cord (caudally to the section) and 6-OHDA-induced lesion, an increase of alpha 2-adrenoceptor density was mainly observed within the distal dorsal horn thus evidencing supersensitivity in this area, while modifications were not detectable in other regions of the spinal gray matter, except at the lumbar level where other dorsal, central, and intermediate zones were significantly enriched.

The action of 1,2,3,5,6,11b-hexahydro-[1]benzothieno [3,2-g]indolizine hydrochloride (ADT 16) on peripheral and central responses mediated by 5-hydroxytryptaminergic and adrenergic systems of the rat

Some acute pharmacological effects have been examined of racemic ADT 16 (1,2,3,5,6,11b-hexahydro[1]benzothieno[3,2-g]indolizine hydrochloride), on peripheral and central responses mediated by 5-HT and adrenergic systems in the rat. In-vitro, ADT 16 (10-1000 nM), similarly to mianserin, antagonized the inhibitory responses to B-HT 920 of the electrically-stimulated rat isolated prostatic vas deferens. High concentrations of ADT 16 (10 microM), also resembled those of mianserin by potentiating twitch responses to electrical stimulation of the tissue. Contractile responses to phenylephrine of rat isolated epididymal vas deferens were antagonized by ADT 16 (0.3-1 microM). In the rat stomach fundus strip, ADT 16 (1-3 microM) antagonized contractions due to 5-HT. ADT 16 (0.1-1 microM) had no effect on responses to acetylcholine of the guinea-pig isolated ileum. In-vivo, in spinalized, decerebrated rats, fenfluramine- or clonidine-induced facilitation of flexor reflex activity of the anterior tibialis muscle was attenuated by ADT 16 (3 and 10 mg kg-1, i.v., and 3 mg kg-1, i.v. respectively). In the anaesthetized rat, L-3,4-dihydroxyphenylalanine (L-dopa)- or L-5-hydroxytryptophan (L-5-HTP)-induced increases in the frequency of spontaneous twitches of the anterior digastricus muscle were attenuated by ADT 16 (1 and 3 mg kg-1, i.v.; n = 4). It is concluded that ADT 16, similarly to mianserin, is a novel peripherally and centrally active antagonist of 5-HT and adrenergic responses in the rat.

NC-1300, a proton-pump inhibitor, requires gastric acid to exert cytoprotection in rat gastric mucosa

Effect of gastric acid suppression on the cytoprotective effect of a single dose of NC-1300 given intragastrically was studied. NC-1300 given intragastrically prevented gastric mucosal damage caused by absolute ethanol in rats in a dose-related manner, while the drug given subcutaneously did not. Pretreatment with NC-1300 given subcutaneously to suppress acid secretion abolished the protective effect of NC-1300 given intragastrically, but not in the presence of 0.1 N HCl. Repeated intragastric administration of NC-1300 for 7 days failed to prevent the ethanol damage. These results suggest that NC-1300 requires gastric acid to exert a protective effect against ethanol in rat gastric mucosa.

Normal distribution of alpha-1-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 1 (alpha 1)-adrenoceptors along the different segments of the spinal cord (cervical, thoracic, lumbar, and sacral) of normal rats has been studied by quantitative autoradiography using the specific alpha 1-antagonist [3H]Prazosin as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 1-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 1-adrenoceptor densities revealed a widespread distribution of these receptors along all segments of the spinal cord with a similar pattern in the various subregions of gray matter studied. This distribution of alpha 1-adrenoceptors was quite well correlated with the distribution of NA terminals, when referring to previous descriptions by immunohistochemistry. After 6-OHDA lesion, as well as caudally to the transection, a significant increase of alpha 1-adrenoceptor densities was observed in all spinal subregions thus evidencing supersensitivity. These results suggest that NA may act in the spinal cord, at least partly, via alpha 1-adrenoceptors and that the expression of these receptors could be influenced by NA dysfunction, as demonstrated here through the effects observed in lesioned animals.

Evidence for the involvement of 5-HT1A receptors in the anticonflict effect of ipsapirone in rats

The anticonflict activity of ipsapirone, a non-benzodiazepine anxiolytic drug, with high affinity for 5-hydroxytryptamine1A (5-HT1A) receptors, was studied in the drinking conflict test in the rat. The drug, administered in doses 1.25-20 mg/kg increased the number of punished licks, with the maximum effect observed after doses of 5-20 mg/kg of ipsapirone. The anticonflict effect of ipsapirone (5 mg/kg) was dose-dependently antagonized by the 5-HT1A receptor, alpha 1-adrenoceptor and dopamine receptor antagonist, NAN-190 (0.25-1 mg/kg) and by the beta-adrenoceptor blocker, SDZ 21009, which also has a high affinity for 5-HT1A and 5-HT1B receptors (2-8 mg/kg). On the other hand, the non-selective 5-HT receptor antagonist, metergoline (2 and 4 mg/kg), the 5-HT2/5-HT1C receptor antagonist, ritanserin (0.25 and 0.5 mg/kg), the selective alpha 1-adrenoceptor blocker, prazosin (0.25-0.5 mg/kg) and the beta-blockers, betaxolol (8 mg/kg) and ICI 118 551 (8 mg/kg), which have no affinity for 5-HT receptors, did not affect the anticonflict action of ipsapirone. The effect of ipsapirone was also not modified in animals with lesions of 5-HT neurones, produced by p-chloroamphetamine (PCA--2 x 10 mg/kg). These results suggest that the anticonflict effect of ipsapirone in the Vogel test, results from its interaction with 5-HT1A receptors, which are probably located postsynaptically.

The effects of desipramine (DMI) and alpha-2 adrenoceptor agonists on flexor reflex activity (FRA) in the spinalized and decerebrate rat

In the spinalized and decerebrate rat, electrically evoked flexor reflex activity (FRA) of the right anterior tibialis muscle was facilitated by DMI (0.1-3 mg/kg i.v.). DMI (3 mg/kg i.v.) had not effect on twitches of the left ant. tib. muscle evoked by electrical stimulation of the distal stump of the severed sciatic nerve (directly evoked twitches). The alpha-1 adrenoceptor antagonist prazosin (300 micrograms/kg i.v.) and the alpha-2 adrenoceptor agonists clonidine (5 and 10 micrograms/kg i.v.) and BHT-920 (100 micrograms/kg i.v.) all suppressed FRA facilitated by DMI (3 mg/kg i.v.). The alpha-2 adrenoceptor antagonists yohimbine (100 micrograms/kg i.v.) and mianserin (30 micrograms/kg i.v.) antagonized the suppressant effect of clonidine (5 micrograms/kg i.v.) on DMI-facilitated FRA. Clonidine (10 micrograms/kg i.v.) had no effect on directly evoked twitches, and at 3-300 micrograms/kg i.v. had no effect on FRA facilitated by L-DOPA (100 mg/kg i.p.). It is suggested that DMI-facilitated FRA can be used as a test for alpha-2 adrenoceptor agonists which may act at presynaptic alpha-2 adrenoceptors in the spinal cord.

Antinociceptive actions of alpha 2-adrenoceptor agonists in the rat spinal cord: evidence for antinociceptive alpha 2-adrenoceptor subtypes and dissociation of antinociceptive alpha 2-adrenoceptors from cyclic AMP

The antinociceptive actions of intrathecal injections of two alpha 2-adrenergic agonists, UK-14,304 and guanfacine, were investigated in rats after pretreatment of the animals with the noradrenaline neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) 14 days in advance. The chronic noradrenaline depletion induced by DSP4 caused a marked increase in sensitivity of the antinociceptive action of UK-14,304 in the tail-flick test. By contrast, the antinociceptive effect of guanfacine was not appreciably affected by the DSP4 treatment. The antinociceptive effects of both UK-14,304 and guanfacine were blocked by intraperitoneal injections of yohimbine, a result indicating that both drugs induced their actions by activating alpha 2-adrenoceptors. Both UK-14,304 and guanfacine were found to reduce the production of cyclic AMP (cAMP) in the spinal cord, as determined using an in vitro radioisotopic method. The cAMP inhibitory effects of both agonists were effectively blocked by yohimbine, but not by prazosin, a finding indicating the alpha 2-adrenergic nature of the response. However, the cAMP inhibitory effect of UK-14,304 was not potentiated by pretreatment with DSP4, a finding in marked contrast with the strong potentiation of the antinociceptive action of UK-14,304 induced by the chronic depletion of endogenous noradrenaline. Moreover, intrathecal injections of forskolin, which increased the endogenous levels of spinal cord cAMP fivefold, did not modify the antinociceptive effects of UK-14,304 or guanfacine in neither normal nor DSP4-treated animals. It is suggested that there exist pharmacologically differing alpha 2-adrenergic receptor pathways capable of mediating antinociceptive effects at the level of the spinal cord. The cAMP inhibitory actions of spinal cord alpha 2-adrenoceptors appear not to be directly linked with the antinociceptive actions of these receptors.

Alpha 2-adrenoceptors mediate inhibition of cyclic AMP production in the spinal cord after stimulation of cyclic AMP with forskolin but not after stimulation with capsaicin or vasoactive intestinal peptide

In slices obtained from the ventral and the dorsal guinea pig spinal cord both forskolin and vasoactive intestinal peptide (VIP) caused a dose-dependent stimulation of the production of cyclic AMP. By contrast capsaicin stimulated cyclic AMP formation only in the dorsal cord; no effect was observed in the ventral cord. The alpha 2-adrenergic agonist UK-14,304 dose-dependently inhibited the production of cyclic AMP in both the dorsal and ventral aspects of the cord when the formation of cyclic AMP had been stimulated with 3 microM forskolin, the maximal inhibition amounting to 25-32%. Also the basal (i.e., unstimulated) production of cyclic AMP was inhibited, the inhibition amounting to about 16-18%. However, after stimulation of cyclic AMP formation in the dorsal cord with capsaicin, UK-14,304 was virtually ineffective in inhibiting the accumulation of cyclic AMP. Also, when the formation of cyclic AMP was stimulated with VIP, UK-14,304 was virtually ineffective in inhibiting the formation of cyclic AMP both in the ventral and the dorsal parts of the cord. When cyclic AMP production had been stimulated with forskolin the ability of UK-14,304 to inhibit the formation of cyclic AMP was not attenuated by capsaicin, either in the ventral or in the dorsal cord. The results are discussed with the notion that cyclic AMP inhibitory spinal cord alpha 2-adrenoceptors are located on cells accessible to stimulation of cyclic AMP with forskolin but not with capsaicin or VIP.

Different effects of norepinephrine and serotonin on the electrical toe stimulation-induced reflexes in the rat spinal cord

1. Electrical toe stimulation was shown to elicit reflex potentials in the ipsilateral common peroneal nerve in the rat. 2. This reflex consisted of the spinal reflex and the spino-bulbo-spinal reflex (SBS reflex). 3. Administration of norepinephrine into the subarachnoid space enhanced the spinal reflex but depressed the SBS reflex. On the other hand, serotonin enhanced both reflexes. 4. These different effects of norepinephrine and serotonin suggest different control of nociceptive reactions by these biogenic amines.

Intrathecal noradrenaline has a dose-dependent inhibitory or facilitatory effect on the flexion reflex in the rat

The effect of intrathecal noradrenaline (NA) on the hamstring flexion reflex to subcutaneous electrical shocks was examined in unanaesthetized, decerebrate, spinalized rats. Low doses of NA (200 ng and 2 micrograms) depressed and high doses (20 micrograms and 200 micrograms) facilitated the reflex. It is suggested that the primary effect of NA in the dorsal horn is inhibitory and in the ventral horn excitatory. Furthermore, the neurons mediating these effects are more sensitive to NA in the dorsal horn than in the ventral horn.

Central action of ipsapirone, a new anxiolytic drug, on serotoninergic, noradrenergic and dopaminergic functions

Ipsapirone (TVX Q 7821, 2-(4-(4-(2-pyrimidinyl)-1-piperazinyl)butyl)-1,2-benzisothiazol-3- (2H)one-1, 1-dioxidehydrochloride), a new anxiolytic drug in respect of the evaluation of its effect on central 5-hydroxytryptamine (5-HT), noradrenaline and dopamine functions was studied. It was found that ipsapirone inhibits induced by 8-OH-DPAT and 5-methoxydimethyltryptamine (agonists of 5-HT1A receptors) behavioural effects (flat body posture and forepaw treading) in normal and reserpinized rats. Ipsapirone partly inhibited in rats but not in mice the 8-OH-DPAT-induced hypothermia. Ipsapirone, administered at high doses, decreased the body temperature in rats and mice, inhibited the 5-hydroxytryptophan-induced head twitches in mice and the tryptamine-induced convulsions and tremor in rats. In the hind limb flexor reflex preparation of the spinal rat only high doses of the drug inhibited stimulation induced by quipazine, m-chlorphenylpiperazine, 8-OH-DPAT and St 587 (an agonist of alpha 1-adrenoceptors). Ipsapirone did not block the fenfluramine- and m-chlorphenylpiperazine-induced hyperthermia in rats at an ambient temperature of 28 degrees C. The drug did not affect clonidine-induced sedation and inconsiderably attenuated clonidine-induced hypothermia in mice. It attenuated the d-amphetamine-induced locomotor hyperactivity in mice and rats but, given alone, decreased the locomotor activity. The obtained results indicate that ipsapirone exhibits 5-HT1A antagonistic effect, and only at high doses it can also produce an inhibitory effect on 5-HT2 and the alpha 1-adrenergic function.