Noradrenergic action of amphetamine following degeneration of descending monoaminergic fibers in the spinal cord

Adrenergic receptors modulate motoneuron excitability, sensory synaptic transmission and muscle spasms after chronic spinal cord injury

The brain stem provides most of the noradrenaline (NA) present in the spinal cord, which functions to both increase spinal motoneuron excitability and inhibit sensory afferent transmission to motoneurons (excitatory postsynaptic potentials; EPSPs). NA increases motoneuron excitability by facilitating calcium-mediated persistent inward currents (Ca PICs) that are crucial for sustained motoneuron firing. Spinal cord transection eliminates most NA and accordingly causes an immediate loss of PICs and emergence of exaggerated EPSPs. However, with time PICs recover, and thus the exaggerated EPSPs can then readily trigger these PICs, which in turn produce muscle spasms. Here we examined the contribution of adrenergic receptors to spasms in chronic spinal rats. Selective activation of the α(1A) adrenergic receptor with the agonists methoxamine or A61603 facilitated Ca PIC and spasm activity, recorded both in vivo and in vitro. In contrast, the α(2) receptor agonists clonidine and UK14303 did not facilitate Ca PICs, but did decrease the EPSPs that trigger spasms. Moreover, in the absence of agonists, spasms recorded in vivo were inhibited by the α(1) receptor antagonists WB4010, prazosin, and REC15/2739, and increased by the α(2) receptor antagonist RX821001, suggesting that both adrenergic receptors were endogenously active. In contrast, spasm activity recorded in the isolated in vitro cord was inhibited only by the α(1) antagonists that block constitutive receptor activity (activity in the absence of NA; inverse agonists, WB4010 and prazosin) and not by the neutral antagonist REC15/2739, which only blocks conventional NA-mediated receptor activity. RX821001 had no effect in vitro even though it is an α(2) receptor inverse agonist. Our results suggest that after chronic spinal cord injury Ca PICs and spasms are facilitated, in part, by constitutive activity in α(1) adrenergic receptors. Additionally, peripherally derived NA (or similar ligand) activates both α(1) and α(2) adrenergic receptors, controlling PICs and EPSPs, respectively.

Role of endogenous release of norepinephrine in muscle spasms after chronic spinal cord injury

The recovery of persistent inward currents (PICs) and motoneuron excitability after chronic spinal cord transection is mediated, in part, by the development of supersensitivity to residual serotonin (5HT) below the lesion. The purpose of this paper is to investigate if, like 5HT, endogenous sources of norepinephrine (NE) facilitate motoneuron PICs after chronic spinal transection. Cutaneous-evoked reflex responses in tail muscles of awake chronic spinal rats were measured after increasing presynaptic release of NE by administration of amphetamine. An increase in long-lasting reflexes, known to be mediated by the calcium component of the PIC (CaPIC), was observed even at low doses (0.1-0.2 mg/kg) of amphetamine. These findings were repeated in a reduced S2 in vitro preparation, demonstrating that the increased long-lasting reflexes by amphetamine were neural. Under intracellular voltage clamp, amphetamine application led to a large facilitation of the motoneuron CaPIC. This indicates that the increases in long-lasting reflexes induced by amphetamine in the awake animal were, in part, due to actions directly on the motoneuron. Reflex responses in acutely spinal animals were facilitated by amphetamine similar to chronic animals but only at doses that were ten times greater than that required in chronic animals (0.2 mg/kg chronic vs. 2.0 mg/kg acute), pointing to a development of supersensitivity to endogenous NE in chronic animals. In summary, the increases in long-lasting reflexes and associated motoneuron CaPICs by amphetamine are likely due to an increased release of endogenous NE, which motoneurons become supersensitive to in the chronic stages of spinal cord injury.

6-Hydroxydopamine treatment of neonatal rats. II. Effects on the development of the hindlimb flexor reflex

The flexor reflex (FR) of the hindlimb was measured in rats at various ages between postnatal days (PND) 12-45, in vehicle-treated control rats and in rats treated with 6-hydroxydopamine (6-OHDA) (100 mg/kg, i.p.) on the first and second days after birth. The hindlimb FR was elicited by graded electrical stimulation of the footpad (0.1-2.0 mA) and quantified by the amplitude of the flexion in grams. The half maximal FR response was evoked by 1.2 mA and was 2-3 g in animals at PND 12-17 and 9-10 g in animals on PND 30 and 45. A similar age dependency was evident in the maximum hindlimb FR evoked by 2.0 mA; the maximum FR was 4.7 +/- 0.5 g on PND 12 and 24 +/- 2 g on PND 45. In rats treated with 6-OHDA, the strength of the FR was similar to that of the controls up to PND 15. However, the FR was increased 25% by PND 17 and 200% by PND 45 in the 6-OHDA-treated animals, versus controls. Pretreatment with clonidine (100 micrograms/kg, i.p.), which activates alpha 1 receptors under the current experimental conditions, did not enhance the FR in control animals. However clonidine pretreatment caused an increase of 400-800% in the FR in the 6-OHDA-treated animals at PND 15 and beyond. Our companion paper demonstrated that as early as PND 5, there was a significant increase (P < 0.05) in alpha 1 receptors in the spinal cord of 6-OHDA-treated animals, versus the controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of central alpha-1 adrenoceptors in canine narcolepsy

The role of central alpha-1 adrenergic receptors in cataplexy was investigated in genetically narcoleptic Doberman pinschers. Treatment of narcoleptic dogs with 25-600 micrograms/kg prazosin, a selective alpha-1 adrenergic receptor blocker, exacerbated cataplexy, whereas treatment with the alpha-1 agonist, methoxamine, ameliorated it. Subsequent studies showed that the beneficial effects of classical treatments of human narcolepsy (amphetamines and tricyclic antidepressants) are antagonized by prazosin, suggesting that these drugs are active through an indirect alpha-1 stimulation (via an increase of norepinephrine in the synaptic cleft). Other studies confirmed that the observed effects were not due to peripheral alpha-1 cardiovascular involvement. Atropine, a central anticholinergic agent, but not methylatropine, a peripheral one, completely suppressed the prazosin effect, which suggests that adrenergic and cholinergic systems act sequentially and not independently to generate cataplexy. Little is known about the physiological role of central alpha-1 adrenoceptors. This series of experiments implicates these receptors in narcolepsy-cataplexy.

Binding of [3H]prazosin and [3H]p-aminoclonidine to alpha-adrenoceptors in rat spinal cord

alpha-Adrenoceptors in spinal cord appear to play a role in a number of physiologic processes including the control of blood pressure, pain and motor function. In order to evaluate more clearly these potential roles, the characteristics of binding of [3H]prazosin ([3H]PRZ) to spinal alpha 1 adrenoceptors and [3H]p-aminoclonidine ([3H]PAC) to spinal alpha 2 adrenoceptors were determined. Binding of each ligand to their respective adrenoceptors was saturable and Scatchard analysis revealed binding of each to a single class of adrenoceptors with characteristics of [3H]PRZ binding of Bmax = 78 fmol/mg protein and Kd = 0.75 nM and [3H]PAC binding Bmax = 70 fmol/mg protein and Kd = 1.39 nM. Whereas [3H]PRZ specific binding (Bmax) was unaltered by guanine nucleotides. [3H]PAC binding was increased with addition of 10 microM guanosine triphosphate (GTP) (P less than 0.05) and decreased with either 50 microM GTP or guanyl-5'-yl-imidodiphosphate [Gpp(NH)p] (P less than 0.01). Competition for specific [3H]PRZ and [3H]PAC binding by various alpha 1 and alpha 2 adrenoceptor agonists and antagonists of known pharmacologic activity revealed that [3H]PRZ defines alpha 1 adrenoceptors (Ki = 2.1 nM for prazosin vs 4300 nM for yohimbine) and [3H]PAC defines alpha 2 adrenoceptors (Ki = 1.06 nM for yohimbine vs 15480 nM for prazosin). Regional spinal cord studies demonstrated that dorsal spinal cord in the lumbar region contains the highest density of both [3H]PRZ (Bmax = 93 +/- 14 fmol/mg protein) and [3H]PAC (Bmax = 101 +/- 6 fmol/mg protein) binding. In contrast, lowest binding was evident in thoracic cord with equal levels in both dorsal and ventral regions (Bmax = 44-48 fmol/mg protein). The regional distribution of both alpha 1 and alpha 2 adrenoceptors in spinal cord compares to the localization previously classified functionally utilizing various pharmacological agonists and antagonists at norepinephrine receptors.

Effect of alpha 1-adrenoceptors blockade with prazosin in canine narcolepsy

The role of central alpha 1-adrenergic receptors in cataplexy was investigated in 4 narcoleptic poodles and 6 genetically narcoleptic Doberman pinschers. Treatment of narcoleptic dogs with prazosin, a selective alpha 1-adrenergic receptor blocker, exacerbated cataplexy in both narcoleptic dog breeds. Control and heterozygous Dobermans were not affected by the drug. Binding studies using [3H]prazosin revealed an increase in alpha 1-receptor binding apparently limited to the amygdala. The present study suggests that central alpha 1-adrenoceptors, whose role is still mostly unknown, play a fundamental role in controlling mechanisms involved in cataplexy and REM sleep.

The effects of clonidine and yohimbine on locomotion and cutaneous reflexes in the adult chronic spinal cat

The effects on the locomotor pattern of a noradrenergic agonist (clonidine) and an antagonist (yohimbine) were studied in 3 adult chronic spinal cats walking on a treadmill. In the early post-transection period, when the cat walked mainly on the tip of its feet, without supporting its own weight, it was observed that clonidine (150 micrograms/kg) could induce a good bilateral foot placement and intermittent complete weight support. When clonidine was given 1-3 months following the transection, at a time when the spinal cats had a stable and regular locomotor performance, the step length increased markedly, especially at low speeds. This was associated with an increase in the duration of the flexor and extensor bursts, as well as an increase of the angular excursion of all joints. These effects, seen during forward locomotion, were also observed during backward locomotion. In addition, the latter was more easily elicited after clonidine. Yohimbine (1.5-3 mg/kg) partially antagonized these effects. The threshold current needed to elicit a small flexion reflex through wires implanted in the dorsum of the paw was 2-3 times higher after clonidine. Trains of shocks in the animal, standing quietly, did not induce the prolonged late discharges normally found in acute spinal cats. Fast paw shaking, elicited by dipping one paw in water, was abolished by clonidine and reappeared after yohimbine. These results indicate that noradrenergic drugs may influence both spinal locomotion and the excitability of cutaneous reflexes. This class of substances could thus play a useful role in the recovery and/or maintenance of locomotor functions after spinal trauma.

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

Clonidine (0.2 mg/kg i.v.) and St 587 (1 mg/kg i.v.), selective agonists of alpha 2- and alpha 1-adrenoceptors, respectively, increased the flexor reflex amplitude in the spinal rat. Yohimbine and rauwolscine, alpha 2-adrenoceptor antagonists, inhibited dose-dependently the effect of clonidine but not of St 587. However, prazosin and clozapine, alpha 1-adrenoceptor antagonists, diminished the action of both agonists in a dose-dependent manner. After central chemosympathectomy caused by 6-OH-DA or DSP-4 the response to clonidine did not differ from that in the sham-operated animals. In 6-OH-DA treated rats yohimbine (1 mg/kg i.v.) antagonized the effect of clonidine to the same extent as it did in unlesioned animals. It is concluded that the results are functional evidence that alpha 1- and alpha 2-adrenoceptors are present in the rat spinal cord, and stimulation of each receptor increases flexor reflex activity.

Relationship of CNS tryptaminergic processes and the action of LSD-like hallucinogens

Tryptamine produces pharmacologic effects in man and the chronic spinal dog which are similar to those produced by LSD, mescaline, psilocin, DMT, DOM and DOB. These effects include tachycardia, tachypnea, mydriasis, hyperreflexia, behavioral changes and in man, hallucinations. Chronic spinal dogs treated chronically with LSD became tolerant to its ability to produce mydriasis, tachycardia, tachypnea and hyperreflexia, and were cross tolerant to the ability of tryptamine, psilocin, mescaline, DMT, DOM and DOB to produce these same effects. Further, it was found that the brain and spinal cord contained tryptamine and could release it. Further tryptamine levels were higher in the brainstem and spinal cord above the level of transection in the chronic spinal dog that in intact dogs, and the same in the spinal cord below the level of transection. These observations suggested that there were both ascending and descending tryptaminergic pathways. Supporting this hypothesis were the observations that L-tryptophan also produced hyperreflexia in the acute, but not the chronic, spinal dog and cat, and that L-tryptophan hyperreflexia was antagonized by alpha-methyldopa but not pCPA. These observations and others argue that the spinal cord and brain have tryptaminergic mechanisms which are distinct from serotoninergic mechanisms, and that LSD-like hallucinogens act in part through a tryptaminergic mechanism.

Comparison of central and peripheral alpha 1-adrenoceptors

A series of alpha-adrenergic agonists and antagonists having diverse chemical structure was examined for both central and peripheral alpha 1-adrenoceptor activity. The agonists tested included several novel aminotetralin derivatives which were potent and selective alpha 1-agonists. Peripheral alpha 1-activity was determined in the isolated rabbit ear artery; central alpha 1-receptor affinity was measured as the ability to inhibit 3H-WB 4101 binding to rat brain homogenates. In the agonist series, an excellent correlation between peripheral alpha 1-activity and central alpha 1-affinity was obtained, providing that partial agonists were excluded. Likewise, the receptor dissociation constant for blockade of the peripheral alpha 1-adrenoceptor correlated well with affinity for the central receptor for all of the alpha-antagonists. These data support the conclusion that central and peripheral alpha 1-adrenoceptors are similar or identical.