Tonic descending inhibition in the stingray''s spinal cord

Reflexes elicited by peripheral nerve stimulation were compared in decerebrated stingrays with the spinal cord intact, after a lesion of the dorsal spinal cord and after spinal transection. Reflexes elicited in decerebrated stingrays are tonically inhibited. Lesions of the dorsal spinal cord release this tonic descending inhibition. The tonic descending inhibitory system in stingrays is comparable to that of mammals.     

Tonic descending modulation of spinal neurons with renal input.

Experiments were conducted to determine the influence of tonically active descending pathways on thoracolumbar spinal neurons that respond to renal nerve stimulation in anesthetized cats. We examined the effect of reversible blockade of spinal conduction on spontaneous activity, responses to renal nerve stimulation and responses to somatic stimuli of 71 spinal neurons. Mid-thoracic cold block resulted in enhanced responses (tonically inhibited neurons), reduced responses (tonically excited neurons), or did not affect neuronal responses. The spontaneous activity of 47 of 69 neurons (68%) increased from 7.3 +/- 2.0 spikes/s before cooling to 23.3 +/- 4.5 spikes/s during cooling. Activity of 8 neurons (12%) decreased while 14 (20%) had no change in activity. Cooling increased the responses of 51 of 71 neurons (72%) to renal nerve stimulation. Renal nerve stimulation evoked a two-fold increase in both short latency (early) and long latency (late) responses. Four neurons had a late response which was revealed by cold block. Cooling decreased responses of 8 of 71 neurons (11%) and 9 neurons (13%) were not affected. Cooling increased the early responses but decreased the late responses of 3 of 71 neurons (4%). All neurons had somatic receptive fields and 33 of 56 exhibited increased responses to somatic stimulation during cooling. In addition, receptive field sizes of 26 neurons increased. Four neurons had a decrease and 25 neurons had no change in receptive field size during cooling. These data indicate that tonically active descending pathways modulate the activity of most spinal neurons with renal input and the major effect of these pathways is inhibitory. This influence may be important in the modulation of spinal circuits that participate in reflexes evoked by renal afferent fibers.     

Evidence against a serotonin involvement in the tonic descending inhibition of nociceptor-driven neurons in the cat spinal cord

To determine whether serotonin (5-HT) is involved in the powerful tonic descending inhibition which exists on dorsal horn nociceptor-driven neurons, their response to a noxious stimulus was tested with drugs that enhance (fluoxetine) or decrease (p-chlorophenylalanine) 5-HT synaptic activity. Neither the response with the spinal cord intact nor the enhanced response with the spinal cord cold blocked was altered by these drugs. Thus we conclude that 5-HT is not involved in this tonic descending inhibition.     

Presynaptic inhibition of the monosynaptic reflex produced by injections of nicotine or eserine in the spinal cat

In completely deafferented cats with spinal cord transected and paralyzed with gallamine, the close arterial injection of nicotine (5 to 33 μg) into the spinal cord circulation or the intravenous injection of eserine (2 mg/kg) produced a transient increase in excitability of the central terminals of primary afferent fibers. Continuous d-c records from dorsal roots during nicotine injections indicated that a depolarization of the terminals was probably responsible for the observed excitability increase. Mecamylamine prevented both the depolarization and the increase in excitability of primary afferent central terminals whereas atropine and gallamine did not. The monosynaptic reflex was always depressed following injections of nicotine or eserine, but the excitability of the motorpool was found to be increased by these drugs. It was concluded that the monosynaptic reflex depression following injections of nicotine or eserine had a significant presynaptic inhibitory component as a consequence of a drug-induced depolarization of the central terminals of Group Ia afferent fibers.     

Properties of the descending limb of the spinobulbospinal micturition reflex pathway in the cat

The micturition reflex is thought to be mediated by a spinobulbospinal reflex pathway passing through the rostral pons. This study examined the properties of the descending limb of the reflex pathway by monitoring the responses of the lower urinary tract to stimulation of the pons in the decerebrate cat. Electrical stimulation (300 μs pulses at 50 Hz intratrain frequencies, 300–500 ms trains, 0.5–15 V) in the region of the locus coeruleus (P 0.5–3.1/L 2–4/H to −2.75) was used to activate the descending excitatory pathway to the sacral parasympathetic nucleus. Low intensity stimulation induced small amplitude, short duration (14 ± 11cm H₂O, 10 ± 3s) bladder contractions in a partially full bladder, whereas higher intensity stimulation induced large amplitude, long duration (69 ± 29cm H₂O, 70 ± 44s) contractions which were similar to distension-induced reflex micturition contractions. The evoked bladder contractions coincided with a reduction in external urethral sphincter (EUS) EMG activity. Following bilateral L₇-S₃ dorsal root transection, electrical stimulation of the pons still elicited the small amplitude bladder contractions, but the larger amplitude, long duration micturition contractions were abolished. During these small evoked bladder contractions, a suppression of EUS activity still occurred following deafferentation, indicating a pontine mediated bladder/EUS synergy. It is concluded that the pons can initiate bladder contractions and coordinated bladder-sphincter activity, but that afferent feedback (via the dorsal roots) is needed to maintain the large amplitude micturition contractions.     

Conduction velocity in spinal descending pathways of baro- and chemoreceptor reflex

Activity in the white rami T3 and L2 or 3 has been recorded and averaged with respect to excitation of the carotid sinus baroreceptor afferents produced by the pulsatile blood pressure (baroreceptor reflex) and with respect to brief trains of electrical stimuli exciting low threshold chemoreceptor afferents in the left carotid sinus nerve (chemoreceptor reflex). Experiments were performed on chloralose anaesthetized cats with both vago-depressor nerves cut. From the latency difference between the onset of the responses at the thoracic and their arrival at the lumbar level the spinal conduction velocity for the pathway of each reflex has been calculated. The baroreceptor reflex pathway has slower spinal conduction velocity 3.3 +/- 0.7 m/sec than the chemoreceptor pathway 5.5 +/- 0.9 m/sec. These results indicate that there are separate descending spinal pathways for the two types of reflexes.     

Presynaptic dopaminergic inhibition of the spinal reflex in rats.

Dopaminergic influence on spinal monosynaptic transmission was examined in rats. Monosynaptic mass reflex (MMR) was recorded from the ventral root L6 following supramaximal stimulation (0.2 Hz; 0.1 ms) to the ipsilateral dorsal root L6 in spinalized rat under pentobarbitone sodium (40 mg/kg, i.p.) anaesthesia. MMR was inhibited by intravenous administration of the dopaminergic agonist, apomorphine (50-200 ug/kg) in a dose-dependent manner. The attenuatory effect of apomorphine (200 ug/kg i.v.) on the reflex could be reversed by the dopaminergic antagonist haloperidol (0.5 mg/kg, i.v.). Under tetanic stimulation (200 Hz; 15s), the pretetanic relative inhibition induced by apomorphine (200 ug/kg, i.v.) was increased only for a short period immediately after the cessation of tetanic stimulation. The results indicate existence of presynaptic dopamine receptors on the afferent terminals converging on the motoneurone which may functionally modulate the spinal motor output.     

Forward and backward classical conditioning of the flexion reflex in the spinal cat

Effects of forward and backward conditioned-unconditioned stimulus (CS-US) intervals on classical conditioning of the flexion reflex were examined in a cat spinal preparation. Interstimulus intervals (ISIs) ranging from +3.0 to -3.0 sec were employed in 9 experimental groups and the results compared with those of an explicitly unpaired control group. Forward conditioning produced an asymmetrical, inverted U-shaped gradient relating magnitude of conditioning to ISI for both acquisition and extinction. The optimum ISI was 1.0 sec. Backward ISIs also produced excitatory conditioning, with optimal conditioning at -0.25 sec. Unlike forward conditioning, backward conditioning produced little sign of retention during extinction trials. The results, which parallel in several ways those of ISI effects in studies of intact animals, support the hypothesis that backward and forward conditioning may be fundamentally different phenomena, under the control of different neural processes.     

Effect of spinal norepinephrine depletion on descending inhibition of the tail flick reflex from the locus coeruleus and lateral reticular nucleus in the rat

The lateral reticular nucleus (LRN) and locus coeruleus-subcoeruleus (LC/SC), brainstem structures which overlap the A1 and A6 noradrenergic nuclei respectively, have been implicated in descending modulation of spinal nociceptive transmission. The present studies were designed to examine the role of norepinephrine (NE) in the mediation of inhibition of the nociceptive tail flick reflex produced by focal electrical stimulation in the LRN and LC/SC. Spinal NE was depleted by intrathecal administration of 6-hydroxydopamine (6-OHDA; 20 micrograms) and the threshold electrical stimulation in the LRN and the LC/SC necessary to inhibit the tail flick reflex in lightly pentobarbital-anesthetized rats was determined 9 and 14 days later. Despite a significant depletion (greater than 85%) of lumbar spinal cord NE content, there was no significant change in the tail flick inhibitory stimulation thresholds in the LRN or LC/SC. NE depletion did, however, potentiate the elevation in the inhibitory stimulation threshold in the LRN produced by intrathecal administration of the alpha 2-adrenoceptor antagonist, yohimbine, suggesting that upregulation of spinal adrenoceptors had occurred following 6-OHDA treatment. Adrenoceptor up-regulation was examined quantitatively by characterizing the dose-dependent antinociceptive potency of the selective alpha 2-adrenoceptor agonist clonidine 3, 7, 10, and 14 days following 6-OHDA administration, and analysis of [3H]rauwolscine binding to lumbar spinal cord 9 days following administration of the neurotoxin. The development of supersensitivity, defined as the leftward parallel shift of the dose-response curves for clonidine administered intrathecally, corresponded to the time course of NE depletion following 6-OHDA treatment on the days tested. Binding of [3H]rauwolscine to lumbar spinal cord revealed an elevation in the estimated Bmax without a change in the estimated Kd of the high affinity binding component 9 days following 6-OHDA administration. This study demonstrates that spinal adrenoceptor denervation supersensitivity develops rapidly following intrathecal administration of 6-OHDA and compensates for the selective destruction of spinal noradrenergic nerve terminals. Thus, the absence of effect of NE depletion on the tail flick inhibitory stimulation threshold in the LRN and the LC/SC does not argue against the hypothesis that spinopetal NE-containing neurons in these brainstem loci are involved in modulation of spinal nociceptive transmission.     

Prolonged inhibition of the flexor reflex by probing the cervix uteri in the cat

In decerebrate or spinal cats, sustained mechanical stimulation of the cervix uteri inhibited the flexor reflex elicited by electrical stimulation of the foot pad during the probing period (160 s). After probing, 3–15 min were required for reflex recovery. No additional inhibition was produced if probing was repeated before recovery, but instead the reflex was facilitated. When probing was applied 5–10 min after reflex recovery the reflex was again abolished. The recovery, however, occurred earlier and was followed by facilitation. Probing the cervix with single mechanical pulses inhibited transiently (140–200 ms) the short latency reflex components, but the components with longer latencies are unaffected or facilitated. Distension of the vaginal wall with balloon also inhibited the flexor reflex, but a transient, mild facilitation appeared several seconds after the distension. In general, whenever the inhibition decreases, the facilitation predominates. Our findings suggest that cervical probing or vaginal distension triggers both a long-lasting inhibition and a concomitant facilitation in different intraspinal flexor reflex pathways.     

Propriospinal control of interneurons in spinal reflex pathways from tendon organs in the cat

After chronic hemisection at C3, stimulation of propriospinal fibers in the dorsal quadrant at Th 10 facilitated disynaptic PSPs from Ib afferents in hindlimb motoneurons. Recording from interneurons monosynaptically activated from group I muscle afferents, and of extracellular focal synaptic potentials around them, revealed monosynaptic EPSPs from long propriospinal neurons. It is suggested that propriospinal neurons originating in the forelimb segments have direct excitatory connexions with interneurons of Ib reflex pathways to hindlimb motoneurons.     

The conduction velocity of the descending spinal pathway to the renal sympathetic nerve in the cat

The conduction velocity of the descending spinal excitatory pathway to the renal sympathetic nerve was measured in five chloralose-anaesthetised, spinal cats (C1 transection). Electrical stimuli were delivered to the dorsolateral funiculus at three levels between segments C3 and T6, and responses recorded from the ipsilateral renal nerve. Spinal conduction velocity was calculated as 4.4 +/- 0.4 m/s (mean +/- SEM), from the latency difference of renal nerve volleys to stimulation at different cord levels. A contralateral pathway to the renal nerve was identified: this also ran in the dorsolateral funiculus, and crossed below segment T5. It conducted at 4.1 and 7.9 m/s (two cats). Renal preganglionic conduction velocity (greater splanchnic nerve) was 4.1 and 5.0 m/s (two cats). As the renal sympathetic nerve is functionally homogeneous, these conduction velocity measurements are of a functionally-defined sympathoexcitatory pathway.     

The role of glutamate in opiate descending inhibition of nociceptive spinal reflexes

The present experiment examined descending inhibition of the nociceptive tail-flick reflex produced by microinjection of morphine and glutamate into the periaqueductal gray (PAG) matter and the neurotransmitters mediating the inhibition at the level of the nucleus raphe magnus (NRM). The longlasting opiate analgesia was significantly reduced by microinjection of excitatory amino acid antagonists 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (PCB, 3.25 mumol) or DL-2-amino-5-phosphono-valerate (APV, 25.38 mumol) into the NRM, whereas the short-lived glutamate analgesia was not. This indicates that although both opiate and non-opiate analgesia may originate in the PAG, the former is relayed through the NRM, whereas the latter is relayed by additional or different nuclei in the medulla. Two observations shed light on the question which receptors mediate the above effect in the NRM. First, PCB blocked morphine analgesia at doses that were 8 times lower than doses of APV that were effective. Second, analgesia produced by injection of glutamate into the NRM was antagonized by PCB (3.25 mumol), whereas APV (25.38 mumol) failed to do so. Together these results indicate that kainate/quisqualate, but not N-methyl-D-aspartate (NMDA), receptors are implicated in the NRM as a relay station in opiate descending inhibition.