Brain-stem areas tonically inhibiting dorsal horn neurones: studies with microinjection of the GABA analogue piperidine-4-sulphonic acid.

In barbiturate anaesthetized cats, tonic inhibition of the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferents was measured by reversibly cooling the spinal cord at the thoraco-lumbar junction. Tonic inhibition was reduced by microinjection of the GABA analogue, piperidine-4-sulphonic acid (2.5 nM in 0.5 microliter) mainly at AP -7, L 2-5 and V -8 to -10. This area in the ventrolateral medulla is just ventral to the facial nucleus and has been shown to be important in cardiovascular control, particularly in relation to fear-defence reactions. It is proposed that tonic inhibition of the nociceptive responses of spinal neurones is part of such a reaction in response to the trauma of surgery. Since previous experiments had shown that the ventrolateral medulla was important in spinal inhibition produced by PAG stimulation, these experiments support the proposal that analgesia does not occur in isolation but is part of a complex behavioural response of an animal in a potentially injurious environment.

A differential effect of naloxone on transmission of impulses in primary afferents to ventral roots and ascending spinal tracts.

Ventral root reflexes and ascending volleys to stimulation of group I muscle afferents, large diameter cutaneous afferents and unmyelinated primary afferents were examined in barbiturate anaesthetized spinal cats. Intravenous naloxone (0.05-0.10 mg/kg) increased reflexes to stimulation of all primary afferent types but of the ascending volleys, only those to stimulation of unmyelinated primary afferents were increased. Thus it appears that opioid peptides have differential effects on transmission of primary afferent impulses to supraspinal areas, an action possibly relevant to analgesia, in contrast to a non-selective suppression of transmission to motoneurones.

Antinociceptive effect of intracisternal carbamazepine evidenced by the bradykinin-induced biting-like response in rats.

Intracisternal injections of carbamazepine produced a rapid and dose-dependent suppressive effect on the biting-like response induced by microapplication of bradykinin onto the tooth pulp (ED50: 2.62 micrograms/rat). Thus, the medullary dorsal horn is probably one of the primary sites for the antinociceptive activity of carbamazepine.

Methysergide and spinal inhibition from electrical stimulation in the periaqueductal grey.

In barbiturate anaesthetized cats, electrical stimulation in the periaqueductal grey (PAG) selectively inhibited the excitation of multireceptive dorsal horn neurons by noxious heating of the skin or by electrical stimulation of unmyelinated primary afferents. Intravenous methysergide (0.2-1.0 mg/kg) had opposing effects on uninhibited responses, increasing excitation by noxious heat but reducing responses to C fibre stimulation. Evidence was obtained that the increases to noxious heat resulted from increased firing of peripheral nociceptors secondary to decreased cutaneous blood flow. Intravenous methysergide reduced inhibition from PAG stimulation. When administered electrophoretically from micropipettes however, methysergide did not reduce such inhibition. The mechanism whereby systemic methysergide reduces PAG spinal inhibition is unknown and cannot necessarily be related to a blockade of spinally released 5-hydroxytryptamine.

Bicuculline and spinal inhibition produced by dorsal column stimulation in the cat.

In barbiturate anaesthetized cats, dorsal column stimulation inhibited ascending volleys recorded in the antero-lateral spinal fasciculus from electrical stimulation of the contralateral tibial nerve and the excitation of neurones of the dorsal horn by noxious heating of the skin. The inhibition was non-selective. Intravenous bicuculline (0.2-0.6 mg/kg) reduced dorsal column induced inhibition of ascending volleys. Bicuculline but not strychnine, administered electrophoretically from micropipettes, reduced dorsal column induced inhibition of the excitation of dorsal horn neurones by noxious heat. These findings suggest that the inhibition studied was produced by release of gamma-aminobutyric acid. This amino acid may play a role in the clinical suppression of pain produced by dorsal column stimulation.

The periaqueductal gray is the site of the antinociceptive action of carbamazepine as related to bradykinin-induced trigeminal pain.

Using freely moving and conscious rats, the antinociceptive effects of microinjections of carbamazepine, into the periaqueductal gray (PAG), nucleus reticularis paragigantocellularis (NRPG) and nucleus raphé magnus (NRM) on the biting-like responses induced by bradykinin applied to the tooth pulp, were investigated to determine the primary site of action of this drug. Microinjections of carbamazepine into the PAG ipsi- and contralateral to the stimulated tooth pulp produced dose-dependent suppressive effects on the biting-like responses within 1 min. The ED50 was 1.57 micrograms per rat, that is about 1,500 times less than that for carbamazepine administered systemically. The antinociceptive effect of carbamazepine administered into the PAG was inhibited by pretreatment with bicuculline but not by phentolamine, propranolol and haloperidol. Microinjections of carbamazepine into the NRPG and NRM were rarely effective in the production of antinociception at doses used (up to 3 micrograms per rat). These results suggest that the PAG is one of the primary target sites for the antinociceptive activity of carbamazepine, and that GABAergic systems are involved this action of carbamazepine.

Lack of interaction between methionine enkephalin and D-phenylalanine on nociceptive and non-nociceptive responses of dorsal horn neurones of the cat.

In the spinal cord of the barbiturate anaesthetized cat, D-phenylalanine (DPA) depressed spontaneous and amino acid-induced firing of multireceptive dorsal horn neurones. The excitation of these neurones by noxious and non-noxious cutaneous stimuli was non-selectively depressed by DPA. DPA did not potentiate the depressant action of methionine enkephalin on these cells. Although naloxone reduced the depressant action of DPA, this was associated with increases in spontaneous firing. Spinal reflexes in this preparation are tonically inhibited by opioid peptides. Intravenous DPA (20-70 mg/kg) did not potentiate this inhibition but produced small increases in spinal reflexes. Collectively these data do not support the hypothesis that analgesia from DPA results from potentiation of endogenously released methionine enkephalin.

Neurotransmitter-blocking agents influence antinociceptive effects of carbamazepine, baclofen, pentazocine and morphine on bradykinin-induced trigeminal pain.

The influence of naloxone (a narcotic antagonist), bicuculline (a GABA antagonist), phentolamine (an alpha-blocking agent), propranolol (a beta-adrenergic blocking agent), haloperidol (a dopaminergic blocking agent), methysergide (a serotonergic blocking agent) and atropine (a muscarinic blocking agent), on the antinociceptive effects induced by carbamazepine, baclofen, pentazocine and morphine, were investigated with a new antinociception test, using the trigeminal pain induced by application of bradykinin onto the tooth pulp of the rat. The antinociceptive effect of carbamazepine was significantly inhibited by bicuculline, phentolamine, propranolol and haloperidol but not by naloxone, methysergide and atropine. The effect of baclofen was significantly reduced by naloxone, bicuculline, propranolol and atropine but not by phentolamine, haloperidol and methysergide. The antinociceptive actions of pentazocine and morphine on trigeminal pain were significantly reduced by naloxone and phentolamine, and by naloxone alone, respectively. These results suggest the involvement of different neurotransmitters in the antinociceptive effects of the four analgesic drugs on trigeminal pain induced by bradykinin.

Analgesic potencies of non-narcotic, narcotic and anesthetic drugs as determined by the bradykinin-induced biting-like responses in rats.

Aversive (nociceptive) biting-like responses induced by micro-application of bradykinin solution onto rat tooth pulp were dose-dependently suppressed by non-narcotic drugs such as baclofen and lidocaine as well as carbamazepine and phenytoin, which are employed for clinical treatment of trigeminal neuralgia. The potency order of these drugs on a molar basis is baclofen (4.20) greater than carbamazepine (1.00) greater than lidocaine (0.94) greater than phenytoin (0.19). Such responses were also inhibited by morphine, pentazocine and cyclazocine (potency ratio of the three general analgesics, 1.00:0.46:8.11), indomethacin (a non-narcotic and anti-inflammatory analgesic) and alpha-chloralose (an anesthetic). The latter drug produced an analgesic effect at doses much lower than those used for anesthesia. These findings suggest that our method is feasible for evaluating the activities of general and particular analgesic drugs in the trigeminal regions.

A mechanism of carbamazepine-analgesia as shown by bradykinin-induced trigeminal pain.

Single neuronal responses induced by micro-application of bradykinin solution to the tooth pulp of the rabbit were recorded in the subnucleus reticularis dorsalis and trigeminal subnucleus caudalis and characterized by intermittent episodes of paroxysmal activities. Carbamazepine, the primary agent for trigeminal neuralgia, suppressed such a paroxysmal response, suggesting that an analgesic action of the drug results from the suppression of synaptic transmission at the subnucleus reticularis dorsalis and trigeminal subnucleus caudalis.

A newly devised reliable method for evaluating analgesic potencies of drugs on trigeminal pain.

A pain-producing substance, bradykinin (0.63--1.25 ng in 0.5--1.0 microliter of distilled water), was applied onto the tooth pulp of the lower incisor of unrestrained rats through a cannula fixed on the surfaces of the incisors using a microsyringe. Such a microapplication of bradykinin induced biting response and other aversive behaviors. When the microapplications were repeated at intervals of 60 min, the biting response seemed most characteristic and reproducible. Therefore, the duration of biting response was used as a measure in studying the effects of drugs on trigeminal pain. If biting duration was inhibited 90% or more after drug administration the effect was considered analgesic. Carbamazepine and phenytoin, which are clinically employed for treating trigeminal neuralgia, and morphine, a narcotic analgesic, produced dose-dependent analgesic effects in this method and the corresponding ED50 values were 13.1 intraperitoneally (ip), 75.0 ip, and 3.00 subcutaneously (sc) mg/kg, respectively. On the other hand, pentobarbital (15 mg/kg, ip), diazepam (1.0 mg/kg, ip), and aspirin (150 mg/kg, ip) were not effective in suppressing the biting response. These results indicate that this newly devised method in the rat is reliable and feasible in evaluating pain related the trigeminal system.

Sites of analgesic action of cyclazocine: a study using evoked-potentials at various regions of the CNS by electrical stimulation of rabbit tooth pulp.

Effects of cyclazocine on the evoked-potentials at the trigeminal subnucleus caudalis, nucleus ventralis posteromedialis of the thalamus, somatosensory S1 area of the cerebral cortex and dorsal hippocampus following electrical stimulation of the tooth-pulp were studied using the rabbit. Pentazocine and morphine were used as reference drugs. Cyclazocine in doses of 0.5-1.0 mg/kg, i.v., significantly suppressed the evoked-potential recorded at the dorsal hippocampus but not those at the other sites. The hippocampal evoked-potential was significantly inhibited also by pentazocine (5.0-10 mg/kg, i.v.) and morphine (2.0-4.0 mg/kg, i.v.). The latter but not the former significantly depressed the evoked-potential at the S1 area of the cerebral cortex. The evoked-potential at the trigeminal subnucleus caudalis was not affected by these three drugs in the doses indicated. These results suggest that cyclazocine as well as pentazocine and morphine inhibit the electrically induced painful impulses at the hippocampus and/or the afferent pathways to the hippocampus from the trigeminal subnucleus caudalis such as the tooth-pulp-hypothalamo-hippocampal pathway.

Analgesic action of cyclazocine: blocking nociceptive responses induced by intra-arterial bradykinin-injection and tooth pulp stimulation.

Cyclazocine, a benzomorphan derivative, suppressed the flexor reflex of the hind-limb of intact rats induced by intra-arterial injection of bradykinin, a potent pain-producing substance, in a dose-dependent manner, without remarkable influence on motor performance. This suppressive effect was antagonized by naloxone, a specific opiate antagonist. The ED50 values for cyclazocine were 0.054 mg/kg s.c., 5.6 mg/kg p.o., and 1/27, 1/11 of those for pentazocine by the respective routes of administration. In spinal rats, however, the inhibitory effect of cyclazocine and pentazocine on the bradykinin-induced flexor reflex was markedly reduced. Furthermore, cyclazocine as well as pentazocine selectively inhibited the EEG arousal response induced by electrical stimulation of the tooth pulp, which elicits the single sensation of pain. These results indicate that cyclazocine in doses used had a specific analgesic action, and that the main site of action probably is in the supra-spinal structures, such as seen in the case of pentazocine.