Inhibition from nucleus raphe magnus of tooth pulp responses in medial reticular neurones of the cat can be antagonized by bicuculline

Ultrastructural evidence for GABA-mediated disinhibitory circuits in the spinal cord of the cat

The synaptic relationships between gamma-aminobutyric acid (GABA)-immunoreactive and enkephalin-immunoreactive profiles in the cat spinal cord were examined using combined pre-embedding immunoperoxidase and post-embedding immunogold electron microscopic immunocytochemistry. Although colchicine was not used, enkephalin-immunoreactive somata and dendrites were detected in regions associated with nociceptive transmission, including laminae I, II, V and X. In each of these laminae, many GABA-immunoreactive terminals were found presynaptic to enkephalin-immunoreactive cell bodies and dendrites. We propose that disinhibition of opioid-containing neurons may be a common feature of pain-related circuits in the cat spinal cord.

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 role of 5-HT, GABA and opioid peptides in presynaptic inhibition of tooth pulp input from the medial brainstem

In decerebrate or chloralose-anaesthetized cats electrical stimulation in the spinal trigeminal nucleus evoked antidromic responses in the mandibular canine tooth pulp. Conditioning stimulation in nucleus raphe magnus (NRM) and in the adjacent contralateral medullary reticular formation, nucleus reticularis gigantocellularis (NRGC) and nucleus reticularis magnocellularis (NRMC), produced a decrease in the threshold for the antidromic responses in a proportion of the tooth pulp inputs. This was interpreted as being due to depolarization of the tooth pulp afferent terminals, reflecting presynaptic inhibition. The primary afferent depolarization (PAD) of tooth pulp afferent terminals by NRM stimulation could be selectively blocked by bicuculline applied intravenously or by iontophoresis in the terminal region. Intravenous naloxone, cinanserin and methysergide had no effect on the PAD evoked from NRM, NRGC or NRMC. Thus NRM appears to exert presynaptic inhibitory control of A delta tooth pulp input to the spinal trigeminal nucleus via GABA-containing neurones.

Naloxone reversible inhibition of reticular neurones in the rat caudal medulla produced by electrical stimulation of the periaqueductal grey matter

Chronic dorsal periaqueductal grey matter electrodes were implanted into adult rats under pentobarbitone anaesthesia. Stimulating these electrodes (25-300 microA) produced behavioural analgesia in 23 of 44 rats tested. In rats given the opiate antagonist naloxone attenuation of this analgesia was seen. In 14 rats displaying behavioural analgesia to periaqueductal grey matter stimulation acute electrophysiological experiments were performed under urethane anaesthesia. Microelectrode recordings were made from neurones, excited by noxious heat or pinch applied to the limbs and tail, and located in the reticular formation of the caudal medulla. Stimulation of the periaqueductal grey matter at an intensity sufficient to produce analgesia in the conscious animal produced direct inhibition of the firing of 62% of neurones tested, excited 23%, had no effect on 14% and attenuated the nociceptive responses of 66%. The inhibitions were characteristically long. Local application of naloxone by microiontophoresis attenuated these long inhibitions in 11 out of 16 neurons tested. Immunohistochemical localization of beta-endorphin containing structures in the vicinity of stimulating and recording sites suggested that the naloxone sensitive inhibition of nociceptive neuronal responses in caudal medulla reticular formation may be due to activation of beta-endorphin fibres descending through the periaqueductal area to the caudal medulla.

Actions of GABA, glycine, methionine-enkephalin and beta-endorphin compared with electrical stimulation of nucleus raphe magnus on responses evoked by tooth pulp stimulation in the medial reticular formation in the cat

In decerebrate, cerebellectomized cats, a comparison was made between the effects of electrical stimulation in nucleus raphe magnus (NRM) and iontophoretic application of GABA, glycine, met-enkephalin and beta-endorphin on the responses of neurones in the medial brain stem reticular formation to tooth pulp stimulation. NRM stimulation, GABA, glycine and enkephalin produced a short lasting inhibition of tooth pulp evoked responses whilst the time course of the inhibition produced by beta-endorphin was much slower, often lasting up to 1 h following a 3-7 min ejection period. The effects of GABA and glycine could be antagonised by iontophoresis of bicuculline and strychnine respectively whilst intravenous injection of naloxone antagonised the inhibition induced by the opioid peptides. In most neurones tested, inhibition of tooth pulp evoked responses by NRM stimulation was blocked by iontophoretic application of bicuculline but not by strychnine or naloxone (i.v.). We conclude that GABA may act as a transmitter which mediates the inhibitory effects of NRM on the responses of reticular neurones to tooth pulp stimulation. Thus GABA may be involved in stimulation produced analgesia.

Descending inhibitions from the nucleus raphe magnus and adjacent reticular formation to the dorsal horn of the rat are not antagonized by bicuculline or strychnine

The effects of iontophoretically applied bicuculline and strychnine on brainstem-and segmental-evoked inhibitions of dorsal horn cells have been examined in the rat. Both antagonists failed to reduce the descending inhibitory influences at concentrations which selectively blocked responses to GABA or glycine and segmentally-derived inhibitions of the same neurones. These results suggest that neither GABA nor glycine is involved in brainstem inhibition of rat dorsal horn cells.

Primary afferent depolarization of tooth pulp afferents by stimulation in nucleus raphe magnus and the adjacent reticular formation in the cat: effects of bicuculline

In decerebrate cats electrical stimulation in nucleus raphe magnus (NRM) and the adjacent region of the reticular formation (NRGC) produced a decrease in the threshold of antidromic responses evoked in the tooth pulp following stimulation in the trigeminal nucleus oralis. This primary afferent depolarization (PAD) of Adelta afferents was unaffected by naloxone given intravenously. Bicuculline, given intravenously or applied directly to the terminal region by iontophoresis, selectively antagonised the PAD evoked by stimulating in NRM. The results suggest that GABA is involved in mediating presynaptic inhibition of Adelta tooth pulp input following stimulation in NRM.