Z.Q. Zhao

Cutaneous stimuli releasing immunoreactive substance P in the dorsal horn of the cat.

In barbiturate-anaesthetized spinal cats, antibody microprobes were used to examine immunoreactive substance P (irSP) release at sites within the spinal cord following cutaneous stimuli. A basal level of irSP release was detected in the region of the substantia gelatinosa of the lumbar spinal cord. No increase in this irSP release was produced by non-noxious thermal or mechanical cutaneous stimulation. Noxious thermal, mechanical or chemical cutaneous stimuli all increased release of irSP in the region of the substantia gelatinosa and in the overlying pia mater. The results support a role for SP in the transmission of information from nociceptors to spinal neurones.

Noxious heating of the skin releases immunoreactive substance P in the substantia gelatinosa of the cat: A study with antibody microprobes

Using a new method, the antibody microprobe technique, the release of immunoreactive substance P (SPiR) in the dorsal horn in response to noxious heating of the skin, was studied in barbiturate anaesthetized spinal cats. Release of SPiR was not produced by immersing the ipsilateral hind paw in water at 37 degrees C. With water at 50 and 52 degrees C, however release was consistently detected in the region of the substantia gelatinosa. These results directly show a central release of SPiR with excitation of nociceptors by heat.

The effects of lesions of medullary midline and lateral reticular areas on inhibition in the dorsal horn produced by periaqueductal grey stimulation in the cat

In barbiturate-anaesthetized cats, the excitation of lumbar dorsal horn neurones by impulses in unmyelinated primary afferent fibres was inhibited by electrical stimulation in the periaqueductal grey matter. This inhibition was slightly reduced by extensive electrocoagulation of the medullary midline and para-medial areas including the raphé, but significantly reduced by small bilateral lesions in the region of the caudal lateral reticular nuclei. When the lateral lesions were made subsequent to midline coagulation, the inhibition from periaqueductal grey stimulation was abolished. An important component of spinal inhibition from periaqueductal grey stimulation appears to relay in lateral reticular areas of the medulla.

Opioid antagonists and spinal reflexes in the anaesthetized cat

Abstract In barbiturate-anaesthetized cats, intravenous naloxone (0.025–0.10 mg/kg) increased the amplitude of monosynaptic reflexes produced by electrical stimulation of the nerves to the biceps-semitendinosus and gastrocnemius muscles and the complex reflexes to electrical stimulation of myelinated afferents of the sural and tibial nerves and reflexes to electrical stimulation of unmyelinated primary afferents of the tibial nerve. Increases in reflexes were also produced by the (−)- but not the (+)-isomer of the opiate antagonist N-furylmethylnormetazocine (both isomers being given in the dose range 0.03–0.20 mg/kg). The doses of naloxone increasing reflexes to C primary afferents had no effect on the responses of some dorsal horn neurones with cutaneous receptive fields to the same stimuli. The results suggest that, in anaesthetized cats, inhibition involving opioid peptides at some stage is present on many motoneurones. This inhibition may have relevance to animal behaviour after injury.

Kelatorphan potentiates the effect of [Met5]enkephalin in the substantia gelatinosa of the cat spinal cord.

In anaesthetized spinal cats, kelatorphan, an inhibitor of enkephalin degradation, was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous peripheral stimuli. When administered near the cell bodies of laminae IV and V neurons, kelatorphan neither altered evoked responses nor potentiated the inhibition by [Met5]enkephalin of these cells. When ejected in the substantia gelatinosa, however, kelatorphan reduced the nociceptive responses of some laminae IV and V neurones, an effect blocked by electrophoretic naloxone. The selective inhibition of nociceptive responses by [Met5]enkephalin administered in the substantia gelatinosa was markedly potentiated by co-administration of kelatorphan, and this effect was also blocked by electrophoretic naloxone. Neurones inhibited by administration of kelatorphan alone in the substantia gelatinosa were excited by administration of naloxone alone at the same site. The results suggest that some dorsal horn neurones are tonically inhibited by an action of opioid peptides in the substantia gelatinosa, and indicate that enzymic degradation limits the action of both exogenous and endogenous enkephalin in this spinal region.

Microelectrophoretic administration of naloxone near motoneurones fails to reproduce the effects of systemic naloxone in anaesthetized cats

Using parallel micropipette assemblies intracellular recordings were obtained from motoneurones of barbiturate-anaesthetized cats while administering amino acids, opioid peptides and naloxone extracellularly. On motoneurones depolarized by DL-homocysteate or hyperpolarized by glycine, the opioid peptides had no effect on membrane potential. Naloxone did not increase evoked EPSP amplitudes and thus failed to reproduce the increases produced by systemic administration of this opioid antagonist. It is probable therefore that the tonic inhibition of motoneurones present in anaesthetized cats which is revealed by naloxone, does not involve release of opioid peptides near motoneurones but rather these compounds control the activity of spinal interneurones.

A function of opioid peptides in the spinal cord of the cat: intracellular studies of motoneurones during naloxone administration.

Abstract In barbiturate anaesthetized spinal cats, doses of naloxone which increased spinal reflexes increased the amplitude of excitatory postsynaptic potentials (EPSPs) recorded intracellularly from motoneurones. In most cases these increases in EPSPs occurred in the absence of changes in membrane potential although small depolarizations following naloxone were recorded with one third of neurones. Somatic membrane conductance was not altered by naloxone. Tonic inhibition involving opioid peptides is thus commonly present on motoneurones under the conditions of these experiments.

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.

The selective effects of metorphamide on dorsal horn neurones of the cat spinal cord

Abstract In anaesthetized cats, the proenkephalin A derivative metorphamide was administered microelectrophoretically while recording the excitation of lumbar dorsal horn neurones by noxious and innocuous cutaneous stimuli. Administered in the substantia gelatinosa or more ventrally near the cell bodies, metorphamide reduced the nociceptive responses and spontaneous firing of laminae IV and V neurones. These effects were reversed by electrophoretic administration of naloxone at the same site. Responses to innocuous stimulation were partially but inconsistently reduced by metorphamide administration near cell bodies, and not reduced by administration in the substantia gelatinosa. These results suggest that metorphamide may be an endogenous ligand at opioid μ-receptors, and may also act at κ-receptors.

Substance P release in the cat spinal cord upon afferent C-fibre stimulation is not attenuated by clonidine at analgesic doses.

In anaesthetized cats, antibody microprobes were used to measure the release of immunoreactive substance P (irSP) in the lumbar dorsal horn during electrical stimulation of primary afferent fibres at intensities suprathreshold for unmyelinated fibres. Release of irSP was detected in the region of the superficial dorsal horn. This evoked release was not reduced by clonidine hydrochloride, administered intravenously or by superfusion of the dorsal cord surface. Microprobes inserted during cord superfusion with lignocaine hydrochloride detected less irSP along their entire length, including in the region of evoked release. The results suggest that the analgesic action of clonidine does not involve reduced release of SP from the central terminals of nociceptors in the spinal cord.