Endomorphin 1 and 2 are two tetrapeptides recently isolated from bovine as well as human brains and proposed to be the endogenous ligand for the μ- opiate receptor Opioid compounds expressing μ-receptor preference are generally potent analgesics. The spinal cord dorsal horn is considered to be an important site for the processing of sensory information including pain. The discovery that endomorphins produced greater analgesia in mice upon intrathecal as compared to intracerebroventricular injections raises the possibility that dorsal horn neurons may represent the anatomic site upon which endomorphins exert their analgesic effects. We report here the detection of endomorphin 2-immunoreactive fiber-like elements in superficial layers of the rat dorsal horn by immunohistochemical techniques. Whole-cell patch recordings from substantia gelatinosa neurons of cervical spinal cord slices revealed two conspicuous effects of exogenously applied endomorphin 1 and 2: (i) depression of excitatory postsynaptic potentials evoked by stimulation of dorsal root entry zone, and (ii) hyperpolarization of substantia gelatinosa neurons. These effects were reversed by the selective μ-opiate receptor antagonist β-funaltrexamine. Collectively, the detection of endomorphin-like immunoreactivity in nerve fibers of the superficial layers and the inhibitory action of endomorphins on substantia gelatinosa neurons provide further support for a potential role of these two peptides in spinal nociception.

Nociceptin, also referred to as orphanin FQ, is believed to be the endogenous ligand for the ORL1. Nociceptin, when injected intracerebroventricularly to mice, produced hyperalgesia in behavioral tests. Recent studies have demonstrated the presence of ORL1 transcript in the spinal cord, and ORL1-like immunoreactivity has been localized to nerve fibers and somata throughout the spinal cord. Here, we report the localization of nociceptin-like immunoreactivity to fiber-like elements of the superficial layers of the rat dorsal horn by immunohistochemical techniques. Whole-cell recordings from substantia gelatinosa neurons in transverse lumbar spinal cord slices of 22-26-day-old rats showed that exogenous nociceptin at low concentrations (100-300 nM) depressed excitatory postsynaptic potentials evoked by stimulation of dorsal rootlets without causing an appreciable change of resting membrane potentials and glutamate- evoked depolarizations. At a concentration of 1 μM, nociceptin hyperpolarized substantia gelatinosa neurons and suppressed spike discharges. The hyperpolarizing and synaptic depressant action of nociceptin was not reversed by the known opioid receptor antagonist naloxone (1 μM). Our result provides evidence that nociceptin-like peptide is concentrated in nerve fibers of the rat dorsal horn and that it may serve as an inhibitory transmitter within the substantia gelatinosa.

Rapid receptive field (RF) reorganization of somatosensory neurons in the rat dorsal horn was examined using extracellular single unit recording. Subcutaneous injection of lidocaine into RFs of dorsal horn neurons results in expansion of their RFs within minutes. The expanded RFs appear adjacent to or/and proximal to original RFs. Out of 63 neurons tested, 36 (58%) show RF reorganization. The data suggest that dorsal horn of spinal cord is one of the initial sites for RF reorganization. The neural mechanisms of this effect are not well understood. We propose that changes in biophysical properties (membrane conductance, length constant) of the neurons resulting from lidocaine injection contribute to RF reorganization. Iontophoretic application of glutamate onto dorsal horn neurons that show lidocaine induced RF's expansion were used to test the model. Application of glutamate produced reduction of reorganized RFs in 9 of 20 (45%) tested cells. Application of NBQX produced no effect on either original or expanded RFs indicate that RF shrinkage effects of glutamate involve NMDA receptors. The results are consistent with the prediction of the proposed model. Subcutaneous injection of capsaicin into tactile RFs of low threshold mechanoreceptive dorsal horn neurons produced no effect on the RF sizes that are consistent with other studies. Following the injection, the original RFs were completely silenced (46%) or remained responsive (54%).