Thyrotropin-releasing hormone- and GABA-like immunoreactivity coexist in neurons in the dorsal horn of the rat spinal cord

Expression of gastrin-releasing peptide by excitatory interneurons in the mouse superficial dorsal horn

Background

Gastrin-releasing peptide (GRP) and its receptor have been shown to play an important role in the sensation of itch. However, although GRP immunoreactivity has been detected in the spinal dorsal horn, there is debate about whether this originates from primary afferents or local excitatory interneurons. We therefore examined the relation of GRP immunoreactivity to that seen with antibodies that label primary afferent or excitatory interneuron terminals. We tested the specificity of the GRP antibody by preincubating with peptides with which it could potentially cross-react. We also examined tissue from a mouse line in which enhanced green fluorescent protein (EGFP) is expressed under control of the GRP promoter.

Results

GRP immunoreactivity was seen in both primary afferent and non-primary glutamatergic axon terminals in the superficial dorsal horn. However, immunostaining was blocked by pre-incubation of the antibody with substance P, which is present at high levels in many nociceptive primary afferents. EGFP⁺ cells in the GRP-EGFP mouse did not express Pax2, and their axons contained the vesicular glutamate transporter 2 (VGLUT2), indicating that they are excitatory interneurons. In most cases, their axons were also GRP-immunoreactive. Multiple-labelling immunocytochemical studies indicated that these cells did not express either of the preprotachykinin peptides, and that they generally lacked protein kinase Cγ, which is expressed by a subset of the excitatory interneurons in this region.

Conclusions

These results show that GRP is expressed by a distinct population of excitatory interneurons in laminae I-II that are likely to be involved in the itch pathway. They also suggest that the GRP immunoreactivity seen in primary afferents in previous studies may have resulted from cross-reaction of the GRP antibody with substance P or the closely related peptide neurokinin A.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

The role of spinal orexin-1 receptors in posterior hypothalamic modulation of neuropathic pain

The posterior hypothalamus (PH) is known to reduce nociceptive pain, but the effect of PH stimulation on neuropathic pain is not known. Because neurons containing the neurotransmitter orexin-A are located in the PH in some strains of rat and intrathecal injection of orexin-A produces antinociception in a neuropathic pain model, we hypothesized that orexin-A from neurons in the PH modifies nociception in the spinal cord dorsal horn. To test this hypothesis, the cholinergic agonist carbachol or normal saline was microinjected into the PH of lightly anesthetized female Sprague-Dawley rats with chronic constriction injury (CCI) and foot withdrawal latencies (FWL) were measured. Carbachol-induced PH stimulation produced dose dependent antinociception as shown by significantly increased FWL compared to saline controls. To investigate the role of orexin-A in PH-induced antinociception, the orexin-1 receptor antagonist SB-334867 or dimethyl sulfoxide (DMSO) for control, was given intrathecally following carbachol-induced PH stimulation. SB-334867 decreased FWL compared to DMSO controls. These data are suggestive that stimulating the PH produces antinociception in a neuropathic pain model and that the antinociceptive effect is mediated in part by orexin-1 receptors in the spinal cord dorsal horn.

Distribution of thyrotropin-releasing hormone immunoreactivity in the brain of the dogfish Scyliorhinus canicula

To improve knowledge of the peptidergic systems of elasmobranch brains, the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers was studied in the brain of the small-spotted dogfish (Scyliorhinus canicula L.). In the olfactory bulbs, small granule neurons richly innervated the olfactory glomeruli. In the telencephalic hemispheres, small TRHir neurons were observed in the superficial dorsal pallium, whereas TRHir fibers were widely distributed in pallial and subpallial regions. In the preoptic region, TRHir neurons formed a caudal ventrolateral group in the preoptic nucleus. In the hypothalamus, the most conspicuous TRHir populations were associated with the lateral hypothalamic recess, but small TRHir populations were found in the posterior tubercle and ventral wall of the posterior recess. The preoptic region and hypothalamus exhibited rich innervation by TRHir fibers. TRHir fibers were observed coursing to the neurohypophysis and the neuroepithelium of the saccus vasculosus, but not to the neurohemal region of the median eminence. Some stellate-like TRHir cells were observed in a few cell cords of the neurointermediate lobe of the hypophysis. The thalamus, pretectum, and midbrain lacked TRHir neurons. Further TRHir neuronal populations were observed in the central gray and superior raphe nucleus of the isthmus, and a few TRHir cells were located in the nucleus of the trigeminal descending tract at the level of the rostral spinal cord. In the brainstem, the central gray, interpeduncular nucleus, secondary visceral region of the isthmus, rhombencephalic raphe, inferior olive, vagal lobe, and Cajal's commissural nucleus were all richly TRHir-innervated. Comparison of the distribution of TRHir neurons observed in the dogfish brain with that observed in teleosts and tetrapods reveals strong resemblance but also interesting differences, indicating the presence of both a conserved basic vertebrate pattern and a number of derived characters.

Distribution of thyrotropin-releasing hormone immunoreactivity in the brain of larval and adult sea lampreys, Petromyzon marinus L

This study investigated the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers in the brain and retina of lampreys. Our results in the brains of large larvae and upstream-migrating adults of the sea lamprey showed the presence of TRHir neurons mainly in the preoptic region and the hypothalamus. A few TRHir neurons were also found in the striatum. The number and staining intensity of TRHir neurons increased from larval stages to adulthood, and the distribution of TRHir populations was wider in adults. The TRHir fibers were more easily traced in adults. Some TRHir fibers entered the neurohypophysis, although most fibers coursed in the different regions of the brain, mostly in the basal region, from the forebrain to the hindbrain. The presence of TRHir stellate cells was observed in the adenohypophysis. In the retina of adult lampreys, but not in that of larvae, TRHir amacrine cells are present.

Thyrotropin releasing hormone stimulation of GABA-gated but not basal chloride ion influx in rat cerebellum

Potential interaction between gamma-amino butyric acid (GABA) and thyrotropin-releasing hormone (TRH) in eliciting a variety of central nervous system (CNS)-related biologic activities is well known; however, the mechanism underlying this interaction is not clearly defined. To gain further insight into this interaction, we examined the effects of TRH and two of its central nervous system selective analogs, DN 1417 and TA 0910, on basal and GABA-mediated chloride ion influx into rat cerebellar neurosynaptosomes. The results of these studies show that TRH may facilitate GABA action by augmenting chloride ion influx and hyperpolarization of cerebellar neurons.

The types of neuron which contain protein kinase C gamma in rat spinal cord

Protein kinase C (PKC) is thought to have a role in sensitization of dorsal horn neurons in certain pain states, and a recent study has reported that mice which lack the gamma isoform (PKCgamma) show reduced neuropathic pain after peripheral nerve injury. Although PKCgamma is present at high levels in the ventral part of lamina II we have limited information concerning the types of neuron in which it is located. In this study we have used immunocytochemistry to characterise the neurons which contain PKCgamma. Immunoreactive neurons were concentrated in ventral lamina II, but were also present in lamina III. Some weakly-immunoreactive neurons were located in the dorsal part of lamina II and in lamina I. The great majority (92%) of cells with PKCgamma were not GABA-immunoreactive, and these cells are likely to be excitatory interneurons. Dual-immunofluorescence labelling showed that PKCgamma was not randomly distributed amongst non-GABAergic neurons, since it was present in 76% of cells with neurotensin and 45% of those with somatostatin, but only 5% of those with the mu-opioid receptor (MOR-1). Cells with the neurokinin 1 receptor are found in lamina I and lamina III, and PKCgamma was present in 22% and 37% of these populations, respectively. These results suggest that excitatory interneurons in laminae II and III which lack the micro-opioid receptor may have a significant role in generating neuropathic pain.

Differential effects of intrathecal thyrotropin-releasing hormone (TRH) on perineal reflexes in male rats

The effects of thyrotropin-releasing hormone (TRH) on the sexual and defecatory reflexes regulated by pudendal motoneurons were investigated. Intrathecal TRH (10 microliters volume; 0.0, 0.01, 1.0 or 100 microM concentration) at lumbosacral spinal segments (L4-S1) in acute preparations produced a dose-dependent increase in external anal sphincter (EAS), but not bulbospongiosus (BS), electromyographic (EMG) activity. Intraspinal (L6) injection of 100 microM TRH (1 microliter/micropipette), significantly increased EAS EMG activity in acute preparations. Electromyographic activity of the BS muscle was unchanged. All doses of intrathecal TRH (10 microliters volume; 0, 10, 50, 100, or 500 microM concentration) in awake animals significantly reduced the proportion of responders to a penile reflex test. Subsequently, all measures of penile reflexes were significantly reduced. Glans tumescence and defecation bouts before or during penile reflex testing were unaffected by intrathecal TRH as were indices of behavioral and motor hyper-reactivity analogous to that produced by serotonin. These data indicate that pudendal motoneurons, in the dorsomedial nucleus, are differentially regulated by neuropeptides present in the lumbosacral spinal cord.

The neuropeptide thyrotropin-releasing hormone modulates GABAergic synaptic transmission on pyramidal neurones of the rat hippocampal slice

The modulatory action of thyrotropin-releasing hormone (TRH), an endogenously occurring neuropeptide, on synaptic potentials mediated by activation of GABAA or GABAB receptors was studied using intracellular recordings from CA1 pyramidal neurones of the rat hippocampal brain slice preparation. Bath-applied TRH (10 microM) produced a reversible depression of fast IPSPs (mediated by GABAA receptors) induced by electrical stimulation of the stratum lacunosum moleculare (LM) or stratum pyramidale (SP). This phenomenon was not associated with changes in the IPSP reversal potential, resting potential, or input resistance. GABAB receptor-mediated slow IPSPs elicited by SP stimulation were found insensitive to TRH whereas those induced by LM stimulation were attenuated by the peptide. AMPA receptor-mediated EPSPs and postsynaptic responses to isoguvacine or baclofen were unchanged by TRH. These data suggest that the action of TRH on GABAergic transmission was probably exerted at presynaptic level within the local circuitry comprising CA1 neurones. Such an effect of TRH represents an interesting example of transient downregulation of inhibitory processes by a physiological neuropeptide and is expected to augment excitability of pyramidal cells.

Galanin is contained in GABAergic neurons in the rat spinal dorsal horn

In order to determine which types of neuron in laminae I-III of the rat spinal dorsal horn contain the peptide galanin, pre-embedding immunocytochemistry with antiserum to galanin was combined with post-embedding detection of GABA- and glycine-like immunoreactivities. Sixty-eight galanin immunoreactive neurons in laminae I-III selected from four rats were examined, and in each case semi-thin sections through the cell body were tested with a monoclonal antibody to GABA and an antiserum to glycine. All of the 68 galanin-immunoreactive neurons tested were GABA-immunoreactive, while only one of them (in lamina III) was glycine-immunoreactive. This suggests that galanin is contained in inhibitory interneurons, and that (like enkephalin, neuropeptide Y and thyrotropin-releasing hormone) it is mainly restricted to GABAergic neurons which do not use glycine as a co-transmitter.

Some inhibitory neurons in the spinal cord develop c-fos-immunoreactivity after noxious stimulation

In order to determine which types of spinal neuron produce c-fos in response to noxious stimulation, we have combined pre-embedding detection of c-fos-like immunoreactivity with post-embedding immunocytochemistry using antibodies against GABA and glycine, 2 h after subcutaneous injection of formalin into a hindpaw of anaesthetized rats. Throughout the spinal cord, the majority of c-fos-immunoreactive neurons (72-81%) did not possess GABA- or glycine-like immunoreactivity, while the remaining cells contained one or both types of immunoreactivity. In the superficial dorsal horn (laminae I and II) and dorsal white matter, between 14 and 20% of c-fos-immunoreactive neurons were GABA-immunoreactive, and some of these were also glycine-immunoreactive. A single neuron in lamina I in one animal was glycine- but not GABA-immunoreactive. In the remainder of the spinal cord, between 21 and 35% of the c-fos-immunoreactive cells were GABA- or glycine-immunoreactive, and the majority of these neurons contained both types of immunoreactivity. These results suggest that some inhibitory neurons in both the superficial and deep parts of the dorsal horn are activated by noxious stimuli. It is known that some of the cells which produce c-fos in response to noxious stimulation are projection neurons, with axons ascending to the brainstem or thalamus, however, because of the large number of c-fos-immunoreactive cells in the dorsal horn, it is likely that many are interneurons, and some of these are probably excitatory cells which use glutamate as a transmitter. It therefore appears that after noxious stimulation c-fos is produced in several types of spinal neuron, including projection cells and both excitatory and inhibitory interneurons.