Electrophysiological Heterogeneity of Spinally Projecting Serotonergic and Nonserotonergic Neurons in the Rostral Ventromedial Medulla

Dept. of Anesthesia, University of Iowa, 200 Hawkins Dr., 6 JCP, Iowa City, IA 52242, USA.
Journal of Neurophysiology (Impact Factor: 2.89). 04/2006; 95(3):1853-63. DOI: 10.1152/jn.00883.2005
Source: PubMed


This study examined the passive membrane and action potential properties of serotonergic and nonserotonergic neurons in the rostral ventromedial medulla (RVM) of the rat using whole cell patch-clamp recording techniques in the slice. Serotonergic neurons were identified by immunoreactivity for tryptophan hydroxylase (TrpH). Spinally projecting neurons were retrogradely labeled with 1'-dioactadecyl-3,3,3',3'-tetramethylindocarbodyanine perchlorate (DiI). Three types of neurons were identified within both spinally projecting serotonergic and nonserotonergic populations. Type 1 neurons exhibited irregular or sporadic spontaneous activity interspersed with periods of quiescence. Type 2 neurons were not spontaneously active and were additionally discriminated by a more negative resting membrane potential and a larger-amplitude action potential. Type 3 neurons fired repetitively without pause. Serotonergic neurons had a higher membrane resistance and greater action potential half-width than their nonserotonergic counterparts and rarely exhibited a fast afterhyperpolarization. Serotonergic type 3 neurons also fired more slowly and regularly than nonserotonergic type 3 neurons. Comparison of electrophysiological and immunohistochemical characteristics suggested that the smallest type 3 serotonergic neurons had an increased risk of immunohistochemical "misclassification" due to failure to detect TrpH, possibly due to more complete dialysis of intracellular contents during lengthy recordings. This risk was minimal for type 1 or 2 serotonergic neurons. The three different types of spinally projecting serotonergic neurons also differed markedly in their responsiveness to the mu opioid receptor agonist D-Ala2, NMePhe4, Gly5-ol]enkephalin. These results provide important new electrophysiological and pharmacological evidence for a significant heterogeneity among spinally projecting serotonergic RVM neurons. They may also provide a basis for resolving the controversy concerning the role of serotonergic RVM neurons in opioid analgesia.

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    • "As expected, DAMGO produced an outward current in serotonergic and non-serotonergic RVM neurons that was completely reversed by 1 μM naloxone (Fig. 4A) and persisted in the presence of 1 μM tetrodotoxin (see Fig. 7 of [50]). However, only 11 of 36 (30.6%) nonserotonergic and 9 of 31 (29.0%) "
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    ABSTRACT: Direct inhibition of pain facilitatory neurons in the rostral ventromedial medulla (RVM) is one mechanism by which mu opioid receptor (MOPr) agonists are proposed to produce antinociception. The antinociceptive and anti-hyperalgesic effects of the MOPr agonist DAMGO are enhanced after intraplantar injection of complete Freund's adjuvant (CFA). This study therefore examined whether CFA treatment similarly enhanced the ability of DAMGO to induce outward currents in spinally projecting RVM neurons. It further examined whether the electrophysiological properties of RVM neurons are altered by CFA treatment. Whole-cell patch clamp recordings were made from three types of serotonergic as well as non-serotonergic spinally projecting RVM neurons obtained from control rats and rats 4h or four days after CFA. Persistent, but not acute inflammatory nociception increased the percentage of Type 2 non-serotonergic neurons that responded to DAMGO from 17% to 57% and the percentage of Type 3 serotonergic neurons that responded to DAMGO from 5% to 55%. These same two populations of RVM neurons exhibited significant differences in their passive membrane properties or spontaneous discharge rate. The outward currents produced by the GABA(B) receptor agonist baclofen were not enhanced, suggesting that the enhancement does not reflect global changes in levels of G(i/o) or activity of G-protein regulated inwardly rectifying potassium channels. These results provide a cellular basis for the enhanced anti-hyperalgesic and antinociceptive effects of MOPr agonists under conditions of persistent inflammatory nociception. These results also provide intriguing, albeit indirect, evidence for two different populations of pain facilitatory neurons in the RVM.
    Full-text · Article · Feb 2010 · Pain
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    • "Electrophysiological investigations of pontomedullary serotonergic neurons in the rat, however, have reported a particularly low discharge rate of 1.8 Hz (Mason 1997), which is similar to the spontaneous activity measured in the present investigation. Another study, also in rat, found that spinally projecting serotonergic neurons displayed a particularly large action potential half width (Zhang et al. 2005), a finding that also corresponds to the results presented here. "
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    ABSTRACT: We have characterized, by intracellular recording and staining combined with immunocytochemistry, a serotonin-immunoreactive neuron in the central olfactory pathway of the male moth Helicoverpa assulta. The neuron joins the unique category of so-called SI antennal-lobe neurons, previously described in several insect species. In similarity with that originally discovered in the sphinx moth Manduca sexta, the neuron identified here has a large soma located posteriorly in the lateral cell cluster of the antennal lobe and an unbranched neurite projecting into the ipsilateral protocerebrum via the inner antennocerebral tract. After bypassing the central body, the axon crosses the midline and extends through the corresponding antennocerebral tract to the contralateral antennal lobe where it innervates the entire assembly of glomeruli including the male-specific macroglomerular complex. The neuron arborizes into several fine branches in bilateral protocerebral regions anterior to the calyces of the mushroom bodies, particularly on the contralateral side. The physiology of the neuron revealed 2 distinctly different spiking amplitudes, 1 small showing a relatively high spontaneous activity and 1 large showing low activity. The small-amplitude spikes displayed increased frequency when pheromones and plant odors were blown over the antenna. The large-amplitude spikes, which had an unusually long duration, showed no observable responses.
    Full-text · Article · Apr 2009 · Chemical Senses
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    • "Electrophysiological classification of RVM neurons into so-called " on " (i.e., pain facilitatory) and " off " (pain inhibitory ) cell groups surprisingly did not include 5HT neurons (Potrebic et al., 1994; Mason, 1997; Gao and Mason, 2000). Rather, serotonergic neurons constitute a heterogeneous population, with slow, regular discharge patterns and variable responses to noxious stimuli and to opioid agonists (Gao and Mason, 2001; Zhang et al., 2006). Indeed , Gao et al. (1998) concluded that neither 5HT nor activity of serotonergic cells is required for the analgesia evoked by opioids. "
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    ABSTRACT: Despite the evidence for a significant contribution of brainstem serotonergic (5HT) systems to the control of spinal cord "pain" transmission neurons, attention has turned recently to the influence of nonserotonergic neurons, including the facilitatory and inhibitory controls that originate from so-called "on" and "off" cells of the rostroventral medulla (RVM). Unclear, however, is the extent to which these latter circuits interact with or are influenced by the serotonergic cell groups. To address this question we selectively targeted expression of a transneuronal tracer, wheat germ agglutinin (WGA), in the 5HT neurons so as to study the interplay between the 5HT and non-5HT systems. In addition to confirming the direct medullary 5HT projection to the spinal cord we also observed large numbers of non-5HT neurons, in the medullary nucleus reticularis gigantocellularis and magnocellularis, that were WGA-immunoreactive, i.e., were transneuronally labeled from 5HT neurons. FluoroGold injections into the spinal cord established that these reticular neurons are not only postsynaptic to the 5HT neurons of the medulla, but that most are also at the origin of descending, bulbospinal pathways. By contrast, we found no evidence that neurons of the midbrain periaqueductal gray that project to the RVM are postsynaptic to midbrain or medullary 5HT neurons. Finally, we found very few examples of WGA-immunoreactive noradrenergic neurons, which suggests that there is considerable independence of the monoaminergic bulbospinal pathways. Our results indicate that 5HT neurons influence "pain" processing at the spinal cord level both directly and indirectly via feedforward connections with multiple non-5HT descending control pathways.
    Full-text · Article · Apr 2008 · The Journal of Comparative Neurology
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