Inhibition of a cAMP-dependent Ca-activated K conductance by forskolin prolongs Ca action potential duration in lamprey sensory neurons

Cyclic AMP stimulates neurite outgrowth of lamprey reticulospinal neurons without substantially altering their biophysical properties

Reticulospinal (RS) neurons are critical for initiation of locomotor behavior, and following spinal cord injury (SCI) in the lamprey, the axons of these neurons regenerate and restore locomotor behavior within a few weeks. For lamprey RS neurons in culture, experimental induction of calcium influx, either in the growth cone or cell body, is inhibitory for neurite outgrowth. Following SCI, these neurons partially downregulate calcium channel expression, which would be expected to reduce calcium influx and possibly provide supportive conditions for axonal regeneration. In the present study, it was tested whether activation of second messenger signaling pathways stimulates neurite outgrowth of lamprey RS neurons without altering their electrical properties (e.g. spike broadening) so as to possibly increase calcium influx and compromise axonal growth. First, activation of cAMP pathways with forskolin or dbcAMP stimulated neurite outgrowth of RS neurons in culture in a PKA-dependent manner, while activation of cGMP signaling pathways with dbcGMP inhibited outgrowth. Second, neurophysiological recordings from uninjured RS neurons in isolated lamprey brain-spinal cord preparations indicated that dbcAMP or dbcGMP did not significantly affect any of the measured electrical properties. In contrast, for uninjured RS neurons, forskolin increased action potential duration, which might have increased calcium influx, but did not significantly affect most other electrical properties. Importantly, for injured RS neurons during the period of axonal regeneration, forskolin did not significantly alter their electrical properties. Taken together, these results suggest that activation of cAMP signaling by dbcAMP stimulates neurite outgrowth, but does not alter the electrical properties of lamprey RS neurons in such a way that would be expected to induce calcium influx. In conclusion, our results suggest that activation of cAMP pathways alone, without compensation for possible deleterious effects on electrical properties, is an effective approach for stimulating axonal regeneration of RS neuron following SCI.

Cholera toxin-B subunit blocks excitatory opioid receptor-mediated hyperalgesic effects in mice, thereby unmasking potent opioid analgesia and attenuating opioid tolerance/dependence

In a previous study we demonstrated that injection (i.p.) of low doses of GM1 ganglioside in mice rapidly attenuates morphine's analgesic effects. This result is consonant with our electrophysiologic studies in nociceptive types of dorsal root ganglion (DRG) neurons in culture, which showed that exogenous GM1 rapidly increased the efficacy of excitatory (Gs-coupled) opioid receptor functions. By contrast, treatment of DRG neurons with the non-toxic B-subunit of cholera toxin (CTX-B) which binds selectively to GM1, blocked the excitatory, but not inhibitory, effects of morphine and other bimodally-acting opioid agonists, thereby resulting in a net increase in inhibitory opioid potency. The present study provides more direct evidence that endogenous GM1 plays a physiologic role in regulating excitatory opioid receptor functions in vivo by demonstrating that cotreatment with remarkably low doses of CTX-B (10 ng/kg, s.c.) selectively blocks hyperalgesic effects elicited by morphine or by a kappa opioid agonist, thereby unmasking potent opioid analgesia. These results are comparable to the effects of cotreatment of mice with morphine plus an ultra-low dose of the opioid antagonist, naltrexone (NTX) which blocks opioid-induced hyperalgesic effects, unmasking potent opioid analgesia. Low-dose NTX selectively blocks excitatory opioid receptors at their recognition site, whereas CTX-B binds to, and interferes with, a putative allosteric GM1 regulatory site on excitatory opioid receptors. Furthermore, chronic cotreatment of mice with morphine plus CTX-B attenuates development of opioid tolerance and physical dependence, as previously shown to occur during cotreatment with low-dose NTX.

Contributions of identifiable neurons and neuron classes to lamprey vertebrate neurobiology

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished.

GM1 ganglioside-induced modulation of opioid receptor-mediated functions

Electrophysiologic studies of dorsal-root ganglion (DRG) neurons in culture have demonstrated both excitatory (Gs-coupled) as well as inhibitory (Gi/Go-coupled) opioid receptor-mediated actions. Brief treatment of DRG neurons with cholera toxin-beta which binds specifically to GM1 sites on neuronal membranes, selectively blocks opioid excitatory but not inhibitory effects. Conversely, after brief treatment of DRG neurons with GM1, but not with GM2, GM3, or other related gangliosides, the threshold concentration of opioid agonists for eliciting excitatory effects is markedly decreased from nM to pM-fM levels and opioid antagonists, for example, naloxone (NLX), at low concentrations paradoxically elicit excitatory effects. These studies suggest that the excitatory opioid supersensitivity of GM1-treated DRG neurons is due primarily to increased efficacy of excitatory opioid-receptor activation of Gs. Recent studies of cloned delta opioid receptors transfected into CHO cells suggest that this supersensitivity of GM1-treated DRG neurons may be further augmented by rapid conversion of many opioid receptors from a Gi/Go-coupled inhibitory mode to a Gs-coupled excitatory mode. The opioid excitatory supersensitivity elicited in DRG neurons by acute elevation of exogenous GM1 provides novel insights into mechanisms underlying opioid tolerance and dependence, since remarkably similar supersensitivity occurs in DRG and other neurons after chronic treatment with morphine or other opioid agonists that upregulate endogenous GM1.

Brief treatment of sensory ganglion neurons with GM1 ganglioside enhances the efficacy of opioid excitatory effects on the action potential

In previous studies, we showed that low (nM) concentrations of opioid prolong the action potential duration (APD) of many mouse dorsal root ganglion (DRG) neurons via Gs-linked excitatory opioid receptors, whereas micromolar opioid levels shorten the APD via Gi/Go-linked inhibitory receptors. In addition, cholera toxin-B subunit (CTX-B) selectively blocks opioid- but not forskolin-induced prolongation of the APD in DRG neurons. Since CTX-B binds with selective high affinity to GM1 ganglioside located on the cell surface, the results suggest that GM1 plays an essential role in regulating excitatory opioid receptor functions. This hypothesis was tested by treating DRG neurons in mouse DRG-cord explants with exogenous gangliosides and determining whether the efficacy of opioid agonists in prolonging the APD is enhanced. The threshold concentration of the opioids, dynorphin(1-13) and morphine required to prolong the APD in many DRG neurons was markedly decreased from nM to fM levels after bath exposure to 10 nM to 1 microM GM1 ganglioside for less than 5 min. In contrast, GM2 and GM3 gangliosides and asialo-GM1 ganglioside were ineffective, even when DRG neurons were exposed to high concentrations (1-10 microM) for periods greater than 1 h. Although GD1a, GD1b and GQ1b gangliosides appeared to be as effective as GM1 when tested at microM concentrations for 15 min, tests at lower concentrations, shorter periods, and/or at lower temperature (24 degrees vs 34 degrees C), showed that they were significantly less effective than GM1.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholera toxin-B subunit blocks excitatory effects of opioids on sensory neuron action potentials indicating that GM1 ganglioside may regulate Gs-linked opioid receptor functions

In a previous study, we demonstrated that cholera toxin-A subunit, as well as the whole toxin, selectively blocks opioid-induced prolongation of the Ca2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons, indicating mediation of this excitatory effect by Gs-linked opioid receptors. The present study shows that pretreatment of DRG neurons with the B subunit of cholera toxin (1-10 ng/ml; greater than 15 min) can also block mu/delta and kappa opioid-induced APD prolongation, but not shortening. Since the B subunit binds selectively to GM1 ganglioside located on the cell surface, these results suggest that this ganglioside may regulate Gs-linked excitatory opioid receptor functions in DRG neurons. Possible contamination of purified B subunit preparations of cholera toxin with traces of the more potent A subunit was eliminated by heating the stock solution to 56 degrees C for 20 min. Exposure of DRG neurons to an affinity-purified anti-GM1 antiserum also blocked opioid-induced APD prolongation, providing further evidence that GM1 ganglioside may play an essential role in excitatory opioid modulation of the action potential of these cells. The blockade by cholera toxin-B subunit and anti-GM1 antibodies of opioid-induced APD prolongation is best accounted for by the following hypothesis: CTX-B interferes with an endogenous GM1 ganglioside component of the excitatory, but not inhibitory, opioid receptor complex on DRG neurons that may allosterically regulate coupling of the receptors via Gs to adenylate cyclase/cyclic adenosine monophosphate-dependent ionic conductances.

Cholera toxin-A subunit blocks opioid excitatory effects on sensory neuron action potentials indicating mediation by Gs-linked opioid receptors

Our previous studies indicated that opioid-induced prolongation of the Ca2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons is mediated by excitatory opioid receptors that are coupled to cyclic AMP-dependent voltage-sensitive ionic conductances. In the present study, DRG neurons were treated with cholera toxin (CTX), or with the A subunit of CTX, in order to determine if these excitatory opioid receptors are positively coupled via the GTP-binding protein Gs to the adenylate cyclase/cyclic AMP system. In contrast, inhibitory opioid receptors have been shown to be linked to pertussis toxin-sensitive Gi/Go regulatory proteins that mediate APD shortening responses. After pretreatment of DRG-spinal cord explants with remarkably low concentrations of CTX-A (1 pg/ml-1 ng/ml; greater than 15 min) or whole toxin (1 pg/ml-1 microgram/ml) the APD prolongation elicited in DRG neurons by 1-10 nM delta/mu (DADLE) or kappa (U-50,488H) opioids was blocked (29 out of 30 cells), whereas APD shortening by microM opioid concentrations was unaffected. Opioid-induced APD prolongation was blocked even when the initial treatment with CTX or CTX-A alone did not prolong the APD. The blocking effects of CTX and CTX-A were reversed in tests made 2 h after return to control medium. The mechanisms underlying the unusually potent blocking effects of CTX and CTX-A on opioid excitatory modulation of the APD of DRG neurons require correlative biochemical analyses.(ABSTRACT TRUNCATED AT 250 WORDS)