Inhibitory action of acetylcholine, baclofen and GTP-gamma-S on calcium channels in adult rat sensory neurons

M2-receptor subtype does not mediate muscarine-induced increases in [Ca(2+)](i) in nociceptive neurons of rat dorsal root ganglia

Multiple muscarinic receptor subtypes are present on sensory neurons that may be involved in the modulation of nociception. In this study we focused on the presence of the muscarinic receptor subtypes, M2 and M3 (M2R, M3R), in adult rat lumbar dorsal root ganglia (DRG) at the functional ([Ca(2+)](i) measurement), transcriptional (RT-PCR), and translational level (immunohistochemistry). After 1 day in culture exposure of dissociated medium-sized neurons (20-35 micrometer diam) to muscarine was followed by rises in [Ca(2+)](i) in 76% of the neurons. The [Ca(2+)](i) increase was absent after removal of extracellular calcium and did not desensitize after repetitive application of the agonist. This rise in [Ca(2+)](i) may be explained by the expression of M3R, which can induce release of calcium from internal stores via inositoltrisphospate. Indeed the effect was antagonized by the muscarinic receptor antagonist atropine as well as by the M3R antagonist, 4-diphenylacetoxy-N-(2 chloroethyl)-piperidine hydrochloride (4-DAMP). The pharmacological identification of M3R was corroborated by RT-PCR of total RNA and single-cell RT-PCR, which revealed the presence of mRNA for M3R in lumbar DRG and in single sensory neurons. In addition, RT-PCR also revealed the expression of M2R, which did not seem to contribute to the calcium changes since it was not prevented by the M2 receptor antagonist, gallamine. Immunohistochemistry demonstrated the presence of M2R and M3R in medium-sized lumbar DRG neurons that also coexpressed binding sites for the lectin I-B4, a marker for mainly cutaneous nociceptors. The occurrence of muscarinic receptors in putative nociceptive I-B4-positive neurons suggests the involvement of these acetylcholine receptors in the modulation of processing of nociceptive stimuli.

Muscarinic M2-receptors in rat thoracic dorsal root ganglia

The occurrence and distribution of the muscarinic M2-receptor subtype (M2R) was investigated in rat thoracic dorsal root ganglia (DRG). Messenger RNA for M2R was demonstrated by RT-PCR in total RNA from DRG. Immunoreactivity to M2R-protein was localized to 26% of sensory neurons, the majority of them (85%) belonging to the size class of 25-40 microm in diameter. Double-labeling (immuno)histochemistry revealed that all M2R-immunoreactive neurons bind the lectin, I-B4, whereas they are generally devoid of substance P-immunoreactivity. These data show the presence of M2R on a subpopulation of presumably nociceptive primary afferent neurons, thereby extending previous pharmacological and electrophysiological studies that indicated a role of M2R and/or M4R in inhibition of calcium channel currents in rat sensory neurons (Wanke, E., Bianchi, L., Mantegazza, M., Guatteo, E., Macinelli, E. and Ferroni, A., Muscarinic regulation of Ca2+ currents in rat sensory neurons: channel and receptor types, dose-response relationships and cross-talk pathways. Eur. J. Neurosci., 6 (1994) 381-391).

Multiple modulatory effects of dopamine on calcium channel kinetics in adult rat sensory neurons

1. The aim of this research was to study the modulatory effects induced on high-voltage-activated (HVA) calcium channels and pharmacologically isolated subtypes through dopamine receptor activation. 2. The experiments were carried out on acutely isolated adult rat sensory neurons, recorded by means of the whole-cell patch-clamp technique. 3. At saturating concentrations dopamine was effective in inducing: (a) a voltage-dependent prolongation of activation kinetics, (b) a voltage-independent scaling down of the currents without any changes in activation and inactivation kinetics, and (c) an acceleration of inactivation kinetics, not affected by a positive conditioning prepulse. 4. These three inhibitory effects were observed on N- and P/Q-type currents, whereas only a voltage-independent scaling up and/or scaling down was observed on L-type current. 5. The inhibitory effects were sometimes observed in isolation in different neurons, but more frequently they were variously combined in the same cell. A correlation analysis of these effects shows no relationship between them, corroborating the conclusion that they are mechanistically distinct. 6. The existence of an inactivating effect accounts for the occurrence of a voltage-dependent inhibitory effect in some cells without an apparent slowing down of activation kinetics, since the increased inactivation may mask the slow component of the activation. 7. The multiple modulatory effects on calcium channels, even on pharmacologically separated N-, L- and P/Q-currents, suggest that pharmacological and functional classifications do not necessarily match completely. 8. The multiple modulatory effects on HVA calcium currents may play a prominent role both in controlling the integrative properties of neurons and in regulating output at a presynaptic level.

Serotonergic inhibition of the T-type and high voltage-activated Ca2+ currents in the primary sensory neurons of Xenopus larvae

The primary sensory Rohon-Beard (R-B) neurons of Xenopus larvae are highly analogous to the C fibers of the mammalian pain pathway. We explored the actions of 5-HT by studying the modulation of Ca2+ currents. In approximately 80% of the acutely isolated R-B neurons, 5-HT inhibited the high voltage-activated (HVA) currents by 16% (n = 29) and the T-type currents by 24% (n = 41). The modulation of the T-type and the HVA currents was mimicked by selective 5-HT1A and 5-HT1D agonists: 8-OH-DPAT and L-694,247. The effects of the agonists were blocked by their respective 5-HT1A or 5-HT1D antagonists: p-MPPI and GR127935, suggesting that both 5-HT1A and 5-HT1D receptors were involved. Approximately 70% of the actions of 5-HT on HVA currents was occluded by omega-conotoxin-GVIA (N-type channel blocker), whereas the rest of the modulation ( approximately 30%) was occluded by <100 nM omega-agatoxin-TK (P/Q-type channel blocker). This suggests that 5-HT acts on N- and P/Q-type Ca2+ channels. Neither the modulation of the T-type nor that of the HVA currents was accompanied by changes in their voltage-dependent kinetics. Cell-attached patch-clamp recordings suggest that the modulation of the T-type channel occurs through a membrane-delimited second messenger. We have studied the functional consequences of the modulation of T-type Ca2+ channels and have found that these channels play a role in spike initiation in R-B neurons. Modulation of T-type channels by 5-HT therefore could modulate the sensitivity of this sensory pathway by increasing the thresholds of R-B neurons. This is a new and potentially important locus for modulation of sensory pathways in vertebrates.

Effects of L-baclofen and D-baclofen on the auditory system: a study of click-evoked potentials from the inferior colliculus in the rat

The drug baclofen is a potential treatment for severe tinnitus, but its action in relieving tinnitus is not known. Baclofen is available as an approved drug only in racemic form with about equal content of the two enantiomers. In the present paper we show that L-baclofen causes a considerable (40.7%) suppression of the amplitude of the second peak in the click-evoked response from the cochlear nucleus. Bipolar recordings from the external nucleus of the inferior colliculus showed that L-baclofen caused a reduction in the amplitude of three or four distinct peaks in this response. D-Baclofen had no detectable effect on the response from the cochlear nucleus, and had only a slight effect on one component of the response from the external nucleus of the inferior colliculus. The demonstrated effect of L-baclofen on excitation in the ascending auditory pathway indicates that this drug may be a potential treatment for hyperactive auditory disorders such as tinnitus and hyperacusis.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin and acetylcholine: further analysis of praziquantel-induced contraction of magnesium-paralysed Schistosoma mansoni

The nature of stimulus-induced flaccid paralysis produced in Mg(2+)-paralysed Schistosoma mansoni was investigated. Serotonin induced a dose-dependent, heterologous flaccid paralysis with an IC50 of 600 nM. This flaccid paralysis was a function of the extracellular Mg2+:Ca2+ ratio and was reversible. Tonic contractions produced by phorbol-12,13-dibutyrate or 60 mM K+ were reversed by the application of serotonin and flaccid paralysis was induced. These actions of serotonin were mimicked by forskolin and synergized by IBMX but the potassium channel blocker, 3,4-DAP, did not produce flaccid paralysis. When Mg(2+)-paralysed parasites were stimulated with 3,4-DAP, IBMX produced a dose-dependent flaccid paralysis with an IC50 of 11 microM. Membrane permeable analogues of cAMP and cGMP did not synergize with IBMX. Cholinergic agonists, but not other inhibitory substances, prevented the serotonin- and forskolin-induced and the IBMX-synergized flaccid paralysis but not that produced by praziquantel. The possible interactions of these agents with the muscle are discussed.

Two distinct modulatory effects on calcium channels in adult rat sensory neurons

D-ala2-D-leu5-enkephalin (100 to 1000 nM) reduces HVA Ca2+ currents of approximately 60% in 92% of the adult rat sensory neurons tested. In 80% of the cells sensitive to enkephalin, the reduction in Ca2+ current amplitude was associated with a prolongation of the current activation that was relieved by means of conditioning pulses in a potential range only about 10 mV positive to the current activation range in control conditions. The time course of the current activation was fitted to a single exponential in control, (tau = 2.23 msec +/- 0.14 n = 38) and double exponential with enkephalin, (tau 1 = 2.18 msec +/- 0.25 and tau 2 = 9.6 msec +/- 1, test pulse to -10 mV, 22 degrees C). A strong conditioning depolarizing prepulse speeded up the activation time course, completely eliminating the slow, voltage-sensitive exponential component, but it was only partial effective in restoring the current amplitude to control values. The voltage-independent inhibitory component that was not relieved could be recovered only by washing out enkephalin. In the remaining 20% of the cells affected, enkephalin decreased Ca2+ current amplitude without prolongation of Ca2+ channel activation. In these cases the conditioning voltage pulse was not effective in relieving the inhibition that persisted also at strong positive test potentials, on the outward currents. The voltage-dependent inhibition occurred slowly after enkephalin superfusion (tau congruent to 12 sec), whereas the voltage-independent one developed about ten times more rapidly. Dopamine (100 microM) could also induce both voltage-dependent and independent modulations. In some sensory neurons the two different effects were separately induced by the two substances. GTP-'y-S (100 ,uM) intracellularly perfused mimicked both the modulatory effects. The two modulations may have different functions in processing nociceptive inputs.

Neurotransmitter modulation of calcium channels is dependent on the charge carrier used in the recording of currents

Currents through calcium channels were recorded using calcium, barium and strontium as charge carriers in NG108-15 cells. The mean normalised peak current amplitude at 0 mV was not significantly different between the charge carriers; however, the sustained component (measured at the end of the 500 ms command step) was ca. 3 times larger in barium and strontium. Further, the inhibition by acetylcholine or noradrenaline, although the same at the peak of the current envelope, was significantly greater on the sustained portion of the current for barium and strontium. Increasing internal calcium-buffering (to reduce calcium-dependent inactivation with calcium as the charge carrier) did not increase the amount of inhibition of the sustained portion of current. These results suggest a cautious approach to analysis of neurotransmitter modulation of calcium currents using other charge carriers than calcium.

Effects of acetyl-L-carnitine on the survival of adult rat sensory neurons in primary cultures

Acetyl-L-carnitine produces a significant increase in the survival time-course of adult rat sensory neurons maintained in primary cultures up to 40 days. The analysis of our data suggests that 200 microM acetyl-L-carnitine added to the medium, slows down neuronal decay especially in the first 10 days in vitro, sparing a fraction of cells which would otherwise be lost. Patch-clamp recordings from these neurons show that superfusion with acetyl-L-carnitine (100-1000 microM) does not induce any membrane current. In addition an agonist muscarinic effect particularly concerning high-voltage activated calcium channel modulation appears to be ruled out. In conclusion our data favour the role of acetyl-L-carnitine in the trophism of sensory neurons in adult rats. In agreement with other in vivo experiments our data reinforce the hypothesis that this substance might be involved in reducing neuronal loss observed in nervous system aging.

Voltage-dependent modulation of calcium current by GTP gamma S and dopamine in cultured frog pituitary melanotrophs

Dopamine (1 microM) reversibly scaled down barium current through high-voltage activated (HVA) calcium channels but had little effect on the time course of current activation in cultured frog melanotrophs. Intracellular perfusion with guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 100 microM) sustained the effect of dopamine. Moreover, GTP gamma S drastically slowed down the current activation kinetics. The latter effect was in part reversed by dopamine. A conditioning prepulse to +70 mV facilitated the current in GTP gamma S-dialyzed cells but not in cells exposed to dopamine. These results suggest the existence of a dual G protein-mediated mechanism for reducing HVA calcium current.