Fast excitatory nicotinic transmission in the chick lateral spiriform nucleus

Cholinergic EPSCs and their potentiation by bradykinin in single paratracheal ganglion neurons attached with presynaptic boutons

We have found that bradykinin (BK) potentiates the nicotine-induced currents in airway paratracheal/parabronchial ganglia (PTG) neurons. In this study, we investigated if BK affects the cholinergic synaptic transmission in rat PTG neurons attached with synaptic buttons. Excitatory postsynaptic currents (EPSCs) were recorded in acutely dissociated PTG neurons attached with presynaptic boutons. EPSC frequency was increased in the high-K(+) external solution without affecting their amplitude. Activation and deactivation kinetics also did not change in the high-K(+) solution. Cd(2+) inhibited the EPSC frequency at 10(-7) M and also amplitude at higher concentrations without changing the kinetics. Mecamylamine inhibited both the amplitude and frequency of EPSCs and reduced the activation and deactivation kinetics. 10(-8) M BK potentiated the EPSC amplitude to 1.37 ± 0.19 times of preapplication control. In addition, its frequency was increased to 2.04 ± 0.41 times. BK did not affect the activation and deactivation kinetics. The effects of BK were mimicked by [Hyp(3)]-BK, a B2 kinin receptor agonist, whereas HOE 140, a B2 kinin receptor antagonist, abolished the effects of BK. In conclusion, BK potentiates the cholinergic synaptic transmission via B2 kinin receptors in the PTG. Since predominant control of airway function is thought to be exerted by cholinergic nerves arising from the PTG, the present findings might underlie at least partly the inflammatory pathological conditions of the lower airway.

Fast synaptic transmission in the goldfish CNS mediated by multiple nicotinic receptors

Usually nicotinic receptors in the central nervous system only influence the strength of a signal between neurons. At a few critical connections, for instance some of those involved in the flight response, nicotinic receptors not only modulate the signal, they actually determine whether a signal is conveyed or not. We show at one of the few such connections accessible for study, up to three different nicotinic receptor subtypes mediate the signal. The subtypes appear to be clustered in separate locations. Depending on the number and combination of the subtypes present the signal can range from short to long duration and from low to high amplitude. This provides a critical connection with a built-in plasticity and may enable it to adapt to a changing environment.

Nicotinic receptor-mediated biphasic effect on neuronal excitability in chick lateral spiriform neurons

Local neuronal circuits integrate synaptic information with different excitatory or inhibitory time windows. Here we report that activation of nicotinic acetylcholine receptors (nAChRs) leads to biphasic effects on excitability in chick lateral spiriform (SPL) neurons during whole cell recordings in brain slices. Carbachol (100 microM in the presence of 1 microM atropine) produced an initial short-term increase in the firing rates of SPL neurons (125+/-14% of control) that was mediated by postsynaptic nAChRs. However, after 3 min exposure to nicotinic agonists, the firing rate measured during an 800 ms depolarizing pulse declined to 19+/-7% (100 microM carbachol) or 26+/-8% (10 microM nicotine) of the control rate and remained decreased for 10-20 min after washout of the agonists. Similarly, after 60 s of electrically-stimulated release of endogenous acetylcholine (ACh) from cholinergic afferent fibers, there was a marked reduction (45+/-5% of control) in firing rates in SPL neurons. All of these effects were blocked by the nAChR antagonist dihydro-beta-erythroidine (30 microM). The inhibitory effect was not observed in Ca(2+)-free buffer. The nAChR-mediated inhibition depended on active G-proteins in SPL neurons and was prevented by the GABA(B) receptor antagonist phaclofen (200 microM), while the GABA(B) receptor agonist baclofen (10 microM) decreased firing rate in SPL neurons to 13+/-1% of control. The inhibitory response thus appears to be due to a nAChR-mediated enhancement of presynaptic GABA release, which then activates postsynaptic GABA(B) receptors. In conclusion, activation of nAChRs in the SPL initiates a limited time window for an excitatory period, after which a prolonged inhibitory effect turns off this window. The prolonged inhibitory effect may serve to protect SPL neurons from excessive excitation.

Nicotinic receptors mediate increased GABA release in brain through a tetrodotoxin-insensitive mechanism during prolonged exposure to nicotine

The effects of nicotine on the spontaneous release of GABA from nerve terminals in the chick lateral spiriform nucleus were examined using whole cell patch-clamp recording in brain slices. Exposure to 1 microM nicotine produced an early immediate increase in the frequency of spontaneous postsynaptic GABAergic currents. This effect was blocked in the presence of 0.5 microM tetrodotoxin. However, a prolonged application of 0.1-1 microM nicotine (>3 min) caused a tetrodotoxin-insensitive increase in the frequency of spontaneous GABAergic currents. This late tetrodotoxin-insensitive effect was blocked by the nicotinic antagonists dihydro-beta-erythroidine (30 microM) and mecamylamine (10 microM), but not by methyllycaconitine (50-100 nM), indicating that activation of high affinity nicotine receptors was mainly responsible for this effect. This enhancement was inhibited by the high threshold Ca(2+) channel blocker Cd(2+) (100 microM), but not by dantrolene or ryanodine. The tetrodotoxin-insensitive enhancement of the frequency of GABA currents by nicotine was reduced by inhibition of cAMP-dependent protein kinase with HA1004 (30 microM), but not by inhibition of protein kinase C with staurosporine (1 microM), and was facilitated by forskolin (10 microM) or bromo-cAMP (50 microM). The results indicate that nicotine-enhanced GABA release can operate through both tetrodotoxin-sensitive and -insensitive mechanisms in a single brain region and that a second messenger cascade may be involved in the tetrodotoxin-insensitive enhancement by nicotine.

Opioid receptor activation attenuates nicotinic enhancement of spontaneous GABA release in lateral spiriform nucleus of the chick

We examined the effects of opioids on the nicotinic enhancement of spontaneous GABA release from presynaptic terminals in the lateral spiriform nucleus (SpL) of the chick. Whole cell recordings from SpL neurons in brain slices were used to monitor spontaneous GABA release. Nicotine (1 microM) produced an 8-fold increase in the frequency of GABA events without changing their amplitude, consistent with an increase of GABA release from presynaptic terminals. L-enkephalin (1 microM) blocked these effects of nicotine on presynaptic GABA release, and the opioid antagonist naloxone (100 nM) antagonized the actions of L-enkephalin. The selective mu agonist DAMGO (300 nM) also attenuated the nicotine-mediated enhancement of GABA release, and the mu selective antagonist CTOP (1 microM) blocked the actions of DAMGO. In contrast, the kappa opioid agonist U50488 (3 microM) and the delta opioid agonist DPDPE (1 microM) had no effect. The results demonstrate that presynaptic release of GABA in the SpL can be regulated by both nicotinic agonists and mu opioids. While mu opioids have little effect on GABA release by themselves, they are able to block the marked enhancement of GABA release normally produced by nicotine. Since both cholinergic and enkephalinergic nerves are present in the SpL, the interactions of these two neurotransmitter systems may serve to precisely regulate GABA release in this brain region.

Cholinergic modulation of purinergic and GABAergic co-transmission at in vitro hypothalamic synapses

The lateral hypothalamus (LH) is an important center for the integration of autonomic and limbic information and is implicated in the modulation of visceral motor and sensory pathways, including those underlying feeding and arousal behaviors. LH neurons in vitro release both ATP and GABA. The control of ATP and GABA co-transmission in LH may underlie the participation of LH in basic aspects of arousal and reinforcement. LH neurons receive cholinergic input from the pedunculopontine and laterodorsal tegmental nuclei as well as from cholinergic interneurons within the LH per se. This study presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated enhancement of GABAergic, but not of purinergic, transmission despite the co-transmission of ATP and GABA at LH synapses in vitro. Facilitation of GABAergic transmission by nicotine is inhibited by antagonists of (alphabeta)*-containing nAChRs, but is unaffected by an alpha7-selective antagonist, consistent with a nAChR-mediated enhancement of GABA release mediated by non-alpha7-containing nAChRs. Activation of muscarinic ACh receptors enhances the release of ATP while concomitantly depressing GABAergic transmission. The independent modulation of ATP/GABAergic transmission may provide a new level of synaptic flexibility in which individual neurons utilize more than one neurotransmitter but retain independent control over their synaptic activity.

Properties of nicotinic receptors underlying Renshaw cell excitation by alpha-motor neurons in neonatal rat spinal cord

We used anatomical and physiological approaches to characterize nicotinic receptors (AChRs) on Renshaw cells of the neonatal rat spinal cord. Confocal imaging of Renshaw cells, identified by their characteristic pattern of gephyrin immunoreactivity, revealed that these neurons are immuno-positive for the alpha4 and beta2 AChR subunits but not for the alpha7 subunit. We used whole cell recording in spinal cord slices to characterize synaptic transmission from alpha-motor neurons to Renshaw cells, which could be identified pharmacologically by the sensitivity of transmission to d-tubocurarine. alpha-Motor neuron-to-Renshaw cell synapses were blocked by 10 microM dihydro-beta-erythroidine (dHbetaE), but not 50 nM methyllycaconitine (MLA), a selective alpha7 antagonist. These findings support a role for alpha4beta2-like AChRs, but not alpha7 AChRs, in rapid excitatory transmission between alpha-motor neurons and Renshaw cells in rat spinal cord.

Frequent occurrence of nicotinic acetylcholine receptors in GABAergic neurons of the chick visual system

Double-labeling immunohistochemical methods were used to investigate the occurrence of the alpha8 and alpha5 nicotinic receptor subunits in presumptive GABAergic neurons of the chick nervous system. Nicotinic receptor immunoreactivity was often found in cells exhibiting GABA-like immunoreactivity, especially in the visual system. The alpha8 subunit appeared to be present in presumptive GABAergic cells of the ventral lateral geniculate nucleus, nucleus of the basal optic root of the accessory optic system, and the optic tectum, among several other structures. The alpha5 subunit was also found in GABA-positive neurons, as observed in the lentiform nucleus of the mesencephalon and other pretectal nuclei. The numbers of alpha8- and alpha5-positive neurons that were also GABA-positive represented high percentages of the total number of neurons containing nicotinic receptor labeling in several brain areas, which indicates that most of the alpha8 and alpha5 nicotinic receptor subunits are present in GABAergic cells. Taken together with data from other studies, our results indicate an important role of the nicotinic acetylcholine receptors in the functional organization of GABAergic circuits in the visual system.

Distinct muscarinic receptors enhance spontaneous GABA release and inhibit electrically evoked GABAergic synaptic transmission in the chick lateral spiriform nucleus

The effects of muscarinic agonists on GABAergic synaptic transmission were examined using whole-cell patch-clamp recording in chick brain slices containing the lateral spiriform nucleus. Bath application of muscarine (10 microM) both increased the frequency of spontaneous GABAergic postsynaptic currents and reduced the amplitude of evoked GABAergic polysynaptic postsynaptic currents elicited by focal afferent fiber electrical stimulation. Both of these muscarinic actions were reversible and dose-dependent. Two M(1) antagonists, telenzepine and pirenzipine, and to a lesser extent the M(2) antagonist methoctramine, protected against muscarine's inhibition of the evoked polysynaptic currents. Other M(2) antagonists (tripitramine and gallamine) as well as the M(3) antagonist 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride) and an M(4) antagonist (tropicamide) provided little or no protection against muscarine in this assay. In contrast, 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride, tropicamide and telenzepine, but not pirenzepine, methoctramine, tripitramine and gallamine, blocked muscarine's enhancement of spontaneous GABAergic currents. McN-A-343 [(4-hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride] and CDD-0097 (5-propargyloxycarbonyl-1,4,5,6-tetrahydropyrimidine hydrochloride), two M(1) agonists, mimicked muscarine's inhibition of the evoked polysynaptic GABAergic currents but did not mimic muscarine's enhancement of spontaneous GABAergic currents. Both actions of muscarine persisted when slices were pretreated with pertussis toxin or N-ethylmaleimide, which inactivate G-proteins coupled to M(2) and M(4) receptors while leaving G-proteins coupled to M(1), M(3) and M(5) receptors intact. Muscarine had no significant effect on the amplitude of the direct postsynaptic current elicited by exogenous GABA in the presence of tetrodotoxin. The results demonstrate that distinct muscarinic receptors oppositely modulate GABAergic transmission in the lateral spiriform nucleus. The receptor mediating the inhibition of evoked GABAergic polysynaptic currents is pharmacologically similar to an M(1) receptor, while the enhancement of spontaneous GABAergic currents appears to be mediated by an M(3) receptor.

Nicotinic synapses formed between chick ciliary ganglion neurons in culture resemble those present on the neurons in vivo

We studied nicotinic synapses between chick ciliary ganglion neurons in culture to learn more about factors influencing their formation and receptor subtype dependence. After 4--8 days in culture, nearly all neurons displayed spontaneous excitatory postsynaptic currents (sEPSCs), which occurred at about 1 Hz. Neurons treated with tetrodotoxin displayed miniature EPSCs (mEPSCs), but these occurred at low frequency (0.1 Hz), indicating that most sEPSCs are actually impulse driven. The sEPSCs could be classified by decay kinetics as fast, slow, or biexponential and, reminiscent of the situation in vivo, were mediated by two major nicotinic acetylcholine receptor (AChR) subtypes. Fast sEPSCs were blocked by alpha-bungarotoxin (alpha Bgt), indicating dependence on alpha Bgt-AChRs, most of which are alpha 7 subunit homopentamers. Slow sEPSCs were unaffected by alpha Bgt, and were blocked instead by the alpha 3/beta 2-selective alpha-conotoxin-MII (alpha CTx-MII), indicating dependence on alpha 3*-AChRs, which lack alpha 7 and contain alpha 3 subunits. Biexponential sEPSCs were mediated by both alpha Bgt- and alpha 3*-AChRs because they had fast and slow components qualitatively similar to those comprising simple events, and these were reduced by alpha Bgt and blocked by alpha CTx-MII, respectively. Fluorescence labeling experiments revealed both alpha Bgt- and alpha 3*-AChR clusters on neuron somata and neurites. Colabeling with antisynaptic vesicle protein antibody suggested that some alpha 3*-AChR clusters, and a few alpha Bgt-AChR clusters are associated with synaptic sites, as is the case in vivo. These findings demonstrate the utility of ciliary ganglion neuron cultures for studying the regulation of nicotinic synapses, and suggest that mixed AChR subtype synapses characteristic of the neurons in vivo can form in the absence of normal inputs or targets.

alpha7-Containing nicotinic receptors are segregated to the somatodendritic membrane of the cholinergic neurons in the avian nucleus semilunaris

Segregation of ion channels and neurotransmitter receptors is an important mechanism for determining the functionality of the nervous system. In the case of nicotinic acetylcholine receptors, electrophysiological and anatomical studies have demonstrated that these receptors can be located at the somatodendritic and the axon terminal portions of neurons. Functionally, somatodendritic nicotinic receptors mediate fast excitatory transmission and possibly regulate other cell functions, while presynaptic nicotinic receptors enhance the release of neurotransmitters from axon terminals. Neurons in the mesencephalic lateral spiriform nucleus of the chick do not appear to restrict the localization of nicotinic receptors to specific membrane compartments, since receptors containing alpha5 and/or beta2 subunits are found both on the cell bodies and on the axonal projections of these neurons [Torrao A. S. et al. (1996) Brain Res. 743, 154-161]. We report here that, in contrast to lateral spiriform neurons, neurons in the nucleus semilunaris do appear to compartmentalize nicotinic receptors. The cholinergic nucleus semilunaris neurons express a high density of alpha7-containing nicotinic receptors on their somas [Britto L. R. G. et al. (1992) J. comp. Neurol. 317, 325-340]. However, when we examined the projections of these neurons in the lateral spiriform nucleus, we found no evidence for expression of alpha7-containing receptors on the cholinergic fibers from nucleus semilunaris neurons. Furthermore, patch-clamp electrophysiological recording from lateral spiriform neurons indicated an absence of presynaptic alpha7-containing nicotinic receptors capable of modulating the release of acetylcholine. We conclude that neurons are capable of segregating alpha7-containing nicotinic receptors to specific areas of their plasma membrane. Such targeting of nicotinic receptors would play an important role in determining their functional role in neurons.

Differential localization of acetylcholinesterase in relation to pre- and postsynaptic nicotinic receptors in the chick brain

Immunofluorescence and confocal microscopy were combined to study the distribution of acetylcholinesterase in relation to the localization of the beta2 subunit of the nicotinic acetylcholine receptors in the chick brain. In several areas where the beta2 subunit is recognizably part of presynaptic receptors, the localization of acetylcholinesterase appeared not to overlap the localization of beta2. On the other hand, acetylcholinesterase and the beta2 subunit exhibited a strictly matching localization in areas where postsynaptic nicotinic receptors are known to be present. These data may represent a morphological substrate for possible differential actions of acetylcholinesterase at presynaptic and postsynaptic nicotinic sites.

Distribution and development of nicotinic acetylcholine receptor subtypes in the optic tectum of Rana pipiens

Acetylcholine allows the elicitation of visually evoked behaviors mediated by the frog optic tectum, but the mechanisms behind its effects are unknown. Although nicotinic acetylcholine receptors (nAChRs) exist in the tectum, their subtype has not been assessed. By using quantitative autoradiography, we examined the binding of [(3)H]cytisine and [(125)I]alpha-bungarotoxin in the laminated tectum. In mammalian systems, these radioligands bind with high affinity to alpha4 nAChR subunits and alpha7 nAChR subunits, respectively. [(3)H]Cytisine demonstrated high specific binding in adult frogs in retinorecipient layer 9, intermediate densities in layer 8, and low binding in layers 1-7 of the tectum. [(3)H]Cytisine binding was significantly higher in the tecta of adults than in those of tadpoles. Lesioning the optic nerve for 6 weeks decreased [(3)H]cytisine binding in layers 8/9 by 70+/-1%, whereas 6-month lesions decreased binding by 76+/-3%. Specific binding of [(125)I]alpha-bungarotoxin in adults was present only at intermediate levels in tectal layers 8 and 9, and undetectable in the deeper tectal layers. However, the nucleus isthmi, a midbrain structure reciprocally connected to the tectum, exhibited high levels of binding. There were no significant differences in tectal [(125)I]alpha-bungarotoxin binding between tadpoles and adults. Six-week lesions of the optic nerve decreased tectal [(125)I]alpha-bungarotoxin binding by 33+/-10%, but 6-month lesions had no effect. The pharmacokinetic characteristics of [(3)H]cytisine and [(125)I]alpha-bungarotoxin binding in the frog brain were similar to those demonstrated in several mammalian species. These results indicate that [(3)H]cytisine and [(125)I]alpha-bungarotoxin identify distinct nAChR subtypes in the tectum that likely contain non-alpha7 and alpha7 subunits, respectively. The majority of non-alpha7 receptors are likely associated with retinal ganglion cell terminals, whereas alpha7-containing receptors appear to have a different localization.

Chick optic lobe contains a developmentally regulated alpha2alpha5beta2 nicotinic receptor subtype

The most widely expressed neuronal nicotinic acetylcholine receptor subtype in chick brain is that containing the alpha4 and beta2 subunits. However, immunoprecipitation and localization studies have shown that some brain areas also contain the alpha2 and/or alpha5 subunits, whose role in the definition of receptor properties is still intriguing. Using subunit-specific polyclonal antibodies, we found that the optic lobe is the chick central nervous system region that expresses the highest level of alpha2-containing receptors. Immunoprecipitation studies of these immunopurified alpha2-containing receptors labeled with the nicotinic agonist [(3)H]epibatidine showed that almost all of them contained the beta2 subunit and that more than 66% contained the alpha5 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha2, alpha5, and beta2 subunits and the absence of the alpha3, alpha4, alpha6, alpha7, alpha8, beta3, and beta4 subunits. The alpha2-containing receptors are developmentally regulated: their expression increases 25 times from embryonic day 7 to posthatching day 1 in the optic lobe, compared with an increase of only 5-fold in the forebrain. The alpha2-containing optic lobe receptors bind [(3)H]epibatidine (K(d) = 29 pM) and a number of other nicotinic agonists with very high affinity and have a pharmacological profile very similar to that of the alpha4beta2 subtype. They form functional cationic channels when reconstituted in lipid bilayers, with pharmacological and biophysical properties different from those of the alpha4beta2 subtype. These channels are activated by nicotinic agonists in a dose-dependent manner and are blocked by the nicotinic antagonist d-tubocurarine.

Nicotine modulates evoked GABAergic transmission in the brain

The effects of nicotine on evoked GABAergic synaptic transmission were examined using whole cell recordings from neurons of the lateral spiriform nucleus in embryonic chick brain slices. All synaptic activities were abolished by the GABA(A) receptor antagonist, bicuculline (20 microM). Under voltage-clamp with KCl-filled pipettes (holding potential -70 mV), nicotine (0.1-1.0 microM) increased the frequency of spontaneous GABAergic currents in a dose-dependent manner. Nicotine enhanced electrically evoked GABAergic transmission only at relatively low concentrations of 50-100 nM (but not 25 nM), which approximate the concentrations of nicotine in the blood produced by cigarette smoking. At higher concentrations nicotine had either no effect (0.25 microM) or diminished (0.5-1.0 microM) evoked GABAergic neurotransmission. Nicotine had no significant effect on the postsynaptic current induced by exogenous GABA (30-50 microM). These data imply that nicotine levels attained in smokers are sufficient to enhance evoked GABAergic transmission in the brain, and that this effect is most likely mediated through activation of presynaptic nicotinic receptors.