Alteration in nicotinic and muscarinic responses of rabbit superior cervical ganglion cells after chronic preganglionic denervation

Changes in cationic selectivity of the nicotinic channel at the rat ganglionic synapse: a role for chloride ions?

The permeability of the nicotinic channel (nAChR) at the ganglionic synapse has been examined, in the intact rat superior cervical ganglion in vitro, by fitting the Goldman current equation to the synaptic current (EPSC) I–V relationship. Subsynaptic nAChRs, activated by neurally-released acetylcholine (ACh), were thus analyzed in an intact environment as natively expressed by the mature sympathetic neuron. Postsynaptic neuron hyperpolarization (from −40 to −90 mV) resulted in a change of the synaptic potassium/sodium permeability ratio (P_K/P_Na) from 1.40 to 0.92, corresponding to a reversible shift of the apparent acetylcholine equilibrium potential, E_ACh, by about +10 mV. The effect was accompanied by a decrease of the peak synaptic conductance (g_syn) and of the EPSC decay time constant. Reduction of [Cl⁻]_o to 18 mM resulted in a change of P_K/P_Na from 1.57 (control) to 2.26, associated with a reversible shift of E_ACh by about −10 mV. Application of 200 nM αBgTx evoked P_K/P_Na and g_syn modifications similar to those observed in reduced [Cl⁻]_o. The two treatments were overlapping and complementary, as if the same site/mechanism were involved. The difference current before and after chloride reduction or toxin application exhibited a strongly positive equilibrium potential, which could not be explained by the block of a calcium component of the EPSC. Observations under current-clamp conditions suggest that the driving force modification of the EPSC due to P_K/P_Na changes represent an additional powerful integrative mechanism of neuron behavior. A possible role for chloride ions is suggested: the nAChR selectivity was actually reduced by increased chloride gradient (membrane hyperpolarization), while it was increased, moving towards a channel preferentially permeable for potassium, when the chloride gradient was reduced.

The nicotinic activation of the denervated sympathetic neuron of the rat

Nicotinic responses to endogenous acetylcholine and to exogenously applied agonists have been studied in the intact or denervated rat sympathetic neuron in vitro, by using the two-microelectrode voltage-clamp technique. Preganglionic denervation resulted in progressive decrease of the synaptic current (excitatory postsynaptic current, EPSC) amplitude, which disappeared within 24 h. These effects were accompanied by changes in ion selectivity of the nicotinic channel (nAChR). The extrapolated EPSC null potential (equilibrium potential for acetylcholine action, E(Syn)) shifted from a mean value of -15.9+/-0.7 mV, in control, to -7.4+/-1.6 mV, in denervated neurons, indicating a decrease of the permeability ratio for the main components of the synaptic current (P(K)/P(Na)) from 1.56 to 1.07. The overall properties of AChRs were investigated by applying dimethylphenylpiperazinium or cytisine and by examining the effects of endogenous ACh, diffusing within the ganglion after preganglionic tetanization in the presence of neostigmine. The null potentials of these macrocurrents (equilibrium potential for dimethylphenylpiperazinium action, E(DMPP); and equilibrium potential for diffusing acetylcholine, E(ACh), respectively) were evaluated by applying voltage ramps and from current-voltage plots. In normal neurons, E(Syn) (-15.9+/-0.7 mV) was significantly different from E(DMPP) (-26.1+/-1.0) and E(ACh) (-31.1+/-3.3); following denervation, nerve-evoked currents displayed marked shifts in their null potentials (E(Syn)=-7.4+/-1.6 mV), whereas the amplitude and null potential of the agonist-evoked macrocurrents were unaffected by denervation and its duration (E(DMPP)=-26.6+/-1.2 mV). It is suggested that two populations of nicotinic receptors, synaptic and extrasynaptic, are present on the neuron surface, and that only the synaptic type displays sensitivity to denervation.

Chronic decentralization of the heart differentially remodels canine intrinsic cardiac neuron muscarinic receptors

The objective of the study was to determine if chronic interruption of all extrinsic nerve inputs to the heart alters cholinergic-mediated responses within the intrinsic cardiac nervous system (ICN). Extracardiac nerve inputs to the ICN were surgically interrupted (ICN decentralized). Three weeks later, the intrinsic cardiac right atrial ganglionated plexus (RAGP) was removed and intrinsic cardiac neuronal responses were evaluated electrophysiologically. Cholinergic receptor abundance was evaluated using autoradiography. In sham controls and chronic decentralized ICN ganglia, neuronal postsynaptic responses were mediated by acetylcholine, acting at nicotinic and muscarinic receptors. Muscarine- but not nicotine-mediated synaptic responses that were enhanced after chronic ICN decentralization. After chronic decentralization, muscarine facilitation of orthodromic neuronal activation increased. Receptor autoradiography demonstrated that nicotinic and muscarinic receptor density associated with the RAGP was unaffected by decentralization and that muscarinic receptors were tenfold more abundant than nicotinic receptors in the right atrial ganglia in each group. After chronic decentralization of the ICN, intrinsic cardiac neurons remain viable and responsive to cholinergic synaptic inputs. Enhanced muscarinic responsiveness of intrinsic cardiac neurons occurs without changes in receptor abundance.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

Are there functional P2X receptors on cell bodies in intact dorsal root ganglia of rats?

P2X purinoceptors have been suggested to participate in transduction of painful stimuli in nociceptive neurons. In the current experiments, ATP (1-10 mM), alpha,beta-methylene-ATP (10-30 microM) and capsaicin (10 nM-1 microM) were applied to neurons impaled with high resistance microelectrodes in rat dorsal root ganglia (L4 and L5) isolated in vitro together with the sciatic nerve and dorsal roots. The agonists were either bath applied or focally applied using a picospritzer. GABA (100 microM) and 40-80 mM K+ solutions gave brisk responses when applied by either technique. Only three of 22 neurons with slowly conducting axons (C cells) showed evidence of P2X-purinoceptor-mediated responses. Only two of 13 cells which responded to capsaicin (putative nociceptors), and none of 29 cells with rapidly conducting axons (A cells), responded to the purinergic agonists. When acutely dissociated dorsal root ganglion cells were studied using patch-clamp techniques, all but four of 30 cells of all sizes responded with an inward current to either ATP or alpha,beta-methylene-ATP (both 100 microM). Our data suggest that few sensory cell bodies in intact dorsal root ganglia express functional purinoceptors.

Effects of axotomy, deafferentation, and reinnervation on sympathetic ganglionic synapses: a comparative study

The main physiological and morphological features of the synapses in the superior cervical ganglia of mammals and the last two abdominal ganglia of the frog sympathetic chain are summarized. The effects of axotomy on structure and function of ganglionic synapses are then reviewed, as well as various changes in neuronal metabolism in mammals and in the frog, in which the parallel between electrophysiological and morphological data leads to the conclusion that a certain amount of synaptic transmission occurs at "simple contacts." The effects of deafferentation on synaptic transmission and ultrastructure in the mammalian ganglia are reviewed: most synapses disappear, but a number of postsynaptic thickenings remain unchanged. Moreover, intrinsic synapses persist after total deafferentation and their number is strongly increased if axotomy is added to deafferentation. In the frog ganglia, the physiological and morphological evolution of synaptic areas is comparable to that of mammals, but no intrinsic synapses are observed. The reinnervation of deafferented sympathetic ganglia by foreign nerves, motor or sensory, is reported in mammals, with different degrees of efficiency. In the frog, the reinnervation of sympathetic ganglia with somatic motor nerve fibers is obtained in only 20% of the operated animals. The possible reasons for the high specificity of ganglionic connections in the frog are discussed.

Lack of long-term potentiation, non-cholinergic transmission and muscarinic inhibition in cat superior cervical ganglia innervated by nodose ganglion cells

In anesthetized cats, in which a nodose-superior cervical ganglion (SCG) anastomosis had been performed 6-9 months earlier, the nictitating membrane contraction evoked by electrical stimulation of the cervical vagus nerve ipsilateral to the anastomosis was recorded. The competence of nodose neurons in regulation the multiple synaptic mechanisms of the sympathetic ganglion was tested by comparing this response with the responses to stimulation of (self-reinnervated SCG). The response of the nictitating membrane ipsilateral to the anastomosis was smaller and had a lower EC50 for hexamethonium (C6) than the responses of the nictitating membrane ipsilateral to the intact or sutured CST. A 40 Hz 10s stimulus train to the intact or sutured CST produced potentiation of ganglionic transmission lasting 1 hour or longer, while a similar stimulus train to the anastomosed cervical vagus nerve produced no potentiation. During block of ganglionic nicotinic transmission with C6, CST or vagus nerve stimulation evoked responses which increased in amplitude with increasing stimulus frequency and were blocked by the selective muscarinic receptor antagonist pirenzepine. When the anticholinesterase eserine was added, the responses evoked by preganglionic stimulation decreased in amplitude in the intact SCG, as previously shown [7], and in the self-reinnervated SCG. This effect, which is due to inhibition mediated by muscarinic receptors selectively blocked by AF-DX116, was absent in the anastomosed SCG. During block of ganglionic transmission with C6 and atropine, a 40 Hz stimulus train to the intact or to the sutured CST evoked a slow, small amplitude contraction that was enhanced by naloxone. This response, most likely mediated by peptides [6], was absent in the anastomosed SCG.(ABSTRACT TRUNCATED AT 250 WORDS)

Sustained cortical metabolic responsivity to physostigmine after nucleus basalis magnocellularis ablation in rats

Unilateral nucleus basalis magnocellularis (NBM) ablation, which causes partial cholinergic denervation of the ipsilateral anterior neocortex, results in an acute but transient depression of regional cerebral metabolic rates for glucose (rCMRglc) in deafferented areas; rCMRglc normalizes within 2 weeks. To seek possible compensatory changes in cholinergic mechanisms following NBM ablation that could lead to rapid metabolic normalization, we studied rCMRglc responses to the receptor agonists nicotine and arecoline and the cholinesterase inhibitor physostigmine in rats at 2 weeks after unilateral NBM destruction. Physostigmine increased rCMRglc in 10 of 30 cortical areas contralateral to the NBM lesion. Compared to the unlesioned side, rCMRglc after physostigmine in the lesioned cortex was significantly lower in 2, significantly higher in 1 and not different (P < 0.05) in 27 areas. Neither arecoline nor nicotine treatment produced rCMRglc asymmetry in lesioned rats. These results demonstrate that responsivity to physostigmine is maintained in most regions of the rat neocortex after extrinsic cholinergic denervation by NBM ablation. This adaptive response appears not to result from cholinergic receptor upregulation and may reflect instead reorganization of cholinergic synapses.

EEG changes induced by acute and chronic quisqualic or ibotenic acid nucleus basalis lesions are stabilized by tacridine

The present study investigated the effects of acute (1 week) and chronic (8 months) quisqualic (quis) and ibotenic (ibo) acid nucleus basalis (NB) lesions on the biochemical activity of the NB cholinergic system (choline acetyltransferase (ChAT) activity) and on neocortical EEG activity. Cortical ChAT activity of quis or ibo NB-lesioned rats did not recover during an 8-month period. Acute and chronic quis and ibo NB lesions increased EEG slow waves and high voltage spindles. Tacridine, an anticholinesterase, dose-dependently suppressed acute and chronic quis and ibo NB lesion-induced EEG changes. The present results suggest that NB cholinergic neurons do not recover after excitotoxin-induced damage during an 8-month period and that cholinergic neuron loss is importantly involved in the acute and chronic lesion-induced EEG changes.

Penile erection in the rat: stimulation of the hypogastric nerve elicits increases in penile pressure after chronic interruption of the sacral parasympathetic outflow

Penile erection, a vascular event mediated by the autonomic nervous system, is often adversely affected by injury to the spinal cord. To further characterize the laboratory rat as an animal model of penile erection and to investigate erectile responses following neural injury, the present study has examined pressor penile responses in intact rats and in animals deprived of sacral parasympathetic outflow. Increases in penile pressure result from graded stimulation of postganglionic parasympathetic fibers. The vasodilator response is insensitive to blockade with atropine, a cholinergic antagonist. Penile tumescence also results from stimulation of the pelvic nerve, but not the hypogastric nerve. However, beginning 3 days after unilateral interruption of the pelvic nerve, stimulation of the ipsilateral hypogastric nerve results in an increase in penile pressure. This novel response, which is blocked by a ganglionic antagonist, is maximally developed at 1 week post-lesion, is stable for at least 3 months and remains confined to the side of the lesion. These results suggest that the rat, although relatively small, can be used to obtain quantitative data on penile erection. Moreover, the model may lend itself to an analysis of the mechanisms of altered control of visceral tissues following injury to the nervous system.

Effects of nBM lesions on muscarinic-stimulation of phosphoinositide hydrolysis

The nucleus basalis magnocellularis (nBM) is believed to be the major path of cholinergic innervation to the frontal cortex. The cerebral cortex is known to contain muscarinic receptors that are coupled to the hydrolysis of phosphoinositides (PI) (9,14). Adult male Sprague-Dawley rats were unilaterally and bilaterally lesioned at the nBM with the excitotoxin ibotenic acid and killed at 7 or 21 to 23 days postsurgery. In rats unilaterally lesioned 7 days previously, the carbachol dose-response curves in lesioned fronto-parietal cortex were identical to control fronto-parietal cortices. In rats studied 21 to 23 days postsurgery, carbachol dose-response curves were again identical in control vs. lesioned fronto-parietal cortices. Similar results are obtained when bilaterally lesioned rats are compared to sham-operated controls. For each group, the hydrolysis is linear with respect to time until 15 minutes with a maximum reached at approximately 40 minutes. Receptor density, as measured by [3H]-QNB binding or agonist competition for [3H]-QNB binding, was not changed by any of the lesions studied. These results suggest that the loss of cholinergic innervation from the nBM does not result in compensatory denervation supersensitivity in cerebral fronto-parietal cortical muscarinic receptors.

Vasopressin-stimulated turnover of phosphatidylinositol in the decentralised superior cervical ganglion of the rat

Vasopressin (AVP) receptors have been demonstrated in the superior cervical ganglion (SCG) linked to the turnover of phosphatidylinositol (PI). We have investigated the effect of changes in autonomic function on the turnover of PI in the rat SCG in response to AVP and carbachol. Decentralisation of the SCG for 3 days causes a significant decrease in the response to AVP, while the response to carbachol is unaffected. The decrease in response to AVP is still present 21 days after decentralisation. A similar pattern of changes is present in ganglia from the homozygous Brattleboro rat. Pretreatment with reserpine has no effect on the response to AVP. The results suggest that AVP may be involved in ganglionic transmission in the autonomic nervous system, but that full expression of AVP-induced turnover of PI is dependent on an intact preganglionic input.

Calibers and microtubules of sympathetic axons are not subject to trophic control by the preganglionic nerve

The caliber and microtubular content of the fibers emerging from the superior cervical ganglion of the cat were studied up to 60 days following preganglionic nerve transection. No significant difference was observed between normal and treated animals. This suggests that the caliber and microtubular content of the sympathetic axon are not subject to trophic control by the preganglionic nerve and that the normal firing rate of the pathway seems insufficient to affect the microtubular content of the fiber.

Lack of nicotinic supersensitivity in frog sympathetic neurones following denervation

The sensitivity of bull-frog sympathetic neurones to nicotinic, cholinergic agonists has been studied in both normal (control) and surgically denervated ganglia. Using gross extracellular recording, the sensitivity to acetylcholine (ACh) increased 18-fold following denervation, while that to carbachol (CCh) was unchanged. Normal ganglia showed a similar sensitivity increase after inhibition of cholinesterase. This suggests that the rise in ACh sensitivity is due to reduced cholinesterase activity, not to true supersensitivity. There was no significant difference in resting membrane potential or input resistance between normal and denervated neurones. Neurones denervated for 7-50 days showed no significant change in sensitivity to ACh or CCh applied iontophoretically at a distance of 10 micron from the cell surface. In control ganglia, localized iontophoretic application of ACh revealed an uneven distribution of sensitivity which is attributed to the localization of receptors to synaptic areas. Fourteen days after denervation, the geometric mean sensitivity to focally applied ACh was not significantly different from that found in control ganglia. The variation in sensitivity to focally applied ACh at randomly chosen sites on denervated neurones was as great as that found in control ganglia. It is concluded that denervation does not cause frog sympathetic neurones to become supersensitive to ACh. The apparent increase in nicotinic ACh sensitivity observed using extracellular recording from whole ganglia is due not to a change in the number or distribution of ACh receptors, but to a decrease in cholinesterase activity.

Modulation of the acetylcholine system in the superior cervical ganglion of rat: effects of GABA and hypoglossal nerve implantation after in vivo GABA treatment

gamma-Aminobutyric acid (GABA) was applied to the superior cervical ganglion (SCG) of CFY rats in vitro and in vivo, with or without implantation of a hypoglossal nerve, to evaluate the effects of these experimental interventions on the acetylcholine (ACh) system, which mainly serves the synaptic transmission of the preganglionic input. Long-lasting GABA microinfusion into the SCG in vivo apparently resulted in a "functional denervation." This treatment reduced the acetylcholinesterase (AChE; EC 3.1.1.7) activity by 30% (p less than 0.01) and transiently increased the number of nicotinic acetylcholine receptors, but had no significant effect on the choline acetyltransferase (acetyl-coenzyme A:choline-O-acetyltransferase; EC 2.3.1.6) activity, the ACh level, or the number of muscarinic acetylcholine receptors. The relative amounts of the different molecular forms of AChE did not change under these conditions. In vivo GABA application to the SCG with a hypoglossal nerve implanted in the presence of intact preganglionic afferent synapses exerted a significant modulatory effect on the AChE activity and its molecular forms. The "hyperinnervation" of the ganglia led to increases in the AChE activity (to 142.5%, p less than 0.01) and the 16S molecular form (to 200%, p less than 0.01). It is concluded that in vivo GABA microinfusion and GABA treatment in the presence of additional cholinergic synapses has a modulatory effect on the elements of the ACh system in the SCG of CFY rats.

Post-tetanic depolarization in sympathetic neurones of the guinea-pig

1. Repetitive intracellular stimulation at a frequency of 5-30 Hz for 1-10 s evoked in neurones of the isolated inferior mesenteric and superior cervical ganglia of the guinea-pig three types of post-spike membrane potential changes: (i) hyperpolarization, (ii) hyperpolarization followed by a slow depolarization, and (iii) a second hyperpolarization following the initial two responses.2. The initial post-spike hyperpolarization had a mean duration of 2.0 s and was often associated with a fall in membrane resistance; it could be elicited in every sympathetic neurone studied. This response was termed the post-tetanic hyperpolarization (PTH).3. The slow depolarization which could be induced only in a portion of neurones had a mean amplitude and duration of 2.2 mV and 27.5 s, respectively; it was termed the post-tetanic depolarization (PTD).4. PTD was associated with a fall in membrane resistance, augmented by membrane hyperpolarization, and reduced by depolarization; its mean extrapolated equilibrium potential was -38 mV.5. PTD was not blocked by nicotinic and muscarinic antagonists, or alpha-and beta-adrenergic receptor antagonists, whereas it was suppressed by adrenaline, noradrenaline, Co(2+) and a low Ca(2+) solution.6. The amplitude of the single spike after-hyperpolarization in normal Krebs solution as well as in high K(+) solution was increased during PTD; furthermore, conditioning hyperpolarization to the level of E(K) increased the amplitude of PTD in normal Krebs as well as in high K(+) solution.7. PTD with similar amplitude, time course and membrane characteristics could be evoked in a portion of neurones of the rabbit superior cervical ganglia; however, PTD was not detected in neurones of the rat superior cervical ganglia.8. Decentralization of the guinea-pig and rabbit superior cervical ganglia for 14 d did not alter the number of neurones in which PTD could be elicited, its amplitude, or its time course.9. Our results suggest that a chemical substance(s) is responsible for the generation of PTD; it may be released from the soma and/or dendrites and acts in an auto-receptive manner on the cells in question. The nature and origin of the second hyperpolarization remain to be clarified.

In vivo modulation of the number of muscarinic receptors in rat brain by cholinergic ligands

Administration of the muscarinic agonist oxotremorine led to a decrease in the number of muscarinic receptors, as determined by specific binding of [3H]quinuclidinyl benzilate ([3H]QNB), in several rat brain regions both during development and at maturity. In contrast, administration of the muscarinic antagonist scopolamine led to an increase in the number of [3H]QNB-binding sites in various brain regions. Scopolamine also prevented the decrease in the number of [3H]QNB-binding sites induced by administration of an organophosphorus drug. The results are compatible with the hypothesis that the number of brain muscarinic receptors, or at least of a sub-class of them, is regulated by their transmitter.

Aging and rat brain muscarinic receptors as measured by quinuclidinyl benzilate binding

Measurement of cholinergic muscarinic receptor binding in various rat brain areas using the ligand [3H]quinuclidinyl benzilate indicates that receptor binding is decreased in striatum and cerebellum of aged female rats (22 months old) as compared to younger rats (4 months old). Decreases were not observed in cortex, hippocampus, hypothalamus, or amygdala areas. Further examination o [3H]quinuclidinyl benzilate binding in subcellular fractions of aged and young rat cerebellum and striatum indicated a decrease in binding in the crude nuclear and crude synaptosomal fractions. Binding data indicate the observed decrease in specific ligand binding is due to a decrease in number of binding sites while receptor affinity does not appear to change.

Release of (3H)norepinephrine from a clonal line of pheochromocytoma cells (PC12) by nicotinic cholinergic stimulation

Release experiments were carried out in vitro with a clonal line of rat pheochromocytoma cells (designated PC12) which synthesize and store catecholamines and which, after treatment with nerve growth factor (NGF), cease cell division and extend neuronal-like processes. In the present study, PC12 cells were exposed to [3H]norepinephrine (NE) which they took up and stored in reserpine-sensitive sites. Exposure of such cells to nicotinic cholinergic agonists resulted in release of [3H]NE into the external medium. Release terminated within 1 min and partially returned after 20 min in the absence of agonist. After 1 min of stimulation with nicotine, NGF-treated cells released 5-6% of their contents of [3H]NE while NGF-untreated cells released 1-2%. Release from both NGF-treated and -untreated PC12 cells was inhibited in the absence of Ca2+ or by elevated Mg2+ and was blocked by the nicotinic antagonists D-tubocurarine and mecamylamine (50% inhibition at 0.1 and 0.06 micrometer, respectively). Release was not affected by the presence of tetrodotoxin. Such findings suggest that release of [3H]NE from PC12 cells may be mediated via stimulation of nicotinic acetylcholine receptors and a consequent stimulation-secretion coupling mechanism.

Dopamine and adenosine 3',5'-monophosphate responses of single mammalian sympathetic neurons

Acetylcholine (ACh), dopamine, and dibutyryl-adenosine 3',5'-monophosphate (dbcAMP) were applied iontophoretically to the rabbit superior cervical ganglion cells from triple-barreled micropipettes, and the response was recorded by intracellular techniques. All ganglion cells tested responded to the depolarizing action of ACh, whereas less than half of the cells that responded to ACh were hyperpolarized by dopamine. This effect was blocked by low concentrations of haloperidol. None of the cells examined responded to dbcAMP applied by iontophoresis. Hence, the present result is not consistent with the concept that a cyclic AMP mechanism underlies the hyperpolarizing effect of dopamine.