Diphasic postsynaptic potential: a chemical synapse capable of mediating conjoint excitation and inhibition

Quantitative DLA-based compressed sensing for T1-weighted acquisitions

High resolution Manganese Enhanced Magnetic Resonance Imaging (MEMRI), which uses manganese as a T₁ contrast agent, has great potential for functional imaging of live neuronal tissue at single neuron scale. However, reaching high resolutions often requires long acquisition times which can lead to reduced image quality due to sample deterioration and hardware instability. Compressed Sensing (CS) techniques offer the opportunity to significantly reduce the imaging time. The purpose of this work is to test the feasibility of CS acquisitions based on Diffusion Limited Aggregation (DLA) sampling patterns for high resolution quantitative T₁-weighted imaging. Fully encoded and DLA-CS T₁-weighted images of Aplysia californica neural tissue were acquired on a 17.2T MRI system. The MR signal corresponding to single, identified neurons was quantified for both versions of the T₁ weighted images. For a 50% undersampling, DLA-CS can accurately quantify signal intensities in T₁-weighted acquisitions leading to only 1.37% differences when compared to the fully encoded data, with minimal impact on image spatial resolution. In addition, we compared the conventional polynomial undersampling scheme with the DLA and showed that, for the data at hand, the latter performs better. Depending on the image signal to noise ratio, higher undersampling ratios can be used to further reduce the acquisition time in MEMRI based functional studies of living tissues.

Role of noradrenergic fibers of the preoptic area in regulating sleep

The preoptic area (POA) has noradrenergic (NE) terminals, and this area controls sleep apart from body temperature and reproduction. The destruction of catecholaminergic (CA) terminals in the POA produced a decrease in sleep in rats. This effect was shown to be due to the destruction of NE and not dopaminergic terminals. The rats, which were hyperthermic after the destruction of CA fibers in the POA, preferred a lower ambient temperature. Though they were unable to have normal amount of sleep after lesion, it did not affect their behavioral thermoregulation. Acute total sleep deprivation for 48 h led to a significant decrease in noradrenaline, increase in the level of metabolites of monoamines, and an enhancement in the number of dendritic spines at the medial preoptic area (mPOA). Enhanced sleep pressure during sleep deprivation could have led to a higher release of noradrenaline, and an increase in dendritic spines in the mPOA. Arousal was produced by application of noradrenaline at the mPOA, whereas the alpha antagonists produced sleep in free-moving rats. This was in contrast to the increased wakefulness produced by the destruction of NE terminals. As wakefulness and sleep, respectively, were induced on local application of alpha-2 antagonist and agonists, it was suspected that the noradrenaline and alpha antagonists might have acted on the alpha-2 receptors, which are predominantly present on the pre-synaptic terminals. Sleep produced by noradrenaline, which was locally applied at the mPOA, after destroying the NE terminals, further confirmed this possibility. Hypothermia and sexual arousal produced by application of alpha- and beta-adrenergic agonists at the mPOA would have contributed towards the wakefulness induced by these drugs in normal rats. Thus, the available evidence shows that the NE fibers in the POA are involved in the induction of sleep.

Dopamine activates two different receptors to produce variability in sign at an identified synapse

Chemical synaptic transmission was investigated at a central synapse between identified neurons in the freshwater snail, Lymnaea stagnalis. The presynaptic neuron was the dopaminergic cell, Right Pedal Dorsal one (RPeD1). The postsynaptic neuron was Visceral Dorsal four (VD4). These neurons are components of the respiratory central pattern generator. The synapse from RPeD1 to VD4 showed variability of sign, i.e., it was either inhibitory (monophasic and hyperpolarizing), biphasic (depolarizing followed by hyperpolarizing phases), or undetectable. Both the inhibitory and biphasic synapse were eliminated by low Ca2+/high Mg2+ saline and maintained in high Ca2+/high Mg2+ saline, indicating that these two types of connections were chemical and monosynaptic. The latency of the inhibitory postsynaptic potential (IPSP) in high Ca2+/high Mg2+ saline was approximately 43 ms, whereas the biphasic postsynaptic potential (BPSP) had approximately 12-ms latency in either normal or high Ca2+/high Mg2+ saline. For a given preparation, when dopamine was pressured applied to the soma of VD4, it always elicited the same response as the synaptic input from RPeD1. Thus, for a VD4 neuron receiving an IPSP from RPeD1, pressure application of dopamine to the soma of VD4 produced an inhibitory response similar to the IPSP. The reversal potentials of the IPSP and the inhibitory dopamine response were both approximately -90 mV. For a VD4 neuron with a biphasic input from RPeD1, pressure-applied dopamine produced a biphasic response similar to the BPSP. The reversal potentials of the depolarizing phase of the BPSP and the biphasic dopamine response were both approximately -44 mV, whereas the reversal potentials for the hyperpolarizing phases were both approximately -90 mV. The hyperpolarizing but not the depolarizing phase of the BPSP and the biphasic dopamine response was blocked by the D-2 dopaminergic antagonist (+/-) sulpiride. Previously, our laboratory demonstrated that both IPSP and the inhibitory dopamine response are blocked by (+/-) sulpiride. Conversely, the depolarizing phase of both the BPSP and the biphasic dopamine response was blocked by the Cl- channel antagonist picrotoxin. Finally, both phases of the BPSP and the biphasic dopamine response were desensitized by continuous bath application of dopamine. These results indicate that the biphasic RPeD1 --> VD4 synapse is dopaminergic. Collectively, these data suggest that the variability in sign (inhibitory vs. biphasic) at the RPeD1 --> VD4 synapse is due to activation of two different dopamine receptors on the postsynaptic neuron VD4. This demonstrates that two populations of receptors can produce two different forms of transmission, i.e., the inhibitory and biphasic forms of the single RPeD1 --> VD4 synapse.

Compartmentalization of information processing in an aplysia feeding circuit interneuron through membrane properties and synaptic interactions

We describe a pair of cerebral-to-buccal interneurons, CBI-5/6, which have outputs and inputs in two ganglia. The soma in the cerebral ganglion received synaptic inputs during buccal motor programs (BMPs) and after mechanical stimulation of the lips. During BMPs the soma received antidromic spikes generated in processes in the buccal ganglion. The soma was driven into a plateau potential by each of these inputs, during which it fired orthodromically at 0-5 Hz. The soma had outputs in the cerebral ganglion consisting of electrical coupling to the adjacent CBI-5/6 and to a cerebral-to-pedal neuron (CPN1). The buccal terminals of CBI-5/6 received inputs that generated rhythmic barrages (up to 25 Hz) of antidromic spikes during BMPs. The buccal terminals had chemical and electrical outputs to motor and premotor elements of feeding circuitry. This combination of synaptic interactions and endogenous properties mean that CBI-5/6 can process information in a number of ways. During the barrage of antidromic spikes, high-frequency firing will produce strong inputs to buccal followers and on their arrival at the soma will transfer excitation electrotonically to CPN1. Subthreshold input to the soma will be transferred electrotonically to cerebral followers but will not be relayed to postsynaptic buccal neurons. Plateau potentials after the antidromic spikes or local cerebral inputs will locally excite CPN1 via electrical coupling but will have little influence on buccal events because of the low orthodromic firing rate. Thus, CBI-5/6 may transmit information locally within the cerebral ganglion or more extensively in both buccal and cerebral ganglia simultaneously.

A synaptic mechanism possibly underlying directional selectivity to motion

A specific synaptic interaction is proposed as the mechanism underlying the directional selectivity to motion of several nervous cells. It is shown that the hypothesis is consistent with previous behavioural and physiological studies of the motion detection process.

Characterization of buccal motor programs elicited by a cholinergic agonist applied to the cerebral ganglion of Aplysia californica

Applying the non-hydrolyzable cholinergic agonist carbachol (CCh) to the cerebral ganglion of Aplysia elicits sustained, regular bursts of activity in the buccal ganglia resembling those seen during biting. The threshold for bursting is approximately 10(-4) M. Bursting begins after a 2 to 5 min delay. The burst frequency increases over the first 5 bursts, reaching a plateau value of approximately 3 per minute. Bursting is maintained for over 10 min. Some of the effects of CCh may be attributed to its ability to depolarize and fire CBI-2, a command-like neuron in the cerebral ganglion that initiates biting. CBI-2 is also depolarized by ACh, and by stimulating peripheral sensory nerves. Excitation of CBI-2 caused by carbachol is partially blocked by the muscarinic antagonist atropine. We examined whether CCh-induced bursting is modified in ganglia taken from Aplysia that previously experienced treatments inhibiting feeding, such as satiation, head shock contingent or non-contingent with food, and training animals with an inedible food. No treatment consistently and repeatedly affected the latency, the peak burst period, the length of time that bursting was maintained, or the threshold CCh concentration for eliciting bursting. However, there was a decrease in the rate of the build-up of the buccal ganglion program in previously satiated animals.

Synaptobrevin/vesicle-associated membrane protein (VAMP) of Aplysia californica: structure and proteolysis by tetanus toxin and botulinal neurotoxins type D and F

Synaptobrevin/vesicle-associated membrane protein (VAMP) and syntaxin are potential vesicle donor and target membrane receptors of a docking complex that requires N-ethylmaleimide-sensitive factor (NSF) and soluble NSF-attachment proteins as soluble factors for vesicle fusion with target membranes. Members of this docking complex are the target of clostridial neurotoxins that act as zinc-dependent proteases. Molecular cloning of the Aplysia californica synaptobrevin cDNA revealed a 180-residue polypeptide (M(r), 19,745) with a central transmembrane region and an atypically large C-terminal intravesicular domain. This polypeptide integrates into membranes at both the co- and posttranslational level, as shown by modification of an artificially introduced N-glycosylation site. The soluble and membrane-anchored forms of synaptobrevin are cleaved by the light chains of the botulinal toxins type D and F and by tetanus toxin involving the peptide bonds Lys49-Ile50, Gln48-Lys49, and Gln66-Phe67, respectively. The active center of teh tetanus toxin light chain was identified by site-specific mutagenesis. His233, His237, Glu234, and Glu270/271 are essential to this proteolytic activity. Modification of histidine residues resulted in loss of zinc binding, whereas a replacement of Glu234 only slightly reduced the zinc content.

Sleep-inducing function of noradrenergic fibers in the medial preoptic area

The aim of the investigation was to find out the role of noradrenergic (NE) terminals of the medial preoptic area (mPOA), in the regulation of sleep-wakefulness. Studies were conducted on free-moving adult male rats with chronically implanted cannulae in the mPOA. Sleep-wakefulness was assessed on the basis of EEG, EMG, and EOG recordings along with behavioral observations. Lesioning of catecholamine terminals (with 6-hydroxydopamine) in the mPOA produced an increase in quiet wakefulness. Prevention of NE fiber destruction, by pretreating the rats with imipramine, prevented this effect. This demonstrated that the increased quiet wakefulness produced by 6-OHDA was the result of NE fiber destruction. Changes in sleep-wakefulness were also assessed after microinjection of NE into the mPOA, in normal and ventral noradrenergic bundle (VNA)-lesioned rats. NE administration induced sleep in VNA-lesioned rats, and arousal in normal rats. The findings suggest that the NE terminals in the mPOA, projecting via VNA, play a role in the induction of sleep.

Synaptic connections in the buccal ganglia of Aplysia mediated by an identified neuron containing a CCK/gastrin-like peptide co-localized with acetylcholine

The identified neuron, B13, located bilaterally in the buccal ganglion of the marine mollusc Aplysia californica, contains a classical neurotransmitter (acetylcholine) and a cholecystokinin/gastrin-like (CCK/G-li) peptide. The following study demonstrates that B13 makes direct synaptic connections with several identifiable postsynaptic follower neurons. These follower neurons also receive convergent input from previously identified cholinergic neurons, B4 and B5, which do not contain a CCK/G-li peptide. The cholinergic responses mediated by B4/B5 and B13 are similar, including in at least one buccal follower, a two-component inhibitory response not seen in previous studies of the buccal ganglia circuits. However, when the cholinergic responses are blocked by appropriate antagonists, a residual, slow depolarizing, chemically-mediated response is observed in two of the identifiable followers when action potentials are evoked in B13 but not when action potentials are evoked in B4 or B5.

Cholinergic receptors in the Aplysia gill

Acetylcholine has been suggested as a neurotransmitter released in the Aplysia gill by peripheral afferents of central neurons and by peripheral neurons within the gill. The perfused gill, isolated from the abdominal ganglion, was examined. At concentrations greater than 1 microM, acetylcholine elicited a slowly developing tonic contraction of the afferent vein that reversed upon washout. This effect was observed on both quiescent and active preparations. At concentrations less than 1 microM, acetylcholine perfusion resulted in a reduction of gill tone. The excitatory effect of acetylcholine was reduced 80 and 60% by the cholinergic antagonists atropine and hexamethonium, respectively. The acetylcholine-evoked contraction was potentiated 2.5-fold when curare was coinfused. Carbachol did not mimic the excitatory effects of acetylcholine. At all concentrations examined (1-100 microM), carbachol infusion reduced baseline tension, the amplitude of spontaneous contractions and contractions evoked by FMRFamide and dopamine. Contractions evoked by perfusion of p-chlorophenylthiocyclic AMP were greatly reduced when carbachol was added to the perfusate. Further addition of curare reversibly blocked carbachol inhibition of the cyclic AMP-evoked contractions. These findings suggest that excitatory and inhibitory cholinergic receptors are involved in the regulation of gill contractile behavior by acetylcholine.

Voltage-dependence of synaptic time course reduces excitation at a diphasic synapse

At a diphasic excitatory-inhibitory synapse of Aplysia, the time constant of decay of the early excitatory synaptic conductance is decreased by depolarization. As a consequence of this reduced excitation, the amplitude of the inhibitory phase is increased and its rise time shortened, and inhibitory charge transfer is greater than excitatory charge transfer at potentials more depolarized than -53 mV. Functional consequences of voltage-dependent time course can thus be identified at a central synapse.

Self-inhibition alters firing patterns of neurons in Aplysia buccal ganglia

The functional consequences of cholinergic self-inhibitory synaptic potentials (SISPs) upon firing patterns were examined in pairs of electrotonically coupled neurons of Aplysia buccal ganglia. In each neuron, the size of the peak SISP current decrements exponentially with increased number of previous conditioning action potentials (APs). To determine the effect of SISPs on the firing patterns of each cell, AP trains elicited by constant-current steps with the SISP intact were compared to those with the SISP blocked by curare. The SISP prolonged initial interspike intervals, providing an early supplement to accommodation, and produced a 75% increase in the sensitivity of firing frequency vs injected current plots for the first ISI. Firing rates were more regular in the presence of the SISP. However, the efficacy of the SISP, like the size of the underlying current, decrements with repetition. SISP effects were also studied in electrotonically coupled pairs of self-inhibitory neurons. Although the SISP altered the shape of the hyperpolarizing component of coupling potentials, DC coupling between the neurons was unaffected. Firing synchrony in coupled pairs stimulated with long DC pulses was assessed with cross-correlation histograms. In 60 mM Ca2+, the SISP sharpens the central peak of synchrony and deepens the flanking troughs, increasing the probability of synchronous firing within +/- 4 msec by 76%. The major determinants of synchrony were found to be common input, SISP-dependent regularity of firing, and the depolarizing phase of the coupling potential, rather than the SISP-enhanced hyperpolarizing phase.

Rate-limiting step of inhibitory post-synaptic current decay in Aplysia buccal ganglia

1. In neurones BL and BR 3, 6, 8, 9, 10 and 11 of Aplysia buccal ganglia, cholinergic inhibitory post-synaptic potentials are produced by activity in either of two presynaptic cells. In order to analyse the synaptic conductance change, neurones were voltage-clamped inhibitory post-synaptic currents (i.p.s.c.) recorded. 2. The synaptic conductance change rises to an average peak value of 0.65 micromho and decays exponentially with single time constant tau of 19 msec. 3. We have attempted to identify the rate-limiting step responsible for i.p.s.c. decay from among the following possibilities: (1) acetylcholine (ACh) supply, (2) ACh removal by diffusion, (3) ACh removal by hydrolysis or (4) a slow unbinding or conformational change closing open synaptic current channels. 4. Cooling prolongs tau, with Q10 of 5.2. Cooling and eserine treatment together produce greatly prolonged, exponentially decaying i.p.s.c.s with tau > 150 msec. These results suggest that ACh removal, either by diffusion or hydrolysis, is not the rate-limiting step. 5. Prolonging synaptic action potential time course with intracellular injection of tetraethylammonium broadens the i.p.s.c. peak but does not affect the decay tail, suggesting that the rate-limiting step is not ACh release. 6. The spectrum of ACh-induced current fluctuations is fitted by a double Lorentzian with cut-off frequencies of 7.8 and 47 Hz. The frequency of the slower component corresponds to the macroscopic i.p.s.c. decay tau. 7. We conclude that a slow conformational change closing open synaptic current channels is likely to determine i.p.s.c. decay. We cannot, however, exclude either delayed diffusion or a late tail of slow ACh release as possibilities.

Substance P: evidence for diverse roles in neuronal function from cultured mouse spinal neurons

Mouse spinal neurons grown in tissue culture were used to examine the membrane mechanisms of action of the peptide substance P. Two functionally distinct actions were observed, one being a rapidly desensitizing excitation, and the other being a dose-dependent, reversible depression of excitatory responses to the putative amino acid neurotransmitter glutamate. These effects on excitability suggest that substance P may play more than one role in intercellular communication in the nervous system.

Inosine may be an endogenous ligand for benzodiazepine receptors on cultured spinal neurons

Mouse spinal neurons grown in tissue culture were used to study the membrane effects of the benzodiazepine flurazepam and the naturally occurring purine nucleoside inosine, which competes for benzodiazepine receptor sites in the central nervous system. Application of inosine elicited two types of transmitter-like membrane effects: a rapidly desensitizing excitatory response and a nondesensitizing inhibitory response. Flurazepam produced a similar excitatory response which showed cross-desensitization with the purine excitation. Flurazepam also blocked the inhibitory inosine response. The results provide electrophysiological evidence that an endogenous purine can activate two different conductances on spinal neurons and that flurazepam can activate one of the conductances and antagonize the other.

Synaptic connections and functional organization in Aplysia buccal ganglia

The buccal ganglion of Aplysia contains three morpho-functional groups (A, B, and C) of large cells and two groups (s1 and s2) of small cells. The A cells evoke monoxynaptic IPSPs in the B cells. We found that s1 cells can evoke large EPSPs in the A cells, IEPSPs in the B cells, and EIIPSPs in the C cells; several s1 cells are able to evoke all three types of responses. Many s2 cells can evoke these same responses, but only in the A and B cells. Furthermore, the s cells can evoke depolarizing PSPs in other s cells; this relation is often reciprocal. All these responses may also be contralateral. Their monosynaptic nature is shown by the consistent 1:1 relationship with the presynaptic spike, and also by the effects of intracellular tetraethylammonium and of high Mg2+ concentration in the bathing medium. d-tubocurarine reversibly suppresses the I phase of the IEPSP evoked by the s cells in the B cells. All the responses evoked by the s cells undergo depression with repetition. The network formed by all these relations is outlined, and a double relationship proposed between s cells and B cells. By electrophysiological tracing of axonal pathways it is shown that the A cells send axons into the 3rd buccal nerve, the B cells into the 2nd and/or 3rd buccal nerve and in two cases into the radular nerve, and the C cells into the gastro-oesophageal nerve. Spontaneous synaptic activity of the buccal neurons appears to be formed mostly by the described PSPs. Spontaneous firing inside the isolated ganglion corresponds well to the alternate pattern of muscular contractions of the buccal mass.

Voltage-clamp analysis of a self-inhibitory synaptic potential in the buccal ganglia of Aplysia

1. In cholinergic neurones BL4, BL5, BR4, and BR5 of Aplysia buccal ganglia, each action potential is followed, in the same cell, by a curare- and high-Mg-sensitive hyperpolarizing after-potential which is enhanced by Ca. 2. In voltage-clamped neurons, substracting currents recorded in curare from currents recorded in sea water reveals that this potential is due to curare-sensitive currents which rise to a peak, then decay exponentially with an apparently voltage-independent time constant of 43 msec. Currents are produced by a voltage-independent, Ca-enhanced, conductance change with a 0-26 mumho peak and a -64 mV reversal potential. The curare-sensitive conductance is also sensitive to high Mg. 3. Both after-potential and curare- or Mg-sensitive current follow each action potential without failures, even in threshold-raising 80 mM-Ca-144-mM-Mg solutions. 4. Both after-potential and current decrease with repetitive firing or short inter-spike interval, possibly due to receptor desensitization. 5. The Mg- and curare-sensitive conductance is also blocked by 1 mM-ACh. 6. The data are consistent with the hypothesis that the hyperpolarization following action potentials in each of these four neurones is produced by a self-inhibitory synaptic mechanism.

Dopamine, serotonin and related compounds: presynaptic effects on synaptic depression, frequency facilitation, and post-tetanic potentiation at a synapse in Aplysia californica

Dopamine, serotonin and related compounds (referred to collectively as biogenic amines) were found to modify transmission at the presumably cholinergic synapse made by an axon in the right visceropleural connective onto cell R15 of the abdominal ganglion of Aplysia californica. (1) With chronic application, dopamine hyperpolarizes R15, and serotonin depolarizes R15. Both actions upon the membrane potential desensitize in 10 min. All the actions described below were studied with chronic perfusion of the biogenic amines after desensitization of this postsynaptic action. (2) The biogenic amines drastically reduce the size of the EPSP evoked at the synapse under investigation; but they do not alter the ACh potential evoked in the soma of R15. (3) The biogenic amines reduce the amplitude of synaptic depression. The relationship between the effects of the amines on the size of an isolated EPSP and on synaptic depression differed from this relationship as affected by post-tetanic potentiation (PTP) or by changes in the Ca2+-Mg2+ balance. (4) The biogenic amines increase frequency facilitation, when the latter is defined as the ratio of the facilitated to the isolated EPSP. However, the absolute magnitude of the facilitated EPSP is always reduced at long times after introduction of the agent; shortly after introduction of the biogenic amines the absolute magnitude of the facilitated EPSP is unaffected in most preparations.

Interaction between two identified cells in the visceral ganglion of the snail, Helix pomatia

An interneurone, making excitatory synaptic connections with a second neurone has been identified in the brain of Helix pomatia. The results suggest that the connection is monosynaptic.

Synapses in the interpeduncular nucleus: electron microscopy of normal and habenula lesioned rats

Four types of synapses have been identified in the rat interpeduncular nucleus. The S synapses were formed by the mainhorizontal plexus of habenulointerpeduncular axons. They contained spherical synaptic vesicles and formed asymmetrical en passant contacts with dendritic processes. The crest synapses had all of these features in common with the S synapses, but occurred in pairs with the two contact zones coextensive and largely parallel on opposite sides of markedly attenuated dendritic processes. Synaptic glomeruli found in this nucleus consisted of several crest synapses engulfed by sheet-like dendritic processes. Identical crest synapses also occurred on dendritic crests. In a limited number of cases, both S and crest synapses arose in common from single axons. Following destruction of the habenular nuclei unilaterally or bilaterally, as expected from the above observations, both S and crest synapses underwent dense degeneration. In some cases with bilateral lesions and in all cases with unilateral lesions, only one of the two axonal endings forming a crest synapse degenerated while the other remained unaffected. Axons containing flat vesicles forming symmetrical axodendritic synapses, axosomatic synapses, and subjunctional bodies associated with S or crest synapses were all minor features of this nucleus. Aspects of the unusual synaptology of this nucleus are discussed in terms of morphology, physiology and transmitter chemistry.

Polyphasic synaptic potentials in the ganglion of the mollusc, Navanax

1. Included in the ensemble of synaptic input received by identified neurones in the ganglion of the marine mollusc Navanax are biphasic synaptic potentials, consisting of a depolarization followed by a hyperpolarization. 2. Both phases are chemically mediated as judged by their susceptibility to a high magnesium medium and neither exhibits depression with repetition. 3. The hyperpolarizing phase has a reversal potential of about -50mV, which varies only with changes in the external chloride concentration. This phase is unaffected by cholinolytics. 4. The depolarizing phase reverses at a more positive potential, is probably the result of a change in sodium conductance and is blocked by hexamethonium and high concentrations of eserine. 5. The biphasic synaptic potentials are therefore similar in many respects to the biphasic response evoked by iontophoretic application of acetylcholine on to these cells, suggesting that the two types of cholinergic receptors previously characterized on these neurones are both functional.

Excitatory, inhibitory and biphasic synaptic potentials mediated by an identified dopamine-containing neurone

1. A giant dopamine-containing cell, situated in the left pedal ganglion of the water snail Planorbis corneus, was identified in isolated living preparations of the central nervous system. Spectrophotofluorimetric analysis confirms that the cell contains dopamine, whereas noradrenaline appears to be absent. The cell is unique in being a repeatedly identifiable dopamine-containing neurone. 2. Stimulation of the giant dopamine-containing cell resulted in excitatory, inhibitory or biphasic (depolarizing-hyperpolarizing) synaptic potentials in a number of follower neurones. The duration of the e.p.s.p.s and i.p.s.p.s was 0-3-5 sec; they ranged from barely detectable responses to ones 7 mV in amplitude in different cells. The depolarizing phase of a biphasic synaptic potential (b.p.s.p.) was usually less than 1 mV in amplitude (max. 3mV) and lasted 40-400 msec. The latency of i.p.s.p.s was long (70-120 msec) compared with that of e.p.s.p.s and b.p.s.p.s (20 msec). Abolition of the depolarizing phase of b.p.s.ps. by tubocurarine left a long-latency (70-120 msec) i.p.s.p. All responses showed summation and marked facilitation. 3. Evidence is presented that the post-synaptic potentials are produced by direct connections from the giant cell and result from a release of dopamine. Of eight putative transmitter substances tested on these different groups of neurones, only dopamine produced a potential change which in each case was of the same polarity as the post-synaptic potential when this was monophasic. However, generally applied dopamine produced only a hyperpolarization in follower cells showing b.p.s.p.s. This result is probably partly due to rapid desensitization of the receptors mediating the depolarization and also to a masking of the depolarization by the more effective hyperpolarizing response. 4. Erogometrine and 6-hydroxydopamine specifically antagonized the i.p.s.p.s and dopamine receptors mediating inhibition. Neither the e.p.s.p.s nor the excitatory dopamine response were blocked by high concentrations of hexamethonium. Hexamethonium was also ineffective in blocking the depolarizing phase of a b.p.s.p., which was, however, selectively eliminated by tubocurarine. 5. It is suggested that dopamine is the transmitter released from the giant cell and that it can mediate excitatory, inhibitory or biphasic responses in different follower neurones.

On the transmitter function of 5-hydroxytryptamine at excitatory and inhibitory monosynaptic junctions

1. Two symmetrical giant neurones located in the cerebral ganglion of Aplysia californica contain 4-6 p-mole 5-hydroxytryptamine (5-HT) and are able to synthesize it (Weinreich, McCaman, McCaman & Vaughn, 1973; Eisenstadt, Goldman, Kandel, Koike, Koester & Schwartz, 1973). Stimulation of each of these neurones evokes excitatory and inhibitory potentials in various cells of the ipsilateral buccal ganglion. In nine buccal neurones it evokes excitatory potentials, in other three, ;classical' inhibitory potentials and in one neurone an ;atypical' inhibitory potential.2. The connexion between the giant cerebral neurone and the cells receiving either an excitatory or a ;classical' inhibitory input from it are monosynaptic. TEA injection into the cerebral giant neurone, which prolongs the presynaptic spike, causes a gradual increase of both the excitatory and the inhibitory potentials. On the other hand, high Ca(2+) media, which block polysynaptic pathways, do not suppress these synaptic potentials.3. The iontophoretic application of 5-HT to the buccal neurones receiving excitatory input from the giant cerebral neurones evokes depolarizations showing the pharmacological properties of both A- and A'-responses to 5-HT (see preceding paper). Antagonists which block only the A-receptors (curare, 7-methyltryptamine, LSD 25) block partially the synaptic depolarizing potentials. Bufotenine, which blocks both the A- and A'-receptors, completely blocks the excitatory potentials. Thus, the post-synaptic membrane of these buccal neurones appears to be endowed with both A- and A'-receptors to 5-HT.4. The ;classical' inhibitory potentials elicited in three buccal neurones are hyperpolarizations which reverse at - 80 mV and are due to an increase in K(+)-conductance. The iontophoretic application of 5-HT to these post-synaptic neurones evokes hyperpolarizing B-responses which are also generated by an increase in K(+)-conductance. Antagonists which block the B-responses (bufotenine, methoxygramine) also block the inhibitory potentials.5. The ;atypical' inhibitory potential evoked in one buccal neurone consists in an hyperpolarization which increases in amplitude with cell hyperpolarization. Iontophoretic application of 5-HT to this buccal cell evokes an hyperpolarizing beta-response which also increases in amplitude with cell polarization and results from a decrease in both Na(+)- and K(+)- conductances. The monosynaptic character of the ;atypical' inhibitory potential is not yet fully proven.6. It can be concluded that the excitatory and inhibitory synaptic effects evoked in the buccal neurones by the stimulation of the 5-HT-containing-giant cerebral neurones are very likely mediated by 5-HT.