Multiple sensory neuronal correlates of site-specific sensitization in Aplysia

Noxious stimulation of a restricted site on the skin of Aplysia (training) causes site-specific sensitization of the tail-withdrawal reflex that is associated with several sensory correlates that are evident both 10 min and 2 hr after training. First, extracellularly recorded afferent activity evoked by test stimulation of the trained site increases, indicating peripheral sensory changes. Second, central sensory alterations are manifested by tail sensory neurons within the pleural VC cluster that innervate the trained site and are activated during training. These mechanosensory/nociceptive cells display a number of differences from unactivated tail sensory neurons innervating other sites: slow depolarization of the soma observed immediately after training, decrease in soma action potential threshold, and enhancement of monosynaptic EPSPs to identified motor neurons. Noxious stimulation of a more extensive region also produces site-specific sensitization of the tail-withdrawal reflex and site-specific enhancement of EPSP amplitude measured 1 d after training. This training produced a novel cellular correlate of behavioral enhancement in Aplysia--regenerative bursting responses (2-35 spikes) in response to brief depolarization of the sensory neuron soma. The changes in peripheral and central excitability appear similar to changes associated with mammalian models of primary hyperalgesia. Site-specific enhancement of nociceptive signaling also occurs during aversive associative conditioning in a noxious unconditioned stimulus (US) pathway. These site-specific changes involve activity-dependent extrinsic modulation (ADEM) of the VC sensory neurons, suggesting a close relationship to changes underlying associative conditioning in conditioned stimulus (CS) pathways in Aplysia.     

Selective short- and long-term effects of serotonin, small cardioactive peptide, and tetanic stimulation on sensorimotor synapses of Aplysia in culture.

Synapses between the sensory and motor cells of Aplysia can be enhanced by heterosynaptic or homosynaptic stimulation. We have used the isolated sensorimotor synapse of Aplysia in cell culture to explore short- and long-term heterosynaptic facilitation produced by 2 facilitatory transmitters and compared these to homosynaptic facilitation produced by posttetanic potentiation. We found that brief application of 5-HT or small cardioactive peptide (SCP) evokes comparable short-lasting enhancement of nondepressed sensorimotor synapses. The effect evoked by SCP diverges from that of 5-HT when the sensorimotor synapse is first depressed by low-frequency homosynaptic stimulation. Whereas 5-HT facilitates sensorimotor synapses whether or not they are depressed, SCP has little or no effect on synapses that have been depressed by more than 75%. The 2 transmitters also differ in producing long-term facilitation. Whereas repeated applications of 5-HT evoke long-term facilitation of the synapses, SCP applications do not. To determine whether these failures to facilitate could be overcome by increasing levels of cAMP, we applied SCP in the presence of phosphodiesterase inhibitors, which resulted in SCP evoking both short- and long-term changes comparable to that of 5-HT. Homosynaptic facilitation by post-tetanic potentiation differed from heterosynaptic facilitation in that tetanic stimulation failed to evoke long-lasting changes in the synapse. These results support recent findings that 5-HT is a critical neuromodulator in behavioral sensitization and dishabituation and suggest that critical levels of cAMP may be required for long- and short-term facilitation of depressed synapses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identified facilitator neurons L29 and L28 are excited by cutaneous stimuli used in dishabituation, sensitization, and classical conditioning of Aplysia

Tactile or electrical stimulation of the skin can be used to produce dishabituation, sensitization, and classical conditioning of the gill- and siphon-withdrawal reflex in Aplysia. These behavioral effects are thought to involve presynaptic facilitation at the synapses from siphon sensory neurons to gill and siphon motor neurons. Facilitation of PSPs onto the motor neurons can also be produced by intracellular stimulation of single identified neurons in the abdominal ganglion, including L29 and L28. In this paper, we further characterize L29 and L28. First, we show that they are excited by cutaneous stimuli similar to those used to produce dishabituation, sensitization, and classical conditioning and may therefore participate in mediating those behavioral effects. The results are also consistent with a possible role of L29 and L28 in higher-order features of conditioning. Second, we show that 5-HT does not mimic some of the PSPs of L29, in agreement with previous evidence that L29 is not serotonergic. Third, we present 2 types of evidence that L29 acts directly to produce facilitation of the sensory cells: (1) L29 comes into close contact with sensory cells in fluorescent double-labeling experiments, and (2) L29 produces facilitation of sensory cells in dissociated cell culture. Together with the results of the preceding paper (Mackey et al., 1989), these results indicate that facilitation of sensory cell synapses contributing to behavioral enhancement of the reflex can be produced by identified neurons that use 2 different transmitters: 5-HT (the transmitter of CB1) and the unknown transmitter of L29.     

Long-term enhancement produced by activity-dependent modulation of Aplysia sensory neurons

We have investigated long-lasting enhancement of signaling effectiveness in the tail sensory neurons of Aplysia using both intracellular and extracellular stimulation. The pairing of high frequency homosynaptic activation with heterosynaptic modulation produced significantly greater enhancement of monosynaptic connections to identified motor neurons than did homosynaptic activity, heterosynaptic modulation, or test stimuli alone. Enhancement of the monosynaptic excitatory postsynaptic potential produced by pairing persisted for at least 4 hr, and the kinetics of decay of this potentiation indicated a time constant of about 5 hr. Although unpaired stimulation produced much weaker enhancement, both homosynaptic activity and heterosynaptic modulation alone produced enhancement lasting more than 90 min. The results are consistent with the possibility that intrinsic electrical activity can amplify the modulatory effects of a paired extrinsic chemical signal to produce long-term changes in synaptic strength. Paired stimulation also produced a relative enhancement of the excitability of the sensory neuron soma as judged by changes in action potential threshold. The lack of generalized changes in the postsynaptic cell and the observation of pairing-induced long-term changes in action potential threshold in the presynaptic cell soma suggest that long-term enhancement produced by pairing has a presynaptic locus in this system. Since pairing-specific enhancement can encode associations between sensory and motivational events in these cells, this form of plasticity may function as a form of associative memory. Similarities between long-term paired enhancement in this system and associative long-term potentiation in other systems suggest that activity-dependent neuromodulation might be involved in cellular memory in other systems as well.     

Effects of interstimulus interval and contingency on classical conditioning of the Aplysia siphon withdrawal reflex

The siphon withdrawal reflex of Aplysia undergoes differential classical conditioning with cutaneous stimulation of the siphon or mantle shelf as the discriminative conditioned stimuli (CS+ and CS-) and shock to the tail as the unconditioned stimulus (US). The reflex has proved to be useful for analyzing the neural mechanisms of conditioning. To test the generality of this experimental system, we have begun to compare the properties of conditioning in Aplysia with those of conditioning in vertebrates. We first examined the effect of the interstimulus interval (ISI) by varying the time between presentation of the CS+ and the US in different groups of animals. Significant differential conditioning was obtained when the onset of the CS+ preceded the onset of the US by 0.5 sec, and marginal conditioning was obtained when the ISI was 1.0 sec. By contrast, no significant conditioning occurred when the CS+ preceded the US by 2, 5, or 10 sec, when the onsets of the stimuli were simultaneous, or when US onset preceded the CS+ by 0.5, 1.0, or 1.5 sec (backward conditioning). We next examined the effect of contingency by giving one group of animals normal differential conditioning, and a second group the same training but with additional USs inserted between the paired trials. Presentation of these additional USs reduced the degree to which the US was contingent on the CS+, but did not change the number of pairings. Animals receiving normal training again showed significant conditioning, whereas animals receiving additional USs showed no conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synapses as associative memory elements in the hippocampal formation

This report analyzes long term potentiation (LTP) and associative interactions between synapses of the ipsilateral and crossed entorhinal cortical (EC) pathways to the dentate gyrus (DG). In the anesthetized rat, conditioning stimulation to one EC-DG pathway reliably elicits LTP at the ipsilateral synapses, while the synapses of the collateral, crossed pathway to the contralateral DG do not exhibit LTP. Furthermore, in the DG ipsilateral to the conditioning stimulation the convergent crossed pathway from the contralateral side, which had not been itself conditioned, failed to exhibit heterosynaptic LTP. These results are consistent with a specific 'synaptic' localization of the changes responsible for LTP, and suggest that some critical number of synapses must be activated in order to observe LTP. While the crossed EC-DG projection never exhibited LTP when conditioned alone, the crossed input could be potentiated under certain circumstances. Specifically, paired conditioning of ipsi- and contralateral inputs by nearly simultaneous conditioning stimulation of the EC bilaterally results in LTP in the crossed system. Furthermore, this associatively induced LTP of the crossed system can be reversed by subsequent conditioning of the ipsilateral system alone. Successive potentiating and depotentiating sequences are possible using paired and non-paired stimulation procedures even after lesions which prevent neural loops through the EC. The results are interpreted as evidence for a 'Hebb' type synapse which has the capability for erasure. This synaptic type is not appropriate for classical conditioning without appendant circuitry, but is suited for other forms of associative learning.     

The contribution of facilitation of monosynaptic PSPs to dishabituation and sensitization of the Aplysia siphon withdrawal reflex.

To examine the relationship between synaptic plasticity and learning and memory as directly as possible, we have developed a new simplified preparation for studying the siphon-withdrawal reflex of Aplysia in which it is relatively easy to record synaptic connections between individual identified neurons during simple forms of learning. We estimated that monosynaptic EPSPs from LE siphon sensory neurons to LFS siphon motor neurons mediate approximately one-third of the reflex response measured in this preparation, which corresponds to siphon flaring in the intact animal. To investigate cellular mechanisms contributing to dishabituation and sensitization, we recorded evoked firing of LFS neurons, the siphon withdrawal produced by stimulation of an LFS neuron, the complex PSP in an LFS neuron, and the monosynaptic PSP from an "on-field" or "off-field" LE neuron to an LFS neuron during behavioral training. Unlike the simplified gill-withdrawal preparation (Cohen et al., 1997; Frost et al., 1997), in the siphon-withdrawal preparation we found no qualitative differences between the major cellular mechanisms contributing to dishabituation and sensitization, suggesting that dissociations that have been observed previously may be attributable to transient inhibition that does not occur for this component of the reflex. Furthermore, in the siphon-withdrawal preparation, all of the various cellular measures, including monosynaptic PSPs from either on-field or off-field LE neurons, changed approximately in parallel with changes in the behavior. These results provide the most direct evidence so far available that both dishabituation and sensitization involve multiple mechanisms, including heterosynaptic facilitation of sensory neuron-motor neuron PSPs.     

Identified FMRFamide-immunoreactive neuron LPL16 in the left pleural ganglion of Aplysia produces presynaptic inhibition of siphon sensory neurons.

The gill- and siphon-withdrawal reflex of Aplysia undergoes transient inhibition following noxious stimuli such as tail shock. This behavioral inhibition appears to be due in part to transient presynaptic inhibition of the siphon sensory cells, which can be mimicked by application of the peptide FMRFamide. Although FMRFamide is widespread in the Aplysia nervous system, an FMRFamide-containing inhibitory neuron has not previously been identified. We have searched for such a neuron by combining FMRFamide immunofluorescence with fluorescent dye backfilling from the abdominal ganglion, the location of the siphon sensory cells. These methods localized a neuron in the left pleural ganglion, which we have named LPL16. LPL16 is FMRFamide immunoreactive; it is excited by tail shock; and stimulation of LPL16 produces inhibition of siphon sensory cell-to-motor cell postsynaptic potentials and narrowing of action potentials in the sensory cells in tetraethylammonium solution. These results indicate that LPL16 participates in the inhibitory effects of tail shock, and support the idea that FMRFamide plays a physiological role in the inhibition.     

Multiple memory traces after associative learning in the honey bee antennal lobe

We investigated the effect of associative learning on early sensory processing, by combining classical conditioning with in vivo calcium-imaging of secondary olfactory neurons, the projection neurons (PNs) in the honey bee antennal lobe (AL). We trained bees in a differential conditioning paradigm in which one odour (A+) was paired with a reward, while another odour (B-) was presented without a reward. Two to five hours after differential conditioning, the two odour-response patterns became more different in bees that learned to discriminate between A and B, but not in bees that did not discriminate. This learning-related change in neural odour representations can be traced back to glomerulus-specific neural plasticity, which depended on the response profile of the glomerulus before training. (i) Glomeruli responding to A but not to B generally increased in response strength. (ii) Glomeruli responding to B but not to A did not change in response strength. (iii) Glomeruli responding to A and B decreased in response strength. (iv) Glomeruli not responding to A or B increased in response strength. The data are consistent with a neural network model of the AL, which we based on two plastic synapse types and two well-known learning rules: associative, reinforcer-dependent Hebbian plasticity at synapses between olfactory receptor neurons (ORNs) and PNs; and reinforcer-independent Hebbian plasticity at synapses between local interneurons and ORNs. The observed changes strengthen the idea that odour learning optimizes odour representations, and facilitates the detection and discrimination of learned odours.     

Differences between the effects of three plasticity inducing protocols on the organization of the human motor cortex

Several experimental protocols induce lasting changes in the excitability of motor cortex. Some involve direct cortical stimulation, others activate the somatosensory system and some combine motor and sensory stimulation. The effects usually are measured as changes in amplitude of the motor-evoked-potential (MEP) or short-interval intracortical inhibition (SICI) elicited by a single or paired pulses of transcranial magnetic stimulation (TMS). Recent work has also tested sensorimotor organization within the motor cortex by recording MEPs and SICI during short periods of vibration applied to single intrinsic hand muscles. Here sensorimotor organization is focal: MEPs increase and SICI decreases in the vibrated muscle, whilst the opposite occurs in neighbouring muscles. In six volunteers we compared the after effects of three protocols that lead to lasting changes in cortical excitability: (i) paired associative stimulation (PAS) between a TMS pulse and an electrical stimulus to the median nerve; (ii) motor practice of rapid thumb abduction; and (iii) sensory input produced by semicontinuous muscle vibration, on MEPs and SICI at rest and on the sensorimotor organization. PAS increased MEP amplitudes, whereas vibration changed sensorimotor organization. Motor practice had a dual effect and increased MEPs as well as affecting sensorimotor organization. The implication is that different protocols target different sets of cortical circuits. We speculate that protocols that involve repeated activation of motor cortical output lead to lasting changes in efficacy of synaptic connections in output circuits, whereas protocols that emphasize sensory inputs affect the strength of sensory inputs to motor circuits.     

Motor cortical plasticity is impaired in Unverricht–Lundborg disease

Patients with Unverricht–Lundborg disease, also referred to as progressive myoclonus epilepsy type 1, exhibit widespread motor symptoms and signs in addition to epileptic seizures, which suggest abnormal excitability of the primary motor pathways. To explore the plasticity of the sensory–motor cortex, we employed a modern neurophysiological method, the paired associative stimulation protocol, which resembles the concept of long‐term potentiation of experimental studies. Seven patients with genetically verified Unverricht–Lundborg disease and 13 healthy control subjects were enrolled in the study to characterize cortical sensory–motor plasticity. In the study protocol, peripheral electric median nerve stimulation preceded navigated transcranial magnetic stimulation targeted to the representation area of thenar musculature on the contralateral primary motor cortex. The protocol consisted of 132 transcranial magnetic stimulation trials at 0.2 Hz, preceded by peripheral sensory stimulation at 25 ms. Motor‐evoked potential amplitudes were analyzed at baseline and after the paired associative stimulation protocol at an intensity of 130% of the individual motor threshold. The patients with Unverricht–Lundborg disease exhibited an average decrease of 15% in motor‐evoked potential amplitudes 30 minutes after paired associative stimulation, whereas in the control subjects, a significant increase (101%) was observed (P < .05), as expected. The results indicate a lack of normal cortical plasticity in Unverricht–Lundborg disease, which stresses the role of abnormal motor cortical functions or sensorimotor integration as possible pathophysiological contributors to the motor symptoms. The impaired cortical plasticity may be associated with the previously reported structural and physiological abnormalities of the primary motor cortex. © 2011 Movement Disorder Society     

Identified serotonergic neurons LCB1 and RCB1 in the cerebral ganglia of Aplysia produce presynaptic facilitation of siphon sensory neurons

Several lines of evidence suggest that 5-HT plays a significant role in presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia. Most recently, Glanzman et al. (1989) found that treatment with the 5-HT neurotoxin, 5,7-DHT markedly reduced both synaptic facilitation and behavioral dishabituation. To provide more direct evidence for a role of 5-HT, we have attempted to identify individual serotonergic facilitator neurons. Hawkins (1989) used histological techniques to locate several serotonergic neurons in the ring ganglia that send axons to the abdominal ganglion and are therefore possible serotonergic facilitators. These include one neuron in the B cluster of each cerebral ganglion, which we have identified electrophysiologically and named the CB1 cells. Both glyoxylic acid histofluorescence and 5-HT immunofluorescence indicate that the CB1 neurons are serotonergic. In a semiintact preparation, the CB1 neurons respond to cutaneous stimulation which produces dishabituation and sensitization (such as tail shock) with an increase in firing, which may outlast the stimulation by 15 min. Intracellular stimulation of a CB1 neuron in a manner similar to its response to tail shock produces facilitation of the EPSPs from siphon sensory neurons to motor neurons, as well as broadening of the action potential in the sensory neurons in tetraethylammonium solution. These results strongly suggest that the identified serotonergic CB1 neurons participate in mediating presynaptic facilitation contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia.     

In vitro analog of operant conditioning in aplysia. II. Modifications of the functional dynamics of an identified neuron contribute to motor pattern selection.

Previously, an analog of operant conditioning was developed using the buccal ganglia of Aplysia, the probabilistic occurrences of a specific motor pattern (i.e., pattern I), a contingent reinforcement (i.e., stimulation of the esophageal nerve), and monotonic stimulation of a peripheral nerve (i.e., n.2,3). This analog expressed a key feature of operant conditioning (i.e., selective enhancement of the probability of occurrence of a designated motor pattern by contingent reinforcement). In addition, the training induced changes in the dynamical properties of neuron B51, an element of the buccal central pattern generator. To gain insights into the neuronal mechanisms that mediate features of operant conditioning, the present study identified a neuronal element that was critically involved in the selective enhancement of pattern I. We found that bursting activity in cell B51 contributed significantly to the expression of pattern I and that changes in the dynamical properties of this cell were associated with the selective enhancement of pattern I. These changes could be induced by an explicit association of reinforcement with random depolarization of B51. No stimulation of n.2,3 was required. These results indicate that the selection of a designated motor pattern by contingent reinforcement and the underlying neuronal plasticity resulted from the association of reinforcement with a component of central neuronal activity that contributes to a specific motor pattern. The sensory stimulus that allows for occurrences of different motor acts may not be critical for induction of plasticity that mediates the selection of a motor output by contingent reinforcement in operant conditioning.     

Postsynaptic processes in rubrospinal neurons of the cat brain exposed to different corticofugal influences

EPSPs of the red nucleus neurons evoked in response to stimulation of the cerebellar nucleus interpositus as well as the sensorimotor and associative parietal region of the cerebral cortex were studied in acute experiments on nembutalized cats. As in the case of the two previous structures mentioned a monosynaptic connection of the associative cortex with the rubrospinal neurons was shown to exist. Unitary EPSPs having latent periods 1.5-2.7 ms, time to peak 1.1-3.1 ms and amplitude of 0.22-0.65 mV were observed in the rubrospinal neurons in response to associative cortex stimulation. Analysis of the time parameters of the unitary EPSPs showed that synapses formed by fibres from the associative cortex are located on the somadendritic membrane of the neuron more proximally comparing with synapses formed by fibres from the sensorimotor cortex.     

Plasticity of the motor cortex in Parkinson's disease patients on and off therapy.

We used the paired associative stimulation (PAS) technique to investigate associative plasticity of the sensorimotor cortex in 16 Parkinson's disease (PD) patients off and on therapy and in 10 age-matched controls. After PAS, motor evoked potential (MEP) amplitudes increased more and the cortical silent period showed a reduced prolongation in patients off therapy than in controls. These changes lasted for at least 30 minutes. In addition, MEP amplitudes increased in a less focal manner in patients off therapy than in controls. After patients received dopaminergic therapy, these abnormalities normalized. The abnormal responsiveness of sensorimotor cortex neurons to PAS in PD patients off therapy probably reflects disordered plasticity within the motor cortex.     

Unilateral ischemic sensorimotor cortical damage induces contralesional synaptogenesis and enhances skilled reaching with the ipsilateral forelimb in adult male rats

Unilateral damage to the forelimb representation area of the sensorimotor cortex (SMC) results in a compensatory reliance on the unimpaired (ipsilateral to the lesion) forelimb as well as reorganization of neuronal structure and connectivity in the contralateral motor cortex. Recently, male rats with unilateral electrolytic SMC lesions were found to have enhanced skilled reaching performance with the ipsilesional forelimb compared with sham-operated controls. The present study was performed to determine whether these behavioral findings are replicable using an ischemic lesion and whether there is a link between the enhanced learning and synaptogenesis in motor cortical layer V opposite the trained limb and lesion, as assessed using stereological methods for light and electron microscopy. Rats were given a sham operation or an endothelin-1 (ET-1) induced ischemic SMC lesion. They were then trained for 20 days on a skilled reaching task with the unimpaired limb or received control procedures. As with previous findings using electrolytic lesions, rats with unilateral ischemic SMC lesions performed significantly better using the unimpaired forelimb than did sham-operates. Lesions, but not training, significantly increased the total number of motor cortical layer V synapses per neuron as well as the number of perforated and multisynaptic bouton (MSB) synapses per neuron compared with shams. Thus, in addition to a net increase in synapses, the improved reaching ability was coupled with an increase in synapse subtypes that have previously been linked to enhanced synaptic efficacy. The failure to induce synaptogenesis in layer V with reach training alone is in contrast to previous findings and may be related to training intensity.     

Effects of movement training on synaptic interface structure in the sensorimotor cortex and hippocampal CA3 area of the ischemic hemisphere in cerebral infarction rats★

BACKGROUND： Movement is an effective way to provide sensory, movement and reflectivity afferent stimulation to the central nervous system. Movement plays an important role in functional recombination and compensation in the brain. OBJECTIVE： To observe movement training effects on texture parameters of synaptic interfaces in the sensorimotor cortex and hippocampal CA3 area of the ischemic hemisphere and on motor function in cerebral infarction rats. DESIGN, TIME AND SETTING： This neural morphology and pathology randomized controlled animal experiment was performed at the Center Laboratory, Affiliated Hospital of Luzhou Medical College, China from November 2004 to April 2005. MATERIALS： A total of 32 healthy male Wistar rats aged 8 weeks were equally and randomly assigned into model and movement training groups. METHODS： Rat models of right middle cerebral artery occlusion were established using the suture occlusion method in both groups. Rats in the movement training group underwent balance training, screen training, and rotating rod training starting on day 5 after surgery, for 40 minutes every day, 6 days per week, for 4 weeks. MAIN OUTCOME MEASURES： Texture parameters of synaptic interfaces were determined using a transmission electron microscope and image analyzer during week 5 following model induction. The following parameters were measured： synaptic cleft width; postsynaptic density thickness; synaptic interface curvature; and active zone length. Motor function was assessed using balance training, screen training, and rotating rod training. The lower score indicated a better motor function. RESULTS： The postsynaptic density thickness, synaptic interface curvature, and active zone length were significantly increased in the sensorimotor cortex and hippocampal CA3 area of the ischemic hemisphere of rats from the movement training group compared with the model group （P 〈 0.05 or 0.01）. Curved synapses and perforated synapses were seen in the sensorimotor cortex and hippocampal CA3 area at     

Differences in excitatory transmission between thalamic and cortical afferents to single spiny efferent neurons of rat dorsal striatum

The striatum is crucially involved in motor and cognitive function, and receives significant glutamate input from the cortex and thalamus. The corticostriatal pathway arises from diverse regions of the cortex and is thought to provide information to the basal ganglia from which motor actions are selected and modified. The thalamostriatal pathway arises from specific thalamic nuclei and is involved in attention and possibly strategy switching. Despite these fundamental functional differences, direct comparisons of the properties of these pathways are lacking. N‐methyl‐d ‐aspartate (NMDA) receptors at synapses powerfully affect postsynaptic processing, and incorporation of different NR2 subunits into NMDA receptors has profound effects on the pharmacological and biophysical properties of the receptor. Utilization of different NMDA receptors at thalamostriatal and corticostriatal synapses could allow for afferent‐specific differences in information processing. We used a novel rat brain slice preparation preserving corticostriatal and thalamostriatal pathways to medium spiny neurons to examine the properties of NMDA receptor‐mediated excitatory postsynaptic currents (EPSCs) recorded using the whole‐cell, patch‐clamp technique. Within the same neuron, the NMDA/non‐NMDA ratio is greater for excitatory responses evoked from the thalamostriatal pathway than for those evoked from the corticostriatal pathway. In addition, reversal potentials and decay kinetics of the NMDA receptor‐mediated EPSCs suggest that the thalamostriatal synapse is more distant on the dendritic arbor. Finally, results obtained with antagonists specific for NR2B‐containing NMDA receptors imply that NMDA receptors at corticostriatal synapses contain more NR2B subunits. These synapse‐specific differences in NMDA receptor content and pharmacology provide potential differential sites of action for NMDA receptor subtype‐specific antagonists proposed for the treatment of Parkinson’s disease.     

Localization of potential serotonergic facilitator neurons in Aplysia by glyoxylic acid histofluorescence combined with retrograde fluorescent labeling

A variety of evidence suggests that 5-HT participates in presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the gill- and siphon-withdrawal reflex in Aplysia. Most recently, Glanzman et al. (1989) have shown that the 5-HT neurotoxin 5,7-DHT markedly reduces both the synaptic facilitation and behavioral dishabituation produced by tail shock. To provide more direct evidence for a role of 5-HT, I have used histological techniques to try to locate individual serotonergic facilitator neurons. I first used a modification of the glyoxylic acid histofluorescence technique to map serotonergic and dopaminergic neurons in the CNS of Aplysia. Intracellular fluorescent labeling combined with histofluorescence indicates that the previously identified L29 facilitator neurons are not serotonergic. Nerve transection experiments suggest that most of the perisomatic 5-HT histofluorescence in the abdominal ganglion (the location of the siphon sensory cells) comes from neurons whose cell bodies are located in the pedal or cerebral ganglia. As there are at least 500 serotonergic neurons in those ganglia, I combined retrograde fluorescent labeling with histofluorescence to identify a small subset of those neurons which send processes to the abdominal ganglion and are therefore potential serotonergic facilitators. In the following paper, Mackey et al. (1989) show that stimulation of 2 of those neurons in the cerebral ganglia (the CB1 cells) produces presynaptic facilitation of the siphon sensory cells contributing to dishabituation and sensitization of the withdrawal reflex.     

Learning insights transmitted by glutamate

Plasticity of the Aplysia sensorimotor synapse plays a crucial role in learning and memory of withdrawal reflexes. During the past ten years, a growing body of evidence has indicated that the sensorimotor synapse is glutamatergic. This new information has guided several studies that implicate AMPA and NMDA receptors in synaptic plasticity. However, further work is necessary to delineate the exact properties of the postsynaptic receptors, and their role in transmission and plasticity. Despite the still incomplete picture of the intrinsic properties of the sensorimotor synapse, identifying the endogenous transmitter has provided a foundation for new avenues of research, the results of which will further improve our understanding of the neurobiology of learning and memory.