电刺激家兔迷走神经中枢端诱导的黑-伯反射中的非联合型学习现象

本文旨在研究电刺激家兔迷走神经诱导的黑-伯（Hering-Breuer,HB）反射中的学习和记忆现象。选择性电刺激家兔迷走神经中枢端（频率10～100Hz,强度20～60μA,波宽0．3ms,持续60s）,观察对膈神经放电的影响。以不同频率电刺激家兔迷走神经可模拟HB反射的两种成分,即类似肺容积增大所致抑制吸气的肺扩张反射和类似肺容积缩小所致加强吸气的肺萎陷反射。（1）长时高频（≥40Hz,60s）电刺激迷走神经可模拟呼吸频率减慢,呼气时程延长的肺扩张反射。随着刺激时间的延长,膈神经放电抑制的程度逐渐衰减,表现为呼吸频率的减慢（主要由呼气时程延长所致）在刺激过程中逐渐减弱或消失,显示为适应性或“习惯化”的现象;刺激结束时呼吸运动呈现反跳性增强,表现为一过性的呼气时程缩短,呼吸频率加快,然后才逐渐恢复正常。长时低频（〈40Hz,60s）电刺激迷走神经可模拟呼吸频率加快、呼气时程缩短的肺萎陷反射。随着刺激时间的延长,膈神经放电增强的程度逐渐衰减,同样表现出“习惯化”现象;刺激结束后,膈神经放电不是突然降低,而是继续衰减,表现为呼气时程逐渐延长,呼吸频率逐渐减慢,直至恢复到前对照水平,表现了刺激后的短时增强效应。（2）HB反射的适应性或“习惯化”程度反向依赖于刺激强度和刺激频率,表现为随着刺激强度和频率的增加,膈神经放电越远离正常基线水平,即爿惯化程度减弱。结果表明,家兔HB反射具有“习惯化”这一非联合型学习现象,反映与其有关的呼吸神经元网络具有突触功能的可翅性,呼吸的中枢调控反射具有一定的适应性。     

Alternative startle motor patterns and behaviors in the larval zebrafish (<Emphasis Type="Italic">Danio rerio</Emphasis>)

In fishes, the C-start behavior, initiated with a C-shaped body bend, is a taxonomically common and widely studied escape response. Its simple neural circuit has made this behavior a model for examining neural control of movement. The S-start, initiated with an S-shaped body bend, is a physiologically distinct escape that occurs in esocid fishes. Here we examine whether zebrafish larvae perform S-starts in order to better understand startle diversity and to attempt to identify the S-start in a system that is tractable for neurobiological studies. We found that larval zebrafish startles varied in the extent of their caudal bending, resulting in C, S and intermediate-shaped responses. We recorded two distinct motor patterns: nearly simultaneous initial activity along one side of the body, characteristic of C-starts, and nearly simultaneous activity rostrally on one side and caudally on the other, characteristic of S-starts. Head stimulation generally elicited C-starts while tail stimulation elicited C- and S-starts. These results demonstrate that the S-start is more common than previously documented and occurs in early developmental stages. We suggest that the S-start may be a fundamental escape behavior in fishes and may provide a comparative model to the C-start for understanding simple neural circuits.     

Transfer of habituation in Aplysia: contribution of heterosynaptic pathways in habituation of the gill-withdrawal reflex

Habituation of the Aplysia gill-withdrawal reflex (and siphon-withdrawal reflex) has been attributed to low-frequency homosynaptic depression at central sensory-motor synapses. The recent demonstration that transfer of habituation between stimulation sites occurs in this model system has prompted the hypothesis that heterosynaptic inhibitory pathways also play a role in the mediation of habituation behavior. To test this hypothesis, the sites and mechanisms of neural plasticity which underlie transfer of habituation in Aplysia were examined. Transfer of habituation is a reduction in the reflex evoked at one stimulation site (siphon) due to repeated presentation of a stimulus to a second site (gill). Centrally mediated transfer of habituation, measured in a preparation lacking the siphon-gill peripheral nervous system (PNS), was associated with a reduced excitatory response in central motor neurons. Repeated tactile stimulation of the gill did not attenuate the gill response evoked by electrical stimulation of the branchial nerve nor the mechanoreceptor response recorded in LE sensory neurons. In contrast, repeated stimulation of siphon or gill at a site which was "off" the sensory field of a specific mechanoreceptor led to a diminution in synaptic transmission between that sensory neuron and its followers (motor neurons and inter-neurons). These data demonstrate that centrally mediated transfer of habituation results from heterosynaptic modulation of synaptic transmission at the sensory-motor (and sensory-interneuron) synapses. Therefore, habituation behavior in Aplysia is mediated through the conjoint action of homosynaptic and heterosynaptic inhibitory processes.     

Parallel molecular pathways mediate expression of distinct forms of intermediate-term facilitation at tail sensory-motor synapses in Aplysia

Three distinct temporal phases of synaptic facilitation (short-, intermediate-, and long-term) are induced by serotonin (5-HT) at sensory (SN) to motor (MN) synapses in Aplysia. Here, we characterize two mechanistically distinct forms of intermediate-term facilitation (ITF) at tail SN-MN synapses. One form, activity-independent ITF, is produced by five spaced pulses of 5-HT in the absence of SN activity. Its induction requires protein synthesis, and its expression requires persistent activation of PKA but not PKC. The other form, activity-dependent ITF, is produced by a single pulse of 5-HT coincident with SN activation. Its induction does not require protein synthesis, and its expression requires persistent activation of PKC but not PKA. These results demonstrate that SN-MN synapses can exhibit two distinct forms of ITF that are mediated by parallel molecular pathways.     

L9 modulation of gill withdrawal reflex habituation in Aplysia.

Repeated tactile stimulation of the siphon in Aphysia normally results in habituation of the gill withdrawal reflex and a concomitant decrease in the amplitude of the excitatory synaptic input ot gill motor neurons in the abdominal ganglion. It was found, however, that induced low-level tonic activity in motor neuron L9, which does not itself elicit a gill withdrawal movement, prevented habituation of the reflex from occurring. Further, in preparations already habituated, this tonic low-level activity brought about a reversal of habituation. Although tonic L9 activity prevented the occurrence of habituation or brought about its reversal, it did not interfere with the synaptic decremental process which normally accompanies gill reflex habituation. Motor neurons L7 and LDG1 were found not to possess this ability of L9 to modulate gill reflex habituation. Evidence suggests that L9's modulatory effect is mediated in the periphery, in the gill and not centrally in the abdominal ganglion.     

Integration of ascending and descending inputs in the auditory midbrain of anurans.

In anurans, the midbrain torus semicircularis is involved in auditory processing and audio-motor integration. In this study, we examined the influence of descending forebrain projections on the auditory response properties and hence the audiomotor transmission of mesencephalic interface neurons. In order to investigate response integration, we performed intracellular recordings from torus neurons in an isolated brain preparation of Discoglossus pictus and Bombina orientalis and stimulated the auditory nerve, striatum, and the dorsal thalamus electrically with single pulses. Stimulation of all three sites could evoke responses in torus neurons that were either excitatory, inhibitory, or a mixture of both, with durations of up to several hundred milliseconds. Further, striatum and thalamus were activated by pulse trains (10-20 Hz, 50 pulses) immediately before stimulating the auditory nerve with single pulses. Thus, responses of torus neurons to "auditory" input were facilitated or suppressed for up to 2 min by striatum stimulation or only suppressed by thalamus stimulation. Intracellular labeling of recorded neurons revealed that response modulation by descending input mostly occurred in laminar nucleus neurons. These results suggest that descending forebrain projections to mesencephalic audiomotor interface neurons may play an important role in modifying acoustically guided behavior in anurans.     

L9 modulation of gill withdrawal reflex habituation in Aplysia

Repeated tactile stimulation of the siphon in Aplysia normally results in habituation of the gill withdrawal reflex and a concomitant decrease in the amplitude of the excitatory synaptic input to gill motor neurons in the abdominal ganglion. It was found, however, that induced low-level tonic activity in motor neuron L₉, which does not itself elicit a gill withdrawal movement, prevented habituation of the reflex from occurring. Further, in preparations already habituated, this tonic low-level activity brought about a reversal of habituation. Although tonic L₉ activity prevented the occurrence of habituation or brought about its reversal, it did not interfere with the synaptic decremental process which normally accompanies gill reflex habituation. Motor neurons L₇ and LDG₁ were found not to possess this ability of L₉ to modulate gill reflex habituation. Evidence suggests that L₉'s modulatory effect is mediated in the periphery, in the gill and not centrally in the abdominal ganglion.     

低频刺激后海马CA1区场兴奋性突触后电位与群体锋电位的变化

在大鼠海马脑片上使用双电极在CA1区进行细胞外记录 ,观察低频刺激 (LFS)诱发同突触长时程抑制 (LTD)时场兴奋性突触后电位 (fEPSP)的斜率 (S EPSP)和群体锋电位 (PS)的幅值 (A PS)的变化。给予 90 0脉冲 1HzLFS后 ,S EPSP和A PS降低的幅度分别是 35 4± 5 3%和 6 8 0± 7 2 % ;而给予 4 5 0脉冲 1HzLFS后 ,S EPSP和A PS分别降低 14 3± 2 3%和 36 8± 6 7%。上述两组中A PS的变化率均显著大于S EPSP (P <0 0 1) ,而 90 0脉冲数组中两个指标的变化率均大于 4 5 0脉冲数组 (P <0 0 5 )。高Mg2 + (4mmol/L)使突触的传递活动减弱 ,但不影响LTD的诱发 ,在高Mg2 + 介质中 ,LFS引起的A PS变化率仍显著大于S EPSP (P <0 0 1)。结果表明 ,由LFS诱发同突触LTD的水平不仅与LFS的脉冲数有关 ,还与评价指标的选择有关     

Desensitization of the peristaltic reflex induced by mucosal stimulation with the selective 5-HT4 agonist tegaserod

The intestinal peristaltic reflex induced by mucosal stimulation is mediated by mucosal release of serotonin (5-HT), which acts on 5-HT(4) receptors located on CGRP-containing afferent nerve terminals. Exposure of the colonic mucosa to the 5-HT(4) receptor agonist tegaserod in the range of 1 nM to 10 muM elicits a peristaltic reflex and stimulates colonic propulsion. The present study was designed to identify the 5-HT(4) receptor subtype mediating the reflex and determine whether functionally effective concentrations of tegaserod desensitize the reflex induced by mucosal stimulation. Exposure of rat colonic mucosa to tegaserod in the range of 5 nM to 5 muM for 5 or 10 min caused rapid time- and concentration-dependent desensitization of the peristaltic reflex induced by mucosal stroking, consistent with the operation of a rapidly desensitizing 5-HT(4b) receptor subtype. Desensitization was accompanied by a decrease in CGRP release. The rate of recovery of peristaltic response depended on the desensitizing concentration of tegaserod: ascending contraction and descending relaxation recovered within 15 min after 5-50 nM tegaserod, 30 min after 0.5 muM, and 60 min after 5 muM. Neither CGRP release nor the peristaltic reflex induced by muscle stretch was affected by 5-HT(4) receptor desensitization, providing further evidence that 5-HT does not mediate the reflex induced by muscle stretch. These results suggest in cases of increased 5-HT availability or prolonged exposure, such as colitis, that it is likely the peristaltic reflex will be blunted.     

Modifications of acoustic habituation by interruption of visual input in quipazine treated cats

The influence of interruption of the visual input on acoustic habituation was studied in cats before and following the administration of quipazine, 3 mg/kg iv. The characteristics of acoustic habituation were analyzed through the magnitude and temporal course of multiunit activity (MUA) responses elicited in the mesencephalic reticular formation (MRF) by repetitive acoustic stimuli (70 db, 50 Hz trains of 2 sec duration) in 6 freely moving cats with cortical electrodes over the parietal cortex and bipolar electrodes chronically implanted in MRF and basolateral amygdala (AMN). The cats were submitted to repetitive acoustic stimulation during one 30 min period before, and three 30 min periods after drug administration in the following conditions: a) with unmasked eyes; b) with masked eyes by means of dark contact lenses. Persistent attentive behavior, catatonic attitudes, hypersynchronous (6 Hz, 100-150 microV) EEG activity and significant increase of spontaneous MUA in FRM and AMN were induced by quipazine both in the cats tested with unmasked and with masked eyes. This increase of MUA was higher immediately following drug administration and progressively decreased, although MUA values remained significantly higher than controls 110 min after quipazine administration. Acoustic habituation, evidenced through the progressive decrease of MUA responses of MRF to acoustic stimuli, was observed before quipazine administration when the cats were tested with unmasked and with masked eyes; as well as in cats tested with unmasked eyes following drug administration. However, the MUA responses to acoustic stimuli did not decrease in cats with masked eyes during acoustic stimulation periods 0-30 min and 40-70 min after quipazine administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effects of excess sympathetic activity on parasympathetic vasodilation in the rat masseter muscle

The present study was designed to examine the effect of sympathetic tonic activity on parasympathetic vasodilation evoked by the trigeminal-mediated reflex in the masseter muscle in urethane-anesthetized rats. Sectioning of the superior cervical sympathetic trunk (CST) ipsilaterally increased the basal level of blood flow in the masseter muscle (MBF). Electrical stimulation of the peripheral cut end of the CST for 2 min using 2-ms pulses ipsilaterally decreased in a dependent manner the intensity (0.5-10 V) and frequency (0.1-5 Hz) of the MBF. The CST stimulation for 2 min at <0.5 Hz with 5 V using 2-ms pulses seems to be comparable with the spontaneous activity in the CST fibers innervating the masseter vasculature, because this stimulation restored the basal level of the MBF to the presectioned values. Parasympathetic vasodilation evoked by electrical stimulation of the central cut end of the lingual nerve in the masseter muscle was markedly reduced by CST stimulation for 2 min with 5 V using 2-ms pulses in a frequency-dependent manner (0.5-5 Hz). Intravenous administration of phentolamine significantly reduced the vasoconstriction induced by CST stimulation in a dose-dependent manner (0.1-1 mg/kg), but pretreatment with either phentolamine or propranolol failed to affect the sympathetic inhibition of the parasympathetic vasodilation. Our results suggest that 1) excess sympathetic activity inhibits parasympathetic vasodilation in the masseter muscle, and 2) alpha- and beta-adrenoceptors do not contribute to sympathetic inhibition of parasympathetic vasodilation, and thus some other types of receptors must be involved in this response.     

Auditory temporal resolution and evoked responses to pulsed sounds for the Yangtze finless porpoises (<Emphasis Type="Italic">Neophocaena phocaenoides asiaeorientalis</Emphasis>)

Temporal cues are important for some forms of auditory processing, such as echolocation. Among odontocetes (toothed whales, dolphins, and porpoises), it has been suggested that porpoises may have temporal processing abilities which differ from other odontocetes because of their relatively narrow auditory filters and longer duration echolocation signals. This study examined auditory temporal resolution in two Yangtze finless porpoises (Neophocaena phocaenoides asiaeorientalis) using auditory evoked potentials (AEPs) to measure: (a) rate following responses and modulation rate transfer function for 100 kHz centered pulse sounds and (b) hearing thresholds and response amplitudes generated by individual pulses of different durations. The animals followed pulses well at modulation rates up to 1,250 Hz, after which response amplitudes declined until extinguished beyond 2,500 Hz. The subjects had significantly better hearing thresholds for longer, narrower-band pulses similar to porpoise echolocation signals compared to brief, broadband sounds resembling dolphin clicks. Results indicate that the Yangtze finless porpoise follows individual acoustic signals at rates similar to other odontocetes tested. Relatively good sensitivity for longer duration, narrow-band signals suggests that finless porpoise hearing is well suited to detect their unique echolocation signals.     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996     

Some observations on habituation of the flexor reflex in the rat: the influence of strychnine, bicuculline, spinal transection, and decerebration

The involvement of inhibition in habituation of the flexor reflex was investigated in intact, spinal, and decerebrate rats. Strychnine and bicuculline were administered in order to determine the contribution of certain forms of central inhibition to the development of habituation. Both strychnine and bicuculline reduced habituation in the intact rat but did not do so in the spinal preparation. Strychnine, in fact, caused a facilitation of habituation in the spinal rat. The impairment of habituation by strychnine was related to the intensity of stimulation used to elicit the reflex. Thus, the effect of strychnine was only demonstrable when relatively high intensities were used. Flexor reflex habituation was shown to be more pronounced in the decerebrate than in the spinal preparation. This difference could be demonstrated only when stimuli of high intensity were given. It is concluded that, in the absence of supraspinal influences, habituation of the flexor reflex does not require inhibitory mechanisms. However, inhibition may play a role in habituation of the component of the reflex that utilizes descending influences.     

Effects of Frequency and Pattern of Medial Forebrain Bundle Stimulation on Caudate Dialysate Dopamine and Serotonin

In vivo microdialysis was employed to detect changes in extracellular dopamine and serotonin in the rat caudate in response to electrical stimulation of the medial forebrain bundle. Extracellular dopamine concentrations increased linearly as a function of the frequency (4-33 Hz) of evenly spaced stimuli in both the presence and absence of cocaine added to the dialysate. Because dopamine neurons are known to fire in single-spike and burst patterns, stimulation pulses were also delivered in a bursting pattern. The response of extracellular dopamine was augmented in both the presence and absence of cocaine when the same number of stimuli were delivered in bursts as compared to an evenly spaced pattern. Serotonin, which was only assessed in the presence of cocaine, similarly increased linearly with frequency, but, in contrast to the dopamine response, levels of serotonin were not augmented by stimuli presented in bursts. These results suggest that microdialysis can be used to detect physiological changes in synaptic transmitter concentrations.     

Long-lasting potentiation of excitatory synaptic signaling to the crayfish lateral giant neuron

The neural circuit that underlies the lateral giant fiber (LG)-mediated reflex escape in crayfish has provided findings relating synaptic change to nonassociative learning such as sensitization and habituation. The LGs receive sensory inputs from the primary sensory afferents and a group of mechanosensory interneurons (MSIs). An increase of excitability by suprathreshold repetitive excitation of this circuit, which is similar to Hebbian long-term potentiation (LTP), has been reported [Miller et al. (1987) J Neurosci 7:1081]. This potentiation was previously thought to result from the enhancement of transmission at cholinergic synapses between primary afferents and MSIs but not the electrical synapses onto LG. In this study, we found that potentiation of synaptic signaling at the electrical synapse onto LG can also be induced when the synapse was activated with subthreshold repetitive pulses or with a few strong suprathreshold shocks. LG LTP was induced in the preparation which had received pulses at limited frequency range. Although whether this LTP is involved in the learning process of escape behavior in crayfish is not clear, the intensity and amount of sensory stimulation used here mimicked those that could easily be produced by a predator trying to catch a crayfish and could be of adaptive significance in life.     

Heterochrony and the development of the escape response: prehatching movements in the rainbow trout Oncorhynchus mykiss

Teleost fishes produce coordinated escape responses (C-starts) at hatching. This implies that essential swimming morphologies and motor behaviors develop during the incubation interval while the embryo is in the chorion. We examined prehatching motor behaviors in rainbow trout Oncorhycus mykiss (considered morphologically mature at hatching) and compared this species with zebrafish Danio rerio (considered morphologically immature) and assessed two hypotheses concerning the development of escape behavior. (1) Escape behavior is associated with the formation of key elements of the musculoskeletal and nervous systems; thus, the escape response appears early in ontogeny, when these elements form. (2) Escape behavior is not directly associated with the formation of underlying morphological elements; instead, it appears at hatching (i.e. when needed). We find that rainbow trout, like zebrafish, respond to touch early in the incubation interval, but do not demonstrate a complete C-start (including the second, propulsive stage) until shortly before hatching. At hatching, rainbow trout and zebrafish are similar in the degree of development of the chondocranium, paired fins and visceral arches (which comprise the larval jaw and gill support); however, rainbow trout have incipient rays in their unpaired fins (dorsal, anal and caudal), whereas zebrafish retain the embryonic fin fold. Although rainbow trout are more mature in axial swimming morphology at hatching, the essential neural and musculoskeletal systems that produce a coordinated escape response are functional at hatching in both species. This finding supports the evolutionary hypothesis that an effective escape response is critical for the survival of newly hatched teleost fishes.     

Short-term exercise training augments sympathetic vasoconstrictor responsiveness and endothelium-dependent vasodilation in resting skeletal muscle

We tested the hypotheses that 4 wk of exercise training would diminish the magnitude of vasoconstriction in response to sympathetic nerve stimulation and augment endothelium-dependent vasodilation (EDD) in resting skeletal muscle in a training intensity-dependent manner. Sprague-Dawley rats were randomly assigned to sedentary time-control (S), mild- (M; 20 m/min, 5% grade), or heavy-intensity (H; 40 m/min, 5% grade) treadmill exercise groups. Animals trained 5 days/wk for 4 wk with training volume matched between groups. Rats were anesthetized and instrumented for study 24 h after the last training session. Arterial pressure and femoral artery blood flow were measured, and femoral vascular conductance (FVC) was calculated. Lumbar sympathetic chain stimulation was delivered continuously at 2 Hz and in patterns at 20 and 40 Hz. EDD was assessed by the vascular response to intra-arterial bolus injections of ACh. The response (% change FVC) to sympathetic stimulation increased (P < 0.05) in a training intensity-dependent manner at 2 Hz (S: -20.2 ± 9.8%, M: -34.0 ± 6.7%, and H: -44.9 ± 2.0%), 20 Hz (S: -22.0 ± 10.6%, M: -31.2 ± 8.4%, and H: -42.8 ± 5.9%), and 40 Hz (S: H -24.5 ± 8.5%, M: -35.1 ± 8.9%, H: -44.9 ± 6.5%). The magnitude of EDD also increased in a training intensity-dependent manner (P < 0.05). These data demonstrate that short-term exercise training augments the magnitude of vasoconstriction in response to sympathetic stimulation and EDD in resting skeletal muscle in a training intensity-dependent manner.     

Nervous reactivity disturbances in patients with late post-traumatic encephalic syndromes.

A polygraphic study performed on 184 patients with late post-traumatic encephalic syndromes has shown marked nervous reactivity disturbances. The disturbances, estimated by testing for the electrographic components of the orienting reflex elicited by a repetitive auditory stimulus and for their habituation, were expressed by alterations in the intensity, resistance to habituation and sequence of habituation of the somatic, autonomic and EEG components. The character and severity of these disturbances depended on the mechanism behind the syndrome-inducing cranio-cerebral trauma, site of the impact and clinical picture of the syndrome.     

Effects of extracellular calcium on low frequency induced potentiation and habituation in the in vitro hippocampal slice preparation

The electrophysiological characteristics of frequency potentiation and habituation were investigated in two afferent systems of the in vitro hippocampal slice preparation. Low frequency stimulation (1 Hz) of the Schaffer collateral - commissural (Sch-comm) fibers results in a short-term potentiation of the amplitude and rate of rise of the EPSP and population spike responses recorded in the CA1 region. In contrast, 1-Hz stimulation of the perforant path (PP) evokes a short-term, habituation-like depression of the dentate granule cell EPSP and population spike. An inverse relationship was observed between stimulus intensity and the magnitude of frequency potentiation or habituation. Changes in afferent fiber volleys or general excitability of postsynaptic membranes did not contribute significantly to the development of either of these forms of short-term plasticity. Perfusion with a medium containing a high calcium - low magnesium concentration (4 mM Ca+2 and 1 mM Mg+2) produced a differential effect on CA1 and dentate evoked potentials. Following a 20-min exposure to this medium, the amplitude of CA1 potentials was increased while dentate responses were decreased. Frequency potentiation of CA1 responses and habituation of dentate responses were depressed or eliminated by the high calcium medium. The opposing influence of extracellular calcium on CA1 and dentate evoked potentials indicates a fundamental difference in the process of transmitter release in these systems, a characteristic that may contribute to the production of frequency potentiation and habituation.