Intracellular study of rat entopeduncular nucleus neurons in an in vitro slice preparation: response to subthalamic stimulation

Responses of rat entopeduncular nucleus (EP) neurons after stimulation of the subthalamic nucleus (STh) and the morphology of the EP neurons were studied using brain slice preparations. EP neurons were classified into two types based on their electrophysiological properties as reported previously. Of 87 EP neurons, 72 were Type I and the rest were Type II. Synaptic responses to STh stimulation were different in these two cell types. STh stimulation evoked excitatory postsynaptic potentials (EPSPs) followed by strong inhibitory postsynaptic potentials (IPSPs) in Type I neurons and EPSPs without strong IPSPs in Type II neurons. The EPSPs were considered to be monosynaptic because no large change in the latency (1.7 ± 0.5ms) resulted by alteration of stimulus intensity. The EPSPs were reversibly suppressed by kynurenic acid in a dose-dependent manner. Bath application of (+)-tubocurarine (10–50 μM) had no effect on EPSPs or IPSPs. Bath application of bicuculline methiodide (50–100 μM) markedly suppressed IPSPs evoked by STh stimulation and at the same time increased the amplitude and duration of EPSPs without affecting the latency. In the presence of bicuculline methiodide, EPSPs could induce plateau potentials and slow action potentials. Some Type I and Type II neurons were intracellularly labeled by biocytin. Type I neurons were located throughout the EP but Type II neurons were located mainly in the dorsal portion of the EP. Medium sized somata of both Type I and Type II neurons were spine-free and fusiform or round in shape. They had 3–4 thick primary dendrites with diameters of 2–5 μm that branched into thin secondary dendrites. The secondary and tertiary dendrites of Type I neurons were sparsely covered with spines. Dendritic terminals of some Type I neurons had complex arborizations with abundant spines and appendages. The dendrites of Type II neurons were generally smooth and had no complex arborizations at their terminals.     

Electrical membrane properties of rat subthalamic neurons in an in vitro slice preparation

The electrical membrane properties of subthalamic (STH) neurons and their response characteristics to stimulation of the internal capsule (IC) were studied in an in vitro slice preparation. Most STH neurons recorded exhibited spontaneous repetitive firing. The input resistance of STH neurons was 146 +/- 48 M omega and showed both an anomalous and a delayed rectification when the membrane was hyperpolarized or depolarized by current injections. In neurons with the membrane potential less negative than 65 mV, depolarizing current pulses generated repetitive firing with the maximum frequency of up to 500 Hz. Two types of tetrodotoxin (TTX)-resistant cobalt-sensitive potentials, slow depolarizing potential and slow action potential, were observed in STH neurons. The slow depolarizing potential had a long duration (over 500 ms in some cases) and was able to trigger repetitive firing. The slow action potential had a duration of about 30 ms and triggered a burst of firing. The slow action potential was seen only when the neurons were hyperpolarized to more negative than 65 mV by a current injection. Electrical stimulation of IC evoked monosynaptic inhibitory postsynaptic potentials (IPSPs) in most of the neurons examined. The polarity of IPSPs was reversed in the depolarizing direction by intracellular injection of Cl-. Bath application of bicuculline markedly suppressed IPSPs and unmasked monosynaptic excitatory postsynaptic potentials (EPSPs). The EPSP was able to trigger a slow depolarization with repetitive firing or a slow action potential with burst of firing when the neuron was hyperpolarized by a continuous current injection. The results demonstrated that STH neurons in an in vitro preparation have spontaneous discharges, high input resistance, capability to generate high-frequency firing, and Ca potentials. The pattern of responses of STH neurons to synaptic inputs is dependent on their membrane potentials.     

Passive electrical membrane properties of rat neostriatal neurons in an in vitro slice preparation

The passive electrical membrane properties of rat neostriatal neurons were studied in in vitro slice preparations. The data are only from neurons having stable resting membrane potentials of more than 50 mV and able to generate action potentials of amplitudes greater than 70 mV evoked by local or intracellular stimulation. All neurons measured for current-voltage relationship (n = 52) showed non-linearity of the input resistance in the hyperpolarizing direction. The mean input resistance at the resting membrane potential was 16.6 M omega. Depolarizing postsynaptic potentials evoked by local stimulation were decreased both in their amplitude and half-decay time by inward current injections exceeding more than 1 nA due to the strong membrane rectification at these levels of hyperpolarization. The mean membrane time constant (tau 0) was 5.3 ms, as measured from the semilogarithmic plots of transmembrane potential shift produced by small hyperpolarizing current pulses. In some neurons, the equalizing term (tau 1) could be determined as well and had a mean value of 1.0 ms. Measurement of (tau 0) using the strength-latency relation showed a similar value (5.0 ms) to that measured from the voltage transients. Intracellular labeling of the recorded neurons with horseradish peroxidase suggested that the recordings were obtained from medium spiny neurons.     

Hyperexcitability of amygdala neurons of Senescence-Accelerated Mouse revealed by electrical and optical recordings in an in vitro slice preparation

In the amygdala (AMG) slices obtained from both the young (4–7 months old) and aged (17–20 months old) groups of Senescence-Accelerated Mouse (SAM) P10, spontaneous bursts were recorded in the medial, central and basolateral nuclei. The spontaneous bursts were also observed in the slices from the young group of SAMR1, whereas the mean frequency was significantly lower than that from the young group of SAMP10. The spontaneous burst was barely detectable in slices from the aged group of SAMR1 during perfusing with the standard solution, while bicuculline methiodide (10 μM), a GABA_A receptor antagonist, or Mg²⁺-free solution induced a similar bursting activity observed in the young group. The burst response was also evoked in the medial, central and basolateral AMG following stimulation of the stria terminalis (ST). Both spontaneous and evoked bursts were completely suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 4 μM), an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist, but not by (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801, 30 μM), an N-methyl- -aspartate receptor antagonist. The hyperexcitability of the AMG neurons was further substantiated by optical recordings. Following stimulation of the ST, the optical signals reflected postsynaptic responses spread into the medial and central AMG areas at 2–5 ms and faded out at 20–30 ms after stimulation. The intensity of the optical signal recorded in the slice from the young SAMP10 was significantly higher than that from SAMR1 or ddY mice. These observations indicate that bursts mediated by AMPA/kainate receptors were transiently generated in the AMG of SAMR1 at the young age, while the bursts with higher frequency were continuously generated in the AMG of SAMP10. The chronic neuronal hyperactivity in the AMG may be partially responsible for the age-related deterioration of memory and learning abilities observed in SAMP10.     

Electrical membrane properties of rat substantia nigra compacta neurons in an in vitro slice preparation

The electrical membrane properties of rat substantia nigra pars compacta (SNC) neurons were studied in an in vitro slice preparation. Some of the recorded neurons were intracellularly labeled with HRP and were found to have morphological characteristics resembling the presumed SNC dopaminergic neurons, as reported by others. The input resistance of SNC neurons at resting membrane potential ranged between 70 and 250 M omega. The membrane resistance showed strong anomalous rectification when the membrane was hyperpolarized by current injection. The anomalous rectification was decreased by the addition of tetraethylammonium bromide (TEA) to the bathing Ringer solution. Injection of depolarizing current or termination of hyperpolarizing current induced slow depolarizing potentials. Their amplitude was dependent on the membrane potential and the current intensity. In neurons treated with tetrodotoxin (TTX) and TEA, slow action potentials were triggered from the slow depolarizing potentials. Both the slow depolarizing potential and slow action potential were TTX resistant and abolished by superfusion of Ca2+-free medium. Long duration hyperpolarizations were observed following the injection of depolarizing current pulses. The hyperpolarization was abolished by the superfusion of Ca2+-free medium or decreased by addition of TEA to the Ringer solution indicating an involvement of a Ca2+-dependent K+-conductance in generation of the hyperpolarization. The long duration hyperpolarization was also observed following action potentials. The spike after hyperpolarization consisted of an initial short duration fast component and a long lasting component. The amplitude of both components seems to be reduced but not abolished by TEA (up to 10 mM). When hyperpolarizing current pulses were applied to neurons that were held either continuously depolarized or were superfused with Ca2+-free medium, the pattern of the membrane potential after the offset of current pulses consisted of an initial fast and a later slow ramp-shaped phase. The latter was associated with a membrane conductance increase and interpreted to be due to an early K+ current. This early K+ current was relatively resistant to TEA. Injections of strong depolarizing currents triggered action potentials with multiple inflections on their rising phase. The amplitudes of action potentials changed abruptly during current application. These data indicate that SNC neurons have multiple generation sites for action potential.     

Thermosensitivity of neurons in the paraventricular nucleus of the rat slice preparation

The thermosensitivity of 65 spontaneously active neurons in the paraventricular nucleus (PVN) was investigated by extracellular recording in the rat hypothalamic slice preparation. The firing rate of these cells was comparatively low, ranging from 0.03 to 10.0 (mean 2.46) impulses/s at 37 degrees C, and only a minority showed a phasic firing pattern. Of 65 neurons tested, 23 (35%) increased their firing rate when the slice was warmed (warm-sensitive neurons) and 9 (14%) showed the opposite response (cold-sensitive neurons). Thermosensitivity was also tested in solutions with reduced [Ca2+] and high [Mg2+]. Eight out of 10 warm-sensitive neurons and 5 of 7 cold-sensitive neurons retained thermosensitivity after synaptic blockade. Out of 6 phasic firing neurons tested, one showed warm-sensitivity and another one showed cold-sensitivity. The thermosensitive neurons were diffusely distributed throughout the PVN and were not located in particular areas of the nucleus. Thus a group of cells in the PVN, including probably both magno- and parvocellular neurons, showed an inherent thermosensitivity, which suggests an important role for the PVN in thermoregulation.     

丘脑底核毁损术对帕金森病大鼠脚内核和黑质网状部的影响

目的 :观察毁损丘脑底核 (STN)对帕金森病 (PD)大鼠脚内核 (EP)及黑质网状部 (SNr)γ -氨基丁酸(GABA)能系统的影响。方法 :将 6 0只Wistar大鼠随机分为 6组 ,每组 10只。对照组采用 6 -OHDA立体定向注入大鼠右侧前脑内侧束 (MFB)和中脑被盖腹侧区 (VTA) ,制成偏侧PD模型。实验组分为第Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ组 ,分别于 6 -OHDA注射前 7d、注射后 1h、2h、3d、7d5个不同时间点 ,局部注射海人藻酸 (KA)破坏STN。 4周后处死大鼠 ,采用免疫组化染色方法 ,定量测量各组大鼠SNr区和EP区的GABA免疫反应阳性区面积和免疫反应强度。实验数据采用方差分析和t检验统计学处理。结果 :GABA免疫组化显示Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ实验组的注射侧EP面积分别为正常侧的 72 9%、83 7%、79 7%、88 1%、90 1%。对照组注射侧为正常侧面积的 139 1% (P <0 0 5、0 0 1)。各实验组注射侧EP的GABA免疫反应强度 (积分光密度 )均较正常侧减少 ,对照组注射侧较正常侧积分光密度增加 (P <0 0 5 ,P <0 0 1)。Ⅰ、Ⅱ、Ⅲ、Ⅳ、Ⅴ实验组的注射侧SNr面积分别为正常侧的 90 6 %、86 9%、87 3%、80 5 %、80 4%。对照组注射侧面积为正常侧的 10 8 1% (P <0 0 5、0 0 1) ,各实验组注射侧SNr的GABA免疫反应强度均较正常侧减少 ,对照组注射     

Postnatal development of neuronal connections in cat visual cortex studied by intracellular recording in slice preparation

Postnatal development of neuronal connections in cat visual cortex (area 17) was studied in slice preparations obtained from kittens aged 1–18 weeks after birth and adult cats by recording intracellularly excitatory (EPSP) and inhibitory postsynaptic potentials (IPSP) evoked in cortical cells by stimulation of white matter. The EPSPs were already present in all cells at 1 week of age. Their efficiency assessed by their maximum rate of rise was low initially and increased progressively with age. In contrast, the IPSPs were absent in half of the cells at 1 week and almost all of the cells came to demonstrate inhibition by 9 weeks except for a few layer II-III cells. At all ages about three-quarters of the IPSPs had GABA_A-mediated early and GABA_B-mediated late components with different time course, reversal potential and sensitivity to GABA antagonists, while the remaining IPSPs had only the early component. The efficiency of both IPSPs assessed by the associated conductance increase showed an increase of more than twice from 1 to 5 weeks, reaching the same level as adults. The time course of the development of inhibition demonstrated in this study paralleled the time course of the development of selective visual responsiveness in cortical cells, suggesting that the postnatal maturation of inhibitory connections is a basis of maturation of visual responsiveness.     

Intracellular recordings from rat nucleus accumbens neurons in vitro

Intracellular recordings were obtained from rat nucleus accumbens (NAC) neurons in brain slice preparations. Local stimulations evoked depolarizing postsynaptic potential (DPSP). Injections of low intensity depolarizing currents decreased the amplitude of the DPSP and reversed a later portion of the DPSP into a hyperpolarizing potential. Superfusion of pentobarbital facilitated the reversal of this later portion of DPSP and bicuculline abolished this polarity reversal. These data suggested that the DPSP evoked by local stimulation consisted of a combination of an excitatory and an inhibitory postsynaptic potential, and that the latter was probably mediated by gamma-aminobutyric acid.     

Effects of 4-aminopyridine (4-AP) on rat neostriatal neurons in an in vitro slice preparation

Effects of 4-aminopyridine (4-AP) on the rat neostriatal neuron were studied using the in vitro slice preparation. The intracellularly recorded neurons had resting membrane potentials of more than 50 mV and were capable of generating action potentials with the amplitude greater than 60 mV. Application of 4-AP in the superfusing media depolarizes the cell membrane and increases its input resistance. Local electrical stimulation induces excitatory postsynaptic potentials (EPSPs) overlapping with inhibitory postsynaptic potentials (IPSPs) in these neurons. 4-AP application enhances the amplitude and duration of the postsynaptic potentials. With application of higher concentration of 4-AP, local stimulation induces a second EPSP and a bicuculline sensitive long duration depolarization. These results indicate that 4-AP clearly has effects on local stimulation-induced postsynaptic responses of neostriatal neurons. Possible mechanisms underlying the 4-AP actions on neurotransmission in the neostriatal slice are discussed.     

Estrogen priming affects active membrane properties of medial amygdala neurons

The medial nucleus of the amygdala (MNA) in the rat is a target tissue for estrogen binding and a sexually dimorphic structure. We used an in vitro slice preparation and intracellular recording techniques to study the effects of β-estradiol priming on active and passive membrane properties of MNA neurons. Two groups of adult female rats were used; ovariectomized (OVX) non-primed rats and OVX rats that were estrogen-primed at least 24 h prior to recording. Estrogen priming increased the occurrence of spontaneous excitatory postsynaptic potentials (EPSPs) in MNA neurons, and of depolarizing afterpotentials (DAPs) observed with a cathodally triggered action potential, and was associated with a lack of accommodation in these cells. Overall, long-term exposure to estrogen markedly increased the spontaneous activity and excitability of the MNA neurons.     

Intracellular recording of lamina X neurons in a horizontal slice preparation of rat lumbar spinal cord

A horizontal slice preparation of postnatal rat lumbar spinal cord has been developed which allows correlative observations of the morphology, electrophysiology, and receptor pharmacology of lamina X neurons. These slices better maintain afferent input and somatodendritic morphology and are amenable to subsequent immunohistochemical processing. Stable intracellular recordings obtained from postnatal day 14–45 animals reveal that a number of different intrinsic membrane conductances contribute to the regulation of excitability in lamina X neurons. In addition, lamina X neurons possess inhibitory GABAergic as well as excitatory glutamate and cholecystokinin receptors. This preparation will be useful in future studies designed to characterize developmental changes in the intrinsic membrane properties, synaptic profiles and neuropeptide responsiveness of lamina X neurons in the rat. Such a characterization is important given that lamina X represents a unique sexually dimorphic region that is a convergence site for somatic and visceral afferent inputs, which includes nociceptive information.     

Intracellular study of rat substantia nigra pars reticulata neurons in an in vitro slice preparation: electrical membrane properties and response characteristics to subthalamic stimulation

The electrical membrane properties of substantia nigra pars reticulata (SNR) neurons and their postsynaptic responses to stimulation of the subthalamic nucleus (STH) were studied in an in vitro slice preparation. SNR neurons were divided into two types based on their electrical membrane properties. Type-I neurons possessed (1) spontaneous repetitive firings, (2) short-duration action potentials, (3) less prominent spike accommodations, and (4) a strong delayed rectification during membrane depolarization. Type-II neurons had (1) no spontaneous firings, (2) long-duration action potentials, (3) a prominent spike accommodation, (4) a relatively large post-active hyperpolarization, and (5) a less prominent delayed rectification. These membrane properties were very similar to those observed in substantia nigra pars compacta (SNC) neurons in slice preparations. Features common to both types of neurons include that (1) the input resistance was similar, (2) they showed an anomalous rectification during strong hyperpolarizations, and (3) they were capable of generating Ca potentials. Intracellular responses of both types of SNR neurons to STH stimulation consisted of initial short-duration monosynaptic excitatory postsynaptic potentials (EPSPs) and a short-duration inhibitory postsynaptic potential (IPSP) followed by a long-duration depolarization. The IPSP was markedly suppressed by application of bicuculline methiodide and the polarity was reversed by intracellular injection of Cl-. In the preparations obtained from internal capsule-transected rats, STH-induced EPSPs had much longer durations than those observed in the normal preparations, while the amplitude of IPSPs and succeeding small-amplitude long-duration depolarizations was small. The results indicated that SNR contains two electrophysiologically different types of neurons, and that both types of neurons receive monosynaptic EPSPs from STH and IPSPs from areas rostral to STH.     

Development of responses of globus pallidus and entopeduncular nucleus neurons to stimulation of the caudate nucleus and precruciate cortex

Extracellular recordings were made from neurons in the globus pallidus and entopeduncular nucleus (GP-ENTO) of anesthetized kittens of 2 to 177 days of age and from four adult cats. Stimulation of the striatum and the precruciate cortex produced responses in GP-ENTO neurons of the youngest kittens tested (2 days of age). In kittens of 1 to 10 days, about 70% of the GP-ENTO neurons responded to either caudate or cortical stimulation with a purely excitatory response (i.e., an evoked action potential). With increasing age the frequency of occurrence of this type of response decreased and the occurrence of inhibitory responses or of sequences of excitation followed by inhibition increased. In addition to these changes in the form of the evoked responses, other response parameters exhibited age-dependent alterations. Latency to response decreased with age and the ability of GP-ENTO neurons to follow repetitive stimuli increased as the kittens became older. These findings suggest that although GP-ENTO neurons are functional as early as 2 days postnatally in the kitten, subsequent maturation of the responsiveness of these neurons continues for several postnatal months.     

A short duration GABAergic inhibition in identified neostriatal medium spiny neurons: in vitro slice study

Inhibition in the neostriatum was investigated in rat in vitro slice preparation using intracellular recording and labeling technique. The initial response recorded following local stimulation is a monosynaptically activated EPSP. In 17% of the neurons tested, IPSPs were observed following EPSPs evoked by local stimulation. In paired shock experiments reduction of test EPSP amplitude or action potentials occurred over interstimulus intervals (ISIs) of 3-38 msec. In some neurons, a pulse injection of depolarizing current was used to trigger an action potential which was in a paired shock, used to condition a test monosynaptically induced EPSP. Test EPSPs were shunted over ISIs less than 45 msec. Paired shock performed on the slices perfused with the medium containing GABA antagonists (e.g., bicuculline methiodide, picrotoxin, or penicillin-G) resulted invariably in potentiation of test EPSPs. Inhibition in the neostriatum in vitro is demonstrated as reduction in test amplitude in paired shock tests, by the presence of IPSPs and by the shunting of EPSPs conditioned by an action potential triggered by direct depolarization. Neurons exhibiting these forms of inhibition were intracellularly labelled with HRP and identified as medium spiny neurons. These results indicate that striatal GABAergic medium spiny neurons which are known to have an extensive axon collateral plexus play in a role in a short lasting inhibition observed in the striatum.     

Evidence for a persistent sodium conductance in neurons from the nucleus prepositus hypoglossi

Intracellular recordings were made from 39 neurons in a slice preparation of the prepositus hypoglossi nucleus from guinea pigs. Morphological characteristics were confirmed by dying neurons with Lucifer yellow. The neurons were spontaneously active, firing in the range of 8–50 spikes/s. Spike duration was short (0.32 ms) and the spikes were followed by fast and slow afterhyperpolarizations. The current vs frequency relationship was linear during steady state firing, but showed dual firing ranges corresponding to the first, third and fifth interspike interval. The instantaneous frequency of the first few interspike intervals could reach 500 spikes/s. Depolarizing and hyperpolarizing responses to square pulses displayed initial sag and rebound responses sensitive to extracellular Cs⁺, pharmacologically classifying the responses as a result of a Q-like current. Substitution of Ca²⁺ in the medium with the inorganic calcium blockers Mn²⁺ or Co²⁺ resulted in oscillatory firing, depolarizing excursions being sensitive to tetrodotoxin (TTX). Mn²⁺ or Co²⁺ in combination with extracellular Cs⁺ elicited TTX-sensitive plateau potentials, blocked in Na⁺-free solution. In conclusion, the prepositus neurons displayed spontaneous activity in the slice preparation and active membrane properties above as well as below the threshold of the action potential. In addition, the prepositus neurons possess a persistent sodium conductance that can be uncovered by inorganic calcium blockers. It may be involved in sustaining the spontaneous discharge.     

Responses of suprachiasmatic nucleus neurons to optic nerve stimulation in rat hypothalamic slice preparation

Stimulation of the optic nerve evoked two positive waves (P₁ and P₂) with short latencies and a following large negative wave (N) in the suprachiasmatic nucleus (SCN) of hypothalamic slice preparations. The conduction velocities of the P₁, P₂ and N waves were assumed to be 2.9 m/s, 0.5 m/s and 0.4 m/s, respectively. The N wave disappeared in low Ca²⁺ medium and all responses disappeared when TTX was added to the medium at a concentration of 10⁻⁶ M. The results indicate that the P₁ and P₂ waves were responses of the optic nerve in the SCN and the N wave was that of SCN neurons. Following optic nerve stimulation 15 out of 212 single SCN neurons were excited; 10 were excited then inhibited, 14 were excited followed by rhythmic oscillation, and 4 were inhibited. The results demonstrate neuronal projections from the optic nerve to the SCN, and suggest that the major role of the excitatory influence on SCN neurons is responsible for the elevation of neuronal activity in the SCN during the daytime.     

Hypocretinergic facilitation of synaptic activity of neurons in the nucleus pontis oralis of the cat

The present study was undertaken to explore the neuronal mechanisms of hypocretin actions on neurons in the nucleus pontis oralis (NPO), a nucleus which plays a key role in the generation of active (REM) sleep. Specifically, we sought to determine whether excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the laterodorsal tegmental nucleus (LDT) and spontaneous EPSPs in NPO neurons are modulated by hypocretin. Accordingly, recordings were obtained from NPO neurons in the cat in conjunction with the juxtacellular microinjection of hypocretin-1 onto intracellularly recorded cells. The application of hypocretin-1 significantly increased the mean amplitude of LDT-evoked EPSPs of NPO neurons. In addition, the frequency and the amplitude of spontaneous EPSPs in NPO neurons increased following hypocretin-1 administration. These data suggest that hypocretinergic processes in the NPO are capable of modulating the activity of NPO neurons that receive excitatory cholinergic inputs from neurons in the LDT.     

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (∼1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.     

Excitatory effects of dopamine on subthalamic nucleus neurons: in vitro study of rats pretreated with 6-hydroxydopamine and levodopa

Increased output from the subthalamic nucleus (STN) following chronic dopamine depletion has been linked to the rigidity and tremor seen in Parkinson's disease (PD). We used extracellular microelectrode recordings from rat brain slices to investigate effects of dopamine on STN neurons. In brain slices prepared from rats that received unilateral 6-hydroxydopamine (6-OHDA) treatment, the spontaneous firing rate of STN neurons was reduced by 63%, and the firing pattern was more irregular, compared to STN neurons from normal rats. However, treatment with levodopa (50 mg/kg, i.p., daily) for 4 weeks normalized the firing rate and pattern of STN neurons in the 6-OHDA-treated rats. Dopamine (3-300 microM), added to the superfusate, significantly increased the firing rates of STN neurons in a concentration-dependent fashion, and also produced a more regular firing pattern in 6-OHDA-lesioned tissue. This excitatory effect of dopamine was mimicked by a D2 receptor agonist (quinpirole), and was reduced by the D2 antagonists haloperidol, clozapine and sulpiride. Antagonists of the D1 receptor (SCH-23390) and ionotropic glutamatergic receptors (CNQX and AP5) could not block the effect of dopamine on firing rate. These results suggest that dopamine exerts a direct excitatory influence on STN neurons via the activation of D2-like receptors.