R A North

Two types of neurone in the rat ventral tegmental area and their synaptic inputs.

1. Intracellular recordings were made from 241 ventral tegmental neurones in slices of rat midbrain. Seventy‐seven per cent of neurones were hyperpolarized by dopamine (principal cells); 16% were hyperpolarized by opioid peptides (secondary cells). 2. Most principal cells fired spontaneously (1‐3 Hz) with a threshold of ‐53 mV; most secondary cells did not fire spontaneously. Action potentials of principal cells were longer (0.9 ms) than those of secondary cells (0.5 ms). 3. Focal electrical stimulation within the ventral tegmental area evoked a biphasic synaptic potential, depolarization followed by hyperpolarization, with a duration of about 200 ms. Experiments with receptor antagonists showed that the depolarizing component resulted from activation of both N‐methyl‐D‐aspartate (NMDA) and non‐NMDA receptors and the hyperpolarizing component resulted from activation of GABAA receptors. 4. A later hyperpolarizing synaptic potential developed after a latency of 50 ms, reached its peak in 250 ms and had a duration of about 1 s. It reversed polarity at ‐108 mV (external potassium concentration was 2.5 mM), was blocked by phaclofen (30 microM‐1 mM) or 2‐hydroxysaclofen (100‐300 microM). In some cells, a phaclofen‐resistant component remained that was increased by cocaine and blocked by sulpiride (1 microM). 5. It is concluded that the ventral tegmental area contains two types of neurone having properties similar to those in the substantia nigra. The cells receive synaptic inputs mediated by excitatory amino acids acting at NMDA and non‐NMDA receptors, GABA acting at GABAA and GABAB receptors, and dopamine acting at D2 receptors.

Membrane properties and synaptic responses of rat striatal neurones in vitro.

1. A tissue slice containing a section of striatum was cut obliquely from rat brain so as to preserve adjacent cortex and pallidum. Intracellular recordings were made from 368 neurones, using either conventional or tight‐seal configurations. 2. Two types of neurone were distinguished electrophysiologically. Principal cells (96%) had very negative resting potentials (‐89 mV) and a low input resistance at the resting membrane potential (39 M omega): membrane conductance (10 nS at ‐65 mV) increased within tens of milliseconds after the onset of hyperpolarization (99 nS at ‐120 mV). Secondary cells (4%) had less negative resting potentials (‐60 mV) and a higher input resistance (117 m omega at the resting potential): hyperpolarization caused an inward current to develop over hundreds of milliseconds that had the properties of H‐current. 3. Most principal cells were activated antidromically by electrical stimulation of the globus pallidus or internal capsule. Intracellular labelling with biocytin showed that principal cells had a medium sized soma (10‐18 microns), extensive dendritic trees densely studded with spines and, in some cases, a main axon which extended towards the globus pallidus. 4. Electrical stimulation of the corpus callosum or external capsule evoked a depolarizing postsynaptic potential. This synaptic potential was reversibly blocked by a combination of 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX, 10 microM) and DL‐2‐amino‐5‐phosphonovaleric acid (APV, 30 microM), but was unaffected by bicuculline (30 microM) and picrotoxin (100 microM). The underlying synaptic current had a fast component (time to peak about 4 ms), the amplitude of which was linearly related to membrane potential and which was blocked by CNQX; in CNQX the synaptic current had a slower component (time to peak about 10 ms) which showed voltage dependence typical of N‐methyl‐D‐aspartate (NMDA) receptors. Both currents reversed at ‐5 mV. 5. Focal electrical stimulation within the striatum (100‐300 microns from the site of intracellular recording) evoked a synaptic potential that was partially blocked (45‐95%) by CNQX and APV: the remaining synaptic potential was blocked by bicuculline (30 microM). The bicuculline‐sensitive synaptic current reversed at the chloride equilibrium potential. 6. The findings confirm that the majority of neostriatal neurones (principal cells, medium spiny neurones) project to the pallidum and receive synaptic inputs from cerebral cortex mediated by an excitatory amino acid acting through NMDA and non‐NMDA receptors. These cells also receive synaptic inputs from intrinsic striatal neurones mediated by GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

5-hydroxytryptamine is a fast excitatory transmitter at 5-HT3 receptors in rat amygdala

A fast excitatory synaptic potential mediated by 5-hydroxytryptamine (5-HT) was recorded in rat lateral amygdala neurons in brain slices. The synaptic potential has brief duration (tens of milliseconds), is mimicked by 5-HT, is potentiated by a 5-HT uptake inhibitor, and is blocked by selective 5-HT3 receptor antagonists. The underlying synaptic current reversed polarity at about 0 mV. This is an example of fast neurotransmission in the mammalian brain mediated by an amine rather than an amino acid. The antiemetic, anxiolytic, and perhaps antipsychotic actions of 5-HT3 antagonists might result from blockade of such synapses.

Multiple subunits of a voltage-dependent potassium channel contribute to the binding site for tetraethylammonium

RNAs encoding a wild-type (RBK1) and a mutant (RBK1(Y379V,V381T); RBK1*) subunit of voltage-dependent potassium channels were injected into Xenopus oocytes. When expressed separately, they made homotetrameric channels that differed about 100-fold in sensitivity to tetraethylammonium (TEA). Mixtures of channels having one, two, or three low affinity subunits were expressed by injecting various proportions of RBK1 and RBK1* RNAs. The affinity for TEA of these three channel species was deduced by fitting concentration-response curves for the inhibition of potassium currents. DNAs were also concatenated to construct a sequence that encoded two connected subunits, and channels that contained four, two, or no TEA-sensitive subunits were expressed. The results suggest that bound TEA interacts simultaneously with all four subunits.

Muscarine increases cation conductance and decreases potassium conductance in rat locus coeruleus neurones

1. Whole‐cell patch‐clamp recordings were made from rat locus coeruleus neurones in slices of brain tissue in vitro. Muscarine (30 microM) caused an inward current of about 100 pA in neurones voltage clamped at ‐60 mV. 2. In about 75% of cells, the current elicited by muscarine was independent of potential in the range ‐60 to ‐120 mV and had no associated conductance change. 3. In about 25% of cells, the current became smaller with hyperpolarization, was associated with a decreased conductance, and reversed polarity between ‐100 and ‐140 mV. The reversal potential changed with the logarithm of the extracellular potassium concentration. Barium and caesium blocked inward rectification and also prevented reversal of the muscarine current. 4. When potassium ions of the intracellular and extracellular solutions were replaced by caesium, the current evoked by muscarine became smaller with depolarization at reversed polarity at +9 mV. This current was associated with an increase in conductance, and was greatly reduced when the extracellular sodium concentration was reduced to 20 mM. 5. The results could be quantitatively accounted for by a model in which muscarine both increases a voltage‐independent cation conductance and decreases the inward rectifier potassium conductance.

Membrane properties and synaptic potentials of three types of neurone in rat lateral amygdala.

1. Intracellular recordings were made from the lateral nucleus of the amygdala in tissue slices cut from rat brain and maintained in vitro. 2. Three types of neurones were distinguished according to the after‐potential that followed an action potential. Type 1 cells (44%, n = 225) had depolarizing after‐potentials, resulting from a calcium‐dependent chloride conductance. Type 2 cells (48%) had long‐lasting (> 250 ms) hyperpolarizing after‐potentials and type 3 cells (8%) had shorter hyperpolarizing after‐potentials. The average resting potentials of the three cell types were ‐78, ‐69 and ‐62 mV respectively. Intracellular labelling with biocytin showed that type 1 cells were pyramidal neurones; type 2 and type 3 cells were non‐pyramidal. 3. Experiments with receptor antagonists identified synaptic potentials mediated by excitatory amino acids and by GABA (acting at GABAA receptors) in all three cell types. A longer duration inhibitory synaptic potential resulting from activation of GABAB receptors was present in type 1 (pyramidal) and type 2 cells. 4. Cholecystokinin (100 nM to 1 microM) depolarized type 2 and type 3 cells but had no effect on type 1 (pyramidal) cells. Baclofen (1‐3 microM) hyperpolarized type 1 and type 2, but not type 3 cells. [Met5]enkephalin (1‐10 microM) hyperpolarized only type 2 cells. 5. It is concluded that the lateral nucleus of the amygdala contains pyramidal neurones and two types of non‐pyramidal neurone; these can be differentiated by membrane properties, synaptic inputs and sensitivities to transmitters.

Current inactivation involves a histidine residue in the pore of the rat lymphocyte potassium channel RGK5

RGK5 is a rat genomic DNA clone that encodes the n-type potassium channel found in T-lymphocytes and other cells. Current through this channel declines (inactivates) over a period of hundreds of milliseconds during a maintained depolarizing pulse, whether in lymphocytes or when expressed in Xenopus oocytes. Here we demonstrate that an amino acid residue near the outer pore of the channel, histidine401, is involved in the inactivation process. Replacement of this residue by tyrosine, the amino acid found in the equivalent position of the homologous but non-inactivating channel RBK1, reduced inactivation of RGK5 over a 5 s depolarizing pulse from 84.3 +/- 1.9% to 18.3 +/- 1.1%. Conversely, replacement of this tyrosine in RBK1 (Tyr379) by histidine increased its inactivation from 21.6 +/- 1.1% to 42.3 +/- 1.5%. These results suggest a mechanism of channel inactivation distinct from that previously described for the A-type potassium channel.