Increased activity of dopamine-containing neurons in the ventral tegmental area is necessary for the reinforcing effects of opioids and other abused drugs. Intracellular recordings from these cells in slices of rat brain in vitro showed that opioids do not affect the principal (dopamine-containing) neurons but hyperpolarize secondary (GABA- containing) interneurons. Experiments with agonists and antagonists selective for opioid receptor subtypes indicated that the hyperpolarization of secondary cells involved the mu-receptor. Most principal cells showed spontaneous bicuculline-sensitive synaptic potentials when the extracellular potassium concentration was increased from 2.5 to 6.5 or 10.5 mM; these were prevented by TTX and assumed to result from action potentials arising in slightly depolarized local interneurons. The frequency of these synaptic potentials, but not their amplitudes, was reduced by opioids selective for mu-receptors. It is concluded that hyperpolarization of the interneurons by opioids reduces the spontaneous GABA-mediated synaptic input to the dopamine cells. In vivo, this would lead to excitation of the dopamine cells by disinhibition, which would be expected to contribute to the positive reinforcement seen with mu-receptor agonists such as morphine and heroin.

SUMMARY1. 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 /LM-1 mM) or 2-hydroxysaclofen (100-300 /M). In some cells, a phaclofenresistant component remained that was increased by cocaine and blocked by sulpiride (1 ,UM).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

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Intracellular recordings were made from 475 rat substantia nigra zona compacta neurons in vitro. The region from which recordings were made was rich in catecholamine fluorescence. Two groups of neuron, termed principal neurons (95% of the total) and secondary neurons (5% of the total) were clearly distinguishable according to one or more of the following 4 electrophysiological properties. Secondary neurons (23 cells) (1) fired spontaneous action potentials at frequencies greater than 10 Hz, or were quiescent (30%); (2) had action potentials less than 1 msec in duration; (3) did not show time-dependent inward rectification with step hyperpolarization; and (4) had slope conductances of about 4 nS (between -75 and -90 mV). In contrast, principal neurons (1) fired spontaneous action potentials in the range 1-8 Hz, or were quiescent (33%); (2) had action potentials greater than 1 msec in duration; (3) showed pronounced time-dependent inward rectification; and (4) had steady-state membrane slope conductances of around 22 nS (between -75 and -90 mV). Secondary cells were not affected by dopamine but were hyperpolarized by baclofen, GABA, and the mu opioid receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO). On the other hand, dopamine and baclofen inhibited firing and/or hyperpolarized all principal cells tested, but mu or delta opioid receptor agonists had no effect. The properties of these 2 cell types broadly correspond with those described by electrophysiological studies in vivo, in which case the majority, or principal, cells are believed to be dopaminergic.(ABSTRACT TRUNCATED AT 250 WORDS)

The neurohormone 5-hydroxytryptamine (5HT or serotonin) exerts its effects by binding to several distinct receptors. One of these is the M-receptor of Gaddum and Picarelli, now called the 5-HT3 receptor, through which 5-HT acts to excite enteric neurons. Ligand-binding and functional studies have shown that the 5-HT3 receptor is widely distributed in peripheral and central nervous tissue and evidence suggests that the receptor might incorporate an ion channel permeable to cations. We now report the first recordings of currents through single ion channels activated by 5-HT3 receptors, in excised (outside-out) membrane patches from neurons of the guinea pig submucous plexus. Whereas application of acetylcholine activated predominantly a 40-pS channel, 5-HT caused unitary currents apparently through two channels of conductances of 15 and 9 pS, which were reversibly blocked by antagonists of the 5-HT3 receptor. Receptors for amine neurotransmitters, including 5-HT1 and 5-HT2, have previously been thought to transduce their effects through GTP-binding proteins: the direct demonstration that 5-HT3 receptors are ligand-gated ion channels implies a role for 5-HT, and perhaps other amines, as a 'fast' synaptic transmitter.

The effects of agonists at ,u and 6 opioid receptors were compared by measuring membrane currents under voltage clamp from neurons of the rat nucleus locus coeruleus and guinea pig submucous plexus. In each tissue, the appropriate selective agonist (Tyr-D-Ala-Gly-MePhe-Gly-ol for ,u receptors in locus coeruleus or Tyr-D-Pen-Gly-Phe-D-Pen for 6 receptors in submucous plexus) increased the conductance of an inwardly rectifying potassium conductance and strongly hyperpolarized the membrane. The properties of the potassium conductance affected by the two opioids could not be distinguished. Experiments with intracellular application of guanosine 5'-[y-thio]triphosphate indicated that a guanine nucleo-

Dopamine-containing neurons of the mammalian midbrain are required for normal behavior and movements. In vivo they fire action potentials in bursts, but in vitro they discharge regularly spaced action potentials. Burst firing in vitro has now been shown to be robustly induced by the glutamate agonist N-methyl-D-aspartate (NMDA) although not by the non-NMDA agonists kainate or quisqualate. The hyperpolarization between bursts of action potentials results from electrogenic sodium ion extrusion by a ouabain-sensitive pump. This mechanism of burst generation in mammalian neurons may be important in the pathophysiology of schizophrenia and Parkinson's disease.

The complementary DNA sequence encoding the cell-surface receptor for ecotropic host-range murine retroviruses (ecoR) shows that it contains 622 amino acids and 14 hydrophobic potentially membrane-spanning sequences. Because this receptor occurs on many or all murine cells and is probably essential for viability of cultured fibroblasts, its normal function might be to transport an essential metabolite. We expressed ecoR in Xenopus laevis oocytes by injecting RNA transcribed from the cloned cDNA. These oocytes specifically bound the gp70 envelope glycoprotein from an ecotropic murine leukaemia virus. An inward current was recorded electrophysiologically when a mixture of amino-acids was applied: this resulted from a stereoselective, saturable uptake of lysine, arginine and ornithine; it was independent of sodium and not substantially altered by gp70. Cysteine and homoserine were also taken up, but sodium was necessary for their transport. These properties of ecoR correspond to those of the y+ amino-acid transporter. Our results demonstrate the subversion of a ubiquitous cell membrane protein, in this case a basic amino acid transporter, for use as a retroviral receptor.