Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and ox ytocin. in addition to playing a role as peptide hormones, also act as neur otransmitters/neuromodulators. A number of arguments support this notion: ( i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohy pophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies. whose axon projects to the limbic system, the brainstem and the spi nal cord. (ii) Vasopressin and oxytocin can be shed from central axons as a re classical neurotransmitters. (iii) Specific binding sites. i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the fi ring rate of selected neuronal populations. (v) In-situ injection of vasopr essin and oxytocin receptor agonists and antagonists can interfere with beh avior or physiological regulations. Morphological studies and electrophysio logical recordings have evidenced a close anatomical correlation between th e presence of vasopressin and oxytocin receptors in the brain and the neuro nal responsiveness to vasopressin or oxytocin. These compounds have been fo und to affect membrane excitability in neurons located in the limbic system . hypothalamus, circumventricular organs, brainstem. and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brai nstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-spec ific cationic channels or by closing K+ channels. These neuropeptides can a lso influence synaptic transmission, by acting either postsynaptically or u pon presynaptic target neurons or axon terminals. Whereas, in cultured neur ons, vasopressin and oxytocin appear to mobilize intracellular Ca+ + in bra instem slices, the action of oxytocin is mediated by a second messenger tha t is distinct from the second messenger activated in peripheral target cell s. In this review, we will summarize studies carried out at the cellular le vel, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxyto cin will be treated together. Though acting via distinct receptors in disti nct brain areas, these two neuropeptides appear to exert similar effects up on neuronal excitability. (C) 2001 Elsevier Science Ltd. All rights reserve d.