Nanomolar oxytocin synergizes with weak electrical afferent stimulation to activate the locomotor CPG of the rat spinal cord in vitro.

Synergizing the effect of afferent fibre stimulation with pharmacological interventions is a desirable goal to trigger spinal locomotor activity, especially after injury. Thus, to better understand the mechanisms to optimize this process, we studied the role of the neuropeptide oxytocin (previously shown to stimulate locomotor networks) on network and motoneuron properties using the isolated neonatal rat spinal cord. On motoneurons oxytocin (1 nM-1 \u3bcM) generated sporadic bursts with superimposed firing and dose-dependent depolarization. No desensitization was observed despite repeated applications. Tetrodotoxin completely blocked the effects of oxytocin, demonstrating the network origin of the responses. Recording motoneuron pool activity from lumbar ventral roots showed oxytocin mediated depolarization with synchronous bursts, and depression of reflex responses in a stimulus and peptide-concentration dependent fashion. Disinhibited bursting caused by strychnine and bicuculline was accelerated by oxytocin whose action was blocked by the oxytocin antagonist atosiban. Fictive locomotion appeared when subthreshold concentrations of NMDA plus 5HT were coapplied with oxytocin, an effect prevented after 24 h incubation with the inhibitor of 5HT synthesis, PCPA. When fictive locomotion was fully manifested, oxytocin did not change periodicity, although cycle amplitude became smaller. A novel protocol of electrical stimulation based on noisy waveforms and applied to one dorsal root evoked stereotypic fictive locomotion. Whenever the stimulus intensity was subthreshold, low doses of oxytocin triggered fictive locomotion although oxytocin per se did not affect primary afferent depolarization evoked by dorsal root pulses. Among the several functional targets for the action of oxytocin at lumbar spinal cord level, the present results highlight how small concentrations of this peptide could bring spinal networks to threshold for fictive locomotion in combination with other protocols, and delineate the use of oxytocin to strengthen the efficiency of electrical stimulation to activate locomotor circuits

Neuroscience 135 4 1075 1086 United States

Evidence is increasingly accumulating in support of a role for the steroid hormone 17beta-estradiol to modify neuronal functions in the mammalian CNS, especially in autonomic centers. In addition to its well known slowly developing and long lasting actions (genomic), estrogen can also rapidly modulate cell signaling events by affecting membrane excitability (non-genomic). Little, however, is known regarding the mechanism(s) by which 17beta-estradiol produces its rapid effects on neuronal membrane excitability. As potassium channels play a crucial role in cell excitability, we hypothesized that 17beta-estradiol caused excitability by modulating potassium flux through the neuronal cell membrane. We tested this hypothesis by examining the effects of 17beta-estradiol on outward potassium currents recorded in cells from the parabrachial nucleus of rats, in vitro. Bath application of 17beta-estradiol (10-100 microM) reversibly reduced voltage-activated outward potassium currents in a concentration-dependent manner. This effect was mimicked by BSA-17beta-estradiol but not mimicked by 17alpha-estradiol and was significantly reduced by ICI 182,780, a selective estrogen receptor antagonist. The inhibitory effect of 17beta-estradiol was dependent on extracellular potassium concentration, with more profound effects observed at lower concentrations. The 17beta-estradiol-induced inhibition of the outward current was blocked by pretreatment with the potassium channel blockers tetraethylammonium and 4-aminopyridine. The time constants of deactivation of tail currents were decreased by 17beta-estradiol over a range of test potentials (-140 to -80 mV). Finally, the inhibitory effect of 17beta-estradiol on the outward potassium currents was blocked following pre-incubation of slices in lavendustin A, a tyrosine kinase inhibitor. Taken together, these results suggest that 17beta-estradiol acts rapidly at an extracellular membrane receptor to reduce tetraethylammonium- and 4-aminopyridine-sensitive outward potassium currents by accelerating the closure of potassium channels. This may be the ionic basis of 17beta-estradiol-induced enhancement of neuronal excitability

Enhanced neurotransmitter release at glutamatergic synapses on oxytocin neurones during lactation in the rat

The increased release of oxytocin during lactation has been shown to be dependent upon glutamatergic transmission and is associated with an increased synaptic innervation of the supraoptic nucleus (SON).To determine whether the glutamatergic synaptic properties of oxytocin neurones are changed during lactation, we recorded excitatory postsynaptic currents (EPSCs) from identified oxytocin neurones in the SON of slices taken from adult virgin and lactating rats.The frequency of AMPA-mediated miniature EPSCs (mEPSCs) more than doubled during lactation. In addition, the decay time constant, but not the amplitude of the mEPSCs was significantly increased in both vasopressin and oxytocin neurones.Paired-pulse facilitation (PPF) was significantly reduced in oxytocin neurones during lactation, whereas no change was observed in vasopressin neurones. Elevating Ca2+ reduced PPF in oxytocin neurones in virgin rats but did not alter PPF in oxytocin neurones from lactating rats.Collectively, our results suggest that excitatory glutamatergic transmission is strengthened in oxytocin neurones during lactation, probably by a combination of an increased number of terminals, slower decay kinetics, and an increase in the probability of release

Adenosine-induced presynaptic inhibition of IPSCs and EPSCs in rat hypothalamic supraoptic nucleus neurones

The effects of adenosine on synaptic transmission in magnocellular neurosecretory cells were investigated using whole-cell patch-clamp recordings in acute rat hypothalamic slices that included the supraoptic nucleus.Adenosine reversibly reduced the amplitude of evoked inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents in a dose-dependent manner (IC50≈ 10 μm for both types of current).Depression of IPSCs and EPSCs by adenosine was reversed by the application of the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 10 μm).When pairs of stimuli were given at short intervals, adenosine inhibitory action was always less effective on the second of the two responses than on the first, resulting in an increased paired-pulse facilitation and suggesting a presynaptic site of action. This observation was confirmed by analysis of spontaneous miniature synaptic currents whose frequency, but not amplitude or kinetics, was reversibly reduced by 100 μM adenosine.CPT had no effect on synaptic responses evoked at a low frequency of stimulation (0.05–0.5 Hz), indicating the absence of tonic activation of A1 receptors under these recording conditions. However, CPT inhibited a time-dependent depression of both IPSCs and EPSCs induced during a 1 Hz train of stimuli.Taken together, these results suggest that adenosine can be released within the supraoptic nucleus at a concentration sufficient to inhibit the release of GABA and glutamate via the activation of presynaptic A1 receptors. By its inhibitory feedback action on the major afferent inputs to oxytocin and vasopressin neurones, adenosine could optimally adjust electrical and secretory activities of hypothalamic magnocellular neurones