The medial preoptic nucleus (MPN) of the hypothalamus plays a major role in many functions involved in maintaining bodily homeostasis, such as thermoregulation and osmoregulation, as well as in the control of complex behaviours, e.g. sexual behaviour. A fundamental basis for the control and execution of these functions is the synaptic communication between neurons of the MPN. However, the functional properties of the synapses involved are largely unknown. The present thesis is a study of ligand-gated ion channels involved in the pre- and post-synaptic aspects of neuronal communication in the MPN of rat. The aim was to clarify synaptic properties in the MPN, to identify the major channel types involved and to obtain a better understanding of their functional properties. By fast application of agonists to isolated neurons, it was first demonstrated that all neurons responded to glutamate with currents mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and a majority of neurons also with currents mediated by N-Methyl-D-aspartate (NMDA) receptors. All neurons also responded to γ-aminobutyric acid (GABA) and glycine with currents mediated by GABAA receptors and glycine receptors, respectively. These findings show that fast-acting excitatory and inhibitory amino-acid transmitters are most likely important for communication between hypothalamic neurons. Application of agonists to isolated neurons revealed cross-talk, detected as an apparent cross-desensitization, between the responses to GABA and those to glycine. Parallel analysis of current and conductance, using gramicidin-perforated patches to avoid perturbing intracellular chloride concentration, showed that the cross-talk was not dependent on a direct interaction between the receptors as previously suggested, but was a consequence of the change in the intracellular chloride concentration during receptor activation. Strengthened by a computer model, the analysis also showed that the current decay in the presence of GABA or glycine was mainly due to a change in the chloride driving force and that receptor desensitization played a minor role only. The role of thermo-sensitive transient receptor potential TRPV1 channels in the regulation of glutamate- and GABA-mediated transmission was studied in the slice preparation, where much of the synaptic connections between neurons are preserved. It was shown that application of the TRPV1 agonist capsaicin increased the frequency of excitatory AMPA receptor- mediated as well as inhibitory GABAA receptor-mediated postsynaptic currents. This effect was partly presynaptic and demonstrates that TRP channels play a role in regulating synaptic transmission in the MPN. The results imply that such mechanisms may possibly contribute to the thermoregulation by MPN neurons.

Brain function implies complex information processing in neuronal circuits, critically dependent on the molecular machinery that enables signal transmission across synaptic contacts between neurons. The types of ion channels and receptors in the neuronal membranes vary with neuron types and brain regions and determine whether neuronal responses will be excitatory or inhibitory and often allow for functional synaptic plasticity which is thought to be the basis for much of the adaptability of the nervous system and for our ability to learn and store memories. The present thesis is a study of synaptic transmission in the medial preoptic nucleus (MPN), a regulatory center for several homeostatic functions but with most clearly established roles in reproductive behaviour. The latter behaviour typically shows several distinct phases with dramatically varying neuronal impulse activity and is also subject to experience-dependent modifications. It seems likely that the synapses in the MPN contribute to the behaviour by means of activity-dependent functional plasticity. Synaptic transmission in the MPN, however, has not been extensively studied and is not well understood. The present work was initiated to clarify the synaptic properties in the MPN. The aim was to achieve a better understanding of the functional properties of the MPN, but also to obtain information on the functional roles of ion channel types for neurotransmission and its plastic properties in general. The studies were carried out using a brain slice preparation from rat as well as acutely isolated neurons with adhering nerve terminals. Presynaptic nerve fibres were stimulated electrically or, in a few cases, by raised external K+ concentration, and postsynaptic responses were recorded by tight-seal perforated-patch techniques, often combined with voltage-clamp control of the post-synaptic membrane potential. Glutamate receptors of α-amino-3-hydroxy-5-methyl-4-izoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) types were identified as mediating the main excitatory synaptic signals and γ-aminobutyric acid (GABA)A receptors as mediating the main inhibitory signals. Both types of signals were suppressed by serotonin. The efficacy of AMPA-receptor-mediated transmission displayed several types of short-term plasticity, including paired-pulse potentiation and paired-pulse depression, depending on the stimulus rate and pattern. The observed plasticity was attributed to mainly presynaptic mechanisms. To clarify some of the presynaptic factors controlling synaptic efficacy, the role of presynaptic L-type Ca2+ channels, usually assumed not to directly control transmitter release, was investigated. The analysis showed that (i) L-type channels are present in GABA-containing presynaptic terminals on MPN neurons, (ii) that these channels provide a means for differential control of spontaneous and impulse-evoked GABA release and (iii) that this differential control is prominent during short-term synaptic plasticity. A model where Ca2+ influx through L-type channels may lead to reduced GABA release via effects on Ca2+-activated K+ channels, membrane potential and other Ca2+-channel types explains the observed findings. In addition, massive Ca2+ influx through L-type channels during high-frequency stimulation may contribute to increased GABA release during post-tetanic potentiation. In conclusion, the findings obtained in the present study indicate that complex neurotransmission mechanisms and different forms of synaptic plasticity contribute to the specific functional properties of the MPN.

Projeções do núcleo pré-óptico medial (MPN) para a porção ventromedial do núcleo ventromedial do hipotálamo (VMHvl) compõe um circuito de controle do comportamento sexual feminino (CSF) influenciado por pistas endógenas e exógenas. Este CSF no adulto só ocorre se o sistema nervoso se desenvolveu corretamente, por meio da ação de hormônios esteroidais. Por isso, objetivamos estudar as projeções do MPN para o VMHvl em fêmeas controle (GC) e ooforectomizados (GO). O GC aos 90 dias de idade foi submetido à cirurgia estereotáxica para injeção iontoforética do traçador anterógrado Phaseolus vulgaris no MPN. Já o GO aos 30 dias de idade, foi submetido à ooforectomia bilateral e aos 90 dias de idade submetido à mesma cirurgia que o GC. Como resultado, nos dois grupos observamos marcação anterógrada no VMHvl, assim como para outras regiões que integram o circuito do CSF; porém observamos uma possível redução das projeções no GO. Sendo assim, estabelecemos a conectividade do MPN com o VMHvl, contudo observamos que no GO há uma alteração na densidade e extensão dessas projeções. / Medial preoptic nucleus (MPN) outputs to the ventromedial portion of the ventromedial hypothalamic nucleus (VMHvl) participates in the female sexual behavior (FSB) control circuit influenced by endogenous and environmental cues. This FSB in adults only occurs if the nervous system has developed correctly, through the action of steroid hormones. Thus, our goal is to study the MPN outputs to the VMHvl in control (CG) and ovariectomized (OG) female animals. For the control group (CG), at 90 days of age, these females underwent stereotactic surgery for iontophoretic injection of the anterograde tracer Phaseolus vulgaris into the MPN. The oophorectomized group (OG), at 30 days of age, underwent oophorectomy and when they reach 90 days of age they underwent the same surgery as CG. As a results, in both experimental groups, we observed anterograde labeling in VMHvl, as well as for other regions that are part of the FBS circuit; but in these nuclei, we observed a possible reduction in the density of the projections on the OG. Thus, we establish the connectivity of the MPN with the VMHvl, however we observe that in the GO there is a change in the density and extension of these projections.