The circadian rhythm (CR) of compound optic nerve potentials (CAPs) from the isolated eye of Aplysia californica can be phase shifted by 5-HT (serotonin). Data are presented in this thesis which suggest that 5-HT acts as a transmitter of temporal information to the circadian pacemaker (CP) in the Aplysia eye. Six h. 5-HT treatments produce both advance and delay phase shifts, are effective at low concentrations (10('-7)M), and alter both the spontaneous activity of the eye and the response of the eye to light. In addition, evidence is presented that the eye contains 5-HT (50ng/mg protein), can synthesis 5-HT from tryptophan, and that the 5-HT stored in the eye can be depleted by Hi-K('+) (depolarizing) treatments. The effects of 5-HT on the eye also show a degree of stereospecificity since three structurally related indoleamines, LSD, Br-LSD, and bufotenine acted as agonists of 5-HT when added to the eye, while other amines (5-hydroxytryptophan, tryptamine, 7-methyltryptamine, and others) and two putative neurotransmitters (Dopamine, Acetylcholine) did not mimic the effects of 5-HT. These results suggest that 5-HT may act as a neurotransmitter in the Aplysia eye. 5-HT appears to be producing its effects on the CR by acting either directly on the CR cell(s), or on cells electrotonically coupled to the CR cell(s), since treatments which block transmitter release (HiMg('2+)-LoCa('2+)-EGTA) do not inhibit phase shifting by 5-HT. The phase shifting effects of 5-HT may be mediated by changes in cellular cAMP, since 8-benzylthio-cAMP (2 x 10('-3)M), a cAMP analog, mimics 5-HT by producing both advance and delay phase shifts when given during the same phases as 5-HT. Papaverine (2 x 10('-4)M), which is a phosphodiesterase inhibitor that has been shown to increase cAMP levels in Aplysia tissue, also mimicked the phase shifting effects of 5-HT. IBMX (5 x 10('-4)M), another phosphodiesterase inhibitor, did not produce significant phase shifts when given alone, but did potentiate the phase shifting effects of subthreshold doses of 5-HT (10('-8)M). The results presented in this thesis are significant because 5-HT is the first endogenous substance shown to produce phase shifts in Aplysia and one of the few natural substances known to produce effects on the CR from any system. The data suggest that 5-HT may be part of a pathway which transmits entrainment information to the CP cell(s) in the eye, and that the effects of 5-HT may be mediated by changing levels of cAMP. 5-HT may provide a natural marker for investigating entrainment pathways and mechanisms in the Aplysia eye.

A semi-intact eye-cup preparation was developed which maintains visual responses and enables intracellular recordings to be made from the optic lobes of the crayfish compound eye. Sustaining fibers (SFs) were impaled with lucifer yellow electrodes near their entrance to the second optic ganglion or Medulla (where the SFs originate). Corneal receptive fields were determined and the SFs identified based on the previous work of Wiersma and Yamaguchi (1966). Following identification of each cell, the dendritic morphology was observed with lucifer yellow iontophoresis and subsequent fluorescence microscopy. An individual SF possesses a dendritic arborization restricted to that portion of the Medulla corresponding to its corneal receptive field. Therefore, the position of the dendritic tree combined with the retinotopic organization within the distal optic neuropils determine each SF's visual receptive field. Contour sensitivity maps were obtained for SF 019. The quantitative variation in sensitivity along a given arc in visual space was correlated with 019 dendritic density along the corresponding portion of the Medulla. The SFs' responses to various stimulus conditions were recorded intracellularly in their integrating segments. No synaptic interactions between SFs were observed. Bursting in response to broad-field intense illumination results from periodic excitatory synaptic input. Other visual interneurons were impaled and dye-filled. These included, (a) Phasic light-on cells, (b) Tonic light-off cells, (c) Phasic light-on and -off cells, (d) Optic tract motion detectors, (e) Nonspiking light-on neurons, (f) Lamina monopolar cells, and (g) Lamina tangential cells (TAN2). A group of neurosecretory cells analogous to the Medulla Externa X-Organ of other crustaceans was discovered in the crayfish. These neurosecretory cells are also interneurons possessing a modality specific (visual) synaptic input. They are inhibited by step increases in illumination and the response lasts for the duration of the stimulus. Transient off-inhibition is commonly present. The IPSPs reverse near resting membrane potentia and appear to be GABA mediated and chloride-dependent. These cells project axons to the proximal edge of the Lamina and have a vertical plane of dendrites in the same layer of the Medulla as the SFs. It is proposed that these cells are involved in the neurosecretory control of the circadian rhythm of screening pigment migration and/or photoreceptor sensitivity in the retina.

The crayfish Sustaining fibers (SFs) are used to analyze the functional organization of the second optic lobe, the Medulla. The SFs are tonic ON visual interneurons whose dendrites lie in a single plane in the Medullary neuropile, and whose axons project via the optic tract to the supraesophageal ganglion, or brain. The SF passive properties are remarkable linear. In the region of the primary neurite generally recorded from, these neurons show linear i-V, i-f and V-f functions. The SF dendrites appear to be entirely passive integrators of their synaptic inputs. The light-driven synaptic input to SFs is delivered by a retinotopic columnar array. The EPSP in response to a light stimulus is large (20-40 mV), and consists of two phases. The transient phase has a reversal potential approximately 50 mV above resting potential. The steady-state phase has a reversal potential about 37 mV above rest, and is associated with a steady-state input conductance which has a peak of about 120% of the resting value. A light-OFF hyperpolarization has a reversal potential 5 mV below rest and is associated with a conductance increase. Cable modelling of SFs revealed that the dendrites are electrotonically compact, about 0.5(lamda) in total width. Passive voltage attenuations in the dendrites are very small (average 6% proximal to distal, 35% distal to proximal). The synaptic parameters (reversal potential and conductance change) are used in conjunction with the model to accurately predict the EPSP voltage. A class of local, non-spiking amacrine (axon-less) cells is described. They are morphologically similar to the SF dendritic trees but with two distinct planes of dendrites and no axons. These neurons respond to light with a response like the SF EPSP but produce no action potentials. The amacrine cells strongly inhibit SF excitatory inputs without directly synapsing on the SFs. Similarities between amacrine cell properties and SF surround inhibition lead to the conclusion that the amacrine cells are directly involved in the formation of SF surround inhibition, and act on the columnar input pathway of the SFs.

Experiments were conducted to determine the currents involved in the pacemaker activity of aggregates and single cells from the embryonic chick heart. Two microelectrode voltage clamp studies of embryonic chick ventricular heart cell aggregates revealed two time-dependent current components in the pacemaker range of potentials ($-$60 to $-$120 mV). Barium (Ba, 5 mM) blocked the more negatively activated time-dependent component unmasking a component which remained inwardly directed for hyperpolarizing steps beyond the potassium equilibrium potential (E$ sb{ rm K}$). This component, which was blocked by cesium (Cs, 2 mM), is consistent with an inward current which activates upon hyperpolarization (the I, model) as proposed by DiFrancesco (1981a,b), for Purkinje fibers. / In order to minimize the problems associated with the accumulation/depletion of ions in the extracellular space during voltage clamp experiments, studies were carried out on single ventricular cells or small clusters of ventricular cells.

No description available.

Embryonic Xenopus muscle cells grown in culture develop patches of high acetylcholine receptor (AChR) density which can be visualized using fluorescent -bungarotoxin. In the present study it was found that these AChR patches often occur at sites of strong adhesion with the culture dish and remain in a fixed position, relatively unchanged for several days. Discrete patches of cholinesterase (ChE) also develop, often at the same sites as the AChR patches, even if the muscle cells have never been innervated in vivo. ChE, like AChRs, also becomes localized at sites of nerve-muscle contact in cultures grown with spinal cord cells whether or not neuromuscular transmission is blocked by curare. The results further indicate that in addition to directing AChR and ChE localization to the site of nerve-muscle contact, the nerve prevents the formation and reduces the survival of AChR and ChE patches elsewhere on the muscle cell.

Mechanisms regulating spinal sympathetic output to the heart and vessels have been examined using the technique of intrathecal administration of neuroactive agents at T2 and/or T9 in the rat. Antagonists and/or agonists of glycine, GABA$ sb{ rm A},$ GABA$ sb{ rm B}$ and excitatory amino acid receptors have been studied to determine how the respective amino acids mechanisms function at the spinal level to regulate sympathetic output. The results indicated that glycine, GABA$ sb{ rm A},$ GABA$ sb{ rm B},$ novel GABA, D-baclofen-sensitive, NMDA and non-NMDA receptors are involved in regulation of sympathetic output at the spinal level. Glycine, GABA$ sb{ rm A},$ GABA$ sb{ rm B},$ NMDA and non-NMDA receptors also participate in regulation of arterial pressure and heart rate under basal conditions. As D-baclofen increases only arterial pressure, it is proposed that there is a differential distribution of some receptors on the sympathetic neurons innervating different end organs. In addition, as the antagonists strychnine and APV alter either heart rate or arterial pressure, it is proposed that different receptors differ in their degree of tonic activation.

No description available.

The effects of various steroid hormones on human chorionic gonadotropin (hCG) production and placental viability were investigated using an explant culture model of human placenta. / Placental hCG production was assessed using two different methods: (a) hCG concentrations in media recovered from cultures were measured by radioimmunoassay and (b) tissue levels of hCG in cultured placentae were determined immunohistochemically; both were evaluated before and after exposure to steroid hormones. In first trimester placentae, progesterone and dehydroepiandrosterone (DHEA) increased hCG concentrations both in collected medium and levels in cultured placentae. Estradiol increased the levels of hCG in tissues but not in media. Cortisol increased concentrations in media but did not alter tissue levels. Testosterone decreased hCG levels in media, but had no effect on hCG placental content. In third trimester cultures, progesterone and DHEA were the only hormones studied which increased concentrations of hCG in media; estradiol, cortisol and testosterone had no effect. Progesterone, estradiol and DHEA, alone or in combination, extended the viability of first trimester placental explant cultures from approximately 7 to 30 days. There was a significant relationship between placental viability and tissue hCG levels (r = 0.73, P $<$ 0.001). The concentrations of hCG, progesterone and estradiol in human placentae were determined at various times through gestation. These studies suggest that a temporal relationship exists between the placental levels of hCG and these steroids, and that they may be significant determinants of growth and differentiation of the placenta in vivo. Furthermore, these investigations support the hypothesis that hCG production by the placenta is subject to paracrine regulation by steroid hormones.

This thesis describes the modulatory effects of acetylcholine (ACh) on neuronal responsiveness in the primary somatosensory cortex of cats. In 52 anesthetized animals, the responses of 440 cells were studied in three series of experiments. In series 1, iontophoretic administration of various doses (7-200 nA) of ACh produced increased firing rates in 16% (33/203) of the neurons tested. Responses to somatic stimuli were modulated by ACh in 29% of the cases. These modulatory effects were predominantly excitatory, with only 5% of the cells being depressed by ACh. In series 2 and 3, 50 nA of ACh applied in the absence of other overt stimula altered the responsiveness of a cell to subsequent tactile stimulation or iontophoretically-applied glutamate. This occurred in 21% (13/61) of the cells. In contrast, when 50 nA of ACh was administered simultaneously with glutamate and/or receptive field stimulation, altered responsiveness occurred in 77% (76/99) of the cases. One-third (32/99) of these cells displayed prolonged changes in responsiveness, lasting from 7 min to over 1 h. / Cells modulated by ACh were found in all cortical layers. The percentage of cholinoceptive cells was highest in layer VI (78%), and was not less than 50% in any layer. Both long and short-term potentiating effects of ACh on responses to somatic stimulation and responses to glutamate administration could be reversibly blocked by low iontophoretic doses of atropine.