Modulation of mechanosensory threshold in Aplysia by serotonin, small cardioactive peptideB (SCPB), FMRFamide, acetylcholine, and dopamine.

Mechanosensory threshold of tail sensory neurons in Aplysia was tested after injecting the tail with neuromodulators known to affect defensive behavior in this animal. Serotonin (5-HT) and small cardioactive peptide (SCPB) reduced peripheral threshold, while FMRFamide, acetylcholine (ACh), and dopamine increased threshold. FMRFamide and ACh also reduced spontaneous activity of sensory neurons. Glutamate and taurine had no effect. SCPB effects persisted for 15-30 min after washout. Functional similarities between peripheral and central effects of these neuromodulators support the hypothesis of coordinate modulation of both regions of the sensory neuron by the same set of modulators following noxious stimulation.

Long-term expansion and sensitization of mechanosensory receptive fields in Aplysia support an activity-dependent model of whole-cell sensory plasticity.

Long-term changes in peripheral receptive field properties of mechanosensory/nociceptive neurons were investigated 1-3 weeks after noxious stimulation. Noxious stimuli consisted of a deep penetrating cut through the middle of the tail, strong electric shock applied to the tail surface, or a combination of deep and superficial tail stimulation. Action potentials evoked in the tail were monitored with intracellular electrodes in central somata of tail sensory neurons. Three long-term changes in receptive field properties were produced in the region of noxious stimulation: (1) mechanosensory thresholds decreased, (2) receptive field areas increased, and (3) the percentage of cells showing receptive field extension across the tail midline increased. Sizes and shapes of individual receptive fields did not vary during extensive testing of tails perfused with artificial seawater or during testing in cobalt solutions that block synaptic transmission. This stability of receptive field geometry, coupled with the observation that increased peripheral excitability in these cells does not increase receptive field size, suggests that long-term receptive field alterations involve growth of peripheral sensory processes. A model is proposed in which the signaling strength of the entire sensory cell increases in response to trauma of its receptive field. In this model long-term enhancement of central and peripheral sensory responsiveness is selectively triggered by activity dependent extrinsic modulation of the centrally located soma, which accelerates synthesis of growth-associated proteins used in collateral and regenerative sprouting of traumatized peripheral processes.

The effects of sex steroid treatments on sexual differentiation in a unisexual lizard, Cnemidophorus uniparens (Teiidae).

Cnemidophorus uniparens is a parthenogenetic unisexual species of lizard in which each individual develops as a female, making it a unique animal model for the study of sexual differentiation. In one study, administration of exogenous testosterone before and/or after hatching influenced the development of the gonads, the accessory reproductive ducts, the renal sex segment of the mesonephric kidney, and the femoral glands, a secondary sex character. Testosterone treatment also affected the cross-sectional area of the gonad and the proportions of cortical and medullary tissues present in the developing gonad. The oviducts and femoral glands of testosterone-treated individuals were hypertrophied; the collecting tubules of the kidney of these animals contained granules, an androgen-dependent, sexually dimorphic character in squamate reptiles. In another study, testosterone, dihydrotestosterone, or estradiol were administered to C. uniparens embryos. No treatment effects on gonadal development were detected on the day of hatching. However, estradiol, but not testosterone and dihydrotestosterone, stimulated development of the oviducts. Taken together these studies suggest that androgen aromatization may play a role in sexual differentiation in lizards.

The effects of early and late androgen treatments on the behavior of Sarotherodon mossambicus (Pisces: Cichlidae).

Groups of Sarotherodon mossambicus were treated with androgen by immersion or oral treatment at various stages of development. Fish were allowed to mature and the effects of treatment on gonadal and behavioral differentiation were examined. The effects of treatment on gonadal differentiation were assessed by determining the sex ratio for each group. Three treatments resulted in sex ratios significantly different from the 1:1 sex ratio obtained in untreated fish. Behavioral differences were detected between groups of males in three measures of territorial defense and aggression, and differences were detected between groups of females in two measures of male-female courtship interaction. A second experiment determined that early-treated females were more sensitive to a second androgen treatment later in life than females not exposed to androgen during development. A number of sex-reversed genetic females functioning as males were detected in two treatment groups with predominantly male sex ratios. There were no differences in the behavior of sex-reversed and non-sex-reversed male fish from the same treatment group. This study establishes that hormone treatments administered during development influence behavioral differentiation in a teleost.