The impact of stress on social behavior in adult zebrafish (Danio rerio).

Stress has adverse effects on social behavior that is mediated by dopamine circuits in the midbrain. The purpose of this research is to examine the effect of chronic stress and dopamine signals on social behavior in zebrafish (Danio rerio). Chronic stress was induced chemically with low dosage of ethanol (0.25% for 5 days), and psychosocially with isolation (3-5 days) or overcrowding (5 days). Dopamine activity was decreased by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure. Social behavior was observed by introducing one treated zebrafish to a group of four control zebrafish and measuring the nearest neighbor distance (NND). Swimming ability was analyzed by measuring total swim distance and average velocity. Analysis of swim ability showed that treatment had no adverse effect upon locomotor functioning. However, stress and MPTP affected social behavior similarly. In all stress conditions, there was a significant increase in NND (7.4±3.9-9.1±4.4 cm). MPTP also caused an increase in NND (8.9±2.7 cm), but MPTP/isolation treatment did not amplify the effect (8.9±5.5 cm). One possible explanation is that chronic stress causes a change in dopamine activity and decreases social behavior, providing insight into the function of dopamine in social behavior.

Genetic dissection of rod and cone pathways in the dark-adapted mouse retina.

A monumental task of the mammalian retina is to encode an enormous range (>10(9)-fold) of light intensities experienced by the animal in natural environments. Retinal neurons carry out this task by dividing labor into many parallel rod and cone synaptic pathways. Here we study the operational plan of various rod- and cone-mediated pathways by analyzing electroretinograms (ERGs), primarily b-wave responses, in dark-adapted wildtype, connexin36 knockout, depolarizing rod-bipolar cell (DBCR) knockout, and rod transducin alpha-subunit knockout mice [WT, Cx36(-/-), Bhlhb4(-/-), and Tralpha(-/-)]. To provide additional insight into the cellular origins of various components of the ERG, we compared dark-adapted ERG responses with response dynamic ranges of individual retinal cells recorded with patch electrodes from dark-adapted mouse retinas published from other studies. Our results suggest that the connexin36-mediated rod-cone coupling is weak when light stimulation is weak and becomes stronger as light stimulation increases in strength and that rod signals may be transmitted to some DBCCs via direct chemical synapses. Moreover, our analysis indicates that DBCR responses contribute about 80% of the overall DBC response to scotopic light and that rod and cone signals contribute almost equally to the overall DBC responses when stimuli are strong enough to saturate the rod bipolar cell response. Furthermore, our study demonstrates that analysis of ERG b-wave of dark-adapted, pathway-specific mutants can be used as an in vivo tool for dissecting rod and cone synaptic pathways and for studying the functions of pathway-specific gene products in the retina.

Test of the paired-flash electroretinographic method in mice lacking b-waves.

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m(-2) (sc cd s m(-2)). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nob versus control mice differed by approximately 60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of L-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.

Regulation of retinal cone bipolar cell differentiation and photopic vision by the CVC homeobox gene Vsx1.

Cone bipolar cells of the vertebrate retina connect photoreceptors with ganglion cells to mediate photopic vision. Despite this important role, the mechanisms that regulate cone bipolar cell differentiation are poorly understood. VSX1 is a CVC domain homeoprotein specifically expressed in cone bipolar cells. To determine the function of VSX1, we generated Vsx1 mutant mice and found that Vsx1 mutant retinal cells form but do not differentiate a mature cone bipolar cell phenotype. Electrophysiological studies demonstrated that Vsx1 mutant mice have defects in their cone visual pathway, whereas the rod visual pathway was unaffected. Thus, Vsx1 is required for cone bipolar cell differentiation and regulates photopic vision perception.

Rod and cone contributions to the a-wave of the electroretinogram of the macaque.

The electroretinogram (ERG) of anaesthetised dark-adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod- and cone-driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone-driven component was isolated by delivering the stimulus shortly after a rod-saturating background had been extinguished. The rod-driven component was derived by subtracting the cone-driven component from the mixed rod-cone ERG. The initial part of the leading edge of the rod-driven a-wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low-pass filter elements. Addition of a simple static saturating non-linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non-linearity or that any significant low-pass filter followed the non-linear stage of the model. A negative inner-retinal component contributed to the later part of the rod-driven a-wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a-wave up to the time of the first zero-crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod-bipolar cell response. The negative inner-retinal component essentially cancelled the early part of the rod-bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a-wave. The leading edge of the cone-driven a-wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod-suppressing background or by the glutamate analogue, cis-piperidine-2,3-dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a-wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ms after the stimulus. The leading edge of the cone-driven a-wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non-linear stage in the cone model was subject to a low-pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod- and cone-driven a-waves of the dark-adapted ERG.

The scotopic threshold response of the dark-adapted electroretinogram of the mouse.

The most sensitive response in the dark-adapted electroretinogram (ERG), the scotopic threshold response (STR) which originates from the proximal retina, has been identified in several mammals including humans, but previously not in the mouse. The current study established the presence and assessed the nature of the mouse STR. ERGs were recorded from adult wild-type C57/BL6 mice anaesthetized with ketamine (70 mg kg(-1)) and xylazine (7 mg kg(-1)). Recordings were between DTL fibres placed under contact lenses on the two eyes. Monocular test stimuli were brief flashes (lambda(max) 462 nm; -6.1 to +1.8 log scotopic Troland seconds(sc td s)) under fully dark-adapted conditions and in the presence of steady adapting backgrounds (-3.2 to -1.7 log sc td). For the weakest test stimuli, ERGs consisted of a slow negative potential maximal approximately 200 ms after the flash, with a small positive potential preceding it. The negative wave resembled the STR of other species. As intensity was increased, the negative potential saturated but the positive potential (maximal approximately 110 ms) continued to grow as the b-wave. For stimuli that saturated the b-wave, the a-wave emerged. For stimulus strengths up to those at which the a-wave emerged, ERG amplitudes measured at fixed times after the flash (110 and 200 ms) were fitted with a model assuming an initially linear rise of response amplitude with intensity, followed by saturation of five components of declining sensitivity: a negative STR (nSTR), a positive STR (pSTR), a positive scotopic response (pSR), PII (the bipolar cell component) and PIII (the photoreceptor component). The nSTR and pSTR were approximately 3 times more sensitive than the pSR, which was approximately 7 times more sensitive than PII. The sensitive positive components dominated the b-wave up to > 5 % of its saturated amplitude. Pharmacological agents that suppress proximal retinal activity (e.g. GABA) minimized the pSTR, nSTR and pSR, essentially isolating PII which rose linearly with intensity before showing hyperbolic saturation. The nSTR, pSTR and pSR were desensitized by weaker backgrounds than those desensitizing PII. In conclusion, ERG components of proximal retinal origin that are more sensitive to test flashes and adapting backgrounds than PII provide the 'threshold' negative and positive (b-wave) responses of the mouse dark-adapted ERG. These results support the use of the mouse ERG in studies of proximal retinal function.