Jun-ichi Toyoda

Analyses of neural mechanisms mediating the effect of horizontal cell polarization.

Polarization of horizontal cells by current through an intracellular electrode elicited a response in certain other horizontal cells and in bipolar cells. Results were consistent with the hypothesis that the feedback from horizontal cells to photoreceptors play an important role in the center-surround antagonism of bipolar cells and also in the opponent color responses of horizontal cells. The feedback from L-type horizontal cells must be mediated by GABA. The red component of RG-type and RGB-type horizontal cells was suppressed by application of GABA antagonists. GABA receptors was not found in these cells, suggesting that the effect was mediated through photoreceptors.

Dual effect of glutamate and aspartate on the on-center bipolar cell in the carp retina

In on-center bipolar cells of the carp retina, glutamate and aspartate applied extracellularly by pressure injection produced hyperpolarizing potential changes. These responses were not blocked by Co2+. Examination of reversal potentials revealed two different conductance changes. One of them appears to be an increase in K+ and/or Cl- conductance and the other a decrease in Na+ conductance. The results are discussed with relation to the difference in ionic mechanisms of cone and rod inputs.

Acute Immobilization Stress and Intraventricular Injection of CRF Suppress Naloxone‐Induced LH Release in Ovariectomized Estrogen‐Primed Rats

The present study was undertaken to evaluate the role and possible interaction of the endogenous opioid peptide (EOP) and corticotropin‐releasing factor (CRF) in the acute stress‐induced suppression of gonadotropin secretion in ovariectomized estrogen‐primed rats. An intravenous (i.v.) injection of naloxone (10 or 20 mg/kg), an EOP antagonist, significantly elevated serum luteinizing hormone (LH) levels within 10 min in non‐stressed animals. The naloxone‐induced LH release was completely eliminated when tested 30 min after the onset of acute immobilization. In a subsequent study, it was found that suppression of the naloxone‐induced LH release occurred as early as 5 min after the stress onset, and was still evident 60 min after the end of a 30‐min period of immobilization. The effect of naloxone was restored 3 h after liberation of the animal from the 30‐min immobilization. An intraventricular (i.c.v.) injection of CRF (1 or 5 μg) also significantly suppressed, in a dose‐related manner, the effect of a subsequent i.v. injection of naloxone. However, an i.c.v. injection of α‐helical CRF(9‐41) (25 or 50 μg), a CRF antagonist, prior to immobilization, could not interfere with the suppressive effect of stress on naloxone‐induced LH release. These results suggest that both acute immobilization stress and CRF can inhibit the LH secretory activity without mediation by EOP neurons. However, the stress‐related suppression may involve non‐CRF mechanism(s).

Permissive role of corticotropin-releasing factor in the acute stress-induced prolactin release in female rats

The present study examined the role of corticotropin-releasing factor (CRF) in the acute stress-induced release of prolactin (PRL) in ovariectomized estrogen-primed female rats. Acute immobilization stress induced a marked increase in serum PRL levels in animals treated with saline intraventricularly (i.c.v.). However, a prior icv injection of alpha-helical CRF(9-41), a CRF antagonist, completely eliminated the immobilization-induced PRL release in the majority of animals, providing evidence for involvement of CRF in the acute stress-induced PRL release. On the other hand, an i.c.v. injection of CRF did not affect basal PRL release at any dose in non-stressed animals, suggesting that the peptide plays a permissive role which enables other undefined stress mediator(s) to stimulate PRL release.

Effects of pinealectomy and melatonin on the timing of the proestrous luteinizing hormone surge in the rat.

Effects of pinealectomy and melatonin replacement on the timing of the preovulatory luteinizing hormone (LH) surge were examined in female rats housed under a light-dark cycle (lights on 06:30-18:30 h). Animals were pinealectomized or sham-operated 21-28 days before bleeding. They were sequentially bled in the afternoon of proestrus through the indwelling cardiac cannula without anesthesia. Proestrous LH surges were observed in all pinealectomized rats, but the onset times of the LH surge in these animals showed a significantly greater variance than those in sham-operated controls. A single melatonin injection at 18:00 or 21:00, shortly before or after the light-dark transition, on the day before proestrus (diestrus II) reduced the variance in the LH surge onset in pinealectomized rats in a dose-dependent manner. In contrast, melatonin injected at 14:00 or 23:00 on diestrus II or at 04:00 on proestrus was ineffective. These results showed that the pineal gland is involved in controlling the timing of the proestrous LH surge in the rat. The pineal signal may be transmitted by melatonin, a major product of the gland.

Color-opponent responses of small and giant bipolar cells in the carp retina

The physiological and morphological properties of color-opponent bipolar cells in the carp retina were studied. Fifty nine OFF-center bipolar cells and 63 ON-center bipolar cells out of about 500 total bipolar cells recorded showed color-opponent responses. The OFF-center color-opponent bipolar cells were classified into three subgroups according to their spectral and spatial responses. Fifty OFF-center color-opponent cells responded with depolarization to a blue light spot and with hyperpolarization to a red spot in the receptive-field center. The polarity of the surround response was opposite to that of center response at each wavelength. Therefore these cells were classified as OFF double-opponent cells (OFF-DO). Eight cells responded with hyperpolarization to a blue and green spot and with depolarization to a red spot. The surround responses of those cells were depolarizing at any wavelength (R+G- cell). One responded with hyperpolarization to a blue and red spot and with depolarization to a green spot. The surround response showed a different spectral characteristic from that of the center response. It responded with depolarization to a blue and green annulus and with hyperpolarization to a red annulus (R-G+B- cell). The ON-center color-opponent bipolar cells were similarly classified into three subgroups. Sixty of ON-center color-opponent cells were the double color-opponent type (ON-DO cell), showing the responses of opposite polarity to the OFF-DO cells. Two cells were classified as R- G+ cell, and one cell as R+G-B+ cell. Both OFF- and ON-DO cells were identified by their morphology as Cajals giant bipolar cells, and R+G-, R-G+, R-G+B-, and R+G-B+ cells as Cajals small bipolar cells. The analysis of the latency and the ionic mechanisms of their responses suggest that DO cells under light-adapted conditions receive direct inputs from long-wavelength (red) cones, RG cells from middle-wavelength (green) cones, and RGB cells from short-wavelength (blue) cones. Possible mechanisms of the opponent inputs to these bipolar cells are discussed.

Bipolar-amacrine synaptic transmission: Effect of polarization of bipolar cells on amacrine cells in the carp retina*

INTRODUCTION The amacrine cells in the teleost retina are often classified as sustained or transient (1.2). Sustained amacrine cells respond to light with sustained depolarization (ON type) or hyperpolarization (OFF type), while transient amacrine cells respond to both onset and offset of light with a transient depolarization (ON-OFF type). Synaptic transmission from bipolar cells to amacrine cells has been studied by measuring membrane resistance changes accompanying light responses (3), and by examining the effect of chemicals on bipolar and amacrine cell responses (4-7). These studies have suggested that the transmission from bipolar to amacrine cells is excitatory. Furthermore, histological observations on dye-injected bipolar cells and amacrine cells have shown that ON and OFF amacrine cells make synaptic connections with bipolar cells at different sublayers of the inner plexiform layer (8). Thus it is widely believed that ON amacrine cells receive excitatory inputs from depolarizing (ON) bipolar cells, OFF amacrine cells from hyperpolarizing (OFF) bipolar cells, and ON-OFF amacrine cells from both ON and OFF bipolar cells. The above inferences are rather indirect, however, and these observations do not elucidate the kinetics of synaptic transmission between bipolar and amacrine cells. For instance, it remains puzzling how the transient nature of ON-OFF responses is produced by summation of the excitatory inputs from the same ON and OFF bipolar cells that drive the sustained amacrine cells. In this study we approached this problem more directly. We simultaneously recorded responses of a bipolar cell and an amacrine cell with intracellular microelectrodes, while polarizing the bipolar cell with current. A step of depolarizing current injected into ON bipolar cells elicited a steady depolarization in sustained ON amacrine cells, but produced only a transient depolarization at the onset of current in ON-OFF amacrine cells. Steady hyperpolarization of OFF bipolar cells, however, elicited a transient depolarization in ON-OFF amacrine cells at the termination

Acute Stress Suppresses the N-Methyl-D-Aspartate-Induced Luteinizing Hormone Release in the Ovariectomized Estrogen-Primed Rat

The effects of N-methyl-D-aspartate (NMDA) and luteinizing hormone releasing hormone (LH-RH) on luteinizing hormone (LH) secretion were examined in ovariectomized estrogen-primed rats under nonstressed and acutely stressed conditions. The basal LH levels were significantly elevated 15 min after the onset of acute immobilization stress, but were not altered in emotionally stressed or nonstressed rats. Intravenous injections of 10 and 40 mg/kg NMDA significantly elevated serum LH levels by 161 and 212%, respectively, from baseline within 10 min in nonstressed animals. However, the NMDA-induced LH release was significantly reduced when tested 30 min after the onset of acute immobilization stress. Acute emotional stress, which did not affect the baseline LH, also suppressed the LH release response to NMDA, suggesting that the reduced LH responses to NMDA in stressed animals was not due to the elevated baseline level. Pituitary LH release responses to LH-RH were not affected by acute immobilization. We conclude from these results: (1) acute immobilization stress exerts both stimulatory and inhibitory effects on LH release, while acute emotional stress has only an inhibitory effect in estrogen-primed ovariectomized rats; (2) this inhibition occurs at the suprapituitary level, and (3) it involves a suppression of the responsiveness of the hypothalamic LH-RH neuronal system to the excitatory amino acid input.

Differential Inhibition of NMDA- and Naloxone-lnduced LH Release by NMDA Receptor Antagonist and CRH in Ovariectomized Estrogen-Primed Rats

A possible functional relationship between endogenous opioid peptides (EOPs), corticotropin-releasing hormone (CRH) and excitatory amino acids (EAAs) in the control of LH secretion was investigated in ovariectomized estrogen-primed rats. An intraventricular (icv) injection of an EAA agonist, N-methyl-D-aspartate (NMDA), or an EOP antagonist, naloxone, produced an abrupt increase in the serum LH level. While icv pretreatment of the animals with 2-amino-5-phosphonovaleric acid, a specific NMDA receptor antagonist, did not affect by itself basal LH levels, it significantly suppressed the NMDA-induced and also the naloxone-induced LH release. An icv injection of CRH also interfered with the naloxone-induced LH release. However, the NMDA-induced LH release was not affected by an icv injection of CRH or of beta-endorphin. These results suggest that the sites of EOP and CRH inhibition may be located upstream of the site of NMDA stimulation on the GnRH neuronal pathway, and that CRH can inhibit LH secretion without mediation by EOP neurons.

A Possible Role of the Pineal Gland in Acute Immobilization-Related Suppression of Naloxone-Induced LH Release in Ovariectomized Estrogen-Primed Rats

It has been recently reported that acute immobilization stress almost completely suppresses the luteinizing hormone (LH) release induced by naloxone, a μ‐opioid antagonist, in ovariectomized estrogen‐primed rats. The present study examined the possible involvement of the pineal gland in the acute immobilization‐related suppression of the naloxone‐induced LH release. An intraventricular (ICV) injection of 15 μg naloxone produced an abrupt increase in circulating LH concentrations in non‐stressed rats. The naloxone‐induced LH release was completely eliminated when tested 60 min after the end of a 30 min session of acute immobilization. The same stress conditions did not affect LH‐releasing hormone (LHRH)‐induced LH release, suggesting that the stress‐related suppression of the naloxone‐induced LH release was a suprapituitary event. In chronically‐pinealectomized rats, but not in sham‐pinealectomized rats, naloxone injected 60 min after the end of the stress session evoked a significant increase in serum LH concentrations. However, naloxone injected ICV during the acute immobilization did not elicit LH release in either pinealectomized or sham‐operated rats. Under non‐stressed conditions, the LH secretory response to naloxone was similar in pinealectomized and sham‐operated animals. The same stress (30 min immobilization) significantly increased pineal melatonin content as well as plasma melatonin concentrations in rats bearing intact pineal glands, indicating that stress actually affected the pineal function. These results provide evidence for a role of the pineal in the suppression of the LH response to naloxone after stress, but not during stress.