Effects of insulin on norepinephrine- and acetylcholine-induced membrane currents of pinealocytes from healthy Wistar and type 2 diabetic GK rats.

The neurohormone melatonin is synthesized by the pineal gland under the stimulation of norepinephrine (NE). Its synthesis is inhibited by acetylcholine (ACh) and by insulin. Type 2 diabetic Goto Kakizaki (GK) rats have higher insulin and lower melatonin plasma levels than healthy Wistar rats. We investigate membrane potentials and currents of isolated pinealocytes in both rat strains and the influence of NE, ACh and insulin by using the perforated patch whole cell clamp technique. Pinealocyte membranes displayed a high resting Na(+) conductance. Stimulation with NE further increased this Na(+) conductance, which led to a slight depolarization in unclamped cells. The amplitude of the NE-evoked current was similar in both rat strains but the current fraction carried by Na(+) was stronger in GK rats. Stimulation with ACh induced a transient inward current and depolarization. These effects were much more pronounced in the pinealocytes of GK rats. The NE-induced current, the ACh-induced current and the membrane depolarization were reduced by pre-administration of insulin in Wistar pinealocytes. Our results provide the first electrophysiological evidence for the modulation, by insulin, of the effects of NE and ACh in pinealocytes of normal rats. The pinealocytes of type 2 diabetic rats were not responsive to insulin. This might explain the reported correlation between the decreased insulin receptor mRNA transcript levels in GK rat pinealocytes and the lack of effect of insulin on ion channels in their cell membranes.

Pigment epithelial changes in a strain of pigmented rabbits with low ERG b-wave amplitudes.

Retinas from a strain of rabbits with low dark-adapted electroretinographic (ERG) b-wave amplitudes, and declining ERG responses with time, were examined by light and electron microscopy. Seven rabbits from the affected strain (13 months to 5 years old) and six control animals (6-26 months old) were included in the study. Small inclusions with an electron dense border, about 0.2-1.5 microm in diameter, were significantly (p<0.01) more numerous within the retinal pigment epithelial (RPE) cells in the affected rabbits than in the control animals. No morphological evidence of retinal degeneration was found. Further studies are needed to establish the functional defect in this strain of rabbits.

Contribution of post-receporal cells to the cone a-wave of the human electroretinogram in congenital stationary night blindness and autoimmune-like retinopathy.

In normal subjects the later part of the cone a-wave to a brief flash increases in amplitude after 50-100 ms darkness due to a contribution from secondary hyperpolarising cells. We recorded these responses along with clinical ON and OFF ERGs in patients with inner retinal dysfunction to see if this part of the a-wave is affected. Patients with autoimmune-like retinopathy and CSNB2 had abnormal ON and OFF responses but the a-wave increased in amplitude in the dark as in normals. Conversely, the OFF-response was normal in CSNB1 but the a-wave did not increase in the dark. Contrary to expectation these results show some hyperpolarising cell function in autoimmune-like disease and CSNB2 and some OFF-pathway abnormality in CSNB1. The a- and d-wave are needed to assess OFF-pathway function.

The effect of GABA and the GABA-uptake-blocker NO-711 on the b-wave of the ERG and the responses of horizontal cells to light.

BACKGROUND: The effects of GABA in the retina have now become of special interest because the anti-epileptic drug vigabatrin, a GABA analogue, can cause visual field loss in humans. Vigabatrin inhibits the GABA-aminotransferase, which finally results in GABA accumulation in the extracellular space. The b-wave of the electroretinogram (ERG), which originates partly in on-bipolar cells, is influenced by both GABAergic horizontal cells (HCs) and GABAergic amacrine cells (ACs). Their influences, however, are difficult to separate. In an attempt to isolate the effect of GABAergic ACs, use has been made of the specific effect of the GABA-uptake-blocker NO-711, which blocks only the GABA transporter GAT1 of GABAergic ACs. METHODS: The ERG and the intracellular responses of HCs to light were recorded in the isolated rabbit retina, and the effects of GABA and NO-711, when added separately to the superfusate, were determined. RESULTS: GABA reduced significantly both the light responses of HCs and the b-wave. NO-711 enlarged the b-wave drastically, but did not affect the responses of HCs to light. CONCLUSIONS: An increase in the extracellular GABA concentration decreases the b-wave; an impairment of the function of ACs increases the b-wave. These conditions are discussed in the context of the lack of consistent changes to the b-wave during therapy with vigabatrin.