Both kainic acid (KA) and N-methyl-d-aspartatic acid (NMDA) depolarize luminosity-type horizontal cells (L-type H cells) in normal turtle retina. The presence of both NMDA and non-NMDA receptors for excitatory amino acids (EAAs) on these cells was highlighted by an unusual effect of the noncompetitive NMDA-antagonist, MK-801. In retinas that had been exposed to MK-801, the action of NMDA was irreversibly altered to one of hyperpolarization, while the depolarizing effect of KA was unaltered. The aim of the present study was to further characterize these receptors on L-type H cells and to extend the investigation to color-opponent H cells (C-type H cells). Intracellular recording was used to study the effects of KA, NMDA, MK-801, the competitive NMDA antagonists, 2-amino-5-phosphonopentanoic acid (AP5) and 2-amino-7-phosphonoheptanoic acid (AP7), and the nonspecific EAA antagonist, kynurenic acid (KYN) on the light responses of L-type and C-type H cells in turtle retina. The effects of combinations of these drugs were also studied. In L-type H cells the agonists caused depolarization and loss of light response, KYN caused hyperpolarization and loss of light response, and MK-801, AP5 or AP7 had no direct effect. However, application of NMDA following MK-801, AP5 or AP7, but not KYN, caused hyperpolarization and loss of light response. The depolarizing effect of KA was unaltered by these antagonists. These data confirm the presence of an unusual NMDA receptor on L-type H cells. In the case of red\green C-type H cells, application of KA caused loss of responses to both red and green light, with loss of green responses preceding loss of red responses. NMDA initially removed responses to both red and green light. The most striking effect of NMDA was seen during early washout where the responses to red were reversed (hyperpolarizing). These responses eventually recovered their normal polarity. These results suggest that the depolarizing response of C-type H cells to red light is mediated by L-type H cells, but not via inhibition of the excitatory input from green cones to C-type H cells.

Previous studies have suggested that melatonin, released from photoreceptors, may modulate retinal dark-adaptive responses by inhibition of dopamine release from retinal interneurons. We have broadened these studies to examine the effect of melatonin on release of another retinal neurotransmitter, acetylcholine (ACh). The ACh system in rabbit retina has been localized to starburst amacrine cells, which release ACh in response to a variety of experimental stimuli, including direct potassium depolarization, flashing light, and glutamatergic as well as GABAergic inputs. The effect of melatonin on release of endogenously synthesized [3H]-ACh was measured in perfusates from retinas or retinal synaptosomes preloaded with [3H]-choline chloride. Melatonin significantly inhibited ACh release stimulated by potassium in intact retina but not in synaptosomes. Stimulation of intact retina by flashing light or by the glutamate receptor agonist, kainic acid, was also inhibited by melatonin. In contrast, there was no significant effect of melatonin on picrotoxin-induced release. These findings suggest that melatonin does have an inhibitory effect on ACh release, either by direct interaction with the cholinergic amacrine cell, or indirectly via GABAergic but not glutamatergic neurons.

Although excitotoxins derived from acidic amino acids are known to damage neurons in the inner nuclear and ganglion cell layers of the retina, little is known about their effects on photoreceptors. This study examines the acute and long-term effects of excitotoxins on photoreceptors of the chick retina. The zinc-iodide-osmium (ZIO) technique, which darkly labels a substantial subpopulation of synaptic vesicles in normal photoreceptor terminals, was used to supplement routine electron microscopy. Two-day-old chicks received a single intraocular injection of either 10, 50, or 200 nmoles kainic acid (KA), 200 nmoles N-methyl-D-aspartic acid (NMDA), or 200 nmoles quisqualic acid (QUIS), and were allowed to survive for either 6 h, 7 d, or 21 d. At 6 h, following exposure to 10, 50, and 200 nmoles KA, there was swelling and disruption of photoreceptor lamellae of the outer segments. At 7- and 21-d survival, 50 and 200 nmoles KA resulted in rounded, condensed synaptic terminals, which contained a high density of synaptic vesicles. However, there was complete loss of ZIO-positive vesicles within these photoreceptors. Outer segments were still disrupted, although small patches of lamellae were evident, suggestive of regeneration. Following exposure to QUIS, there was extensive swelling of outer segment lamellae at 6 h survival. Synaptic ribbons in terminals were also swollen. At longer survival periods, QUIS exposure resulted in a reduction of ZIO-positive vesicles, as well as swollen lamellae in outer segments. NMDA exposure, at either short or long-term survival, did not alter photoreceptor morphology, including the pattern of ZIO stain. The prolonged effects of KA, and to a lesser extent QUIS, on photoreceptors suggests that these drugs have a long-term effect on photoreceptor function. The ZIO technique provides a novel and potentially useful approach for identification of damaged photoreceptors.

The existence of multiple areas of extrastriate visual cortex raises the question of how the response properties of each area are derived from its visual input. This question was investigated for one such area in the cat, referred to here as the Clare-Bishop area (Hubel & Wiesel, 1969); it is the region of lateral suprasylvian cortex that receives input from area 17. A novel approach was used, in which kainic acid was injected locally into the Clare-Bishop area, making it possible to record directly from afferent inputs.

The response properties of the great majority of a sample of 424 presumed afferents resembled cells in areas 17 and 18. Thus, a systematic comparison was made with cells from area 17's upper layers, the source of its projection to the Clare-Bishop area (Gilbert & Kelly, 1975), to see whether these afferents had distinctive properties that might distinguish them from cells projecting to areas 18 or 19. Some differences did emerge: (1) The smallest receptive fields typical of area 17 were relatively scarce among afferents. (2) Direction-selective afferents were more abundant than were such cells in area 17. (3) End-stopped afferents were extremely rare, although end-stopped cells were common in area 17's upper layers.

Despite these differences, afferents were far more similar in their properties to cells in areas 17 and 18 than to cells in the Clare-Bishop area. Compared to the latter, afferents showed major discrepancies in receptive-field size, in direction selectivity, in end-stopping, and in ocular dominance distribution. These differences seem most likely to stem from circuitry intrinsic to the Clare-Bishop area.

The present study examines the differential effects of three excitotoxins, kainic acid (KA), N-methyl-D-aspartate (NMDA), and α-amino-2,3-amino-2,3-dihydro-5- methyl-3-oxo-4- isoxazolepropanoic acid (AMPA) on neurons within the genglion cell layer (GCL) of the chick retina. Two-day-old chicks were given a single, 5 μl, intravitreal injection of KA, NMDA, or AMPA at a range of doses. Following treatment with 40 nmol KA, there was a 21% loss of neurons in the GCL. At 200 nmol KA, the loss increased to 46%. Exposure to KA eliminated mainly small neurons of soma area 5–15μm2, and medium-sized ganglion cells of soma area 15–25μm2. Large ganglion cells (>25μ,2) remained unaffected. The vast majority of small cells were probably displaced amarcrine cells. At a does of 3000 nmol NMDA, no further loss of cells was evident. Exposure to 200 nmol AMPA resulted in a 30% loss of large and some medium-sized ganglion cells. In a further series of experiments, exposure to excitotoxin was followed by a retinal scratch, which eliminated retinal ganglion cells within the axotomized region. The results indicate that only a small proportion of displaced amacrine cells are destroyed by NMDA and AMPA, whereas virtually all displaced amarine cells are sensitive to KA. The findings of this study indicate the existence of subclasses of ganglion cells with specificity towards different types of excitatory amino acids (EAA).

The astrocytic enzyme adenosine kinase (ADK) is a key negative regulator of the brain’s endogenous anticonvulsant adenosine. Astrogliosis with concomitant upregulation of ADK is part of the epileptogenic cascade and contributes to seizure generation. To molecularly dissect the respective roles of astrogliosis and ADK-expression for seizure generation, we used a transgenic approach to uncouple ADK-expression from astrogliosis: in Adk-tg mice the endogenous Adk-gene was deleted and replaced by a ubiquitously expressed Adk-transgene with novel ectopic expression in pyramidal neurons, resulting in spontaneous seizures. Here, we followed a unique approach to selectively injure the CA3 of these Adk-tg mice. Using this strategy, we had the opportunity to study astrogliosis and epileptogenesis in the absence of the endogenous astrocytic Adk-gene. After triggering epileptogenesis we demonstrate astrogliosis without upregulation of ADK, but lack of seizures, whereas matching wild-type animals developed astrogliosis with upregulation of ADK and spontaneous recurrent seizures. By uncoupling ADK-expression from astrogliosis, we demonstrate that global expression levels of ADK rather than astrogliosis per se contribute to seizure generation.

Epilepsy is characterized by both neuronal and astroglial dysfunction. The endogenous anticonvulsant adenosine, the level of which is largely controlled by astrocytes, might provide a crucial link between astrocyte and neuron dysfunction in epilepsy. Here we have studied astrogliosis, a hallmark of the epileptic brain, adenosine dysfunction and the emergence of spontaneous seizures in a comprehensive approach that includes a new mouse model of focal epileptogenesis, mutant mice with altered brain levels of adenosine, and mice lacking adenosine A1 receptors. In wild-type mice, following a focal epileptogenesis-precipitating injury, astrogliosis, upregulation of the adenosine-removing astrocytic enzyme adenosine kinase (ADK), and spontaneous seizures coincide in a spatio-temporally restricted manner. Importantly, these spontaneous seizures are mimicked by untreated transgenic mice that either overexpress ADK in brain or lack A1 receptors. Conversely, mice with reduced ADK in the forebrain do not develop either astrogliosis or spontaneous seizures. Our studies define ADK as a crucial upstream regulator of A1 receptor-mediated modulation of neuronal excitability, and support the ADK hypothesis of epileptogenesis in which upregulation of ADK during astrogliosis provides a crucial link between astrocyte and neuron dysfunction in epilepsy. These findings define ADK as rational target for therapeutic intervention.

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, including its timing, its relationship to other developmental changes, factors that may influence its progress and its potential role in brain development. Specifically, we discuss some of the theories proposed for its occurrence and agree with the classic notion that REM sleep is, at least, an active mechanism that may play a role in the maturation of the central nervous system (CNS), specifically contributing to the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until after puberty in the human, but in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. This purported role for REM sleep may change to involve a number of other functions with age. We describe recent findings showing that intrinsic morphological and physiological properties as well as serotonergic, n-methyl-D-aspartic acid (NMDA) and kainic acid (KA) synaptic inputs to mesopontine cholinergic neurons change dramatically at this critical period in development, perhaps driving what has been proposed as a REM sleep inhibitory process (RIP). We hypothesize that a dysregulation of this process could result in life-long disturbances in REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders which have a mostly postpubertal age of onset. Finally, we also hypothesize that the role of normal cyclic increases in vigilance, observable during both sleep and waking, may be related, at least in part, to cortical blood flow.