Addition of human melanopsin renders mammalian cells photoresponsive.

A small number of mammalian retinal ganglion cells act as photoreceptors for regulating certain non-image forming photoresponses. These intrinsically photosensitive retinal ganglion cells express the putative photopigment melanopsin. Ablation of the melanopsin gene renders these cells insensitive to light; however, the precise role of melanopsin in supporting cellular photosensitivity is unconfirmed. Here we show that heterologous expression of human melanopsin in a mouse paraneuronal cell line (Neuro-2a) is sufficient to render these cells photoreceptive. Under such conditions, melanopsin acts as a sensory photopigment, coupled to a native ion channel via a G-protein signalling cascade, to drive physiological light detection. The melanopsin photoresponse relies on the presence of cis-isoforms of retinaldehyde and is selectively sensitive to short-wavelength light. We also present evidence to show that melanopsin functions as a bistable pigment in this system, having an intrinsic photoisomerase regeneration function that is chromatically shifted to longer wavelengths.

Synaptically released glutamate reduces gamma-aminobutyric acid (GABA)ergic inhibition in the hippocampus via kainate receptors.

Exogenous application of agonists at the kainate subtype of glutamate receptors has been shown to depress evoked monosynaptic inhibition by gamma-aminobutyric acid (GABA)ergic interneurons in the hippocampus. This observation has led to the hypothesis that synaptic release of endogenous glutamate might have a disinhibitory effect on neuronal circuits, in addition to depolarizing neurons via postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate, and N-methyl-D-aspartic acid (NMDA) receptors. It is not known, however, if glutamate released from excitatory neurons has the same kainate receptor-mediated effect on monosynaptic inhibitory transmission as exogenous agonist application. Indeed, the recent demonstration that excitatory synaptic signals elicited in interneurons are partly mediated by kainate receptors suggests that these receptors may have a pro- rather than disinhibitory role. Here, we examine the effect of synaptically released glutamate on monosynaptic inhibitory signaling. In the presence of antagonists to AMPA and NMDA receptors, brief bursts of activity in glutamatergic afferent fibers reduce GABAergic transmission. This depression of inhibition is reversibly abolished by blocking kainate receptors. It persists when GABA(B) receptors are blocked and is enhanced by blocking metabotropic glutamate receptors, possibly explained by presynaptic regulation of glutamate release from excitatory afferents by metabotropic autoreceptors. We conclude that the net kainate receptor-mediated effect of synaptically released glutamate is to reduce monosynaptic inhibition. Since this form of disinhibition may contribute to seizure initiation, kainate receptors may constitute an important target for anticonvulsant drug development.

Epileptogenic action of caffeine during anoxia in the neonatal rat hippocampus.

Excessive maternal caffeine consumption can lead to fetal and neonatal pathology, but the underlying mechanisms have not been determined. Here, we report that low doses of caffeine generate seizures when applied in conjunction with brief anoxic episodes in the hippocampus of neonatal rats in vitro. In control conditions, brief (4-6 minutes) anoxic episodes reversibly depressed evoked synaptic responses and blocked the physiological pattern of network activity. In the presence of caffeine (50 microM), similar anoxic episodes generated ictal (29%) or interictal (33%) epileptiform activities often followed during reoxygenation by recurrent spontaneous seizure activity that persisted for several hours. These effects are likely mediated by a blockade of adenosine receptors by caffeine because (1) in control conditions, caffeine antagonized the inhibitory effect of selective A1 receptor agonist N6-cyclopentyladenosine on excitatory synaptic responses, and (2) epileptogenic effects of caffeine were reproduced by selective A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and theophylline. Our findings suggest that endogenous adenosine released during anoxia acting via A1 receptors prevents seizures in the neonatal hippocampus and that the antagonism of these receptors by caffeine leads to epileptogenesis. This study suggests concerns about the safety of caffeine in the fetus and newborn.

Maturation of kainate-induced epileptiform activities in interconnected intact neonatal limbic structures in vitro.

In vivo studies suggest that ontogenesis of limbic seizures is determined by the development of the limbic circuit. We have now used the newly-developed in vitro intact interconnected neonatal rat limbic structures preparation to determine the developmental profile of kainate-induced epileptiform activity in the hippocampus and its propagation to other limbic structures. We report gradual alterations in the effects of kainate during the first postnatal week on an almost daily basis; from no epileptiform activity at birth, through interictal seizures around postnatal day (P) 2 and ictal seizures by the end of the first week. The developmental profile of kainate-induced hippocampal seizures is paralleled by the expression of postsynaptic kainate receptor-mediated currents in CA3 pyramidal cells. Intralimbic propagation of the hippocampal seizures is also age-dependent: whereas seizures readily propagate to the septum and to the contralateral hippocampus via the commissures on P2, propagation to the entorhinal cortex only takes place from P4 onwards. Finally, repeated brief applications of kainate to the hippocampus induce recurrent spontaneous glutamatergic ictal and interictal discharges which persist for several hours after the kainate is washed away and which replace the physiological pattern of network activity. Paroxysmal activities are thus generated by kainate in the hippocampus at an early developmental stage and are initially restricted to this structure. Before the end of the first week of postnatal life, kainate generates the epileptiform activities that may perturb activity-dependent mechanisms that modulate neuronal development. Although at this stage neurons are relatively resistant to the pathological effects of kainate, the epileptiform activities that it generates will perturb activity-dependent mechanisms that modulate neuronal development.