Efficacy of nonselective optogenetic control of the medial septum over hippocampal oscillations: the influence of speed and implications for cognitive enhancement.

Optogenetics holds great promise for both the dissection of neural circuits and the evaluation of theories centered on the temporal organizing properties of oscillations that underpin cognition. To date, no studies have examined the efficacy of optogenetic stimulation for altering hippocampal oscillations in freely moving wild-type rats, or how these alterations would affect performance on behavioral tasks. Here, we used an AAV virus to express ChR2 in the medial septum (MS) of wild-type rats, and optically stimulated septal neurons at 6 Hz and 30 Hz. We measured the corresponding effects of these stimulations on the oscillations of the MS and hippocampal subfields CA1 and CA3 in three different contexts: (1) With minimal movement while the rats sat in a confined chamber; (2) Explored a novel open field; and (3) Learned and performed a T-maze behavioral task. While control yellow light stimulation did not affect oscillations, 6-Hz blue light septal stimulations altered hippocampal theta oscillations in a manner that depended on the animal's mobility and speed. While the 30 Hz blue light septal stimulations only altered theta frequency in CA1 while the rat had limited mobility, it robustly increased the amplitude of hippocampal signals at 30 Hz in both regions in all three recording contexts. We found that animals were more likely to make a correct choice during Day 1 of T-maze training during both MS stimulation protocols than during control stimulation, and that improved performance was independent of theta frequency alterations.

The anticonvulsant action of the galanin receptor agonist NAX-5055 involves modulation of both excitatory- and inhibitory neurotransmission.

The endogenous neuropeptide galanin is ubiquitously expressed throughout the mammalian brain. Through the galanin receptors GalR1-3, galanin has been demonstrated to modulate both glutamatergic and GABAergic neurotransmission, and this appears to be important in epilepsy and seizure activity. Accordingly, galanin analogues are likely to provide a new approach to seizure management. However, since peptides are generally poor candidates for therapeutic agents due to their poor metabolic stability and low brain bioavailability, a search for alternative strategies for the development of galanin-based anti-convulsant drugs was prompted. Based on this, a rationally designed GalR1 preferring galanin analogue, NAX-5055, was synthesized. This compound demonstrates anti-convulsant actions in several animal models of epilepsy. However, the alterations at the cellular level leading to this anti-convulsant action of NAX-5055 are not known. Here we investigate the action of NAX-5055 at the cellular level by determining its effects on excitatory and inhibitory neurotransmission, i.e. vesicular release of glutamate and GABA, respectively, in cerebellar, neocortical and hippocampal preparations. In addition, its effects on cell viability and neurotransmitter transporter capacity were examined to evaluate potential cell toxicity mediated by NAX-5055. It was found that vesicular release of glutamate was reduced concentration-dependently by NAX-5055 in the range from 0.1 to 1000 nM. Moreover, exposure to 1 µM NAX-5055 led to a reduction in the extracellular level of glutamate and an elevation of the extracellular level of GABA. Altogether these findings may at least partly explain the anti-convulsant effect of NAX-5055 observed in vivo.

Status Epilepticus Induced Spontaneous Dentate Gyrus Spikes: In Vivo Current Source Density Analysis.

The dentate gyrus is considered to function as an inhibitory gate limiting excitatory input to the hippocampus. Following status epilepticus (SE), this gating function is reduced and granule cells become hyper-excitable. Dentate spikes (DS) are large amplitude potentials observed in the dentate gyrus (DG) of normal animals. DS are associated with membrane depolarization of granule cells, increased activity of hilar interneurons and suppression of CA3 and CA1 pyramidal cell firing. Therefore, DS could act as an anti-excitatory mechanism. Because of the altered gating function of the dentate gyrus following SE, we sought to investigate how DS are affected following pilocarpine-induced SE. Two weeks following lithium-pilocarpine SE induction, hippocampal EEG was recorded in male Sprague-Dawley rats with 16-channel silicon probes under urethane anesthesia. Probes were placed dorso-ventrally to encompass either CA1-CA3 or CA1-DG layers. Large amplitude spikes were detected from EEG recordings and subject to current source density analysis. Probe placement was verified histologically to evaluate the anatomical localization of current sinks and the origin of DS. In 9 of 11 pilocarpine-treated animals and two controls, DS were confirmed with large current sinks in the molecular layer of the dentate gyrus. DS frequency was significantly increased in pilocarpine-treated animals compared to controls. Additionally, in pilocarpine-treated animals, DS displayed current sinks in the outer, middle and/or inner molecular layers. However, there was no difference in the frequency of events when comparing between layers. This suggests that following SE, DS can be generated by input from medial and lateral entorhinal cortex, or within the dentate gyrus. DS were associated with an increase in multiunit activity in the granule cell layer, but no change in CA1. These results suggest that following SE there is an increase in DS activity, potentially arising from hyperexcitability along the hippocampal-entorhinal pathway or within the dentate gyrus itself.

Regulation of glucose and insulin release following acute and repeated treatment with the synthetic galanin analog NAX-5055.

The neuropeptide galanin is widely expressed in both the central and peripheral nervous systems. However there is limited understanding of how individual galanin receptor (GalR1, 2, and 3) subtypes mediate the physiological activity of galanin in vivo. To address this issue we utilized NAX-5055, a systemically available, metabolically stable galanin analog. NAX-5055 displays a preference for GalR1 receptors and possesses potent anticonvulsant activity in vivo, suggesting that NAX-5055 engages central galanin receptors. To determine if NAX-5055 also modulates the activity of peripheral galanin receptors, we evaluated the effect of NAX-5055 on blood glucose and insulin levels in mice. Acute and repeated (once daily for four days) systemic administration of NAX-5055 (4 mg/kg) significantly increased blood glucose levels compared to vehicle treated mice. However, a hyperglycemic response was not observed following systemic administration of NAX-805-1, a scrambled analog of NAX-5055, with critical receptor binding residues, Trp(2) and Tyr(9), reversed. These results suggest that chemical modifications independent of the galanin backbone of NAX-5055 are not responsible for the hyperglycemic response. The effect of NAX-5055 on glucose homeostasis was further evaluated with a glucose tolerance test (GTT). Mice administered either acute or repeated (once daily for four days) injections of NAX-5055 (4 mg/kg) displayed impaired glucose handling and reduced insulin response to an acute glucose (1g/kg) challenge. Here we have shown that systemic administration of a centrally active GalR1-preferring galanin analog produces acute hyperglycemia and an inhibition of insulin release in vivo and that these effects are not attenuated with repeated administration. NAX-5055 thus provides a new pharmacological tool to further the understanding of function of both central and peripheral GalR1 receptors in vivo.

Anticonvulsant neuropeptides as drug leads for neurological diseases.

Anticonvulsant neuropeptides are best known for their ability to suppress seizures and modulate pain pathways. Galanin, neuropeptide Y, somatostatin, neurotensin, dynorphin, among others, have been validated as potential first-in-class anti-epileptic or/and analgesic compounds in animal models of epilepsy and pain, but their therapeutic potential extends to other neurological indications, including neurodegenerative and psychatric disorders. Disease-modifying properties of neuropeptides make them even more attractive templates for developing new-generation neurotherapeutics. Arguably, efforts to transform this class of neuropeptides into drugs have been limited compared to those for other bioactive peptides. Key challenges in developing neuropeptide-based anticonvulsants are: to engineer optimal receptor-subtype selectivity, to improve metabolic stability and to enhance their bioavailability, including penetration across the blood­brain barrier (BBB). Here, we summarize advances toward developing systemically active and CNS-penetrant neuropeptide analogs. Two main objectives of this review are: (1) to provide an overview of structural and pharmacological properties for selected anticonvulsant neuropeptides and their analogs and (2) to encourage broader efforts to convert these endogenous natural products into drug leads for pain, epilepsy and other neurological diseases.

Developing novel antiepileptic drugs: characterization of NAX 5055, a systemically-active galanin analog, in epilepsy models.

The endogenous neuropeptide galanin and its associated receptors galanin receptor 1 and galanin receptor 2 are highly localized in brain limbic structures and play an important role in the control of seizures in animal epilepsy models. As such, galanin receptors provide an attractive target for the development of novel anticonvulsant drugs. Our efforts to engineer galanin analogs that can penetrate the blood-brain-barrier and suppress seizures, yielded NAX 5055 (Gal-B2), a systemically-active analog that maintains low nanomolar affinity for galanin receptors and displays a potent anticonvulsant activity. In this report, we show that NAX 5055 is active in three models of epilepsy: 1) the Frings audiogenic seizure-susceptible mouse, 2) the mouse corneal kindling model of partial epilepsy, and 3) the 6 Hz model of pharmacoresistant epilepsy. NAX 5055 was not active in the traditional maximal electroshock and subcutaneous pentylenetetrazol seizure models. Unlike most antiepileptic drugs, NAX 5055 showed high potency in the 6 Hz model of epilepsy across all three different stimulation currents; i.e., 22, 32 and 44 mA, suggesting a potential use in the treatment of pharmacoresistant epilepsy. Furthermore, NAX 5055 was found to be biologically active after intravenous, intraperitoneal, and subcutaneous administration, and efficacy was associated with a linear pharmacokinetic profile. The results of the present investigation suggest that NAX 5055 is a first-in-class neurotherapeutic for the treatment of epilepsy in patients refractory to currently approved antiepileptic drugs.

Design, synthesis, and characterization of high-affinity, systemically-active galanin analogues with potent anticonvulsant activities.

Galanin is an endogenous neuropeptide that modulates seizures in the brain. Because this neuropeptide does not penetrate the blood-brain barrier, we designed truncated galanin analogues in which nonessential amino acid residues were replaced by cationic and/or lipoamino acid residues. The analogues prevented seizures in the 6 Hz mouse model of epilepsy following intraperitoneal administration. The most active analogue, Gal-B2 (NAX 5055), contained the -Lys-Lys-Lys(palmitoyl)-Lys-NH(2) motif and exhibited high affinity for galanin receptors (K(i) = 3.5 nM and 51.5 nM for GalR1 and GalR2, respectively), logD = 1.24, minimal helical conformation and improved metabolic stability. Structure-activity-relationship analysis suggested that cationization combined with position-specific lipidization was critical for improving the systemic activity of the analogues. Because the anticonvulsant activity of galanin is mediated by the receptors located in hippocampus and other limbic brain structures, our data suggest that these analogues penetrate into the brain. Gal-B2 may lead to development of first-in-class antiepileptic drugs.