M. J. Cano-Cebrián

Intestinal Absorption Enhancement Via the Paracellular Route by Fatty Acids, Chitosans and Others: A Target for Drug Delivery

Peroral delivery of hydrophilic drugs is one of the greatest challenges in biopharmaceutical research. Hydrophilic drugs usually present low bioavailability after oral administration. One of the causes of this low bioavailability is their poor intestinal permeation through the paracellular pathway. This pathway is actually restricted by the presence of tight junctions at the apical side of the enterocytes. In the last few years, great interest has been focused on the structure and cellular regulation of tight junctions, materializing in more in-depth knowledge of this intestinal barrier. Simultaneously, and on the basis of this understanding, continuous efforts are being made to develop agents that can modulate tight junctions and magnify the paracellular permeability of hydrophilic compounds without causing significant intestinal damage. This review focuses on strategies to improve the paracellular permeation of poorly absorbed drugs as a way to enhance their bioavailability after oral administration. Most of the research on this subject has been carried out using in vitro models (mainly Caco-2 cell monolayers), which yield useful information on the potential effects and mechanisms of action of absorption-enhancing compounds. However, in vivo studies, which are much more scarce, are needed to confirm the effects of potential enhancers and to evaluate the suitability of including these compounds as excipients in drug formulation. We review the in vitro and in situ studies involving the most promising paracellular permeation enhancers (e.g., medium chain fatty acids and chitosan and its derivatives), analyzing the degree of drug absorption enhancement achieved, as well as the potential associated toxicity. The few studies performed in vivo are also presented. In addition, the findings of recent absorption enhancers, such as zonula occludens toxin or thiolated polymers, are reviewed.

Hippocampal dopamine receptors modulate the motor activation and the increase in dopamine levels in the rat nucleus accumbens evoked by chemical stimulation of the ventral hippocampus.

A number of studies have shown that chemical stimulation (using N-methyl-D-aspartate (NMDA) infusions) or electrical stimulation of the ventral hippocampus (VH) elicits locomotor activation and sustained increases in nucleus accumbens (NAc) dopamine (DA) levels in rodents. How DA neurotransmission in NAc is involved in these effects has also been well established. However, the modulatory role of the DA receptors located in VH is not yet fully understood. The purpose of this study was to characterize the role played by VH D1 and D2 subtype receptors in both the locomotor activation and NAc DA increases induced by NMDA stimulation of the VH. This was assessed by studying how retrodialysis application of NMDA (50 mM, 10 min) affects motor activity and NAc DA levels during simultaneous retrodialysis administration of the D1/D5 receptor antagonist SCH 23390 (100 and 250 μM, 60 min) or the D2 receptor antagonist raclopride (100 and 250 μM, 60 min). SCH 23390 attenuated or completely abolished NMDA-evoked locomotor activation and the concurrent increase in NAc DA levels. On the other hand, raclopride was initially able to attenuate the effects of VH NMDA stimulation. However, in the last phase of the experiments, animals showed an important increase in clonic seizure activity with a simultaneous and dramatic increase in NAc DA levels. Our results show that the NMDA receptor-mediated effects in the VH require both D1 and, probably, D2 receptors and suggest that DA in VH strongly modulates the excitatory outputs from this brain area.

Hippocampal dopamine receptors modulate cFos expression in the rat nucleus accumbens evoked by chemical stimulation of the ventral hippocampus.

Recently, we have shown that D1 and D2 receptors in the ventral hippocampus (VH) modulate both the locomotor activation and the increase in dopamine (DA) levels in the rat nucleus accumbens (NAc) induced by NMDA stimulation of the VH. In the present study we analyze the possible role of VH D1 and D2 receptors in the modulation of the cFos expression in NAc (core and shell subregions) and in dorsal striatum. This was assessed by immunohistochemical analysis of cFos expression in the rat brains after retro-dialysis application of NMDA (50mM, 10 min) into VH, in absence and in presence of either the D1/D5 receptor antagonist SCH 23390 (100 and 250 microM, 60 min) or the D2 receptor antagonist raclopride (100 and 250 microM, 60 min). NMDA induced a robust increase in the cFos expression in the NAc shell, both in the ipsilateral and contralateral side. No statistically significant increases were observed in the NAc core and in the dorsal striatum. Simultaneous application of SCH 23390 and NMDA into the VH attenuated the NMDA-evoked cFos expression in NAc shell. In contrast, raclopride had no significant effect. Our present results show that the NMDA receptor mediated effects in the VH require D1 receptors and suggest that DA in VH strongly modulates the excitatory outputs from this brain area.

Improved effect of the combination naltrexone/D-penicillamine in the prevention of alcohol relapse-like drinking in rats

Opioid antagonists are licensed drugs for treating alcohol use disorders; nonetheless, clinical studies have evidenced their limited effectiveness. Preclinical findings indicate that opioid receptor (OR) antagonists, such as naltrexone (NTX), reduce the alcohol deprivation effect (ADE). However, a detailed analysis of published data shows the existence of a delayed increase in ethanol consumption after continuous OR blockade, a phenomenon originally called as ‘delayed ADE’. We have recently reported that D-penicillamine (DP) is able to prevent ADE through a mechanism dependent on the inactivation of acetaldehyde, the main metabolite of ethanol. Hypothetically, OR activation would be triggered by acetaldehyde after ethanol consumption. Hence, we conjecture that the combination of NTX and DP, due to their distinct but complementary mechanisms to impede OR activation, may be more efficacious in the prevention of the ADE and, specifically, the ‘delayed ADE’. Herein, we compare the effects of the combination NTX/DP (NTX: 2×5 mg/kg SC injection daily/DP: SC infusion (0.25 mg/h)) versus NTX on the ADE in long-term ethanol-experienced rats. As expected, NTX-treated animals displayed a delayed ADE. However, NTX/DP treatment prevented this delayed effect. Our present data indicate that this combination therapy shows an adequate anti-relapse preclinical efficacy being able to overcome the preclinical limitations of NTX alone.

Regional differences in mu-opioid receptor-dependent modulation of basal dopamine transmission in rat striatum

The nigrostriatal dopamine system is implicated in the regulation of reward and motor activity. Dopamine (DA) release in dorsal striatum (DS) is controlled by the firing rate of DA neurons in substantia nigra pars compacta. However, influences at terminal level, such as those involving activation of mu opioid receptors (MORs), can play a key role in determining DA levels in striatum. Nonetheless, published data also suggest that the effect of opioid drugs on DA levels may differ depending on the DS subregion analyzed. In this study, in vivo microdialysis in rats was used to explore this regional dependence. Changes in basal DA levels induced by local retrodialysis application of DAMGO (selective MORs agonist) in three different subregions of DS along the rostro-caudal axis were studied. Our results indicate that whereas administration of 10μM DAMGO into the rostral and caudal DS significantly reduced DA levels, in medial DS an increase in DA levels was observed. These data reveal a regional-dependent MOR modulation of DA release in DS, similar to that described in the ventral striatum. Our findings may lead to a better understanding of the nigrostriatal DA system regulation.

Pre-Clinical Studies with D-Penicillamine as a Novel Pharmacological Strategy to Treat Alcoholism: Updated Evidences

Ethanol, as other drugs of abuse, is able to activate the ventral tegmental area dopamine (VTA-DA) neurons leading to positively motivational alcohol-seeking behavior and use, and, ultimately to ethanol addiction. In the last decades, the involvement of brain-derived acetaldehyde (ACD) in the ethanol actions in the mesolimbic pathway has been widely demonstrated. Consistent published results have provided a mechanistic support to the use of ACD inactivating agents to block the motivational and reinforcing properties of ethanol. Hence, in the last years, several pre-clinical studies have been performed in order to analyze the effects of the sequestering ACD agents in the prevention of ethanol relapse-like drinking behavior as well as in chronic alcohol consumption. In this sense, one of the most explored interventions has been the administration of D-Penicillamine (DP). These pre-clinical studies, that we critically summarize in this article, are considered a critical step for the potential development of a novel pharmacotherapeutic strategy for alcohol addiction treatment that could improve the outcomes of current ones. Thus, on one hand, several experimental findings provide the rationale for using DP as a novel therapeutic intervention alone and/or in combination to prevent relapse into alcohol seeking and consumption. On the other hand, its effectiveness in reducing voluntary ethanol consumption in long-term experienced animals still remains unclear. Finally, this drug offers the additional advantage that has already been approved for use in humans, hence it could be easily implemented as a new therapeutic intervention for relapse prevention in alcoholism.