Anticonvulsants As Anxiolytics, Part 2 Pregabalin And

B R A I N S T O R M S Clinical Neuroscience Update

Anticonvulsants as Anxiolytics, Part 2

Pregabalin and Gabapentin as α2δ Ligands at Voltage-Gated Calcium Channels

Stephen M. Stahl, M.D., Ph.D.

α δ Issue: Anticonvulsants that act as ligands at 2 subunits of voltage- gated calcium channels may also prove to be novel anxiolytics.

ctivation of fear circuits tion of neurotransmitter release.5Ð10 If increased during neurotransmission, is a leading hypothesis this reduction happens in amygdala- neurotransmitter release is thus en- A for explaining symptoms centered fear circuits, it might have hanced. On the other hand, when the 1Ð3 α δ in anxiety disorders, and returning anxiolytic actions. 2 ligands pregabalin and gabapen- neurotransmission in these circuits to tin bind to these channels and thereby a more normal pattern may reduce KNOW YOUR CALCIUM CHANNELS: decrease calcium flow through them, certain symptoms.4 For example, anti- VOLTAGE-SENSITIVE OR the release of several neurotrans- convulsants may theoretically reduce LIGAND-GATED? mitters from presynaptic neurons is seizures by decreasing excessive out- decreased.5Ð10 put from epileptic neurons, and could, Most clinicians have heard of cal- Another subtype of voltage-gated by analogy, reduce symptoms of anxi- cium channels, but only recently has it calcium channels is an L channel, ety if these agents were also able to become clear that there are multiple which resides in membranes of vascu- decrease neuronal activation within subtypes of calcium channels, some lar smooth muscle and is blocked by fear circuits.1Ð4 A newly discovered regulated directly by voltage and antihypertensives commonly known mechanism of reducing neurotrans- others regulated directly by neuro- as “calcium channel blockers.”11 Such mission is employed by the anticon- transmitters, with each having unique drugs lower blood pressure but have vulsants pregabalin and gabapentin: physiologic functions as well as differ- neither anticonvulsant nor anxiolytic they bind to a specific subunit of one ential selectivity for specific drugs.11 actions. type of calcium channel—namely, the For example, calcium channels regu- α δ 2 subunit of voltage-sensitive cal- lated by the charge across the mem- Ligand-Gated Channels cium channels—which leads to reduc- brane where they reside are called An example of a ligand-gated “voltage sensitive” or “voltage gated” calcium channel is the NMDA (or whereas calcium channels regulated N-methyl-D-aspartate) glutamate re- by neurotransmitters are called “ligand ceptor complex, one of the key

BRAINSTORMS is a monthly section of The gated.” mediators of excitatory postsynaptic Journal of Clinical Psychiatry aimed at neurotransmission.12 A novel drug providing updates of novel concepts emerging from the neurosciences that have relevance to Voltage-Sensitive Channels for the treatment of Alzheimer’s dis- the practicing psychiatrist. Two subtypes of calcium channels ease, memantine, binds loosely to From the Neuroscience Education Institute in the voltage-sensitive family— the NMDA receptor complex, and in Carlsbad, Calif., and the Department of Psychiatry at the University of California San known as N and P/Q channels— the hallucinogen phencyclidine binds Diego. regulate neurotransmitter release dur- tightly to the NMDA receptor com- 11 α δ Reprint requests to: Stephen M. Stahl, M.D., ing synaptic neurotransmission. On plex, but 2 ligands do not bind Ph.D., Editor, BRAINSTORMS, Neuroscience Education Institute, 5857 Owens Street, Ste. 102, the one hand, when calcium flow to the NMDA receptor complex. Carlsbad, CA 92009. through these presynaptic channels is Thus, postsynaptic ligand-gated cal-

cium channels may cooperate with presynaptic voltage-gated calcium Take-Home Points channels during neurotransmission, ◆ but their functions and pharmacology Activation of neurotransmission in fear circuits may underlie are quite unique. symptoms in anxiety disorders. ◆ α δ Agents such as pregabalin and gabapentin that target the 2 COULD α2δ LIGANDS BE NOVEL ANXIOLYTICS? subunits of voltage-gated calcium channels can reduce neurotransmission in activated neuronal circuits by reducing the Preclinical studies have estab- release of neurotransmitters. α δ lished the anxiolytic actions of the 2 ◆ Reducing neurotransmission in fear circuits could hypothetically 13 ligand pregabalin. Some preliminary reduce symptoms in anxiety disorders. clinical data have suggested that the α δ 2 ligand gabapentin may have anxiolytic properties after a case series re- ported marked clinical improvement Compared with studies for gaba- ine,20 and these findings have been with gabapentin as adjunctive therapy pentin, much better designed studies filed with the U.S. FDA for marketing in patients with treatment-refractory of anxiety have been conducted for approval in this indication. Prelimi- anxiety disorders.14 Also, a placebo- pregabalin, a higher-potency analog nary findings with pregabalin in so- controlled study15 in social phobia to gabapentin with better bioavail- cial phobia are also promising, and has shown that gabapentin reduced ability and potentially more con- studies in other anxiety disorders, in- anxiety symptoms. Another study16 sistent clinical effects. Multicenter, cluding panic disorder, are ongoing. in panic disorder found no overall placebo-controlled comparator trials Thus, it appears that the high-potency α δ gabapentin/placebo differences, only of pregabalin in generalized anxiety 2 ligand pregabalin is promising to improvement in the more severely ill disorder suggest comparable efficacy become a new anxiolytic with a novel patients. to benzodiazepines17Ð19 and venlafax- mechanism of action.

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