In 1995, when leptin was discovered and shown to make fat mice thinner, hopes quickly soared for a pill to treat the nation's pervasive obesity problem. Yet while some clinical trials are already under way, it is becoming clear that no such drug is yet within reach.
To wary observers of drug development, the leptin story represents yet another cautionary tale of society's insatiable appetite for quick fixes, soon spoiled by the realization that much more basic research is needed.
| Joel Elmquist tries to sort out the brain pathways of leptin, a hormone studied intensely because of its potential role in human obesity. |
But to researchers like Joel Elmquist, assistant professor of neurology at Beth Israel Deaconess, leptin has opened up a fascinating area of inquiry. Leptin piqued the curiosity of this neuroanatomist, because even though it is made by fat cells throughout the body, it acts on the brain. In the January 20 Proceedings of the National Academy of Sciences, Elmquist, Clifford Saper, the James Jackson Putnam professor of neurology, and Jeffrey Flier, professor of medicine, both at BID, and others describe some of the complex brain pathways that leptin activates. The researchers are also beginning to connect these pathways to some of the hormone's many effects in an effort to understand how leptin informs the brain about the body's metabolic state and how the brain responds.
"To understand leptin in disease and attempt to treat it, you have to understand the basic way it works. And it is clear that it exerts its major effects through the brain, so this is where you have to look," Elmquist says.
In the Driver's Seat
In only three years, researchers have learned that leptin affects almost every neuroendocrine axis--chains of hormonal command originating in the brain--and that it does so by acting on nerve cells in the hypothalamus. This region, deep inside the brain, has far-reaching control. Through connections with the pituitary gland, the hypothalamus regulates the body's hormonal state, and through connections to the autonomic nervous system, it helps run many body functions without our conscious control: blood pressure, body temperature, biological rhythms, eating behavior, and weight.
Made when energy stores are adequate, leptin decreases appetite while increasing body temperature and energy expenditure. It affects production of growth hormone and thyroid hormone and influences reproduction. Insufficient leptin is associated with infertility in rodents. Leptin is such a multifarious molecule that ultimately, when researchers understand it better, it may be considered one of the most important hormones we have, says Elmquist.
Previous research had already identified one area in the hypothalamus, the arcuate nucleus, as being involved in leptin function. But Elmquist and his collaborators reasoned that one pathway was not possibly enough to accommodate all the "traffic" that would be generated as leptin exerts its diverse effects.
The researchers started their study by injecting four nanoliters of a tracer dye into a brain area of rats, which they suspected of being the main output area for responses to leptin, the paraventricular nucleus of the hypothalamus (PVH, see image). Nerve endings in the PVH took up the dye and transported it back to the cell body. Then the scientists injected the rats with intravenous leptin and tested for the presence of a marker that neurons sensitive to leptin express after exposure to the hormone. Neurons labeled twice, with the tracer and the activity marker, were those that responded to leptin and sent that signal on to the PVH.
| Leptin Circuits: The fat-derived hormone leptin enters the brain's hypothalamus to exert its many effects. An area called the PVH (paraventricular hypothalamic nucleus) probably uses its connections to the pituitary gland and the autonomic nervous system to execute the brain's responses to leptin. The dorsomedial hypothalamic nucleus (DMH) assists the PVH by controlling the flow of information to it. The subparaventricular zone (SPVZ) integrates metabolic information with time information to adjust eating-related hormone rhythms. |
The researchers found two new pathways. Even though receptors for leptin are found spread all over the hypothalamus, suggesting that neurons everywhere in this region somehow participate in leptin biology, neurons from only one area, the dorsomedial hypothalamic nucleus, project directly to the PVH. Elmquist suspects that this connection mediates leptin's effects on the autonomic nervous system, possibly helping to adjust insulin levels and calm the body after a meal, for example.
The researchers suggest that the dorsomedial hypothalamic nucleus serves as a gatekeeper controlling the flow of information to the PVH, like an assistant outside the chief executive's office. The analysis and processing of leptin information, Elmquist suggests, might happen elsewhere, before information flows to the PVH.
The second leptin pathway illustrates that point. Leptin-responsive neurons in another region, the ventromedial hypothalamic nucleus, project to an area called the subparaventricular zone (SPVZ). Serendipity helped Elmquist find this route. The PVH, into which he aimed to inject his dye, is only two hundredths of an inch wide. Occasionally, he missed and squeezed dye into the neighboring SPVZ, visualizing neurons that project there, not to the ventromedial hypothalamic nucleus.
This ventromedial hypothalamic nucleus-SPVZ connection, it turned out, made perfect sense in light of previous findings. The area hosting the circadian pacemaker, which generates daily rhythms based on light, sends most of its nerve output to the SPVZ. Moreover, daily hormone levels are known to rise and fall in rhythmic patterns, which change when eating routines change. Finally, leptin is known to regulate hormone secretion. Tying all this together, the Harvard researchers suggest that the SPVZ receives nutritional information from leptin-sensitive neurons in the ventromedial hypothalamic nucleus and integrates that with circadian information coming from the pacemaker. It then sends its output--possibly orders to change certain hormone levels--to the dorsomedial hypothalamic nucleus gatekeeper, which forwards the message to the PVH for the order to be executed through contacts at the pituitary gland.
Blazing Trails
This work represents just the beginning of scientists' attempts to unravel leptin's brain connections, says Elmquist. In 1995, Saper wrote that nerve groups involved in autonomic control form an "incestuous web" in which most nerve groups are connected to most other groups.
These days, neuroanatomists try to untangle this web by adding activity markers--such as the gene used in this study--to their usual tracing methods. That way, researchers can visualize only those pathways that are active when the information of interest, in this case leptin, travels through the brain, ignoring the bewildering jumble of unrelated connections. That approach visualizes major thoroughfares, such as the dorsomedial hypothalamic nucleus-PVH link, as well as smaller side streets where special tasks are processed, such as the ventromedial hypothalamic nucleus-SPVZ link.
--Gabrielle Strobel