Efferent projection from the preoptic area for the control of non-shivering thermogenesis in rats

Abstract

1. To investigate the characteristics of efferent projections from the preoptic area for the control of non-shivering thermogenesis, we tested the effects of thermal or chemical stimulation, and transections of the preoptic area on the activity of interscapular brown adipose tissue in cold-acclimated and non-acclimated anaesthetized rats. 2. Electrical stimulation of the ventromedial hypothalamic nucleus (VMH) elicited non-shivering thermogenesis in the brown adipose tissue (BAT); warming the preoptic area to 41.5 C completely suppressed the thermogenic response. 3. Injections of d, l-homocysteic acid (DLH; 0.5 mM, 0.3 microliter) into the preoptic area also significantly attenuated BAT thermogenesis, whereas injections of control vehicle had no effect. 4. Transections of the whole hypothalamus in the coronal plane at the level of the paraventricular nucleus induced rapid and large rises in BAT and rectal temperatures. This response was not blocked by pretreatment with indomethacin. The high rectal and BAT temperatures were sustained more than 1 h, till the end of the experiment. Bilateral knife cuts that included the medial forebrain bundle but not the paraventricular nuclei elicited similar rises in BAT and rectal temperatures. Medial knife cuts had no effect. 5. These results suggest that warm-sensitive neurones in the preoptic area contribute a larger efferent signal for non-shivering thermogenesis than do cold-sensitive neurones, and that the preoptic area contributes a tonic inhibitory input to loci involved with non-shivering thermogenesis. This efferent inhibitory signal passes via lateral, but not medial, hypothalamic pathways.

Figure 1. Locations of the tips of thermode assembly (A) and locations of electrode tips (B)

The filled circles indicate the locations of the thermode tip (A) and the electrode tip (B). AC, anterior commissure; OX, optic chiasm; ic, internal capsule; f, fornix; mfb, medial forebrain bundle; ml, medial lemniscus; mt, mammillothalamic tract; opt, optic tract; DM, dorsomedial hypothalamic nuclei; 3V, 3rd ventricle; VMH, ventromedial hypothalamic nuclei; TS, triangular septal nuclei; MPO, medial preoptic nuclei.

Figure 2. Changes in brown adipose tissue thermogenic response following electrical stimulation of the ventromedial hypothalamic nucleus before, during and after warming of the preoptic area

A, temperature of interscapular brown adipose tissue (T_bat), rectal temperature (T_re), and hypothalamic temperature (T_hy); the filled bars indicate electrical stimulation (ES) of the VMH: 0.075 mA, 33 Hz, 0.5 ms. B, the filled circle indicates the location of the thermode tip (left), and the electrode tip (right). Abbreviations as in Fig. 1.

Figure 3. Increases in brown adipose tissue temperature in response to VMH electric stimulation before, during and after preoptic (PO) warming to 41.5 °C

Electrical stimulation (ES) of the VMH (0.075 mA, 33 Hz, 0.5 ms) was applied for 2 min before (•), during (▪), and after (○) preoptic warming. Values are means and vertical bars are ± s.e.m. (n = 7).

Figure 4. Locations of cannula tips for drug microinjection (A) and electrode tips for electrical stimulation (B)

The filled circles indicate the locations of cannula and electrode tips in the DLH-injected group and the open triangles indicate those in the control group. Abbreviations as in Fig. 1.

Figure 5. Effects of injection of DLH (A) or control vehicle (B) into the preoptic area on the rise in T_bat and T_re by electrical stimulation of the VMH in separate cold-acclimated rats

A and B, the open bars indicate application of DLH (0.5 mm, 0.3 μl) or control vehicle (phosphate-buffered saline, PBS) and the filled bars indicate electrical stimulation (ES, 0.075 mA, 33 Hz, 0.5 ms). C, the filled circles indicate the location of DLH injection and electrical stimulation in the DLH-injected rat and the open circles indicate the location of PBS injection and electrical stimulation in the control rat. Abbreviations as in previous figures.

Figure 6. Changes in brown adipose tissue temperature in response to VMH electrical stimulation before and after the injection of DLH (A) or control vehicle (B) into the preoptic area

Values are means and vertical bars are ± s.e.m. (n = 6). Asterisks indicate significant difference from the value before drug injection: *P < 0.05; **P < 0.01.

Figure 7. Extent of knife cuts

The extent of transection in complete (A, n = 2), lateral (B, n = 3) and medial cuts (C, n = 3). LV, lateral ventricle; sm, stria medullaris thalami; PV, paraventricular nuclei; LH, lateral hypothalamic area. Other abbreviations as in previous figures. Different forms of hatching and shading indicate the extent of transection in different animals.

Figure 8. Effects of complete cuts on brown adipose tissue thermogenesis

L and R indicate knife cuts applied on the left and right sides, respectively. In the inset, the hatched areas indicate the extent of the knife cut. Abbreviations as in previous figures.

Figure 9. Effects of lateral cuts on brown adipose tissue thermogenesis

L and R indicate knife cuts applied on the left and right sides, respectively. In the inset, the hatched areas indicate the extent of the knife cut. Abbreviations as in previous figures.

Figure 10. Lack of effect of medial cuts on brown adipose tissue thermogenesis

A, L and R indicate knife cuts applied on the left and right sides, respectively. B, effect of preoptic warming was tested after the knife cut. C, the filled circle indicates the location of the thermode tip (left), and the electrode tip (right). The hatched areas indicate the extent of the knife cut (middle). Abbreviations and symbols as in previous figures.

Figure 11. Lack of effect of indomethacin injection (s.c. 15 mg kg⁻¹) on brown adipose tissue thermogenesis induced by complete knife cuts

L and R indicate knife cuts applied on the left and right sides, respectively. In the left inset, the hatched areas indicate the extent of the knife cut. In the right inset, the filled circle indicates the location of the electrode tip. Abbreviations as in previous figures.