**3. Thermoregulatory neuronal networks comprising PO/AH thermosensitive neurons**

The neuronal network controlling brow adipose tissue (BAT) thermogenesis and the fever response has been studied extensively. Thermal and chemical stimulation in the PO/AH combined with selective hypothalamic transections have shown that warm-sensitive PO/AH neurons send efferent signals to loci involved in the control of BAT thermogenesis) [14,15]. PO/AH warming or injection of glutamate suppressed BAT thermogenesis thus suggesting that it is controlled by warm-sensitive neurons ([14]). Studies using combined retrograde labeling and immunocytochemistry revealed that EP3 prostanoid receptor-positive GABAergic PO/AH neurons project to the sympathetic premotor neurons in the rostral raphe pallidus (rRPA). The projections are either direct or via the dorsomedial hypothalamus (DMH) [16,17]. Bilateral microinjections of GABA-A receptor agonists or antagonists into the rRPa or DMH, blocked the fever induced by intra-PO/AH PGE2 applications. The central role of EP3-receptors in PO/AH neurons in the fever response was demonstrated also by local knockdown of its expression [18]. The role of the DMH in the control of BAT thermogenesis was proven also by direct chemical or electrical stimulation [19]. These studies clearly revealed a tonic GABAergic inhibition of the DMH and rRPA by the PO/AH as crucial for basal thermoregulation and hyperthermic responses.

194 Hyperthermia

thermosensitive neuronal populations [2].

thermosensitivity during NREM sleep [7].

frequency of IPSPs and of miniature IPSPs [5,13].

**thermosensitive neurons** 

**3. Thermoregulatory neuronal networks comprising PO/AH** 

The neuronal network controlling brow adipose tissue (BAT) thermogenesis and the fever response has been studied extensively. Thermal and chemical stimulation in the PO/AH combined with selective hypothalamic transections have shown that warm-sensitive PO/AH neurons send efferent signals to loci involved in the control of BAT thermogenesis) [14,15]. PO/AH warming or injection of glutamate suppressed BAT thermogenesis thus suggesting that it is controlled by warm-sensitive neurons ([14]). Studies using combined retrograde

hypothalamic temperature (Thy). Hammel and collegues proposed that a particular net thermoregulatory response was proportional to (Thy –Tset), where Tset was conceived as a hypothetical set reference, a complex parameter representing the state of activity of

Since the first extracellular single-unit study [3], which found that some PO/AH neurons, termed "warm-sensitive", increase their firing rates when Thy increases, it has been considered that they represent the central thermoreceptors. The other PO/AH neurons, which display little temperature-dependent changes in firing rate, are considered temperature-insensitive. PO/AH thermosensitive neurons respond not only to changes in local and peripheral temperature, but also to hormones, osmolarity and glucose concentration (reviewed in [1]). These findings suggest that these neurons, or a subgroup of them, also play a role in the integration of thermoregulation with other homeostatic processes such as control of metabolic rate (glucose sensing). The thermosensitivity of PO/AH neurons is a plastic property both *in vivo* and *in vitro*. It has been found that the thermosensitivity can change rapidly in the presence of the pyrogens PGE2 [4] or IL-1 [5,6]. Slower changes are observed in some warm-sensitive PO/AH neurons which decrease their

The mechanism of intrinsic thermosensitivity of PO/AH neurons is controversial. Boulant and colleagues consider that the increased firing rate is solely due to an increased rate of rise of the prepotential which precedes an action potential (reviewed in [8]. Other studies describe strong depolarizations (10 mV or larger) in response to heating which cause the increased firing rate in warm-sensitive neurons ([9]). In cultured PO/AH neurons both phenomena are present, however they occur also in temperature-insensitive neurons [10]. Finally, the warming-activated inward current was found to be tetrodotoxin (TTX) insensitive in some studies [9,10,11] and TTX-sensitive (i.e. mediated by voltage-gated Na channels) in others [12]. The question remains open as to whether all warm-sensitive PO/AH neurons have some intrinsic thermosensitivity or if they can also display thermosensitive firing that is synaptically-driven [10]. We have shown that prostaglandin E2 (PGE2), a well established endogenous pyrogen, increases the thermosensitivity and firing rates of PO/AH neurons by decreasing the frequency of IPSPs [4]. In contrast, IL-1β hyperpolarizes a different set of PO/AH neurons and reduces their thermosensitivity by increasing the Recent studies have established also the existence of direct glutamatergic projections from the PO/AH [20,21] as well as from the lateral hypothalamus [22] to the rRPA that control thermoregulation. Some glutamatergic PO/AH neurons projecting to rRPa are also peptidergic [20].

Shivering, a different mechanism of thermogenesis, is also controlled by the PO/AH.

Injections of excitatory amino acids as well as PO/AH warming inhibited cold-induced shivering suggesting that this mechanism, similar to BAT thermogenesis, is controlled by PO/AH warm-sensitive neurons [15]. In contrast, cooling of the PO/AH had little effect on cold-induced shivering. The efferent signals mediating shivering descend in the medial forebrain bundle [23].

Evaporative heat loss is also controlled by a network originating in the PO/AH since it is the only brain region that induces salivary secretion when warmed [23]. Preoptic warming, glutamate injections as well as electrical stimulation facilitate salivary secretion [5,13,15] as well as body extension [24], another aspect of evaporative heat loss.

The neuronal network controlling cutaneous blood flow also originates in the PO/AH.

Warming the PO/AH elicits skin vasodilation [25], by activation of warm-sensitive neurons [15]. The efferent pathway descends through the medial forebrain bundle [23]. It is believed that warm-sensitive neurons in PO/AH send excitatory signals to vasodilator neurons and inhibitory signals to vasoconstrictor neurons. PO/AH neurons controlling cutaneous blood flow project to the rostral medullary raphe region directly [26], suggesting that distinct populations of PO/AH neurons control thermogenesis and cutaneous vasomotion. This concept is supported by the observation that the two thermoregulatory mechanisms are activated at different threshold temperatures [27].

Little is known about the local networks comprising warm-sensitive and temperatureinsensitive neurons. One study found little thermosensitivity in the frequency of spontaneous IPSPs and EPSPs recorded in either warm-sensitive or temperature-insensitive PO/AH neurons, suggesting that the former do not send local projections [28]. This study also compared the morphologies of w-s and t-i PO/AH neurons filled with Lucifer yellow or biocytin. The dendritic arbors were characterized, however the axonal projections could not be described. This finding may reflect technical limitations or the fact that PO/AH neurons send few local projections [28]. Our studies in mice have not found evidence for local projections of PO/AH GABAergic neurons but have revealed reciprocal connections of PO/AH glutamatergic neurons [21].

Histaminergic Modulation of Body Temperature and Energy Expenditure 197

glutamate [40], acetylcholine and noradrenaline [41]. H3Rs are coupled to Gi/o and inhibit high voltage activated Ca2+ channels, a typical mechanism for the regulation of transmitter release. In rat, there are three functional splice variants of the H3R. In mouse, both RNase protection assay experiments and PCR results indicate that only one isoform of the H3R is present [42] which is coupled negatively to cAMP. H3Rs also activated the phospholipase

In summary, H1Rs and H2Rs have mostly excitatory actions on neurons or potentiate excitatory inputs. By contrast, H3-receptor activation causes autoinhibition of TMN neurons and inhibition of neurotransmitter release. Recent morphological and physiological studies

The role of CNS histamine in thermoregulation has been established in various organisms from invertebrates [46] to lower vertebrates [47] as well as mammals. Early studies in mammals have reported a role of hypothalamic histamine in the control of body temperature [48]. The preoptic area/ anterior hypothalamus (PO/AH), region which contains temperature-sensitive neurons and regulates the thermoregulation setpoint, is the main locus in which histamine affects body temperature [49]. Histamine injected in the medial preoptic nucleus (MPON) induces hyperthermia. Similarly, intra-MPON injection of a histamine-N-methyltransferase inhibitor (which results in a local increase of histamine concentration) also produces hyperthermia [50]. Behavioral temperature selection studies suggest that preoptic histamine signaling affects both the set point of the hypothalamic thermostat, as well as heat loss mechanisms [51]. Both H1 and H2 receptors have been

Some studies suggest a hyperthermic tone due to histamine signaling. Thus, premedication with a H2R antagonist before general anesthesia augments core hypothermia during this procedure [52]. In pathological conditions histamine appears to mediate hypothermic responses. Ionizing radiation induces hypothermia that can be blocked by H1R and H2R antagonists applied centrally [53]. Exposure of the head to ionizing radiation appers to

Peripherally, histamine is involved in the rise of skin blood flow during whole body heating [54]. Similarly, combined H1R and H2R antagonists diminish the alcohol-induced flushing

More recent observations using transgenic models further indicate a role of histamine signaling in thermoregulation are. Thus H3R-/- transgenic mice display a lowered core body temperature suggesting that these receptors mediate a tonic hyperthermic action [56]. Other studies point to a hypothermic action of histamine, mediated by H1 subtype receptors. Thus, anaphylaxis induced hypothermia is not observed in HDC(-/-) mice or in the presence of H1R antagonists [57]. Also, IL-1β-induced thermogenesis is potentiated by depletion of

A2 (PLA2) via the Gi/o proteins which results in production of arachidonic acid [43].

suggest the presence of H3 receptors also postsynaptically [21,44,45].

stimulate histamine release from brain-resident mast-cells [53].

implicated in these responses.

in individuals of Oriental origin [55].

hypothalamic histamine [58].

**5. Central histaminergic modulation of core body temperature** 
