Temperature-insensitive Neurons Research Articles

Arginine vasopressin effects on membrane potentials of preoptic area temperature-sensitive and -insensitive neurons in rat hypothalamic tissue slices

Arginine vasopressin (AVP) plays a hypothermic regulatory role in thermoregulation and is an important endogenous mediator in this mechanism. In the preoptic area (POA), AVP increases the spontaneous firing and thermosensitivity of warm-sensitive neurons and decreases those of cold-sensitive and temperature-insensitive neurons. Because POA neurons play a crucial role in precise thermoregulatory responses, these findings indicate that there is an association between the hypothermia and changes in the firing activity of AVP-induced POA neurons. However, the electrophysiological mechanisms by which AVP controls this firing activity remain unclear. Therefore, in the present study, using in vitro hypothalamic brain slices and whole-cell recordings, we elucidated the membrane potential responses of temperature-sensitive and –insensitive POA neurons to identify the applications of AVP or V1a vasopressin receptor antagonists. By monitoring changes in the resting potential and membrane potential thermosensitivity of the neurons before and during experimental perfusion, we observed that AVP increased the changes in the resting potential of 50% of temperature-insensitive neurons but reduced them in others. These changes are because AVP enhances the membrane potential thermosensitivity of nearly 50% of the temperature-insensitive neurons. On the other hand, AVP changes both the resting potential and membrane potential thermosensitivity of temperature-sensitive neurons, with no differences between the warm- and cold-sensitive neurons. Before and during AVP or V1a vasopressin receptor antagonist perfusion, no correlation was observed between changes in the thermosensitivity and membrane potential of all neurons. Furthermore, no correlation was observed between the thermosensitivity and membrane potential thermosensitivity of the neurons during experimental perfusion. In the present study, we found that AVP induction did not result in any changes in resting potential, which is unique to temperature-sensitive neurons. The study results suggest that AVP-induced changes in the firing activity and firing rate thermosensitivity of POA neurons are not controlled by resting potentials.

The hyperthermic response to intra-preoptic area administration of agmatine in male rats

Agmatine is an endogenous biogenic amine that exerts various effects on the central nervous system. The hypothalamic preoptic area (POA, thermoregulatory command center) has high agmatine immunoreactivity. In this study, in conscious and anesthetized male rats, agmatine microinjection into the POA induced hyperthermic responses associated with increased heat production and locomotor activity. Intra-POA administration of agmatine increased the locomotor activity, the brown adipose tissue temperature and rectum temperature, and induced shivering as demonstrated by increased neck muscle electromyographic activity. However, intra-POA administration of agmatine almost had no impact on the tail temperature of anesthetized rats. Furthermore, there were regional differences in the response to agmatine in the POA. The most effective sites for the microinjection of agmatine to elicit hyperthermic responses were localized in the medial preoptic area (MPA). Agmatine microinjection into the median preoptic nucleus (MnPO) and lateral preoptic nucleus (LPO) had a minimal effect on the mean core temperature. Analysis of the in vitro discharge activity of POA neurons in brain slices when perfused with agmatine showed that agmatine inhibited most warm-sensitive but not temperature-insensitive neurons in the MPA. However, regardless of thermosensitivity, the majority of MnPO and LPO neurons were not responsive to agmatine. The results demonstrated that agmatine injection into the POA of male rats, especially the MPA, induced hyperthermic responses, which may be associated with increased BAT thermogenesis, shivering and locomotor activity by inhibiting warm-sensitive neurons.

Hyperthermia

There is scientific support for the concept of a temperature set point that is, maintenance of an optimal temperature for the metabolic processes that life depends on Nervous activity in the preoptic-anterior hypothalamus of the brain triggers heat losing (sweating, etc.) or heat generating (shivering and muscle contraction, etc.) activities through stimulation of the autonomic nervous system. The pre-optic anterior hypothalamus has been shown to contain warm sensitive, cool sensitive, and temperature insensitive neurons, to determine the body's temperature setpoint. As the temperature that these neurons are exposed to rises above 37 °C (99 °F), the rate of electrical discharge of the warm-sensitive neurons increases progressively. There is scientific support for the concept of a temperature set point that is, maintenance of an optimal temperature for the metabolic processes that life depends on Nervous activity in the preoptic-anterior hypothalamus of the brain triggers heat losing (sweating, etc.) or heat generating (shivering and muscle contraction, etc.) activities through stimulation of the autonomic nervous system. The pre-optic anterior hypothalamus has been shown to contain warm sensitive, cool sensitive, and temperature insensitive neurons, to determine the body's temperature setpoint. As the temperature that these neurons are exposed to rises above 37 °C (99 °F), the rate of electrical discharge of the warm-sensitive neurons increases progressively.

The Role of Prostaglandin E2 Synthesized in Rat Lateral Parabrachial Nucleus in LPS-Induced Fever

Introduction: The lateral parabrachial nucleus (LPBN) is considered to be a brain site of the pyrogenic action of prostaglandin (PG) E2 outside of the preoptic area. Yet, the role of the LPBN in fever following a systemic immune challenge remains poorly understood. Methods: We examined the expression of cyclooxygenase-2 (COX-2) and microsomal PGE synthase-1 (mPGES-1) in the LPBN after the intraperitoneal injection of lipopolysaccharide (LPS). We investigated the effects of LPBN NS-398 (COX-2 inhibitor) on LPS-induced fever, the effects of direct LPBN PGE2 administration on the energy expenditure (EE), brown adipose tissue (BAT) thermogenesis, neck muscle electromyographic activity and tail temperature, and the effects of PGE2 on the spontaneous firing activity and thermosensitivity of in vitro LPBN neurons in a brain slice. Results: The COX-2 and mPGES-1 enzymes were upregulated at both mRNA and protein levels. The microinjection of NS-398 in the LPBN attenuated the LPS-induced fever. Direct PGE2 administration in the LPBN resulted in a febrile response by a coordinated response of increased EE, BAT thermogenesis, shivering, and possibly decreased heat loss through the tail. The LPBN neurons showed a clear anatomical distinction in the firing rate response to PGE2, with the majority of PGE2-excited or -inhibited neurons being located in the external lateral or dorsal subnucleus of the LPBN, respectively. However, neither the firing rate nor the thermal coefficient response to PGE2 showed any difference between warm-sensitive, cold-sensitive, and temperature-insensitive neurons in the LPBN. Conclusions: PGE2 synthesized in the LPBN was at least partially involved in LPS-induced fever via its different modulations of the firing rate of neurons in different LPBN subnuclei.

Electrophysiological properties of thermosensitive neurons in slices of rat lateral parabrachial nucleus.

Both warm- and cold-sensitive neurons are found in the lateral parabrachial nucleus (LPB), a crucial relay for skin temperature information from the spinal cord to the preoptic area. The aims of this study were to investigate the electrophysiological properties of temperature-sensitive and -insensitive neurons in brain slices, and elucidate the basic mechanisms underlying the thermosensitivity of rat LPB neurons. In warm-sensitive neurons, temperature exerted no significant effects on resting membrane potential (RMP), threshold potential, and amplitude of the afterhyperpolarizing potential. However, warming significantly increased the prepotential rates of depolarization and the inactivation rates of potassium A current (IA) in warm-sensitive neurons, which in turn shortened their interspike interval and elevated the firing rate. In contrast, temperature had no significant effects on the depolarizing prepotentials and inactivation rate of IA in temperature-insensitive neurons. Besides, in cold-sensitive neurons, cooling and warming produced membrane depolarization and hyperpolarization, respectively, and there was a strong correlation between firing rate and membrane potential thermosensitivity. Nevertheless, temperature exhibited no significant effect on the depolarizing prepotential of cold-sensitive neurons. These results suggest that LPB neuronal warm sensitivity may reside in the temperature-dependent prepotentials and IA, while neuronal cold sensitivity might be mainly due to heat-induced changes in RMP.

Arginine vasopressin differentially modulates GABAergic synaptic transmission onto temperature-sensitive and temperature-insensitive neurons in the rat preoptic area.

The preoptic area (POA) of the hypothalamus, containing temperature-sensitive and temperature-insensitive neurons, plays a key role in specific thermoregulatory responses. Although arginine vasopressin (AVP) has been shown to induce hypothermia by increasing the firing activities of warm-sensitive neurons and decreasing those of cold-sensitive and temperature-insensitive neurons, the effects of AVP on POA GABAergic transmission remain unknown. Herein, inhibitory postsynaptic currents (IPSCs) of temperature-sensitive and temperature-insensitive neurons in POA slices were recorded using whole-cell patch clamp. By monitoring changes in GABAergic transmission during AVP treatment, we showed that AVP decreased the amplitudes and frequencies of spontaneous IPSCs in mostly warm-sensitive neurons and in some temperature-insensitive neurons but increased these parameters in other temperature-insensitive neurons. The IPSC amplitude was reduced for only cold-sensitive neurons. RT-PCR and Western blot analyses further confirmed the POA expression of V1a receptors and GABAA receptors, including the subunits α1, α2, α3, β2, β3 and γ2. The effects of AVP on IPSCs in temperature-sensitive and temperature-insensitive neurons were dependent on G proteins and intracellular Ca2+ . AVP-mediated modulation was associated with changes in the kinetic parameters (decay time, 10-90% rise time, half-width). Together, these results suggest that AVP, acting via V1a receptors but not V1b receptors, differentially modulates GABAergic synaptic transmission and fine-tunes the firing activities of temperature-sensitive and temperature-insensitive neurons in the rat POA.

Arginine vasopressin antagonizes the effects of prostaglandin E2 on the spontaneous activity of warm-sensitive and temperature-insensitive neurons in the medial preoptic area in rats.

Arginine vasopressin (AVP) plays an important role in thermoregulation and antipyresis. We have demonstrated that AVP could change the spontaneous activity of thermosensitive and temperature insensitive neurons in the preoptic area. However, whether AVP influences the effects of prostaglandin E2 (PGE2) on the spontaneous activity of neurons in the medial preoptic area (MPO) remains unclear. Our experiment showed that PGE2 decreased the spontaneous activity of warm-sensitive neurons, and increased that of low-slope temperature-insensitive neurons in the MPO. AVP attenuated the inhibitory effect of PGE2 on warm-sensitive neurons, and reversed the excitatory effect of PGE2 on low-slope temperature-insensitive neurons, demonstrating that AVP antagonized the effects of PGE2 on the spontaneous activity of these neurons. The effect of AVP was suppressed by an AVP V1a receptor antagonist, suggesting that V1a receptor mediated the action of AVP. We also demonstrated that AVP attenuated the PGE2-induced decrease in the prepotential's rate of rise in warm-sensitive neurons and the PGE2-induced increase in that in low-slope temperature-insensitive neurons through the V1a receptor. Together, these data indicated that AVP antagonized the PGE2-induced change in the spontaneous activity of warm-sensitive and low-slope temperature-insensitive neurons in the MPO partly by reducing the PGE2-induced change in the prepotential of these neurons in a V1a receptor-dependent manner.

In vitro thermosensitivity of rat lateral parabrachial neurons

The lateral parabrachial (LPB) neurons play a pivotal role in the thermoregulatory afferent pathway by transmitting cutaneous thermosensory signals received from spinal neurons directly to the thermoregulatory command center, the preoptic area (POA). The present study was conducted to electrophysiologically characterize the local temperature responsiveness of rat LPB neurons in brain slices to evaluate their local thermosensitivity and permit comparison with thermosensitive neurons in POA and spinal cord slices under consistent experimental conditions. In current clamp, warm- and cold-sensitive neurons were recorded in LPBel, LPBc and LPBd, the three LPB subnuclei responsible for the transmission of cutaneous feedforward signals. Of the 92 spontaneously firing LPB neurons, 27% were warm sensitive, 10% were cold sensitive, and 63% were temperature insensitive, and the spontaneous firing rate of the warm-sensitive neurons was significantly greater than that of the temperature-insensitive neurons. These proportions and spontaneous activity are similar to results obtained in the POA and spinal cord. Furthermore, the thermosensitivity was also present in 38% of silent neurons evoked by injection of a small amount of depolarizing current. Warm-sensitive neurons in the LPB were similar in thermoresponsiveness to those in the POA and spinal cord. However, cold sensitivity in the LPB was distinct from that in the POA. The firing rate of most cold-sensitive neurons changed steeply at a relatively narrow band of temperature, and some of them were silent near thermoneutrality. The percentages of thermosensitive and insensitive neurons within the three LPB subnuclei were not significantly different, nor were the mean maximal thermal coefficients of the thermosensitive neurons. These results suggest that LPB have local thermosensory functions as POA and spinal cord, and might be an important extrahypothalamic “thermoregulator”.

Neurological Basis of AMP-Dependent Thermoregulation and its Relevance to Central and Peripheral Hyperthermia

Therapeutic hypothermia is of relevance to treatment of increased body temperature and brain injury, but drugs inducing selective, rapid, and safe cooling in humans are not available. Here, we show that injections of adenosine 5'-monophosphate (AMP), an endogenous nucleotide, promptly triggers hypothermia in mice by directly activating adenosine A1 receptors (A1R) within the preoptic area (POA) of the hypothalamus. Inhibition of constitutive degradation of brain extracellular AMP by targeting ecto 5'-nucleotidase, also suffices to prompt hypothermia in rodents. Accordingly, sensitivity of mice and rats to the hypothermic effect of AMP is inversely related to their hypothalamic 5'-nucleotidase activity. Single-cell electrophysiological recording indicates that AMP reduces spontaneous firing activity of temperature-insensitive neurons of the mouse POA, thereby retuning the hypothalamic thermoregulatory set point towards lower temperatures. Adenosine 5'-monophosphate also suppresses prostaglandin E2-induced fever in mice, having no effects on peripheral hyperthermia triggered by dioxymetamphetamine (ecstasy) overdose. Together, data disclose the role of AMP, 5'-nucleotidase, and A1R in hypothalamic thermoregulation, as well and their therapeutic relevance to treatment of febrile illness.

Effects of arginine vasopressin on firing activity and thermosensitivity of rat PO/AH area neurons

It is well known that the preoptic–anterior hypothalamus (PO/AH), containing temperature-sensitive and -insensitive neurons plays an important role in precise thermoregulatory responses. Previous in vivo studies suggest that the arginine vasopressin (AVP) is an important endogenous mediator in thermoregulation, since AVP and V1a vasopressin receptor antagonist can induce hypothermia and hyperthermia, respectively. In the present study, intracellular electrophysiological activity was recorded from temperature-sensitive and -insensitive neurons in rat PO/AH tissue slices, using a whole-cell patch clamp. By monitoring neuron’s changes of firing activity and thermosensitivity when perfused with AVP or V1a vasopressin receptor antagonist, we found that AVP increased the spontaneous firing rate in 65% of warm-sensitive neurons and decreased it in nearly 50% of cold-sensitive and temperature-insensitive neurons. These changes are due to the AVP enhancing the rise rate of depolarization prepotential in warm-sensitive neurons and reducing it in the other neurons. Moreover, AVP increased the thermosensitivity of warm-sensitive neurons while it decreased thermosensitivity of cold-sensitive and temperature-insensitive neurons. V1a vasopressin receptor participated in these responses. Since excited warm-sensitive neurons or inhibited cold-sensitive and temperature-insensitive neurons promote heat loss or suppress heat production and retention. These results that AVP excites warm-sensitive neurons and inhibits cold-sensitive and temperature-insensitive neurons suggest a probable mechanism of AVP-induced hypothermia.

Effects of ZD7288 on firing pattern of thermosensitive neurons isolated from hypothalamus

The role of the hyperpolarization-activated current (Ih) mediated by HCN channels in temperature sensing by the hypothalamus was addressed. In warm-sensitive neurons (WSNs), exposure to ZD7288, an inhibitor of Ih mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, decreased their action potential amplitudes and frequencies significantly. By contrast, ZD7288 had little or no effect on temperature-insensitive neurons (TINs). Exposure of WSNs to ZD7288 led to a significant increase in the duration of the inter-spike interval and a reduction of Ih irreversibly. These results suggest that ZD7288 have the contrasting effects on the firing patterns of WSNs versus TINs, which implies HCN channels play a central role in temperature sensing by hypothalamic neurons.

The Effects of leptin on thermosensitive neurons in the anterior hypothalamus of the rat

The hormone leptin is thought to play a role in regulating body fat and metabolism in mammals through its ability to alter the activity of anterior hypothalamus (AH) neurons that play a role in homeostatic control. Recent studies have shown that when leptin is injected either intravenously or into the ventricles in rodents, an increase in body temperature is detected, which may be a possible through which leptin inhibits feeding behavior. On the basis of current models for thermoregulation, we hypothesized that warm sensitive neurons in the AH would exhibit a decrease in firing rate in response to leptin while temperature insensitive neurons would respond with an increase in firing rate. Using a tissue slice preparation, the extracellular activity of AH neurons was recorded to classify their responses to temperature and to leptin (100 μM). The majority of warm sensitive neurons decreased their firing rates. While nearly 50% of the temperature insensitive neurons responded with an increase in firing rate, the majority of the remaining temperature insensitive neurons showed decreases in firing rate. This contrast in responses may relate to the specific efferent connections of these thermoregulatory neurons which will be characterized in a future study. This research was supported by grants from the NIH (NS053794, NS065461 & NS053794-01S1) and a Howard Hughes Medical Institute grant to the College of William and Mary.

The effects of the alpha‐2 adrenoreceptor agonist clonidine on the activity of thermally classified neurons in anterior hypothalamus of the rat

In response to infection, noradrenergic vagal afferents to the hypothalamus may be responsible for a rapid biphasic shift in thermoregulatory control. An initial hypothermic shift may result from the activation of alpha-2 adrenoreceptors, which is followed by an enduring hyperthermic response that is initiated by activation of alpha-1 andrenoreceptors and prolonged by the presence of prostaglandin E2 (PGE2). While previous studies from our lab have characterized the effects of PGE2 (Ranels & Griffin, 2003) and the alpha-1 agonist Cirazoline (Imbery et al., 2007), this study recorded the in vitro single-unit activity of thermally classified anterior hypothalamic neurons in response to the alpha-2 adrenoreceptor agonist Clonidine (100 μM). Based on their responses to temperature, warm sensitive neurons (m > 0.8 impulses.s-1.oC-1) showed significant increases in firing rate while temperature insensitive neurons showed significant decreases in firing rate. These changes would result in a hypothermic response according to Hammel's model, demonstrating the role of this adrenoreceptor subtype in temperature regulation. This research was supported by grants from the NIH (NS053794, NS065461 & NS053794-S1) and in part by a Howard Hughes Medical Institute grant through the Undergraduate Science Program to the College of William and Mary.

Calcitonin gene-related peptide alters the firing rates of hypothalamic temperature sensitive and insensitive neurons

BackgroundTransient hyperthermic shifts in body temperature have been linked to the endogenous hormone calcitonin gene-related peptide (CGRP), which can increase sympathetic activation and metabolic heat production. Recent studies have demonstrated that these centrally mediated responses may result from CGRP dependent changes in the activity of thermoregulatory neurons in the preoptic and anterior regions of the hypothalamus (POAH).ResultsUsing a tissue slice preparation, we recorded the single-unit activity of POAH neurons from the adult male rat, in response to temperature and CGRP (10 μM). Based on the slope of firing rate as a function of temperature, neurons were classified as either warm sensitive or temperature insensitive. All warm sensitive neurons responded to CGRP with a significant decrease in firing rate. While CGRP did not alter the firing rates of some temperature insensitive neurons, responsive neurons showed an increase in firing rate.ConclusionWith respect to current models of thermoregulatory control, these CGRP dependent changes in firing rate would result in hyperthermia. This suggests that both warm sensitive and temperature insensitive neurons in the POAH may play a role in producing this hyperthermic shift in temperature.

TRPV blockade by ruthenium red does not suppress thermosensitivity in hypothalamic neurons

The thermoregulatory preoptic‐anterior hypothalamus (POAH) contains both temperature sensitive and insensitive neurons. While controversial, recent studies suggest that POAH neuronal thermosensitivity is due to vanilloid‐sensitive transient receptor potential (TRPV) channels. This hypothesis was tested by determining the responses of POAH neurons to ruthenium red, a ryanodine receptor antagonist that blocks TRPV‐mediated responses. Whole‐cell patch microelectrodes recorded the intracellular activity of POAH neurons in rat hypothalamic tissue slices perfused with control ACSF and experimental media containing 1–100 μM ruthenium red. Each neuron was characterized by its spontaneous firing rate at 36°C and its firing rate thermosensitivity (impulses/sec/°C) during changes in tissue temperature. Ruthenium red did not reduce the firing rate thermosensitivity nor the membrane potential thermosensitivity of POAH warm sensitive neurons. This supports our previous studies indicating that hypothalamic neuronal thermosensitivity is not determined by TRP channels nor by thermally‐induced changes in the resting membrane potential. [Supported by NIH grants NS‐14644 and NS‐045758]

The effects of Cirazoline, an alpha-1 adrenoreceptor agonist, on the firing rates of thermally classified anterior hypothalamic neurons in rat brain slices

Peripheral exposure to LPS induces a biphasic fever thought to be initiated via vagal afferents to the preoptic area of the anterior hypothalamus (POAH), an important thermoregulatory control center in the brain. Previous studies have shown that norepinephrine synaptically mediates this Prostaglandin E 2 (PGE 2)-dependent change in temperature through the selective activation of alpha-2 adrenoreceptors (AR). However, there is clear evidence that alpha-1 AR activation of thermoregulatory hypothalamic neurons will result in a rapid hyperthermia that is not dependent on PGE 2. This direct action of norepinephrine in the POAH was tested in the present study by recording the single-unit activity of POAH neurons in a tissue slice preparation from the adult male rat, in response to temperature and the selective alpha-1 AR agonist Cirazoline (1–100 μM). Neurons were classified as either warm sensitive or temperature insensitive. Warm sensitive neurons responded to Cirazoline with a decrease in firing rate, while temperature insensitive neurons showed a firing rate increase. These responses are similar to those reported for PGE 2 and suggest that both warm sensitive and temperature insensitive neurons in the POAH are important in mediating this alpha-1 AR-dependent hyperthermic shift in body temperature.

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA A-receptor agonist muscimol, GABA A-receptor antagonist bicuculline, GABA B-receptor agonist baclofen and GABA B-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 μM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 μM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA B-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased ( p < 0.05) by baclofen and significantly decreased ( p < 0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.

The effects of cirazoline on the firing rates of thermally classified neurons in the anterior hypothalamus of the rat

Peripheral exposure to LPS induces a biphasic fever thought to be initiated via vagal afferents to the anterior hypothalamus (AH), an important thermoregulatory control center in the brain. Previous studies have shown norepinephrine (NE) synaptically mediates this Prostaglandin E2 (PGE2)-dependent change in temperature through the selective activation of α-2 adrenoreceptors (AR). However, there is clear evidence that α-1 AR activation of thermoregulatory hypothalamic neurons will result in a rapid hyperthermia that is not dependent on PGE2. This direct action of NE in the AH was tested in the present study by recording the single-unit activity of these neurons in a tissue slice preparation from the adult male rat, in response to temperature and the selective α-1 AR agonist Cirazoline (10–100 μM). Neurons in the AH were classified as either warm-sensitive (m ≥ 0.8 impulses.s−1.°C−1), or temperature-insensitive. Warm-sensitive neurons responded to cirazoline with a decrease in firing rate, while temperature-insensitive neurons showed an initial firing rate increase. These responses are similar to those reported for PGE2 and suggest that both warm-sensitive and temperature-insensitive neurons in the AH are important in mediating this α-1 AR-dependent hyperthermic shift in body temperature. (Supported by NSF: IBN-9983624, NIH: NS053794 and a HHMI grant to the College of William and Mary.)

Role of hyperpolarization‐activated currents in hypothalamic neuronal thermosensitivity

The thermoregulatory preoptic-anterior hypothalamus (POAH) contains both temperature sensitive and insensitive neurons. Our previous studies predict that neuronal warm-sensitivity is due to thermosensitive pacemaker currents that include both the inactivation of transient outward K+ currents and the activation of Ih (cation currents activated by hyperpolarization). The latter include inward currents through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Our recent modeling study (Wechselberger et al, Am. J. Physiol. 2006) predicts that HCN channels contribute to neuronal warm-sensitivity, while K+ leak channels contribute to temperature insensitivity. Since POAH neurons show differences in their HCN immunoreactivity, the present study tested the models’ predictions by measuring Ih in different types of POAH neurons recorded in rat hypothalamic tissue slices. During current-clamp recordings, each neuron was characterized by its spontaneous activity at 36°C and by its firing rate thermosensitivity (impulses/sec/°C) when the tissue slice was warmed and cooled. Voltage-clamp recordings measured Ih and the effects of temperature on this current. In support of the models’ predictions, our experiments to date indicate that Ih is often associated with neurons displaying pacemaker activity and warm-sensitivity. [Supported by NIH grants NS14644 and NS045758]

Carbon dioxide and pH effects on temperature-sensitive and -insensitive hypothalamic neurons

The preoptic-anterior hypothalamus (POAH) controls body temperature, and thermoregulatory responses are impaired during hypercapnia. If increased CO(2) or its accompanying acidosis inhibits warm-sensitive POAH neurons, this could provide an explanation for thermoregulatory impairment during hypercapnia. To test this possibility, extracellular electrophysiological recordings determined the effects of CO(2) and pH on the firing rates of both temperature-sensitive and -insensitive neurons in hypothalamic tissue slices from 89 male Sprague-Dawley rats. Firing rate activity was recorded in 121 hypothalamic neurons before, during, and after changing the CO(2) concentration aerating the tissue slice chamber or changing the pH of the solution bathing the tissue slices. Increasing the aeration CO(2) concentration from 5% (control) to 10% (hypercapnic) had no effect on most (i.e., 69%) POAH temperature-insensitive neurons; however, this hypercapnia inhibited the majority (i.e., 59%) of warm-sensitive neurons. CO(2) affected similar proportions of (non-POAH) neurons in other hypothalamic regions. These CO(2) effects appear to be due to changes in pH since the CO(2)-affected neurons responded similarly to isocapnic acidosis (i.e., normal CO(2) and decreased pH) but were not responsive to isohydric hypercapnia (i.e., increased CO(2) and normal pH). These findings may offer a neural explanation for some heat-related illnesses (e.g., exertional heat stroke) where impaired heat loss is associated with acidosis.