Electrical Stimulation Of Hypothalamus Research Articles

Chapter 28 - The neurobiology of cluster headache

Cluster headache is a primary headache form occurring in paroxysmal excruciatingly severe unilateral head pain attacks usually grouped in periods lasting 1-2months, the cluster periods. A genetic component is suggested by the familial occurrence of the disease but a genetic linkage is yet to be identified. Contemporary activation of trigeminal and cranial parasympathetic systems-the so-called trigemino-parasympathetic reflex-during the headache attacks seem to cause the pain and accompanying oculo-facial autonomic phenomena respectively. At peripheral level, the increased calcitonin gene related peptide (CGRP) plasma levels suggests trigeminal system activation during cluster headache attacks. The temporal pattern of the disease both in terms of circadian rhythmicity and seasonal recurrence has suggested involvement of the hypothalamic biological clock in the pathophysiology of cluster headache. The posterior hypothalamus was investigate as the cluster generator leading to activation of the trigemino-parasympathetic reflex, but the accumulated experience after 20 years of hypothalamic electrical stimulation to treat the condition indicate that this brain region rather acts as pain modulator. Efficacy of monoclonal antibodies to treat episodic cluster headache points to a key role of CGRP in the pathophysiology of the condition.

Increase in c-Fos and Arc protein in retrosplenial cortex after memory-improving lateral hypothalamic electrical stimulation treatment

Post-training Intracranial self-stimulation (ICSS) of the lateral hypothalamus (LH), a kind of rewarding deep-brain stimulation, potentiates learning and memory and increases c-Fos protein expression in specific memory-related brain regions. In a previous study, Aldavert-Vera et al. (2013) reported that post-acquisition LH-ICSS improved 48h retention of a delay two-way active avoidance conditioning (TWAA) and induced c-Fos expression increase in CA3 at 90min after administration. Nevertheless, this c-Fos induction was only observed after the acquisition session and not after the retention test at 48h, when the ICSS improving effect was observed on memory. This current study aims to examine the hypothesis that post-training ICSS treatment may stimulate c-Fos expression at the time of the TWAA retention test in retrosplenial cortex (RSC), a hippocampus-related brain region more closely related with long-lasting memory storage. Effects of ICSS on Arc protein, a marker of memory-associated synaptic plasticity, were also measured by immunohistochemistry in granular and agranular RSC. The most innovative results are that the ICSS treatment potentiates the c-Fos induction across TWAA conditions (no conditioning, acquisition and retention), specifically in layer V of the granular RSC, along with increases of Arc protein levels in the granular but not in agranular areas of RSC ipsilaterally few hours after ICSS. This leads us to suggest that plasticity-related protein activation in the granular RSC could be involved in the positive modulatory effects of ICSS on TWAA memory consolidation, opening a new approach for future research in ICSS memory facilitation.

Stimulation of Fetal Hypothalamus Induces Uterine Contractions in Pregnant Rats at Term1

The fetal brain is thought to have a role in the onset and progression of labor. Evidence also exists for fetal oxytocin release just before and during parturition. The present study examined whether activation of the fetal brain could induce uterine myometrial contractions through oxytocin receptors in the dam. Under urethane anesthesia, electrical stimulation of the hypothalamus of fetal rats that were still connected with the dams by an intact umbilical cord induced uterine contractions in term pregnant rats. Intraperitoneal injections of synthetic oxytocin in fetuses induced uterine contractions in the dams similar to those induced by electrical stimulation of the fetal hypothalamus. Maternal intravenous injections of an oxytocin antagonist immediately attenuated uterine contractions induced by fetal oxytocin injections and electrical stimulation of the fetal hypothalamus. These findings suggest the possibility that oxytocin released from the fetal hypothalamus is involved in parturition.

Purinergic transmission into the NTS mediates cardiovascular adjustments during hypothalamic electrical stimulation

This study sought to determine the role of NTS purinergic receptors in cardiovascular adjustments to hypothalamic defense area electrical stimulation (HDAS). Male Wistar rats were anesthetized and prepared for mean arterial pressure (MAP), heart rate (HR), hindquarter blood flow (HQBF) and vascular conductance (HQVC) recordings. HDAS (150 μA; 0.6 ms; 100 Hz; N=6) evoked hypertension (21±5 mmHg), tachycardia (38±7 bpm) and hindquarter vasodilation indicated by the maximum peak (161±13% of baseline) and the area under the HQVC curve (AUC: 5958±778%). After bilateral microinjections of suramin (10 mM) into the NTS, pressor and HR responses to HDAS were unchanged (35±11 mmHg; 29±6 bpm respectively), whereas hindlimb vasodilation was extensively reduced (peak:71±7%, AUC: 782±180%). In another group (N=5) bilateral microinjections of PPADS (5mM) into the same NTS sites markedly reduced hypertension (16±4 mmHg vs 7±1 mmHg), tachycardia (7±2 bpm vs 2±1 bpm) and hindlimb vasodilation (max: 114±23% vs 28±9%, AUC: 3107±824% vs 145±49%) induced by HS. Basal levels of MAP, HR and HQVC remained unchanged after suramin or PPADS microinjections into the NTS. Our results indicated the participation of NTS different purinergic receptors subtypes in the hindquarter vasodilation during defense reactions.

Feeding, body weight, and sensitivity to non-ingestive reward stimuli during and after 12-day continuous central infusions of melanocortin receptor ligands

The brain melanocortin system mediates downstream effects of hypothalamic leptin and insulin signaling. Yet, there have been few studies of chronic intracerebroventricular (i.c.v.) melanocortin receptor (MCR) agonist or antagonist infusion. Although there is evidence of interaction between melanocortin and dopamine (DA) systems, effects of chronic MCR ligand infusion on behavioral sensitivity to non-ingestive reward stimuli have not been investigated. The objective of this study was to investigate effects of chronic i.c.v. infusion of the MCR agonist, MTII, and the MCR antagonist, SHU9119, on food intake, body weight, and sensitivity to rewarding lateral hypothalamic electrical stimulation (LHSS) and the reward-potentiating (i.e., threshold-lowering) effect of d-amphetamine. The MCR antagonist, SHU9119 (0.02 μg/h) produced sustained hyperphagia and weight gain during the 12-day infusion period, followed by compensatory hypophagia and an arrest of body weight gain during the 24-day post-infusion period. At no point during the experiment was sensitivity to LHSS or d-amphetamine (0.25 mg/kg, i.p.) altered. The MCR agonist, MTII (0.02 μg/h) produced a brief hypophagia (3 days) followed by a return to control levels of daily intake, but with body weight remaining at a reduced level throughout the 12-day infusion period. This was followed by compensatory hyperphagia and weight gain during the 24-day post-infusion period. There was no change in sensitivity to non-ingestive reward stimuli during the infusion of MTII. However, sensitivity to d-amphetamine was increased during the 24-day post-infusion period. It therefore seems that changes in ingestive behavior that occur during chronic MCR ligand infusion may not affect the response to non-ingestive reward stimuli. However, it is possible that the drive to re-feed and restore body weight following MCR agonist treatment includes neuroadaptations that enhance the incentive effects of drug stimuli.

CLUSTER

Cluster headache is the most severe of the primary headaches. Positron emission tomography and functional MRI studies have shown that the ipsilateral posterior hypothalamus is activated during cluster headache attacks and is structurally asymmetric in these patients. These changes are highly specific for the condition and suggest that the cluster headache generator may be located in that brain area; they further suggest that electrical stimulation of that region might produce clinical improvement in chronic cluster headache sufferers refractory to medical therapy. In five patients with severe intractable chronic cluster headache, hypothalamic electrical stimulation produced complete and long‐term pain relief with no relevant side‐effects. We therefore consider it essential to propose criteria for selecting chronic cluster headache patients for hypothalamic deep brain stimulation before this procedure is undertaken at other academic medical centers. Comment: This article describes the five patients operated on in Italy for chronic intractable cluster with the stimulator in the cluster hypothalamic generator with good results and “no relevant side‐effects.” However, the authors' admonition for criteria for patient selection is now underlined by Dr. Schoenen's platform presentation at the American Academy of Neurology meeting in San Francisco in 2004, at which the Belgian experience was described: “The procedure was uneventful in 4/5 patients … Unfortunately, one patient developed massive brainstem and basal ganglia hemorrhage 4 hours after surgery and died after 2 days … In this small series, deep brain stimulation of posterior hypothalamus proved effective to control otherwise intractable chronic cluster headache … Serious adverse events can occur even with a single electrode … Pooling … patients with other deep brain stimulator procedures performed in our department (38 for Parkinson's disease, essential tremor, and generalized dystonia) … the overall rate of significant hemorrhage was 2.3%.”[Vendenheede M, Maertens de Noordhout AS, Remacle JM, Mouchamps M, Schoenen J. Deep Brain Stimulation of Posterior Hypothalamus in Chronic Cluster Headache. Neurology. 2004;62(suppl 5):A356.] This new information takes some of the enthusiasm out of everyone's initial reaction to this procedure. Obviously, hemorrhage and death are not consequences of the other surgical procedure for cluster that is often successful, radiofrequency trigeminogangliorhizolysis. —Stewart J. Tepper, MD

Deep brain stimulation for intractable chronic cluster headache: proposals for patient selection.

Cluster headache is the most severe of the primary headaches. Positron emission tomography and functional MRI studies have shown that the ipsilateral posterior hypothalamus is activated during cluster headache attacks and is structurally asymmetric in these patients. These changes are highly specific for the condition and suggest that the cluster headache generator may be located in that brain area; they further suggest that electrical stimulation of that region might produce clinical improvement in chronic cluster headache sufferers refractory to medical therapy. In five patients with severe intractable chronic cluster headache, hypothalamic electrical stimulation produced complete and long-term pain relief with no relevant side-effects. We therefore consider it essential to propose criteria for selecting chronic cluster headache patients for hypothalamic deep brain stimulation before this procedure is undertaken at other academic medical centres.

Synergistic effects of cocaine and dizocilpine (MK-801) on brain stimulation reward

The rewarding effects of lateral hypothalamic electrical stimulation were assessed in animals treated with the combination of cocaine and dizocilpine (MK-801), a noncompetitive N-methyl- d-aspartate antagonist. Eight male Long-Evans rats were trained to perform a lever-press operant to deliver trains of cathodal rectangular pulses directly into the lateral hypothalamus. Response rate was determined across the range of effective stimulation frequencies. For each rat the frequency threshold was defined as the lowest frequency that sustained minimal responding. After thresholds had stabilized each rat was tested under 4 treatment conditions; saline+saline, dizocilpine (0.05 mg/kg, i.p., 30 min before test)+saline, saline+cocaine (4 mg/kg, i.p., 5 min before test) and dizocilpine+cocaine. The saline+saline, dizocilpine+saline and saline+cocaine treatments each failed to cause significant changes in threshold or maximum response rates. The dizocilpine+cocaine treatment produced a large reduction in thresholds indicating a synergism between the two drugs and the rewarding stimulation. These synergistic effects of dizocilpine and cocaine stand in contrast to the putative antagonism by dizocilpine of cocaine's psychomotor-sensitizing action.

Feeding, drug abuse, and the sensitization of reward by metabolic need.

The incentive-motivating effects of external stimuli are dependent, in part, upon the internal need state of the organism. The increased rewarding efficacy of food as a function of energy deficit, for example, has obvious adaptive value. The enhancement of food reward extends, however, to drugs of abuse and electrical brain stimulation, probably due to a shared neural substrate. Research reviewed in this paper uses lateral hypothalamic electrical stimulation to probe the sensitivity of the brain reward system and investigate mechanisms through which metabolic need, induced by chronic food restriction and streptozotocin-induced diabetes, sensitizes this system. Results indicate that sensitivity to rewarding brain stimulation varies inversely with declining body weight. The effect is not mimicked by pharmacological glucoprivation or lipoprivation in ad libitum fed animals; sensitization appears to depend on persistent metabolic need or adipose depletion. While the literature suggests elevated plasma corticosterone as a peripheral trigger of reward sensitization, sensitization was not reversed by meal-induced or pharmacological suppression of plasma corticosterone. Centrally, reward sensitization is mediated by opioid receptors, since the effect is reversed by intracerebroventricular (i.c.v.) infusion of naltrexone, TCTAP (mu antagonist) and nor-binaltorphimine (kappa antagonist). The fact that these same treatments, as well as i.c.v. infusion of dynorphin A antiserum, block the feeding response to lateral hypothalamic stimulation suggests that feeding and reward sensitization are mediated by a common opioid mechanism. Using in vitro autoradiography, radioimmunoassays and a solution hybridization mRNA assay, brain regional mu and kappa opioid receptor binding, levels of prodynorphin-derived peptides, and prodynorphin mRNA, respectively, were measured in food-restricted and diabetic rats. Changes that could plausibly be involved in reward sensitization are discussed, with emphasis on the increased dynorphin A1-3 and prodynorphin mRNA levels in lateral hypothalamic neurons that innervate the pontine parabrachial nucleus, where mu binding decreased and kappa binding increased. Finally, the possible linkage between metabolic need and activation of a brain opioid mechanism is discussed, as is evidence supporting the relevance of these findings to drug abuse.

Effects of corticosteroid synthesis inhibitors on the sensitization of reward by food restriction

Chronic food restriction sensitizes animals to the rewarding effects of food, drugs and lateral hypothalamic electrical stimulation. The present study employed a curve-shift analysis of lateral hypothalamic self-stimulation (LHSS) to evaluate whether the elevated plasma corticosterone levels that accompany food restriction mediate the sensitization of reward. In Experiment 1, two adrenocorticoid synthesis inhibitors, aminoglutethimide and metyrapone, were administered to food-restricted rats and the magnitude of plasma corticosterone suppression was determined at two post-administration time points. In Experiment 2, these compounds were administered to ad libitum fed and food-restricted rats whose LHSS behavior was evaluated at a time coincident with suppression of corticosterone. It was found that neither compound reversed the sensitizing effect of food-restriction on the rewarding efficacy of brain stimulation. However, aminoglutethimide (50 mg/kg) produced an increase in maximal response rates (a performance factor) across groups while metyrapone (100 mg/kg) produced a decrease. The most interesting result of this study was that 2 h after aminoglutethimide administration, when corticosterone levels had recovered from suppression, the rewarding efficacy of LHSS increased markedly in food-restricted rats. Possible explanations for this effect, including adrenocortical rebound, alterations in neurosteroid synthesis, and exacerbation of metabolic need are discussed.

Effects of corticosteroid synthesis inhibitors on the sensitization of reward by food restriction

Chronic food restriction sensitizes animals to the rewarding effects of food, drugs and lateral hypothalamic electrical stimulation. The present study employed a curve-shift analysis of lateral hypothalamic self-stimulation (LHSS) to evaluate whether the elevated plasma corticosterone levels that accompany food restriction mediate the sensitization of reward. In Experiment 1, two adrenocorticoid synthesis inhibitors, aminoglutethimide and metyrapone, were administered to food-restricted rats and the magnitude of plasma corticosterone suppression was determined at two post-administration time points. In Experiment 2, these compounds were administered to ad libitum fed and food-restricted rats whose LHSS behavior was evaluated at a time coincident with suppression of corticosterone. It was found that neither compound reversed the sensitizing effect of food-restriction on the rewarding efficacy of brain stimulation. However, aminoglutethimide (50 mg/kg) produced an increase in maximal response rates (a performance factor) across groups while metyrapone (100 mg/kg) produced a decrease. The most interesting result of this study was that 2 h after aminoglutethimide administration, when corticosterone levels had recovered from suppression, the rewarding efficacy of LHSS increased markedly in food-restricted rats. Possible explanations for this effect, including adrenocortical rebound, alterations in neurosteroid synthesis, and exacerbation of metabolic need are discussed.

Withdrawal from chronic amphetamine elevates baseline intracranial self-stimulation thresholds

Intracranial self-stimulation was assessed before, within, and after a chronic amphetamine treatment regimen. Amphetamine was given twice daily 5 days per week for 6 weeks at dosages escalating from 1 to 10 mg/kg per injection. Lateral hypothalamic self-stimulation rate-frequency functions were taken 36 h after the last injection in each weekly series and weekly for 3 weeks following the last injection. Frequency thresholds increased and maximal response rates decreased progressively as a function of amphetamine withdrawal during treatment; each returned to near normal levels within 2 weeks of the last injection. When subsequently tested under amphetamine, animals previously receiving the 6-week amphetamine treatment regimen had self-stimulation thresholds and maximal response rates that did not differ significantly from those of saline-treated control animals. These data confirm that chronic amphetamine treatment results in a dependence syndrome characterized in part by a phasic depression in the brain mechanism mediating the reinforcing effects of lateral hypothalamic electrical stimulation.

Effects of chronic food restriction on prodynorphin-derived peptides in rat brain regions

Chronic food restriction produces a variety of physiological and behavioral adaptations including a potentiation of the reinforcing effect of food, drugs and lateral hypothalamic electrical stimulation. Previous work in this laboratory has revealed that the lowering of self-stimulation threshold by food restriction is reduced by μ- and κ-selective opioid antagonists. In the present study, the effect of chronic food restriction on levels of three prodynorphin-derived peptides, namely dynorphin A 1–17 (A 1–17), dynorphin A 1–8 (A 1–8) and dynorphin B 1–13 (B 1–13) were measured in eleven brain regions known to be involved in appetite, taste and reward. Food restriction increased levels of A 1–17 in dorsal medial (+ 19.6%), ventral medial (+ 24.2%) and medial preoptic (+ 82.9%) hypothalamic areas. Levels of A 1–17 decreased in the central nucleus of the amygdala (− 35.1%). Food restriction increased levels of A 1–8 in nucleus accumbens (+ 34.4%), bed nucleus of the stria terminalis (+ 24.5%) and lateral hypothalamus (+ 41.9%). Food restriction had no effect on levels of B 1–13. A 1–17 is highly κ-preferring and the brain regions in which levels increased all have a high ratio of κ:μ and δ receptors. A 1–8 is less discriminating among opioid receptor types and the brain regions in which levels increased have a low ratio of κ:μ and δ receptors. The present results suggest that food restriction alters posttranslational processing within the dynorphin A domain of the prodynorphin precursor, possibly leading to a change in the balance between κ and non-κ opioid receptor stimulation in specific brain regions.

The role of multiple opioid receptors in the maintenance of stimulation-induced feeding

Feeding induced by lateral hypothalamic electrical stimulation is sensitive to opioid antagonism and has previously been blocked by naloxone and antibodies to dynorphin A fragments. In the present study, high affinity receptor-selective antagonists were used to determine the particular opioid receptor type(s) that mediates stimulation-induced feeding (SIF). Separate groups of rats were used to conduct i.c.v. dose-response studies with TCTAP (μ), naltrindole (δ) and norbinaltorphimine (κ). TCTAP, at the highest dose tested (i.e. 5.0 nmol) and norbinaltrophimine, at doses of 10.0 and 50.0 nmol, increased the brain stimulation frequency threshold for eliciting SIF. Naltrindole, at doses up to 50.0 nmol, had no effect. Results of another study, recently conducted in this laboratory, indicate that the present doses of TCTAP and norbinaltrophimine have no efefct on thresholds for lateral hypothalamic self-stimulation. This suggests that μ and κ opioid activity are associated with feeding, rather than the SIF test, where 5 determinations of threshold are obtained in serial order, naloxone characteristically increases thresholds toward the end of a test while conventional appetite suppressants increase thresholds uniformly throughout a test. TCTAP and norbinaltorphimine produce a ‘naloxone-like’ pattern of threshold elevation, suggesting that μ and κ receptors are involved in the process whereby endogenous opioid activity sustains feeding once initiated.

Response to acoustic stimuli increases in the ventral cochlear nucleus after stimulus pairing.

Recordings of activity in response to click and hiss were made from 364 units of the ventral cochlear nucleus of cats. The unit response to acoustic stimuli increased after forward or backward pairing of the stimuli with glabella tap and hypothalamic electrical stimulation. The results provide evidence against the widely held view that transmission through this initial brain stem relay of the auditory system is invariant, and suggest, instead, that the activity of the ventral cochlear nucleus changes to support increased attentiveness to acoustic signals after variably ordered pairing of conditioned and unconditioned stimuli.

Brain neuronal mechanisms of motivation and reinforcement: systemic organization of behavior.

The concept of systemic quantitation of behavior was used to study the interaction between dominant motivation and reinforcement on brain neurons. Feeding and escape were used as experimental models. Results showed that feeding and escape motivations are based on the mechanisms of ascending activating influences of lateral or ventromedial hypothalamic centers, respectively, on cortical neurons. Feeding and escape motivations evoked by electrical stimulation of hypothalamus, or the ones occurring naturally, modify the responses of single neurons of sensorimotor and insular cortex to sensory stimuli, to microiontophoretic application of neurotransmitters and neuropeptides, and especially to reinforcement. Dominant motivations are reflected in the burst-like activity and in a specific pattern of interpulse intervals of single units in various brain structures. Reinforcement transforms this activity into a regular pattern and also changes the neuronal responses of hypothalamic feeding and escape centers, to electrical stimulation and to microiontophoretic application of oligopeptides. Specific patterns of the unit responses to conditioned stimuli in the course of consecutive reinforcement was determined. After several instances of reinforcement, especially after repetitive reinforcement, the brain neurons influenced by the dominant motivation begin to produce specific protein molecules that are important for organization of an appropriate type of behavior. This process can be blocked by protein synthesis inhibitors.

The influence of stimulation of the hypothalamus on the choice of food during maintenance of rats on various salt and water diets.

Electrical stimulation of the lateral hypothalamus of rats maintained on a normal or salty diet with water deprivation leads to an increase in drinking excitability and to a decrease in the thresholds for salty food consumed. The stimulation promotes attenuation of the effects of the "mental" desalting of food that is achieved by a spatial co-placement of the nonsaltiness signal with salty reinforcement, and accelerates the conversion of the signal significance of the conditional stimuli associated with nonsalty and salty reinforcement as well. Electrical stimulation of the hypothalamus accelerates the formation of conditioned reflex aversion to salty food and the inhibition of conditioned reflexes in the direction of the corresponding food dispenser, and intensifies the existing state of thirst as well, imparting to it characteristics of a dominant motivation.

What psychological processes mediate feeding evoked by lateral hypothalamic electrical stimulation?

What psychological processes mediate feeding evoked by lateral hypothalamic electrical stimulation?

Hypothalamic sites affecting masticatory neurons in rats

The effects of hypothalamic electrical stimulation and dl-sulpiride injections on the unit activity of masticatory trigeminal neurons were assessed in rats. Unilateral electrodes or bilateral cannulas were implanted in the perifomical hypothalamus. The animals which exhibited eating to electrical stimulation or to sulpiride injections were selected. Then, under urethane anesthesia, electrical stimuli or sulpiride injections were applied at sites which elicited eating while a passive jaw movement-related neuron was recorded. Electrical stimulation or sulpiride injections affected the basal firing rate of 15 out of 32 (53%), and 13 out of 17 (76%) jaw movement-related neurons, respectively. The basal firing rate of 3 out of 18 (16.6%) was affected by electrical stimulation of noneliciting feeding nearby places. Facilitation was observed in 7 out of those 13 jaw movement-related trigeminal neurons after intrahypothalamic sulpiride injections. When dopamine was injected in the hypothalamus 3 minutes before sulpiride, this drug could only affect the basal firing rate of 1 out of 12 (8%) jaw movement-related neurons. The electrical stimulation and sulpiride injections into the hypothalamus had the same inhibitory or excitatory effect on a given trigeminal neuron as the passive jaw movement did. A similar phenomenon was observed with the sulphide-induced facilitation. These results suggest that hypothalamic D 2 satiety receptors modulate brain stem feeding reflexes.

Brown adipose tissue thermogenic responses of rats induced by central stimulation: effect of age and cold acclimation.

1. Urethane-anaesthetized, age-matched cold-(4 degrees C) and room (21 degrees C)-acclimated groups of Sprague-Dawley rats were given repeated (three) ventromedial hypothalamic electrical stimulations. Interscapular brown adipose tissue (IBAT), colonic, surface tail temperatures and blood pressure were monitored before and after each electrical stimulation. Propranolol HCl (2.5 mg kg-1 i.v.) was given 10 min prior to the last (third) ventromedial hypothalamic stimulation. 2. Repeated electrical stimulation of the ventromedial hypothalamic (VMH) nucleus to either small or large rats (kept at 21 degrees C) caused no significant change in interscapular brown fat, colonic or surface temperatures compared to respective pre-stimulation control values whereas mean arterial pressure was slightly but significantly increased during the 30 s stimulation period. 3. Electrical stimulation of the ventromedial hypothalamic nucleus of small or large rats, kept at 4 degrees C for 3 weeks prior to testing, caused significant (greater than 0.25 and greater than 0.40 degrees C, respectively) rises in interscapular BAT temperature from respective prestimulation control values. Colonic temperatures increased following ventromedial hypothalamic electrical stimulation only in the small, 4 degrees C-acclimated group whereas surface tail temperatures did not significantly change after stimulation of either cold-exposed group. Mean arterial pressures were significantly increased during ventromedial hypothalamic electrical stimulation in both 4 degrees C-acclimated groups, compared to pre-stimulation control levels and, in addition, were above those of age-matched rats kept at 21 degrees C. Intravenous propranolol, which decreased interscapular brown fat and colonic temperatures in all groups, blocked the rise in interscapular brown fat and colonic temperatures of the 4 degrees C-acclimated rats following the last electrical stimulation of the ventromedial hypothalamic nucleus. 4. In vitro biochemical analysis of the interscapular brown fat pads of another four groups of age-matched, small and large 21 and 4 degrees C-acclimated rats revealed that the thermogenic capacity of the 4 degrees C-acclimated groups was, in all cases, significantly increased from age-matched groups previously kept at 21 degrees C, as shown by significant increases in brown adipose tissue mass, BAT DNA and protein content, BAT mitochondrial protein and BAT mitochondrial GDP binding.(ABSTRACT TRUNCATED AT 400 WORDS)