Microstimulation mapping of the basal forebrain in the anesthetized rat: the "preoptic locomotor region"

Preoptic and hypothalamic regulation of multi-tiered, chronologically arranged female rat sexual behavior

As in many mammalian behaviors, sexual behavior exhibits structure. Each modular components of the structure, that are linked together over time, occur in probabilistic manner. Endocrine milieu, in particular sex hormones, define the probability to synchronize the behavior with the production of gametes. Developmental experience and environmental cues affect the hormonal milieu of the brain. This is especially true in female mammals, in which ova mature with certain intervals along with ovarian secretion of sex hormones. Estrogens secreted by mature ovarian follicles support both affiliative and executive components of female sexual behavior. In the absence of the ovarian steroids, females avoid males when possible, or antagonize and reject males when put together. Female sexual behavior is intimately linked with the estrous cycle in many species such that females are only receptive for a brief period at the estrus stage surrounding ovulation. Thus, in the rat, females strongly influence the outcome of mating encounter with a male. Affiliative or solicitatory behavior shown by females in estrus leads to the female adapting the lordosis posture, which is characterized by hindleg postural rigidity and lordotic dorsiflexion of the spine, in response to touch-pressure somatosensory stimuli on the skin of the flanks, rump-tail base, perineum region given by male partner. The posture facilitates intromission and consequently fertilization. Although dependence on estrogens is the most important feature of female rat sexual behavior, cervical probing combined with palpation of the hindquarter skin acts as a supranormal stimulus to elicit lordosis. Thus, lordosis behavior is a hub of multi-tiered, chronologically arranged set of behaviors and estrogen appear to alter excitability of neural network for lordosis.

The Role of the Medial Septum-Associated Networks in Controlling Locomotion and Motivation to Move

The Medial Septum and diagonal Band of Broca (MSDB) was initially studied for its role in locomotion. However, the last several decades were focussed on its intriguing function in theta rhythm generation. Early studies relied on electrical stimulation, lesions and pharmacological manipulation, and reported an inconclusive picture regarding the role of the MSDB circuits. Recent studies using more specific methodologies have started to elucidate the differential role of the MSDB's specific cell populations in controlling both theta rhythm and behaviour. In particular, a novel theory is emerging showing that different MSDB's cell populations project to different brain regions and control distinct aspects of behaviour. While the majority of these behaviours involve movement, increasing evidence suggests that MSDB-related networks govern the motivational aspect of actions, rather than locomotion per se. Here, we review the literature that links MSDB, theta activity, and locomotion and propose open questions, future directions, and methods that could be employed to elucidate the diverse roles of the MSDB-associated networks.

Hypothalamic Dopamine Neurons Control Sensorimotor Behavior by Modulating Brainstem Premotor Nuclei in Zebrafish

Dopamine (DA)-producing neurons are critically involved in the production of motor behaviors in multiple circuits that are conserved from basal vertebrates to mammals. Although there is increasing evidence that DA neurons in the hypothalamus play a locomotor role, their precise contributions to behavior and the circuit mechanisms by which they are achieved remain unclear. Here, we demonstrate that tyrosine-hydroxylase-2-expressing (th2+) DA neurons in the zebrafish hypothalamus fire phasic bursts of activity to acutely promote swimming and modulate audiomotor behaviors on fast timescales. Their anatomy and physiology reveal two distinct functional DA modules within the hypothalamus. The first comprises an interconnected set of cerebrospinal-fluid-contacting DA nuclei surrounding the 3^rd ventricle, which lack distal projections outside of the hypothalamus and influence locomotion through unknown means. The second includes neurons in the preoptic nucleus, which send long-range projections to targets throughout the brain, including the mid- and hindbrain, where they activate premotor circuits involved in swimming and sensorimotor integration. These data suggest a broad regulation of motor behavior by DA neurons within multiple hypothalamic nuclei and elucidate a novel functional mechanism for the preoptic DA neurons in the initiation of movement.

Lateral preoptic and ventral pallidal roles in locomotion and other movements

The lateral preoptic area (LPO) and ventral pallidum (VP) are structurally and functionally distinct territories in the subcommissural basal forebrain. It was recently shown that unilateral infusion of the GABAA receptor antagonist, bicuculline, into the LPO strongly invigorates exploratory locomotion, whereas bicuculline infused unilaterally into the VP has a negligible locomotor effect, but when infused bilaterally, produces vigorous, abnormal pivoting and gnawing movements and compulsive ingestion. This study was done to further characterize these responses. We observed that bilateral LPO infusions of bicuculline activate exploratory locomotion only slightly more potently than unilateral infusions and that unilateral and bilateral LPO injections of the GABAA receptor agonist muscimol potently suppress basal locomotion, but only modestly inhibit locomotion invigorated by amphetamine. In contrast, unilateral infusions of muscimol into the VP affect basal and amphetamine-elicited locomotion negligibly, but bilateral VP muscimol infusions profoundly suppress both. Locomotor activation elicited from the LPO by bicuculline was inhibited modestly and profoundly by blockade of dopamine D2 and D1 receptors, respectively, but was not entirely abolished even under combined blockade of dopamine D1 and D2 receptors. That is, infusing the LPO with bic caused instances of near normal, even if sporadic, invigoration of locomotion in the presence of saturating dopamine receptor blockade, indicating that LPO can stimulate locomotion in the absence of dopamine signaling. Pivoting following bilateral VP bicuculline infusions was unaffected by dopamine D2 receptor blockade, but was completely suppressed by D1 receptor blockade. The present results are discussed in a context of neuroanatomical and functional organization underlying exploratory locomotion and adaptive movements.

Estradiol-sensitive projection neurons in the female rat preoptic area

Electrical stimulation of the preoptic area (POA) interrupts the lordosis reflex, a combined contraction of back muscles, in response to male mounts and the major receptive component of sexual behavior in female rat in estrus, without interfering with the proceptive component of this behavior or solicitation. Axon-sparing POA lesions with an excitotoxin, on the other hand, enhance lordosis and diminish proceptivity. The POA effect on the reflex is mediated by its estrogen-sensitive projection to the ventral tegmental area (VTA) as shown by the behavioral effect of VTA stimulation as well as by the demonstration of an increased threshold for antidromic activation of POA neurons from the VTA in ovariectomized females treated with estradiol benzoate (EB). EB administration increases the antidromic activation threshold in ovariectomized females and neonatally castrated males, but not in neonatally androgenized females; the EB effect is limited to those that show lordosis in the presence of EB. EB causes behavioral disinhibition of lordosis through an inhibition of POA neurons with axons to the VTA, which eventually innervate medullospinal neurons innervating spinal motoneurons of the back muscle. The EB-induced change in the threshold or the axonal excitability may be a result of EB-dependent induction of BK channels. Recordings from freely moving female rats engaging in sexual interactions revealed separate subpopulations of POA neurons for the receptive and proceptive behaviors. Those POA neurons engaging in the control of proceptivity are EB-sensitive and project to the midbrain locomotor region (MLR). EB thus enhances lordosis by reducing excitatory neural impulses from the POA to the VTA. An augmentation of the POA effect to the MLR may culminate in an increased locomotion that embodies behavioral estrus in the female rat.

Relationship of arousal to circadian anticipatory behavior: ventromedial hypothalamus: one node in a hunger-arousal network

The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Inactivation of the medial preoptic area/anterior hypothalamus by lidocaine reduces male sexual behavior and sexual incentive motivation in male rats

Permanent bilateral lesions of the medial preoptic area anterior hypothalamus (MPOA/AH) produce a drastic inhibition of male sexual behavior in all species studied to date. The present experiment was designed to evaluate if temporal inactivation of the MPOA/AH by infusions of lidocaine also inhibits sexual behavior in male rats. This would allow us to rule out the possibility that the behavioral effects observed after damage of the MPOA/AH could be associated with plastic changes induced by the lesion in other brain regions. We also evaluated sexual incentive motivation in males after the infusion of lidocaine in a test in which copulation is not possible but where males maintain approach behavior to the estrous females despite repeated testing. The percentage of animals displaying mounts, intromissions and ejaculation was significantly reduced while mount and intromission latency were prolonged after infusion of lidocaine. No changes were observed in sexual behavior after infusion of lidocaine in animals with cannulae outside the MPOA/AH suggesting that the inhibitory effects are specific to this brain region. Sexual incentive motivation was also affected by administration of lidocaine. Males consistently showed a clear preference for the sexually receptive female except when infused with lidocaine. After the infusion of the compound a significant reduction in the time spent in the incentive zone of the stimulus female was observed. These results support the hypothesis that neurons of the MPOA/AH are involved in the control of male sexual motivation.

Convergence of nociceptive information in the forebrain of female rats: reproductive organ response variations with stage of estrus

Neurons in the preoptic area (POA) of the hypothalamus and the bed nucleus of stria terminalis (BST) play an important role in the neuroendocrine control of the reproductive cycle, mating behaviors and nociception. Single unit extracellular recordings were performed in the POA and BST region of 20 urethane anesthetized female rats during either the proestrus (elevated levels of estrogen/progesterone) or metestrus (low circulating hormones) stage of the estrous cycle. A total of 118 neurons in the POA and 65 neurons in the BST responded to the search stimuli, bilateral electrical stimulation of the viscerocutaneous branch of the pelvic nerve and/or sensory branch of the pudendal nerve (i.e., dorsal nerve of clitoris). Most of the neurons responding to the electrical search stimuli received a high degree of somatovisceral convergence, including inputs from the abdominal branches of the vagus, cervix, vagina, colon and skin territories on the perineum and trunk. Mean neuronal response thresholds for vaginal and cervical stimulation but not colon distention were significantly higher for animals tested during proestrus. Also, there was a shift in POA and BST neuronal responsiveness towards more inhibition and less excitation during proestrus for a variety of somatovisceral inputs. These data demonstrate that the changes in hormonal status affect the properties of POA and BST neurons, which likely relates not only to the functional importance of these inputs for reproductive behaviors but also for nociceptive processing as well.

Estradiol and tamoxifen reverse ovariectomy-induced physical inactivity in mice

Decreased physical activity and increased body mass are associated with estrogen deficiency.

PURPOSE

To determine whether estrogen or the estrogen analog, tamoxifen, could reverse those detrimental effects after surgical ovariectomy in mice.

METHODS

Ten-week-old C57BL/6 mice were sham operated (sham, N = 6) or ovariectomized (OVX, N = 9). After 4 wk of voluntary wheel running, placebo (OVX-P) or 17beta-estradiol (OVX-E2) pellets were implanted and the mice ran an additional 4 wk. A second study followed in which mice received placebo, 17beta-estradiol, or tamoxifen (OVX-Tam) simultaneously with ovariectomies. Distances run per 24 h and body masses were analyzed by two-way ANOVA with repeated measures.

RESULTS

During the initial 4 wk, OVX mice ran approximately 80% less and had approximately 20% greater body masses compared with sham mice (P < 0.001). Estradiol replacement quickly reversed the inactivity as OVX-E2 mice increased their running from 1.9 +/- 0.3 km x 24 h(-1) to 6.9 +/- 0.7 km within a week of replacement, which was equivalent to shams (8.1 +/- 0.7 km), whereas OVX-P mice ran only 0.5 +/- 0.2 km (P < 0.01). OVX-E2 mice tended to maintain body mass after estradiol replacement, whereas the OVX-P mice continued to increase mass. OVX mice that received tamoxifen had high running activity, approximately 9 km x 24 h(-1), and maintained body mass.

CONCLUSION

The removal of ovarian hormones caused mice to become inactive and gain body mass. Hormone therapy in the form of 17beta-estradiol or tamoxifen rapidly stimulated voluntary wheel running and reversed body mass gains, indicating that estrogen receptor binding was involved in regulating physical activity.

Cholinergic stimulation in the lateral septal area activates anterior hypothalamic area neurons via excitatory amino acid receptors in rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats. It is suggested that there exist neuronal projections from the lateral septal area (LSV) to the AHA in rats. In this study, we examined whether neurons in the LSV are involved in activation of AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of carbachol into the LSV caused an increase in firing rate of AHA angiotensin II-sensitive neurons. The carbachol-induced increase of firing rate of AHA angiotensin II-sensitive neurons was inhibited by pressure application of the excitatory amino acid receptor antagonist kynurenate but not by the AT1 receptor antagonist losartan onto the same neurons. Microinjection of carbachol into the LSV also increased the firing rate of AHA ACh-sensitive neurons, and the carbachol-induced increase of firing rate of ACh-sensitive neurons was again abolished by pressure application of kynurenate but not by the muscarinic receptor antagonist scopolamine onto the same neurons. Microinjection of the muscarinic receptor antagonist 4-DAMP into the LSV did not affect the firing rate of AHA angiotensin II-sensitive neurons. These findings indicate that neurons in the LSV are involved in activation of AHA angiotensin II-sensitive neurons. It seems likely that the carbachol-induced activation of AHA angiotensin II-sensitive neurons is mainly mediated via excitatory amino acid receptors at AHA neurons.

Cholinergic mechanism in the lateral septal area is involved in the stress-induced blood pressure increase in rats

Previously, we demonstrated that the rostral part of the ventral zone of the lateral septal area (LSV) was involved in the restraint stress-induced pressor response. It is suggested that there exist acetylcholine receptors responsible for blood pressure increase in the caudal part of the lateral septal area. In this study, we examined whether acetylcholine receptors responsible for pressor responses also exist in the rostral part of the LSV and whether these acetylcholine receptors are involved in the stress-induced pressor response in rats. Microinjection of either carbachol (10-100pmol) or physostigmine (0.46 and 1.5nmol) into the LSV caused a dose-dependent increase in blood pressure. The pressor response to carbachol (30pmol) was inhibited by the M1 antagonist pirenzepine and the M3 antagonist 4-DAMP mustard but not by the M2 antagonist methoctramine injected into the LSV. Bilateral microinjections of the M1/M3 antagonist 4-DAMP (1nmol) inhibited the restraint stress-induced pressor response. These findings suggest that M1/M3 muscarinic receptors responsible for blood pressure increase exist in the rostral part of the LSV and they are partly involved in the stress-induced pressor response.

Activation of hypothalamic angiotensin receptors produces pressor responses via cholinergic inputs to the rostral ventrolateral medulla in normotensive and hypertensive rats

We have previously reported that the angiotensin system in the anterior hypothalamic area (AHA) is enhanced in spontaneously hypertensive rats (SHR) and that this enhancement is involved in hypertension in SHR. In addition, acetylcholine (ACh) release is increased in the rostral ventrolateral medulla (RVLM) of SHR, which has also been shown to be involved in hypertension in SHR. In this study, we examined whether the enhanced angiotensin system in the AHA of SHR is related to the increase in cholinergic inputs to the RVLM. Electrical stimulation in the AHA produced a pressor response and an increase in firing rate of RVLM barosensitive neurons. These responses were inhibited and enhanced by RVLM application of the muscarinic receptor antagonist scopolamine and the cholinesterase inhibitor physostigmine, respectively. AHA stimulation also produced release of ACh in the RVLM. Microinjections of angiotensin II and carbachol into the AHA produced pressor responses. The pressor response to angiotensin II was inhibited by scopolamine microinjected into the RVLM, although this produced no effect on the response to carbachol. In SHR, although not in Wistar-Kyoto rats, microinjection of losartan into the AHA inhibited pressor responses to physostigmine. However inhibition was not observed in response to the directly acting muscarinic receptor agonist carbachol, injected into the RVLM. These findings demonstrate that angiotensin receptor activation or electrical stimulation in the AHA produce a pressor response via an increase in ACh release in the RVLM. In addition, the present study suggests that the enhanced angiotensin system in the AHA of SHR increases cholinergic inputs to the RVLM, which leads to increases in blood pressure.

The lateral septal area is involved in mediation of immobilization stress-induced blood pressure increase in rats

Immobilization stress increased the number of neurons with Fos immunoreactivity, mainly in the ventral zone of the rostral part of the lateral septal nucleus (LSV) in rats. Immobilization stress caused an increase in blood pressure, and the stress-induced pressor response was inhibited by the GABA(A) receptor agonist, muscimol (8 and 80 pmol), injected bilaterally into the rostral part of the LSV in a dose-dependent manner. Intracerebroventricular injection of muscimol (16 pmol) did not affect the immobilization stress-induced pressor response. These findings suggest that the rostral part of the LSV is involved in mediation of the stress-induced pressor response.

Stress-induced relapse to drug seeking in the rat: role of the bed nucleus of the stria terminalis and amygdala

There is growing interest in the role that the bed nucleus of the stria terminalis (BNST) and central nucleus of the amygdala (CeA), components of the extended amygdala, play in drug addiction. Within the BNST and CeA, there is an extensive system of intrinsic, primarily GABAergic, interconnections known to synthesize a variety of neuropeptides, including corticotrophin-releasing factor (CRF). The actions of CRF at extrahypothalamic sites,including the BNST and CeA, have been implicated in stress responses and in the aversive effects of withdrawal from drugs of abuse. Most recently, we have shown a critical role for extrahypothalamic CRF in stress-induced reinstatement of drug seeking in rats. In attempting to determine which brain circuitry mediates the effect of stress on relapse and, more specifically, where in the brain CRF acts to initiate the behaviours involved in relapse, we focused on the BNST and CeA. In the present paper, we summarize studies we have conducted that explore the role of these brain sites in stress-induced relapse to heroin and cocaine seeking, and then consider how our findings can be understood within the more general context of what is known about the role of the BNST and CeA in stress-related and general approach behaviours, such as drug seeking.

Projections from the nociceptive area of the central nucleus of the amygdala to the forebrain: a PHA-L study in the rat

The lateral capsular division (CeLC) of the central nucleus (Ce) of the amygdala, in the rat, has been shown to be the main terminal area of a spino(trigemino)-parabrachio-amygdaloid nociceptive pathway [Bernard & Besson (1990) J. Neurophysiol. 63, 473-490; Bernard et al. (1992) J. Neurophysiol. 68, 551-569; Bernard et al. (1993) J. Comp. Neurol. 329, 201-229]. The projections to the forebrain from the CeLC and adjacent regions were studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) restricted in subdivisions of the Ce and the basolateral amygdaloid nucleus anterior (BLA). Our data showed that the entire CeLC projects primarily and extensively to the substantia innominata dorsalis (SId). The terminal labelling is especially dense in the caudal aspect of the SId. The other projections of the CeLC in the forebrain were dramatically less dense. They terminate in the bed nucleus of the stria terminalis (BST) and the posterior hypothalamus (pLH). No (or only scarce) other projections were found in the remaining forebrain areas. The Ce lateral division (CeL) and the Ce medial division (CeM), adjacent to the CeLC, also project to the SId with slightly lower density labelling. However, contrary to the case of the CeLC, both the CeL and the CeM extensively project to the ventrolateral subnucleus of the BST (BSTvl) with a few additional terminals found in other regions of the lateral BST. Only the CeM projects densely to both the interstitial nucleus of the posterior limb of the anterior commissure and the caudal most portion of the pLH. The projections of the BLA are totally different from those of the Ce as they terminate in the dorsal striatum, the accumbens nucleus, the olfactory tubercle, the nucleus of olfactory tract and the rostral pole of the cingulate/frontal cortex. This study demonstrates that the major output of the nociceptive spino(trigemino)-parabrachio-CeLC pathway is to the SId. It is suggested that the CeLC-SId pathway could have an important role in anxiety, aversion and genesis of fear in response to noxious stimuli.

Reproduction-related behaviors of Swiss-Webster female mice living in a cold environment

Based on a molecular neuroendocrine theory about cold environments, thyroid hormone levels, and liganded thyroid hormone receptor interference with estrogen receptor function, experiments were designed to test female mouse reproductive behaviors in the cold. Because natural seasonal temperature declines would usually be associated with decreased photoperiods and reduced food supplies, we combined cold temperatures with short days and metabolic challenge. The simplest hypothesis was that lordosis quotients would be significantly reduced as a result of cold temperatures. That hypothesis was denied. Instead, female approaches to the stud male declined. Because cold temperatures also led to significant reductions of activity in locomotor wheels, a straightforward reduction of activity could explain the female's behavior during mating tests. We suggest that cold temperatures accompanied by reduced photoperiod and reduced metabolic fuel can reduce overall activity in female mice, thus indirectly blocking untimely reproductive behaviors.

Reproduction-related behaviors of Swiss-Webster female mice living in a cold environment

Based on a molecular neuroendocrine theory about cold environments, thyroid hormone levels, and liganded thyroid hormone receptor interference with estrogen receptor function, experiments were designed to test female mouse reproductive behaviors in the cold. Because natural seasonal temperature declines would usually be associated with decreased photoperiods and reduced food supplies, we combined cold temperatures with short days and metabolic challenge. The simplest hypothesis was that lordosis quotients would be significantly reduced as a result of cold temperatures. That hypothesis was denied. Instead, female approaches to the stud male declined. Because cold temperatures also led to significant reductions of activity in locomotor wheels, a straightforward reduction of activity could explain the female's behavior during mating tests. We suggest that cold temperatures accompanied by reduced photoperiod and reduced metabolic fuel can reduce overall activity in female mice, thus indirectly blocking untimely reproductive behaviors.

Biological determinants of spontaneous physical activity

A decline in daily physical activity levels is clearly a major factor contributing to the current obesity epidemic affecting both developed and developing countries in the world. This escalating problem is associated with increased morbidity and mortality and reduced psychosocial health. Thus, increasing physical activity has become the strategy of choice in public health strategies to prevent obesity. Efforts to improve levels of physical activity in the population rely upon an accurate understanding of the determinants of physical activity. Most research has focused on environmental and social influences, while the potential for physical activity to be controlled by intrinsic biological processes has been largely overlooked. This review presents some of the compelling and diverse evidence that has emerged recently showing that physical activity energy expenditure is a critical factor in both the successful regulation of energy balance in normal individuals, as well as the abnormal regulation of energy balance that characterizes obesity. Although the metabolic and genetic factors involved in these regulatory processes remain mostly unidentified, some novel discoveries have been made in this area recently and these are described within this review.

Stress-induced relapse to heroin and cocaine seeking in rats: a review

Studies in humans suggest that exposure to stress increases the probability of relapse to drug use, but until recently there has been no animal model to study the mechanisms that mediate this effect. We have developed a reinstatement procedure that allows us to study the effect of stress on relapse to drug seeking in rats. Using this procedure, we have shown that exposure to intermittent footshock stress reliably reinstates heroin and cocaine seeking after prolonged drug-free periods. In the present paper, we summarize results from several studies on stress-induced reinstatement of heroin and cocaine seeking in rats. We first assess the degree to which the phenomenon of stress-induced relapse generalizes to other stressors, to behaviors controlled by other drugs of abuse, and to behaviors controlled by non-drug reinforcers. We then review evidence from studies concerned with the neurotransmitters, the brain sites, and the neural systems involved in stress-induced reinstatement of drug seeking. Finally, we consider the mechanisms that might underlie stress-induced relapse to drug seeking and the possible implications of the findings for the treatment of relapse to drug use in humans.

Neuronal activity in female rat preoptic area associated with sexually motivated behavior

Single unit activities were recorded from 31 neurons in the preoptic area (POA) of female rats engaging in sexual interactions. Concurrent videotape recordings were used to establish a relationship between neuronal activity and particular behavioral events. In 14 of the 31 neurons, the firing rate changed in association with bouts of sexual activity. The remaining 17 fired with more variability regardless of episodes of sexual interactions. Peri-event histograms identified four types of neurons: type 1 (n=4) increased their firing rate when the female rats initiated proceptive behavior; type 2 (n=4) showed a brief activation when the male mounted; type 3 (n=4) fired in response to intromission, and type 4 (n=2) were inhibited prior to and throughout the display of lordosis reflex. Type 1 neurons fired at significantly higher rates during the solicitatory period, from the initiation of solicitatory locomotion to the male mounts. Their activity was suppressed when the males mounted successfully with intromission. Types 1-3 neurons were recorded from the transitional region between the medial and lateral POAs. Type 4 neurons were located more medially in the medial POA. Systemic injection of pimozide, a dopamine receptor blocker, diminished firing in type 1 neurons and abolished proceptivity. The firing pattern in type 1 neurons appeared to embody the motivational state of the animal with an implication for a consummatory value of penile intromission. Visceral or somatosensory inputs may be responsible for short bursts in types 2 and 3 neurons. Type 4 neurons behaved exactly as if they inhibit the execution of the lordosis reflex. The results showed separate sets of POA neurons each specifically associated with proceptive and receptive components of female rat sexual behavior.

Hypothalamic serotonergic activity correlates better with brain temperature than with sleep-wake cycle and muscle tone in rats

The activity of the serotonergic system varies in phase with the sleep-wake cycle, which is associated with changes in several physiological functions, including electroencephalographic activity, brain temperature, and locomotion. The aim of the present study was to clarify which of these parameters correlates better with serotonergic activity in spontaneous conditions. Voltammetric recordings by telemetry of serotonergic metabolism in the medial preoptic area and polygraphic recordings of sleep-wake activity (by means of electroencephalographic delta band, brain cortical temperature and neck electromyographic activity recordings) were simultaneously performed in freely moving rats. Univariate analyses of variance revealed that each variable under investigation was statistically correlated with serotonergic metabolism. When the variables were entered into the model simultaneously, both partial correlation and step-wise multiple regression analyses indicated that the highest correlation exists between serotonergic metabolism and brain cortical temperature. The present data show that serotonergic activity in the medial preoptic area is closely linked to physiological changes in brain temperature.

Connections of the rat lateral septal complex

The organization of lateral septal connections has been re-examined with respect to its newly defined subdivisions, using anterograde (PHAL) and retrograde (fluorogold) axonal tracer methods. The results confirm that progressively more ventral transverse bands in the hippocampus (defined by the orientation of the trisynaptic circuit) innervate progressively more ventral, transversely oriented sheets in the lateral septum. In addition, hippocampal field CA3 projects selectively to the caudal part of the lateral septal nucleus, which occupies topologically lateral regions of the transverse sheets, whereas field CA1 and the subiculum project selectively to the rostral and ventral parts of the lateral septal nucleus, which occupy topologically medial regions of the transverse sheets. Finally, the evidence suggests that progressively more ventral hippocampal bands innervate progressively thicker lateral septal sheets. In contrast, ascending inputs to the lateral septum appear to define at least 20 vertically oriented bands or subdivisions arranged orthogonal to the hippocampal input (Risold, P.Y. and Swanson, L.W., Chemoarchitecture of the rat lateral septal nucleus, Brain Res. Rev., 24 (1997) 91-113). Hypothalamic nuclei forming parts of behavior-specific subsystems share bidirectional connections with specific subdivisions of the lateral septal nucleus (especially the rostral part), suggesting that specific domains in the hippocampus may influence specific hypothalamic behavioral systems. In contrast, the caudal part of the lateral septal nucleus projects to the lateral hypothalamus and to the supramammillary nucleus, which projects back to the hippocampus and receives its major inputs from brainstem cell groups thought to regulate behavioral state. The neural system mediating defensive behavior shows these features rather clearly, and what is known about its organization is discussed in some detail.

Autoradiographic distribution of cerebral blood flow increases elicited by stimulation of the nucleus basalis magnocellularis in the unanesthetized rat

The nucleus basalis magnocellularis (NBM) of the rat, equivalent of Meynert's nucleus in the primate, is the origin of the main cholinergic innervation of the cerebral cortex. Stimulation of this area has been previously shown to induced marked, cholinergically mediated, blood flow increases in the frontal and parietal cortices. However, the complete distribution of the cerebrovascular effects of NBM stimulation within the whole brain has not been determined. In the present study, we used the [14C]iodoantipyrine autoradiographic method to measure local cerebral blood flow (CBF) in the unanesthetized rat, chronically implanted with a stimulation electrode. We performed unilateral electrical stimulation of the NBM in order to compare both the interhemispheric differences in blood flow and the differences with a group of sham-stimulated rats. Considerable blood flow increases were found in most neocortical areas, exceeding 400% in the frontal area, compared to the control group. Marked responses also appeared in discrete subcortical regions such as the zona incerta, some thalamic nuclei and structures of the extrapyramidal system. These responses were mostly ipsilateral to the stimulation. The significance and the distribution of these blood flow increases are related first, to anatomical and functional data on mainly the cholinergic projections from the NBM, but also non-cholinergic pathways connected with the NBM, second, to biochemical data on the basalocortical system, and third, to the limited ultrastructural data on the innervation of microvascular elements. This cerebrovascular study represents a step in the elucidation of the function of the basalocortical system and provides data which may be related to certain deficits of degenerative disorders such as Alzheimer's disease in which this system is consistently affected.

Priming of locomotor initiation by electrical stimulation in the hypothalamus and preoptic region in the anesthetized rat

Electrical stimulation at a locomotor site can prime (i.e., shorten the latency to initiate) stepping elicited by subsequent stimulation of the same or a different site. We tested for the priming effect in representative sites along the medial forebrain bundle, and determined if its magnitude showed regional differences. Rats (n = 20) were anesthetized with Nembutal and held in a stereotaxic apparatus over a wheel. Stepping was detected by accelerometers attached to the hindlimbs. Priming and test trains of stimulation (0.5-ms cathodal pulses, 50 Hz, 25-75 microA, 7-9-s train duration) separated by 20 s were delivered every 90 s. When the priming and test stimulations were applied to the same site, the priming effects were similar along the entire extent of the medial forebrain bundle. When the priming and test sites were different, the priming effect depended on their relative positions. Anterior stimulation primed posterior sites at magnitude comparable to those produced by stimulating the same posterior site. Posterior stimulation primed anterior sites at a level half of that produced by stimulation of the same anterior site. This pattern was found for priming and test sites that were ipsilateral and contralateral. Priming is a general and robust phenomenon with properties that may be useful for studying locomotor initiation pathways.

Diametrically opposite effects of estrogen on the excitability of female rat medial and lateral preoptic neurons with axons to the midbrain locomotor region

Electrical stimulation of the midbrain locomotor region (MLR) in 76 ovariectomized, urethan-anesthetized female rats elicited antidromic action potentials in 252 preoptic neurons. Thresholds and refractory periods for the activation ranged from 60 to 1550 microA and 1.3 to 5.0 ms, respectively. The probability distribution for the peak-to-peak amplitude (2-14 mV) or the overall duration (0.7-4.4 ms) was bell-shaped, whereas that for the latency (1.8-33.5 ms) was distinctively bimodal with a division at 12.0 ms. Two groups of preoptic neurons of a similar soma size therefore project to the MLR presumably via different routes. In 121 neurons with latencies < or = 12.0 ms, estrogen lowered the antidromic activation thresholds (nested analysis of variance, P < 0.02), but 131 neurons with latencies > 12.0 ms had their thresholds increased (P < 0.005) and refractory periods prolonged (P < 0.02) by estrogen. Even though both overlapped in part, many potentials with the shorter latencies were recorded from the medial part of the lateral preoptic area (mLPO), lateral to the recording sites of the longer-latency potentials in the medial preoptic area (MPO). The observed antagonistic effects of estrogen on the two groups of preoptic neurons with axons to the MLR may contribute to increased locomotor activity in female rats in estrus.

Neural activity in the midbrain correlated with hindlimb extension initiated by locomotor stimulation of the hypothalamus of the anesthetized rat

Midbrain neuronal activity that correlated with the initiation of locomotion produced by hypothalamic stimulation was studied. Locomotion was elicited by electrical stimulation in the perifornical hypothalamus of 59 rats anesthetized with Nembutal. The first hindlimb extension indexed stepping onset. Single and multiple neurons were recorded ipsilateral to the stimulation site at 2230 sites in the anterior and posterior midbrain. To classify responses, activity patterns averaged around stimulation onset and around the extension onset were examined. Responses with specific correlations to extension onset were Type I; responses not specifically related to the extension onset were Type II. In the anterior midbrain, 6% of sites were Type I and 8% were Type II. The larger Type I responses were frequent in the anterior tegmentum near the central gray. The relative frequency of Type I patterns in the posterior ventrolateral tegmentum was similar. Other regions showed relatively more Type II responses; they included the ventral tegmental area, and the regions near the superior cerebellar peduncle and the posterior central gray. Regional population profiles showed that during the initiation of locomotion, neurons in the posterior peribrachial region responded early and neurons in the anterior dorsal and the posterior ventrolateral tegmentum responded later. The initiation-related activity of Type I neurons in the anterior and posterior midbrain tegmentum suggest that they warrant further study for a role in locomotor initiation.

Organization of projections from the anterior hypothalamic nucleus: a Phaseolus vulgaris-leucoagglutinin study in the rat

Anterior hypothalamic nucleus (AHN) projections were examined with the Phaseolus vulgaris-leucoagglutinin (PHA-L) method in adult male rats. Labeled axons from the AHN follow three major routes. 1) A large ascending pathway ends densely in the telencephalon, particularly in the lateral septal nucleus. Axons along this route provide moderate to dense input to the medial and lateral preoptic areas, and a few are also observed in the septofimbrial nucleus and fimbria; the latter end in the temporal hippocampus. A few axons reach the amygdala through the bed nuclei of the stria terminalis, which receive a moderate input, and then the stria terminalis, and others reach it by way of the ansa peduncularis. 2) The second pathway travels dorsal to the AHN, ending densely in rostral perifornical regions of the lateral hypothalamic area, and the rostral ventrolateral tip of the nucleus reuniens. The parataenial and rostral paraventricular thalamic nuclei also receive a significant input. Some fibers and boutons were also observed in the rhomboid, interanterodorsal, and mediodorsal nuclei, and others course through the stria medullaris to the lateral habenula. 3) the largest pathway descends through dorsal and ventral routes in the medial hypothalamic zone before ending massively in the periaqueductal gray. Dorsal route fibers provide inputs to the zona incerta and posterior hypothalamic nucleus, whereas more ventral axons generate dense terminal fields in the ventromedial nucleus capsule and core, and dorsal premammillary nucleus. The retrochiasmatic area, dorsomedial nucleus, and medial supramammillary nucleus also receive significant inputs, and a few axons end in the subparafascicular nucleus, superior colliculus, and mammillary body. The caudalmost axons were seen in the pontine central gray and reticular formation. These pathways are bilateral, usually with a distinct ipsilateral predominance. The overall pattern of efferents from anterior, central, and posterior parts of the AHN is similar, whereas the relative densities of particular terminal fields may vary considerably. Projections from adjacent parts of the retrochiasmatic and perifornical areas are also described. The results are discussed in terms of neural circuitry that may be involved in mediating interactions between animals.

Noradrenergic inputs to sleep-related neurons in the preoptic area from the locus coeruleus and the ventrolateral medulla in the rat

Responses of sleep-related neurons in the preoptic area (POA) to stimulation of the locus coeruleus (LC) and the ventrolateral medulla (VLM), components of the reticular activating system, were recorded in the unanesthetized, head-restrained rat. Single-pulse stimulation of the LC and the VLM, respectively, inhibited 50% and 54% of 30 sleep-active neurons and excited 47% and 67% of 34 waking-active neurons. The remaining neurons were mostly unaffected. Seventy-three neurons that were not related to a sleep-wake state were mostly (i.e., 73-80%) unresponsive to stimulation. The high incidence of responses by sleep-related neurons suggests that neural inputs from the LC and VLM regulate the hypnogenic mechanisms in the POA. Stimulation of the LC antidromically activated 15% of sleep-active neurons and 11% of waking-active neurons. Thus, some of the sleep-related neurons in the POA may regulate LC neurons. In a later stage of the experiment, we used isoflurane-anesthetized rats that had been used for recording sleep-related neurons. Antagonists for adrenoceptors at a concentration of 10 microM were applied to neurons through a multibarrel micropipette to examine the involvement of noradrenaline in the responses as a neurotransmitter. Application of the alpha 2-blocker, yohimbine, attenuated the inhibitory responses in all 7 neurons tested. The beta-blocker, timolol, and the alpha 1-blocker, prazosin, did not alter any of the inhibitory responses. On the other hand, timolol attenuated the excitatory responses in 4 of 7 neurons, and prazosin attenuated the excitatory responses in 5 of 12 neurons. Yohimbine did not affect the excitatory responses. Thus, the LC and the VLM probably inhibit sleep-active neurons through alpha 2-adrenoceptors and excite waking-active neurons through either beta- or alpha 1-adrenoceptors.

Preoptic and hypothalamic neurons and the initiation of locomotion in the anesthetized rat

Despite its insensate condition and apparent motoric depression, the anesthetized rat can provide useful information about the systems involved in locomotor initiation. The preparation appears to be particularly appropriate for the study of the appetitive locomotor systems and may be more limited for the study of the circuits involved in exploratory and defensive locomotion. In the anesthetized rat, pharmacological evidence indicates that the preoptic basal forebrain contains neurons which initiate locomotor stepping. Mapping with low levels of electrical stimulation indicates, but does not prove, that a region centered in the lateral preoptic area might be the location of these neurons. Several lines of evidence indicate that locomotor stepping elicited by electrical stimulation of the hypothalamus is mediated by neurons in the perifornical and lateral hypothalamus. Locomotor effects of hypothalamic stimulation persist in the absence of descending fibers of passage from the ipsilateral preoptic locomotor regions but are severely impaired by kainic acid lesions in the area of stimulation. Injections of glutamate into the perifornical and lateral hypothalamus elicit locomotor stepping at short latencies. Anatomical evidence suggests that the two regions are components of a network for appetitive locomotion. The recognition that multiple systems initiate locomotion both clarifies and complicates the study of locomotion. It provides a framework that incorporates disparate findings but it also underscores the need for increased attention to behavioral issues in studies of locomotor circuitry.

Socio-sexual behavior in male rats after lesions of the medial preoptic area: evidence for reduced sexual motivation

Different hypotheses have been put forward trying to explain the mechanisms associated with the disruption of male sexual behavior after lesions of the medial preoptic area (MPOA). It has been suggested that sexual motivation, motor execution or both are affected by MPOA lesions. In the present experiment, the socio-sexual behavior of male rats bearing extensive MPOA lesions, that abolished sexual behavior, was compared with that of sham-lesioned animals and to prelesion levels. The socio-sexual interactions were recorded for 10 min in one prelesion and two postlesion tests. The frequency and duration of the following behaviors were recorded: rearing, sniffing, self-grooming, grooming partner, genital exploration, pursuit and resting. The analysis of the socio-sexual interactions showed that the frequency and duration of pursuit was reduced in the first and second tests after the lesion in comparison to both prelesion levels and to a sham-lesioned group. There is strong evidence that pursuit is the only precopulatory behavior that can consistently predict the appearance of sexual behavior. When pursuit is reduced the transition from the precopulatory to the copulatory phase is made more difficult. Therefore, it appears that the MPOA lesions reduce the subject's motivation to engage in sexual behavior.