Amphetamine acts within the lateral hypothalamic area to elicit affectively neutral arousal and reinstate drug-seeking

Abstract

Psychostimulants, including amphetamine (AMPH), exert robust arousal-enhancing, reinforcing and locomotor-activating effects. These behavioural actions involve drug-induced elevations in extracellular norepinephrine (NE) and dopamine (DA) within a variety of cortical and subcortical regions. The lateral hypothalamic area (LHA), including the lateral hypothalamus proper, perifornical area and adjacent dorsomedial hypothalamus, is implicated in appetitive- and arousal-related processes. The LHA is innervated by both NE and DA projections and systemically administered AMPH has been demonstrated to activate LHA neurons. Combined, these and other observations suggest the LHA may be a site of action in the behavioural effects of psychostimulants. To test this hypothesis, we examined the degree to which AMPH (10 nmol, 25 nmol) acts within the LHA to exert arousing, locomotor-activating and reinforcing actions in quietly resting/sleeping rats. Although intra-LHA AMPH robustly increased time spent awake, this occurred in the absence of pronounced locomotor activation or reinforcing actions, as measured in a conditioned place preference (CPP) paradigm. Arousing and stressful conditions or drug re-exposure can elicit relapse in humans and reinstate drug-seeking in animals. Given the LHA is also implicated in the reinstatement of drug-seeking behaviour, additional studies examined whether AMPH acts within the LHA to reinstate an extinguished CPP produced with systemic AMPH administration. Our results demonstrate that AMPH action within the LHA is sufficient to reinstate drug-seeking behaviour, as measured in this paradigm. Collectively, these observations demonstrate that psychostimulants act within the LHA to elicit affectively neutral arousal and reinstate drug-seeking behaviour.

Fig. 1.

Left panel: photomicrograph depicting placement of the infusion needle within the lateral hypothalamic area (LHA). Arrow indicates the ventral most extent of the needle track. Right panel: raw cortical electroencephalograph (EEG) and electromyograph (EMG) traces before and after intra-LHA amphetamine (AMPH). Shown are 5 min recordings of EEG/EMG prior to (pre-infusion) and beginning 2 min following the end of a unilateral intra-LHA infusion of AMPH (25 nmol; post-infusion). Prior to the infusion, the animal spent the majority of time asleep, reflected in large-amplitude, slow-wave EEG activity combined with low-amplitude EMG activity. Following AMPH infusion, the animal spent a majority of time awake, reflected in a decrease in EEG amplitude and an increase in EMG amplitude, including large deflections indicative of active movement. 3 V=third ventricle; fx=fornix; mtt=mammillothalamic tract; opt=optic tract.

Fig. 2.

Intra- lateral hypothalamic area (LHA) infusions of amphetamine (AMPH) dose-dependently increases waking and decreases slow-wave and REM sleep. Shown are the effects of unilateral infusion of vehicle, 10 nmol AMPH,25 nmol AMPH, and bilateral infusion of 25 nmol AMPH infused into the LHA on time spent in electroencephalograph (EEG)/electromyography (EMG)-defined waking, slow-wave sleep, and REM sleep states. Symbols represent mean (±S.E.M.) time (s) spent awake per 30 min epoch. PRE1 and PRE2 represent pre-infusion epochs. POST1–POST3 represent post-infusion epochs. *p<0.05; **p<0.01 compared to vehicle-treated animals.

Fig. 3.

Schematic depiction of the location of unilateral infusion of 25 nmol amphetamine (AMPH) and the amount of time spent awake associated with each infusion. Symbols indicate the position of the ventral tip of the infusion needle determined from histological sections. Numerals specify the range for time spent awake in the first 60 min following the infusion (1=0–600 s; 2=601–1200 s; 3=1201–1800 s; 4=1801–2400 s; 5=2401–3000 s). No animals were awake >3000 s following infusion. Vehicle-treated rats displayed a mean time awake in this period of 739±81 s, reflecting the fact that typically animals will spontaneously wake for relatively brief periods to groom, eat and drink and then return to sleep. AMPH-induced waking was observed with infusions placed within the majority of the medial–lateral extent of the dorsal hypothalamus. Infusions outside of this dorsal region were substantially less effective at eliciting waking beyond levels observed in either untreated pre-infusion epochs or following vehicle treatment. AHN, anterior hypothalamic nucleus; ARH, arcuate nucleus hypothalamus; cpd, cerebral peduncle; DMH, dorsomedial nucleus hypothalamus; fx, columns of the fornix; LHA, lateral hypothalamic area; mtt, mammillothalamic tract; opt, optic tract; V3, third ventricle; VMH, ventromedial nucleus hypothalamus; ZI, zona incerta. Numbers below panels refer to coronal level relative to bregma (modified from Swanson, 1992).

Fig. 4.

Reinforcing effects of s.c. amphetamine (AMPH) but not intra-lateral hypothalamic area (LHA) AMPH, as measured by conditioned place preference (CPP). Left panel: a wake-promoting dose of s.c. administered AMPH (AMPH 0.25 mg/kg s.c.) elicited a significant increase in time spent in the drug-paired side. Right panel: in contrast, a wake-promoting dose of AMPH bilaterally infused into the LHA (25 nmol) did not increase time spent in the drug-paired side. Bars display mean (±S.E.M.) time (s) spent in the drug-paired side over a 10 min testing period. **p<0.01 vs. PRE COND.

Fig. 5.

Systemically administered amphetamine (AMPH) reinstates an extinguished AMPH-induced conditioned place preference (CPP). Systemically administered AMPH (0.25 mg/kg s.c.) elicited a significant CPP (PRE COND vs. POST COND). Following extinction training, significantly less time was spent in the drug-paired side (EXTINCTION). Subsequent treatment with AMPH (0.10 mg/kg s.c.) reinstated the CPP (AMPH REINST). Bars display mean (±S.E.M.) time (s) spent in the drug-paired side over a 10 min testing period. **p<0.01 vs. PRE COND; ^##p<0.01 vs. POST COND; ^%p<0.05 vs. EXTINCTION.

Fig. 6.

Amphetamine (AMPH) directly infused into the lateral hypothalamic area (LHA) reinstates an extinguished conditioned place preference (CPP). Left panel: systemically administered AMPH (systemic; 0.25 mg/kg s.c.) elicited a significant CPP (PRE COND vs. POST COND 0.25 mg/kg). After conditioning, these same animals underwent extinction training (EXTINCTION). Following extinction, one-half of these animals then received bilateral intra-LHA vehicle infusions (REINST VEH) while the other half received bilateral intra-LHA AMPH (REINST AMPH 25 nmol). Following each treatment, animals were re-tested for extinction, and then received the other treatment (e.g. AMPH if they had first received vehicle). Middle panel: bilateral infusion of vehicle into the LHA had no effect on time spent in the drug-paired side in extinguished animals. Right panel: in contrast, bilateral intra-LHA AMPH (25 nmol) reinstated the extinguished CPP, significantly increasing time spent in the drug-paired side. Bars display mean (±S.E.M.) time (s) spent in the drug-paired side over a 10 min testing period. **p<0.01 vs. PRE COND; ^##p<0.01 vs. POST COND; ^%p<0.05 vs. EXTINCTION.