The aim of the study was to test whether the synthesis of substance P (SP) and that of its receptor (also known as NK1 receptor) are coordinately regulated after chronic pharmacologic intervention in two neural systems, the spinal cord and basal ganglia. In one set of experiments, capsaicin was administered subcutaneously during the early postnatal period (day 3 after birth) to induce degeneration of afferent sensory neurons in the spinal cord. In the other set of experiments, interruption of dopaminergic transmission was achieved by two methods: (a) The neurotoxin 6-hydroxydopamine was used to denervate dopaminergic neurons during the early postnatal period, and (b) haloperidol was used in adult animals to block dopaminergic transmission by receptor blockade. The spinal cord, striatum, or both were used for the quantification of tachykinin [SP and neurokinin A (NKA)] and opioid peptides [[Met5]-enkephalin (ME) and dynorphin A (1-8) (DYN)] by radioimmunoassays. The abundance of total SP-encoding preprotachykinin (PPT) mRNA and SP receptor (SPR) mRNA in spinal cord (C5 to T1 segments), striatum, or microdissected substantia nigra was determined by northern blot or solution hybridization analysis. Amines and their acid metabolites were quantified by HPLC. Capsaicin administration (subcutaneously) during the early postnatal period increased latency in a hot-plate test, decreased SP and NKA levels, increased levels of PPT mRNAs, and did not affect SPR mRNA levels in the spinal cord. Intraspinal SP systems may attempt to compensate for the loss of afferent SP input, whereas spinal cord receptor mRNA levels do not appear to be altered.(ABSTRACT TRUNCATED AT 250 WORDS)